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• - 9 - ORDER NO.R5-2002-0148 INFORMATION SHEET MUSCO FAMILY OLIVE COMPANY AND THE STUDLEY COMPANY WASTEWATER TREATMENT AND LAND DISPOSAL FACILITY SAN JOAQUIN COUNTY alternatives for calculating the background concentrations exist. The Discharger has proposed use of off-site domestic/agricultural/industrial wells as data sources and has presented a discussion in a 7 August 2002 "Revised Background Water Quality Preliminary Evaluation"report prepared by Kleinfelder. It is noted that the report does not calculate background groundwater values.and instead proposes additional investigations. Staff approached the problem of determining the background groundwater concentrations several ways, while considering the available data and the time constraints in arriving at a decision. Five alternatives for calculating background groundwater concentrations were evaluated. Each of the alternatives are presented below. Staff determined that Alternative No. 5 was the best selection of the alternatives available at this time. Alternative No 1 —Use of On-Site Groundwater Monitoring Wells To estimate the wastewater effluent limits, staff's initial approach was to use on-site groundwater monitoring wells to define background groundwater quality. This approach resulted in the effluent limits published in the Tentative WDRs. In the Tentative WDRs analysis,the locations of the wells, groundwater flow direction, and concentrations observed in the wells were considered in selecting wells to represent background groundwater quality. Wells MW-1 and MW-8 were selected as representative. Well MW-2 was considered but removed from the data set as an anomaly; staff believe the extreme concentrations observed in Well MW-2 are the result of an unknown site specific source and deserve more analysis and interpretation before this well can be considered a background well. The background groundwater quality was calculated by adding one standard deviation to the average concentration and then rounding up. The resulting background concentrations (and therefore the wastewater effluent limits) were: total dissolved solids 1,500 mg/L, chloride 150 mg/L, and sodium 290 mg/L. Alternative No 2 Revised Use of On-Site Groundwater Monitoring Wells Staff reviewed the additional data collected in the June 2002 sample event and recalculated the background groundwater quality. The background groundwater quality was again calculated by adding one standard deviation to the average concentration of Wells MW-1 and MW-8 and then rounding up. The resulting background concentrations, and wastewater effluent limitations,were: total dissolved solids 1,350 mg/L, chloride 240 mg/L, and sodium 290 mg/L. Alternative No 3 Use of On-Site Monitoring Wells and CCR Title 27 Procedures The Discharger's documents identify a perched groundwater zone and a deeper groundwater zone. The 31 July 2002 Groundwater Monitoring Report prepared by Kleinfelder states that Wells MW-1, MW-2, MW-3,MW-5,MW-6, MW-10, MW-11,MW-13, and W-2 are perched zone aquifer wells, while Wells MW-4, MW-7, MW-8, MW-9, and MW-12 are classified as deep zone wells. Staff believe additional investigation is required at the site before the aquifer zones can be classified and disagree with the classification of the shallow zone wells as perched. While staff disagree with the nomenclature of the zones, staff agrees that two groundwater zones have been identified by the monitoring well installations.