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7 Mr. Benjamin Hall 0 -3 - 15 October 2004 Musco Family Olive Company Groundwater Impacts from Impounded Wastewater Groundwater monitoring data collected from monitoring well 16 (MW-16), approximately 220 feet downgradient of the dam, showed a steady and pronounced increase in calcium, magnesium, and alkalinity shortly after wastewater was impounded in the reservoir. The memorandum concludes this is sufficient evidence that waste constituents from impounded wastewater have impacted groundwater. The report in contrast attributes the recent steady and pronounced increase in calcium, magnesium, and alkalinity to natural weathering and/or past agricultural practices. The report does not explain why the effects of natural weathering and/or past unspecified agricultural practices can be discerned in MW-16 only shortly after the impoundment of wastewater. Staff's interpretation of the data and conclusion are based on abundant evidence in Regional Board files that show high-strength organic waste (e.g., food processing wastewater discharges, unlined sludge drying beds)moving through soil has the potential to result in or contribute to the degradation of groundwater from calcium,magnesium, and alkalinity, as well as from other waste constituents and byproducts. Microbial decomposition of organic waste creates organic acids and carbon dioxide, and the latter dissolves in soil-pore liquid to create a weak acid. The generation of these byproducts appears to accelerate the natural soil mineral weathering process and elevate concentrations of calcium, magnesium, and alkalinity in soil in liquid. To our knowledge, no scientific evidence in the technical literature better explains why groundwater passing under soils loaded with high-strength organic waste often contains elevated concentrations of calcium,magnesium, and alkalinity, and it is reasonable to attribute the change in quality detected in MW-16 to the same reaction phenomenon. The report also states it is unlikely that impounded wastewater infiltrating to groundwater would have traveled significant lateral distances to reach MW-16 within the first 18 months of reservoir operation. In the technical report Packground Groundwater Quality Report, dated 30 July 2004 and received 2 August 2004, the velocity of area groundwater is estimated as ranging from 73 feet/year for silt and 372 feet/year for silty sand. Using the faster velocity, wastewater from the reservoir could theoretically travel to MW-16 within about 17 months after initial impoundment. Depending upon the location and extent of sandy/gravelly lenses between the bottom of the reservoir and MW-16, as well as the increase in hydraulic head resulting from the impoundment of wastewater in the reservoir, the groundwater travel time could be significantly faster. Blanket Drain Discharge Both your letter and the report conclude that the discharge from the blanket drain consists primarily of naturally occurring groundwater that is flowing through the blanket drain as a result of an artificial artesian condition. Using groundwater elevation data obtained in December 2003 and an assumed hydraulic gradient of 0.01 foot/foot, the report projects the elevation of groundwater in the area of the dam to be about the same as the natural ground surface, or about 12 to 22 feet above the reservoir's blanket drain at 298 feet MSL. The report concludes these apparent hydraulic head differences are sufficient to induce a vertical upward hydraulic gradient in the area of the dam that prevents or significantly impedes the infiltration of impounded wastewater through the soils underlying.the reservoir. The report appears not to consider the hydraulic head attributable to the impounded wastewater, which can approach 40 feet when the dam is operated at its maximum pool elevation of 350 feet MSL. Additional information and technical reasoning from Musco are necessary to persuade staff the hydraulic head created by the reservoir and its construction does not affect the groundwater flow dynamics in this area.