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Recommendations
<br /> Extent of plume
<br /> Monitoring points are needed downgradient of the site,and the vertical extent of the plume
<br /> directly beneath the source area needs to be determined. A work plan is currently being
<br /> drafted to address these issues.
<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.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 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.
<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 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 /> -90, SVE-South,and SVE-North be sampled during the next monitoring event.
<br /> Monitoring wells MW-DIS-3,-4,-5,-6,-7S,-71),-8S,-813,-9,and-10 will not be sampled.
<br /> We propose to perform one additional monitoring event for 1999,scheduled for late October
<br /> to early November.After the results of that monitoring event have been analyzed, the
<br /> monitoring program shall be re-evaluated.
<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 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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