Topic Area: 2f. Challenges in Application of Bioremediation Tools

Formatted Title:

Enhanced Reductive Dechlorination for Large-Scale CVOC Treatment in Challenging Bedrock and Overburden Geology

Background/Objectives:

A release of trichloroethene (TCE) occurred at a historical landfill and has resulted in a downgradient chlorinated volatile organic compounds (CVOCs) plume. Before 1899, the site functioned as a stone quarry, which was later backfilled with dried wastewater sludge. The highest CVOC impacts, including dense non-aqueous phase liquid (DNAPL), are present in bedrock: a competent, banded gneiss (Nashoba Formation) encountered at approximately 10 to 35 feet below grade. A highly interconnected and transmissive fracture network is the primary pathway of contaminant transport. Static groundwater elevation within the fracture network is approximately 10 feet higher than the hydraulic head of groundwater within the overburden aquifer. As a result, lesser overburden impacts due to groundwater upwelling have been observed. Overburden soils are predominantly fine sandy silts. Remediation objectives were to significantly reduce contaminant mass within bedrock and overburden aquifers and to create a reactive barrier to off-site contaminant flux.

Approach/Activities:

Enhanced in situ dechlorination (EISD) was selected as the remedy. The treatment approach included a large construction and drilling program. The program included the installation of 23 open borehole bedrock injection wells installed in a 750-foot-long permeable reactive barrier (PRB) along the property boundary. Borehole geophysics was performed at 24 bedrock injection wells to identify fractures for treatment and approximate fracture aperture size. Bedrock groundwater samples were collected using straddle packers from identified fracture depths to quantify CVOC impacts over the length of the barrier. In addition, 65 injection wells established an overburden PRB. Photoionization detector (PID) screening measurements were collected from overburden soils during drilling and from injection well casings to provide insight into relative CVOC impacts across the barrier.

The EISD treatment consisted of hydraulic fracturing to emplace zero-valent iron (ZVI), sand proppant, and emulsified vegetable oil (EVO) into bedrock fractures and low-pressure injection of EVO, sodium lactate, pH buffer, and bioaugmentation into overburden injection wells and bedrock injection wells via straddle packers. Tilt metering of hydraulic fracturing processes was conducted to quantify bedrock fracture propagation during injection.

Samuel Marshall

Affiliation

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(Group 2, Poster Board #51) Enhanced Reductive Dechlorination for Large-Scale CVOC Treatment in Challenging Bedrock & Overburden Geology