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Human Health. Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br />" Tracy, California <br /> as the soil sol t <br /> solution remains liquid (Huddleston and Cresswell, 19 t,). The highest rate of <br /> degradation apparently occurs at soil temperatures above 20" C (Dibbl;, and Bartha, 1979). <br /> Oxygen is often a limiting factor in biodegradation. It has been shown .hat biodegradation rates <br /> ' decrease as the oxygen supply is depleted (Huddleston and Cresswell, 1976. The aeration of <br /> soils is dependent on the amount of air-filled and water filled pore spa-,;s, or capillaries, and the <br /> ' size of the pores. If 100% of the available pore space is saturated w th water, then too little <br /> oxygen will be present for metabolization. While "water logging" of sc ill will reduce the oxygen <br /> content, some moisture content is necessary. A 10% capillary aqueous saturation is associated <br /> ' with marginal metabolic activity due to the reduction of available watt i (Atlas, 1984). Dibble <br /> and Bartha (1979) concluded that 20% capillary saturation is -.fficient for petroleum <br /> ' hydrocarbon biodegradation and found no detectable difference in degradation rates between 20 <br /> and 30% capillary saturation. <br /> ' Several studies (Vanloocke et al., 1975; Dibble and Bartha, 1979; Han-')ricke et al., 1980) have <br /> shown that the rate of biodegradation is higher in alkaline than acidic :61s. Jones et al., (1970) <br /> concluded that the role of fungi increases and the role of bacteria decreases with decreasing pH. <br /> Optimum soil pH conditions for fungal and bacterial biodegradation ranges from neutral to <br /> 1 slightly alkaline. <br /> The presence of nitrogen and phosphorous salts have been shown Co stimulate degradation <br /> (Kincannon, 1972; Verstraete et al., 1976; Dibble and Bartha, 1979). However, the degree of <br /> this effect and the time required to produce and effect is a matter of debate (Lehtomaki and <br />' Niemela, 1975; Jobson et al., 1974; Raymond et al., 1976). <br /> ' 3.1.3 Toxicokinetics <br />' The toxicokinetics of gasoline and diesel fuels are poorly characterized because of the complex <br /> composition of the petroleum mixtures. Therefore, the toxicokinetics of i;aaoline and diesel fuels <br /> are approximated through the indicator chemicals selected (benzene, ethylbenzene, toluene, and <br /> rxylenes). <br /> 30-0136-11 <br /> 3-3 <br /> ........... .. <br />