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ERT 22D03 W01 former Unocal Service Station 5098 Stockton Cal form November 12 1998 Status of Residual Petroleum Hydrocarbons in Soil During September to December 1988, Unocal excavated soil containing petroleum hydrocarbons to a maximum depth of 53 feet below grade (approximate depth of groundwater during excavation activities) or to the vertical extent of residual petroleum hydrocarbons Based on current water level measurements in the nearest wells to the excavation, the bottom of the excavation extends approximately 12 feet below the current water level Approximately 6,000 cubic yards of soil were removed from the site (Unocal, August 15, 1996) Based on the results of remedial efforts to remove soil containing petroleum hydrocarbons, the Central Valley Regional Water Quality Control Board (Regional Board) provided a letter (dated August 25, 1989) which "confirmed the soil contamination at the subject site has been adequately cleaned up" A copy of the August 25, 1989 Regional Board letter is provided in Attachment B The locations of excavations and samples as well as corresponding results of analyses are attached (Attachment C) The concentrations of residual hydrocarbons remaining in soil (zone of groundwater fluctuation) are shown on Plate 6 Status of Dissolved Petroleum Hvdrocarbons Maps depicting the concentrations of dissolved total purgeable petroleum hydrocarbons as gasoline (TPPHg) and benzene are shown on Plates 7 and 8 respectively Irri ation Well Data Unocal requested ERI personnel to delineate the location of the irrigation well utilized by Delta College The location of the well is shown on Plate 2 According to the Water Well Drillers Report, the well was constructed in 1950 to a total depth of 325 feet with perforations beginning at 40 feet below grade Based on conversations with Delta College representatives, the well is typically pumped between the months of May through October at a rate of approximately 240,000 gallons per night (7 00 p m to 6 00 a m ) Groundwater samples for laboratory analysis have previously been collected and analyzed by Unocal and Montgomery Wards The analytical results available to date from the Delta College well are provided in Attachment D PROPOSED WORK ERI and Unocal will contract to drill seven borings and construct single and dual completion (side-by- side) monitoring wells in the borings at locations shown on Plate 2 The proposed «ork will be performed to 1) address the County's concern with submerged well screens in the existing groundwater monitoring wells, and, 2) further evaluate the lateral and vertical extent of petroleum hydrocarbons extending from the former Unocal property towards the Montgomery Ward property and the Delta College irrigation well The rationale for each well and a description of the proposed location are provided below Well MW10A MW10A is proposed to be located near existing groundwater monitoring well MW10, is in an upgradient location (based on winter gradient information), and will be of single casmglshallow construction with the well screen above and below the water table as encountered at the time of drilling This well is intended to specifically respond to the request of the County for replacement of a