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STAFF REPORT 0 • 9 MUSCO FAMILY OLIVE COMPANY AND THE STUDLEY COMPANY CONSIDERATION OF WDRS AND A C&A ORDER 6 September 2002 Regional Board Agenda 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. The Discharger has submitted two quarters (April 2002 and July 2002) of water quality data for the site. With limited data, staff calculated the background groundwater monitoring wells in each zone by calculating the average value, and adding two standard deviations to establish a background water quality for the site. This procedure is consistent with CCR Title 27 Section 20415. Because two saturated zones have been identified at the site, each zone must include a unique water quality value. Wells MW-1 and MW-8 were selected as the background wells for the water table aquifer (i.e., Kleinfelder's"perched zone") and the confined unit (Kleinfelder's "deep zone") respectively. Well MW-1 was selected because it is the furthest upgradient shallow zone well. Although MW-8 is not the furthest upgradient confined zone well, it is downgradient of the processing facility which is not considered a source area, and it is located in an area that has not historically been used for land application until recently. The other confined zone wells were evaluated for use and determined to be inappropriate for the following reasons: Well MW-12 is located adjacent to the Title 27 surface impoundments, Well MW-9 is dry, and Well MW-4 is located adjacent to the 1 Mgal settling pond. Well MW-7 might be appropriate, however it is located downgradient of the Title 27 surface impoundments and therefore it was not selected. Staff calculated background groundwater quality values for both the water table and confined zones, as shown on the table below. For the water table aquifer, the limits are: TDS 850 mg/L, chloride 150 mg/L, and sodium 260 mg/L; for the confined aquifer they are: TDS 1,300 mg/L, chloride 400 mg/L, and sodium 300 mg/L. Well Date Units TDS Chloride Sodium Water Table Aquifer