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Groundwater Gradient CASGEM data indicate the groundwater gradient is generally southwest to northeast, following the ground contours. Based DWR well monitoring records, depth to groundwater ranges from approximately 25' bgs to 80' bgs. Contribution of Rainfall An estimate of the quantity and quality of rainfall was required for the nitrate loading analysis. The Hantzsche and Finnemore method utilizes the "average recharge rate of rainfall" in calculating potential adverse nitrate impacts to the aquifer. It is assumed that not all rainfall will percolate to the aquifer, as some water may be lost to evaporation or transpiration. However, most of the rainfall in California is during the cooler months when evaporation is less. Hence, if water is held in detention for long periods, and the holding basin soils are permeable, much of the water will percolate. Rainfall recharge (or deep percolation of rain) is a significant factor in the nitrate loading estimate in the Hantzsche and Finnemore model. There are a variety of rainfall data that span various timeframes for the Tracy, California area. For analysis, 90% of the 30 year average of 13.05 inches/year was used to estimate annual precipitation, resulting in a value of 11.75 inches/year. Due to requirements for new developments to detain site runoff, some of the storm water will be held and either percolate into the groundwater or evaporate. Water that remains in the basins after 72 hours will be pumped off-site. Soil borings for the property show that the soil texture at the base of the detention basins is similar to the top soil, with high clay content. Thus, percolation of storm drainage will by limited. For purposes of determining nitrate loading, a conservative estimate of 10% rainfall retention and percolation was used for the analysis. A standard nitration concentration for rainfall of 1 mg/L-N was assumed (Hantzsche and Finnemore). Denitrification Reductions The Project will treat wastewater to allow unrestricted non -potable use for landscape irrigation. The proposed wastewater treatment process performance indicates a nitrogen reduction from 75 mg/I to 10 mg/I or lower. Thus, nitrogen concentrations will be significantly reduced prior to land application. Subsequently, soil denitrification is a process that occurs when certain bacteria metabolize the oxygen in nitrate (NO3) in the wastewater and release nitrogen gas (N2) to the atmosphere. Due to the biologic processes in the shallow soils, nitrogen may undergo a reduction between 10 percent to 35 percent. Soils with higher clay content, moist soil conditions, high pH, and organic material denitrify by about 35 percent, and soils with high sand content and fast percolation rates tend to denitrify at a lower rate of about 10 percent. Using the treated effluent to irrigate plants, grasses, and trees will further reduce nitrogen concentrations. For purposes of this study, a denitrification factor of 25 percent was used, as soils in the area consist of clay/clay loam, and vegetation will assist with removal of both volume and nitrogen concentrations. 61 Page