SITE INFORMATION AND CORRESPONDENCE

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ORDER NO.R5-2002-0148 11 - INFORMATION SHEET MUSCO FAMILY OLIVE COMPANY AND THE STUDLEY COMPANY WASTEWATER TREATMENT AND LAND DISPOSAL FACILITY SAN JOAQUIN COUNTY • Kleinfelder recommends use of off-site wells to establish background groundwater quality. Some data on off-site wells was presented in the RWD and is presented in the Tentative WDRs. The use of the wells is questionable because screened (perforated) intervals are unknown in most cases, the wells are not in close proximity to the facility (over 9,000 feet in one case, and the closest being approximately 800 feet), no information on the potential for any of the wells to have been individually contaminated is available, and the data quality for the little data that is available is unknown. • Staff do not agree with the unsubstantiated assumption that off-site wells are screened in shallow aquifers. There is nothing to base this assumption on, and in fact there is reason to consider it in error. Agricultural and industrial wells are rarely screened only in shallow zones because a reliable water source is important to their business operation. Fluctuating groundwater elevations and/or drawdown in pumping wells could dewater wells and stop irrigation or business operations if wells were of limited depth. • Kleinfelder implies that the onsite geology is heterogeneous and that more interpretation is necessary to fully understand the geology and the resulting placement of the monitoring wells. Staff agree. However, Kleinfelder does not address the fact that the geology in the surrounding area is also assumed to be heterogeneous, so that the use of wells from a large area around the facility is not appropriate because they are not expected to accurately describe the site conditions. Alternative No. 5 —Use of the Discharger's On-Site Production Well Staff's review of the hydrogeology reports revealed that the site conditions are not adequately characterized and understood to establish a final background groundwater quality value. Additional subsurface investigation and groundwater monitoring is required to better understand the site conditions. After reviewing the alternatives for determining a background groundwater quality value, staff determined use of the on-site production well was the best interim measure of groundwater quality that is in use at the site. This well is screened from 50 to 607 feet below ground surface, so the analytical data is a composite of the concentrations in the shallow and deeper aquifers. In addition, five sampling events were conducted at this well, leading to better statistical confidence. Staff used the same method to calculate the background values as described in Alternative No. 3. The average of the five monitoring events was calculated, and two standard deviations were added, in a method consistent with CCR Title 27 Section 20415. Staff recognize this method of calculating the background groundwater quality is not the optimum method but based on the information and data available, believe it is appropriate for an interim concentration limit. The background groundwater quality, and resulting effluent limits, are: TDS 2,047 mg/L, chloride 601 mg/L, and sodium 597 mg/L. Additional Hvdrogeologic Investigations The Discharger proposed additional investigations to better characterize the site and regional hydrogeology. The investigations are described in the 7 August 2002 Revised Background Water Quality Preliminary Evaluation Report,prepared by Kleinfelder. While the proposed studies may provide information on the regional groundwater quality, staff believe that additional studies should be focused on the Discharger's facility. Investigations of groundwater conditions in the region may not be