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LOGE 1832b: De Casas NLS Addendum Page 2 April 17, 2020 NITRATE LOADING CALCULATION As in the previous report prepared by our office, the method utilized for the nitrate loading analysis is based on a mass balance formula discussed in a published paper by Hantzsche and Finnemore (1992). The Hantzsche and Finnemore equation is a conservative approximation of ground-water nitrate-nitrogen concentration resulting from the combined effect of on-site sewage disposal systems and percolating recharge waters. Percolating Recharge Waters For this addendum, the recharge input for the Hantzsche and Finnemore equation is derived from average monthly precipitation of rainfall and evapotranspiration rates for the area. The calculation first determines whether the average monthly precipitation exceeds average monthly potential evapotranspiration. If so, then the difference is the percolating recharge water into the aquifer. Runoff and stream infiltration drainage are not factored into the calculation. This approach likely under-estimates recharge to the aquifer and, consequently, over-estimates nitrate loading. The Site is located between the CIMIS stations at Holt (station 248) and Staten Island (station 242); taking both stations into account will yield a value more representative of the Site area. Data from the Holt station were available for 2017 through 2019; based on the calculation shown, the estimated rate of recharge from percolating waters for the Holt station is determined to be 4.04 inches (Plate 1). A statistical error of 21 percent is associated with this value. Data for the Staten Island station were available for the past four years; based on the calculation shown, the estimated rate of recharge from percolating waters for the Staten Island station is determined to be 11.78 inches (Plate 2). A statistical error of 45 percent is associated with this value. The two stations are then averaged, for a regional deep percolation value of 7.91 inches. Quality of Rainwater Hantzsche and Finnemore listed 1 mg/L-N as the nitrate concentration of rainwater for their original calculation; in the past, Live Oak has also utilized this value for the nitrate loading calculation. In May 2019, Live Oak collected rainwater during two storm events in Lodi; in March 2020, Live Oak collected an additional sample at the same location. Each sample was collected in a clean, dry, stainless steel bowl, then decanted into a plastic container and placed on ice for transport. The samples were analyzed for nitrate by FGL Environmental of Stockton. Nitrate was detected in the May 17, 2019 sample at a concentration of 0.4 mg/L-N, in the May 19, 2019 sample at a concentration of 0.2 mg/L-N, and in the March 16, 2020 sample at a concentration of 0.2 mg/L. The laboratory analytical results from 2020 are