SITE INFORMATION AND CORRESPONDENCE

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�r v Explanation of Groundwater Treatment System Water Balance Table The monthly water balance for the aeration pond, treatment system, and spray field are provided in the accompanying table. The monthly input and losses from the pond, treatment, and spray field are calculated in gallons for every month of the design year (100-year wet season), and pumping rates are listed in gallons per minute (GPM) when appropriate. Inputs include groundwater pumping and precipitation from the 100-year wet season. The hundred year wet season was chosen after consultation with Mike Mosbacher of the Regional Board to be the wettest recorded year in the past 100 years. The nearest records found by Condor with a 100-year history were from Stockton (U.S. Weather Bureau). The 1982-1983 water year (July-June) was chosen as the year of greatest rainfall (27.89 inches). Losses are from evaporation from the pond surface, evaporation from the spray/air- stripping systems, and infiltration, (the pond is currently being used to provide aerated groundwater to a bioremediation zone). Monthly evaporation values were taken from the 1951 records for the Tracy Station, Dept. Water Resources Bulletin 73-79. These values were the lowest evaporation rates found for any area in the vicinity of the facility, and are lower than the 20-year low at the Tracy Pumping Station. Losses from aeration, air- stripping, and spray were estimated from irrigation efficiency numbers to be 25% of pumped spray from April through September, and 10% of pumped spray from October through March. Infiltration rate was taken from a site-specific pumping test performed in well MW4A near the pond. The pump test measures primarily horizontal flow, whereas infiltration though the pond bottom is vertical. The pond bottom will also be clogged by decayed organic matter and silt. Therefore, a correction of 0.1 is applied to the pump test data and a gradient of one is assumed. It is anticipated that with experience at operating the system, refinements to these estimates of spray losses and infiltration will be possible. The water balance analysis was designed to maintain the pond at half capacity to provide a storage buffer for down time. The pond water elevation in feet above mean sea level is calculated at the end of each month (row 21), as well as the available water storage in gallons (row 19) and in days (row 20). There is an average storage capability of 4.6 days/month in the pond to cushion unforeseen breakdowns or other events. The water balance shows that pond losses outpace inputs for the summer months of April through September, and groundwater pumping rates could be increased. During the winter months, when excess water must be removed from the pond, the application rate in the orchard spray field is well below the maximum application rate for the Stomar Soil. It is assumed for the sake of calculations that the water is applied for 6 hours over 4 acres.