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June 13, 2006 NOA Project Number: E05164A the SSS. 2.3 Current and Future On-Site Ground Water Uses One domestic water well exists on the property and is associated with the produce market on the northwest corner of the subject property. Use of the well will continue for the produce market. 3.0 SOURCES AND IMPACT OF NITRATE TO GROUND WATER F13.1 Current Off-Site Sources of Nitrate F, We recognize that septic systems, agriculture, and dairies in the area may be the most significant sources of nitrate. Septic tank use in the vicinity is discussed in Section 3.4 of the SSS. The water sample was analyzed for nitrate and DBCP. Nitrate was found at 55.0 mg/L-NO3,which is above the maximum contaminant level (MCL) set by the US EPA of 45 mg/L-NO.. DBCP was not detected in the water sample. Though the well water sample indicated elevated nitrates, this would not be unusual for a shallow water well in San Joaquin County. We would recommend that the well be inspected and consultation with EHD be performed to determine whether the well could be repaired. We would recommend not using the well for drinking water. 3.2 Contribution of Rainfall An estimate of the quantity and quality of rainfall was required for the nitrate loading analysis. The estimate of deep percolation of rain is based on the average monthly precipitation of rain fall and evapotranspiration for the Manteca area of California. An estimate of potential deep percolation of rain water recharge into the ground is required for the nitrate loading calculation. Since the site is fairly sandy and a moderate and rapid percolation rates were measured,we will make the assumption that nearly all the rainfall will infiltrate'the ground fairly rapidly. Therefore,we estimate that roughly 90 percent of the rain fall will enter the ground water as recharge. The 10 percent reduction is to allow for some evaporation, transpiration of plants. Runoff is not expected. This method is not appropriate for sites which do not have relatively high infiltration rates. l Based on this method, an estimate for the deep percolation of rain for the Site is 11.69 inches. Due to to the sandy soils which allow fairly rapid infiltration ,we estimate the dee percolation of rain is ased� on 90 ercent of the long-term avers o rain fallor the Manteca-area (www.cunts.water.ca.gov): 12.96 inches x0. = 11. inches. The nitrate concentration of rainfall was estimated to be 1 mg/L-N (Hantzsche and Finnemore, 1992). �� 3