Trihydro designed and implemented semi-passive bioremediation technologies at a Resource Conservation and Recovery Act (RCRA) facility to remediate the offsite migration of chlorinated solvent impacts, leading to upwards of 99% reduction in concentrations of tetrachloroethene (PCE) and trichloroethene (TCE).

Trihydro provides geologic, hydrogeologic, and environmental engineering consulting services at an active Resource Conservation and Recovery Act (RCRA) facility. Impacts at the site are associated with an onsite solid waste management unit and primarily consist of chlorinated solvents.

Trihydro’s initial focus was to determine the nature and extent of soil and groundwater contamination associated with historical facility operations. The project site footprint is only 25 acres, but due to the complex geology and its effect on contaminant fate and transport, the investigation area included over 100 acres with more than 100 soil borings, temporary groundwater monitoring points, and 28 groundwater monitoring wells.

After identifying the extent of the contamination, Trihydro developed corrective measures alternatives to treat the contaminant plume. The team evaluated active remediation technologies, passive technologies, and the use of institutional controls and identified a combination of in situ bioremediation (ISB) and institutional controls as the preferred strategy. Trihydro documented the selected strategy in a Response Action Plan (RAP) for submission to the Texas Commission on Environmental Quality (TCEQ).  Trihydro designed, constructed, and provided operations and maintenance (O&M) on the remedial system.

Trihydro then designed a pilot test to produce information to design and scope a full-scale ISB system. The ISB pilot test resulted in PCE concentrations of 95% to 99% within five months of implementation. Monitoring data indicated that the treatment zone expanded to downgradient areas due to natural groundwater flow. The pilot test indicated that the treatment zone would expand over time, meaning a relatively large area could be treated by injection focused on a smaller area.

With the pilot test complete, Trihydro designed and implemented a full-scale ISB program, using pilot test data to model multiple treatment scenarios. The full-scale design consisted of source area treatment and biobarrier treatment, with emulsified vegetable oil (EVO) bioremediation amendment delivered to dedicated injection wells. The source area treatment consisted of installing injection wells in a grid pattern in the area of highest tetrachloroethene (PCE) and trichloroethene (TCE) concentrations in soil and groundwater. The biobarriers consisted of installing three linear arrays of injection wells aligned perpendicular to the direction of groundwater flow. A reactive treatment zone was established in each biobarrier, with PCE-impacted groundwater flowing into the treatment zone and treated groundwater exiting the treatment zone. The treatment zone will expand downgradient of each biobarrier over time due to advection of bioremediation amendment with ambient groundwater flow.

Groundwater samples collected within the treatment zone after full-scale ISB indicate 80% to 99% reductions in PCE concentrations, with concentrations of PCE and degradation products continuing to decrease over time. The ISB program has consisted of injection of EVO during a month-long field program followed by two to three years of in-situ treatment without further active intervention. The project has transitioned to monitored natural attenuation (MNA) as a polishing step.

Groundwater samples collected within the treatment zone after full-scale ISB indicate 80% to 99% reductions in PCE concentrations, with concentrations of PCE and degradation products decreasing over time. 

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