The site is underlain by about 2 feet of unconsolidated deposits that overlie quartz monzonite granitic bedrock. Groundwater is encountered within bedrock fractures approximately 34 feet below the source area. The deeper bedrock has much smaller fractures, significantly decreasing groundwater flow. The depth to groundwater at the site averages from 23 to 118 feet below ground surface (bgs). A 12-inch aplite dike also was identified in the subsurface.
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
Operations at a fuel storage area led to a very large plume contaminated with petroleum commingled with trichloroethylene (TCE). Contamination ranges from 250 to 600 feet below ground surface (bgs) and the TCE plume is about 1.2 miles long, 0.5 miles wide, and at least 250 feet deep in the source area. TCE was released into fractured bedrock as pure product. While dense non-aqueous phase liquid (DNAPL) was not encountered during previous investigations, dissolved-phase concentrations greater than 1 percent of the free-phase solubility of TCE indicate that DNAPL is present.
Major Contaminants and Maximum Concentrations:
- Trichloroethene (34,000 µg/L)
- N-Nitrosodimethylamine (var)
- Dioxane (var)
No technologies selected.
- Chemical Oxidation (In Situ)
Comments:
A feasibility study was conducted to assess remedial alternatives. A treatability study (TS) was conducted to evaluate use of an infiltration gallery in the area where the previous TCE release occurred to deliver oxidants for in situ chemical oxidation (ISCO) treatment of the contaminated groundwater. More specifically, the TS was conducted to evaluate whether sodium permanganate: (1) can be effectively delivered via the infiltration gallery at the Site 25 source area; (2) can effectively destroy TCE in the TS area once it reaches groundwater; and (3) affects the concentrations of N-nitrosodimethylamine (NDMA) and 1,4-dioxane in the TS area. The ISCO TS involved the injection of sodium permanganate through an infiltration gallery that was installed in the TCE release area and above a 12-inch-thick aplite dike discovered in the subsurface during a 1999 near-surface fracture study. The highly fractured and vertically oriented aplite dike may have acted as a preferred pathway for the movement of contaminants. In January 2010, an L-shaped infiltration gallery was installed in a newly excavated trench measuring approximately 115 feet long by 8 feet wide by 5.5 feet deep. Three perforated polyvinyl chloride (PVC) drainage pipes surrounded by crushed rock and encapsulated in geotextile fabric were placed into the trench segments, then backfilled to ground surface with clean native soil. After installing the monitoring well network, completing baseline sampling, and assembling the oxidant delivery system, the sodium permanganate was injected in April 2010 followed by 16 months of performance monitoring.
Not documented.
The gallery delivered sodium permanganate from the top of fractured bedrock to groundwater, but only reached three shallow wells (55 feet deep) that were within 20 horizontal feet of the gallery. Sodium permanganate was not observed in any of the more distant shallow wells or the deeper wells (100 feet deep). The oxidant moved progressively to the northeast (NE), approximately parallel to the strike of the aplite dike and roughly perpendicular to the west southwest direction of groundwater flow.
Once reaching the groundwater, sodium permanganate was effective at oxidizing high concentrations of TCE
(up to 34,000 micrograms per liter) to non-detectable concentrations within the fractured bedrock. However, sodium permanganate was not effective at oxidizing NDMA or 1,4-dioxane. Sodium permanganate persisted in the monitoring wells throughout the 16-month monitoring period. Based on challenges and costs associated with remediation and modeling of the plume migration and degradation, the selected remedy may include a technical impracticability waiver request for the contaminated zone.
References:
Battey, T. 2012. "Delivery of Oxidant Through an Infiltration Gallery to a Commingled Plume in Fractured Granitic Bedrock." 8th International Conference Remediation of Chlorinated and Recalcitrant Compounds. May 21-24.
EPA. Web Site with Information on Edwards Air Force Base, including Site 25. Accessed at: http:yosemite.epa.govr9sfundr9sfdocw.nsfce6c60ee7382a473882571af007af70d130468772dfd94ff88257007005e9413!opendocument
Edwards Air Force Base. 2011. You Tube Video on Cleanup of Site 25: Includes Discussion of ISCO Tests. Uploaded May 9. http:www.youtube.comwatch?v=hsNyzOzRF90
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