SITE INFORMATION AND CORRESPONDENCE

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Bert E. Van Voris _ 10 - 17 September 2004 Supervising Engineer Notwithstanding the issue of the blanket drain discharge and the extent to which it is composed of impounded wastewater, evidence that impounded wastewater is impacting groundwater is seen in the changing quality of groundwater in MW-16, a shallow monitoring well 250 feet downgradient of the dam. Since wastewater impoundment, quarterly monitoring of groundwater passing through this well has shown an increase in TDS and sodium, as well as in calcium, magnesium and alkalinity. The RWD cites the low permeability of native soils under the reservoir(estimated at 10"6 to 10-7 cm/sec) as evidence that the increased concentrations of TDS and sodium in MW-16 cannot be attributable to the percolation of impounded wastewater. The RWD does not acknowledge that native soils under the reservoir also contained significant deposits of highly permeable material—material that had to be excavated in the area of the dam prior to its approval by the Division of Dam Safety.10 Also,the RWD does not mention the increasing concentration of calcium, magnesium and alkalinity in MW-16. The 3 June 2004 letter from Musco's consultant dismissed the trend of increasing calcium, magnesium and alkalinity in MW-16 as evidence of organic overloading and instead attributed it to "natural weathering (mineralization of groundwater interacting with the aquifer matrix) and/or increases in carbon dioxide levels in soil due to past agricultural uses." While this is not the forum for a full discussion of the causes for elevated calcium, magnesium, and alkalinity in groundwater impacted by high-organic discharges, Regional Board files do contain substantial evidence that decomposition of organic material within the soil profile,particularly the release of carbon dioxide gas, has the potential to accelerate soil weathering. The elevated concentrations in groundwater of calcium, magnesium, and other soil minerals suggest this has occurred. Until irrefutable evidence is presented to indicate otherwise(e.g., from a tracer dye study), it will remain my professional opinion that monitoring data from MW-16 indicates impounded wastewater has affected groundwater quality. The matter of the reservoir's permeability and containment integrity must be resolved as part of the current WDRs and TSO, as well as to consider Musco's proposed use of the reservoir for treatment. This use has the potential to impact groundwater for organics and decomposition byproducts, especially as a sludge layer will form and thicken along the reservoir's bottom. It will be necessary to implement BPTC measures to prevent these impacts. For example, either the reservoir must be equipped with a liner to preclude the infiltration of waste constituents to soil or the wastewater discharged to the reservoir must be treated to a level that will not release organics in a mass that will cause elevated concentrations of decomposition byproducts in groundwater. Water Balance Introductory Comments: Water balances submitted in support of a land discharge must employ the maximum anticipated discharge flow and,through identification of available disposal area, storage, percolation losses, etc., demonstrate that there is sufficient effluent storage and disposal area to accommodate the maximum discharge flow without discharging offsite during exceptionally wet years (i.e., 100-year return period).