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., N, �0 u The detectable concentrations of TEX and TPH-D in the., groundwater samples collected from monitoring wells MW3 and MW4 indicates groundwater has been impacted by the leakage; of diesel product. The presence of dissolved hydrocarbons in the down groundwater gradient well MW3 and the .absence in. upgradient well MW1 is consistent with what would LQ expected based on the current groundwater flow conditions. Using the period of time since source mitigation (about 1.5 years) and the distance of contaminant migration (58 feet to MW3) the approximate minimum rate of migration (ignoring retardation) is 0.1 feet per day. Given the groundwater gradient, this is a reasonable rate for silty soil types as encountered in .the saturated zone at the ,site. The non- detectable concentrations. ofdissolved hydrocarbons in. well MW2 suggests the contaminant plume has not migrated towards the on site irrigation well. As with the soil samples, the wells MW1 and MW2 define the extent of groundwater f contamination to the northwest and southwest of the tank. The non-detectable levels of Benzene and low concentrations of toluene, ethylbenzene and xylene in the water sample from MW4 indicates, 1), the original product had low concentrations of STEX or, 2), degradation of the more m; volatile hydrocarbon fraction (BTEX) has taken place. The STEX concentrations in groundwater samples from all four wells are significantly below the Maximum Contaminant Level (MCL) for drinking water established by the State Department of health Services (DHS). No MCL exists for TPH-D although nuisance conditions related to taste and/or odor would have to be considered. The concentration of fecal coliform in w all four wells exceed state water quality standards indicating poor water quality and restricted beneficial use already exists at the site. Based on the data collected to date, soil and groundwater have been impacted by varying concentrations of diesel hydrocarbons. The extent of soil contamination below 200 ppm has only..been defined in the southerly direction (MW1). The :extent of groundwater contamination to non-detectable levels has been completed to the northwest and southwest of the tank (MW1 and MW2); The nearest surface water resource of beneficial use is Smiths Canal located approximately 150 feet south of the diesel tank. Degradation of this resource diesel hydrocarbons is not threatened because 1), under current groundwater flow conditions the river lies up gradient of the contamination and 2), analysis of a groundwater sample, MW1, collected between the river and the tank did not _.detect the presence of _ _hydrocarbon constituents. The closest known groundwater well, the on- site on- site irrigation well, is to be tested but should contain -- diesel hydrocarbon based on the current site conditions. This well is scheduled to be abandoned per county requirements, if not impacted by diesel or coliform contamination. 6