Rapid, Efficient Delineation From VI Potential of A Large Soil Gas - - PowerPoint PPT Presentation

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Jan 12, 2024 162 likes •314 views

Rapid, Efficient Delineation From VI Potential of A Large Soil Gas Plume Using HAPSITE and Other Lines of Evidence Chris Lutes and Jennifer Knoepfle, Ph.D., P.G Site Setting Small town in Indiana; EPA led NPL Site Residential and

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Rapid, Efficient Delineation From VI Potential

f A Large Soil Gas Plume Using HAPSITE

and Other Lines of Evidence

Chris Lutes and Jennifer Knoepfle, Ph.D., P.G

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2 Presentation Title

Small town in Indiana; EPA led NPL Site
Residential and Commercial (pop. = ~5,000)
Unconsolidated sand and gravel aquifer (0-~90 ft bgs) overlies shale bedrock
Municipal water extracts (~10-20 µg/L) (drawn from entire aquifer 15 – 90 ft bgs)

exceed the MCL (5 µg/L) for PCE

Five of 36 monitoring wells screened over the “shallow” aquifer (5-23 ft bgs) have PCE

concentrations in groundwater greater than the residential VISL (~26 µg/L) (14% of wells exceed)

Permanent soil vapor probes (SVP) installed to a depth of one foot above the top of

water table. Top of water table ranged from 8 to 12 bgs. A total of 15 of 18 SVPs have PCE concentrations that exceed the residential subslab VISL (~360 µg/m3). (83% of SVPs exceed)

Some residents still use private wells screened in the shallow aquifer

Site Setting

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Basic History

1986-1991: Industrial dry cleaner (source)

was operational (PCE contamination)

2003 – 2008: EPA removal action (soil

excavation + AS-SVE in and adjacent to source)

2010: IDEM noticed PCE impacted municipal

wells exceed PCE MCL and town treats water through carbon filtration

2015: Region/CH2M began RI efforts (4 field

events)

2016: Subslab and Indoor Air Sampling

Underway by EPA

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Recent Investigation Approach

August 2015: PCE (and TCE) soil vapor plume – 15 of 18 permanent

SVP exceeded the PCE VISL

Five locations exceeding the IDEM action levels and EPA Removal

Management Levels (RMLs) for PCE (2 locations for TCE) – Soil Vapor Max concentrations 170,000 µg/m3 (PCE) and 12,000 (TCE)

August 2015: Bulk soil concentrations (discrete depth) for PCE and

TCE exceeded residential RSLs in one permanent SVP location. Soil considered delineated.

October 2015: PCE “residual” groundwater plume – considered

delineated (120 µg/L= max GW PCE concentration)

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December 2015: HAPSITE soil vapor investigation

80 soil gas locations around the site, to delineate the extent
f the soil vapor plume and guide EPA Emergency Response

(size, shape, chemical nature of plume);

80 points completed in 5 days; 8 ft depth (42% exceeded

VISL). Project included 8 confirmatory lab samples

2016 Removal branch/START has recently performed targeted

subslab and indoor air sampling (results pending)

May 2016: 2nd HAPSITE soil gas investigation is proposed

based on results from December 2015 to complete soil vapor delineation

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Comparison of Effected GW/Soil Vapor Plume Dimensions

Plume based on locations were GW PCE concentrations exceeded

VISLs – ~9 buildings – ~164,000 ft2 (two small plumes 60,000 ft2 and 104,000 ft2 each) – ~3.9 acres (two small plumes 2.4 acres and 1.4 acres each)

Plume based on results from results from permanent soil gas points

– ~18 buildings – ~585,000 ft2 plume area – ~13.4 acres

Plume based on results from First HAPSITE investigation

– ~178 buildings – ~3,796,900 ft2 plume area – ~87.1 acres

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Plume Map Based on Groundwater VISL Exceedances

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HAPSITE Planned Locations –Based On Permanent Soil Gas Results

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HAPSITE Results Substantially Increases Soil Gas Footprint in Several Directions

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Temporary Soil Vapor Probe Installation and Sampling Set-Up

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HAPSITE Instrument

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Cost Comparison

Conventional Approach: 18 permanent soil vapor points

installed to 9-12 ft. bgs; one round of laboratory analysis = $43K about $2,400 a point.

HAPSITE/TRIAD Approach: 78 temporary soil vapor points,

installed to 8 ft bgs, with HAPSITE analysis on all, 8 confirmation samples $74K about $950 a point.

This comparison does not include the additional benefits of

speed and eliminating unnecessary sample points with field decision making.

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HAPSITE Output

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Methods Inter-comparison (Eight samples)

2 Points 1 10 100 1000 1 10 100 1000 10000 HAPSITE Result (ug/m3) Laboratory Result (ug/m3)

Comparision of HAPSITE to Confirmatory Laboratory Samples for PCE

data Ideal Agreement VISL (PCE) VISL (PCE)

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Lessons Learned

HAPSITE provided efficient delineation consistent with

TRIAD principles of field adaptive decision making.

HAPSITE with temporary soil vapor probe mobilization

was rapid – 2 months for planning and mobilization.

External soil gas sampling with HAPSITE allowed a

greater density of data points then the permanent monitoring well network.

Soil gas points appear to exceed corresponding VISLs by