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21 June 2019 AdvancedGeo AGE Project No. 19-4745 Environmental Page 10 of 14 9.0. IMPACT OF NITRATE TO GROUNDWATER Due to the proposed commercial use of the property and the potential for nitrate leaching into the groundwater from the existing septic systems, a nitrate loading analysis for the future use of the existing septic system has been 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 agricultural, commercial and residential lands 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) Holt Station #248. The average rain fall data for the area was gathered from the Western Regional Climate Center summary. The averages were calculated based on data collected between 1949 and 2006, the annual rain fall average for the area is 17.63 inches. O 9.3. CURRENT ON-SITE SOURCES OF NITRATE Septic systems, rainfall and agriculture (adjacent property to the east) are sources of nitrates (fertilizers) present on or around the property. The current site usage includes a small modular office building. The office contains one toilet and one sink that are used by employees on shift and visitors. Use of the office space includes hand washing and use of toilets. The reasonable or maximum usage of wastewater for the office building by workers and visitors is 30 gallons per day (gpd) per bathroom (5 gallons per person). The total nitrate-nitrogen (NO3_N) average concentrations in domestic waste ranges between 20 to 85 milligram total nitrogen liter (mg/1-N) with an average of 45 mg/I-N when washing and sanitation is considered. The weighted average calculation for total nitrogen concentrations for onsite commercial flow is proposed as the following: 0 Commercial volume 30 gallons per day (gpd).