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Proposal 2106b: Ruvalcaba SS/NLS Addendum Page 2 February 4, 2021 Assumptions for Nitrate Loading Calculation Variable Value Units Description Q 200 gpd Effluent flow rate, 3-bedroom house Nw 35 mg/L-N Effluent stream concentration A 1.50 acres Site area Nb 0.30 mg/L-N Concentration of rain d 0.35 constant Denitrification factor R 2.25 inches per year Recharge rate of rainfall W 1.8 inches per year Uniform waste water loading (calculated) Nc 10.3 mg/L-N Long-term average concentration of percolating effluent The US EPA recommended Maximum Contaminant Level (MCL) for nitrate in drinking water is 10 mg/L-N. In a worst-case situation, the maximum buildup of nitrates in the aquifer from the use of a septic system for a single three-bedroom house on the Site is predicted to be 10.3 mg/L-N. The result of the nitrate loading estimate is roughly equivalent to the maximum drinking water standard. As discussed in the original report, due to the conservative nature of the calculation, clayey soil present, and lack of existing nitrate exceedances in the area, the actual nitrate loading experienced on the Site over time may be less than predicted. For protection of the ground water, it is recommended that only shallow septic systems utilizing leach lines or filter beds be installed on the Site. Recent San Joaquin County Environmental Health Department policy has been to evaluate each project on a case- by-case basis to determine whether additional measures to mitigate nitrate should be employed. Water Test Results San Joaquin County Environmental Health Department collected a water sample from the domestic well on the Site on May 23, 2019. The sample was analyzed for coliform, general minerals, and pH. The results are attached. Nitrate was detected in the sample at a concentration of 7.2 mg/L-N, below the MCL of 10 mg/L-N. According to the included Drinking Water Interpretation, iron and manganese were detected above their MCLs. According to the Coliform Bacteria Analysis, both total coliform and E. coli were listed as "failure" due to the presence of E. coli. We recommend that you contact the San Joaquin County Environmental Health Department for recommendations about drinking water quality on the Site.