In Place Groundwater Remedy Next to Canal Protects Surface Water Quality: A Case Study

Groundwater at an industrial site in Texas was impacted with Chlorinated Volatile Organic Carbon’s (CVOCs), primarily PCE and TCE, from historical waste disposal. The topography at the site slopes steeply toward a channel where groundwater discharges. Depth to groundwater ranges from approximately 20 ft bgs at the source area to 1 ft bgs close to the channel. Tight soils consisting of silty clays make up the geology of the site. Remedial objectives were to achieve the Texas Risk Reduction Program Tier 1 Residential Protective Concentration Levels in surface and ground water.  

Remediation of groundwater at the site entailed a combined approach employing chemical oxidation at the source area and a reductive Permeable Reactive Barrier (PRB) for passive plume treatment before discharge to the channel. Environmentally friendly reductive technologies, including zero valent iron (ZVI) and a source of carbon, were chosen adjacent to the channel in order to protect the water body and aquatic life.  Fine-grained soils and a shallow water table presented challenges for hydraulic fracturing and direct push technologies. A closely spaced injection array, thicker slurry consistency, and lowered injection rates minimized surfacing during direct push injections.

The combined oxidation and reduction treatments have resulted in > 90% reduction in total chlorinated ethenes in groundwater at the source area (from a maximum of 2,211 ppm measured in 2002 to 0.216 ppm measured in January 2009). Performance monitoring in the channel confirmed > 98% removal of total chlorinated ethenes directly downgradient from the reductive treatment zone (from a maximum of 1.815 ppm measured in April 2005 prior to the installation of the reductive zone to 0.028 ppm measured in January 2009).  Remedial objectives have been met.