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i <br /> Lead scavengers EDB and EDC have been detected in monitoring wells MW-DIS-1,NP-1- <br /> 90',NP-1-80',NP-1-70', and SVE=South. <br /> Recommendations <br /> Extent of Plume <br /> Additional groundwater monitoring wells are needed to monitor the cross-gradient and <br /> downgradient extent of the hydrocarbon/1,2-DCA plume present at the Disco site. <br /> Frequency of sampling <br /> The Disco site has accumulated a large data set of groundwater sample results since <br /> monitoring began in 1991. Monitoring wells containing petroleum hydrocarbons have either <br /> stable or reducing concentrations. Monitoring wells outside of the dissolved hydrocarbon <br /> plume have been consistently non-detect. <br /> A Groundwater Monitoring Frequency Decision Tree,Figure 4,is presented to evaluate the <br /> i frequency of sampling for each monitoring well at the site,based upon the location of the <br /> well with respect to the plume and gradient direction.A similar decision tree has been <br /> approved by DTSC and the Central Valley Regional Water Quality Control Board for a <br /> PG&E Former MGP site, located in downtown Stockton. <br /> The decision tree organizes the monitoring wells as upgradient, downgradient, <br /> crossgradient,or within the plume as follows: <br /> • Upgradient wells:MW-DIS-4,-5,-6, -7S,-7D,-8S,and-8D. <br /> • Downgradient wells: Currently,no downgradient wells exist. <br /> • Crossgradient:MW-DIS-3,-9,and-10 <br /> • Within Plume:MW-DIS-1,-2 NP-1-70',0 -80 -90' SVE-South and SVE- r <br /> No th. <br /> Table 7 presents the recommended sampling frequency for all wells at the Disco site. Due to <br /> the recently discovered presence of 1,2-DCA,EDB,and TBA,we recommend that all wells <br /> be <br /> sampled for these compounds on a quarterly bans to assess the extent of these <br /> compounds at the site. Modifications to the sampling plan will be made after four rounds of <br /> sampling for these compounds. Monitoring wells MW-DIS-1,MW-DIS-2,NP-1-70,-80,-90, <br /> SVE-South,and SVE-North will continue to be sampled semi-annually for TPH-gas and <br /> BTEX compounds. <br /> Note:In order to continue the accurate assessment of the groundwater flow direction, <br /> water level measurements in all wells and evaluation of the groundwater gradient will <br /> continue to be performed on a quarterly basis. <br /> i ' <br /> I ' <br /> 4 <br />