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the western base of the Sierra Nevada. A characteristic of the San Joaquin Valley's groundwater basins l has been the tendency for water levels to vary, both seasonally and annually, over short periods of time. Seasonal fluctuations reflect the rainy and dry season. Rainy season infiltration raises the water table, and dry season pumping lowers the water table. The 1994 EIR reported that, in the project area, water levels ely 10 feet due to agricultural pumping demand and have dropped over the last 20 years by approximat ter bearing formation as approximately 70 feet below drought. The 1994 EIR identified the uppermost wa the base of the landfill,with the hydraulic gradient to the north-northeast. The beneficial uses of project area groundwater are domestic, municipal, and agricultural. A Solid Waste Water Quality Assessment Test(SWAT)in 1989 found a significant groundwater quality impact by regulated volatile organic compound(VOC)chemicals at contamination levels above the drinking water standard. The 1994 EIR reported the key VOC contaminants as tetrachloroethene(PCE), trichloroethene(TCE), cis-1,2- dichloroethene(DCE), and vinyl chloride (VC). VOC concentrations found in the downgradient monitoring wells included VC (11 pg/L), PCE(210 pg/L),TCE (39 pg/L), and DCE(36 pg/L). Supplemental sampling results indicated that VOC's were detected in CYA wells at levels equal to or slightly above drinking water action levels. However,confirmation sampling of the CYA wells in 1990 did not detect any VOC's. Sampling at a private well at 8106A Austin Road did find VOC's, and this was confirmed by further sampling. The City constructed a well between the well at 8106A Austin Road and the confirmed plume location in 1994. This well did not encounter the plume. Additional sampling and analysis were determined to be needed to determine if the plume extends to 8106A Austin Road. A Corrective Action Plan (CAP)was proposed by the City on April 24, 1991, to address the VOC impacts to groundwater. The plan was approved by the Regional Water Quality Control Board (RWQCB) staff on June 10, 1991. wo extraction wells were installed with the capacity to extract about 300 gallons per minute (gpm). The g tower. The treated water was discharged to surface extracted groundwaterwas treated through a strippin water(Little Johns Creek)under an NPDES permit. A 1994 finding of VOC contamination in an agricultural well located on CYA property adjacent to Austin I: Road, approximately 3,750 feet downgradient of the landfill, extended the suspected area of contamination. Since the 1994 EIR, various investigations, programs, and reports have been completed that have an it impact on the Austin Road Landfill environmental setting,impacts,and mitigation measures. VIII Groundwater flows in the project area are generally to the'north-northwest. The groundwater gradient is also reflective of the weather cycles and pumping, with a steeper north trending gradient in dry years and a more gentle northeast gradient in wet years (Kleinfelder, 1988; CDM, 1999). The dry year gradient is 'II apparently affected by greater drawdown in the area northeast of Stockton. Perched water may occur intermittently during the wet season, above relatively impermeable silts or clays in the sediment sequence. Hydrogeological date obtained from the adjacent Forward Inc. Landfill site indicate that vertical and horizontal transmissivities are highly varied, and there is substantial interconnection between upper and lower aquifers. The development of a regional cone of depression east of Stockton,in response to pumping of agricultural and municipal wells, indicates the connectedness of the regional aquifers. The flow directions and gradients are consistentwith the groundwater quality results collected during CDM's 1999 investigation. The plume of PCE and TCE trends in a northeasterly direction from the landfill. In addition, water contamination has been found at shallower depths near the landfill but at deeper depths further from the landfill, indicating that the plume is moving downward as well. The downward migration VOC's emanating from the landfill may be influenced by the location of sand layers described and also by vertical flow conditions. The VOC contaminant plume extends from the northern edge of the landfill I 0 feet to the northeast of the site, based on 1999 data(CDM, 1999A). This perimeter to approximately 4,00 750-foot distance reported in the 1994 EIR. The primary difference in the is only nominally more than the 0 feet ata is the indication that the plume has migrated deeper plume movement since the 1994 EIR summarized dI at the leading edge of the plume than appeared in the past. The primary contaminants of concern are still PCE and TCE, with concentration levels up to 69 and 48 pg/L, respectively,reported in CDM 1999A. The San Joaquin County GP-99-7, ZR-99-6, UP-99-17, ER-99-21City of Stockton Community Development Page 15 l