A Conceptual Understanding of Leakage During Soil-Gas Sampling

Transcription

1 A Conceptual Understanding of Leakage During Soil-Gas Sampling Dominic C. DiGiulio, Ph.D. U.S. Environmental Protection Agency Office of Research and Development National Risk Management Research Laboratory Ground Water and Ecosystems Restoration Division Ada, Oklahoma Presented at the: 17th Annual Association for Environmental Health and Sciences (AEHS) Meeting Workshop on Soil-Gas Sample Collection and Analysis San Diego, CA March 21 22, 2007

2 Leak Testing

3 Questions to Ponder Can leakage be reduced by reducing flow? Can leakage be reduced by reducing applied vacuum? Should concentrated solvents be used for leak testing? What level of leakage is acceptable?

4 Conceptual Model of Leakage During Sampling During soil-gas sampling, ambient air may directly enter a sampling vessel through loose fittings or through openings and cracks in and/or around concrete and bentonite seals. If an excessive amount of air is extracted, ambient air may also enter a sampling vessel indirectly through surrounding soil. Leakage is defined here as the direct movement of ambient air into a screened interval. Understanding direct and indirect mechanisms of entry is critical to devising tests and strategies to deal with these issues.

5 Heuristic Model of Leakage During Sampling Only vertical flow is allowed down a compromised borehole having an integrated gas permeability of k 1 (no flow boundary at surface). Only radial flow is allowed to a screened interval from surrounding soil having a gas permeability of k 2 (no flow boundaries above and below screen). Atmospheric pressure is established some radial distance r 2 from the screen. This configuration allows easy manipulation of equations outlined in Appendix A.

6 Mathematical Representation of Leakage 1 Leakage = 1+ C ( k ) 2 k 1 2bL C = r ln r r ( ) Leakage is expressed as a fraction between 0 and 1. Leakage is a function of the permeability contrast between the borehole and surrounding media and geometric factors.

7 Mathematical Representation of Leakage 1 Leakage = 1+ C ( k ) 2 k 1 As borehole permeability (k 1 ) approaches 0 (perfect seal), leakage goes to zero. As borehole permeability (k 1 ) approaches infinity (open borehole), leakage approaches 1. As formation permeability (k 2 ) approaches zero (no flow from formation), leakages approaches 1. As the ratio of formation to borehole permeability (k 2 /k 1 ) decreases, leakage increases (i.e, leakage is more likely in less permeable soil)

8 This Means That: Leakage does not increase with increased flow. Leakage does not increase with increased applied vacuum or pressure unless structural changes occur in the borehole. Leakage can only be reduced properly sealing the borehole.

9 Should Shaving Cream Be Used for Leak Testing? Propellents such as pentane, propane, and butane in shaving cream are unknown. Soil-gas may contain hydrocarbons present in soil-gas.

10 Should Concentrated Solvents Like Isopropanol (IPA) be Used for Leak Testing? Cross-contamination issues If not enclosed, unknown concentration around probe. Small leak (e.g., less than 0.1% could compromise analysis of vapors of concern. For instance, if IPA is present at only 10% of its vapor pressure above a probe, leakage of only 0.1% would result in a concentration of 14,300 ug/m3 in a sampling vessel.

11 Do we really need concentrated solvents? An acceptable level of leakage is subjective. The State of New York requires maximum leakage at 10%. Flooding a chamber with helium and use of a helium detector capable of reaching 100 ppmv can attain a sensitivity of 0.01% and provide results in real time ppmv 6 10 ppmv = = % We do not have to inject tracer the entire time of sampling. If tracer concentration is held constant in a chamber and a tracer is known not to be present in soil-gas, leakage can be quantified by: Leakage = C C measured chamber t tracer application t sample

12 Conclusions Leakage is a function of the permeability contrast between the formation and borehole. Decreasing flow and vacuum does not decrease leakage. If sampling system can be enclosed, a readily available gas such as helium can provide high sensitivity for evaluating leakage.

13 Blayne?

14 Liquid Method EPA SGWS In this method, a liquid is applied to a rag or paper towel and the towels emplaced at various points along the probe and sampling train. This method is qualitative. 1

15 Gas Method EPA SGWS In this method, a shroud is filled with a tracer gas and the gas concentration measured in the shroud and in the sample. Typical gases utilized are helium, carbon dioxide, propane, isobutylene, or sulfur hexafluoride. This method is quantitative. 2

16 Tent Shroud EPA SGWS If covering the entire sampling train is desired, a tent shroud can be used. The tracer, either a liquid or gas, is emplaced in the shroud. The concentration can be measured in the tent and in the sample with a handheld meter or on-site lab. 3

17 Tent Shroud EPA SGWS Working in the tent shroud. 4

18 Tent Shroud EPA SGWS Working in the tent shroud. 5

19 Liquid Method Pros Fast & easy Can cover multiple spots easy Very conservative (100 ug/l = 0.1% leak) Cons Typically qualitative Don t know results in real-time without lab Small leak can raise DLs of VOC analysis OK Method if Lab On-site EPA SGWS

20 Typical Liquids Isopropanol partitions into skin Shaving Cream (butane) - partitions into skin Freons readily available Pentane, hexane: not readily available EPA SGWS

21 Gas Method Pros Quantitative Real-time results with portable meters Cons More complicated and slower. Increases costs Harder to cover multiple locations, esp with DP Best Method if No Lab On-site EPA SGWS

22 Typical Gaseous Tracers Helium meter exp, low MW Carbon dioxide not in areas of high bio Isobutylene can use hand-held PID SF6 meter & gas exp Have to Ensure Gases Clean! EPA SGWS

23 Summary Both gaseous & 1iquid methods OK Choice should be up to collector If sample volume small & sample flows easy, leak compound likely not necessary EPA SGWS A summary of the key points in this presentation. 10