WORK PLANS_FILE 2 - Laserfiche WebLink

Home Browse Search

1 injection) to the 8/10/00 event (approximately 3 months after HRC injec(ion): TCR declined from 1,000 to 478 ug/1; cis-DCE declined from 7,300 to 3,090 ug/I; and VC declined from 870 to 7,12 ug 1. Trans-DCE showed au insignificant increase from 71 to 78 ug/l over this time period. COC concentrations have shown a significant decrease since the HRC. injection. We conclude that the HRC has sucecssililly stimulated natural attenuation of COCl by reductive dechlorination. DISCUSS[ON Ole GEOCFIENUCAL DATA To provide au anaerobic state to support the development of anaerobic microorganisms,all oxygen and other electron acceptors(such as nitrate and sulfate)have to be consumed. This condition can be achieved by providing substrates such is lactic acid to the saturated zone. HRC is a source of lactic acid. Anaerobic microorganisms metabolize lactic acid to Carbon dioxide and water depleting electron acceptors. Subsequently, the oxidation reduction potential(ORP)shift's from a positive to a negative value. Fermentative microorganisins and the HRC-derived lactic acid, in Conjunction, ton'm pyruvic acid and acetic acid. This decomposition process provides the hydrogen required for reductive dechlorination (replacing the chlorine in the COCl with hydrogen). Naturally occurring microorganisms capable of reductive dechlorination then use the hydrogen to progressively remove chlorine atoms from chlorinated hydrocarbon contaminants (i.e., in a simplistic sense,to convert PCI:to TCE to DCE to VC to ethene or ethane)(Wiedctncier ct al., 1996). 'fable I presents pertinent grotuulwater geochemical data collected from MW-3 and 10 since May I,2000(immediately prior to the HRC injection event). Analysis of perched groundwater samples in and around the source area indicate that the saturated zone is now in an anaerobic condition, which has been promoted by the injection of 1TRC. ORP measurements range from --307 millivolts (mV) in source well MW-3 to -39 mV in MW-10, and ORP has reduced significantly in MW-3 and MW-10 since the application of TTRC. An ORP of less than 50 mV is required for effective reductive dechlorination, with values of less than—100 mV being optimal(Wiedcmcier et al., 1996). Ili general,the ORP will reduce only when the electron ncccptors (nitrates and sulfates, for example) are consumed. Furthermore,the presence of low quantities of methane in monitoring wells in and around the source area indicates that the aquifer is slightly methanogenic(indicative of anaerobic conditions). Elevated levels of HRC-derived metabolic acids(lactic, propionic, pyruvie, butyric, and acetic acids) in MW-3 and MW-10 indicate that these acids have been ef'tectivcly released into the shallow aquifer and are being utilized to generate hydrogen within the aquifer to stimulate reductive dechlorination. Permanent gases ethane and ethene have been detected in source well MW-3. The presence of ethane and ethene indicates that the reductive dechlorination pathway his proceeded through DCE and VC to the end-product ethene and/or ethane. Gcochcnnical sampling results indicate conclusively that tine HRC injection has been successful in stimulating the reductive dechlorination process.