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Soil Suitability Nitrate Loading Study Manteca Unified School District 14600 Austin Rd.,Manteca,California Page 7 Since shallow groundwater at the Site is approximately 45 feet bgs within a clay layer that begins at approximately 15 to 17.5 feet bgs, it is unlikely that infiltration of discharge from the septic system will reach groundwater. 2.2 GROUNDWATER ANALYSIS 2.2.1 Depth to Groundwater, Seasonal Fluctuations of Groundwater Depth,Directional Flow and Gradient As stated in Section 1.4, Condor explored subsurface conditions by means of multiple soil borings at the Site on February 15, 2019. Groundwater was encountered at a depth of 45 feet bgs during drilling. Groundwater was not encountered in the percolation test hole excavated to a depth of 3 feet bgs on October 10, 2019. While the depth to groundwater is expected to fluctuate in response to both seasonal rainfall and irrigation of surrounding farmland,there is a practical limit to the anticipated temporary rise in groundwater levels at the Site. A review of groundwater elevations in nearby water supply wells, as presented in annual reports prepared by the San Joaquin County Flood Control & Water Conservation District, shallow groundwater levels near the Site have been steadily declining since the 1960s. 2.2.2 Existing or Potential Groundwater Contamination Issues in the General Location of the Site The Site is located in rural residential and farmland involved in agricultural practices. Chemical applications, livestock waste, fertilizers, and uncontained or failed septic tank waste can contribute to the occurrence of nitrate in groundwater. The most recent available consumer confidence report for the Site well (2018 Consumer Confidence Report, Appendix E) indicates no violations for tested constituents. A source water assessment conducted for the well in August 2001 determined that the well is considered most vulnerable to activities associated with irrigated crops. Available well analytical data from the GAMA database (Appendix E)also shows no constituents of concern above regulatory levels. 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 Current and expected future use of groundwater from the Site well are for domestic and irrigation purposes. 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 dozens of buildings/residences within 0.5 miles. It is assumed that each building has at least one septic system. The up-gradient area is predominantly agricultural farmland with dispersed rural residential housing. Refer to Section 1.3.5 for description 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 rainfall for Manteca,California, based on the nearest weather station to the Site. 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) average monthly climate summary average rainfall int? CONDOR 1