Laserfiche WebLink
Weston Solutions, Inc. <br /> Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California <br /> PREVIOUS INVESTIGATIONS <br /> 2.2 LOW-RISK CLOSURE REQUEST AND RECENT INVESTIGATIONS <br /> In May 2012, a Low-Risk Closure Request was submitted to EHD based on the following <br /> information: <br /> ■ The Site had been adequately characterized and the lateral and vertical extent of <br /> contamination has been defined. <br /> ■ No significant sources of petroleum hydrocarbons(USTs or free product)were present <br /> on-site. <br /> ■ Results from two monitoring wells (MW-2 and MW-2A) in the contaminant plume <br /> showed the petroleum hydrocarbons degrading overtime. However, 3,000 gg/L of <br /> benzene remained in groundwater beneath the Sutter Street sidewalk where <br /> contaminated soil was not removed during the 1989 excavation event. <br /> ■ The contaminant plume was stable and was not migrating based on results from 20 <br /> years of groundwater monitoring of downgradient wells. <br /> ■ The Site had been developed and paved with little surface water infiltration. <br /> ■ The Site was owned by San Joaquin County, and current land use did not warrant deed <br /> restrictions or institutional controls. <br /> ■ Modeling of benzene was below acceptable risk levels for subsurface vapor intrusion <br /> into buildings. <br /> ■ There were no nearby drinking water wells or other sensitive receptors. <br /> In September 2012,EHD requested the following concerns be addressed prior to giving further <br /> consideration of the closure request: <br /> ■ Demonstrate the downgradient extent (between wells MW-6 and MW-7) of impacted <br /> groundwater from MW-2, the most impacted monitoring well. <br /> ■ Provide an estimate of the residual dissolved and sorbed contaminant masses. <br /> ■ Provide an estimate of the time required for dissolved contaminant concentrations to <br /> achieve background conditions of groundwater. <br /> ■ Provide a demonstration that the secondary source in the area of MW-2 has been <br /> remediated to the extent practicable. <br /> ■ Completion of the conceptual site model (CSM) with plume map and description of <br /> plume disposition and behavior. <br /> 2-5 Febraary 2021 <br />