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03 June 2021 Project No. 21-6431 Page 11 of 15 8.3. CURRENT AND FUTURE USE OF GROUNDWATER AT THE SITE The existing water supply well at 6055 Garnica Court (site) conforms to DWR and EHD standards. The well is currently utilized for the potable water source at the property and will continue to be used to supply the subject property with a domestic water source. 9.0. IMPACT OF NITRATE TO GROUNDWATER Due to the planned residential use of the property and the potential for nitrate leaching into the groundwater from the proposed septic system, a nitrate loading analysis for the future use of the septic system is required by the EHD. 9.1. CURRENT OFF-SITE SOURCES OF NITRATE Septic systems and agriculture are sources of nitrates in the area of the site. The property is bordered by residential land, and is near agricultural land, which might provide a nitrate source. There are several residences within 1,000 feet of the site. 9.2. IMPACT OF RAINFALL CONTRIBUTION TO NITRATE LOAD The rainfall of the area has been estimated for the nitrate loading analysis. Deep percolation is estimated by calculating the average monthly rainfall and evapotranspiration of the site area. If average monthly rainfall exceeds average monthly potential for evapotranspiration (ETo) there is a potential for deep percolation without consideration for runoff. Average potential ETo was estimated from data obtained from the California Irrigation Management Information System (CIMIS) Manteca Station #70. Manteca Station #70 has been collecting ETo and precipitation data since January 2014. Deep percolation (average annual rainfall recharge) was estimated utilizing the monthly ETo and precipitation data collected from Manteca Station #70 between 2015 and 2020. The annual rainfall average for the area between 2015 and 2020 was 11.52 inches (CIMIS); the annual ETo average ranged from 0.48 inches in 2020 to 4.41 inches in 2017. The estimated deep percolation (R) was determined by calculating the average of the ETo between 2015 and 2020. The estimated deep percolation (R) is 3.20 inches (Appendix E). 9.3. CURRENT ON-SITE SOURCES OF NITRATE Septic systems, rainfall and agriculture are sources of nitrates (fertilizers) present on or around the property.