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10 May 2021 <br />Project No. 21-6390 <br />Page 10 of 14 <br />9.0. IMPACT OF NITRATE TO GROUNDWATER <br /> <br />Due to the planned commercial use of the property and the potential for nitrate leaching <br />into the groundwater from the proposed septic system, a nitrate loading analysis for the <br />future use of the septic system is required by the EHD. <br /> <br /> <br />9.1. CURRENT OFF-SITE SOURCES OF NITRATE <br /> <br />Septic systems and agriculture are sources of nitrates in the area of the site. The property <br />is bordered by agricultural land on all sides. <br /> <br /> <br />9.2. IMPACT OF RAINFALL CONTRIBUTION TO NITRATE LOAD <br /> <br />The rainfall of the area has been estimated for the nitrate loading analysis. Deep <br />percolation is estimated by calculating the average monthly rainfall and <br />evapotranspiration of the site area. If average monthly rainfall exceeds average monthly <br />potential for evapotranspiration (ETo) there is a potential for deep percolation without <br />consideration for runoff. Average potential ETo was estimated from data obtained from <br />the California Irrigation Management Information System (CIMIS) Holt Station #248. Holt <br />Station #248 has been collecting ETo and precipitation data since June 2016. <br /> <br />Deep percolation (average annual rainfall recharge) was estimated utilizing the monthly <br />ETo and precipitation data collected from Holt Station #248 between 2017 and 2020. The <br />annual rainfall average for the area between 2017 and 2020 was 10.50 inches (CIMIS); <br />the annual ETo average ranged from 0.08 inches in 2020 to 4.68 inches in 2019. The <br />estimated deep percolation (R) was determined by calculating the average of the ETo <br />between 2017 and 2020. The estimated deep percolation (R) is 3.05 inches (Appendix D). <br /> <br /> <br />9.3. CURRENT ON-SITE SOURCES OF NITRATE <br /> <br />Septic systems, rainfall, and agriculture (fertilizers) are sources of nitrates present on or <br />around the property. <br /> <br /> <br />9.4. FUTURE ON-SITE SOURCES OF NITRATE <br /> <br />The future site usage includes the addition of a commercial office building for commercial <br />use. The office building will be open Monday through Friday from 6:00 AM to 3:00 PM. <br />The septic system will be utilized 260 days per year by approximately forty (40) full-time <br />employees. The maximum usage of wastewater for the office building is 30 gpd per <br />person, for a total of 1,200 gpd. <br />