SITE INFORMATION AND CORRESPONDENCE

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Concentrations of nitrate ranged from 1,700 ug/l in well MW-5 to 69,000 ug/l in well MW-4(Table 2). Figure 4 shows the October 2011 nitrate concentrations in groundwater. Petroleum hydrocarbons (TPH-g, TPH-d, BTEX, and MtBE) were analyzed at MW-1 due to its proximity to the former UST. MtBE was also analyzed downgradient of the former UST at well MW-5. In well MW-1,TPH-d and TPH-g were reported at 260 and 140 ug/l,respectively. Benzene was reported at 1.2 ug/l. Toluene,ethyl benzene,total xylenes, and MtBE were not detected above reporting limits. MtBE was detected near the reporting limit in well MW-5 at 0.55 ug/l (Table 2, Appendix Q. The sample from well MW-3 was analyzed for b-BHC, a member of the organochlorine class of pesticides(OCPs),and was not detected above the reporting limit of 0.10 ug/l(Table 2,Appendix B). 4.2 GROUNDWATER MONITORING TRENDS The shallow groundwater flowed primarily southeast beneath the site through most of 1999 (IT, 2000). During the April 2002 monitoring event(HydroGeoLogic,2002)groundwater flow direction was consistent with previous years. In the October 2002 event (HydroGeoLogic, 2003), groundwater flow direction was more southerly,but in March 2004,was interpreted to be flowing in a westerly direction (Karsyn, 2004). JDS interpreted the flow directions to be south-southwest in October 2004, and east-southeast in October 2005 and 2011. These shifts in water table flow direction are not well understood. The site's water table does not contain abundant water as shown from pump test results (IT Corp., 1999), and it exhibits a relatively flat gradient. These characteristics likely make the flow direction sensitive to any regional influences such as changes in surface drainage and infiltration patterns, or possibly tidal influences from the nearby Port of Stockton. Trends in groundwater elevation at the site are shown in Figures 5 and 6. The site has had periodic monitoring since 1989. Figure 5 depicts data from the more recent monitoring events conducted,and Figure 6 illustrates all historic data combined. Both Figures exhibit the strong seasonal variation of water levels that correspond to the wet and dry seasons indicative of the region. In the late 1980's and early 1990's, groundwater elevations ranged from-8 feet to-14 feet relative to mean sea level (feet msl), rising to nearly 0 feet msl (sea level) in the late 1990's. The large rise in shallow groundwater elevation(>10 feet)at the site between the late 1980's and late 1990's is consistent with observations made throughout the region and is likely a result of El Nino weather patterns that brought several years of above-normal precipitation. The October 2011 groundwater elevations were higher than those observed in the Fall of 2002,2004, and 2005, and consistent with those observed in the late 1990's. Nitrate Relative to October 2005,the concentration of nitrate in groundwater decreased significantly in every well tested except MW-1, which exhibited a modest increase. Based on the predominant east- southeast groundwater flow direction,and that relatively higher nitrate concentrations occur at wells MW-3 and MW-4,it is feasible that off-site nitrate continues to impact the site. Figure 7 depicts the nitrate concentrations with time in selected wells. MW-1 and MW-7 were not plotted. cdwn 4go-l1 5 JD.m h cm wmg MIM I