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m <br /> WOM <br /> i <br /> unconfined aquifer may not be accurately represented when the <br /> water column of a monitoring well is recharged through a vertical <br /> interval of the aquifer where a contaminant concentration <br /> gradient exists. Thus, for hydrocarbon sampling, where maximum <br /> concentrations are near the top of the water table, the shortest <br /> 3 <br /> r <br /> feasible screened interval should provide samples most represen- <br /> tative of the maximum contamination concentration (Robbins 1989 ) . <br /> n <br /> SJCEHD has recommended that new groundwater monitoring wells be <br /> installed to provide for continued quarterly monitoring of <br /> groundwater beneath this site. If, over the wet winter season, <br /> groundwater levels do not. increase enough .to permit proper <br /> sampling of the 60-foot wells MW3, MW4, and MW5, Exxon will <br /> f submit a work plan to install a. new monitoring well or wells. <br /> 1 .2.2 Concerns Expressed by the Regional Water Quality Control <br /> Board Central Valley Region <br /> The specific,,concerns expressed in <br /> p the RWQCS letter of 28 jek <br /> September 1989 (Appendix,A) are as follows: <br /> 1 . The extent of groundwater contamination cannot be deter <br /> ( mined because groundwater levels are too low to permit <br /> proper.'sampling of some wells. ! <br /> ;- <br /> . 2... . The groundwater gradient direction reported from `past <br /> gauging data may not be valid if the wells do not all <br /> intercept the same aquifer. <br /> 3 The observed decrease in groundwater contaminant concen- <br /> trations with time may not be .due to 'hydrocarbon degra- <br /> dation, but may be Niue to improper sampling. <br /> p 4.. Analysis of well headspace vapor samples indicates that <br /> hydrocarbons are present .in. the unsaturated zone soils. <br /> e35/71050/2-90/tx 4 <br />