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January 8, 1998 �, ► Pare 4 • health UNOCAL's proposed installation of an additional monitoring well (GS1 work plan, October 9, 1997 - Attachment D) is consistent with this approach in that the well will further define the extent of migration of hydrocarbons toward the nearest downgradient water-supply well It is PEG's opinion that additional monitoring wells screened in discrete intervals are not required to fully define the vertical and lateral extent of the hydrocarbon plume With the installation of the additional downgradient well, as proposed in the GSI work plan, the lateral extent of the plume should be defined All existing wells are screened across the entire upper portion of the saturated zone and are able to detect migration of hydrocarbons throughout the impacted soil zone It is PEG's opinion that existing and proposed wells are sufficient to characterize both the vertical and lateral extent of the hydrocarbon plume The soils beneath the site are stratified with thick clay layers between intermittent sand zones (see cross-sections in Attachment B) With the lack of any nearby water-supply wells, no strong downward hydraulic gradients are anticipated It has been PEG's experience that hydrocarbons generally do not migrate vertically, but rather are restricted to lateral movement in the uppermost water-bearing zone Additionally, at UST sites where PEG has installed monitoring wells in a deeper water-bearing zone or in a deeper portion of the shallowest water-bearing zone, hydrocarbons were not detected, or present at low concentrations PEG does not see any benefit of further definition of the extent of hydrocarbon-impacted • soil, the majority of which is currently beneath the groundwater table The central issue is the ability of these hydrocarbons to impact groundwater Quality The mobility of these hydrocarbons can be evaluated through the existing groundwater monitoring system Drilling of soil borings will not produce any additional meaningful information on the _ risk the hydrocarbons pose to groundwater resources The long distance to the nearest downgradient water supply well leads to the conclusion that the risk to groundwater resources is low It is PEG's opinion that performance of sparge/vent pilot remedial studies is unnecessary There are no data suggesting that active remediation is required for protection of human health or groundwater resources Existing data show that hydrocarbon migration through the shallow groundwater has been limited Data also show that hydrocarbon concentrations are decI ring Even the recent detection of SPH in Well UV-1 does not change the fact that the downgradient extent of the plume appears to be Iess than 200 feet, a distance typical of most: hydrocarbon plumes The additional groundwater monitoring proposed by GSI will provide data to assess the plume stability and risk to groundwater resources O The 17 ai mw 111 wh1ch flee 1)efifione7 15 crgg7 ieved Actions required by PHS/EHD will result in expenditure of significant funds PEG . estimates the cost of required actions to be approximately $50,000 This cost was based I I O I SVAH)i 87REVIEW