SR0085504 - Laserfiche WebLink

Home Browse Search

Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California GEOLOGY AND HYDROGEOLOGY cross-section (shown on Figure 3-1). Analytical results from recently collected groundwater samples are also shown on the cross section. Groundwater elevations have historically ranged from approximately 20 feet bgs in 2020 to 50 feet in 1991. The original wells (MW-1 though MW-3) were installed in December 1988; however, the water table dropped 15 feet shortly thereafter, and deeper wells (MW-4 through MW-8) were installed in August 1991 as well as replacement monitoring wells (MW-2A and MW-3A) in 1995. The water table has rebounded 25 feet since 1991, with a small decline between 2001 and 2006. Seasonal fluctuations in groundwater elevations of less than 2 feet are apparent. The groundwater flow direction—calculated using water level data from MW-3A, MW-6 and MW-9—has historically flowed east to northeast at a gradient ranging from 0.0015 to 0.004 feet/foot. The groundwater flow direction during the recent period of high potentiometric surface elevations has been northeast. A groundwater elevation contour map from the 4th quarter 2020 monitoring event is shown on Figure 3-2. A tidal study was conducted on well MW-4 on 9 November 1991 where a 0.10-foot decrease in elevation was observed between high tide and low tide over a 4.5-hour period. The tidal change recorded at the port of Stockton was a decrease of 1.7 feet. A buried stream channel leading from McLeod Lake, located 0.33 mile northwest of MW-4, is likely the tidal hydraulic conduit. A historical map of Stockton from 1849 was reviewed and shows a former slough connecting McLeod Lake to Mormon Slough running through the HSA property. Tidal variations are not believed to have any measurable impact on contaminant transport at the Site. The hydraulic conductivity of the shallow groundwater zone was estimated by conducting slug tests in wells MW-4, MW-6, and MW-8 in October 1991. The rising head data were analyzed using Aqtesolv software to determine the hydraulic conductivity. Wells MW-4 and MW-6 had similar hydraulic conductivity of 0.00 15 foot/minute for the silt aquifer material. Well MW-8 had a slightly greater hydraulic conductivity of 0.002 foot/minute for the silt and sand aquifer material. 3-2 February 2021