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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. 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 /> j A Groundwater Monitoring Frequency Decision Tree,Figure 3,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. As shown on the decision tree,
<br /> quarterly groundwater samples will continue to be collected on those wells within the
<br /> I plume (MW-DIS-1,-2,NP-1-70',-80',-90',SVE-South,and SVE-North), at least through 1999
<br /> i (includes two more monitoring events). At that time, the monitoring program shall be re-
<br /> evaluated.
<br /> Note:In order to continue the accurate assessment of the groundwater flow direction,
<br /> water level measurements and evaluation of the groundwater gradient will continue to be
<br /> performed on a quarterly basis.
<br /> The decision tree organizes the monitoring wells as upgradient,downgradient,
<br /> crossgradient,or within the plume as follows:
<br /> i • Upgradient wells: MW-DIS-4,-5,-6,-7S,-713,-8S,and-81).
<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 6 presents the recommended sampling frequency for all wells at the Disco site. Based
<br /> on the decision tree,we recommend that only wells MW-DIS-1,MW-DIS-2,NP-1-70,-80,
<br /> and-90 be sampled during the third quarter 1999.
<br /> Oxygenate Sampling; Del Monte proposes to continue oxygenate testing through the end of
<br /> 1999 for all monitoring wells within the plume (MW-DIS-1,-2,NP-1-70',-80',-90',
<br /> SVE-North, and SVE-South). If oxygenates are not detected in these sampling events,we
<br /> propose to discontinue testing for oxygenates.
<br /> As summarized above,TBA has not been confirmed in any monitoring well at the Disco site.
<br /> It has been detected once,in five different wells. Based on the period of operation of the
<br /> tank,we believe it is unlikely that additives or oxygenates were present in the gasoline
<br /> released from the Disco site.
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