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Soil Suitability Nitrate Loading Study Manteca Unified School District 5700 East Nile Ave., Manteca, CA Page 8 2.2.3 Current Use of Groundwater at the Site and of the Future Expected Use of Groundwater as Related to the Proposed Development Project Shallow groundwater in much of San Joaquin County has been impacted by agricultural chemicals, including nitrates, TCP and a variety of herbicides/pesticides as a result of historical agricultural use. Testing of Site water supply wells have confirmed the presence of nitrates and TCP in groundwater beneath the Site. Safe drinking water is being provided to the school using bulk storage tanks to deliver the drinking water. On-site wells are strictly used to provide irrigation water. 2.3 SOURCES AND CALCULATED IMPACT OF NITRATE TO GROUNDWATER 2.3.1 Current Off-Site Sources of Nitrate to Soil and Groundwater in the General Up Gradient Area and the Contribution of Nitrate Load from These Sources to the Project Site Septic systems and agriculture in the area may be the most significant sources of nitrate. As discussed in Section 1.3, septic system density for the vicinity of the Site was based on 16 buildings/residences within 0.5 miles of the Site. It is assumed that at least each building has one septic system. The upgradient area is predominantly agricultural farmland with intermittent residential housing. Refer to Section 1.3.5 for descriptions of septic tanks in the general area. 2.3.2 Description of Impact of Rainfall Contribution of Nitrate Load to the Project Site 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 for Manteca, California, based on the nearest weather station to the site available. The estimate first determines whether the average monthly precipitation exceeds average monthly potential evapotranspiration. If so, then the difference is potential deep percolation. Based on the California Irrigation Management Information System (CIMIS) monthly report and the Western Regional Climate Center (www.wrcc.dri.edu), the average monthly climate summary average rainfall was found to be 10.41 inches per year (inches/year) for the years 1971 through 2000. Based on this method, the average deep percolation of rain for the Nile Garden School area is estimated to be 0.99 inches/year. The nitrate concentration of rain fall was estimated to be 1 mg/L-N (Hantzsche and Finnemore, 1992). Precipitation data is provided in Appendix F. 2.3.3 Current On-Site Sources of Nitrate to Soil and Groundwater and the Contribution of Nitrate Load from These Sources to the Project Site Current on-site sources of nitrate include existing septic tanks as discussed and identified in Section 1.3 and agricultural use in the area. 2.3.4 Proposed Future On-Site Sources of Nitrate to Soil and Groundwater and the Contribution of Nitrate Load from These Sources to the Project Site Proposed on-site sources of nitrate are discussed in Section 1.3.7. 2.3.4.1 Denitrification Estimate Denitrification is an important process which occurs within the leach field whereby certain bacteria utilize the oxygen in the nitrate (NO3) and release nitrogen gas (N2) to the atmosphere. Denitrification is expressed as a percentage, and the normally accepted range is 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 are estimated to denitrify at a lower rate, about 10 percent to 15 percent. 114t_ CONDOK