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0 ■ <br />-JjZrWMA <br />Working To Restore Nature <br />(CH) was observed between approximately 1 i and 16 feet in boring MW3 Copies of boring <br />Iogs are included in Attachment 1 <br />SOIL EXCAVATION <br />In December 1993 approximately 1400 cubic yards of hydrocarbon impacted soils were <br />excavated from the vicinity of the former fuel storage tanks The extent of excavation is <br />depicted on Figure 3 Laboratory analysis of confirmation samples collected subsequent to <br />excavation indicated that residual petroleum hydrocarbon impacted soils remained along the east <br />wall of the pit beneath the existing structure, between approximately 6 and l I feet below grade <br />at its thickest <br />Assuming that the impacted zone is uniformly 4 feet thick, it was estimated that 7 5 pounds (I 1 <br />gallons) of diesel fuel remains in the soil The calculations used to determine the volume of <br />impacted soil and the amount of diesel within the soil are provided in Attachment 2 Similar <br />calculations for benzene, toluene, ethylbenzene, xylenes, and gasoline fuel are also included <br />The estimated extent of residual diesel fuel remaining in the soil is shown on Figure 4 The <br />estimated extent of residual benzene remaining in the soil is shown on Figure 5 Based upon <br />analytical results of confirmation soil samples summarized in Table 1, the distribution of residual <br />toluene, ethylbenzene, xylenes, and gasoline fuels in the soil is similar to that of benzene (Figure <br />3) <br />'40 FATE OF CONTAMINANTS <br />The concentration of a substance in groundwater (Cw) can be predicted by the concentration in <br />soil (Cs) with which it is in contact This is accomplished by use of the soil distnbution <br />coefficient (Kd) (Hounslow, 1990) Distribution coefficients are substance specific, unitless <br />relationships defined as <br />Kd = Cs/Cw <br />The larger the value of Kd the greater the tendency of a substance to remain within the soil <br />Kd values are expected to increase with increasing organic carbon and clay content in the soil <br />One study (EPA, 1978) directly determined soil distribution coefficients for eleven organic <br />compounds in sediment from Coyote Creek in Santa Clara County, California The organic <br />carbon content of the sediment was 1 4%4, similar to that in the Jacktone Clay at the Comgan <br />site Utilizing the experimental data, a relationship between the soil distribution coefficients for <br />organic substances and their respective solubilities was derived <br />log Kd = 5 35 - 0 66 og (ol (ppb) <br />150054\11 -AD -494 FHL 2 <br />�W_ <br />