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i <br /> confined conditions within this unit. Occasional minor perched water zones are encountered in this <br /> formation, particularly in the Mokelumne River.area near Lodi and northern Stockton. <br /> Mehrten Formation.This formation is exposed in the easternmost part of the subbasin where it forms <br /> readily identifiable,nearly flat-topped hills. The formation is late Miocene to Pliocene in age and is <br /> composed of moderately to well indurated andesitic sand to sandstone interbedded with conglomerate, <br /> tuffaceous siltstone, and claystone (Wagner and others, 1990). The Mehrten Formation is <br /> approximately 440 feet thick in eastern surface outcrops s to over 600 feet thick in the.subsurface near <br /> Ilf PP Y P <br /> Stockton. It is reported to be 1,300=L feet thick at McDonald Island(Page, 1986). <br /> The top of the Mehrten Formation occurs at depths of approximately 800 to 1,000 feet in the Stockton <br /> area. Regional studies.indicate that Mehrten Formation sands commonly yield on the order of 1,000 <br /> gpm from wells. The formation appears to be semi-confined at least locally in the Stockton area,due <br /> to the inferred extensive fine-grained beds in its upper part. <br /> Groundwater Level Trends'Measurements over the past 40 years show a fairly continuous decline in <br /> groundwater levels in Eastern San Joaquin County (San Joaquin County Flood Control and Water <br /> Conservation District,2008). Groundwater levels have declined at an average rate of 1.7 feet per year <br /> and have dropped as much as 100 feet'in some areas. It is estimated that groundwater overdraft <br /> during the past 40 years has°reduced storage in the basin by as much as 2 million acre feet. Due to the <br /> continued overdraft of groundwater within.the subbasin, significant groundwater depressions are <br /> i present below the City of Stockton and east of Stockton (SJCFC&WCD, 2008). Several of these <br /> groundwater depressions extend to depths of about 100 feet below ground surface(or.more than 40 <br /> feet below mean sea level). <br /> Groundwater levels near the ST&E site may have actually risen since 1999 as several wells in this <br /> area monitored by the Northeastern San Joaquin County Groundwater Banking Authority (2004) <br /> a have shown recent rises ofthe groundwater surface on the order of five to eight feet or more since <br /> i <br /> 1999. <br /> Env ironmenta]\STE\PARS CR09102008.wpd 16 <br /> f <br /> I <br /> 'E <br />