Soil Gas Sampling Methods

Transcription

1 Soil Gas Sampling Methods Soil Gas Methods Sampling Details Sampling Strategies Chlorinated-HCs vs. Hydrocarbons This part of the training will focus specifically on soil gas sampling since it is a commonly used approach around the country and, if you have a vapor intrusion problem, the chances are high you will be required to oversee or work with soil gas data.

2 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 Sampling protocols vary Commonly Used in VI Assessments Measurement of soil gas is a common investigatory approach around the country. Actual soil gas data are reflective of subsurface properties, are less expensive than indoor air measurements, and allow real-time results. The screening levels are also higher so there is less chance to be chasing blanks. There are some drawbacks, including the lack of knowledge of the transfer rate, very restrictive default screening levels, and debate over how & where to collect samples.

3 Which Soil Gas Method? Passive (limited use) Flux Chambers (limited use) Active Active Method Most Often Employed for VI There are three types of soil gas methods. Active refers to actively withdrawing vapor out of the ground. It gives quantitative values. Passive refers to burying an adsorbent in the ground and letting the vapors passively contact and adsorb onto the collector. It does not give quantitative data and hence can not be used for risk applications, except for screening. Surface flux chambers were discussed previously. The active method is the one most applicable to VI risk assessments.

4 Pros: Passive Soil Gas 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 methods consist of the burial of an adsorbant into the ground for a period of time (typically 5 to 10 days) and the subsequent retrieval of the adsorbant for measurement. The contaminants passively diffuse and adsorb onto the collector over time. The method is easy to deploy and is proven to find contamination zones. However, the method does not yield concentration values and thus can t be used for risk-based applications. Ongoing efforts to calibrate the method to give concentration data are inconclusive. As a result, this method is considered by most regulatory agencies, including EPA, to be a screening method. It can be used to locate contamination zones on larger parcels where the contamination is not already defined.

5 Passive Soil Gas Sampling Media Hydrophobic Adsorbant provided by Beacon Environmental Adsorbent inside vial with membrane Adsorbent inside vapor permeable, waterproof membrane Examples of passive collectors. 5

6 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 Chamber Surface flux chambers are attractive because they give a direct measurement of the flux into the structure or out of the soil. This eliminates the need to know the effective diffusivity and the uncertainty inherent in the models. The biggest drawback with chambers is whether they can be placed in the proper locations in an existing structure. Also, few regulators, consultants, or vendors have used them, so they are unfamiliar of the protocols to use and how to interpret the data. As a result, regulatory acceptance is limited. In slab-on-grade structures or undeveloped lots or crawl spaces, surface flux chambers are a viable method to use. Chambers can also prove useful to support the presence of bioattenuation. 6

7 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 A static application of the flux chamber allows for the collection of the actual air rising up from the subsurface. The static flux chamber can be used to collect continuous readings (such as with a Landtec at a landfill), or it can be used to collect a sample over a given amount of time, as shown by the photo on the right. In this example, the collector would take an initial sample immediately upon setting the chamber (to obtain a background sample), then collect a final sample after 8 hours* to determine the VOC flux into the chamber over that time period. *8 hours is just an example; 10 hours, 24 hours, etc.

8 Active Soil Gas Sampling Overview 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 Active soil gas consists of inserting a probe or tube into the ground and actively withdrawing a volume of the soil gas. A number of issues need to be considered when conducting active soil gas investigations which will be covered in the following slides. The Appendix in the 2007 ITRC Guidance Document includes a convenient check list to aid users who are collecting or overseeing soil gas data. 8

9 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) When collecting active soil gas samples, you are most likely going to use either a direct push drilling method or an auger method. Direct push includes truck mounted hammers, limited access hammers, or manually driven rods. Auger includes hollow stem or hand auger methods. Other methods of drilling, such as sonic, air rotary, or mud rotary, are typically not employed for soil gas sampling due to the interference with the subsurface and unknown equilibration times.

10 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) There are two ways to obtain a reliable soil vapor data from the subsurface. The first is by pushing a direct push rod into the ground and collecting the sample through the rod. There are options for connecting the tubing prior to drilling, which is often referred to as Drive Point Sampling. The other option is to connect feed the tubing down the rod after reaching the target depth, referred to as Post Run Tubing (PRT) Sampling. The benefit for sampling through the direct push rod is that no foreign material is introduced, which means that the equilibration times are typically a lot shorter, on the order of minutes. Also, with the drive point method in particular, very little space is needed for drilling, as the rod is manually driven into the ground (perfect for small offices, back yards, etc). These methods are advantageous for those sites requiring only one round of sampling. The downfall is that this technique is also not effective in consolidated lithologies, both due to risks of refusal and the inability to pull a sample from such a small inlet if there is little to no porosity (i.e. clays). Creating a seal at the base of the rod is recommended to avoid leaks. With the PRT method in particular, the sampler cannot check the connection of the tubing at the fitting until the rod is removed from the ground (after the sample has been

11 collected).

12 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 The second way to obtain reliable soil vapor data from the subsurface is to create an open boring with either direct push or auger methods, then build a vapor well within the boring, using sand and bentonite buried in the ground. The vapor wells consist of small diameter, inert tubing and offer advantages when vertical profiles are desired or when repeated sampling events are likely. Multiple tubes can be nested in the same borehole.

13 Soil Vapor Probe Construction Single Depth Nested Probes Typical construction diagrams with example sand and bentonite thicknesses A schematic of a multi-depth nested vapor well. In the following slides, we will go over the materials themselves. 12

14 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 Some of the issues that need to be considered when installing probes include: Tubing Type: Small diameter tubing offers advantages over large PVC pipe. Flexible tubing tends to leak. Probe tip: Metal tips may have blanks due to the cutting process. Materials used to seal the probe.

15 Soil Vapor Probe Construction H&P, Inc. Tubing Type: Small diameter tubing offers advantages over large PVC pipe. Flexible tubing tends to leak. Probe tip: Metal tips may have blanks due to the cutting process.

16 Tubing Test Tubing TCE #1 TCE #2 Average (ug/m3) (ug/m3) SS PEEK Nylaflow Teflon Polyethylene* Cu nd 170? * Typical Geoprobe Tubing A test of the influence of tubing type was performed in February 2008 at the Lemoore NAS research site. Six different tubings were nested together in the same borehole. Soil gas concentrations were measured on 2 occasions two days apart. The sampling order was reversed from day 1 to day 2 to eliminate any purge volume effects. 15

17 Soil Vapor Probe Construction 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 H&P, Inc. The sand pack zone is the actual filter zone for the vapor probe (the filter at the bottom of the tubing is intended only to keep debris from entering the tubing, so you want to be sure that your sand pack is in the zone in which you want to sample. Sand is usually 6 12 thick. Above the sand, a small layer of dry bentonite is placed, mainly to protect the sand pack from having any water enter it during the hydration of the seal above. The annular seal of the vapor probes is constructed with a hydrated bentonite, or a bentonite/cement mixture. Many direct push drillers will hydrate the bentonite in lifts (a few inches of bentonite + a few inches of water, repeat). Some auger drillers will hydrate the bentonite at the surface, and pour as a slurry down the boring.

18 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 Sub-slab soil gas samples are soil gas samples that are collected immediately below a structure (slab, basement, etc.). The sample points can be temporary or permanent, and are constructed in much the same way as soil vapor wells (just in a smaller scale). Permanent points are convenient for repeat sampling, but the sample point should be flush mounted and sealable to minimize potential for damage, prevent vapor infiltration, maintain cosmetic appearance and room functionality in family homes. Temporary points need to be sealed effectively to prevent infiltration of vapor, water, etc. 17

19 Some Lessons Learned Watch what you use to seal holes Loaded with TCE Loaded with TBA Examples of sealants used to seal probes that contain VOCs

20 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? Depending on your data quality objectives, equipment blanks are strongly recommended.

21 This is a chromatogram of the air from inside polyethylene tubing off of a direct-push rig. 20

22 Active Soil Vapor Sampling Considerations - Overview 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. Now that we have covered installation methods and materials, we will now move on to the sampling considerations for active soil gas collection.

23 Equilibration Time Vapor Wells ~60% after 2 hrs, 70% after 4 hrs, 85% after 8 hrs A study done for EPA-ORD on the time required for soil gas concentrations to stabilize after probe installation. In the case of burying tubing in the ground, 4 to 8 hours were required to reach 70% to 85% of the final concentration. 22

24 Equilibration Time Through Rod ~80% immediately, 100% after 1 hr A study done for EPA-ORD on the time required for soil gas concentrations to stabilize after probe installation. In the case of sampling through a probe rod inserted into the ground, concentrations were within 80% of final concentrations immediately after probe rod insertion and at 100% within an hour. 23

25 Equilibration Time Varying regulations across the country (this table shows the varying equilibration times in CA, for example) The suggested equilibration times for active soil vapor probes are variable across the country, or even across a state. This table is just an example of the different equilibration times that may apply to projects in California. The point of this slide is just to remind you to consider your equilibration period, and make sure that the consultant and regulator are on the same page with regards to what is appropriate. 24

26 Purge either with a syringe (smaller volumes) or a pump (larger volumes) Purging H&P, Inc. H&P, Inc. A simple and fast procedure to purge soil vapor probes is to use a disposable gas-tight syringe. Volumes can be carefully measured, the collector can get a feel for the permeability of the sampling zone from the syringe resistance, low vacuums and flow are applied, and no bulky hardware or power is required.

27 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) Potentially the volume of Dry Bentonite Pore Space as well Typically, a total of 3 Purge Volumes (3PV) are removed prior to sampling

28 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 Recommended to maintain a vacuum of <100 water (7-8 Hg) 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. During this slide, the instructors will show an example of what high vacuum looks like and feels like when purging.

29 Leak Check Methods H&P, Inc. Liquid Tracer Gaseous Tracer, Full Shroud Gaseous Tracer, Small Shroud Leak testing methods using liquid tracers and gaseous tracer compounds. 28

30 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 Leak testing methods using liquid tracers and gaseous tracer compounds. 29

31 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. Video Placeholder H&P, Inc. In addition to creating an adequate seal for the soil vapor probe, additional steps can be taken at the surface prior to sampling to ensure that leaks do not occur from poor connections in your sample train. A shut-in test is not a necessary part of sampling, but it protects the sampler from having an obvious leak and can be performed PRIOR to sampling. 30

32 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. In addition to creating an adequate seal for the soil vapor probe, additional steps can be taken at the surface prior to sampling to ensure that leaks do not occur from surface seals. 31

33 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. Even with solid connections and a robust surface seal, communication of ambient air into the subsurface can still occur through cracks in the slab, preferential pathways, etc. Having a leak and not knowing about it can lead to gross misinterpretation of the data. 32

34 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 Cons H&P, Inc. 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 If using a liquid leak check, 1,1-Difluoroethane is strongly recommended. IPA can cause reporting problems if detected, and also cannot be reported with as low of a reporting limit as 1,1-DFA. Butane is used by some, but NOT RECOMMENDED because the source (shaving cream), contains high levels of BTEX and other petroleum related compounds.

35 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) If no mobile lab is on-site, gaseous tracers are the safest approach to use since you can get real-time results with portable meters. Lab grade helium can be difficult to obtain. Party helium is an OKAY alternative, but some sources of party helium do contain other natural gas impurities (i.e. benzene, etc).

36 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 What is your acceptance threshold for evaluating a hit of the leak check compound? The answer to this will depend on your local regulations, as well as your data quality objectives. Although liquid tracers such as 1,1-DFA, are qualitative, studies conducted by H&P show that the surface concentration of 1,1-DFA can be expected to be in the realm of 1,000 ug/l. This is when the compound is applied correctly, however. The compound does dissipate quickly and needs to be reapplied every 10 minutes or so during purging/sampling. Gaseous tracer application includes a measurement of the shroud, so the surface concentration is known. Using this information, the percentage of leak can be calculated. Some DQOs call for just the probe to be tested, while others also want to test the sample to ensure that a leak did not occur. It s always more conservative to test the summa, but this is especially recommended with increased sample volumes (6L, for example), since the risk of leakage increases with sample volume.

37 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 Picture of the post-run tubing (PRT) connector used by most direct-push rigs. Fitting can leak if threads are not sealed properly. There is no way to tell for sure since the operator can not see the connection.

38 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 Lower detection levels requires more careful protocols. Important sampling considerations include sample volume, container type, flow rate, and leak testing to ensure valid samples are collected. Smaller volumes require less complicated sampling systems and minimize the chances for leakage from the surface and desorption off soil. Recent studies have shown no difference in soil gas values regardless of whether small (0.5 L) or large (100 L) volumes are collected. Sample containers must be inert, clean, and handled properly (no cooling or heat). Canisters have longer holding times, but have the potential for blanks (carry-over from previous samples), cost more, and can be trickier to fill. Tedlar bags are good for ~2 days, are less expensive, and suitable for concentrations of 1 ppbv or higher. Sample flow rate is of concern to many agencies, but recent data are showing it not to be a factor. Tracer/leak compounds are generally required to ensure sample integrity because small leaks can create significant effects at such low concentrations. The larger the volume extracted and the more complicated the sampling system, the greater the potential for leaks.

39 Summa Canister Volumes A 6-liter Summa can is about the size of a basketball. A 400 cc mini-can is about the size of a baseball. Lower volumes give more control on sample location, require less time to collect, and minimize chances of breakthrough from the surface or other sampling zones in nested wells. For soil gas samples, most labs only require 100 cc of sample, so small canisters (<1 liter) are sufficient volume.

40 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 Although the Advisory recommends sample volumes of 1 Liter or less in multiple sections, some projects still utilize 6 Liter canisters (often to obtain required reporting limits). IF sampling with a 6L canister is performed, additional checks should apply The Shut-In Test (i.e. equipment vacuum test) of 60 seconds may not suffice for a canister that is now going to take 30 minutes to fill. To maintain the concentration of the tracer gas at the surface, either check the helium levels frequently, or reapply the liquid multiple times (i.e. every 10 min) Particularly for subslab, the zone of influence may include additional points of entry for ambient air not addressed by the leak check compound.

41 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. Proper storage and management of summa canisters is critical for a successful project 40

42 Conc. Vs Sample Volume This slide shows soil gas results for different collection volumes ranging from 500 cc to 100 liters collected by EPA-ORD at a test site. There is no significant variation in concentration. Slide courtesy of Dr. Dominic DiGuilio, EPA-ORD

43 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! The use of tedlar bags for soil gas samples has many advantages and is allowed by many agencies. Be Sure Lab Has Tested Bag for Stability of COC

44 Sample Transfer Soil vapor samples can be easily transferred from a tedlar bag into an evacuated canister in the field for longer holding times or for more security during sample shipping.

45 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 44

46 SVOC TO-17 Sampling Typical & Complex Simple! Top photo: A typical sampling arrangement used for collection of samples on adsorbents. Note the abundance of fittings and the need for duplicate cartridges for breakthrough. A very complicated set-up, prone to leaks. Bottom photo: A much simpler sampling arrangement for adsorbent tubes with better control on actual vapor volumes passed through the adsorbent. 45

47 3/31/14 Plan Accordingly: The Life of a Canister 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 Typical cycle that a canister goes through from cleaning to sample collection to sample analysis. Slide courtesy of Taryn McKnight, TestAmerica Ambient Air - Standardized Term and Conditions 46

48 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 A few additional sampling considerations.

49 Soil Gas Sampling Strategies Overview Where to Collect Samples Exterior vs. Interior (sub-slab) How Often to Sample Hydrocarbons vs. Chlorinated Compounds In this part of the training, we will discuss soil gas sampling strategies.

50 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 Soil gas, like soil, is not homogenous in most cases. So you need enough data to give decent coverage near, around, or under the receptor. Simpler collection systems with small volumes are advantageous as there is less to go wrong and enable higher production per day (20+ samples per day). Less expensive analytical methods (8021, 8260, TO-14 short list) enable more analyses for reasonable cost. Real-time data can be extremely helpful to track soil gas contamination laterally and vertically. 49

51 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? There is currently much debate on where to collect samples and no existing protocols or guidance. So, common sense comes into play. If the source is not directly below, collect samples between the structure and the source at a depth that is deep enough to give repeatable results. Collect in any known preferential pathways, such as utility lines. If the source is below, it may be okay to collect around the structure before going inside the structure. Spatial averaging allows a better representation of what s below the structure. One approach is to collect samples on all sides of the house and use an averaging method to get a value under the structure footprint. If real-time data exist, add additional points depending upon the results. If not, collect extra samples spatially and analyze if necessary.

52 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 The closer to the surface, the more the potential temporal variation. Depths of 3 to 5 below the surface are generally considered deep enough to get repeatable data and resampling is not required by most agencies. For chlorinated VOCs, generally deeper data are preferred. For petroleum VOCs, shallower samples are better due to shallow bioattenuation. 51

53 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 The closer to the surface, the more the potential temporal variation. Depths of 3 to 5 below the surface are generally considered deep enough to get repeatable data and resampling is not required by most agencies. Historical radon data around houses show variations less than a factor of 7 in cold climates. Recent VOC data from Endicott show soil gas variations less than a factor of 4 over 15 months. Larger variations may be likely in areas of extreme temperature variation (northern climates), during heavy periods of precipitation, and when the structure s heating or ventilation systems are operative. In general, if the soil gas concentrations are below allowed levels by a factor of 10, there should be no need to repeat sampling.

54 Meteorological Effects on Soil Gas wind direction Location time [d] -20 Time [d] Wind direction No Correlations Pressure differential Time [d] Wind speed rainfall 1.8 barometric pressure Time [d] Barometric pressure rainfall [mm] At last 5 studies over the past 8 years have shown little correlation between soil gas data & common meterological parameters.

55 Soil Gas Temporal Study EPA-ORD 2007 Probe A3 (TCE - Normalized) bgs 1.4 Normalized Concentration bgs P P P :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 3 bgs Time (3/16/07 to 4/10/07) <20% change over 25 days This is a plot of data recently collected for an EPA funded study by an automated instrument at at Vandenberg AFB site from three probes at the same location but at different depth (3, 8, & 17 bgs). This plot consists of over 500 points per probe collected once per hour over a 4 week period from mid March to mid April The soil gas concentrations varied by less than 10% over these four days even for probes only 3 feet below the surface.

56 Sub-slab Soil Gas Data SSP 2 PCE Sub-Slab Variations do NOT Occur Over Short Time Periods (days) day /2012 1/2013 2/2013 3/2013 P Sub-slab soil gas data collected at a sub-slab location by a continuous analyzer shows little variation in values over 3 months from December 2012 to March The red bands show about 1-day of time. Observed variations are small and occur over long time frames, weeks to months, not over short time frames (days). This means that grab samples are suitable for sub-slab samples. There is no advantage to 8-hr or 24-hr time period sub-slab samples. 55

57 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 There are some differences in soil gas sampling for petroleum hydrocarbon VOCs than for chlorinated solvents. If samples at deeper depths exceed allowable values, shallower samples may need to be collected to document the effect of bioattenuation. Oxygen data should always be collected to document presence of the aerobic zone. Soil phase data may be needed to document the presence of a clean soil layer. 56

58 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 So called exclusion criteria, meaning criteria to eliminate sites from further vapor intrusion assessment based upon source concentration and distance between the source & receptor have fundamentally changed the process for investigating the VI pathway at petroleum sites. 57

59 Clean Soil Model for HC Vapors Bio-barrier Reaction Zone Conceptual model for clean soils shows that above the hydrocarbon reaction zone there is no more hydrocarbon vapor migration if there is oxygen present. The key to identifying if sites are clean or dirty is to determine if there is sufficient oxygen present in the vadose zone over an interval of several feet. 58

60 Sub-Slab vs. Near-Slab Samples Are these better than these? Are sub-slab samples better to collect? This issue is highly contested around the country and has huge ramifications. Sub-slab samples are much more intrusive, require access agreements, and often attorneys get involved, especially for private residences.

61 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 The draft EPA OSWER VI guidance advocates sub-slab samples and some State agencies agree. MDEQ does not require sub-slab sampling. Some are fearful that the contaminants build-up under the slab ( ponding effect ). But, sub-slab sampling is intrusive and often leads to legal complications. By Fick s law, the sub-slab concentration can be no higher than the source concentration, so if the source is below, collection of exterior soil gas samples at the source depth or at some depth closer to the source will give useful data and not create as many legal headaches. For hydrocarbons, if high oxygen levels exist all around the slab at a shallow depth, and the slab is small, it is likely that bioattenuation under the slab is occurring and shallow soil gas will be representative of sub-slab soil gas.

62 VI Assessment Issue 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 Problems with using & interpreting sub-slab data include the large spatial variation under the slab (10x to 100x), whether to use the highest value or lowest value and the poor correlation between sub-slab concentrations and indoor air concentrations.

63 Indoor Air & Sub-Slab Concentrations EPA OSWER Database 1000 Indoor Air Concentration (µg/m 3 ) Soil Gas Concentration (µg/m 3 ) This is the plot of co-located indoor air and subslab data from the EPA- OSWER database. The correlation between indoor air and subslab soil gas is very poor. This means that sub-slab soil gas concentrations are poor predictors of indoor air concentrations.

64 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 A plot of sub-slab soil gas concentrations versus indoor air concentrations at a school shows a poor correlation. This indicates that sub-slab soil gas is not a good indicator of higher indoor air levels.

65 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 The most common way to do VI assessments is to start with exterior data (groundwater or soil gas) to identify structures, then do sub-slab or near-slab sampling, then do indoor air sampling. This is definitely the best approach for petroleum VOCs, but maybe not so for chlorinated VOCs, especially considering that sub-slab data are a poor predictor of indoor air concentrations. 64

66 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) Some general recommendations on what investigatory method to use. 65

67 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 Some general recommendations on what investigatory method to use. 66