A New Passive Diffusion Sampler (PDS) for Soil Gas Sampling

Transcription

1 A New Passive Diffusion Sampler (PDS) for Soil Gas Sampling Cynthia J. Paul, John Wilson, Ken Jewell U.S. Environmental Protection Agency Office of Research and Development National Risk Management Research Laboratory Ground Water and Ecosystems Restoration Division Ada, Oklahoma Ryan Sharma U. of WI 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 Conventional Practice: Measures concentration of BTEX in ground water beneath the building using a conventional well screened across the water table. Assumes concentration of contaminants in the capillary fringe is the average concentration of contaminants produced from the monitoring well. Uses screening models (i.e. Johnson Ettinger) to estimate the affect of biodegradation on removal of hydrocarbon vapors.

3 Considerations If oxygen is available at the capillary fringe, biodegradation may remove the fuel vapors before they have a chance to diffuse upward into buildings Our approach replaces the estimates of a mathematical model with monitoring, to see if there are vapors in the soil gas immediately above the capillary fringe that can diffuse into buildings

4 Laboratory Study

5 40 ml VOA vial Passive Diffusion Sampler (PDS) Teflon septa replaced with 25 mm membrane filter (0.2 µm) 0.4 gm Na 3 PO 4 as preservative Filled w/deionized water

6 Equilibration Jar Diffusion membrane o-ring Open vial with sim. gasoline PDS Experimental set-up Vials filled w/deionized water, capped and placed in jar. Vial containing simulated gasoline in excess was also placed in jar. Jars were stored at 4º C.

7 Experimental Variables Simulated gasolene Concentration Equilibration Time Benzene, 0.4% 1 day PDS - Capped o-xylene and MTBE in hexane 0.004% % 2 days 1 week 2 weeks 4 weeks w/ membrane (0.2 µm pore size) Control - Uncapped

8 0.4% MTBE Concentration in control (ug/l) day 2 day 1 wk 2 wk 4 wk Concentration in PDS (ug/l)

9 0.004% MTBE 1000 Concentration in control (ug/l) Concentration in PDS (ug/l) 1 day 2 day 1 wk 2 wk 4 wk

10 0.4% Benzene Concentration in control (ug/l) day 2 day 1 wk 2 wk 4 wk Concentration in PDS (ug/l)

11 0.004% Benzene 100 Concentration in control (ug/l) Concentration in PDS (ug/l) 1 day 2 day 1 wk 2 wk 4 wk

12 0.4% o-xylene 1000 Concentration in control (ug/l) day 2 day 1 wk 2 wk 4 wk Concentration in PDS (ug/l)

13 Results 4 weeks is sufficient for benzene and MTBE, but not o-xylene Repeat lab study with longer equilibration time

14 Field Study Conducted at A1 Moving site, Madison, WI Series of depth discrete monitoring wells at two locations (EPA-2 and EPA-4) Sampled with PDS (soil gas and ground water) and peristaltic pump (ground water) monthly Conventional monitoring wells with 10-foot screens)

15 A1 Moving Site

16 PDS and Messenger for 2-inch well Messenger PDS

17 Monitoring well cluster installed at discrete depths from 4 to 13.5 ft with 2-in screens Messenger holds PDS in place while deployed within each well

18 A1 Moving Site: Madison, WI PL0713 MW-11 HPT-2 PL0708 PL0783 MW-1 HPT-9* PL0705 PL0704* HPT-1 PL0786 PL0703 MW-10 PL0781* EPA PASSIVE DIFFUSION 4 PL0701 EPA-5 SAMPLER NEST HPT-3 PL0706* APPROXIMATE LOCATION OF FENCE PL0709 HPT-4 MW-2 MW-8 MW-6P HPT-6 EPA-2 PL0710* HPT-8 PL0785* MW-3 PASSIVE DIFFUSION SAMPLER NEST EPA 2 MW-6 EPA-1 MW-4 HPT-5* HPT-7 PL0711 PL0790 MW-7 PL0782 & PL0784 PL0712 MW-5 MW-9 GPS data collected 5/18/06 and 5/19/06 Feet ± Meters * = APPROXIMATE WELL LOCATION Ryan Sharma

19 Concentration TPHg (mg/kg) Ground Surface Depth (ft) Clay Sand with Clay Plastic Clay Ground Water Flow Friable Clay Reduced Sand EPA 2 TPHg May 2006 Oxidized Sand EPA-1 EPA-2 Water Table 7.09'

20 Ground Surface Depth (ft) Concentration (ug/l) Grey Plastic Clay Grey Clay Water Table 6.68' Grey Sand/Clay Oxidized Sand Reduced Sand Oxidized Sand MW5-78 ug/l GW PDS Ground water Flow EPA 4 Benzene - June 2006

21 Ground Surface Depth (ft) Concentration (ug/l) Clay Plastic Clay Friable Clay Water Table 7.40' Sand with Clay Reduced Sand Oxidized Sand GW PDS MW ug/l Ground Water Flow EPA 2 Benzene June 2006

22 Advantages of PDS Allows more discrete samples from smaller volume of subsurface material Detect sharp vertical gradients of BTEX compounds in soil gas and ground water Averages concentrations over several days - less susceptible to temporal variation No purge water produced

23 Does not extract fluids - minimal affect on concentration gradients during sampling - less question of where the sample really came from. The PDS is indifferent to the position of the water table provides a sample from either ground water or soil gas. Low cost to analyze PDS samples compared to other methods.

24 Limitations of PDS Requires installation of several discrete wells to sample along a vertical concentration gradient. Sampler may not come to equilibrium with short term exposures - they must be emplaced during one site visit and recovered on a second visit. Samples can t be duplicated. Water analysis by GC not low enough detection limits for vapor intrusion assessment

25 Future Studies Sparge sampling points with Argon Conduct gas permeability studies Conduct active gas sampling Conduct similar study at different site

26 Although the research described in this presentation has been funded wholly or in part by the U.S. Environmental Protection Agency, it has not been subjected to the Agency s peer and administrative review and therefore may not necessarily reflect the views of the Agency and no official endorsement may be inferred.