A Pratical Strategy for Assessing the Subsurface Vapor-to-Indoor Air Migration Pathway at Petroleum Hydrocarbon Sites

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1 Cllecting and Interpreting Sil Gas Samples frm the Vadse Zne A Pratical Strategy fr Assessing the Subsurface Vapr-t-Indr Air Migratin Pathway at Petrleum Hydrcarbn Sites Regulatry Analysis and Scientific Affairs PUBLICATION NUMBER 4741 NOVEMBER 2005

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3 Cllecting and Interpreting Sil Gas Samples frm the Vadse Zne A Practical Strategy fr Assessing the Subsurface Vapr-t-Indr Air Migratin Pathway at Petrleum Hydrcarbn Sites Regulatry and Scientific Affairs PUBLICATION NUMBER 4741 NOVEMBER 2005 PREPARED UNDER CONTRACT BY: Lesley Hay Wilsn, Ph.D. Sage Risk Slutins LLC Paul C. Jhnsn, Ph.D. Department f Civil and Envirnmental Engineering Arizna State University James R. Rcc Sage Risk Slutins LLC

4 SPECIAL NOTES API publicatins necessarily address prblems f a general nature. With respect t particular circumstances, lcal, state, and federal laws and regulatins shuld be reviewed. Neither API nr any f API s emplyees, subcntractrs, cnsultants, cmmittees, r ther assignees make any warranty r representatin, either express r implied, with respect t the accuracy, cmpleteness, r usefulness f the infrmatin cntained herein, r assume any liability r respnsibility fr any use, r the results f such use, f any infrmatin r prcess disclsed in this publicatin. Neither API nr any f API s emplyees, subcntractrs, cnsultants, r ther assignees represent that use f this publicatin wuld nt infringe upn privately wned rights. API publicatins may be used by anyne desiring t d s. Every effrt has been made by the Institute t assure the accuracy and reliability f the data cntained in them; hwever, the Institute makes n representatin, warranty, r guarantee in cnnectin with this publicatin and hereby expressly disclaims any liability r respnsibility fr lss r damage resulting frm its use r fr the vilatin f any authrities having jurisdictin with which this publicatin may cnflict. API publicatins are published t facilitate the brad availability f prven, sund engineering and perating practices. These publicatins are nt intended t bviate the need fr applying sund engineering judgment regarding when and where these publicatins shuld be utilized. The frmulatin and publicatin f API publicatins is nt intended in any way t inhibit anyne frm using any ther practices. API is nt undertaking t meet the duties f emplyers, manufactures, r supplies t warn and prperly train and equip their emplyees, and thers expsed, cncerning health and safety risks and precautins, nr undertaking their bligatins t cmply with authrities having jurisdictin. Infrmatin cncerning safety and health risks and prper precautins with respect t particular materials and cnditins shuld be btained frm the emplyer, the manufacturer r supplier f that material, r the material safety data sheet. All rights reserved. N part f this wrk may be reprduced, stred in a retrieval system, r transmitted by any means, electrnic, mechanical, phtcpying, recrding, r therwise, withut prir written permissin frm the publisher. Cntact the Publisher, API Publishing Services, 1220 L Street, N.W., Washingtn, D.C Cpyright 2005 American Petrleum Institute ii

5 Frward Nthing cntained in any API publicatin is t be cnstrued as granting any right, by implicatin r therwise, fr the manufacture, sale, r use f any methd, apparatus, r prduct cvered by letters patent. Neither shuld anything cntained in the publicatin be cnstrued as insuring anyne against liability fr infringement f letters patent. Suggested revisins are invited and shuld be submitted t the Directr f Regulatry Analysis and Scientific Affairs, API, 1220 L Street, NW, Washingtn, DC The infrmatin included in this publicatin is intended as general guidelines and nt specific recmmendatins fr all sites. Site-specific cnsideratins, prfessinal judgment and regulatry requirements will dictate the methds and prcedures used at any particular site. This publicatin is nt intended t replace the advice f qualified prfessinals. Trademarks: Cali-5-bnd is a registered trademark f Calibrated Instruments, Inc. Luer-lk is a registered trademark f the Bectn, Dickinsn and Cmpany Crpratin Tedlar is a registered trademark f the E. I. Du Pnt De Nemurs and Cmpany Crpratin Tefln is a registered trademark f the E. I. Du Pnt De Nemurs and Cmpany Crpratin Tenax is a registered trademark f the Buchem B.V. Crpratin iii

6 Acknwledgements API and the authrs wuld like t acknwledge and thank the fllwing peple fr their cntributins f time, cmments, and expertise during this study and in the preparatin f this reprt: API STAFF CONTACT Harley Hpkins, Regulatry Analysis and Scientific Affairs Department (RASA) MEMBERS OF THE SOIL AND GROUNDWATER TECHNICAL TASK FORCE Curtis Stanley, Shell Glbal Slutins (U.S.), Chairman Brian Davis, Chevrn Crpratin Rick Greiner, CncPhillips Dan Irvin, CncPhillips Vic Kremesec, Atlantic Richfield Cmpany, A BP Affiliated Cmpany Matt Lahvis, Shell Glbal Slutins (U.S.), Inc. Paul Lundegard, Chevrn Crpratin Mark Malander, ExxnMbil Crpratin Tm Peargin, Chevrn Crpratin Tdd Ririe, Chevrn Crpratin OUTSIDE REVIEWERS Bart Eklund, URS Crpratin Rbert Ettinger, GeSyntec Cnsultants Blayne Hartman, H&P Mbile Gechemistry Rbert Pirkle, Micrseeps Gina M. Plantz, Severn Trent Labratries, Inc. Chris VanCantfrt iv

7 Executive Summary This dcument fcuses n the cllectin f sil gas samples fr assessing the significance f the subsurface-vapr-t-indr-air expsure pathway. While sil gas cllectin is nt the nly means f assessing this pathway, sil gas data play a prminent rle in recent guidance published by the American Petrleum Institute (API 1998) and the United States Envirnmental Prtectin Agency (USEPA 2002a). Fr example, these data can be used t help make decisins cncerning: Resurce Cnservatin and Recvery Act (RCRA) crrective actin envirnmental indicatrs (EI) fr human health expsures Current expsure scenaris in existing buildings Future expsure scenaris in existing buildings Future expsure scenaris in future buildings. This dcument is intended t cmplement API 1998 and USEPA 2002a. It prvides mre indepth infrmatin n issues assciated with sil gas sampling and data interpretatin as applied t pathway assessment. This dcument is specifically fcused n petrleum hydrcarbn impacted sites. Hwever, much f the infrmatin presented is applicable t all sil gas sampling. Sil gas sampling has been used fr many years fr site assessment and remediatin system mnitring purpses. The user, hwever, will find that the data quality bjectives and acceptable methds f sampling fr pathway assessment are different frm thse that are cmmnly acceptable fr using sil gas data fr delineatin, site assessment, r mnitring remediatin systems. This dcument is unique in that it emphasizes cnceptual mdels fr vapr transprt, describes hw t chse sample lcatins and depths, explains hw t check the data fr incnsistencies and als prvides checklists n each f these tpics t assist field prject mangers. This dcument allws fr flexibility in the selectin and refinement f practicable and defensible sampling methds. The fcus here is n identifying key issues assciated with sil gas sampling and data interpretatin. Field prject managers shuld find this dcument useful when develping scpe-f-wrk requirements fr site-specific wrk plans and bid requests. T supprt preparatin f site-specific wrk plans, scpe-f-wrk actin items are included at the end f Sectins 4.0 thrugh 7.0. Sectin 1.0 prvides a brief intrductin. Sectin 2.0 discusses sil gas transprt, with emphasis n petrleum hydrcarbn vaprs, and presents a brief synpsis f expected sil gas prfiles based n empirical analysis f existing data. Sectin 3.0 discusses the cnceptual vapr-migratin mdel. Sectin 4.0 fcuses n sampling lcatins, depths, and sampling frequency. Sectin 5.0 fcuses n mnitring installatins and sample cllectin prcedures. Sectin 6.0 discusses methds f sil gas analysis. Sectin 7.0 discusses interpretatin f sil gas data. Appendix A prvides a site infrmatin checklist. Appendix B prvides wrksheets fr three typical scenaris that can be used fr planning sampling lcatins. Appendix C prvides mre details n sample cllectin. Appendix D gives supprting infrmatin n analytical methds, and Appendix E prvides tls t be used in the data evaluatin. v

8 Recmmendatins presented here are based n the experience and prfessinal judgment f the authrs and are designed t be bradly applicable. This des nt imply that the recmmendatins are universally applicable r that there are nt situatins fr which ther methds r prcedures wuld be better suited. The user f this manual is cautined t cnsider all f the site-specific infrmatin and t make decisins based n site-specific circumstances, experience, and prfessinal judgment. In additin, sme regulatry agencies have expressed preferences fr sampling methds and techniques. The applicable regulatry preferences shuld be examined fr each site. This dcument des nt address all ptential strategies fr assessing the vapr-t-indr-air pathway. The user f this manual is directed t USEPA 2002a, and applicable state regulatry guidance fr ther methds t assess the vapr-t-indr-air pathway (see PaDEP 2002; MaDEP 2002; WDHFS 2003; NJDEP 2004; CRWQCB 2003; CSDDEH 2003). This dcument als des nt address safety- and hazard-mitigatin effrts t prevent fires r explsins resulting frm the accumulatin f hazardus vaprs. It assumes that these situatins have been cntrlled by emergency r immediate respnse actins befre the planning f a sil-gas-sampling prgram is initiated. If the results f the sil-gas-sampling prgram indicate that there is an immediate cncern fr human expsures t vapr-phase chemicals f cncern, then emergency respnse r interim actins shuld be implemented as required under state r federal regulatins. vi

9 Cntents 1.0 Intrductin Sil Gas Transprt and Sil Gas Prfiles at Petrleum Hydrcarbn Impacted Sites Expectatins fr Sil Gas Prfiles at Petrleum Hydrcarbn Impacted Sites Measured Sil Gas Prfiles at Petrleum Hydrcarbn Impacted Sites Cnceptual Migratin Mdel fr Subsurface Vapr t Indr Air Develpment f a Strategy fr Sil Gas Sampling General Apprach Pint Sampling Transects and Vertical Prfiles Selectin f Lateral Psitins fr Sil Gas Transects Vertical Prfiles Summary f Sampling Depth and Lcatin Selectin Cnsideratins Sme Cmments n Sample Cllectin Adjacent t and Beneath Buildings Sampling Frequency Additinal Cnsideratins t Increase Cnfidence in Data Sets and the Interpretatin f Sil-Gas- Sampling Results Sil Gas Sample Cllectin Basic Mnitring Installatin Optins Permanent Prbes Temprary Driven Prbes Cmparisn f Mnitring Installatins Other Cnsideratins fr Sampling Prbe Installatins Sample Cllectin Prcedures Sil Gas Equilibratin Sample Prbe Purging Sample Cllectin Sample Cllectin Vacuum Ways t Avid Cmmn Prblems with Sil Gas Sampling Alternatives t Sil Gas Sampling Passive Implant Samplers Flux Chambers Analytical Methds Analytical Methd Selectin Field Analytical Methds Cmmn Analytical Methds Data Quality Analysis and Interpretatin f Sil Gas Sampling Data Data Organizatin Data Analysis Data Quality Analysis Data Cnsistency Analysis Expsure Pathway Assessment Expsure Pathway Cmpleteness Expsure Pathway Significance Further Evaluatin References Additinal Reading Analytical Methds Bidegradatin Data Analysis General Mdeling Sample Cllectin Methds Site Characteristics and Cnceptual Vapr-Migratin Mdels vii

10 Appendix A. Appendix B. Appendix C. Appendix D. Appendix E. Characteristics Checklist Selectin f Sil Gas Sample Lcatins Sil Gas Sample Cllectin Analytical Methds Data Evaluatin Figures Figure 2-1. Typical cnventinal cnceptual mdel f sil gas migratin... 3 Figure 2-2. Revised cnceptual mdel f sil gas migratin at petrleum hydrcarbn impacted sites Figure 2-3. Sil gas prfiles (Rggemans et al. 2002) Figure 2-4. Sil gas prfile at a site with methane prductin in the surce zne (Jhnsn et al. 2003). This figure shws the sil gas prfiles fr xygen (circles) and methane (diamnds) Figure 2-5. Nrmalized sil gas cncentratin distributin fr xygen and hydrcarbn underging aerbic bidegradatin with first-rder rate λ = 0.18 (h -1 ) and vapr surce at cncentratins f 20 mg/l, 100 mg/l and 200 mg/l lcated underneath a basement fundatin at a depth f 8 m belw grund surface. Hydrcarbn and xygen cnturs are nrmalized t the surce and the atmspheric cncentratins, respectively. Frm Abreu (2005)...12 Figure 2-6.Nrmalized sil gas cncentratin distributins fr xygen and hydrcarbn underging aerbic bidegradatin with a first-rder rate λ = 0.18 (h -1 ) and a vapr surce cncentratin f 200 mg/l lcated beneath a slab-n-grade fundatin at depths f 1 m, 3 m, 5 m and 8 m belw grund surface. Hydrcarbn and xygen cnturs are nrmalized t the surce and the atmspheric cncentratins, respectively. Frm Abreu (2005) Figure 4-1. Cnsideratins fr vertical prfiles at relatively flat sites (e.g., cnsistent distance between the grund surface and the vapr surce depth) with cnsistent stratigraphy Figure 4-2. Cnsideratins fr vertical prfiles at sites with significant spatial variability in the distance between grund surface and the vapr surce depth Figure 4-3. Sub-slab-t-indr-air attenuatin Figure 7-1. Flwchart fr data evaluatin Tables Table 4-1. Cnsideratins fr Samples Cllected Immediately abve the Vapr Surce Table 4-2. Cnsideratins fr Samples Cllected Laterally Mid-Way between the Vapr Surce and the Building Lcatin Table 4-3. Cnsideratins fr Samples Cllected Adjacent t the Base f an Existing Building Fundatin r Basement Table 4-4. Cnsideratins fr Samples Cllected Immediately belw the Building Fundatin r Basement Table 4-5. Cnsideratins fr Samples Cllected within the Ftprint f a Future Building Lcatin Table 6-1. Cmmn Analytical Methds Table 7-1. Example Cmparisns f Bidegradatin Stichimetry and Fluxes viii

11 1.0 Intrductin Tpic: This sectin prvides general infrmatin abut the subsurface-vapr-t-indr-air expsure pathway. Purpse: T define the fllwing activities fr cllecting and interpreting sil gas samples: Cllecting infrmatin Identifying sample lcatins Determining sample cllectin methds Selecting sample analytical methds Interpreting results. Significance: Sil gas data can be used t assess fr significant cncentratins f petrleum hydrcarbn vaprs in the subsurface, t determine if vapr migratin t a building is ccurring, t estimate pssible indr air cncentratins, and t identify significant attenuatin f vapr transprt by natural prcesses. Sil gas sampling has been cnducted fr many years as a tl fr evaluating the distributin f chemicals f cncern in sil and grundwater, fr guiding site characterizatins, and fr mnitring remedial actin prgress. Hwever, sil gas data cllected fr a site characterizatin are generally fcused n develping an understanding f the lcatin and distributin f chemicals f cncern in envirnmental media (e.g., sil, grundwater) and nt n determining the definitive cncentratins f chemicals f cncern in sil gas. As a result, the methdlgies used fr traditinal site characterizatins may nt be apprpriate fr evaluating the subsurface-vaprt-indr-air expsure pathway. When assessing the subsurface-vapr-t-indr-air pathway, an initial screen is cnducted t identify thse sites where further site-specific investigatin and assessment are warranted. As the sampling f indr air pses many practical and technical challenges, the ensuing site-specific pathway assessment ften fcuses n sil gas cllectin and analysis (as described in API 1998). Fr the evaluatin f the subsurface-vapr-t-indr-air expsure pathway, sil gas samples are cllected t: Establish a snapsht f the cncentratins f chemicals f cncern in sil gas at a lcatin alng the expsure pathway between the surce and the building lcatin Analyze the ptential fr human receptrs t be expsed in indr envirnments Predict the expected indr air cncentratin based n the sil gas cncentratins using an estimated attenuatin factr 1

12 Accunt fr the fate and transprt prcesses between a sampling lcatin and the indr envirnment. In the case f petrleum hydrcarbns, aerbic bidegradatin has the ptential t attenuate cncentratins f chemicals f cncern in sil gas and fluxes significantly as the vaprs mve tward buildings frm sils r grundwater cntaining cncentratins f petrleum hydrcarbn chemicals f cncern. Therefre, sil gas data cllected fr the subsurface-vapr-t-indr-air expsure pathway need t be fcused n the purpse and use f the data and be based n these specific data-quality bjectives. The cllectin and use f sil gas data t evaluate the subsurface-vapr-t-indr-air expsure pathway is a relatively new apprach fr this expsure pathway, and limited infrmatin is available n the apprpriate methdlgies. T address this need, ptins fr the cllectin, analysis, and interpretatin f sil gas data are presented in this dcument. This dcument emphasizes cnceptual mdels fr vapr transprt in the frmulatin f sampling plans and data analysis and presents infrmatin needed t supprt the selectin f sample lcatins and depths. The verall prcess f cllectin and analysis f sil gas samples is supprted thrugh imprtant reminders and checklists at the end f each sectin. A systematic prcess is prvided in the data analysis sectin t check the cllected data fr incnsistencies and fr determinatin f situatins requiring further study. Specifically, five basic activities have been identified fr the cllectin and interpretatin f sil gas samples. These activities include the fllwing: 1. Cllecting infrmatin t understand the characteristics f the site (Sectin 3.0, Appendix A) 2. Identifying the lcatin r lcatins fr sil gas sample cllectin (Sectin 4.0, Appendix B) 3. Determining the methd r methds fr cllecting sil gas samples (Sectin 5.0, Appendix C) 4. Selecting the methd r methds t analyze sil gas samples (Sectin 6.0, Appendix D) 5. Evaluating and interpreting the results (Sectin 7.0, Appendix E). T present a basic understanding f the prcesses affecting the subsurface-vapr-t-indr-air expsure pathway and a fundatin fr decisins n sil gas sampling, the user is first prvided with a discussin f sil gas transprt (Sectin 2.0). This is fllwed by a discussin f the five activities identified fr the cllectin and interpretatin f sil gas samples. 2

13 2.0 Sil Gas Transprt and Sil Gas Prfiles at Petrleum Hydrcarbn Impacted Sites Tpic: The cnceptual mdel fr sil gas migratin and expectatins fr sil gas prfiles at petrleum hydrcarbn sites are discussed here. Purpse: T shw hw chemicals vlatilize frm impacted sil r grundwater and migrate t regins f lwer chemical cncentratin (e.g., the atmsphere, cnduits, basements). Significance: The migratin f hydrcarbn vaprs thrughut mst f the subsurface is thught t be primarily the result f diffusive transprt prcesses. Other prcesses, such as advective sil gas flw, xygen migratin, and aerbic bidegradatin, als can significantly affect the sil gas migratin t buildings. Sil gas migratin is typically cnceptualized as shwn Figure 2-1. Chemicals vlatilize frm impacted sil r grundwater and migrate t regins f lwer chemical cncentratin (e.g., the atmsphere, cnduits, basements). Althugh Figure 2-1 schematically depicts huses, vapr intrusin t cmmercial buildings and ther structures als can be f cncern. This guidance is bradly applicable t all building scenaris. { Atmsphere and Indr Air { Vadse Zne Sil Gas basement crawl-space Chemical Vapr Migratin slab Sil Surce (residual r LNAPL) Grundwater Surce Figure 2-1. A typical cnventinal cnceptual mdel f sil gas migratin. 3

14 Vapr Migratin Vapr migratin results frm the fllwing prcesses: Diffusin the randm kinetic energy f mlecules results in a net mvement f chemicals frm areas f higher cncentratin t areas f lwer cncentratin. The diffusive flux f chemicals is prprtinal t the cncentratin gradient (change in cncentratin divided by distance) and the effective prus medium diffusin cefficient (typical values are in the cm 2 /s range fr many chemicals and typical vadse zne cnditins [see Figure 4 in Jhnsn 2002]). Advectin the mvement f chemicals with the bulk mtin f the sil gas, where the flw f the bulk sil gas is the result f a sil gas pressure gradient caused by subsurface-building-pressure differences r atmspheric pressure fluctuatins. Diffusin is likely the dminant prcess far frm a building and near the surce, and advectin is likely the dminant prcess near the building. The effect f atmspheric pressure fluctuatins are nt yet easily quantifiable, but it is likely that the resulting scillatry mvement f sil gas has a net effect n chemical migratin that is small in shallw sils and f similar magnitude t diffusin in deeper sils (Massmann and Farrier 1992; Parker 2003). This migratin f hydrcarbn vaprs thrughut mst f the subsurface is thught t be primarily the result f diffusive transprt prcesses. Hwever, atmspheric pressure fluctuatins als culd cntribute t vapr spreading, with the significance f the vapr spreading depending n the thickness and cmpsitin f the vadse zne. Near buildings r ther enclsed spaces, significant advective sil gas flw fields may exist due t under- r verpressurizatin f a building (e.g., resulting frm the peratin f indr appliances, such as a clthes dryer r the heating, ventilatin, and air cnditining (HVAC) system; temperature differences; building-wind interactins). Prcesses causing hydrcarbn vapr transprt als can bring xygen frm the atmsphere t the subsurface. With petrleum hydrcarbn impacted sils, the sil gas cmpsitin and verall vapr migratin als can be affected by aerbic and anaerbic bidegradatin. Fr example: Oxygen mving dwn int the subsurface frm the atmsphere and hydrcarbn vaprs mving upward frm the surce will be cnsumed partially (r cmpletely) in regins f active aerbic bidegradatin, and carbn dixide (CO 2 ) will be prduced. This can result in the attenuatin f hydrcarbn vapr fluxes (mass per ttal area per time) t the atmsphere and enclsed spaces. In sme cases, this als can result in the creatin f xygen-depleted znes in the subsurface near and immediately abve the surce. Anaerbic decmpsitin f residual LNAPL in sils and disslved chemicals f cncern in grundwater may ccur in the xygen-depleted surce znes, and methane (CH 4 ) may be generated. The methane gas then migrates upward in the directin f enclsed spaces and the atmsphere, and it can underg aerbic bidegradatin in the mre xygen-rich subsurface regins (as d the hydrcarbn vaprs riginating frm the surce as discussed abve). Thus, a mre cmplex cnceptual mdel is needed t adequately describe subsurface petrleum hydrcarbn vapr migratin. One such cnceptualizatin is shwn in Figure

15 Advective Sil Gas Flw { Atmsphere Oxygen and Indr Air { Vadse Zne Sil Gas Methane Prductin? (this ccurs frequently when the vapr surce zne is anaerbic) Vapr Migratin basement Oxygen Vapr Migratin Aerbic Bidegradatin Hydrcarbn Vapr Migratin Grundwater Surce Aerbic Zne Anaerbic Zne Sil Surce (residual r LNAPL) Figure 2-2. Revised cnceptual mdel f sil gas migratin at petrleum hydrcarbn impacted sites. Fr further reading abut sil gas migratin prcesses as they relate t the vapr-t-indr-air expsure pathway, see API 1998; Jhnsn 2002; Jhnsn et al. 1999; Little et al. 1992; and Rggemans et al Expectatins fr Sil Gas Prfiles at Petrleum Hydrcarbn Impacted Sites It is useful, when selecting apprpriate sil-gas-sampling lcatins and depths, t be able t anticipate the qualitative nature f sil gas prfiles. This infrmatin als is critical t assessing data quality after sampling and analyses are perfrmed as discussed in Sectin 7.0. In this sectin, qualitative expectatins fr sil gas prfiles are discussed, fllwed by the presentatin f sample graphs f sil gas prfiles in Sectin 2.2. Based n the preceding discussin, the fllwing bservatins are qualitatively expected at petrleum hydrcarbn impacted sites: The highest cncentratins f chemicals f cncern in sil gas will be fund near the surce. Cncentratins f chemicals f cncern in sil gas will decrease with distance away frm the surce zne absent advectin and in the directin f the grund surface and enclsed spaces. Cncentratins f chemicals f cncern in sil gas als will decrease with distance laterally away frm the surce zne. Hwever, it is pssible fr hydrcarbn vaprs t becme trapped belw a layer f fine-grained sils r a misture barrier. This wuld affect the cncentratin distributin. The cmpsitin f the hydrcarbn vaprs at the surce will reflect the cmpsitin f the petrleum liquid and the chemical prperties f thse cmpnents. Hydrcarbn 5

16 vaprs at the surce als may cntain a significant fractin f methane if the sil gas in the vapr surce zne is xygen-depleted (anaerbic) and methangenesis is ccurring. Oxygen cncentratins in the sil gas abve impacted sils and grundwater will decrease with depth dwn thrugh the sil, reaching zer directly abve the surce in mst sils with shallw hydrcarbn impacts. Decreasing xygen cncentratins with depth als can be caused by backgrund xygen demand, especially in sils with high levels f natural rganic matter. The cntributin f backgrund xygen utilizatin can be assessed thrugh sil gas mnitring in nearby un-impacted areas. In regins where xygen cncentratins decline with depth, increasing CO 2 cncentratins with depth are generally bserved thrugh the aerbic regin f the subsurface. CO 2 is prduced during the aerbic bidegradatin f petrleum hydrcarbns and the aerbic bidegradatin f methane that might be prduced under anaerbic cnditins clser t sme surce znes. The decreases in xygen with depth and the increases in CO 2 with depth shuld be cnsistent with each ther as discussed later in Sectin 7.0. There will likely be sme regin f the subsurface in which aerbic bidegradatin ccurs. The significance f this prcess with respect t attenuatin f cncentratins f chemicals f cncern in sil gas will depend primarily n: Estimating the Time Necessary T Achieve Near-Steady-State Cnditins API (1998) and Jhnsn et al. (1999) discuss the imprtance f cnsidering whether sil gas prfiles have reached near-steady cnditins, and they prvide an equatin fr estimating the time necessary t achieve near-steady cnditins, T ss : T ss > R v θ v L 2 eff D v where D v eff is the effective diffusin cefficient discussed abve (generally having values in the range cm 2 /s), L is the distance frm the surce t grund surface (cm), θ v is the vaprfilled vid vlume (ften in the range cm 3 -vids/cm 3 -sil), and R v is the vapr-phase retardatin factr (chemicals like the prpanes, butanes, and pentanes, and xygen will have R v clse t unity, while chemicals mst ften f cncern because f health cnsideratins [e.g., mnarmatic hydrcarbns, MTBE] will have vapr-phase retardatin factrs n the rder f 10 < R v < 100). Using these values, ne can estimate times t reach near-steady cnditins f a few hurs t a few days fr shallw sites (< 1 m depth t the vapr surce); a few mnths t a few years fr intermediate-depth sites (up t 3 m depth t the vapr surce); and as much as a year t decades fr deeper vapr surces (> 10 m depth). Aerbic bidegradatin, if significant, will aid in reducing thse times t reach near-steady cnditins because the path length ver which cncentratins reduce significantly will be shrter. Surface cnditins (t the extent that they limit xygen migratin int r hydrcarbn vapr migratin ut f the subsurface) Subsurface cnditins (e.g., misture cntent, lithlgy, nutrient availability) 6

17 Cncentratins f chemicals f cncern and the ttal hydrcarbn cncentratin in sil gas at the surce Thickness f the aerbic regin Rate f bidegradatin reactins. Because xygen transprt and aerbic bidegradatin play a significant rle when sil gas prfiles are established, the sil gas prfiles measured near buildings may be quite different frm thse that wuld be measured beneath them, especially if the building fundatin reduces the xygen flux t the subsurface. In thse cases, the effects f aerbic bidegradatin n the sil gas prfile will be minimal beneath the fundatin. The discussin f sil gas prfiles assumes that the releases that have generated the vapr surce have been in place fr sme perid f time and that the cncentratins f chemicals f cncern in sil gas have reached a near-steady cnditin. Sme data indicate that sil gas prfiles are affected by seasnal changes; therefre, the near-steady cnditins still exhibit sme tempral variability. 2.2 Measured Sil Gas Prfiles at Petrleum Hydrcarbn Impacted Sites T better illustrate the cnnectin between the cnceptual mdel shwn in Figure 2-2 and measured vertical sil gas prfiles, sample sil gas prfiles are shwn in Figure 2-3 and Figure 2-4. Each f these prfiles is cnsistent with the cnceptual mdel, yet each is qualitatively different frm the thers. In these plts, nrmalized sil gas cncentratins (actual values divided by the maximum cncentratin at that site) are pltted as a functin f depth belw grund surface (z/l = actual depth t sil gas sample/depth t the surce at that site). Rggemans et al. (2002) perfrmed an empirical assessment f sil gas prfiles frm petrleum hydrcarbn impacted sites and classified the data in terms f generalized hydrcarbn-xygen sil gas prfiles. Figure 2-3 represents specific examples f these prfiles; all data riginate frm sites impacted by gasline r ther petrleum prducts. Nte that mst f the prfiles presented by Rggemans et al. (2002) were measured near buildings r beneath paved surfaces; few were measured beneath buildings. The prfiles in Figure 2-3 and Figure 2-4 shw xygen utilizatin (as evidenced by decreasing cncentratins with depth belw grund surface) and sme level f hydrcarbn cncentratin reductin, althugh it is variable. In prfile A f Figure 2-3, the xygen penetrates abut half f the distance dwn t the vapr surce, but then is cnsumed by aerbic bidegradatin ver a shrt distance. This aerbic bidegradatin is als reflected in the hydrcarbn cncentratin prfile that shws the hydrcarbn cncentratin decreasing several rders f magnitude ver a shrt distance near the anaerbic/anxic transitin zne. Prfile A was the mst frequently bserved by Rggemans et al. (2002). In prfile B, the xygen is present thrughut the vadse zne, except at the vapr surce zne interface. The crrespnding hydrcarbn prfile reflects reductin in hydrcarbn cncentratin by aerbic bidegradatin with distance abve the vapr surce. The effect f aerbic 7

18 bidegradatin, hwever, is less dramatic than in prfile A. Prfile B might be bserved at shallw sites where transprt distances are shrt and bidegradatin is slw relative t the xygen diffusin time scale thrugh the vadse zne, r where vapr surce cncentratins are relatively lw with respect t atmspheric xygen cncentratins (as might be the case abve disslved hydrcarbn grundwater plumes). Prfile C was cllected beneath a basement verlying a high cncentratin vapr surce. It is distinguished frm the ther prfiles by the lack f xygen at the mnitring pints and less attenuatin f the hydrcarbn vapr cncentratin. Relative t prfile A, which als crrespnds t a high cncentratin vapr surce (but ne beneath an uncvered surface) data suggest that in this case the building affects the xygen transprt and significance f the resulting aerbic bidegradatin. Prfile D has an xygen prfile similar t that in prfile B, but the hydrcarbn attenuatin with distance away frm the surce is much mre significant (i.e., a fur-rder-f-magnitude decrease in cncentratin ver a very shrt depth). This prfile ccurs when the surce is lcated in a zne having lwer diffusin rates than the verlying sils; fr example, this data set crrespnds t a case where the vaprs riginate frm within, r belw, the capillary fringe. Figure 2-4 presents vapr cncentratin prfiles frm a site impacted by heavier hydrcarbns (the surce is cmpsed f hydrcarbns in the C 12 t C 24 range). Of interest in Figure 2-4 is the prductin f methane gas frm hydrcarbn decmpsitin and the subsequent attenuatin f the methane gas within a shrt distance by aerbic bidegradatin prcesses. Qualitatively, Figure 2-4 is similar t prfile A in Figure 2 3, with the exceptin that methane is the dminant cmpnent f the hydrcarbn vapr cncentratin. 8

19 z/l Prfile A VW-96 D = 18 m Cmax (hydrcarbn)= 73 mg/l uncvered surface z/l Prfile B Ostendrf and Kampbell (1991) M30 March 1989 D = 2 m Cmax (hydrcarbn)= 27 mg/l paved surface hydrcarbn xygen C/Cmax hydrcarbn xygen C/Cmax Prfile C Prfile D Lahvis and Baehr (1996) VW-8 D = 1 m Cmax (hydrcarbn)= 20 mg/l uncvered z/l Paulsbr Basement D = 6 m Cmax (hydrcarbn)= 60 mg/l beneath basement z/l hydrcarbn xygen hydrcarbn xygen C/Cmax C/Cmax Figure 2-3. Sil gas prfiles (Rggemans et al. 2002). 9

20 z/l Uncal GOF Site (2001) SVH3 D = 20 m Cmax (methane)= 9500 ppmv uncvered methane xygen C/Cmax Figure 2-4. Sil gas prfile at a site with methane prductin in the surce zne (Jhnsn et al. 2003). This figure shws the sil gas prfiles fr xygen (circles) and methane (diamnds). Rggemans et al. (2002) attempted t crrelate sil gas prfiles with site characteristics and surface cver cnditins, but were unable t find any bvius crrelatins (i.e., sil gas prfiles belw paved surfaces did nt necessarily resemble thse belw buildings, and sil gas prfiles beneath paved surfaces varied frm site t site). Thus, the ability t anticipate the reductin in hydrcarbn vapr flux caused by bidegradatin, based n site prperties, is limited at this time. Sil-gas-prfile data, therefre, are critical t understanding the subsurface prcesses and the net effect n hydrcarbn vapr migratin t enclsed spaces. T help visualize the impact that xygen and aerbic bidegradatin can have n vapr prfiles at a site, Figure 2-5 and Figure 2-6 frm Abreu (2005), shw results frm three-dimensinal numerical simulatins f vapr transprt and aerbic bidegradatin under hmgeneus cnditins near a building with a basement. Figure 2-5 presents hw significantly the chemical f cncern and xygen sil gas prfiles and the attenuatin factr can be affected by changes in the surce cncentratin, fr a prgressin f surce cncentratins frm 20 mg/l t 200 mg/l. Fr reference, the Rggemans et al. (2002) reprt suggests that surce vapr cncentratins > 200 mg/l wuld be representative f gasline surce znes abve the water table, while surce sil vapr cncentratins < 20 mg/l wuld be expected t ccur near disslved plumes dwngradient f surce znes. In the 200 mg/l case, the effect f bidegradatin is minimal relative t simulatins withut bidegradatin, while the 2 mg/l surce vapr cncentratin case crrespnds t attenuatin that is six rders f magnitude different frm the 200 mg/l case (α=5.6 x versus 7.1 x 10-5 ). The majr difference between the figures is the xygen penetratin depth beneath the building. In the 20 mg/l case, xygen is fund at elevated levels beneath the building ftprint, s that chemical vaprs are subjected t aerbic bidegradatin alng mst f the transprt pathway. It shuld be nted that these results are presented here simply t visualize trends and that they are specific t this depth; the influence f cncentratin 10

21 n attenuatin factrs fr aerbically bidegradable chemicals is expected t be mre significant as the surce depth is increased (Abreu 2005). Figure 2-6 illustrates the effect f depth n sil gas prfiles and vapr attenuatin cefficients fr aerbically bidegradable chemicals. Simulatins are shwn fr slab-n-grade fundatins and relatively high (200 mg/l) sil vapr surce cncentratins at depths ranging frm 1 m t 8 m belw grund surface (Abreu 2005). The effect f bidegradatin relative t nn-bidegradatin simulatins is minimal fr the shallwer depths, but is very significant fr the surce lcated 8 m belw grund surface. Overall, the simulatins illustrate that the significance f the effect f aerbic bidegradatin is expected t be linked t the presence f xygen beneath a fundatin, and that attenuatin due t aerbic bidegradatin will increase with increasing surce depth and decreasing surce cncentratin. 11

22 Hydrcarbn Csurce = 200 mg/l 1E-6 α = 7.1Ε 5 1E E E Oxygen Depth bgs (m) α = 7.16Ε Csurce = 100 mg/l 1E-4 1E-3 1E-7 1E-6 1E α = 5.6Ε 11 1E-4 1E Csurce = 20 mg/l 1E-7 1E-6 1E x (m) Figure 2-5. Nrmalized sil gas cncentratin distributin fr xygen and hydrcarbn underging aerbic bidegradatin with first-rder rate λ = 0.18 (h -1 ) and vapr surce at cncentratins f 20 mg/l, 100 mg/l and 200 mg/l lcated underneath a basement fundatin at a depth f 8 m belw grund surface. Hydrcarbn and xygen cnturs are nrmalized t the surce and the atmspheric cncentratins, respectively. (Abreu 2005). 12

23 0-1 α = 1.08 Ε Hydrcarbn Oxygen α = 1.13 Ε E Depth bgs (m) α = 3.48 Ε E E α = 8.25 Ε E E-6 1E-5 1E x (m) Figure 2-6. Nrmalized sil gas cncentratin distributins fr xygen and hydrcarbn underging aerbic bidegradatin with a first-rder rate λ = 0.18 (h -1 ) and a vapr surce cncentratin f 200 mg/l lcated beneath a slab-n-grade fundatin at depths f 1 m, 3 m, 5 m and 8 m belw grund surface. Hydrcarbn and xygen cnturs are nrmalized t the surce and the atmspheric cncentratins, respectively. (Abreu 2005)

24 3.0 Cnceptual Migratin Mdel fr Subsurface Vapr t Indr Air Tpic: This sectin prvides backgrund infrmatin t assist in develping a cnceptual migratin mdel fr a specific site. The cnceptual migratin mdel is imprtant fr planning a sil-gas-sampling prgram. Purpse: T define a cnceptual migratin mdel fr describing the vapr surce characteristics, the current and future building lcatins and features, and the gelgic prfile f the subsurface. The mdel cnveys a wrking hypthesis f the mvement f vapr-phase chemicals f cncern within the subsurface t a current r future building. Significance: The cnceptual migratin mdel shuld effectively cmmunicate the imprtant features f the site gelgy, hydrgelgy, and petrleum hydrcarbn distributin and cmpsitin relevant t vapr migratin. It is especially imprtant that the site be reasnably well characterized fr the purpses f planning sil gas sampling and labratry analyses. Prir t selecting sampling lcatins and depths, a site-specific cnceptual migratin mdel shuld be develped. The cnceptual migratin mdel describes the vapr surce characteristics, including the vapr surce lcatin, size, envirnmental media, the cncentratins f chemicals f cncern and the ptential fr these t change with time. It als shuld include a descriptin f the expected sil gas cncentratin distributin, and a discussin f: Vapr Transprt alng Utility Cnduits whether r nt this distributin has reached near-steady cnditins under current site cnditins (e.g., present lcatins f buildings and surface cver), whether r nt future site uses might alter the sil gas distributin (e.g., future building lcatins and surface features), and hw the sil gas prfile is expected t be influenced by gelgic features. The cnceptual mdel shuld identify the current and reasnably ptential future subsurface-vapr-t-indr-air expsure pathways that may be present. The expsure If vapr transprt frm the surce area t the building culd ccur alng utility cnduits, then vapr sampling inside the utility cnduits, manhles, r sumps shuld be cnsidered in additin t vadse-zne-sil-gas sampling. Specific guidance fr utility sampling is beynd the scpe f this dcument; hwever, cnsideratin shuld be given t field instrument screening at utility access pints as an initial step t determine if the utility is acting as a cnduit fr vaprs. It is imprtant t establish in the cnceptual migratin mdel whether it appears that vapr migratin is taking place alng the utility backfill r if there is actual vapr transprt inside the utility itself. Any utility sampling prgram must include safety precautins t prtect persnnel (e.g., xygen and cmbustible gas mnitring, cnfined-space entry requirements) and t avid damage t utilities. 14

25 pathways shuld be defined by the vapr surce (e.g., sil, grundwater, light nn-aqueus phase liquids [LNAPL]) and lcatins where the subsurface-vapr-t-indr-air expsure pathway is f cncern (e.g., current r ptential future building). Specific buildings and future building sites shuld be identified. A descriptin f the existing r anticipated building cnstructin type (e.g., slab n grade, basement, multiple stries), design (e.g., basement r flr-slab thickness), and use (e.g., residential, cmmercial, industrial) shuld als be identified. As part f the develpment f the cnceptual migratin mdel, it is imprtant t review available infrmatin fr the site t determine the state f knwledge abut subsurface cnditins and t identify specific infrmatin that is necessary t evaluate the subsurface-vapr-t-indr-air expsure pathways. The cnceptual migratin mdel shuld effectively cmmunicate the imprtant features f the site gelgy, hydrgelgy, and petrleum hydrcarbn distributin and cmpsitin relevant t vapr migratin. It is especially imprtant that the site be reasnably well characterized fr the purpses f planning sil gas sampling and labratry analyses, including infrmatin related t the: Types f petrleum hydrcarbns with vlatile chemicals f cncern (e.g., gasline, jet fuel, diesel) that are currently r previusly stred r handled at the site Petrleum hydrcarbn chemicals f cncern and their cncentratins in sil and grundwater (e.g., benzene, tluene, ethylbenzene, xylenes, MTBE, naphthalene). The particular chemicals f cncern fr a site, the use f the data, and the regulatry requirements are used t determine the apprpriate labratry analytical methds. Ptential surces and surce areas f vaprs (e.g., sil, grundwater, LNAPL) Presence f LNAPL, which may represent an expanding vapr surce Presence f residual LNAPL, which may represent a stable r reducing vapr surce Fr grundwater surces f vaprs, it is useful t knw whether the grundwater plume is expanding, stable, r shrinking as this will give an indicatin f the tempral variability f cncentratins f chemicals f cncern in sil gas and whether future cncentratins in sil gas at a specific lcatin are likely t be higher r lwer than current cncentratins. In additin, grundwater table fluctuatins shuld be evaluated when assessing the tempral variability f grundwater surces. Gelgy and hydrgelgy in the area f the site Distinct sil strata and qualitative cntrasts between them (e.g., finer- r carsergrained sils, higher r lwer misture cntent strata). Finer-grained sils are generally mre mist and tend t be the znes where there are significant cncentratin gradients. The carser-grained sils ften represent preferential vaprmigratin znes. 15

26 It is als imprtant t cnsider whether there are relatively thin, finer-grained sil layers that may have a significant impact n vapr migratin but can be difficult t detect during drilling. Apprximate lcatin f vapr surces in the subsurface and distances (lateral and vertical) between the vapr surces and the building lcatin. The distances are imprtant in determining the number f and lcatins fr sil-gas-sampling prbes. Pssible preferential vapr-migratin cnduits (e.g., utility cnduits, sewers) Of particular interest are the utilities that intersect a vapr surce and als cnnect t a building If there is significant vapr migratin in preferential pathways, then the measurement f cncentratins f chemicals f cncern in sil gas may nt be representative f vapr cncentratins that wuld migrate int an indr envirnment. Other investigative techniques (e.g., utility vapr screening, indr air measurements) fr the subsurface-vapr-t-indr-air expsure pathway may prvide better data t evaluate the pathway. The user f this manual is directed t USEPA 2002a, and applicable state regulatry guidance fr ther methds t assess the vapr-t-indrair pathway (see PaDEP 2002; MaDEP 2002; WDHFS 2003; NJDEP 2004; CRWQCB 2003; CSDDEH 2003). Cnstructin features f existing r future buildings (e.g., size, age, presence f fundatin cracks, entry pints fr utilities). Appendix A prvides a summary f the infrmatin that is useful fr understanding the site. 16

27 4.0 Develpment f a Strategy fr Sil Gas Sampling Tpic: This sectin deals with articulating the questins t be answered by the sil gas sampling and develping a strategy fr sil gas sampling. Purpse: T develp a strategy fr sil gas sampling based n: The practicality f cllecting sil gas samples at a particular site A cnceptual migratin mdel fr subsurface vapr t indr air Questins t be answered by the sampling Regulatry requirements Technical issues. Significance: The develpment f a cnceptual migratin mdel, a discussin f the sampling ptins, a list f key cnsideratins, and an explratin f reasnable sampling scenaris fr petrleum hydrcarbn impacted sites are needed t establish an effective sil-gas-sampling prgram. Sectin 3.0 discusses the develpment f the cnceptual migratin mdel fr subsurface vapr t indr air. This sectin discusses ptins fr cllecting sil gas samples. Because the strategy fr sil gas sampling may vary frm site t site, prescriptive guidance n sampling lcatins, depths, and frequencies is nt prvided. Hwever, a discussin f the sampling ptins, a list f key cnsideratins, and reasnable sampling scenaris fr petrleum hydrcarbn impacted sites are discussed in this sectin. Lcal regulatry agencies may have sampling guidance that differs frm that presented here. Appendix B prvides wrksheets fr three typical scenaris that can be used fr planning sampling lcatins. The ptins fr sil gas sample cllectin discussed in this sectin are based n cnsideratin f: The practicality f cllecting sil gas samples at a particular site A cnceptual migratin mdel fr subsurface vapr t indr air Questins t be answered by the sampling Regulatry requirements Technical issues. Infrmatin abut site cnsideratins fr sampling is included in Sectin 5.0 and in Appendix C. In develping a sil-gas-sampling strategy, the questins t be answered by the sil gas sampling shuld be clearly articulated prir t designing the sil-gas-sampling plan. The questins t be answered by sil gas sampling at petrleum hydrcarbn impacted sites are ften similar t ne r mre f the fllwing: 1. Is the subsurface-vapr-t-indr-air expsure pathway currently cmplete fr individual chemicals f cncern? 17

28 2. Are cncentratins f chemicals f cncern in sil gas currently abve applicable regulatry actin levels r ther levels f cncern? 3. Is bidegradatin cntributing t the attenuatin f hydrcarbn vaprs between the surce and building at this site? 4. Hw might the answers t questins (1) t (3) change when cnsidering plausible future activity and land-use scenaris? The sampling apprach selected fr a site will depend n the site-specific cnditins and the questins t be answered abut the site. The selectin f sampling lcatins and depths needs t cnsider the cnceptual migratin mdel (e.g., lcatin f the vapr surces and the buildings f cncern, the expected vapr distributin, and characteristics f the subsurface). It is likely that a phased prgram f sil gas sampling may be used at sme sites, while at ther sites the cnditins may indicate that specific types f sampling and specific sampling lcatins are apprpriate. The basic sampling appraches include: Pint samples at specific depths in ne r mre lateral lcatins. These may be cllected using temprary driven prbes r by installing permanent sil-gas-sampling prbes. Vertical prfiles f samples at tw r mre depths. These can be installed at ne r mre lateral lcatins, called transects. These sil gas samples may be cllected using temprary driven prbes r by installing permanent sil-gas-sampling prbes. 4.1 General Apprach Belw are three basic steps in the selectin f sil-gas-sampling lcatins. 1. Develp a cnceptual migratin mdel describing the current r ptential future subsurface-vapr-t-indr-air expsure pathways. The mdel may require mdificatin as sil gas data are cllected. 2. Identify the questins t be answered by the sil gas sampling and any regulatry requirements. 3. Select the sampling apprach, the lcatins, and depths t prvide infrmatin sufficient t assess the pathway and t answer the questins psed fr the site. 4.2 Pint Sampling Varied Sample Lcatins and Depths It is a gd idea t design the sampling prgram t cllect sil gas samples at ne r mre lcatins r depths where cncentratins are expected t be abve the labratry detectin limits s that the field and labratry methds can be verified. If all f the analytical results are belw the labratry detectin limits, it is nt easy t cnfirm that the field methds were implemented crrectly (e.g., sample cllectin withut dilutin). In additin, there is increased cnfidence in the data when there are samples at tw r mre depths, given that the sample results are internally cnsistent and are cnsistent with the cnceptual migratin mdel. Pint sampling is the cllectin f an individual sil gas sample frm a specific depth at a single sample lcatin. Often, sil gas sampling begins with pint sampling. Cnsideratin shuld be 18

29 given t hw representative ne sample will be f the cncentratins f chemicals f cncern in sil gas either spatially r temprally. It may be necessary t cllect several pint sil gas samples t develp an understanding f the distributin f cncentratins f chemicals f cncern in sil gas. A discussin f sampling frequency is given in Sectin 4.6. Pint sampling is ften implemented when cnducting an initial assessment f the subsurfacevapr-t-indr-air expsure pathway. Pint samples are cllected at a specified depth, and the cncentratins f chemicals f cncern in sil gas are cmpared with chemical- and depthspecific target levels. The cncentratins f chemicals f cncern in sil gas als may be cmpared t measured cncentratins f the chemicals f cncern in ambient air samples. The mst cmmn lcatins fr pint sampling are immediately adjacent t, r beneath, a building fundatin and at the vapr surce. Use f the latter presumes that the lcatin f the vapr surce is knwn with cnfidence, and this may r may nt be the case at a given site. If the cncentratin f a chemical f cncern in sil gas is abve the target level, then a mre sitespecific analysis shuld be cnducted r remedial actin shuld be implemented. Cncentratins f chemicals f cncern in sil gas belw target levels indicate that the subsurface-vapr-tindr-air expsure pathway may nt be significant. See Sectin 7.0 fr mre infrmatin abut data analysis. Anther applicatin f pint sampling may be t evaluate the results f simple, cnservative mdels f vapr transprt in sil r frm grundwater. Where mdeling indicates that there may be cncentratins f chemicals f cncern abve target levels at a building, it might be apprpriate t cllect several pint sil gas samples t cmpare t the mdel results and further investigate if the pathway is actually cmplete. Pint samples may be cllected using temprary driven prbes r permanent prbes as described in Sectin 5.1. It is imprtant that the sil gas sample cllected frm a pint sample is apprpriate fr the cmparisn. Therefre, the decisin n the lcatin and depth f the pint sample shuld cnsider the vapr surce (e.g., sil, grundwater, LNAPL), the mbility f the surce (e.g., grundwater), and the impacts f hetergeneity in sil. 4.3 Transects and Vertical Prfiles Questins (1) and (2) at the beginning f this sectin arguably can be assessed with the cllectin f representative sil gas samples either at the vapr surce r immediately adjacent t r beneath a fundatin. In sme situatins, that infrmatin may be sufficient. In ther circumstances, the added knwledge that cmes frm lateral transects and vertical prfiles may be valuable fr the fllwing reasns: The assessment f the subsurface-vapr-t-indr-air expsure pathway is typically triggered by the knwledge that a vapr surce (e.g., petrleum hydrcarbn impacted sils r grundwater) is present in clse prximity t a building (r future building lcatin). If a single sample is cllected near the building fundatin and that sample results in nn-detectable cncentratins f chemicals f cncern in sil gas, then there may be sme questin abut the sil gas cllectin methd r representativeness f that single sil gas sample. If a sample als were cllected near the vapr surce and this 19

30 sample were t have detectable cncentratins f the expected magnitude, then the cnfidence in the validity f the near-fundatin sample increases. The majr difference between petrleum hydrcarbn impacted sites and sites impacted with ther vlatile chemicals f cncern (e.g., chlrinated hydrcarbn impacted sites) is that aerbic bidegradatin can be a significant cntributr t the verall attenuatin f petrleum hydrcarbn chemical f cncern vaprs as they migrate in sil frm the surce tward the building and grund surface. At sme sites, very high ttal hydrcarbn vapr cncentratins (e.g., within 10- percent f the lwer explsive limits) Aerbic Bidegradatin Establishing the significance f aerbic bidegradatin as it relates t hydrcarbn fluxes and indr air cncentratins requires a series f samples cllected alng the subsurfacevapr-migratin pathway and analyzed fr chemicals f cncern and respiratin gases. At many sites, the surce is lcated directly beneath a building and vertical prfiling is sufficient in thse cases. At ther sites where the surce is displaced laterally frm the building, a lateral series f vertical prfiles between the surce and building may be necessary. will be detected near the surce, and very lw, r nn-detectable, cncentratins will be present near the buildings f cncern. In this case, the cllectin f sil gas samples at pints between the surce and building, and review f the ttal hydrcarbn cncentratin, as well as chemical f cncern cncentratin and xygen-sil-gas prfiles, will help t establish aerbic bidegradatin as a key attenuatin mechanism. Cnfidence in the verall understanding f vapr migratin at a site is increased when cnsistency exists between the cnceptual migratin mdel and the sil gas prfiles and transects. This discussin has been included here t prvide sme insight t the underlying emphasis n vertical prfiles and transects in this dcument. As stated abve, pint samples at key lcatins may be sufficient t determine whether the expsure pathway is cmplete r significant. One f the unique features f petrleum hydrcarbn impacted sites is the ptential fr chemical f cncern vapr fluxes and cncentratins t be significantly attenuated by aerbic bidegradatin. T establish the ccurrence f aerbic bidegradatin, a single sample might be sufficient in particular, a sample shwing depleted xygen levels (and increased carbn dixide); hwever, the estimatin f attenuatin rates resulting frm aerbic bidegradatin requires a sil gas prfile r transect between the vapr surce and the building. 20

31 Lateral Distance Threshld Recently there has been discussin cncerning the need fr a threshld lateral distance criterin. Fr example, the USEPA (2002a) prpses that the pathway is nt cmplete and sampling is nt necessary if the distance between a knwn surce and the building is greater than 100 feet. Fr reference, the USEPA criterin was based primarily n the authrs prfessinal judgment and practical cnsideratins. Based n theretical cnsideratins, it is knwn that this distance criterin shuld cnsider the depth t the vapr surce, the vapr surce strength, the indr-air target levels, and lcal gelgy (Abreu and Jhnsn 2004; Lwell and Eklund 2004). Thus, the USEPA s 100-ft distance might, r might nt, be prtective at every site. Fr petrleum hydrcarbn impacted sites, the 100-ft distance is likely sufficient fr mst sites, prvided that the vapr surce edge is well defined, vertical migratin f xygen is nt significantly impeded by surface r subsurface features, and cnditins that culd prmte lateral migratin are nt present (e.g., landfill gas prductin, highly layered sils) In additin, cnfidence in the cnceptual migratin mdel fr a site and in quantifying the attenuatin is increased when vertical prfile r transect data are available. As discussed abve, this cnfidence increases when there is cnsistency within the sil gas data set (e.g., cncentratins decrease alng the path between the surce and building, decreasing xygen cncentratins with depth) and when there is cnsistency between the gelgic prfile develped under the cnceptual migratin mdel and the sil gas data (e.g., higher cncentratin gradients thrugh finer-grained sils) Selectin f Lateral Psitins fr Sil Gas Transects Sil gas transects can be used when the vapr surce (e.g., sil, LNAPL, grundwater) is nt lcated directly beneath the building r lcatin f a future building. Sil gas transects cnsist f tw r mre sampling lcatins between the surce and the building f cncern. The purpse f cllecting sil gas transects is t demnstrate and quantify the attenuatin f cncentratins f chemicals f cncern in sil gas mving laterally frm the surce t the building. A basic sampling prgram might invlve three sampling lcatins. Fr example: One sampling lcatin at the edge f the surce zne clsest t the building f cncern One sampling lcatin mid-way between the surce and building One sampling lcatin at the building. While these three sampling lcatins are likely sufficient, based n the experience f the authrs, users might cnsider additinal intermediate pints if the spacing between sampling prbes is mre than abut 50 feet. Transect samples may be cllected using temprary driven prbes r permanent prbes as described in Sectin 5.1. The lcatin and depth f the transect samples shuld cnsider the vapr surce (e.g., sil, LNAPL grundwater), the mbility f the surce medium (e.g., grundwater), and the impacts f hetergeneity in sil. 21

32 4.3.2 Vertical Prfiles A sil-gas vertical prfile cnsists f tw r mre samples cllected frm a single lcatin between the tp f the surce and the grund surface r building fundatin. The purpse f cllecting sil-gas vertical prfiles is t demnstrate and quantify the attenuatin f cncentratins f chemicals f cncern in sil gas vertically frm the tp f the surce t the grund surface r building fundatin. Fr sites where the vapr surce is directly beneath the building, sil-gas vertical prfiles are typically used t determine the ptential fr and magnitude f vapr migratin int the building. The fllwing shuld be cnsidered in selecting vertical sampling depths: T estimate the maximum cncentratin f chemicals f cncern in sil gas, a sample is needed immediately abve the vapr surce T estimate the cncentratin f chemicals f cncern in sil gas near the fundatin, samples shuld be cllected immediately adjacent t, r beneath, the building fundatin T establish the ccurrence and significance f aerbic bidegradatin, tw r mre sil gas samples shuld be cllected between the vapr surce and the building fundatin The vertical distance between adjacent samples based n practical cnsideratins is usually nt less than abut 2 feet, and at sme sites, this practical cnstraint might limit the number f samples cllected Fr deeper surces, where the distance between the surce and the building fundatin is greater than apprximately 40 feet, prfiles cnsisting f five r mre sampling depths are ften necessary t prvide reslutin f the sil gas prfile Sampling lcatins als may be selected based n the lithlgy f the site (e.g., in mre permeable sil znes, whether natural r artificial, r at lcatins where changes in cncentratin are expected) When assessing sites where impacted grundwater, LNAPL, r a smear zne is the vapr surce, a clser spacing f samples shuld be cnsidered, relative t the remainder f the prfile (e.g., several samples spaced 2 feet apart) near the grundwater table/capillary fringe, as the cncentratin gradients are ften greatest at that depth. A vertical prfile culd be develped using nly three sample depths. Hwever, depending n site-specific circumstances (e.g., depth t tp f surce area, lithlgy) fur r mre sample depths may be apprpriate. Sme situatins preclude the use f vertically-nested sil gas prbes r vertical prfiling. As a rule f thumb, when grundwater is within a few feet f a fundatin, it is likely that nly ne sil-gas-sampling depth is practical. The site characteristics and physical cnstraints will factr int the number f vertical samples. Vertical prfile samples may be cllected using temprary driven prbes r permanent prbes as described in Sectin 5.1. The decisin n the number f vertical prfile samples shuld cnsider the vapr surce (e.g., sil, grundwater, LNAPL), the depth f the surce (e.g., grundwater, LNAPL), and the impacts f hetergeneity in sil. The lithlgic prfile, surface tpgraphy, and 22

33 depth t the vapr surce shuld be cnsidered when installing vertical prfile prbes at multiple lateral psitins. Fr relatively flat sites (e.g., cnsistent distance between the grund surface and the vapr surce depth) and cnsistent stratigraphy (e.g., depths t distinct gelgic units), use f cnsistent sampling depths is preferred s that sampling results can be cmpared mre easily (see Figure 4-1). Fr cases with significant spatial variability in the distance between grund surface and the vapr surce depth, it is preferable t cllect all f the deepest samples at the elevatin f the vapr surce and all f the shallwest samples at the fundatin r basement elevatin. (see Figure 4-2) Given these cnstraints, the selectin f depths fr intermediate pints shuld target depths where significant changes in cncentratin are expected (e.g., immediately abve and belw fine-grained sils, near the capillary fringe fr grundwater surces). basement Surce Figure 4-1. Cnsideratins fr vertical prfiles at relatively flat sites (e.g., cnsistent distance between the grund surface and the vapr surce depth) with cnsistent stratigraphy. 23

34 basement Surce Figure 4-2. Cnsideratins fr vertical prfiles at sites with significant spatial variability in the distance between grund surface and the vapr surce depth. 4.4 Summary f Sampling Depth and Lcatin Selectin Cnsideratins Based n the discussins abve, Table 4-1 thrugh Table 4-5 have been prepared t summarize the use f data frm the varius sample depth lcatins and t ffer sme key cmments and cautins fr each. Fr each lcatin, the user shuld cnsider whether the samples are representative f the cncentratins f chemicals f cncern at the lcatin. Fr any site, the utility f the lcatins and depths will vary depending n the cnceptual migratin mdel and the questins f interest. 24

35 Table 4-1. Cnsideratins fr Samples Cllected Immediately abve the Vapr Surce Sample Depth r Lcatin Sil gas samples cllected immediately abve the vapr surce (e.g., highest cncentratins f chemicals f cncern in sil r grundwater). grundwater surce Use f Data These samples shuld represent the highest cncentratins f chemicals f cncern present in sil gas. In assessing if the pathway is cmplete r significant, these samples can be used t generate a cnservative expsure cncentratin estimate fr a present r future building scenari. Cmments and Cautins These cncentratins are generally greater than cncentratins f chemicals f cncern in sil gas at the building. Understanding (thrugh mdeling r empirical data analysis) f the estimated sil-gas-vapr attenuatin factr is needed t estimate cncentratins f chemicals f cncern in the building. In general, the mdels used with these cncentratins are the mre cnservative screening mdels that d nt cnsider bidegradatin. As a result, the estimates f indr air cncentratins are likely t be biased twards values that are higher than what is likely t ccur at the site. As the distance between the sample lcatin and the building increases, the uncertainty in estimating cncentratins f chemicals f cncern in indr air will likely increase. These cncentratins reach near-steady cnditins quickly and tend t be stable seasnally and are relatively unaffected by nearsurface changes (e.g., surface cver, weather changes). If the cncentratins in these samples are belw target levels, then the vapr-t-indr-air pathway is nt likely t be significant (see Sectin 7.0). 25

36 Table 4-2. Cnsideratins fr Samples Cllected Laterally Mid-Way between the Vapr Surce and the Building Lcatin Sample Depth r Lcatin Sil gas samples cllected frm a lcatin laterally mid-way between the vapr surce and the building lcatin. grundwater surce Use f Data Used in cnjunctin with surce vapr sampling, this sample may indicate sitespecific attenuatin alng the subsurface-vapr-migratin pathway. Cmments and Cautins Understanding (thrugh mdeling r empirical data analysis) f the estimated sil-gas-vapr attenuatin factr is needed t estimate the cncentratins f chemicals f cncern in the building. In general, the mdels used with these cncentratins are the mre cnservative screening mdels that d nt cnsider bidegradatin. As a result, the estimates f cncentratins f chemicals f cncern in indr air are likely t be biased twards values that are higher than what is likely t ccur at the site. These cncentratins are difficult t interpret in the absence f cncentratins f chemicals f cncern in sil gas at the surce zne and an accurate cnceptual migratin mdel. 26

37 Table 4-3. Cnsideratins fr Samples Cllected Adjacent t the Base f an Existing Building Fundatin r Basement Sample Depth r Lcatin Sil gas samples cllected adjacent t the base f an existing building fundatin r basement. grundwater surce Use f Data Because these samples are cllected clse t the expsure lcatin, they may be a useful predictr f cncentratins f chemicals f cncern in indr air. Cmments and Cautins There is likely less attenuatin f cncentratins f chemicals f cncern alng the subsurface-vapr-t-indr-air expsure pathway at this lcatin than fr mre distant sample lcatins (e.g., Table 4-1 and Table 4-2). Depending n where subsurface vaprs enter a building thrugh the building fundatin and n the xygen distributin adjacent t and beneath the building, this sample lcatin may r may nt be representative f the actual cncentratins f chemicals f cncern in sil vapr that are entering a building. Depending n the distance frm the surce t the building, these cncentratins may nt reach near-steady cnditins fr sme time after the release (see Sectin 2.1). These samples are mre likely t be affected by changes in nearsurface cnditins (e.g., temperature, precipitatin, barmetric pressure fluctuatins). 27

38 Table 4-4. Cnsideratins fr Samples Cllected Immediately belw the Building Fundatin r Basement Sample Depth r Lcatin Sil gas samples cllected immediately belw the building fundatin r basement. grundwater surce Use f Data Because these samples are cllected clse t the expsure lcatin, they may be a useful predictr f cncentratins f chemicals f cncern in indr air. Cmments and Cautins Lgistical issues are assciated with sample cllectin (e.g., building access, placing sampling prbes, maintenance f sampling prbes). These samples are mre likely t be variable with time as they are affected by changes in near-surface cnditins (e.g., temperature, precipitatin, barmetric pressure fluctuatins, HVAC systems). The cncentratins f chemicals f cncern in sil gas under a building fundatin may als be spatially variable. Mre than ne sampling lcatin may be required t develp a representative cncentratin. In general, empirical relatinships are used with these cncentratin data t estimate cncentratins f chemicals f cncern in indr air. Because the empirical relatinships already reflect the effects f bidegradatin between the vapr surce and the building, the estimated indr air cncentratins are nt inherently biased high r lw. Depending n where subsurface vaprs enter a building thrugh the building fundatin and the distributin f xygen adjacent t and beneath the building, this sample lcatin may r may nt be representative f the actual cncentratins f chemicals f cncern in sil gas that are entering a building. Depending n the distance frm the surce t the building, these cncentratins may nt reach near-steady cnditins fr sme time after the release (see Sectin 2.1). 28

39 Table 4-5. Cnsideratins fr Samples Cllected within the Ftprint f a Future Building Lcatin Sample Depth r Lcatin future building Sil gas samples cllected within the ftprint f a future building lcatin. grundwater surce Use f Data In cmbinatin with mdeling r empirical analysis, the cncentratins f chemicals f cncern in sil gas may prvide an estimate f future subsurface-vapr-t-indr-air expsure pathway impacts. Cmments and Cautins The samples cllected at the surce depth will likely be representative f future cnditins (unless grundwater is very shallw). Intermediate depth and shallw samples may nt be representative f future cnditins, depending n the surface cver, as the building fundatin and building HVAC system wuld have sme effect n xygen transprt (and therefre n the significance f aerbic bidegradatin). 4.5 Sme Cmments n Sample Cllectin Adjacent t and Beneath Buildings Sampling directly thrugh fundatin slabs f existing buildings (i.e., thrugh-slab sampling) presents significant lgistical and practical issues, including: Access issues Disturbance f residents r building ccupants Representativeness f the samples (depending n the actual vapr entry pints t the building, these sample lcatins may r may nt represent vaprs that are entering the building) Ability t install permanent sampling installatins that can be used fr multiple sampling events Maintenance f permanent sampling lcatins Limitatins n the types f sampling installatins and depths that can be used. Fr these reasns, sampling (and the placement f permanent sil-gas-sampling prbes) adjacent t a building ften is cnsidered. T date, n well recgnized study has been cnducted t 29

40 defensibly address the representativeness f samples cllected adjacent t a building, r beneath a building, fr cncentratins f chemicals f cncern that may be entering the building. As a result, there isn t any cnsensus n which lcatin cnsistently prvides data that is mre representative, but it is the case that sme regulatry agencies recmmend thrugh-slab sampling. The fllwing shuld be cnsidered fr sampling lcatins near buildings: When the building verlies the vapr surce, samples cllected at the surce zne depth are generally nt affected by changes in surface cnditins. Therefre, samples measured at surce zne depth adjacent t and beneath a building shuld yield cmparable cncentratins. Thus, if the pathway assessment can be cnducted using the deeper sil gas samples, then samples cllected adjacent t a building shuld be sufficient. The cnstructin f the building fundatin, its size, age (particularly as it relates t the existence f cracks), and entry pints f utilities all factr int the ptential fr migratin f vaprs frm sils utside f the building int the indr envirnment. These building attributes shuld be cnsidered in the cnceptual migratin mdel and in the selectin f sample lcatins. When it is imprtant t demnstrate the significance f attenuatin via aerbic bidegradatin, the need t sample beneath the building becmes mre critical. This is because xygen transprt t regins beneath the building ftprint may be limited relative t areas utside the building ftprint. As a result, xygen may becme depleted, and the effect f attenuatin by aerbic bidegradatin wuld be reduced. Differences between samples cllected next t and beneath buildings will prbably be mst significant fr shallw vapr surces (e.g., less than a 10-ft vertical separatin between the basement r building fundatin and the vapr surce). There shuld be less difference fr deeper vapr surces (e.g., greater than a 40-ft separatin between the basement r building fundatin and the vapr surce). When cmmitted t perfrming thrugh-slab sampling, a site-specific sampling and data analysis plan shuld be develped prir t perfrming the fieldwrk. The plan shuld include the number and lcatin f samples t be cllected, the analytical methds t be used, the required detectin limits, and when apprpriate, the selectin and use f tracers. The plan shuld include a discussin f the ratinale fr each f the elements (e.g., sample lcatins) and specificatins (e.g., assessment f variability) in the plan. Cllectin f tracer gas samples (e.g., radn) within the building and beneath the fundatin may prvide site-specific insight int the sub-slab attenuatin factr (α) used t assess indr impacts fr the petrleum hydrcarbn chemicals f cncern. If tracer gas samples are t be used t assess the site-specific, sub-slab attenuatin factr, ne shuld be wrking with measured tracer indr air cncentratins that are greater than ten-times the reasnably expected backgrund cncentratins r analytical detectin limits. Radn may nt be an apprpriate tracer at sme lcatins because f lw cncentratins in sil gas r because f indr surces (e.g., building materials) f radn (Hartman, 2004a). The attenuatin factr is defined as: 30

41 C α = C indr sub slab and is represented in Figure 4-3. C indr C sub-slab surce C surce grundwater Figure 4-3. Sub-slab-t-indr-air attenuatin. 4.6 Sampling Frequency Sil gas prfiles tend t be relatively stable with time, unless the cnditins near the vapr surce change significantly, prperties f the vadse zne change significantly, r cnditins at the grund surface change. Fr example, rising r fluctuating grundwater levels will limit vapr migratin frm residual LNAPL in sil if the water level rises abve the tp f the residual LNAPL. Vapr prfiles als might be affected by infiltratin events, including lawn r landscape irrigatin, if they significantly alter the air-filled prsity in the subsurface, submerge the surce, r temprarily dilute the cncentratins f chemicals f cncern in shallw grundwater. If multiple sampling events are desired, sampling frequencies shuld be selected t cincide with seasnal changes at the site. Fr example, samples shuld be cllected during wet and dry seasns, r during seasnal high and lw grundwater level perids. It shuld be nted that ne sampling event might be sufficient fr a number f sites, especially thse sites having cncentratins f chemicals f cncern in sil gas in the surce area belw target cncentratins. In additin, at these sites: ther data are cnsistent with the measured sil gas cncentratin (e.g., grundwater and sil data cnsistency as assessed thrugh equilibrium partitining calculatins, see Sectin 4.7 and Appendix E), and all ther lines f evidence gathered fr the site supprt the cnclusin that sil gas cncentratins wuld nt increase t cncentratins f cncern in the future. 31

42 Multiple sampling events are likely t be necessary at sites with higher cncentratins f chemicals f cncern in sil gas (e.g., cncentratins greater than 1,000 times the indr air target level), r where a demnstratin f the stability f the vapr cncentratins shuld is needed (e.g., if the time since the release is nt lng enugh t feel cnfident that near-steady vapr prfiles have develped, see Sectin 2.1). 4.7 Additinal Cnsideratins t Increase Cnfidence in Data Sets and the Interpretatin f Sil-Gas-Sampling Results As discussed in Sectin 4.2, pint sampling may be sufficient t demnstrate pathway cmpleteness at sme sites; hwever, fr ther sites, the fllwing ptins fr increasing the cnfidence in the assessment f this pathway may be useful: Cllectin f sil gas vertical prfiles and transects generally prvides an added level f cnfidence in the data set. Cnfidence in the data set increases if the data set includes bth lw, r nn-detect, cncentratins f chemicals f cncern near the grund surface and higher cncentratins near the surce. Cnfidence in the data set and interpretatin increases when the sil gas prfiles and transects are cnsistent with the cnceptual migratin mdel and the sil gas prfiles (e.g., hydrcarbn, xygen) bserved at ther sites (see Sectin 2.2 and Sectin 7.2). Grundwater Cncentratins Calculatin f a sil gas cncentratin based n a grundwater cncentratin prvides an estimate f the expected sil gas cncentratin. Hwever, a number f factrs wuld result in the actual sil gas cncentratin being higher r lwer than the estimated sil gas cncentratin. These factrs include: a submerged LNAPL surce, an LNAPL surce in the vadse zne, grundwater well screens that are lng, rapid bidegradatin in the vadse zne, and, a sil gas sampling interval that is shallw relative t the grundwater table. In additin, it is nt pssible t cllect vapr samples immediately abve the water table (because f high water saturatins in the capillary fringe). There is a decreasing cncentratin gradient mving up thrugh the capillary fringe twards grund surface, s vapr cncentratins in vapr samples cllected abve the capillary fringe are expected t be less than thse predicted t be in equilibrium at the water table. The cmparisn f measured sil gas cncentratins t the calculated expected cncentratins is intended as a relative measure f the cnfidence in the data set and an indicatr f the applicability f the cnceptual migratin mdel. Cnfidence in the data set increases when the near-surce cncentratins f chemicals f cncern are rughly equal t expected values. Fr example, fr grundwater surces, a grundwater sample culd be cllected and grundwater elevatin culd be measured at the same time that the sil gas samples are cllected. The grundwater samples can be cllected frm the same lcatin as the sil gas samples r 32

43 in the immediate vicinity. The grundwater sample can be used t calculate an expected sil gas cncentratin and this estimated value can be cmpared t the measured cncentratin as a check n the data that were cllected. See Appendix E fr additinal discussin and a calculatin wrksheet. Grundwater elevatin measurements als can be used t evaluate the ptential fr submerged surces t be present that wuld affect sil gas prfiles based n changes in grundwater elevatin. The cllectin f samples frm mre than ne sampling event generally increases cnfidence in the data set, as this helps assess the tempral changes expected due t a fluctuating water table and ther seasnal variatins. In additin, the fllwing supplemental data, which may have already been generated during the site investigatins, are beneficial in develping a better understanding f vapr migratin at a given site: A pht lg f a sil cre accmpanied by labratry analysis f the fllwing basic physical prperties, as identified by Ririe et al. (2002), fr each significant sil layer f the vadse zne: Sil misture Bulk density Air-filled prsity Water-filled prsity Ttal rganic carbn Hydraulic cnductivity Air permeability. Recent precipitatin recrd fr the area (easily btained frm weather-mnitring data) Surface cver (based n visual inspectin) Grundwater elevatin histry (frm grundwater elevatin measurements at the site r frm nearby sites). These and ther data needs are included in Appendix C. 33

44 Scpe-f-Wrk Actin Items: Develp the cnceptual migratin mdel t determine the applicable subsurface-vaprt-indr-air expsure pathways cnsidering the fllwing imprtant site characteristics: Surce characteristics Depth t grundwater Distance frm surce t expsure lcatin Current building r future building Lcatins f utilities and prcess piping Sil types Stratified vadse zne r hmgeneus vadse zne. Define questins t be answered by the sil gas sampling Determine if there are any regulatry requirements fr the sampling lcatins r frequencies Determine the applicable target levels and whether ambient air sampling will be cnducted Develp the sampling plan: Cnsider using the crss-sectins in Appendix B r ther visual representatins t plan sampling lcatins and depths Determine if a phased apprach t sampling is apprpriate Select sampling lcatins, depths, and frequencies. 34

45 5.0 Sil Gas Sample Cllectin Tpic: Cllectin methds fr sil gas samples are described in this sectin. Purpse: T discuss cmmn challenges assciated with sil gas sampling and t present pssible alternatives. Significance: Each sil-gas-mnitring methd has advantages and limitatins. Methd selectin shuld cnsider site-specific cnditins, including the lithlgy f the vadse zne, the site cnfiguratin, the depth t the vapr surce, and the prjected sampling frequency. This sectin prvides an verview f the basic sil-gas-sample cllectin methds and the equipment used t cllect thse samples. Mre infrmatin is prvided in Appendix C, including additinal references and resurces. It will nt be pssible t use sil-gas-mnitring installatins at all sites. Fr instance, with very fine-grained sils and high misture cntents, it may be difficult t cllect representative sil gas samples in a reasnable time perid using typical equipment and reasnable vacuum levels. The use f permanent prbes fr shallw vapr surces (< 3 feet belw grund surface) als may be undesirable given the difficulties assciated with cllecting representative sil gas samples at thse depths (e.g., increased ptential f surface air leakage). In additin, site access restrictins and physical cnstraints may limit the lcatins available fr installatin and maintenance f any kind f sil gas prbe, including temprary driven prbes. Cnsider the practicality f each silgas-sampling methd at a given site prir t prceeding thrugh the tasks discussed in this sectin. 5.1 Basic Mnitring Installatin Optins Sil-gas-mnitring installatin ptins include: Permanent prbes Temprary driven prbes. The mnitring installatins are described in the fllwing sectins. The descriptins included in these sectins are representative f typical installatins; hwever, variatins n these cnfiguratins are equally valid. Sme regulatry agencies have develped guidance that specifies sil-gas-mnitring installatin requirements. As fr any mnitring installatins, prper field prcedures shuld be fllwed during drilling (e.g., decntaminatin f equipment and tls, health and safety measures, inspectin f materials prir t use). Fr further reading n sample cllectin methds, see ASTM 1992; CRWQCB 2003; CSDDEH 2003; DeVitt et al. 1987; Eklund 1985; Eklund 1992; Hartman 2002, 2003, 2004a; USEPA

46 5.1.1 Permanent Prbes Permanent prbes, installed individually r as a nested grup, are typically cnstructed in similar fashin t grundwater mnitring wells using augered sil brings r direct push techniques. Fr augered sil brings, a sil bring is augered t the depth f the lwest identified mnitring interval, and a sampling prbe is set in sand-pack material (usually abut a 1-ft interval) at the bttm f the bring. The sampling prbe generally cnsists f small-diameter (e.g., < 1/4 inch inside diameter) tubing (e.g., cpper, stainless steel, nyln) running frm grund surface t the sampling depth. The end f this tubing may be cvered with fine screen, r it may be cnnected t a shrt (e.g., 6- t 12-inch) perfrated sectin f a 1-inch- r smaller-diameter pipe, r t a small-diameter metal mesh tube. The small-diameter tubing may be attached t a mre rigid supprt, such as a 1-inch PVC pipe that extends frm grund surface t the bttm f the brehle (CRWQCB 2003; Lahvis 2002; Hartman 2002, Hartman 2004a). When three r mre sil gas prbes are set in ne bring, the prbes are generally placed at an interval just abve the vapr surce, at ne r mre intermediate depths, and at a shallw depth. The bring is sealed with bentnite abve the sand-pack interval t the depth f the next deepest sampling interval, where anther prbe is set in sandpack material. The bring is again sealed abve the sand-pack interval with bentnite t the next sampling interval r the grund surface in the case f the shallwest prbe. Typically, the surface seal (the seal abve the last sampling interval) will be apprximately 3 feet in thickness. With direct-push techniques, a single sampling prbe is installed by pushing it t the desired depth. These driven prbes can be made f rigid tubing with dispsable drive pints; and in sme cases, the rds are driven dwn and als left in place. Alternatively, prbes can be installed via hllw, remvable drive rds. In this case, the rds are driven dwn, the prbe assembly is lwered int the drive rd, and the drive rd is remved while the prbe assembly is held in place. Often, the installers rely n natural cllapse f the frmatin arund the prbes, but n sme ccasins, a sand pack and a seal are installed thrugh the drive rd as it is remved. The prbe assemblies are Sampling Prbe Seals It is imprtant that any sil-gas-sampling prbe be sealed t minimize the exchange f atmspheric air with the sil gas and t maximize the ptential that the sample being retrieved is actually sil gas frm the sampling depth. Leak testing is a challenge fr sil-gas-sampling prbes because air is being sampled; the leaks cannt be detected visually. In general, ne relies n the results f the sampling vlumes, pressures, and labratry analytical results t determine if the sil-gas-sampling prbes were sufficiently airtight. It is imprtant t fllw the field prcedures (e.g., fr permanent prbes, seal abve each sampling interval) and t install the sil-gas-sampling prbes with care. It als is imprtant t ensure that the sampling tubing is sealed at grund surface between sampling events (e.g., with stpccks r ther valves), as barmetric pressure fluctuatins will induce cyclical inhalatin f ambient air and exhalatin f sil gas, bth f which can cmprmise the representativeness f the sil gas sample. 36

47 generally small-diameter, (e.g., 1/8- t 1/2-inch) 6-inch-lng stainless steel mesh tubes cnnected t the grund surface by a length f small-diameter (e.g., 1/8- t 1/4-inch) flexible tubing. The sampling interval fr the prbes is generally shrt (e.g., 6 t 12 inches) (CRWQCB 2003; Hartman 2002; Lahvis 2002, BP 1998) Temprary Driven Prbes Hllw metal rds (e.g., stainless steel) equipped with dispsable r retrievable drive pints may be driven int the sil by hand r with the aid f direct-push equipped vehicles. The prbes are driven t the desired depth. Then, depending n the design, they are either sampled at that depth r pulled up a few inches t expse a sampling tip. Sampling is cnducted thrugh tubing cnnected directly t the expsed sampling tip r by extracting vaprs thrugh the drive rds (CRWQCB 2003; Hartman 2002). After sampling, the rds are remved, r the device is pushed deeper t the next sample depth. In sme circumstances (e.g., finer-grained sils, deeper drilling depths), it can be difficult t remve the drive rds. 5.2 Cmparisn f Mnitring Installatins As discussed in the previus sectins, the different sampling methds are applicable t different field cnditins. The utility f each f the sampling methds is summarized here. Permanent installatins Permanent prbes can be sampled ver time t develp a tempral recrd f the cncentratins f chemicals f cncern in sil gas at a cnsistent set f lcatins and depths. Augered permanent sil-gas-sampling prbes An advantage f the augered bring installatin f vertically-nested prbes is a smaller ftprint f the sil-gas-mnitring installatin. Augered permanent prbes prvide the pprtunity t carefully seal each sampling interval and install a sand pack arund each sampling interval t ensure that adequate vapr vlumes can be cllected. Augered prbes can be installed in the vadse zne fr mst sil lithlgies. When grundwater ccurs at a shallw depth (e.g., within a few feet f a fundatin), it may nt be pssible t install vertically-nested prbes. Direct push permanent prbes Installatin time fr direct-push permanent prbes may be shrter than fr augered installatins. In additin, the disruptin t the subsurface will be minimized and the ptential fr generating sils that must be dispsed f will be minimized. 37

48 Time fr the sil gas t re-equilibrate after the drilling peratin may be shrter fr a direct-push installatin than fr an augered installatin. Driven prbes may nt be useful in all frmatins. Fr example, the presence f cbbles may hinder r preclude the use f direct-push technlgies, direct-push installatins may nt ffer any significant advantages ver auger installatins fr deeper sampling depths (> 100 feet), and direct-push samplers smetimes have difficulty cllecting sil gas samples in finer-grained sils (relative t augered installatins with a mre permeable sand pack f significant vlume). There is als a ptential fr the driven prbes t clse ff pre spaces alng the brehle walls during installatin in finer-grained sils, which can affect the representativeness f the sil gas samples. Permanent installatin prvides the pprtunity t carefully seal each sampling prbe t minimize the intrductin f atmspheric air. Frm a practical perspective, it is mre difficult t install sand packs and seals thrugh the inside f direct-push rds than t install sand pack in the larger diameter temprary casings f augered prbes. Temprary installatins Temprary driven prbes can be installed, sampled, and then remved with minimal surface disruptin. Their use may be advantageus in situatins where access restrictins preclude the installatin f permanent prbes and when samples will nt be cllected ver time. The use f temprary driven prbes is ften advantageus in situatins where the grundwater r vapr surce zne is relatively shallw (e.g., less than 6 feet belw grund surface). The use f temprary driven prbes is beneficial when an n-site labratry is used s that field decisins can be made t adjust r expand the sampling prgram (Hartman 2002). Temprary driven prbes als are useful fr cllecting data t identify suitable lcatins fr permanent prbes and in situatins where cncentratins f chemicals f cncern in sil gas are nt expected abve target levels s that multiple sampling events are nt needed. Where temprary driven prbes are used, additinal mbilizatin and direct-push events are needed t develp tempral trends in the cncentratins f chemicals f cncern in sil gas. Sil gas samples cllected within a few feet laterally f each ther shuld prduce similar results, s it is pssible t develp a histric recrd using a sequence f temprary direct-push sampling events. Temprary driven prbes allw vertically-nested prbes t be installed t prvide a prfile f the cncentratins f chemicals f cncern in sil gas in the subsurface. 38

49 Hwever, the use f temprary driven prbes t evaluate vertical prfiles ver time may be impractical because f additinal mbilizatins and the need fr additinal temprary driven prbes t be installed in clse prximity t previus temprary driven prbes. The ptential fr crss-cntaminatin f sampling depths shuld be cnsidered when using the same push rd fr multiple samples. 5.3 Other Cnsideratins fr Sampling Prbe Installatins As nted at the beginning f this sectin, it is recmmended that the practicality f sil gas sampling be assessed fr any site befre embarking n a sil-gas-sampling plan. Hwever, it is even mre f a cncern at sites with finer-grained, very mist sils. At these sites, it is advisable t install ne r tw sample prbes and evaluate the integrity f the sil gas samples frm thse first installatins (i.e., using the data evaluatin methds recmmended in Sectin 7.0) prir t implementing an extensive sampling prgram. See Appendix C fr suggestins fr testing t determine if sil gas sampling is practicable. 5.4 Sample Cllectin Prcedures Prcedures fr cllecting sil gas samples are discussed belw. Sme regulatry agencies have guidance fr sample gas cllectin, and users shuld familiarize themselves with guidance specific t their site befre designing a sil-gas-sample cllectin plan. Fr further reading abut sil gas cllectin methds, see ASTM 1992; CRWQCB 2003; CSDDEH 2003; Devitt et al. 1987; Hartman 2002, 2004a; USEPA 1996, Sil Gas Equilibratin The subsurface sil gas prfile shuld be allwed t re-equilibrate after any disturbance caused by installing the sil-gas-mnitring prbes (r the peratin f remediatin systems). Based n experience, the fllwing rules f thumb shuld be cnsidered: Direct-push prbes appear t have minimal disturbance n sil gas prfiles, and samples can be cllected immediately after reaching the driven depth The disturbance caused by the installatin f permanent prbes by drilling methds varies with the drilling methd. (See Appendix C fr mre infrmatin abut equilibratin time fr different drilling methds). If the sampling plan calls fr multiple sampling events ver several mnths, the data may be used t establish if the sil gas prfile has equilibrated. Hartman (2004) suggests if the sampling plan nly calls fr ne set f sil gas samples, several samples can be cllected ver a few days r weeks t determine the stabilized cncentratins. 39

50 5.4.2 Sample Prbe Purging Fllwing equilibratin, the sampling equipment shuld be cnnected t the sampling prbe. The sampling equipment shuld be checked fr leaks prir t purging f the sampling prbe. The sampling prbe and equipment is purged t cllect representative samples f sil gas at the sample interval. Based n experience, the purging and sampling shuld be cnducted t minimize the purged vlume (see Sectin 5.5 and Appendix C) Sample Cllectin Once the sampling prbe has been purged, the sil gas sample and any replicates can be cllected. Typically, sil gas is cllected using small-diameter tubing and gas-tight valves and fittings. The sample may be drawn ut f the grund by generating a vacuum using a pump, a syringe, r a sampling vessel that is under vacuum. The specific sample cllectin cntainers (e.g., SUMMA canisters, synthetic bags) are determined based n the analytical labratry, r field analysis, requirements and the detectin limits needed fr the evaluatin (see Sectin 6.0) Sample Cllectin Vacuum Sme regulatry agencies specify upper limits n the vacuum that can be applied during sample cllectin, with the thught that higher vacuums will affect the cncentratins r relative prprtins f chemicals f cncern in the sample. In ther wrds, sme scientists feel that increasing the vacuum enhances the vlatilizatin f mre vlatile cmpunds frm a mixture f chemicals f cncern n sil, in residual LNAPL, r in grundwater. In additin, as the vacuum levels are increased, the chance fr leakage in the sil-gas-sampling prbes and abvegrund equipment is increased. Because f these ptential issues assciated with increased vacuums n a sil gas sample, the fllwing shuld be cnsidered when evaluating the sample cllectin vacuum: Based n thermdynamic cnsideratins, the applicatin f typical vacuums (< 100 inches f water) shuld have a small effect n the cncentratin r cmpsitin f a sil gas sample. In thery, the applicatin f a vacuum shuld nt change the relative cncentratins f chemicals f cncern in a sil gas sample. Increasing vacuums, hwever, will increase the abslute cncentratins f all chemicals f cncern in the sil gas sample when thse cncentratins are expressed as a cncentratin under standard cnditins f 1 atmsphere (atm)(as is typical fr mst labratries). At mst, this increase in cncentratin wuld be apprximately (P atm /[P atm -P gauge ]), where P gauge is the gauge vacuum and P atm is 1 atm (=406 inches f water). Fr example, a 100-inch water vacuum might cause a 33-percent increase in cncentratins. Based n experience, atmspheric gas shrt-circuiting and leakage during sampling is mre likely when higher vacuums are applied. 40

51 5.5 Ways t Avid Cmmn Prblems with Sil Gas Sampling When planning a sil-gas-sampling prgram and develping a site-specific scpe f wrk, the fllwing issues shuld be cnsidered. A list f issues is presented here, and pssible slutins are included with each item. Minimize leakage alng sampling prbes Leakage ccurs because f imprper installatin f sil gas prbes (e.g., when the tube r rd is nt prperly sealed in the grund), r because f the type f surface cver. Vapr prbes can be tested fr surface leakage using a tracer gas (e.g., prpane, butane, isprpanl) at the grund surface. The sil gas samples shuld be analyzed fr the tracer. The tracer, hwever, shuld be a cmpund nt present in the sil gas (prpane and butane may be detectable as part f a gasline release at a site) and ne that can be detected with sufficient sensitivity (CRWQCB 2003; Hartman 2002). Tracking xygen cncentratins at depth where vertically nested prbes have been installed can be a means t determine if the sampling prbes r equipment are leaking. If the xygen cncentratins d nt decrease with depth and hydrcarbn vapr surces are present in that vicinity, then there is a ptential fr leakage alng the sampling prbes. Sealing at the surface f shallw sil gas prbes shuld be cnsidered fr temprary driven prbes and shuld be installed fr permanent prbes. Prbes can be sealed using bentnite. Fr temprary driven prbes, sealing the surface with bentnite is difficult t wrk with when trying t maximize sampling efficiency (e.g., extra time t clean equipment, wet bentnite at the grund surface). Sealing might be als accmplished by wetting the grund surface arund the prbe. (See the sidebar Sampling Prbe Seals in Sectin 5.1.1). Based n experience, sampling intervals shuld be placed n clser t the grund surface than 3 feet. Shallwer depths may be apprpriate, but greater care is needed t limit shrt-circuiting and t ensure representative sample cllectin (e.g., cmpetent surface seals, prper prbe cnstructin). Further, Hartman (2002) indicates that large purged vlumes can increase the ptential fr shrt-circuiting, especially fr shallw sil-gas-sampling prbes. In general, the prbes, tubing, and fittings shuld be selected t minimize the verall internal vlume f the equipment. Careful installatin prcedures (e.g., prper placement f sand-pack and bentnite seals) shuld be used in the cnstructin f permanent prbes. Avid lateral mvement f prbes nce they have been installed t minimize any separatin between the sils and the utside f the prbe. 41

52 When pre-manufactured prbe bundles are used fr vertically-nested prbes, they shuld be inspected at the surface prir t installatin. The prbe bundles shuld be inspected t be sure the cnnectins are tight and there are n visible hles r defects. Minimize the ptential fr shrt-circuiting due t utility lines r ther cnduits When the sil-gas-mnitring installatin is placed t clse t a utility, vaprs are nt being drawn frm the vadse zne but instead are being drawn frm within the utility. A thrugh understanding f the lcatin f all utilities and prcess piping shuld be develped prir t any sil gas sampling. Sil-gas-sampling lcatins shuld be placed at a sufficient distance frm utility r piping backfill areas t prtect the utilities and prcess piping and t btain representative samples. Cnsideratin shuld be given t using passive implant samplers (see Sectin 5.6) t investigate preferential flw pathways assciated with utilities r prcess piping rather than drilled sil gas prbes. Minimize vertical sampling intervals Often, sil-gas-sampling prbes are installed with sampling intervals that are t lng. This increases the uncertainty in the interpretatin f the measurements because the cncentratin is averaged ver a larger area. At sites with hetergeneus stratigraphy, sampling intervals that verlap several different sil layers that may have significantly different prperties will decrease the value f thse data fr pathway assessment. Sil-gas-sampling prbes with shrt (e.g., 6-inch) intervals are generally preferred. Sil-gas-sampling prbes with sampling intervals that are cnsistent with the site stratigraphy shuld be installed. Avid dilutin f samples The sampling shuld minimize the ptential fr ambient air t be inadvertently pulled int the sample. Once the sampling equipment has been installed at a sil vapr prbe, all f the fittings shuld be checked fr leaks prir t cllecting the sil gas sample (see Appendix C). Prvide cnsistent methds ver time It is very imprtant that the field prcedures used in sil gas sampling are cnsistent within each sampling event and acrss multiple sampling events ver time. Often, the results f sil gas sampling cmpared ver time r acrss a site fr the same sampling event d nt crrespnd with ther site infrmatin. Based n experience, this is ften because f variatins in field prcedures. All field prcedures shuld be dcumented each time a sampling event is cnducted (e.g., purged vlumes, sampling equipment used, number f field blanks, duplicates). The purged vlume selected fr a sampling event shuld be cnsistent and recrded fr all f the sample lcatins acrss the site (Lahvis 2002, CSDDEH 2003). 42

53 When planning each sampling event, the recrds frm prir events shuld be thrughly reviewed and cnsistent prcedures fllwed. Sampling shuld be cmpleted as quickly as is practical t prduce an internally cnsistent data set (Lahvis 2002). Fr typical sites, all f the sil gas samples can be cllected ver the curse f a day t a week. Cnsider cllecting grundwater samples at the same time and in similar lcatins as sil gas samples. Ensure adequate site characterizatin The prper placement f sil-gas-sampling prbes and the assciated sampling intervals is dependent n gd site characterizatin. Site characteristics discussed in Sectin 3.0 and Appendix A shuld be reviewed fr the site. If there are deficiencies in the data, cllectin f additinal infrmatin r site data shuld be cnsidered prir t implementing the sil-gas-sampling prgram. 5.6 Alternatives t Sil Gas Sampling As discussed previusly, it will nt be practicable at all sites t cllect sil gas samples using the mnitring installatins described in this sectin. Alternative sampling methds may be useful at sme f these sites t assist in the evaluatin f the subsurface-vapr-t-indr-air expsure pathway. Tw such methds, passive implant samplers and flux chambers, are briefly described here Passive Implant Samplers Typically, passive implant samplers are an adsrbent material in a cntainer that is placed in a small-diameter bring in the vadse zne sils. The passive implant samplers are cmmnly installed at shallw depths (e.g., less than 3 feet). The cntainer permits the flw f sil gas, and the adsrbent cllects the vapr-phase chemicals f cncern. The samplers are placed in the grund fr a perid f time (e.g., 1 r 2 weeks) and then they are retrieved. The analysis f the adsrbent material estimates a ttal mass f chemicals that has been cllected. It is difficult t accurately estimate the vlume f sil gas that has cntacted the sampler, and therefre, the mass cannt be cnverted easily t a vapr-phase cncentratin (CSDDEH 2003, Hartman 2002). In general, the applicatin f passive samplers is t determine the existence f vapr-phase chemicals f cncern (e.g., lcating surces, identifying preferential pathways) in the vadse zne and nt t quantify the cncentratins f vapr-phase chemicals f cncern fr expsure pathway assessment. Basic cnsideratins fr the use f passive implant samplers include the fllwing: Passive implant samplers are nt typically used t develp vertical prfiles Passive implant samplers prvide an alternative in very fine-grained sils where the practicality and integrity f samples frm sil gas prbes may be in questin (CSDDEH 2003) 43

54 Passive implant samplers wuld give an indicatin f whether the subsurface-vapr-tindr-air expsure pathway is incmplete if the masses detected in samplers lcated at varius distances away frm a vapr surce were decreasing at an acceptable rate Passive implant samplers may be useful fr identifying preferential pathways assciated with utility cnduits Flux Chambers Flux chambers are cntainers that are typically dme- r rectangular-shaped and are placed n the grund surface r ver a suspected fundatin intrusin area. A sweep gas flw is induced acrss the flux chamber, and the effluent passes thrugh a srbent trap r int a large sample cllectin cntainer (e.g., synthetic bag, SUMMA canister). Sme flux chamber measurements are cnducted withut a sweep gas flw. This is a static test. The flux chamber is left in place t cllect effluent vaprs n a srbent trap r in a large-vlume sample cntainer, r t allw buildup f vapr cncentratins in a static flux chamber. The time fr a flux chamber test depends n a number f factrs, including whether a sweep gas flw is used, the expected vapr cncentratins, and the type f sample cntainers used. At the end f the test, r at multiple times during the test, the cncentratin is determined. Using the time perid fr each measurement, the ftprint area f the flux chamber, and the cncentratin and vlume f the effluent vaprs, the mass flux emissin frm the subsurface can be calculated (CSDDEH 2003; Eklund 1992; Hartman 2002, 2003). Care shuld be taken when extraplating the results frm a small ftprint flux chamber t the ftprint f a residence r cmmercial building. Use f flux chambers requires knwledge f the sil-gas entry pints fr a given building, and these are ften difficult t identify even when cnducting a thrugh survey f the ptential lcalized vapr intrusin pints. Entry pint cncerns are less imprtant if the flux chamber is used in a crawl space f a building. 44

55 Scpe-f-Wrk Actin Items: Determine if sil gas sampling is practicable Determine if there are regulatry requirements fr the sample cllectin Lcate utilities and prcess piping Use the cnceptual vapr migratin mdel t determine whether: Pint samples r vertical prfiles are t be cllected Temprary r permanent prbes are t be installed. Define detailed field prcedures and a methd t dcument actual field prcedures Cnsider what additinal field prcedures might be needed based n bservatins during installatins and sampling (e.g., installatin f additinal prbes). 45

56 6.0 Analytical Methds Tpic: Analytical methds typically used in sil-gas-sampling prgrams at petrleum hydrcarbn sites and the data quality cnsideratins that arise are examined in this sectin. Purpse: T determine the apprpriate methds fr analyzing sil gas samples frm a specific petrleum hydrcarbn site. Significance: Different analytical methds can be used t quantify cncentratins f vapr-phase petrleum hydrcarbns in sil gas samples, and the particular methds selected will need t be cnsistent with the data quality bjectives fr that site. The analytical methds apprpriate fr analyzing sil gas samples will depend n the sampling methds and the data quality bjectives fr the site. Mre than ne different level f analytical precisin may be required; fr example, the analysis f specific chemicals f cncern may require a mre stringent level f quantificatin than the analysis f fixed and respiratin gases (e.g., N 2, O 2, CO 2, CH 4 ). It is imprtant, hwever, that the analytical methds used are cnsistent within each sampling event. In additin, the stringent analytical requirements fr specific chemicals f cncern may preclude the use f many field-mnitring devices cmmnly used t evaluate sil gas (e.g., hand-held pht-inizatin detectr [PID], flame-inizatin detectr [FID], and explsimeter) as these d nt prvide the necessary accuracy r specificity. 6.1 Analytical Methd Selectin Analytical methds are generally defined by the USEPA under the air txic prgram (USEPA 1999), slid waste prgram (USEPA 1998b), r ther prmulgated methds (USEPA 1996a, 2002a). A number f different analytical methds can be applied t quantify cncentratins f chemicals f cncern in sil gas samples. Hwever, the particular methd selected will depend n the use f the data and any regulatry requirements. Therefre, prir t selecting a methd fr analyzing sil gas samples, the fllwing questins shuld be answered: What are the specific chemicals f cncern r ther analytes (e.g., natural attenuatin parameters) that need t be identified by the analysis? The specific analytes (e.g., benzene, naphthalene) fr the subsurface-vapr-t-indr-air expsure pathway shuld be identified. Generally, these will be the vlatile and semi-vlatile chemicals f cncern identified during the verall investigatin at a site. Many regulatry agencies have identified specific chemicals f cncern that shuld be included in the analyte list. Hwever, if specific chemicals f cncern are nt identified, an analytical methd shuld be selected based n its ability t detect the range f analytes (e.g., vlatile rganic cmpunds [VOC], semi-vlatile rganic cmpunds [SVOC]) that may be present at a site. 46

57 Even thugh risk-based pathway assessment is based n cncentratins f chemicals f cncern, it is imprtant als t measure the ttal petrleum hydrcarbn cncentratin in sil gas samples at petrleum hydrcarbn impacted sites. This is necessary t understand adequately the subsurface prcesses, especially as they relate t bidegradatin. Fr example, the attenuatin by bidegradatin f any single chemical is influenced by the presence f all hydrcarbns in the sil gas, as they determine the ttal xygen demand and xygen-penetratin depth. The ttal petrleum hydrcarbn measurement shuld be f the full range f detectable hydrcarbns, nt f a specific prduct range f carbn numbers (e.g., as in a sil r water analysis that quantifies gasline range rganics ). What analytical methd reprting limits are required t evaluate adequately the ptential expsures? It is imprtant t determine the smallest cncentratins f chemicals f cncern in sil gas r ther analytes that are expected t be required fr purpses f evaluating the subsurface-vapr-t-indr-air expsure pathway. Fr evaluatin f this expsure pathway, indr air target levels fr chemicals f cncern r ther analytes shuld be identified. These indr air target levels can be used t identify the necessary detectin limits fr the sil gas analyses. Generally, the detectin limits fr cncentratins f chemicals f cncern in sil gas shuld be n mre than 100 times the target indr air cncentratin s that nn-detect cncentratins can be cnclusively evaluated. Appendix D includes additinal infrmatin n reprting limits and a wrksheet fr defining detectin limits. D sil r grundwater analytical results, r ther field data, indicate that cncentratins f chemicals f cncern in sil gas will be high? If cncentratins f chemicals f cncern r ther analytes in sil gas are anticipated t be high, then the analytical methd selected shuld be designed t address high cncentratins. In cases where high cncentratins are anticipated, slid waste prgram methds (USEPA 1998b) may be apprpriate. There is sme cncern that the slid waste prgram methds may be biased lw fr sme chemicals f cncern. Hartman (2004a) indicates that fr benzene, tluene, ethylbenzene, xylenes and chlrinated hydrcarbns the slid waste prgram methds and air txics methds prduce similar results. Hw are the samples t be cllected? The analytical methd selected, in many cases, will define the cllectin methd (e.g., SUMMA canister) that shuld be used and typically the sample preparatin that is required t analyze a sample. D the regulatry agencies require certificatin f the labratry r that specific analytical methds be used? Many state regulatry agencies require that samples be analyzed by specific methds. They may als require the labratry that is cnducting the analysis t be certified under a state prgram. In sme cases, this may limit the use f field analytical methds. Are there shrt turnarund times required fr analytical results? Turnarund times will be influenced by shipping requirements, hlding times, labratry backlg, and analytical methd. Depending n the pririties f the subsurface-vapr-t-indr-air expsure 47

58 pathway evaluatin, field analysis may be preferable t shipment t a labratry. Field analysis can prvide nearly real-time results. Are the analytical methds apprpriate fr the sil gas samples? The analytical methds ften are updated with newer techniques. It is suggested that the user cnsult with the regulatry agency and a qualified analytical labratry t identify analytical methds apprpriate fr the specific site. Appendix D prvides a summary f typical analytical methds apprpriate fr sil gas samples Field Analytical Methds Prtable gas chrmatgraphs (GC) can achieve sensitivities and specificity similar t a fixed labratry GC. In additin, recent develpments in prtable mass spectrmeters (MS) nw permit GC/MS analysis in the field. Hwever, fr a field GC t prvide equivalent results t a fixed labratry, it shuld fllw the same analytical prcedures (e.g., sample pre-cncentratin n a srbent trap fllwed by desrptin t cncentrate very dilute samples) and implement cmparable quality-cntrl measures t the fixed labratry. Respiratin gas analyses (e.g., xygen, carbn dixide) als can be cnducted in the field using instruments specific fr thse analyses Cmmn Analytical Methds Table 6-1 identifies cmmn analytical methds used fr sil gas samples at petrleum hydrcarbn sites. A mre detailed list f analytical methds is prvided in Appendix D. Table 6-1. Cmmn Analytical Methds Analyte Field Methd Fixed Labratry Methd Benzene, Tluene, Ethylbenzene, Xylenes, MTBE GC by Methd 8260 GC by Methd TO15 r 8260 Ttal Petrleum Hydrcarbns GC by Methd 8015 GC by Methd 8015 Oxygen Field meter w/ electrchemical cell (BP 1998) GC/TCD by Methd 3C Carbn Dixide Field meter w/ infrared analyzer (BP 1998) GC/TCD by Methd 3C Methane Field meter (CRWQCB 2003) GC/TCD by Methd 3C Nitrgen Field meter GC/TCD by Methd 3C 6.2 Data Quality The accuracy f an analytical methd is dependent n the handling and preparatin f the sample and the maintenance f the analytical equipment. Mst analytical methds prescribe minimum quality-cntrl measures that are designed t mnitr the perfrmance f the analytical prcedures. Hwever, additinal quality-cntrl measures can be implemented by the labratry r the analyst. At a minimum, the quality-cntrl measures shuld include calibratin f the instruments and an assessment f the analytical accuracy and precisin (USEPA 2001). Details f 48

59 the analytical methd quality-cntrl measures are included in the dcumentatin fr the selected analytical methd (USEPA 1998b, 1999, 2002). Scpe-f-Wrk Actin Items: Determine target analyte list, including any desired tracer cmpunds Determine required detectin limits Cnsult analytical labratry t determine suitable analytical methds, required sample cntainers, cnstraints n hlding times, and necessary shipping prcedures Define numbers f samples, replicates, blanks, and any standards that are needed fr the field prgram. 49

60 7.0 Analysis and Interpretatin f Sil Gas Sampling Data Tpic: In this sectin, guidance is prvided n analyzing sil-gas-sampling data fr the evaluatin f the subsurface-vapr-t-indr-air expsure pathway. Purpse: T evaluate the cmpleteness f the expsure pathway and t prvide guidance fr presenting data and fr assessing data quality and cnsistency. Significance: If the expsure pathway is ptentially cmplete and likely t be imprtant, then further evaluatin (e.g., vapr-migratin transprt mdels) r ther actins shuld be cnsidered. This sectin addresses the use f sil gas data fr assessing the significance f the subsurfacevapr-t-indr-air expsure pathway. It is assumed that the sampling lcatins, sampling methds, and chemical analyses are cnsistent with the recmmendatins given in previus sectins f this dcument. It is imprtant t nte that the discussin belw is specific t the assessment f petrleum hydrcarbn impacted sites. A suggested step-by-step prcess fr evaluating the subsurface-vapr-t-indr-air expsure pathway is described belw and graphically presented in Figure 7-1. In this step-by-step prcess, the data are rganized and analyzed fr quality and cnsistency. If the data quality and cnsistency are sufficient t assess the subsurface-vapr-t-indr-air expsure pathway, cmpare the data set t indr air target levels fr the chemicals f cncern established fr the site t determine if the expsure pathway is ptentially cmplete. If the expsure pathway is determined t be ptentially cmplete, then an attenuatin factr is applied t the indr air target levels t determine subsurface sil gas target levels. The data set is then cmpared t the subsurface sil gas target levels t determine if the expsure pathway is likely t be f significance. If the expsure pathway is ptentially cmplete and likely t be f significance, then further evaluatin (e.g., vapr-migratin transprt mdels) r ther actins shuld be cnsidered. 50

61 Data Organizatin (7.1) Tabulate, plt, and present sil gas data Data Analysis (7.2) Review data fr: Sufficient quality t be used fr expsure pathway assessment Cnsistency within the data set and with cnceptual migratin mdel Reasnable representatin f site cnditins Is data set f sufficient quality? (7.2.1) Yes Is data cnsistent? (7.2.2) N N Additinal Data cllectin needed Yes Expsure Pathway Assessment (7.3) Cmpare sil gas data t target indr air cncentratins and analyze trends between surce and building Cmpare sil gas data t target subsurface vapr cncentratins and analyze trends between surce and building Yes Is expsure pathway ptentially cmplete? (7.3.1) Yes Is expsure pathway f significance? (7.3.2) N N Subsurface Vaprt-indr air expsure Pathway Nt Cmplete Yes Further Actin (7.4) Additinal evaluatin (vapr transprt mdeling) Vapr mitigatin Remedial actin Figure 7-1. Flwchart fr data evaluatin. 51

62 7.1 Data Organizatin Tabulate, plt, and present sil gas data in a manner that facilitates review and quick determinatin f spatial and tempral trends, relatinships between the varius data pints, and data cnsistency. The fllwing data rganizatin and presentatin frmats are suggested: A table f all data listing sample lcatin identifier, sample depth, sample date, sampling methds, chemical analysis methds, labratry detectin limits, and results f chemical analyses Plan view maps displaying the spatial lcatins f sampling prbes, physical structures, utilities and prcess piping, and any infrmatin related t the extent and magnitude f chemicals f cncern in ther media (e.g., grundwater, sil, indr air) Crss-sectin figures shwing the vertical psitin f sample lcatins, gelgic descriptins f the subsurface, and any infrmatin related t the extent and magnitude f chemicals f cncern in ther media (e.g., grundwater, sil, indr air) Fr each vertical sampling lcatin, plts shwing cncentratins f analytes in sil gas (e.g., xygen, ttal petrleum hydrcarbns, methane, individual chemicals f cncern, carbn dixide) as a functin f depth (e.g., see Figure 2-3). Fr surces displaced laterally frm the building r lcatin f a future building, twdimensinal plts shwing cncentratins f analytes in sil gas (e.g., xygen, ttal petrleum hydrcarbns, methane, individual chemicals f cncern, carbn dixide) as a functin f depth and distance Identify indr-air target levels, sil-gas target levels, ambient utdr-air cncentratins, and backgrund indr-air cncentratins f chemicals f cncern Sil-gas target levels may be determined by dividing the indr air target levels by an accepted cnservative vapr attenuatin factr (e.g., 0.01 and are vapr attenuatin, r alpha, factrs appearing in USEPA [2002a]). In sme cases, regulatry agencies may have defined sil gas target levels. There als may be mre than ne sil gas target level; fr example, ne that applies near the building and ne that applies near the surce. Cncentratins f chemicals f cncern in ambient utdr air may be measured at the site, r may be frm ranges f reference values (MaDEP 2002; Wallace 1989; Wn and Crsi 1998). Typical backgrund cncentratins f specific chemicals f cncern in indr air frm sites nt affected by subsurface petrleum hydrcarbns releases can be identified frm the literature r the regulatry agency (MaDEP 2002; PaDEP 2002; Wallace 1989; Wn and Crsi 1998; USEPA 1998a). 52

63 7.2 Data Analysis Review the sil gas data t ensure that the quality and cnsistency f the data are sufficient t assess the significance f the subsurface-vapr-t-indr-air expsure pathway. This review shuld first address the quality f the data fr use in the expsure pathway assessment and then the cnsistency f the data within the data set and with the cnceptual migratin mdel. The fllwing questins shuld be answered during the data analysis: Is the data set f sufficient quality t be used fr expsure pathway assessment? Is the data set reasnably self-cnsistent? Is the data set cnsistent with the cnceptual migratin mdel? Is the data set reasnably representative f site cnditins? The users shuld answer the questins listed abve and decide if they have sufficient cnfidence in the data and the cnceptual migratin mdel t prceed with the next step. The quality and cnsistency analyses are discussed in mre detail in the fllwing tw sectins Data Quality Analysis The data quality analysis addresses the questin f whether the data are sufficient t be used fr expsure pathway assessment. Data shuld be reviewed fr cnsistency with published r demnstrated limits f the analysis methds by cnducting the fllwing activities: Cmpare analytical results t detectin limits t identify the measured cncentratins f chemicals f cncern. Nte any qualifiers that the labratry has placed n any f the cncentratin values. Cmpare analytical results t quantitatin limits t understand the cnfidence in the cncentratin values. If the quantitatin limit is clse t the cncentratin f interest (e.g., cncentratin < 5 times the quantitatin limit), then there may be mre uncertainty in the results. Cmpare replicate results t assess variability in the sampling methds and labratry analytical prcedures Review labratry analytical detectin limits t verify cnsistency with the detectin limits selected fr the site (See Sectin 6.1) Evaluate uncertainty in analytical results by cmparing duplicate and replicate samples and by identifying the frequency f anmalus data (e.g., 20-percent xygen cncentratins at the vapr surce, r vapr prfiles that d nt exhibit the expected cncentratin trends discussed in Sectin 2.0) Review analytical results fr the blank samples (e.g., field blanks, labratry blanks, and trip blanks) t determine if there are any issues with the labratry r field prcedures that may have affected the results. 53

64 7.2.2 Data Cnsistency Analysis The data cnsistency analysis addresses the questin f cnsistency within the data set and with the cnceptual migratin mdel. In additin, the data cnsistency analysis addresses the questin f whether the data set is reasnably representative f site cnditins. The entire data set shuld be reviewed fr internal cnsistency and fr cnsistency with the site cnceptual mdel. Fr example: The spatial distributin f cncentratins f chemicals f cncern in sil gas shuld be qualitatively and semi-quantitatively cnsistent with the spatial distributin f grundwater cncentratin and sil cncentratin data. Fr example, the highest cncentratins f chemicals f cncern in sil gas shuld be bserved in areas with the highest cncentratins f chemicals f cncern in sil and grundwater. Within a set f samples acrss a site, higher cncentratins f chemicals f cncern and lwer xygen cncentratins might be bserved beneath buildings relative t unpaved areas f the site. Cncentratins f chemicals f cncern in sil gas shuld be cnsistent with the expected cncentratins f chemicals f cncern in sil gas given the vapr surce (See Sectin 4.7 and Appendix E). The vertical distributin f cncentratins f chemicals f cncern in sil gas shuld be qualitatively and quantitatively cnsistent with the cnceptual migratin mdel fr the site (see Sectin 3.0) and shuld be cnsistent with published sil gas prfiles frm petrleum hydrcarbn impacted sites (e.g., Figure 2-3). Fr example, xygen cncentratins shuld nt increase with depth, and cncentratins f chemicals f cncern in sil gas shuld decrease with distance away frm the surce areas. Vertical gradients f xygen, methane, carbn dixide, and chemicals f cncern shuld be semi-quantitatively cnsistent with each ther (i.e., xygen, carbn dixide, methane, and hydrcarbn gradients shuld be self-cnsistent accrding t knwn aerbic bidegradatin stichimetry). Table 7-1 illustrates sme examples fr the cmparisn f xygen (O 2 ) and ttal hydrcarbn fluxes. In the table, the mass flux values are based n the mlecular weights f the mlecules and the stichimetry f the reactin equatin (e.g., 5 xygen mlecules: 5 x 2 x 16 = 160). Fluxes are calculated as the change in sil gas cncentratin (expressed in mass per vlume units) divided by the change in distance, multiplied by the estimated effective diffusin cefficient. See Appendix E fr infrmatin abut cncentratin unit cnversins. 54

65 Table 7-1. Example Cmparisns f Bidegradatin Stichimetry and Fluxes Aerbic Bidegradatin Reactin Expected Hydrcarbn t O 2 Rati f Mass Fluxes When this is the Dminant Aerbic Reactin C 4 H O 2 -> 4CO 2 + 2H 2 O 58:160 C 6 H /2O 2 -> 6CO 2 + 3H 2 O 78:240 CH 4 + 2O 2 -> CO 2 + 2H 2 O 16:64 Tempral trends shuld be evaluated. Fr example, if the vapr pathway assessment is predicated n near-steady vapr cncentratins, multiple lines f evidence (e.g., tempral trends in sil gas data and estimates f the time-t-near-steady cnditins Sectin 2.1) shuld supprt the cnceptualizatin. If there are significant tempral variatins with time, ne shuld assess if these are cnsistent with tempral trends in the factrs that might influence vapr cncentratin prfiles (e.g., changes in vapr cncentratins with rising r lwering grundwater levels, changes in precipitatin). When data incnsistencies are identified, the sampling plans and prtcls shuld be reviewed t identify any inherent biases r reasns fr the data incnsistencies. A decisin shuld then be made cncerning the necessity f cllecting additinal data, r revising the sampling prtcls r cnceptual migratin mdel. These issues shuld be reslved prir t further evaluatin f the subsurface-vapr-t-indr-air expsure pathway. 7.3 Expsure Pathway Assessment Once the data quality and cnsistency has been determined as sufficient t assess the subsurfacevapr-t-indr-air expsure pathway, evaluate the data t determine if the subsurface-vapr-tindr-air expsure pathway is cmplete (r likely t be cmplete fr future-use scenaris). If the expsure pathway is cmplete r likely t be cmplete, evaluate the data t determine if the expsure pathway is likely t be significant. Appraches fr determining if the expsure pathway is cmplete based n sil gas data are discussed in the fllwing sectins Expsure Pathway Cmpleteness T determine if the subsurface-vapr-t-indr-air expsure pathway is cmplete (r likely t be cmplete), the sil gas data review shuld include the fllwing cmparisns: If the cncentratins f chemicals f cncern in sil gas everywhere in the subsurface are belw the indr air target levels established fr the site, then the expsure pathway is nt cmplete 55

66 If the cncentratins f chemicals f cncern in sil gas are abve the indr air target levels at ne r mre sample lcatins, determine if the cncentratins decline (either laterally r vertically) t less than indr air target levels between the vapr surce and the building. If cncentratins f chemicals f cncern d nt decline t less than the indr air target levels, then the expsure pathway is ptentially cmplete and further evaluatin shuld be cnducted t determine if the subsurface-vapr-t-indr-air expsure pathway is likely t be significant. In bth cases, the determinatins are based n existing data and current cnditins. Users shuld decide if future-use scenaris are likely t affect the sil gas prfiles. Fr petrleum hydrcarbn impacted sites, future changes in surface cver are typically f mst cncern, as increased lwpermeability surface cver (e.g., pavement, fundatins) culd inhibit xygen supply t the subsurface, which in turn can result in lessening the effect f attenuatin due t aerbic bidegradatin, and als can lead t methane prductin when petrleum hydrcarbn impacted sils are present. Migratin f chemicals f cncern in grundwater culd cause changes in cncentratins f chemicals f cncern in sil gas. Installatin f utility cnduits thrugh r near petrleum hydrcarbn impacted sils als culd affect sil gas prfiles and vapr transprt lcally. In making the abve determinatins, the discussin in Sectin 4.0 cncerning cnfidence in shallw versus deep sil gas samples, as well as the influence f surface cver n sil gas prfiles at petrleum hydrcarbn impacted sites, shuld be cnsidered Expsure Pathway Significance T determine if a ptentially cmplete subsurface-vapr-t-indr-air expsure pathway is likely t be significant, the sil gas data review shuld include the fllwing cmparisns: If the cncentratins f chemicals f cncern in sil gas everywhere in the subsurface are belw sil gas target levels established fr the site then the expsure pathway is nt likely t be significant. If cncentratins f chemicals f cncern in sil gas are abve sil gas target levels at ne r mre sample lcatins, determine if cncentratins decline (either laterally r vertically) t less than sil gas target levels between the vapr surce and the building. If cncentratins f chemicals f cncern d nt decline t less than sil gas target levels between the vapr surce and the building, then the pathway is likely t be significant. In each case, the determinatins are based n existing data and current cnditins, and the cautins discussed in Sectin are applicable. 7.4 Further Evaluatin If cncentratins f chemicals f cncern in sil gas are abve the sil gas target levels and d nt decline (either laterally r vertically) t less than the sil gas target levels between the vapr surce and the building, then further actins shuld be implemented. These further actins may include: Using the existing sil gas data in cnjunctin with vapr-migratin transprt mdels. Jhnsn et al. (1999) discuss a number f analytical appraches apprpriate fr 56

67 petrleum hydrcarbn impacted sites. One apprach mdels the subsurface as ne r mre specific gelgic layers specifying different sil prperties fr each. Anther apprach uses empirical matching f sil gas data with mdificatins t the Jhnsn and Ettinger (1991) algrithm t accunt fr bidegradatin. While it will nt happen at all sites, it is nt unreasnable t expect cncentratins f chemicals f cncern in sil gas t attenuate by factrs f 10,000 t 100,000 between a hydrcarbn vapr surce and indr air. Cnsidering the discussin in Sectin 4.0 cncerning cnfidence in shallw versus deep sil gas samples when validating a vapr transprt mdel with sil gas prfiles. Cnducting additinal sil gas sampling t develp a mre cmplete understanding f the prfiles f cncentratins f chemicals f cncern in sil gas and ptential fr bidegradatin f vapr-phase hydrcarbns in the subsurface and any tempral trends. Implementing indr air sampling and analysis, and analysis f indr backgrund cncentratins, t crrelate cncentratins f chemicals f cncern in sil gas with cncentratins f chemicals f cncern in indr air and t evaluate the subsurfacevapr-t-indr-air expsure pathway. Cmparing cncentratins f chemicals f cncern in sil gas t ambient utdr air cncentratins t crrelate cncentratins f chemicals f cncern in sil gas with cncentratins f chemicals f cncern in utdr air and t evaluate the subsurfacevapr-t-indr-air expsure pathway. Implementing remedial actins t reduce the cncentratins f chemicals f cncern, alng the subsurface-vapr-t-indr-air expsure pathway, r ther analytes in sil gas (e.g., methane) thrugh surce reductin r vapr mitigatin at the building. Imprtant Reminders fr Interpretatin f Sil Gas Data: Organize the sil gas data in tables, graphs, and maps s that trends and incnsistencies can be identified. Cmpare the data with the cnceptual migratin mdel t determine if the data reasnably represent the site, r if the cnceptual migratin mdel shuld be revised. Identify applicable indr air target levels, sil gas target levels, indr backgrund cncentratins, and ambient air cncentratins fr cmparisn t measured data. Identify if additinal data shuld be cllected. 57

68 8.0 References Abreu, L.D.V A Transient Three-Dimensinal Numerical Mdel t Simulate Vapr Intrusin Int Buildings. Ph.D. Dissertatin. Arizna State University. Abreu, L. and P.C. Jhnsn Effect f Vapr Surce-Building Separatin and Building Cnstructin n Sil Vapr Intrusin as Studied with a Three-Dimensinal Numerical Mdel. Accepted fr Publicatin. Envirnmental Science & Technlgy. American Petrleum Institute (API) Assessing the Significance f Subsurface Cntaminant Vapr Migratin t Enclsed Spaces, Site-Specific Alternatives t Generic Estimates. Publicatin N Health and Envirnmental Sciences Department. Washingtn D.C. American Sciety fr Testing and Materials (ASTM) Standard Practice fr Analysis f Refrmed Gas by Gas Chrmatgraphy. D (2000), Vl West Cnshhcken, PA. American Sciety fr Testing and Materials (ASTM) Standard Guide fr Sil Gas Mnitring in the Vadse Zne. D , Vl West Cnshhcken, PA. American Sciety fr Testing and Materials (ASTM) Standard Test Methd fr Analysis f Natural Gas by Gas Chrmatgraphy. D , Vl West Cnshhcken, PA. British Petrleum (BP) SPP-110 Vadse Zne Sampling at BP Crrective Actin Sites. Glbal Envirnmental Management, a BP Affiliated Cmpany. Lisle IL. Charbeneau, R.J Grundwater Hydraulics and Pllutant Transprt. Upper Saddle River, NJ: Prentice Hall. Califrnia Reginal Water Quality Cntrl Bard Ls Angeles Regin (CRWQCB) and the Department f Txic Substances Cntrl (DTSC) Advisry Active Sil Investigatins. January. Cunty f San Dieg, Department f Envirnmental Health (CSDDAH) Site Assessment and Mitigatin Manual (SAM Manual 2002). San Dieg, CA. January 22. Devitt, D.A., R.B. Evans, W.A. Jury, T.H. Starks, B. Eklund, and A. Ghlsn Sil Gas Sensing fr Detectin and Mapping f Vlatile Organics. EPA/600/8-87/036. U.S. EPA- EMSL. Las Vegas, NV. Eklund, B Detectin f Hydrcarbns in Grundwater by Analysis f Shallw Sil Gas/Vapr. American Petrleum Institute Publicatin N May. Eklund, B "Practical Guidance fr Flux Chamber Measurements f Fugitive Vlatile Organic Emissin Rates." Jurnal f the Air & Waste Management Assciatin 42 (12): Hartman, B Hw t Cllect Reliable Sil-Gas Data fr Risk-Based Applicatins, Part 1: Active Sil-Gas Methd. LUST Line Bulletin 42. Octber. 58

69 Hartman, B Hw t Cllect Reliable Sil-Gas Data fr Upward Risk Assessments, Part 2: Surface Flux Chamber Methd. LUST Line Bulletin 44. August. Hartman, B cmmunicatin frm B. Hartman, HP Labs t H. Hpkins, API. June 3. Hartman, B. 2004a. Hw t Cllect Reliable Sil-Gas Data fr Risk-Based Applicatins Specifically Vapr Intrusin, Part 3: Answers t Frequently Asked Questins. LUST Line Bulletin 48. Nvember. Jhnsn, P.C Sensitivity Analysis and Identificatin f Critical and Nn-Critical Parameters fr the Jhnsn and Ettinger (1991) Vapr Intrusin Mdel. API Sil and Grundwater Research Bulletin N. 17. API. May. Jhnsn, P.C. and R.A. Ettinger Heuristic Mdel fr Predicting the Intrusin Rate f Cntaminant Vaprs int Buildings. Envirnmental Science & Technlgy. 25 (8): Jhnsn, P.C., R.L. Jhnsn, and M.W. Kemblwski Assessing the Significance f Vapr Migratin t Enclsed-Spaces n a Site-Specific Basis. Jurnal f Sil Cntaminatin. 8 (3): Jhnsn, P.C., P. Lundegard, J. Catts, K. DiSimne, D. Eley, and K. Schreder Surce Zne Natural Attenuatin Field Measurements, Data Interpretatin, and Data Reductin at the Frmer Guadalupe Oil Field, San Luis Obisp Cunty, Califrnia. Lahvis, M.A Guidance n Use f Sil-Gas Surveys t Assess Vapr Transprt t Indr Air. Shell Glbal Slutins (U.S.), Inc. Hustn, TX. Lewis R.J., Sr. (Revised by) Hawley's Cndensed Chemical Dictinary, 14th Editin. New Yrk: Jhn Wiley & Sns, Inc. Little, J.C., J.M. Daisey, and W.M. Nazarff Transprt f Subsurface Cntaminants int Buildings: An Expsure Pathway fr Vlatile Organics. Envirnmental Science & Technlgy 26 (11): Lwell, P.S., B. Eklund VOC Emissin Fluxes as a Functin f Lateral Distance frm the Surce. Envirnmental Prgress. 23 (1) Massachusetts Department f Envirnmental Prtectin (MaDEP) Indr Air Sampling and Evaluatin Guide. WSC Plicy N Cmmnwealth f Massachusetts. Bstn. MA. April. Massmann, J. and D. F. Farrier Effects f Atmspheric Pressures n Gas Transprt in the Vadse Zne. Water Resurces Res 28: Parker, J Physical Prcesses Affecting Natural Depletin f Vlatile Chemicals in Sil and Grundwater. Vadse Zne Jurnal 2: Ririe, G.T., R.E. Sweeney, and S.J. Daugherty A Cmparisn f Hydrcarbn Vapr Attenuatin in the Field with Predictins frm Vapr Diffusin Mdels. Jurnal f Sil and Sediment Cntaminatin 11 (4):

70 Rggemans, S., C.L. Bruce, and P.C. Jhnsn Vadse Zne Natural Attenuatin f Hydrcarbn Vaprs: An Empirical Assessment f Sil Gas Vertical Prfile Data. American Petrleum Institute Technical Reprt. United States Envirnmental Prtectin Agency (USEPA) Sil Gas Sampling. Standard Operating Prcedure N Envirnmental Respnse Team. Washingtn, D.C. June. United States Envirnmental Prtectin Agency (USEPA). 1996a. CFR Prmulgated Test Methds. Methd 3. Technlgy Transfer Netwrk, Emissin Measurement Center. Washingtn, D.C. June. United States Envirnmental Prtectin Agency (USEPA). 1998a. Inside IAQ. EPA's Indr Air Quality Research Update. EPA/600/N-98/002. Office f Research and Develpment. Research Triangle Park, NC. Spring/Summer. United States Envirnmental Prtectin Agency (USEPA). 1998b. SW-846 Manual. Revisin 5. Office f Slid Waste. Washingtn, D.C. April. United States Envirnmental Prtectin Agency (USEPA) Cmpendium f Methds. Secnd Editin. Technlgy Transfer Netwrk, Ambient Mnitring Technlgy Infrmatin Center. Washingtn, D.C. January. United States Envirnmental Prtectin Agency (USEPA) CFR Prmulgated Test Methds. Methd 16. Technlgy Transfer Netwrk, Emissin Measurement Center. Washingtn, D.C. February. United States Envirnmental Prtectin Agency (USEPA). 2002a. OSWER Guidance fr Evaluating the Vapr Intrusin t Indr Air Pathway frm Grundwater and Sils (Subsurface Vapr Intrusin Guidance). Nvember Draft. United States Envirnmental Prtectin Agency (USEPA) Methd 8015D Revisin 4 Nnhalgenated Organics Using GC/FID. Office f Slid Waste. Washingtn, D.C. June. USEPA and the United States Army Crps f Engineers Field Analytical Technlgies Encyclpedia (FATE). Washingtn D.C. Updated January. 60

71 9.0 Additinal Reading 9.1 Analytical Methds American Sciety fr Testing and Materials (ASTM) Standard Practice fr Analysis f Refrmed Gas by Gas Chrmatgraphy. D (2000), Vl West Cnshhcken, PA. American Sciety fr Testing and Materials (ASTM) Standard Test Methd fr Analysis f Natural Gas by Gas Chrmatgraphy. D , Vl West Cnshhcken, PA. United States Envirnmental Prtectin Agency (USEPA). 1996a. CFR Prmulgated Test Methds. Methd 3. Technlgy Transfer Netwrk, Emissin Measurement Center. Washingtn, D.C. June. United States Envirnmental Prtectin Agency (USEPA). 1998b. SW-846 Manual. Revisin 5. Office f Slid Waste. Washingtn, D.C. April. United States Envirnmental Prtectin Agency (USEPA) Cmpendium f Methds. Secnd Editin. Technlgy Transfer Netwrk, Ambient Mnitring Technlgy Infrmatin Center. Washingtn, D.C. January. United States Envirnmental Prtectin Agency (USEPA) CFR Prmulgated Test Methds. Methd 16. Technlgy Transfer Netwrk, Emissin Measurement Center. Washingtn, D.C. February. United States Envirnmental Prtectin Agency (USEPA) Methd 8015D Revisin 4 Nnhalgenated Organics Using GC/FID. Office f Slid Waste. Washingtn, D.C. June. USEPA and the United States Army Crps f Engineers Field Analytical Technlgies Encyclpedia (FATE). Washingtn D.C. Updated January. 9.2 Bidegradatin DeVaull, G.E., R.A. Ettinger, J.P. Salinitr, and J.B. Gustafsn Benzene, Tluene, Ethylbenzene and Xylenes (BTEX) Degradatin in Vadse Zne Sils During Vapr Transprt: First Order Rate Cnstants. Prceedings f the API/NGWA Petrleum Hydrcarbns and Organic Chemicals in Grundwater: Preventin, Detectin, and Remediatin Cnference. Hustn, TX. Nvember De Visscher, A., D. Thmas, P. Beck, and O. Van Cleemput Methane Oxidatin in Simulated Landfill Cver Sil Envirnments. Envirnmental Science & Technlgy 33:

72 Hers, I., J. Atwater, L. Li., and R. Zapf-Gilje Evaluatin f Vadse Zne Bidegradatin f BTX Vaprs. Jurnal f Cntaminant Hydrlgy. Vlume 46, Issues 3 4, Pages Jhnsn, P.C., P. Lundegard, J. Catts, K. DiSimne, D. Eley, and K. Schreder Surce Zne Natural Attenuatin Field Measurements, Data Interpretatin, and Data Reductin at the Frmer Guadalupe Oil Field, San Luis Obisp Cunty, Califrnia. Lewis, M. A. and A. L. Baehr Estimatin Of Rates Of Hydrcarbn Bidegradatin By Simulatin Of Gas Transprt In The Unsaturated Zne. Wat. Resur. J. 32: Lundegard, P.D., and P.C. Jhnsn Surce Zne Natural Attenuatin Investigatins at a Frmer Oil Field. Prceedings f the Internatinal Petrleum Envirnmental Cnference. Hustn, Texas. Nvember. Oremland, R.S. and C.W. Culbertsn Imprtance f Methane-Oxidizing Bacteria in the Methane Budget as Revealed by the Use f a Specific Inhibitr. Nature 356: April. Pateris, G., D. Werner, K. Kaufmann, and P. Hhner Vapr Phase Bidegradatin f Vlatile Fuel Cmpunds in the Unsaturated Zne: A Large Scale Lysimeter Experiment. Envir. Sci. Technl. 36: Rggemans, S., C.L. Bruce, P.C. Jhnsn, and R.L. Jhnsn Vadse Zne Natural Attenuatin f Hydrcarbn Vaprs: An Empirical Assessment f Sil Gas Vertical Prfile Data. API Sil and Grundwater Technical Task Frce Bulletin 15. December. 9.3 Data Analysis American Petrleum Institute (API) Assessing the Significance f Subsurface Cntaminant Vapr Migratin t Enclsed Spaces, Site-Specific Alternatives t Generic Estimates. Publicatin N Health and Envirnmental Sciences Department. Washingtn, D.C. Jhnsn, P.C., R.L. Jhnsn, and M.W. Kemblwski Assessing the Significance f Vapr Migratin t Enclsed-Spaces n a Site-Specific Basis. Jurnal f Sil Cntaminatin. 8 (3): General Charbeneau, R.J., Grundwater Hydraulics and Pllutant Transprt. Upper Saddle River, NJ: Prentice Hall. Cnnr, J.A., Ahmad, F.A., and McHugh, T.E Develpment f Simple Screening Criteria fr the Indr Air Pathway. API/NGWA Petrleum Hydrcarbns Cnference. Hustn, TX. Nvember. Interstate Technlgy and Regulatry Cuncil (ITRC) Vapr Intrusin Issues at Brwnfields Sites. ITRC. December. 62

73 Massachusetts Department f Envirnmental Prtectin (MaDEP) Indr Air Sampling and Evaluatin Guide. WSC Plicy N Cmmnwealth f Massachusetts. Bstn. MA. April. PaDEP (Pennsylvania Department f Envirnmental Prtectin) Land Recycling Prgram Technical Guidance Manual. Sectin IV.A.4. Vapr Intrusin int Buildings frm Grundwater and Sil under the Act 2 Statewide Health Standard Harrisburg PA. Octber. Draft. Wiscnsin Department f Health and Family Services (WDHFS) Chemical Vapr Intrusin and Residential Indr Air. Madisn, WI. February. USEPA Assessing Ptential Indr Air Impacts fr Superfund Sites. Office f Slid Waste and Emergency Respnse. EPA/540-R-95/128. PB May. USEPA. 1998a. Inside IAQ. EPA's Indr Air Quality Research Update. EPA/600/N-98/002. Office f Research and Develpment. Research Triangle Park, NC. Spring/Summer. USEPA. 2002a. OSWER Guidance fr Evaluating the Vapr Intrusin t Indr Air Pathway frm Grundwater and Sils (Subsurface Vapr Intrusin Guidance). Nvember Draft. Wallace L.A Majr Surces f Benzene Expsure. Envirnmental Health Perspectives 32: Wn, D. and R.L. Crsi Develpment f an Indr Envirnment Fate Mdel. Vlume I. Literature Review. Department f Civil Engineering, University f Texas at Austin. Austin, TX. 9.5 Mdeling Abreu, L.D.V A Transient Three-Dimensinal Numerical Mdel t Simulate Vapr Intrusin Int Buildings. Ph.D. Dissertatin. Arizna State University. Abreu, L. and P.C. Jhnsn Effect f Vapr Surce-Building Separatin and Building Cnstructin n Sil Vapr Intrusin as Studied with a Three-Dimensinal Numerical Mdel. Accepted fr Publicatin Envirnmental Science & Technlgy. Garbesi, K. and R.G. Sextr Mdeling and Field Evidence f Pressure-Driven Entry f Sil Gas int a Huse thrugh Permeable Belw-Grade Walls. Envirnmental Science & Technlgy. 23 (12): Garbesi, K., R.G. Sextr, W.J. Fisk, M.P. Mdera, and K.L. Revzan Sil-Gas Entry int an Experimental Basement: Mdel Measurement Cmparisns and Seasnal Effects. Envirnmental Science & Technlgy. 27 (3): Hers, I., R. Zapf-Gilje, L. Li., and J. Atwater Validatin f Mdels Used t Predict Indr Air Quality frm Sil and Grundwater Cntaminatin. Prceedings f the API/NGWA Petrleum Hydrcarbns and Organic Chemicals in Grundwater: Preventin, Detectin, and Remediatin Cnference. Anaheim, CA, Nvember

74 Hers, I, R. Zapf-Gilje, P.C. Jhnsn, and L. Li Evaluatin f the Jhnsn and Ettinger Mdel fr Predictin f Indr Air Quality. Grund Water Mnitring & Remediatin 23 (2): Jhnsn, P.C Identificatin f Critical Parameters fr the Jhnsn and Ettinger (1991) Vapr Intrusin Mdel. API Sil and Grundwater Research Bulletin N. 17. API. May. Jhnsn, P.C. and R.A. Ettinger Heuristic Mdel fr Predicting the Intrusin Rate f Cntaminant Vaprs int Buildings. Envirnmental Science & Technlgy. 25 (8): Little, J.C., J.M. Daisey, and W.M. Nazarff Transprt f Subsurface Cntaminants int Buildings: An Expsure Pathway fr Vlatile Organics. Envirnmental Science & Technlgy 26 (11): Lwell, P.S., B. Eklund VOC Emissin Fluxes as a Functin f Lateral Distance frm the Surce. Envirnmental Prgress. 23 (1) Stein, V.B., J.P.A. Hettiaratchi, and G. Achari A Numerical Mdel fr Bilgical Oxidatin and Migratin f Methane in Sils. ASCE Practice Peridical f Hazardus, Txic, and Radiactive Waste Management 5 (4): Sample Cllectin Methds American Sciety fr Testing and Materials (ASTM) Standard Guide fr Sil Gas Mnitring in the Vadse Zne. D , Vl West Cnshhcken, PA. British Petrleum (BP) SPP-110 Vadse Zne Sampling at BP Crrective Actin Sites. Glbal Envirnmental Management, a BP Affiliated Cmpany. Lisle IL. Califrnia Reginal Water Quality Cntrl Bard Ls Angeles Regin (CRWQCB) and the Department f Txic Substances Cntrl (DTSC) Advisry Active Sil Investigatins. January. Cunty f San Dieg, Department f Envirnmental Health (CSDDEH) Site Assessment and Mitigatin Manual (SAM Manual 2002). San Dieg, CA. January 22. Devitt, D.A., R.B. Evans, W.A. Jury, T.H. Starks, B. Eklund, and A. Ghlsn Sil Gas Sensing fr Detectin and Mapping f Vlatile Organics. EPA/600/8-87/036. U.S. EPA- EMSL. Las Vegas, NV. Eklund, B Detectin f Hydrcarbns in Grundwater by Analysis f Shallw Sil Gas/Vapr. American Petrleum Institute Publicatin N May. Eklund, B "Practical Guidance fr Flux Chamber Measurements f Fugitive Vlatile Organic Emissin Rates." Jurnal f the Air & Waste Management Assciatin 42 (12): Hartman, B Hw t Cllect Reliable Sil-Gas Data fr Risk-Based Applicatins, Part1: Active Sil-Gas Methd. LUST Line Bulletin 42. Octber. 64

75 Hartman, B Hw t Cllect Reliable Sil-Gas Data fr Upward Risk Assessments, Part 2: Surface Flux Chamber Methd. LUST Line Bulletin 44. August. Hartman, B. 2004a. Hw t Cllect Reliable Sil-Gas Data fr Risk-Based Applicatins Specifically Vapr Intrusin, Part 3: Answers t Frequently Asked Questins. LUST Line Bulletin 48. Nvember. Huntingtn Beach Methane Cdes (HBMC) City Specificatin N. 429 Methane District Building Permit Requirements. Reference t HBMC Sectin Huntingtn Beach, CA. Draft. Lahvis, M.A Guidance n Use f Sil-Gas Surveys t Assess Vapr Transprt t Indr Air. Shell Glbal Slutins (U.S.), Inc. Hustn, TX. Ls Angeles Methane Cdes (LAMC) High Ptential Methane Zne Requirements. City Building Regulatins Sectin Ls Angeles, CA. New Jersey Department f Envirnmental Prtectin (NJDEP) Draft Vapr Intrusin Guidance Dcument. January. USEPA Sil Gas Sampling. Standard Operating Prcedure N Envirnmental Respnse Team. Washingtn, D.C. June. Wang, Y., S. Raihala, A. P. Jackman, and R. St. Jhn Use f Tedlar Bags in VOC Testing and Strage: Evidence f Significant VOC Lsses. Envr. Sci. Technl. 30: Site Characteristics and Cnceptual Vapr-Migratin Mdels Fitzpatrick, N.A. and J. J. Fitzgerald An Evaluatin f Vapr Intrusin Int Buildings Thrugh a Study f Field Data. 11 th Annual Cnference n Cntaminated Sils. University f Massachusetts at Amherst. Massachusetts Department f Envirnmental Prtectin. Fischer, M.L., A.J. Bentley, K.A. Dunkin, A.T. Hdgsn, W.W. Nazarff, R.G. Sextr, and J.M. Daisey Factrs Affecting Indr Air Cncentratins f VOCs at a Site f Subsurface Gasline Cntaminatin. Envirnmental Science & Technlgy. 30: Fisher, L.J., L.M. Abrila, R.H. Kummler, D.T. Lng, and K.G. Harrisn Evaluatin f the Michigan Department f Envirnmental Quality s Generic Grundwater and Sil Vlatilizatin t Indr Air Inhalatin Criteria. Michigan Envirnmental Science Bard. Lansing, MI. May. Fugler D. and M.Admait Indr Infiltratin f Vlatile Organic Cntaminants: Measured Sil Gas Entry Rates and Other Research Results fr Canadian Huses. Jurnal f Sil Cntaminatin 6 (1): Jhnsn, P.C., R.A. Ettinger, J. Kurtz, R. Bryan, and J.E. Kester Migratin f Sil Gas Vaprs t Indr Air: Determining Vapr Attenuatin Factrs Using a Screening-Level Mdel and Field Data frm the CDOT-MTL. API Sil and Grundwater Research Bulletin N. 16. API. April. 65

76 Kindzierski, W.B Influence f Sil Gas Entry, Outdr and Indr Surces n Indr VOCs at Petrleum Cntaminated Settings. Prceedings f the Air and Waste Management Cnference. Trnt, Ontari, Canada. June Kramer, W.H Evaluatin f Subsurface Utilities and Indr Air Envirnments as Migratin Pathways and Pints f Expsure in a RBCA Site Assessment. Prceedings f the API/NGWA Petrleum Hydrcarbns and Organic Chemicals in Grundwater: Preventin, Detectin, and Remediatin Cnference. Hustn, TX. Nvember Laubacher, R.C., P. Barthlmae, P. Velasc, and H.J. Reisinger An Evaluatin f the Vapr Prfile in the Vadse Zne abve a Gasline Plume. Prceedings f the API/NGWA Petrleum Hydrcarbns and Organic Chemicals in Grundwater: Preventin, Detectin, and Remediatin Cnference. Hustn, TX. Nvember Ririe, G.T., R.E. Sweeney, S.J. Daugherty A Cmparisn f Hydrcarbn Vapr Attenuatin in the Field with Predictins frm Vapr Diffusin Mdels. Jurnal f Sil and Sediment Cntaminatin 11 (4):

77 Cllecting and Interpreting Sil Gas Samples frm the Vadse Zne A Practical Strategy fr Assessing the Subsurface Vapr-t-Indr Air Migratin Pathway at Petrleum Hydrcarbn Sites APPENDIXES

78 Appendix A. Characteristics Checklist Tpic: The infrmatin included in this checklist shuld be useful fr develping the site-specific cnceptual migratin mdel and planning the sil gas sampling. The user shuld cnsider cpying this checklist and cmpiling the infrmatin fr each site. Utilities and Prcess Piping Lcate and map ut all undergrund utilities near the sil r grundwater impacts; pay particular attentin t utilities that cnnect impacted areas t ccupied buildings. Lcate and map ut all undergrund prcess piping near the sil r grundwater impacts. Buildings Lcate and map ut identified existing and ptential future buildings. Identify the ccupancy and use f the identified buildings (e.g., residential, cmmercial). Describe the cnstructin f the building, including materials (e.g., wd frame, blck,), penings (e.g., windws, drs), and height (e.g., ne-stry, tw-stry, multiple-stry); identify any elevatr shafts in the building. Describe the fundatin cnstructin, including: Type (e.g., basement, crawl space, slab n grade) Flr cnstructin (e.g., cncrete, dirt) Depth belw grade. Describe the HVAC system in the building, including: Furnace/air cnditining type (e.g., frced air, radiant) Furnace/air cnditining lcatin (e.g., basement, crawl space, utility clset, attic, rf) Surce f return air (e.g., inside air, utside air, cmbinatin) System design cnsideratins relating t indr air pressure (e.g., psitive pressure is ften the case fr cmmercial buildings). Describe sub-slab ventilatin systems r misture barriers present n existing buildings, r identify building- and fire-cde requirements fr sub-slab ventilatin systems (e.g., fr methane) r misture barriers belw fundatins fr future buildings. Surce Area Lcate and map ut the surce area fr the vapr-phase petrleum hydrcarbns related t the subsurface-vapr-t-indr-air expsure pathway. Describe the presence, distributin, and cmpsitin f any LNAPL at the site. A-1

79 Identify the vapr-phase petrleum hydrcarbns that are t be cnsidered fr the subsurface-vapr-t-indr-air expsure pathway. Describe the status and results fr the delineatin f petrleum hydrcarbns in envirnmental media, specifically sil and grundwater, between the surce area and the ptentially impacted buildings. Describe the envirnmental media (e.g., sil, grundwater, bth) cntaining petrleum hydrcarbns. Describe the depth t surce area. Describe the ptential migratin characteristics (e.g., stable, increasing, decreasing) fr the grundwater distributin f petrleum hydrcarbns. Gelgy/Hydrgelgy Review all bring lgs and sil sampling data t understand the lcatins f: Surces Finer-grained sil layers Higher-permeability layers that may facilitate vapr migratin. Describe distinct strata (sil type and misture cntent e.g., mist, wet, dry ) and the depth intervals between the vapr surce and grund surface. Describe the depth t grundwater. Describe grundwater characteristics (e.g., seasnal fluctuatin, hydraulic gradient). Site Characteristics Estimate the distance frm edge f grundwater plume t building. Estimate the distance frm vapr surce area t building. Describe the surface cver between the vapr surce area and the ptentially impacted building. A-2

80 Appendix B. Selectin f Sil Gas Sample Lcatins Tpic: The scenari diagrams included in this appendix may be helpful in planning sil-gas-sampling lcatins and depths. The user shuld cnsider cpying the scenari diagram that mst-clsely matches the site cnditins and using it t recrd the site-specific dimensins. Once the diagram has been custmized fr the site, the lcatins and depths fr sil gas samples can be identified. B.1 Scenari 1 Building Nt Over Surce r Grundwater Distributin f Petrleum Hydrcarbns 1. Depth t tp f surce 2. Depth t tp f grundwater 3. Distance frm surce t prperty line 4. Distance frm surce t building 5. Distance frm edge f grundwater plume t building 6. Depth f fundatin belw grund surface 7. Type f surface cver ff site 8. Type f surface cver n site 9. Width f building B-1

81 B.2 Scenari 2 - Building Over Grundwater Distributin f Petrleum Hydrcarbns Prperty Line basement 6 2 surce grundwater Depth t tp f surce 2. Depth t tp f grundwater 3. Distance frm surce t prperty line 4. Distance frm surce t building 5. Distance frm surce t edge f grundwater plume 6. Depth f fundatin belw grund surface 7. Type f surface cver ff site 8. Type f surface cver n site 9. Width f building B-2

82 B.3 Scenari 3 - Building Over Surce f Petrleum Hydrcarbns basement 4 2 surce grundwater Depth t tp f surce 2. Depth t tp f grundwater 3. Distance frm surce t edge f grundwater plume 4. Depth f fundatin belw grund surface 5. Type f surface cver 6. Type f surface cver 7. Width f building B-3

83 Appendix C. Sil Gas Sample Cllectin Tpic: This appendix includes cnsideratins fr sil-gas-mnitring installatins, sil-gas-cllectin prcedures, and dcumentatin f activities. C.1 Basic Mnitring Installatin Methds This sectin prvides useful cnstructin infrmatin and details fr the mnitring installatin methds. Please nte that the infrmatin is intended as general guidelines and nt specific recmmendatins fr all sites. Site-specific cnsideratins, prfessinal judgment, and regulatry requirements will dictate the methds and prcedures used at any particular site. C.1.1 Permanent Prbes Based n experience, and as described in CRWQCB (2003), Lahvis (2002), Hartman (2002), and BP (1998), the fllwing cnstructin details shuld be cnsidered fr the installatin f permanent prbes: Use shrt individual sampling intervals (e.g., 6 t 12 inches). Clr cde r tag tubing r prbes at the surface s that the sampling depth is easily identifiable fr future sampling events. Cmplete and seal permanent prbes at the grund surface (e.g., rad bxes, lcked caps, vapr-tight valves). If multiple sampling intervals are installed as vertically-nested prbes, cnsider installing a grundwater sampling prbe as part f the sil-gas-sampling cluster, especially if a grundwater plume is the vapr surce. When using augered brings fr the installatin f sil-gas-sampling prbes, the fllwing shuld be cnsidered: Install sampling prbes with sand-pack intervals f abut 1 ft. Seal each sampling interval with bentnite r grut abve and belw the sand pack in the annulus f the bring. If dry bentnite is placed in the bring, care shuld be taken t fully hydrate the bentnite. Placing the bentnite in small increments (e.g., < 6 inches) fllwed by water is helpful. Alternatively, the bentnite can be added using a cmbinatin f dry and hydrated bentnite, r in slurry frm if the bring is f sufficient diameter. Use dwn-hle supprt rds, which may ffer practical benefits during installatin, particularly fr deeper prbes (CRWQCB 2003). See Figure C 5 fr a schematic f augered permanent prbes. C-1

84 When using direct-push brings fr the installatin f sil-gas-sampling prbes, the fllwing shuld be cnsidered: Avid lateral mvement f the prbes nce they are in the grund t prevent leakage f atmspheric air. Installing sand-pack intervals and seals in small-diameter brings may be difficult. See Figure C 6 fr a schematic f direct-push permanent prbes. C.1.2 Temprary Driven Prbes Based n experience, and as described in CRWQCB (2003) and Hartman (2002), the fllwing cnstructin details shuld be cnsidered fr the installatin f temprary driven prbes: Seal prbes at the surface with bentnite prir t sampling. Warning: sealing temprary prbes at the grund surface can make the field peratins difficult and a bit messy due t the expsed, wet clay. If a sampling tube is used inside the driven rds, seal it inside the rd t prevent shrt-circuiting. Attach the sil-gas-sampling prbe tip t the sampler tubing r t the driven rds, depending n the methd used. See Figure C 7 fr a schematic f direct push temprary prbes. C.2 Field Activities during Sil Gas Sampling This sectin prvides infrmatin abut related field activities that shuld be cnsidered during the installatin f sil-gas-sampling prbes r during sil-gas-sampling events. C.2.1 C.2.2 General Site Cnditins Cnduct a vapr survey with a field instrument (e.g., PID r FID) f all undergrund utilities t determine if the utilities are preferential vapr-migratin pathways. Nte the current weather cnditins (e.g., temperature, barmetric pressure, humidity, sunny/cludy). Nte the date f the last precipitatin event and the apprximate rainfall depth. Sil Cnditins If permanent prbes are installed, make a pht recrd f the sil cre, if cllected, and cllect several sil samples fr misture cntent analysis. If the vapr surce, r sil surce, is nt well defined, then cllect sil samples during the installatin f the sil-gas-sampling prbes at each sample interval fr labratry analyses f chemicals f cncern. Field screening f the sil samples als shuld be cnducted, and ther mre qualitative indicatrs f impacts shuld be nted (e.g., drs and staining). C-2

85 C.2.3 Grundwater Cnditins Cllect grundwater samples fr labratry analyses f chemicals f cncern at the same time and in similar lcatins as the sil-gas-sampling lcatins. See nte abut grundwater sampling under Sectin C.1.1. C.3 Sample Cllectin Prcedures This sectin prvides infrmatin abut sample cllectin prcedures that shuld be cnsidered when planning the sil-gas-sampling prgram. As an example, phts frm ne site shwing the sample cllectin equipment are included at the end f this appendix. C.3.1 Testing t Determine if Sil Gas Sampling is Practicable As a qualitative test, a 20-cubic-centimeter (cc) syringe culd be cnnected t the silgas-sampling tubing t determine if a sample can be withdrawn (Hartman 2004). If the sil-gas-sampling tubing has a greater vlume than 20 cc, the test will be incnclusive because vaprs will nt be drawn frm the subsurface. Anther example f a test that can be perfrmed t determine if sil gas sampling is practicable at a site is the fllwing: 1) Install a T-cnnectin at the end f the sil-gas-sampling tubing. 2) Cnnect a vacuum gauge t ne branch f the T-cnnectin. 3) Cnnect a syringe fitting (e.g., a Luer-lk fitting) and a 60-mL r larger syringe t the remaining branch f the T-cnnectr. (See Figure C 1) 4) With the syringe cnnected, pull the plunger back t the full-scale reading and hld in that psitin. 5) Mnitr the vacuum created and its relaxatin. 6) If it des nt relax within a few minutes t an hur, it is unlikely that sil gas sampling is practicable at the site. C-3

86 Vacuum gauge >100 ml syringe Pull back plunger and watch relaxatin f the vacuum Figure C 1. Example test apparatus t determine if sil gas sampling is practicable. C.3.2 Purging Calculate the dead vlume based n the length and inner diameter f the sampling prbe and the cnnected sampling tubing and equipment (i.e., it is assumed that the sampler and brehle sand-pack vapr space is equilibrated with surrunding vaprs [as it shuld be if the prbe has been well sealed]) s that the bring vapr space is nt included in the dead vlume. Purge sampling prbe and equipment in rder t cllect representative samples f sil gas at the sample interval. The number f purged vlumes is generally between ne and five dead vlumes (CSDDEH 2003, Hartman 2002, Lahvis 2002). Based n experience, minimizing the purged vlumes is apprpriate. In additin, Hartman (2004) indicates that minimizing the purged vlume may reduce the uncertainty abut where in the subsurface sil vaprs are being drawn. The number f dead vlumes purged may be based n a fixed value (e.g., three dead vlumes), r ther prcedures such as: Analyzing the purged gas with a field vapr analyzer (e.g., PID r FID) until the cncentratins f ttal hydrcarbns stabilize (BP 1998), r using field instruments t measure respiratin gases (e.g., O 2, CO 2 ) and assessing cnsistency acrss sequential purged vlume samples. Cnducting a purged vlume test t determine the number f dead vlumes t remve that crrespnds t the highest recvered vapr cncentratins (CRWQCB 2003). C-4

87 C.3.3 C.3.4 C.3.5 The purged vlume selected shuld be cnsistent at all f the sample lcatins acrss the site and shuld be recrded (CSDDEH 2003; Lahvis 2002). Testing Mnitring Installatins fr Shrt-Circuiting During sil gas sampling, cnsider cnducting a leak test at sme prprtin f, r all, sil-gas-sampling lcatins using tracer cmpunds. Fr example: Use xygen as a high-end indicatr f shrt-circuiting. Elevated xygen measurements in sil gas analytical results may indicate significant shrt-circuiting. CRWQCB (2003) recmmends leak tests be cnducted fr the belw grund equipment using tracer cmpunds (e.g., butane, prpane, isprpanl), sme f which are ingredients in cnsumer prducts such as shaving cream. Fr example, the shaving cream is applied at the surface where air culd enter the sil gas prbes. The sil gas sample is analyzed fr the tracer cmpund using a methd that can achieve less than 10 ug/l detectin limit. Butane and prpane may be detectable as part f a release f gasline at a site, s judgment shuld be used abut whether this leak testing methd can be used at a specific site. Hartman (2002) recmmends that leak tests be cnducted at sites where leakage may be a cncern (e.g., shallw sample intervals, large sample vlumes). Checking Abvegrund Sampling Equipment fr Leaks One methd fr checking the sampling equipment fr leaks is described belw: 1) Cnnect all abvegrund sampling equipment (everything that is t be cnnected t the in-the-grund sil gas prbe tubing) t a 10-L Tedlar bag cntaining a tracer gas f a knwn cncentratin (e.g., diluted helium). 2) Place a valve between the tracer gas bag and the abvegrund sampling equipment, and a vacuum gauge dwn-stream f this valve. 3) Begin t draw tracer gas thrugh the abvegrund sampling equipment and adjust the valve t create a vacuum similar t what will be used when withdrawing sil gas frm the in-the-grund sil gas prbe tubing. 4) Analyze the tracer gas that is cllected t see if it has the same cncentratin as the riginal Tedlar bag frm which the sample was withdrawn. If nt, then the abvegrund sampling equipment is leaking. Decntaminating Equipment Clean abvegrund sampling equipment shuld be used fr all parts f the sil gas sample cllectin. This can be implemented by using dispsable parts r using prper prcedures t clean reusable equipment. If reusable sample cntainers are used (e.g., Summa canisters), the supplier shuld be able t prvide analytical results demnstrating the cntainers are clean. C-5

88 External sampling equipment parts may be washed in the field. If tubing is nt dispsable, then five r mre vlumes shuld be purged between samples (CSDDEH 2003). Tubing shuld nt be re-used fr shallwer samples (lwer cncentratins expected) nce it has been used t cllect deeper samples (higher cncentratins expected). C.3.6 C.3.7 Sample Cllectin Flw Rates Select a flw rate that des nt bias the sample. Minimize the flw rate, and therefre, the vacuum, t practical levels during sampling. Fr example, flw rates shuld nt exceed abut 1 L/min, and flw rates as slw as abut 1L/h d nt create significant lgistical issues fr sampling plans. Lahvis (2002) recmmends a vacuum f less than 10 inches f water, althugh vacuums as high as 50 t 100 inches f water shuld nt adversely bias samples, prvided that the sampling equipment des nt leak at thse vacuums (see Sectin 5.4.4). Based n experience, sampling rates in the 1 L/min t 1 L/h range are practicable. Fr shallw grundwater situatins, minimize the sample cllectin flw rate t prevent grundwater frm entering the sample cntainer. Measure and recrd the vacuum at which the samples were cllected fr each sampling prbe. Mnitr the vacuum during sampling with an in-line gauge (Lahvis 2002). Sample Cntainers The sample cntainers chsen fr a specific site will depend n the sampling equipment and analytical requirements. Examples f different sample cntainers include: Specially-treated canisters, including Summa plished r glass-lined canisters These are available in vlumes ranging frm apprximately 400 ml t 6 L. They are under vacuum, and the vacuum shuld be verified befre sampling in case the valve has been leaking in shipment. The slid surfaces f the cntainers mean the samples are likely t be mre stable in shipping. Typically, sampling is cnducted using a flw regulatr n the canister (CRWQCB 2003). Tedlar bags These are available in vlumes ranging frm apprximately 1 t 100 L. Fr shipping, the bags need t remain at standard pressure (Hartman 2002). Cali-5-Bnd bags (infrmatin frm These are available in vlumes up t apprximately 200L. C-6

89 They are five-layer bags designed t limit gas diffusin int r ut f bags. Syringes These are available in vlumes ranging frm apprximately 0.5 ml t 10 ml. Recvery f heavier VOCs may be biased lw. They are typically used fr n-site analysis nly. Srbent sampling tubes These are used t cncentrate samples (e.g., Tenax srbent). They can be single r multi-layer srbent tubes. The srbents are specialized materials based n the chemicals f cncern. They are used fr EPA TO-17 analyses (USEPA 1999). C.3.8 C.3.9 Equilibratin Time After the installatin f sil-gas-sampling prbes, allw sufficient time fr the sil gases t equilibrate with the air inside f the sampling prbes prir t cnducting purging and sampling activities. CRWQCB (2003) recmmends an equilibratin time f 20 t 30 minutes fr temprary driven prbes, and 48 hurs fr prbes installed using augered brings. The disturbance caused by the installatin f permanent prbes by drilling methds varies with the drilling methd. Traditinal auger-based drilling will intrduce air thrughut the sil clumn, but that disturbance may be limited t the immediate vicinity f the brehle and within the sand packs. If that is the case, thughtful use f sampler purging may be sufficient t cllect representative samples sn after the seals are set (within a few days f installatin). Other mre invasive methds, such as air-rtary drilling, wuld be expected t significantly disturb the sil gas prfile in a wide area arund the sil-gassampling installatin, and it culd take weeks t mnths fr the sil gas prfile t reequilibrate. Mre quantitative estimates f the time necessary fr re-establishment f near-steady sil gas prfiles can be generated using the equatin discussed in the sidebar in Sectin 2, Estimating the Time Necessary T Achieve Near Steady-State Cnditins. Fr this applicatin, cnsider chsing the travel distance L t be the depth t the vapr surce r the lateral distance t the undisturbed vapr prfile, whichever is less. If the sampling plan calls fr multiple sampling events ver several mnths, the data may be used t establish if the sil gas prfile has equilibrated. Hartman (2004) suggests if the sampling plan nly calls fr ne set f sil gas samples, several samples can be cllected ver a few days r weeks t determine the stabilized cncentratins. Other Sample Cllectin Issues Sil gas samples can be cllected by different methds: C-7

90 Grab sample a ne-time sample cllected ver a shrt perid f time (e.g., less than 15 minutes). Grab samples are generally used in sil gas sampling. Cmpsited grab samples fr finer-grained sils, multiple samples frm the same depth at lw flw rates can be cllected ver time in rder t minimize sampling vacuum while cllecting enugh samples fr labratry analysis (Lahvis 2002). Cllect field blanks and trip blanks fr labratry analyses. Cllect duplicate samples fr labratry analyses frm 10-percent f the sampling intervals (Lahvis 2002). Analyze samples as quickly as pssible t prevent lss f vlatiles r degradatin f the analytes. The times may depend n the analytical methd hlding times, r regulatry guidance. Fr example: Analyze Tedlar bags and gas-tight vials within 48 hurs (CSDDEH 2003), up t 72 hurs, SW-846, methd 0040 (USEPA 1998b). Analyze Summa canisters within 72 hurs (CRWQCB 2003), up t 30 days, methd TO-15 (USEPA 1999). Analyze samples cllected fr field analysis within 30 minutes (CRWQCB 2003). D nt chill sil gas samples fr shipping t a labratry because the vlatiles may cndense ut f the vapr phase at the lwer temperature (Hartman 2002). Prir t cllecting a sil gas sample, check the individual sampling prbe t be sure that it is nt blcked (e.g., measure the vacuum n the sampling prbe, pull a vacuum n the sampling prbe then release the vacuum. Be sure that the pressure returns t the initial value). C.4 Dcumentatin Cnsider the fllwing dcumentatin tasks: Recrd the depth fr each sil-gas-sampling interval Fr permanent prbes, prepare a sketch r table t recrd the dimensins fr each sil-gas-sampling lcatin. Maintain recrds f all field prcedures, including any leak testing, purging, and sampling fr each sampling lcatin. Recrd the time t cmplete each activity. Maintain recrds f the field activities (e.g., general site cnditins, sil and grundwater cnditins) cnducted as part f the sil-gas-sampling prgram. C.5 Phts All phts curtesy f P. Lundegard, Chevrn Crpratin, Brea, CA. C-8

91 Figure C 2. Vertically-nested sampling prbes with sealed Luer-lk fittings and a sampling syringe. C-9

92 Figure C 3. Vertically-nested sil-gas-sampling prbes sealed at the grund surface with a cncrete pad and a vertical surface casing. C-10

93 Figure C 4. Sampling syringe cnnected t ne sampling tube. Nte that each sampling tube is labeled and that the syringe and the sampling tube each have sample valves. C-11

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