ARCHIVED REPORTS XR0005572 - Laserfiche WebLink

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• Vertically, the highest concentrations of petroleum hydrocarbons in subsurface soil were detected within the zone of groundwater fluctuation (i e , capillary fringe) The greatest concentration of TPH-as-diesel (8,800 mg/kg) was detected in SB-7 at a depth of 15 feet bgs The soil sample collected at the maximum drilling depth of 20 feet bgs from SB-7 contained 460 mg/kg of TPH-as- diesel As such, the vertical extent of impacted sod is undefined in the vicinity of the former UST Based on the results of this investigation, Wright Environmental recommends the following course of corrective action 1 Further define the lateral extent of impacted sod and groundwater to the north of the former UST (within the State Highway 99 right-of-way), the lateral extent of impacted groundwater to the east of MW-1, and the vertical extent of impacted soil beneath the former UST These objectives could be achieved in one field day using a Geoprobe® direct-push drilling rig Within the shoulder area of the highway nght-of-way, at least three sod borings should be advanced for soil and groundwater-grab sampling One of these borings should be angled to the north under the highway A groundwater grab sample should be collected on-site east of MWA to define the lateral extent of impacted groundwater in that direction One additional soil boring should be advanced at the former UST location to define the vertical extent of impacted sod at that location • 2 Excavate the heavily impacted sod under and in proximity to the former UST The excavation limits should depend on field observation and screening Based on the soil sample data collected to date, the excavation limits should extent approximately 20 to 30 feet from the former UST in all directions to a depth of between 15 and 20 feet bgs This remedial effort would substantially removed the source of continuing impact to groundwater Residual petroleum hydrocarbons left in the sod would likely biodegrade passively over time 3 Monitor the concentration and stability of the groundwater contaminant plume Each of the groundwater monitoring wells should be monitored and sampled on a quarterly basis for at least one, one-year hydrologic cycle Groundwater samples should be analyzed for TPH-as• diesel, TPH-as-gasoline, BTEX and MTBE by Modified EPA Methods 8015 and 8020 Quarterly groundwater monitoring reports should be prepared and submitted to PHS/EHD and the Central Valley Regional Water Quality Control Board 4 Model the future fate and transport characteristics of the residual petroleum hydrocarbons to assess the risk to human health and the environment Based on the risk assessment, submit a request for regulatory closure or a plan for additional environmental corrective action 11