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1 , 04/05/2000 14:43 209-579-222 MODESTO ATC PAGE 03 <br /> The detectable concentrations of TEX and TPH-D in the <br /> groundwater samples collected from monitoring wells MW3 and <br /> MW4 indicates groundwater has been impacted by the leakage <br /> of diesel product. The presence of dissolved hydrocarbons <br /> in the down groundwater gradient well MW3 and the absence in <br /> upgradient well MW1 is consistent with what would be <br /> expected based on the current groundwater flow conditions. <br /> Using the period of time since source mitigation (about 1. 5 <br /> years) and the distance of contaminant migration ( 58 feet to <br /> MW3) the approximate minimum rate of migration (ignoring <br /> retardation) is 0.1 feet per day. Given the groundwater <br /> gradient, this is a reasonable rate for silty soil types as <br /> encountered in the saturated zone at the site. The non- <br /> detectable concentrations of dissolved hydrocarbons in well <br /> MW2 suggests the, contaminant plume has not migrated towards <br /> the on site irrigation well. As with the soil samples, the <br /> wells MW1 and MW2 define the extent of groundwater <br /> contamination to the northwest and southwest of the tank. <br /> The non-detectable levels of benzene and low concentrations <br /> of toluene, ethylbenzene and xylene in the water sample from <br /> MW4 indicates, 1 ) , the original product had low <br /> concentrations of BTEX or, 2 ) , degradation of the more <br /> volatile hydrocarbon fraction (BTEX) has taken place. The <br /> BTEX concentrations in groundwater samples from all four <br /> wells are significantly below the Maximum Contaminant Level <br /> (MCL) for drinking water established by the State Department <br /> of Health Services (DHS) . No MCL exists for TPH-D although <br /> nuisance conditions related to taste and/or odor would have <br /> to be considered. The concentration of fecal coliform in <br /> all four wells exceed state water quality standards <br /> Indicating poor water quality and restricted beneficial use <br /> already exists at the site. <br /> Based on the data collected to date, soil and groundwater <br /> have been impacted by varying concentrations of diesel <br /> hydrocarbons. The extent of soil contamination below 200 <br /> ppm has only been defined in the southerly direction (Mwl ) . <br /> The extent of groundwater contamination to non-detectable <br /> levels has been completed to the northwest and southwest of <br /> the tank (MW1 and MW2) . The nearest surface water resource <br /> Of beneficial use is Smiths Canal located approximately 150 <br /> feet south of the diesel tank. Degradation of this resource <br /> diesel hydrocarbons is not threatened because 1) , under 1 <br /> current groundwater flow conditions the river lies up <br /> gradient of the contamination and 2 ) , analysis of a <br /> groundwater sample, MWI, collected between the river and the <br /> tank did not detect the presence of hydrocarbon <br /> constituents. The closest known groundwater well, the—=- <br /> site <br /> he—off site irrigation well, is to be tested but shouldcontain <br /> diesel hydrocarbon based on the current site conditions. <br /> This well, is scheduled to be abandoned per county <br /> requirements, if not impacted by diesel or coliform <br /> contamination. <br /> 6 <br />