Groundwaters from sand aquifer

A half-life of about 40 days was reported for hexachloroethane in an unconfined sand aquifer (Criddle et al. 1986). Laboratory studies with wastewater microflora cultures and aquifer material provided evidence for microbial reduction of hexachloroethane to tetrachloroethylene under aerobic conditions in this aquifer system (Criddle et al. 1986). In anaerobic groundwater, hexachloroethane reduction to pentachloroethane and tetrachloroethylene was found to occur only when the water was not poisoned with mercury chloride (Roberts et al. 1994). Pentachloroethane reduction to tetrachloroethylene occurred at a similar rate in both poisoned and unpoisoned water. From these results, Roberts et al. (1994) suggested that the reduction of hexachloroethane to tetrachloroethylene occurred via pentachloroethane. The first step, the production of pentachloroethane, was microbially mediated, while the production of tetrachloroethylene from pentachloroethane was an abiotic process. 

Two of the larger LNAPL hydrocarbon occurrences, site No. 1 and 4 (see Ligure 12.23), formerly reinjected coproduced groundwater into generally the same hydros-tratigraphic zone from which it is withdrawn site No. 1 reinjected without treatment into the Gage aquifer, whereas site No. 4 reinjected into the Old Dune Sand aquifer. Because of the presence of dissolved hydrocarbons, notably benzene, in the coproduced water that is typically returned to the aquifer during LNAPL recovery operations, immediate application of the EPA toxicity characteristic rule may result in classification of the reinjected water as disposal of a hazardous waste. This, in turn, would terminate use of UIC Class V wells (which many of these operations currently... 

East and Gulf coasts. In the Potomac-Raritan-Magothy Formation of coastal plain New Jersey, for example, Pco, decreases in the direction of flow from about 10 to 10 bar while the pH rises from 5.8 to 8.0 over a 19 km distance downdip (Langmuir 1969). (See Fig. 5.3.) Chapelle (1983) and Chapelle and Knobel (1983) also note a pH increase and COj decrease with groundwater flow in the coastal plain Aquia aquifer of Maryland. The aquifer is chiefly composed of quartz sand (-55%), glauconite clay (-30%), and carbonate shell material (-8%). Over a downgradient-flow distance of about 64 km, the groundwater pH increases from about 7.2 to 8.9, and the CO2 pressure drops from about 10 to as low as 10 bar. 

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