soil gas sampling: a practical perspective 2012 asgi soil gas sampling.pdf · soil gas sampling: a...
TRANSCRIPT
Soil Gas Sampling: A
Practical Perspective
Theo Johnson
Geological Services Unit
Department of Toxic Substances Control
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
53
LABORATORY CERTIFICATION
For soil gas TO methods, labs must have
the designated instrumentation/setup for
those methods.
QUESTIONS?