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Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California 3 GEOLOGY AND HYDROGEOLOGY 3.1 LOCAL GEOLOGIC INFORMATION The Site lies at an approximate elevation of 16 feet above mean sea level in the northern San Joaquin Valley. McLeod Lake is located 1,700 feet northwest of the Site. The lake drains to the Stockton Deep Water Channel and the San Joaquin River. Subsurface investigations were conducted at the Site from 1988 to 2019 for the assessment of the lateral and vertical extent of petroleum hydrocarbons historically released from former Site operations. The soil beneath the Site was collected using hollow stem auger drill rigs, and the lithology was described by a geologist based on visual observations with the exception of recent investigations, which were drilled using direct push methods. Boring logs were created for all 12 groundwater monitoring wells installed at the Site and from two soil borings(BH-13 and BH-14). Soil borings BH-1 through BH-12 were drilled to groundwater(ranging from 25 to 40 feet bgs)to estimate the extent of contamination prior to the excavation of impacted soil in 1989 and were not logged for lithologic descriptions. Soil beneath the Site is comprised of interconnected layers of sand, silt, and clay. The Site is generally underlain by clay to a depth of approximately 14 feet bgs, silt to 18 feet bgs, and sand to 48 feet bgs. Silt and sand layers separated by clay are located beneath the Site from 54 feet to 105 feet. Three groundwater zones were encountered while drilling the deepest well at the Site (MW-10). The shallow water table groundwater zone is located in the upper sand layer, and an intermediate water zone is located in a silt layer between 54 and 78 feet bgs separated from the sand layer above by 6 feet of clay. However, the upper and intermediate groundwater zones are contiguous at well MW-7 located downgradient from the Site.The lower groundwater zone is located between 74 and 105 feet bgs and is separated from the intermediate zone by 5 feet of clay. The lower groundwater zone was not logged in boring BH-14 located 40 feet to the southwest. The lower groundwater zone appears to be semi-confined based on rising groundwater levels measured in the well shortly after removing the well plug; however, the static water level in well MW-10 is similar to those in other nearby monitoring wells. Boring logs from selected wells and borings (shown on Figure 2-2) were used to create a geologic 3-1 Peb,v,n,r 2021