Using soil gas concentration mapping to predict soil vapor extraction radius of influence variances and optimize remedial system design

1. Abstract

Soil vapor extraction (SVE) is one of the most commonly used technologies for the remediation of unconsolidated soils impacted by chlorinated volatile organic compounds (cVOCs). Most SVE remediation engineers incorporate an array of site data, contaminant details, and remedial goals during their design process. Foremost among these critical design criteria is having a comprehensive understanding of the source area, contaminant distribution, subsurface vacuum response, and lithologic complexities at the Site. Vadose zone soil grain size analysis, effective porosity, pilot testing results, and soil cVOC concentrations are routinely included as part of this design process. Even with this plethora of data, many sites still fall victim to improperly designed remediation systems. 

In order to help refine the SVE system design process, soil gas plume mapping was evaluated for its ability to provide useful data regarding preferential airflow pathways. An assessment of multiple sites with cVOC releases, pre-remedial soil gas investigations, and SVE system operational data was conducted to determine if pre-remediation soil gas mapping could yield beneficial design inputs for SVE systems installed in unconsolidated vadose soils.

2. Objective

This analysis was conducted to evaluate if a consistent, predictive pattern could be determined to link soil gas plume behavior with SVE vacuum ROI. If found, this predictive data could be used to help refine SVE system designs for sites with limited site-specific data or those where SVE pilot testing was not possible.

3. Evaluation Criteria

Representative sites were evaluated based upon the following criteria:

  1. Specific lithologic patterns, consisting of a cohesive surface cap underlain by unconsolidated vadose zone soils.
  2. The similarity of soil characteristics (moisture content, grain size, etc.).
  3. The completion of a pre-remedial soil gas investigation.
  4. The installation and operation of an SVE system affecting the unconsolidated vadose zone soils.

Sites were screened out from this analysis if multiple source areas were identified, significant down-gradient off-gassing from groundwater was present, or inadequate soil gas, lithologic, or SVE system data was collected.

4. Lithologic Characteristics

Representative sites were evaluated based upon the following criteria:

  • The shallow cohesive unit across all Sites yielded an average thickness of approximately 8.5 ft, ranging from a minimum of 6 to a maximum of 18 ft.
  • The average thickness of the unconsolidated vadose units was 16 ft, ranging from 13.5 to 21 ft.
  • The average depth to water across all Sites was 26 ft, ranging from a minimum depth of 22 to a maximum of 30 ft. 
  • The average (and ranges) of the unconsolidated grain size classification percentages are as follows:
    • Sand and Gravel: 89.6% (85.9 – 95%)
    • Silt: 5.75% (<1 – 9.9%)
    • Clay: 5.3% (2.8 – 7.7%)
  • The average moisture content in soil samples collected from the unconsolidated vadose soils was 7.78%, with values ranging from 3 to 17%.

Site #1: Example 2-D Cross Section

The cross-section for Site #1 is featured in the down-gradient direction of groundwater flow, from east to west, and contains a two main lithologic features consistent across all four Sites in this study: An upper cohesive unit and a lower unconsolidated unit.

  • Average shallow cohesive unit thickness of 7 ft.
  • Average unconsolidated vadose unit thickness of 13.5 ft
  • Average depth to water of 22 ft
  • Grain size classification percentages:
  • Sand and Gravel: 89%, Silt: 7%, Clay: 4%
  • Average moisture content in soils: 9.6%

Site #1: Soil Gas Plume With SVE ROI

With a single extraction well setup, this ROI mimics the generalized circular nature of the soil gas ROI. The vacuum propagates slightly more to the north than the soil gas plume, but the general overlap is fairly consistent across the Site.

  • PCE was detected at a maximum concentration of 51,600 µg/m3in the source area
  • The soil gas plume has an area of roughly 66,051 ft2(100 µg/m3)
  • The SVE applied vacuum is 5.5 in-Hg
  • The SVE system ROI is roughly 80,424 ft2(0.1 in-wt)

Site #2: Example 2-D Cross Section

The cross-section for Site #2 is featured in the down-gradient direction of groundwater flow, from north to south. The vadose soils for Site #2 display similar features to those in the cross-section for Site #1, with the exception that lower consolidated soils are acting as the vertical confining unit as opposed to groundwater. The extraction well and soil gas points are installed within this upper vadose zone.

  • Average shallow cohesive unit thickness of 6 ft
  • Average upper unconsolidated vadose thickness of 15 ft
  • Average depth to water of 27 ft
  • Average moisture content in soils: 9.6%
  • The upper and lower cohesive units bound the soil gas and SVE vacuum ROI in the vertical directions

Site #2: Soil Gas Plume With SVE ROI

The soil gas plume for Site #2 was expansive, and the single extraction well utilized during SVE system operation could not generate a large enough ROI to reach the extents of this plume. The highest concentrations of onsite soil gas spread laterally before rounding out towards the edges. The SVE ROI displayed similar characteristics, expanding east to west before rounding into an oval shape.

  • PCE was detected at a maximum concentration of 1,480,000 µg/m3in the source area
  • The soil gas plume has an area of roughly 1,583,362 ft2(100 µg/m3)
  • The SVE applied vacuum is 5.0 in-Hg
  • The SVE system ROI is roughly 125,663 ft2(0.1 in-wt)

Site #3: Example 2-D Cross Section

The cross-section for Site #3 depicts more variance than the other two, but keeps the main themes of unconsolidated vadose soils bound by an upper cohesive unit and saturated soils below. Groundwater flow direction is generally east to west, as is the cross-section.

  • Average shallow cohesive unit thickness of 9.5 ft
  • Average upper unconsolidated vadose thickness of 21 ft
  • Average depth to water of 25 ft
  • Four pairs of co-located deep and shallow extraction wells were installed at the Site

Site #3: Soil Gas Plume With SVE ROI

The soil gas plume for Site #3 isn’t as symmetrical as the others, displaying more angular features. Soil gas propagation to the east is limited, as is vacuum propagation. The highest concentrations of soil gas appear to extent to the northwest, with the greatest extents to the southwest. Vacuum propagation generally advanced in a more evenhanded fashion towards the west.

  • PCE was detected at a maximum concentration of 375,000 µg/m3in the source area
  • The soil gas plume has an area of roughly 173,494 ft2(100 µg/m3)
  • The SVE system ROI is roughly 152,053 ft2(0.1 in-wt)
  • The SVE applied vacuum is 9.5 in-Hg

Site #4: Example 2-D Cross Section

The lithology depicted on cross-section for Site #4 maintains the overall thematic principles contained on the other 3 cross-sections, while also displaying a vertically oriented fining of the sands into silty-clay soils. There is a greater degree of transition at this Site compared to the others. Groundwater flow direction is generally north to south, as is the cross-section.

  • Average shallow cohesive unit thickness of 12 ft
  • Average upper unconsolidated vadose thickness of 15 ft
  • Average depth to water of 30 ft
  • Two extraction wells are located pairs of co-located deep and shallow extraction wells were installed at the Site

Site #4: Soil Gas Plume With SVE ROI

The soil gas plume for Site #4 is elongated from north to south, while the SVE vacuum ROI extends furthest to the northeast. While there is mild disagree between the concentration vs. vacuum propagation at the Site, the ultimate extents of both line up well.

  • PCE was detected at a maximum concentration of 1,400,000 µg/m3in the source area
  • The soil gas plume has an area of roughly 180,955 ft2(1,000 µg/m3)
  • The SVE applied vacuum is 13.5 in-Hg
  • The SVE system ROI is roughly 196,349 ft2(0.1 in-wt)

Discussion

  • The SVE system ROI overlay was compared against the soil gas plume to determine how closely the concentration trends aligned with the vacuum ROI extents.
  • Multi-extraction well systems (Site #3 and #4) displayed a similar degree of propagation similarity as single extraction well systems (Site #1 and #2).
  • System SCFM and applied vacuum were compared to ROI to determine if higher or lower ratios affected similarity to the soil gas plume.
    • Site #1: 396 SCFM – 5.5 in-Hg
    • Site #2: 433 SCFM – 5.0 in-Hg
    • Site #3: 210 SCFM – 9.5 in-Hg
    • Site #4: 287 SCFM – 13.5 in-Hg
  • It appears that lower applied vacuums (Site #1 and 2) yield more symmetrical ROIs, while the volume of extracted air appears to have a similar bearing on ROI symmetry.
  • These same symmetrical traits appear in the soil gas plume for Site #1 and 2 as well.
  • There appears to by some dynamic shaping variances within the highest soil gas concentrations and highest applied vacuums, but similarities tended to prevail at the extents of both.
  • It appears that the subsurface characteristics governing the dispersion and extents of soil gas also play an important factor in the propagation of applied vacuum from SVE systems.

Conclusions

While not strong enough to replace pilot testing data, soil gas plume mapping can provide indicators for anticipating non-uniform vacuum propagation in the subsurface, which can be beneficial during extraction well placement and airflow, vacuum, and blower sizing.