SUCCESSFUL REMEDIATION OF A CHLORINATED ETHANE PLUME IN GROUNDWATER

A release of used oil from a machine shop located in the Midwest impacted a shallow groundwater aquifer. Source characterization and subsequent source area remediation was implemented in the early 1990’s. Persistent detections of chlorinated VOCs in groundwater necessitated a re-evaluation of the remedial approach. Reductive dechlorination was implemented in early 2001 to enhance the transformation of 1,1,1-TCA into daughter products and benign end products. Ninety four injection points were advanced in the VOC plume area and dosed with approximately 9100 pounds of a lactate-based carbon substrate (HRC) manufactured by Regenesis.

As part of an upcoming property transfer, an additional source area was located trans-gradient to the used oil UST in 2002. Due to the high concentrations of 1,1,1-TCA detected in the new source area, an interim dual-phase extraction (DPE) system was installed in the source area. Over the course of nine months approximately 100 pounds of chlorinated VOCs were removed from the source area. A stability groundwater monitoring program was implemented to evaluate the rebound in the 1,1,1-TCA concentrations in the source area after the DPE was halted.

To understand the fate and transport of the chlorinated hydrocarbons in the shallow groundwater, two fate and transport models, BIOSCREEN and BIOCHLOR were utilized. The output from the BIOSCREEN model (state agency required model) predicted that a source area concentration of 13.55 mg/L of 1,1,1-TCA would be an acceptable concentration to remain in the groundwater and not exceed MCLs at the down-gradient property line. The results of the stability monitoring program in the source area indicated that additional remediation was required. Reductive dechlorination was again selected to be the preferred remedial alternative. Approximately 3200 pounds of a lactate-based carbon substrate (3DME) was injected into 55 points in March 2008.

Performance groundwater monitoring was implemented from June 2008 through March 2010. Five quarters of rebound groundwater monitoring were implemented between June 2010 and June 2011. In July 2011, the state agency requested a groundwater closure monitoring plan be prepared and implemented. Closure monitoring indicates concentrations remain less than the SSTLs and closure of the site has been requested.