Soil Gas Sampling: A

Practical Perspective

Theo Johnson

Geological Services Unit

Department of Toxic Substances Control

tjohnson@dtsc.ca.gov

September 25, 2012

DOCUMENT OVERVIEW

• Combines 2003 Advisory and 1997 LARWQCB 1997 Interim Guidance for Active Soil Gas Investigations into one comprehensive document

• Incorporates over 450 comments from consultants

WHY SOIL GAS?

• Measure the extent of vapor phase

contaminants

• Use in combination with conventional

sampling to delineate VOC contamination

• Soil matrix type (gravel, sand, silt, clay)

• Reduces volatilization and biodegradation

potential

• Real time decisions

• Time and cost

2012 Advisory

• Expanded workplan section

– Addition of Site Conceptual Model (CSM) • Purpose of CSM

• Basic components in a CSM

– Elements of the Workplan

• Report section

– Assess and revise CSM

– Identify data gaps based on revised CSM

2012 Advisory

• Provides investigation flexibility using Data

Quality Objectives (DQOs) instead of

prescriptive language.

– Sample spacing

– Sample depths

– Detection limits

– Analytical methods

2012 Advisory

• Initial sampling at 2 depths based on

DQOs, CSM and LITHOLOGY!

– 5 feet (ft) below ground surface (bgs)

– 10 to 15 ft bgs

• Link to Vapor Intrusion (VI) investigations

– Vapor sample depths for VI investigations

• Vertical profiling section

– Based on DQOs

2012 Advisory

• Sample tubing

– Appropriate tubing for chemicals sampled

• No longer recommend copper or low density

polyethylene tubing

• Batch (blank) sampling for tip and tubing

recommended

– Appendix B lists chemicals and appropriate

tubing composition for vapor sampling and

probe design.

2012 Advisory

• Drilling Considerations

– Post-run tubing method may be used

depending on DQOs

– Equilibration times (i.e., sampling delay

following probe installation)

• Direct Push – 2 hours

• Hollow stem auger or hand auger – 48 hours

• Rotosonic or air rotary methods – weeks

Probe Construction

• Sand Pack

– Place probe in middle of sand pack

– Use treme pipe on wells ≥ 15 ft deep

• Dry Bentonite

– Place on top of Sand Pack

– Prevents moisture from infiltrating into Sand

Pack

• Hydrated Bentonite

– Hydrate at the surface

Probe Construction

2012 Advisory

• Sub-slab sampling

– Sampled at the same rate (100 to 200

mL/min) as vadose zone sampling

– Maintain low vacuum (<100 inches H20)

– Use smallest sample containers (0.4 to 1 L)

– References DTSC Vapor Intrusion Guidance,

Appendix G for sub-slab probe installation

– Figure 2 for sub-slab probe typical diagram

SOIL GAS ASSEMBLY TESTS

Complete shut-in, leak, and

purge volume tests before

collecting soil gas samples after

the soil gas well has equilibrated

SOIL GAS ASSEMBLY TESTS

• Shut-in Test

– Checks for leaks in the above-ground valves,

lines and fittings downstream from the top of

the probe to the sample container

– Evacuate the system to a minimum measured

vacuum of about 100 inches of water for at

least 1 minute

– Observe any vacuum loss and correct loose

fittings

SOIL GAS ASSEMBLY TESTS

• Shut-in Test

– After the test is completed try not to move the

sampling train.

– A shut-in test is not a replacement for

a leak test

SOIL GAS ASSEMBLY TESTS

• Leak Test

– Evaluates whether ambient air is

introduced into the soil gas sample

during the purging and sampling

process

– Conducted at every soil gas well each

time a soil gas sample is collected

Revisions to the Advisory

• Leak Check

– New list of compounds

• Liquid tracer compounds, such as hexane,

pentane, n-propanol

• Gaseous tracer compounds, such as helium or

sulfur hexafluoride can be used along with

appropriate shrouding or tenting

• Select a leak check compound based on the

project’s DQOs and site historical uses

SOIL GAS ASSEMBLY TESTS

• Leak Test

– Leak check evaluation

• A soil gas well should be decommissioned if the

leak cannot be corrected. Replacement soil gas

wells should be installed at least five feet from the

original location

• Sources of leaks: poorly constructed wells, poor

quality fittings, stripped, over tightened, dirty or

worn threads, and excessive sampling train

connections.

SOIL GAS ASSEMBLY TESTS

• Leak Test

– Leak check evaluation

• A data adjustment factor based upon the

concentration of the leak check compound is

inappropriate

• Leak check compound field screening prior to

laboratory analysis is recommended for laboratory

bound sample containers

• leak check compounds, both liquid and gaseous,

may contain impurities

SOIL GAS ASSEMBLY TESTS

• Leak Check (cont.)

– Qualitative vs. Quantitative

• Qualitative analysis

– Quantified at detection limit (DL) of target

analytes.

– If the concentration of leak check compound is

10 times the DL for target analyte(s), then

corrective action is necessary.

SOIL GAS ASSEMBLY TESTS

• Leak Check (cont.)

– Quantitative

• Use of gaseous tracer with shrouding or tenting to

determine percent of ambient air leak into a

collection vessel.

– Leaks in excess of 5% of sample should be

mitigated.

• Procedures for quantitative leak test are

described in Appendix C.

To Shroud or Not to

Shroud

SOIL GAS ASSEMBLY TESTS

• Purge Volume Test

– Completed after the shut-in test

– Removes ambient air from the sampling

system

– Establishes the optimal purge volume for a

lithology

– One, three and ten purge volumes

– Use the same analytical method as the site’s

constituents of concern

SOIL GAS ASSEMBLY TESTS

• Purge Volume Test

• One purge volume includes:

– The internal volume of the tubing and probe

tip

– The void space of the sand pack around the

probe tip

– The void space of the dry bentonite in the

annular space

PURGE/SAMPLE FLOW RATE AND

APPLIED VACUUM

• Purge and Sampling Rates

– Purge rates between 100 to 200 mL/min and

vacuums <100 inches of water for

standard small diameter (1/8 to 1/4 inch)

tubing should be maintained

– Higher flow rate for excessive purge times,

such as for deep wells with larger-diameter

tubing. Maintain vacuums <100 inches of

water

Sample Handling and Transport

• Sample Containers

– Use of plastic syringes is discouraged due to

sorption of COCs. Glass syringes are preferred

– Samples collected using polymer gas sample

bags should be analyzed within 6 hours after

collection

– Glass bulbs with surrogates added within 15

minutes of collection should be analyzed within

24 hours after collection (see Appendix E).

Sample Collection

• Soil Gas Sampling in Low-Permeability Soil

• Modified purging and sampling procedures

– Purge or sample probe until vacuum is > 100 in H2O

– Let vacuum re-equilibrate and begin purging and

sampling;

or

– Reinstall vapor probe with larger sand pack (~10 L)

• Further detail on sampling in low-permeability

soil are discussed in Appendix D.

Revisions to the Advisory

• Sample Collection

– Polymer Gas Sampling Bags

• A vacuum chamber or lung sampler should be

employed when using gas sampling bags.

– Allows filling gas sample bag without passing vapor

through pump.

• The pump draws a vacuum within chamber or

container and resulting pressure differential causes

sample to be drawn through sampling tube into

gas sample bag.

Soil Gas Sampling

• Sorbent Tubes for Method TO-17

– Soil gas sampling for analysis by Method

TO-17 is described in Appendices E and

F

– TO-17 may be used for other volatile

constituents depending on project DQOs.

Tip breaker

Tu

be

Ho

lde

rs

Tube to pump and sample

Sorbent

cartridges

Cap

Two Tenax Tubes in Series

Adapted from Air Toxics 2008

Field Conditions

• Rainfall Appendix G)

– Soil gas sampling should not be conducted:

• during or within 5 days of a significant rain event

(1/2 inch or greater); or

• where the following conditions occur:

– Irrigation or watering of soil,

– Standing or ponded water areas.

Field Conditions

• Barometric Pressure (Appendix G)

– Soil gas sampling should be delayed until frontal

systems have passed the area.

– Alternatively, soil gas sampling times and depths

may be chosen to minimize the effects of changes

in barometric pressure.

Field Conditions

• Corrective measures for low flow

conditions

1.Evaluate site lithologic logs or collect new

continuous soil cores

2.Use alternate low flow sampling methods

(passive soil gas methods described in

Appendix A)

3.Use soil matrix sampling EPA Method 5035

4.Evaluate the need for the sampling location

Revisions to the Advisory

• Detection Limits (DLs)

– Project-specific DLs should be based on

DQOs.

– DLs should be quantitative and < reporting

limit.

– Tracer compound DLs should be adjusted

to be in same concentration range as

analytes.

Revisions to the 2003 Advisory

Appendix A – Passive Soil Gas Method

Appendix B – Canister Holding Time, Use of Tedlar®

Bags, and Tubing Types

Appendix C – Quantitative Leak Testing Using Shroud

and Helium

Appendix D – Soil Gas Sampling in Low-Permeability Soil

Appendix E – Naphthalene Soil Gas Collection

Appendix F – Soil Gas Analytical Method Review

Appendix G – Barometric Pressure, Rain Fall, and Soil

Drainage

Appendix H – Reporting Format and Parameters

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LABORATORY CERTIFICATION

Amendment process for currently certified

labs

Application process for new labs

DO NOT CONTACT ELAP YET!

Details need to be worked out!

2012 Advisory

• On DTSC website at:

http://www.dtsc.ca.gov/SiteCleanup/upload/VI_Activ

eSoilGasAdvisory_FINAL_043012.pdf

• Soon to be posted:

– FAQ

– Revision 1

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LABORATORY CERTIFICATION

For soil gas TO methods, labs must have

the designated instrumentation/setup for

those methods.

QUESTIONS?