Part 5 Soil Gas Sampling Methods. Soil Gas Measurement. VI Requires Much Lower DLs

Transcription

1 Part 5 Soil Gas Sampling Methods Soil Gas Methods Sampling Details Sampling Strategies Chlorinated-HCs vs. Hydrocarbons Soil Gas Measurement Pros: Representative of Subsurface Processes Higher Screening Levels Relatively Inexpensive Can Give Real-time Results Cons: Transfer Rate Unknown Ultra Low Proposed Screening Levels Protocols still debated Preferred Approach By Many Agencies VI Requires Much Lower DLs Typical Soil Gas Concentrations MTBE & Benzene near gasoline soil: >1,000 ug/l PCE under dry cleaner: >1,000 ug/l Soil Gas Levels a Threat to GW: MTBE: >10 ug/l BTEX/PCE: >100 ug/l 10,000x Soil Gas Levels Exceeding EPA VI Levels At 5 : Benzene: ug/l, TCE: ug/l Greater Chance for False Positives 1

2 Which Soil Gas Method? Active? Passive? (limited use) Flux Chambers? (limited use) Active method most often employed for VI Passive Soil Gas Pros: Easy to Deploy Can Find Contamination Zones Low Permeability soils Cons: Does not Give Concentration No Less Expensive Considered as Screening Tool Passive Soil Gas Sampling Media Hydrophobic Adsorbant provided by Beacon Environmental Adsorbent inside vial with membrane Adsorbent inside vapor permeable, waterproof membrane 2

3 Direct Flux Measurement (Flux Chambers) Pros Direct measurement of intrusion Cons Proper location (may miss preferential paths) Protocols debated How to use data? Unsophisticated audience Regulatory acceptance limited Schematic of Surface-Flux Chambe Static Flux Chamber Equipped with Landtec GEM 2000 for real-time CO2, O2, CH4 used to collect data continuously Set up to collect the flux of vapor from the subsurface, then connect a summa canister from the chamber for analysis Flux Chamber Site Receptors Water at 3 bgs 3

4 Active Soil Gas Sampling Considerations Probe installation Equilibration and purge volumes Flow rate, vacuum, and leak tests Sample containers Temporal effects Sample density and locations Checklist in 2007 ITRC Guidance Probe Installation Methods Drilling Techniques Direct Push (Truck/LAR or Manual) Auger (Hollow Stem Auger or Hand Auger) Other techniques are typically not recommended (i.e. sonic, mud/air rotary, etc) Probe Installation: Sampling through the Direct Push Rod Drive Point (tubing connected prior to drilling) Post Run Tubing (PRT connected after target depth is reached) 4

5 Probe Installation: Vapor Wells / Soil Gas Implants Quick & easy to install/remove Allows for repeated sampling Multiple depths possible, and can nest probes within the same boring Soils can be removed concurrently Nested Probes (three probes in one boring) Single Depth Probes (one probe per boring) Depths are marked in a variety of ways. Be sure to take GOOD NOTES for future sampling Soil Gas Implants Soil Vapor Probe Construction Single Depth Nested Probes Typical construction diagrams with example sand and bentonite thicknesses 5

6 Soil Vapor Probe Construction Tubing Type Rigid wall tubing ok (nylon, teflon, SS) Flexible tubing not (tygon, hardware store) Probe Tip Beware metal tips (may have cutting oils) Materials Used to Bury Probes Sand, cement Soil Vapor Probe Construction H&P, Inc. Tubing TCE #1 (ug/m3) Tubing Test TCE #2 (ug/m3) Average SS PEEK Nylaflow Teflon Polyethylene* Cu nd 170? * Typical Geoprobe Tubing 6

7 Teflon or Nylaflow Tubing? The two most commonly requested tubing types are Nylaflow (1/8 ) and Teflon (1/4 ). The CA DTSC Advisory references the Hayes study, and leads readers to conclude that Nylaflow is inferior to Teflon, for Naphthalene in particular. Recent recovery studies conducted by H&P simulate typical soil vapor sampling practices and indicate that there is not a significant difference between the two tubing types for Naphthalene in particular. Data review in process and findings to be presented at a future date. For longevity in permanent probe construction, 1/8 Nylaflow is more flexible and performs better coiled within a well box than the more rigid 1/4 Teflon. Tubing Types Soil Vapor Probe Construction H&P, Inc. Sand Pack Typically #3 kiln dried sand Dry Bentonite Important to protect the sand pack from excess hydration above Hydrated Bentonite to Seal Either hydrated in lifts or poured into the boring as a slurry CALIFORNIA: Preference for slurry hydration, but when employed for small diameter direct push, this method has time/cost implications 7

8 Sub-Slab Soil Gas Sampling Soil gas most likely to enter structure May detect chemicals originating within building Sample at slab base and/or at depth Permanent or temporary sample points Large Spatial Variability Permanent Implants Some Lessons Learned Do not mark sample locations with spray paint: toluene What NOT to do!!!! Some Lessons Learned Watch what you use to seal holes Loaded with TCE Loaded with TBA 8

9 Some Lessons Learned Watch where you collect your samples Some Lessons Learned Always be prepared for the unexpected!!! Equipment Blanks Background contamination from a variety of sources: Filter Tip Stainless Steel tips are machined and may contain cutting oils and/or cleaning solvents Tubing Where is the tubing stored? Was it flushed with nitrogen or ambient air prior to installation? Unexpected Equipment Contamination What type of leak check compound was used? Does it contain impurities? Blooper reel of sealing materials and field oversights Was decon performed, and if so, was it performed correctly? 9

10 Decon Procedures: More is not always Better! Full strength cleaning materials contain VOCs* *Headspace analysis of undiluted Liquinox DILUTE according to instructions and be sure to triple wash to avoid accidental cross-contamination. Benzene =1530 ug/m3 Active Soil Vapor Sampling Considerations Equilibration times Purging and calculation of purge volume Flow rate and probe vacuum Leak check compounds and procedures Sample container types and volumes Temporal effects H&P, Inc. Equilibration Time Vapor Wells ~60% after 2 hrs, 70% after 4 hrs, 85% after 8 hrs 10

11 Equilibration Time Through Rod ~80% immediately, 100% after 1 hr Equilibration Time Example of the varying equilibration times for sites in Southern California Purge either with a syringe (smaller volumes) or a pump (larger volumes) Purging H&P, Inc. H&P, Inc. 11

12 Purge Volume Purpose: Remove ambient air that was entrained in the system during installation prior to sampling Calculate: Volume of Tubing (πr 2 * Length) Volume of Sand Pack Pore Space (πr 2 * Porosity * Thickness) CA SPECIFIC Also Volume of Dry Bentonite Pore Space Typically, a total of 3 Purge Volumes (3PV) are removed prior to sampling Purge and Sample: Flow Rate and Vacuum Rule of Thumb: Flow rate 200mL/min Studies show that this does not have a significant impact on results, and often larger volumes are removed at 5-10L/min More Important: Avoid high vacuum Maintain a vacuum of <100 water Vacuum may indicate that there is likely too little permeability to obtain an uncompromised sample High vacuum increases potential for preferential pathways, leaks, and partitioning of compounds into vapor phase H&P, Inc. Leak Testing Methods Overview Leak Prevention Steps - Shut-In Test (or Vacuum Test) - Water Dam (i.e. creating a robust surface seal) Leak Check Compound Applications - Liquid Tracers - Gaseous Tracers Evaluation of Leaks, if they occur - Thresholds and Interpretation - Reminder about checking PRT fittings 12

13 Leak Prevention Step: Shut-In Test (or Vacuum Test) Connect sample train and all above ground fittings Apply vacuum to the system, close all ports, and observe vacuum for at least 1 minute (or greater) If vacuum holds, no leaks! If vacuum drops, a leak is present and can be addressed prior to sampling. Shut-In Test Video H&P, Inc. Leak Prevention Step: Reinforcing the Surface Seal with Water or Bentonite Water Dam Place a small container at the base of the tubing where it enters the subsurface, around the tubing. Pour water into the container, creating a robust seal. Photo courtesy of Cox-Colvin Concept can also be used to apply a mound of hydrated bentonite at the base of the tubing where it enters the ground. Leak Prevention Steps IMPORTANT TIP: Neither the Shut-In Test or the Water Dam are a replacement for using a leak check compound! These aid the sampler in preventing leaks, but do not detect leaks. 13

14 Leak Check Methods H&P, Inc. Liquid Tracer Gaseous Tracer, Full Shroud Gaseous Tracer, Small Shroud Liquid Tracer Method Compounds Commonly Used 1,1-Difluoroethane (duster spray) Isopropyl Alcohol Butane (shaving cream) Pros Easy to obtain and apply Cost effective Excellent with Mobile Lab H&P, Inc. Cons Qualitative (surface concentration is not measured each time) If using a fixed lab, real-time results are not available Even a small leak of IPA or Butane can necessitate Sample dilution, raising target compound RLs Gaseous Shroud Tracer Method Compounds Commonly Used Helium Isobutylene, CO2, etc Pros Quantitative Real-time results prior to sample collection using a meter Cons Higher cost and more time in field Requires extra equipment and experience Recommended to test on-site, and again at the fixed lab, especially for subslab and/or sample containers 1L or greater (where the potential for leakage is greater) 14

15 Evaluation of Leak Check Detections Threshold for a leak check detection? Liquid Leak Check is QUALITATIVE Common threshold is 10 ug/l, with some states as high as 100 ug/l, others as low as 0.05 ug/l The surface concentration is estimated at 1,000 ug/l Gaseous Leak Check is QUANTITATIVE Shroud concentration is measured; leak % can be calculated Common threshold is 10%, with some states as high as 15%, others as low as 5% Do you test the sample in addition to the probe? PRO TIP if using a 6L summa for your sample, best to test the sample back at the fixed base lab Post-Run Tubing (PRT) Fitting Remember to evaluate the connection of the tubing at the O-Ring upon removing the PRT fitting from the ground Containers Sample Container Types and Volumes Canisters: More blank potential. Higher cost Tedlars: Good for ~2 days. Easy to collect Sorbent Media: Can be used for modified active soil gas sampling Sample Volume Greater the volume, greater the uncertainty Smaller volumes faster & easier to collect 15

16 Summa Canister Volumes Considerations for Soil Gas Sample Volumes >1 Liter Sample volumes of 1L or less are recommended, but some projects use 6L (due to requested RLs and/or lab inventory) IF sampling with a 6L: Longer shut in test Maintain concentration of leak check compound Consider the larger zone of influence and the effect on the sample results, particularly for subslab samples Summa Canister Volumes OR (40) 400mL Summas (40) 6L Summas 6-Liter Summa canisters not only cost more to ship, but they require a lot of space for transportation and for sample management on-site. 16

17 Conc. Vs Sample Volume Tedlar Bags Pros: Easy to use with minimal training Fill with a peristaltic pump, syringe, or lung box ~$20/each and easy to have on hand for fast sampling Provides sufficient volume for repeat on-site analysis and confirmation at a fixed lab. Cons: Limited holding times = rush analytical costs at fixed lab Can pop/leak in shipment Not all are created equal! Sample Transfer 17

18 Sorbent Sampling Media used for Active Soil Gas Sampling Commonly used for TO-17 analysis of Naphthalene and Diesel Soil vapor is actively pulled through the sorbent tube at a known rate and volume. Sample volume is used to calculate a quantitative result from the analysis. H&P, Inc. Example showing a sorbent tube and summa being collected in-line together SVOC TO-17 Sampling Typical & Complex Simple! Beware of the Hardware 18

19 Let s Make it Complex Plan Accordingly: The Life of a Summa Certification: 48 hr Leak Check: Overnight Send to field: 1 to 3 days Clean: 24 to 72 hrs Analysis: 24 hrs to 2 weeks Sample and send back: Days to weeks Additional Active Soil Gas Sampling Considerations Vacuum Pumps Collect on upstream side. Watch vacuum applied Time-Integrated Samples? Existing data does not show large variations Certified Clean Canisters Not needed if DL > 1ppbv Residual Vacuum in Canisters Bad idea for soil gas samples 19

20 Additional (Optional) Equipment Used for VI Investigations Site Specific Weather Data Very Important for Ambient Air Samples Thermal Imagery Help Locate Entry Points and Very Useful for Crawl Space Evaluations Landfill Gas Meters Real-time CO2, O2 and Methane Soil Gas Sampling Strategies Overview Where to Collect Samples Exterior vs. Interior (sub-slab) How Often to Sample Hydrocarbons vs. Chlorinated Compounds Where to Sample Spatially Source Not Immediately Below Collect on side towards source Collect on other sides of structure Preferential pathways at edges & conduits Source Below Collect around structure or under structure Get decent coverage Representativeness Need enough points Average the data? 20

21 Location 3 Spatial Variability What to Do? Soil Gas Not Homogeneous Spatial & Vertical Variations Exist Don t Chase 1-Point Anomalies Get Enough Data Near/Around/Under On-site Analysis Enables Real-Time Decisions How Deep to Sample? Depth Below Surface 3 to 5 bgs generally considered stable For Cl-VOCs: Closer to Source (>10 bgs) Sub-slab For PVOCs: Away from Source, Closer to Receptor 3 to 5 foot Below Receptor Meteorological Effects on Soil Gas 400 wind direction time [d] Wind direction Time [d] No Correlations Pressure differential Time [d] Wind speed rainfall 1.8 barometric pressure Time [d] Barometric pressure rainfall [mm] 21

22 How Often to Sample? Seasonal Effects How Important? Most studies show less than 2x to 10x Extreme Conditions? Heavy rain Rising/falling groundwater Extreme heating/cooling Soil Gas Temporal Study EPA-ORD 2007 Probe A3 (TCE - Normalized) bgs Normalized Concentration bgs :50:09 10:50:26 7:50:44 4:51:04 1:51:43 22:53:46 5:43:29 2:43:47 23:44:04 20:44:22 17:44:39 14:44:57 11:45:15 8:45:32 16:02:01 13:02:18 10:02:36 7:02:55 4:03:12 1:43:56 22:44:15 7:06:41 4:06:59 1:07:19 22:07:58 19:11:25 P P P 3 bgs Time (3/16/07 to 4/10/07) <20% change over 25 days 22

23 Sub-slab Soil Gas Data SSP 2 PCE Sub-Slab Variations do NOT Occur Over Short Time Periods (days) day P /2012 1/2013 2/2013 3/2013 PVI Specific Sampling Issues Might Need to Sample <5 bgs If samples >5 bgs exceed allowable levels How to know? On-site analysis best If not, collect samples anyway Always Collect Oxygen Data Might Need Soil Phase Data to Exclude Out PVI Pathway Assessment Steps Step 1: Can Site be Screened Out Exclusion criteria applicable? Use Biovapor model for pre-existing data? Step 2: Can Screen-Out Data be Collected? Step 3: Do a PVI Assessment Makes PVI Assessments Much Simpler & Less Expensive 23

24 Clean Soil Model for HC Vapors Bio-barrier Reaction Zone O 2 Profiling - Approach 18 Locations Throughout Neighborhood Vertically Every Foot Down to 8-10 bgs Used direct-push (not PRT) Oxygen by portable meter (& CO2 & CH4) Soil Samples at 1 & 5 bgs (backup) Did All Locations in 11 Hours! Sub-Slab vs. Near-Slab Samples Are these better than these? 24

25 Sub-Slab vs. Near-Slab Samples EPA Prefers. Some States Require Very Intrusive; Legal Complications HCs: If O 2 Present, Near-slab OK Cl-HC: Deeper (>10 ) exterior data more reflective of sub-slab VI Assessment Point #5: Are Sub-Slab Data Useful? Large Spatial Variation What Value to Use? Poor Correlation with Indoor Air Recent ASU Study Suggests LOW Conc is Likely Entry Point SITE 1: Soil Vapor & Sub-slab Vapor PCE F (mcg/m3) C D Slide from Bill Wertz - NYDEC 25

26 Indoor Air & Sub-Slab Concentrations EPA OSWER Database 1000 Indoor Air Concentration (µg/m 3 ) Soil Gas Concentration (µg/m 3 ) Indoor Air vs Subslab Soil Gas Recent School site SubSlab SG vs IA Highest IA at Lowest SS! 80 Indoor Air ,000 40,000 60,000 80, , , , , ,000 SubSlab Soil Gas Which Direction to Sample? Outside-In Definitely for PVOCs Most common approach for all VOCs Many agencies prefer for all VOCs Sometimes not a good idea for Cl-VOCs Inside-Out Better in some situations for Cl-VOCs Less expensive if lots of structures 26

27 A Practitioner s Cheat Sheet: Which Investigatory Method To Use? Indoor Air Probably not for hydrocarbons PCE: Yes (CA excluded) TCE: Probably Groundwater Data Seldom - over-predicts risk, especially for HCs Soil Phase Data Yes for tank sites (Exclusion Criteria) A Practitioner s Cheat Sheet: Which Investigatory Method To Use? Exterior Soil Gas Data Yes for hydrocarbons Yes in States where modeling allowed No if OSWER guidance applies Sub-Slab Soil Gas Data Yes at dry cleaner sites Yes if you have IA detections Yes for defining areas to mitigate No if you are trying to predict IA Practical Strategies (Things to Do) Get Enough Data CL-HCs: Vertical Profiles Around Structure HCs: Oxygen Profiles Around Structure Use Radon for Slab-Specific Alpha Measure Ventilation Rate Have Competent Subs 27