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STAFF REPORT 10 MUSCO FAMILY OLIVE COMPANY AND THE STUDLEY COMPANY CONSIDERATION OF WDRS AND A C&A ORDER 6 September 2002 Regional Board Agenda Well Date Units TDS Chloride Sodium MW-1 4/10/02 mg/L 690 67 240 MW-1 6/24/02 mg/L 770 110 250 Average -- mg/L 730 88.5 245 St. Dev. mg/L 57 30 7 Background -- mg/L 850 150 260 Confined Aquifer MW-8 4/10/02 mg/L 1,300 130 280 MW-8 6/24/02 mg/L 1,300 270 290 Average -- mg/L 1,300 200 285 St. Dev. mg/L 0 99 7 Background mg/L 1,300 400 300 Alternative No. 4—Use of Regional Wells The 7 August 2002 Revised Background Water Quality Preliminary Evaluation Report described problems with the use of the onsite monitoring wells, and instead proposes the use of regional domestic/agricultural/industrial wells to determine the background groundwater quality. Staff disagree with a number of the conclusions in the report and consider a number of the conclusions unsupported based on the technical information presented in the reports. For example: • 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