Comparative Effects of HRC Application at Four Project Locations (PDF)

Four site locations in the Southern or Midwest United States were the venues for pilot testing of a commercially available injectable bioremediation product (HRC™) to enhance the reductive dechlorination of chlorinated solvents. This activity occurred from 1997 through the end of 2000. Prior to injection, each site exhibited bioattenuation of parent compounds such as PCE, TCE, carbon tetrachloride, or chlorofluorocarbons, but at very slow rates. Enhanced bioattenuation was desired so an injection of HRC™, a food grade, polylactate ester was made at each site location using direct push technology. Injections were made in a variety of grid patterns in either source areas or distal portions of the respective plumes.
HRC™ has been demonstrated at numerous sites to slowly release fermentable lactic acid, which through a series of conversions, releases hydrogen at each step that becomes available for the reductive dechlorination process. When injected, the semi-solid HRC™ product remains in place, but a plume of lactic acid develops as a result of microbial activity upon the product.

Injections at the various site locations were made in approximately 250-sq. ft. study areas at depths of up to 45 ft. below ground surface. The thickness of the injection zones varied from 7 ft. to 15 ft. Monitoring wells or direct push points located in the pilot study areas were used for groundwater sampling and measurement of field parameters.

Groundwater samples were collected from each site’s monitoring array for selected monitored natural attenuation parameters, the parent compound(s) and daughter products. Field parameters such as temperature, pH, conductivity, dissolved oxygen (DO), and redox were made using a flow-through cell. Samples were collected monthly to bi-monthly. Acclimation periods, where changes in geochemistry occurred without significant changes in chlorinated solvent concentrations, occurred at the different sites over a period of one to two months. The emergence of lactic acid and its breakdown products were observed in specific monitoring wells at the sites within a one to three month period. This typically coincided with a significant decrease in the sulfate concentration and an increase in sulfide. Redox fell to –100 to -300 mv at various sites and TOC increases from one to two orders of magnitude.

Analytical results indicated that lactic acid often travels along preferential flow paths at the different sites, but the areas impacted showed mass reduction of chlorinated compounds between 40 and 80% over 90-120 day studies. Observations have also been made on the persistence of effects from HRC™ depending upon the soil type present, the groundwater flow velocity, and the configuration of the injection grid. Combined, these observations will aid in the future design of HRC™ injection grids under a variety of field conditions.

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