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Wnght Environmental Services,Inc 3.0 Site Stratigraphy Overview 9 The site drilling revealed geology and stratigraphy similar to that revealed in previous exploratory bonngs Complexly interbedded sandy silt, clayey silt and thin discontinuous beds of sand, cemented sand and, clay and silt beds underlie the site (see Figures 1 and 2) These deposits are interpreted as and mixed braided/meandering stream and flood plain deposits Numerous thin discontinuous stream channels are interbedded with and cut through the fine- grained floodplain deposits These vertical and lateral facies changes within the aquifers and aquicludes create complex interbedding interpreted to have variable degrees of hydraulic conductivity A few "clean" sandy beds are probably channel deposits, otherwise, much dispersed fine-grained sediment occurs in aquifers and aquicludes As a general rule these °clean sandy„ channel deposits do not appear widespread in the area 4.0 Regional Hydrogeology Regional groundwater level variations have occurred in the past, with an overall rise in recent years The San Joaquin County Flood Control and Water Conservation District, San Joaquin County Planning Department, collects countywide groundwater data and prepares regional groundwater maps (see Lines of Equal Depth to Water In Wells, Fall-1974 and Spring-1999) A review of these groundwater contour maps shows that the regional levels of groundwater have risen about 30 feet (from a depth of about 80 feet to a depth of about 50 feet) over that 25-year period Although there is no specific on-site data it is assumed that the groundwater levels could have risen under the site on the order of tens of feet The current depth to groundwater in site monitoring wells (Fall 2003) is about 41 to 43 feet bgs The historic regional groundwater flow direction is west-southwest although groundwater contours become complex in the site area (Spring 1999 data) and would be expected to show different or variable local groundwater flow directions, such as that observed on-site 5.0 Site Hydrogeology and Hydrostratigraphy The aquifer strata that have been mapped in previous studies by Ramage are enlarged by the work presented in this report On the basis of the information collected to date, the site stratigraphy is grouped together into two aquifer strata and two aqulclude strata (hydrostratigraphy) present under the site The uppermost aquifer is termed the A-aquifer and the underlying aquifer is termed the B-aquifer for this report (see Figures 1, 2 and 3) The interpreted aquifer contacts are overlaid upon the geologic cross sections to show both grouping of strata and its internal complexity within aquifers and aquicludes The overall site hydraulic conductivity is interpreted as moderately low A-Aquifer The A-aquifer occurs at depths of between about 45-48 feet to about 60 The aquifer is composed of clayey silt, silt and thin beds of sand and silty sand The A-aquifer appears to thicken slightly to the east and contains interbeds with clay and clayey silt The aquifer . contains much dispersed silt and clay In all strata that in interpreted to lower overall hydraulic conductivity This aquifer appears to be weakly confined to unconfined Stefano's-CPT Report Page 4 of 9 I