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Mr. Peter MacNicholl <br /> 6/12/2007 <br /> Page 2 <br /> Background <br /> The site is a logistics depot located near the center of the San Joaquin valley in central <br /> California, and covers about 908 acres. The site has about 65 feet of relief, sloping <br /> from 110 feet msl in the southwest to about 45 feet msl to the northeast. The base <br /> occupies former agricultural land west of the San Joaquin River. Contaminants at the <br /> site chiefly consist of the VOCs PCE and TCE. The groundwater model will be used to <br /> analyze capture zones of extraction wells, optimize existing groundwater extraction <br /> systems, and predict cleanup times. <br /> Soils and geology at the base consist of about 200 feet of alluvial-fan and interfan <br /> sediments of the Upper Tulare Member of the Tulare Formation, derived from erosion of <br /> the Diablo range to the west. These sediments contain local linear channel deposits of <br /> coarser sediment, and more extensive flood and overbank deposits of finer-grained <br /> material. The base of the alluvium at about 200 feet BGS is a 100-foot thick fine- <br /> grained unit that is identified as the Corcoran Clay, a lagoonal or lacustrine deposit. <br /> The model has been divided horizontally into six layers that do not represent <br /> hydrostratigraphic units, with a basal no-flow boundary. It is assumed that the alluvium <br /> represents effectively one hydrostratigraphic unit. Data input for the model included <br /> about 130 aquifer and slug tests, pumping rates for wells where known, assumed <br /> vertical recharge rates, and calculated subsurface inflow and outflow rates. <br /> Specific Comments <br /> Section 2. <br /> 1. Section 2.3.3.3 Figures 2-6 and 2-7 suggest that there are some identifiable <br /> areas of substantial vertical flow and other areas where little occurs. Vertical <br /> downward gradients are indicated both above and below the Below Lower <br /> Geologic Horizon. Near LM055B, the sections show substantial upward flow <br /> through a clay horizon, indicating the clay horizon is not a good aquitard. The <br /> south half of the Upper geologic horizon shows substantial vertical flow within the <br /> model zone. Perhaps it would be better to describe the system as a somewhat <br /> haphazard gradational system grading from unconfined to confined with some <br /> local communication between layers. While the Corcoran clay probably allows <br /> orders of magnitude less vertical flow than the other intervals modeled, it too may <br /> contain breaches, natural and manmade within the model domain. The section <br /> should be revised. <br /> 2. Section 2.3.4.1 While agricultural pumpage has slowed or ceased near the base, <br /> municipal pumpage with similar temporal patterns has increased. As stated <br /> above, even though most offsite pumping is from beneath trio Curcoran, the <br />