PRACTICAL ASPECTS OFCONDUCTING SOIL VAPOR INVESTIGATIONS in general accordance with the 2015 DTSC Advisory

Transcription

1 PRACTICAL ASPECTS OFCONDUCTING SOIL VAPOR INVESTIGATIONS in general accordance with the 2015 DTSC Advisory CA CUPA 2017, San Diego, February 2017 Suzie Nawikas, H&P Mobile Geochemistry, Inc Dave Balkenbush, H&P Mobile Geochemistry, Inc Blayne Hartman, Hartman Environmental Geosciences

2 HOW ARE THE SUGGESTIONS IN THE DTSC ADVISORY BEING IMPLEMENTED? Varying suggestions from different regulatory bodies (County, EPA, etc) Varying preferences and priorities from consultants and clients Varying interpretations and opinions from consultants and contractors based on personal experience Certain topics stand out as being the most alternatively interpreted parameters and/or inconsistently addressed items. The discussion that follows is from the perspective of a subcontractor that performs soil vapor surveys on a daily basis (18,000+ soil vapor samples/year) Implementation of the Advisory

3 CURRENT TOPICS OF INTEREST 1. Replicate/Duplicate samples and expected reproducibility 2. Equilibration times with relation to purge volumes HANDS-ON STATION 1 3. Tubing types for vapor probes, focusing on Teflon vs. Nylaflow 4. Considerations for sample volumes greater than 1-Liter 5. Leak check compounds, application, and evaluation HANDS-ON STATION 2 6. Drive point sampling 7. Hydration methods for the annular seal of a vapor probe 8. Sampling in low permeability conditions HANDS-ON STATION 3 9. Alternative methods for air evaluation 10.Continuous monitoring for low level TCE Implementation of the Advisory

4 TOPIC OF INTEREST #1 Reproducibility of Primary and Duplicate Samples The guidance provides an example of 50% RPD, but many work plans and expectations differ (i.e. 25%-100% RPD expected/allowed) Research presented by H&P at AWMA VI Conference on Soil Vapor Reproducibility With consistent and proper sampling and analytical techniques, RPDs should be expected to be within 25%-30% (or within analytical precision). When factors such as different instrumentation, different container types, and different analytical methods are removed, natural variation does not have a large impact on the differences between primary samples and duplicate/replicate samples. Reproducibility

5 TOPIC OF INTEREST #2 Equilibration Times and Purge Volumes The Advisory has clear definitions of equilibration times for various installation methods. However, there is confusion regarding when the sand pack is considered to be in equilibrium and no longer requires purging, due to conflicting language in various parts of the Advisory, and conflicting instructions from DTSC Staff on project specific occurrences. Study of Soil Vapor Sampling Methods and Equilibration Times Which is the Best Approach? Kevin Green & Justin Rauzon of SCS Engineers, in conjunction with Suzie Nawikas & Louise Adams of H&P, Inc. Aimed to examine if current recommendations meet the most representative and practical assessment goals, and to clear up confusion surrounding drive point sampling. To be presented at AEHS, San Diego, March Currently in review for journal submittal. Equilibration

6 HANDS-ON STATION 1 3. Tubing types for vapor probes, focusing on Teflon vs. Nylaflow 4. Sample sizes for soil vapor and subslab vapor samples 5. Leak check compounds, application, and evaluation HANDS-ON STATION 2 6. Drive point sampling 7. Hydration methods for the annular seal of a vapor probe 8. Sampling in low permeability conditions HANDS-ON STATION 3 9. Alternative methods for air evaluation 10. Continuous monitoring for low level TCE Implementation of the Advisory

7 HANDS-ON STATION 1 Tubing types for vapor probes (Teflon vs. Nylaflow) Sample sizes for soil vapor and subslab vapor samples Leak check compounds, application, and evaluation Presented by Suzie Nawikas, H&P Suzie.Nawikas@handpmg.com Hands-On Station 1

8 TOPIC OF INTEREST #3 Teflon or Nylaflow Tubing? The two most commonly requested tubing types are Nylaflow (1/8 ) and Teflon (1/4 ). The 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

9 TOPIC OF INTEREST #4 Considerations for Sample Volumes >1 Liter 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. Sample Container Size

10 TOPIC OF INTEREST #5 Evaluation of Leak Check Compounds Extremely different tolerances for the two different methods of performing leak checks Gaseous Tolerance is 5% Liquid Leak Tolerance is below 1% - Multiple studies and field tests show that the surface concentrations of liquid tracers are 1,000,000+ ug/m3. With RLs typically ranging from ug/m3, estimated thresholds of 10x the RL are 0.005% - 0.1% Sliding scale approach toward the liquid leak check threshold leads to more leniency on sites which have higher reporting limit goals, as well on samples that require dilutions. Leakage (i.e. communication) is common with subslab samples, leading practitioners to use the shroud application method to obtain higher thresholds. Not all projects include a budget to analyze the summa canister for the gaseous compound. Leak Check Evaluation

11 HANDS-ON STATION 2 Drive point sampling Hydration methods for the annular seal of a vapor probe Sampling in low permeability conditions Presented by Dave Balkenbush, H&P Dave.Balkenbush@handpmg.com Hands-On Station 2

12 TOPIC OF INTEREST #6 The Use of Drive Point Sampling Methods The Advisory leads many consultants to infer that Drive Point Sampling is not preferred by DTSC, therefore it is rarely considered as an alternative to building soil vapor wells in a boring. Drive Point Sampling is effectively and commonly used in other parts of CA and the U.S., and is useful at sites where installing probes and/or waiting for equilibration may be logistically difficult. The equilibration period for Drive Point Sampling It is not defined in the Advisory. Some work plans defer to 2 hours, while others reference studies showing minute equilibration times. SCS/H&P Equilibration Study (to be presented AEHS in March) USEPA Study conducted at VAFB, as referenced in the San Diego County s 2011 SAM Manual Photo courtesy of AMS Samplers Drive Point Sampling

13 TOPIC OF INTEREST #7 Hydration of Bentonite to Create an Annular Seal The Advisory suggests to hydrate the annular seal material at the surface then pouring into the boring, rather than hydrating in lifts. Practical experience with Leak Testing and Vacuum Testing has shown that hydration in lifts creates a sufficient seal between probe depths and to the surface. Hydration of bentonite and/or neat cement bentonite mixtures at the surface ALSO creates a sufficient seal, but adds significant time and cost to a project. Consultants are faced with a range of budgetary options, yielding similar outcomes, which often leads them to choose the more cost effective option (hydrating in lifts). Annular Seal Hydration

14 TOPIC OF INTEREST #8 Sampling in Low Permeability Conditions per Appendix D Alternative Sampling Method does not provide an upper limit on the time allowed for the vacuum to dissipate (i.e. pressure rebound cycle). This leads to uncertainty in the field regarding if/how to proceed. Reinstallation Method suggests that purging just the internal volume of the tubing is acceptable, if the probes have been in a state of equilibration for 2 weeks or more. This is often extrapolated in the field to purge a minimum of one volume of tubing from the existing probe as is (or more, if vacuum allows), then proceed with sample collection. Low Permeability

15 HANDS-ON STATION 3 Methods for air evaluation Continuous monitoring for low level TCE (See Presentation by Hartman for slides) Presented by Blayne Hartman blayne@hartmaneg.com Hands-On Station 3