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Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California PREVIOUS INVESTIGATIONS Groundwater monitoring well MW-1 was destroyed during the construction of the HSA building in 1989. In 1989, a 500-gallon fuel oil UST was removed from between Market Street and the northern portion of the HSA property. Soil samples collected from the ends of the tank did not contain detectable concentrations of petroleum hydrocarbons. The former UST is shown on Figure 1-2. In 1991, a groundwater investigation included the installation of five additional monitoring wells(MW-4 through MW-8),aquifer testing,and a tidal influence study.Dissolved petroleum hydrocarbons were detected in all five new monitoring wells. Subsequent quarterly groundwater monitoring results indicated that the highest concentrations of dissolved hydrocarbons were present in monitoring well MW-8, which was located on Market Street near USTs owned by the City of Stockton. The locations of the 1991 groundwater monitoring wells are shown on Figure 1-2. In 1995, two deeper monitoring wells (MW-2A and MW-3A) were installed to replace wells MW-2 and MW-3,which went dry due to a drop in the regional groundwater table.Well MW-3 was abandoned.During the installation of monitoring well MW-2A,free product was observed in soil samples and a sheen was observed in groundwater samples. Petroleum hydrocarbons were not detected in soil or groundwater samples collected from monitoring well MW-3A. Groundwater monitoring wells installed in 1995 are also shown on Figure 1-2. In 1996, a soil and groundwater investigation was conducted in the vicinity of MW-2A, and soil boring BH-13 was drilled to assess the vertical extent of soil contamination below the groundwater table. Groundwater monitoring well MW-9 was installed in the parking lot across Washington Street and south of the former USTs to assess groundwater conditions cross- gradient of well MW-2A. The investigation and groundwater monitoring confirmed residual contamination in the vicinity of monitoring well MW-2A and groundwater contamination was not present south of Washington Street. The location of monitoring well MW-9 is shown on Figure 1-2, and the location of soil boring BH-13 is shown in Figure 2-2. 2-2 Febraary 2021