Full-scale application of ELS® microemulsion Technology for the Treatment of an Aquifer Contaminated with perchloroethylene and trichloroethylene via Ehnanced Reductive Dechlorination

Abstract

Mixed plumes of chlorinated organics and oxidized metals are a common contaminant at many sites. The oxidized metals can be mediated by the establishment of moderately reducing conditions. The chlorinated organics have been demonstrated to be degradable by specific dechlorinating microrganisms in anaerobic environment such as Dehalococcoides sp. Enhanced biological dechlorination requires the presence of an effective electron donor to provide molecular hydrogen (H2) to completely degrade chlorinated ethenes. Distribution of the electron donor results in the biologically mediated establishment of highly reducing conditions in the treatment zone. This process also results in the reduction and precipitation of the oxidized metals via sulphate-reducing conditions. Peroxychem LLC has developed an innovative electron donor, ELS® Microemulsion Reagent (ELS) for in situ treatment of chlorinated organics and metals. This substrate has been successfully applied at numerous sites to address a variety of contaminants. ELS® is an organic electron donor composed of an easily fermentable organic substrate based on lecithin, and designed to enhance in situ anaerobic bioremediation aquifers contaminated by organochlorine compounds and heavy metals such as hexavalent chromium Cr[VI]. The product is easy to mix, dilute and inject into the subsurface. Once in the groundwater, indigenous microorganisms utilize ELS to rapidly generate highly reducing conditions, favoring biotic dechlorination reactions and the reduction of oxidized metals such as Cr[VI]. This innovative technology was successfully applied to a former manufacturing site in the center of Italy, where groundwater was historically contaminated with Tetrachloroethylene (PCE > 5.5 milligrams per Liter; mg/L), Trichloroethylene (TCE > 2 mg/L), 1,2-Dichloroethene (1,2-DCE > 1 mg/L) and, to a lesser extent, Vinyl Chloride (VC) and 1,2-Dichloropropane (DP). A pump-and-treat system (P&T) installed in the source was active as a source containment measure and to speed up the overall groundwater remediation. However, there was concern that the pumping could affect the ELS treatment effectiveness because of the increased groundwater flow velocity and the potential for removal of the injected bioremediation substrate. To mitigate this potential some wells were switched off the flow rates of others was adjusted to ensure compatibility with the planned product injection. In particular, an upstream low-flow-rate pump and treat system was maintained over the ELS® treatment period, primarily to delay the fast-downstream diffusion of the amendments in the aquifer, thus enhancing the source treatment. Following the calibration of the P&T system, approximately 4,900 kg of ELS® concentration was injected under high pressure at 51 locations into the source area. In about 12 months from injection of ELS® Microemulsion into the groundwater in the main source area, concentrations of PCE, TCE and the recognized catabolites, such as DCE and VC, rapidly reduced, compared to the pre-treatment concentrations, until they reached the statutory national limits (CSC D.lgs 152/06) in the main monitoring piezometers of the area, also highlighting the establishment of clear and enhanced biotic reducing conditions. No rebound effects have been observed in the next three years of monitoring.