Why Wait? Soil Vapor Sampling Methods and Equilibration Times

Transcription

1 Why Wait? Soil Vapor Sampling Methods and Equilibration Times Kevin Green, SCS Engineers AEHS, San Diego, March 22, 2017 Coauthors Louise Adams and Suzie Nawikas H&P Mobile Geochemistry Inc. Justin Rauzon SCS Engineers 2 1

2 Basics Installation of sample probes disturbs subsurface conditions, therefore, equilibration period is required to collect representative samples Advisory Active Soil Gas Investigations, by Cal EPA, DTSC, and LA and SFB RWQCBs (the Advisory ) Two hour soil gas equilibration period for direct push methods 3 Two Questions Does 2 hour equilibration period produce higher quality data than a shorter equilibration period? Which direct push method delivers the best quality data within a practical timeframe for fast paced active soil gas investigations? 4 2

3 Outline for Today Background / Guidance for Equilibration Experimental Setting Original Investigation Experimental Investigation Analytical Method Data Analysis and Discussion Summary 5 Background / Guidance Advisory Direct Push Methods: 2003 Advisory: minutes 2012 and 2015 Advisories: 2 hours Hand Auger and HSA: 2 days Air Rotary and Rotosonic: 2 weeks Or, Collect Time-series Data To Confirm Equilibrium 6 3

4 Other Equilibration Guidance San Diego County DEH Temp probes with sand pack: 8 hours Probe Rod and Post-run Tubing (PRT): minutes Arizona DEQ Probe rod: minutes 7 Other Equilibration Guidance Other Studies Sand Pack Wells: 3 hours for a 1.5 dia. probe PRT: 1 hour for 80% equilibration Sand Pack: 60% - 2 hrs, 70% - 4 hrs, 85% - 8 hours 8 4

5 Experimental Setting Former Dry Cleaner, Riverside County, CA Santa Ana RWQCB Regulatory Oversight Building Razed, Property Graded, 8 Feet of Soil Stockpiled on Property Probes installed at 13 and 23 feet below grade, corresponding to 5 and 15 feet below the original grade Clayey to sandy silts No groundwater impacts at ~90 ft bgs 9 Original Investigation A total of 37 probe locations installed in grid pattern at ~40-ft intervals Direct push temporary probes double nested with sand pack above and below the filter port/vapor well screens, and at least a 2-hour wait prior to purging and sampling In most cases, vapor probes not sampled until 24 hours (1,440 minutes) or later. 10 5

6 11 Original Invest. (Cont.) Primary Constituent: PCE in every sample at concentrations between 2.74 and 193 ug/l. Degradation components (i.e., TCE, etc.) not consistently detected 12 6

7 PCE at 13 ft bgs 13 Experimental Investigation Conducted at the completion of the original investigation Five locations selected with a range of original PCE concentrations: SV4 (111 ug/l) SV8 (76.1 ug/l) SV14 (12.4 ug/l) SV25 (63.5 ug/l) SV37 (3.09 ug/l) 14 7

8 Exp. Investigation (Cont.) Three additional vapor samples collected in a radial pattern, 2 feet from each original probe location Three sampling methods: PRT Post-run Tubing SWP Sand Pack, Wait, Purge (original) SPW Sand Pack, Purge, Wait Three wait times: 30, 60 and 120 minutes

9 PRT Ready for Sampling 17 Sand Pack Well Construction 18 9

10 Analytical Methods Samples collected and analyzed for VOCs using H&P Method 8260SV, the DoD ELAP certified equivalent of modified EPA Method 8260B for soil gas samples Certified On-site Laboratory Accredited SOP, including QC such as: Initial 5-point calibration, daily and continuing calibrations, lab control spike, internal standards, surrogates, method blank and field blank 19 Analytical Methods (Cont.) Shut in test for 60 seconds Liquid leak check at the surface Probe vacuum and flow rate monitoring during sampling Purge volume test (3 purge selected) Samples analyzed within 15 minutes of collection No light exposure Field logs and COC 20 10

11 Data PCE Concentrations Boring Identification Number SV4 SV8 SV14 SV25 SV37 Sample Method Wait Time in Minutes micrograms per liter (µg/l) SPW SWP PRT SPW = Sand, Purge, and Wait Method 2. SWP = Sand, Wait, and Purge Method 3. PRT = Post Run Tubing Method

12

13

14 27 Summary PRT method had higher PCE concentrations at 30 and 60 minutes than other methods, and in 4 out of 5 cases at 120 minutes. The PRT sampling method is least disruptive of equilibrium conditions and generally obtains the highest PCE concentrations with any given equilibrium time

15 Summary Sand, Purge and Wait (SPW): Generally had the lowest PCE concentrations, regardless of wait time. A research dead end. Regardless of sampling method, in 4 of 5 cases, PCE concentrations were lower with a 60 minute equilibration then with a 30 minute calibration. 29 Summary Because of inherent variability with soil vapor, the Advisory allows a wider variance, such as a 50% Relative Percent Difference (RPD), when comparing field duplicate/replicate samples

16 Summary Across all three sampling methods, the lowest PCE concentration differed from the highest PCE concentration by greater than 50% in 4 of 5 locations. Sampling method matters. However, within the PRT sampling method, variation was less than 50% across all three equilibration times. 31 Summary The only sampling method for which 24 hour data was available was SWP. The 24 hour data were higher than any of the 30, 60 or 120 minute data, indicating more than 120 minutes is needed to establish equilibrium for SWP. PRT with a 30 minute equilibration time provides similar results to the SWP method with 24 hour equilibration

17 Summary Under most circumstances, PRT sampling and a 30 minute equilibration time would be expected to be less time consuming and provide higher quality data than other methods that might be considered equivalent under current regulatory guidance. 33 Questions Answered Does 2 hour equilibration period produce higher quality data than a shorter equilibration period? Not Necessarily. Which direct push method delivers the best quality data within a practical timeframe for fast paced active soil gas investigations? Post-Run Tubing (PRT) 34 17

18 Final Observation While the shape of the PCE vapor plume shown on the map may shifted based on the method and equilibration times, any of the methods and equilibration times would have identified the general location of the VOC source and the extent of the vapor plume. 35 Questions? Kevin Green, 36 18