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STAFF REPORT • 0 $ ' MUSCO FAMILY OLIVE COMPANY AND THE STUDLEY COMPANY CONSIDERATION OF WDRS AND A C&A ORDER 6 September 2002 Regional Board Agenda Well Date TDS Chloride Sodium NO, as N Sulfate 6/24/02 Well was dry. MW-10 4/10/02 2,700 1,300 820 16 53 6/24/02 2,500 1,300 780 29 91 MW-11 4/10/02 2,000 690 620 9.7 470 6/24/02 2,000 600 630 22 380 MW-12 4/10/02 2,100 580 500 14 890 6/24/02 2,300 530 570 34 960 MW-13 4/10/02 3,000 1,400 790 37 41 6/24/02 2,900 1,500 790 46 51 W2(1985) 4/10/02 690 33 240 3.2 17 6/24/02 790 180 290 6.7 26 Because of the limitations of the data and poorly understood hydrogeology of the site, several 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.