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stable or reducing concentrations.Monitoring wells outside of the dissolved hydrocarbon <br /> plume have been consistently non-detect. Given the demonstrated stability of the <br /> groundwater flow direction and the consistent analytical sample results of existing <br /> monitoring wells,we recommend that the frequency of sampling be reduced.for select <br /> monitoring wells at the Disco site. <br /> A Groundwater Monitoring Frequency Decision Tree,Figure 4,is presented to evaluate the <br /> 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,-713,-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',-80',-90',SVE-South,and SVE-North. <br /> Table 7 presents the recommended sampling frequency for all wells at the Disco site. Based <br /> on the decision tree,we recommend that wells MW-DIS-1,MW-DIS-2,NP-1-70,-80,-90, <br /> SVE-South,and SVE-North be sampled semi-annually.During 1999,these wells were <br /> sampled on a quarterly basis. However,given the demonstrated stability of the <br /> groundwater flow direction and the consistent analytical sample results for these <br /> monitoring wells,reducing the sampling to semi-annual will provide sufficient information <br /> for assessing the extent and concentration trends of the plurne. <br /> Semi-annual groundwater sampling events will be scheduled in the spring(April) and fall <br /> (October)to ensure the events occur during periods of highest and lowest groundwater <br /> elevations. <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 /> Oxygenate and Additives:We propose to continue testing for oxygenates, and additives <br /> EDB and 1,2-DCA by EPA Method 8260 next sampling event. <br /> I <br /> 4 <br />