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Except for PCI., there are no California State of U.S. AFtARs for <br /> soils. The 'central Valley Regional Water Quality board has a <br /> recommended approach for estimating soluble threshold limit con- <br /> centrations (STLCs) in soils (DHS 1986) . An STLC is an accept- <br /> able limit of contaminant in soil which will protect the quality <br /> of surface and ground waters. It is defined as 100 times the <br /> drinking water criterion. Urging the U.S. MCL for benzene and <br /> MCLGs for toluen- and xylene (Table 6) the STLCs for these com- <br /> pounds are 0 .5 mg/kg, 200 mg/kg and 44 mg/kg, respectively. <br /> 3.2 FATE AND TRANSPORT EVALUATION <br /> 3.2. 1 Mechanisms of Attenuation <br /> In ground water a number of mechanisms, including biodegradation, <br /> volatilization, and sorption, will serve to attenuate the <br /> dissolved hydrocarbons (API 1985b) . Fuel hydrocarbons, <br /> represented here by benzene, C6H6, are readily biodegraded under <br /> aerobic conditions (Raymond et al. 1976) . Stoichiometrically, <br /> i this degradation occurs according to the following reaction: <br /> C6H6 + 7-1/2 01 --> 6CO2 + 3H2O <br /> Complete oxidation requires an oxygen/hydrocarbon ratio (by <br /> weight) of approximately 3/1 . Typically, following a subsurface <br /> spill the first hydrocarbons to contact ground water rapidly <br /> deplete the available oxygen, resulting in anaerobic conditions. <br /> Assuming an initial oxygen content of 8 mg/l, approximately 2.6 _ <br /> mg/L of hydrocarbons may be degraded in this fashion. <br /> Anaerobic biodegradation of fuel hydrocarbons has recently been <br /> documented both in field and laboratory research (Battermann and <br /> Werner 1984; Schink 1985; Choteau et al. 1982; Kuhn and Zeyer <br /> 1986; Vogel and Grbic-Galic 1986; Wilson et al. 1986) , although <br /> generally at rates lower and somewhat less predictable than those <br /> characterizing aerobic biodegradation (Healy and Daugherton 1986) . <br /> 16 <br />