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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 area. A review of the rainfall data from various sources indicate a range of average rainfall somewhere between 10 inches and 14 inches per year. For purposes of this report,the more conservative value of 10.5 inches was used for determining nitrate loading. Due to requirements for new developments to retain the site runoff, nearly all of the storm water for the parcel will be held in on-site detention basins. As such, it is estimated that most of the rainfall will percolate into the ground. Historically, most of the precipitation that fell on the property would run off, since the site slopes at approximately 1%to 2%,and the top soil has a high clay content. Thus, development will greatly increase the volume of groundwater recharge at the site. For purposes of this study, percolation of rainfall is assumed to occurfrom November through March, since evaporation will exceed rainfall in other months. In winter months,five(5)days of evaporation were assumed (corresponding to rainfall events),thereby reducing the total percolation value. This resulted in a 61%average rainfall percolation value. Nitrogen concentration for rainfall was estimated to be 1 mg/L-N (Hantzsche and Finnemore). Denitrification Reductions Denitrification is a process that occurs in the septic system drain field whereby certain bacteria metabolize the oxygen in nitrate(NO3) in the waste water and release nitrogen gas (N2)to the atmosphere. Due to the biologic processes in the shallow soils, nitrogen in the drain field 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. For purposes of this study, a denitrification factor of 25 percent was used, as soils in the area have moderate clay content. Nitrate Loading Estimate A nitrate loading analysis was performed to determine the impact of on-site sewage disposal from the project on local ground water. The analysis uses assumptions as discussed. Table 2 provides a summary of assumptions used for the nitrate loading calculations. Amore detailed review of assumptions and calculations is provided in Figure 3. 61 Page