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n = effective porosity <br /> h/I = gradient <br /> 3.7.6 REGIONAL WATER QUALITY <br /> The San Joaquin hydrologic basin has experienced overdraft for many years. In the <br /> Central San Joaquin Irrigation District, which encompasses Forward Landfill, water <br /> levels have dropped approximately 70 feet in the past 30 years. Declining water <br /> levels throughout the Stocleton area have induced the eastward movement of <br /> poor-quality water,f'rom-1 delta sediments,-A4igration of these saline waters had <br /> already impacted the utility of-groundwater as far east as Stockton at the time of a <br /> 1981 study by the U.S. Geological Survey (USGS) (San Joaquin County Flood <br /> Control and Water Conservation District, 1990). The San Joaquin County Flood <br /> Control and Water Conservation District has documented a decline in water <br /> quality for the Stockton area due to a general-increase in chlorides. <br /> The regional water quality in the area around Forward Landfill meets the State's <br /> Drinking Water Standard. The water is a calcium-magnesium/bicarbonate water <br /> type, with total dissolved solids (TDS) concentrations under 500 mg/I. <br /> 3.7.7 SITE GROUNDWATER QUALITY <br /> Groundwater quality at the existing Forward Landfill is summarized in the quarterly <br /> and annual monitoring reports submitted to the RWQCB and DISC. Generally, <br /> groundwater quality at the site is good. VOCs have been detected sporadically, <br /> and at low concentrations since groundwater monitoring began in 1987. Until <br /> their abandonment in 1993, VOCs were most commonly detected in monitoring <br /> wells MW-5A and MW=513, near the southern border of WMU B (Figure 7),..Since <br /> then, VOCs have been most commonly detected in the northeast corner of the <br /> existing Forward Landfill, in the main entrance facilities area. <br /> In September 2000, the RWQCB requested that Forward assess the source and <br /> potential impacts of low-level VOC concentrations in the groundwater adjacent to <br /> the landfill. The assessment was performed by Geologic Associates (GLA) and <br /> summarized in the Low-Level VOC Detections in Monitoring Wells letter report <br /> (February 2001). The report found that other than acetone and methyl chloride <br /> (common laboratory contaminants), VOCs were detected in samples from 8 <br /> monitoring wells during 1999 and 2000. VOCs were most commonly detected at <br /> Forward Landfill jTD 3-14 <br /> L:\AI l ied\2000.193\Reports\fi naljtd:Sec-3.0:08/20/01 <br /> BRYAN A.STIRRAT&ASSOCIATES <br />