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LIVE OAK GEO ENVIRONMENTAL, INC. 407 W. Oak St., Lodi, CA 95240 :- (209) 369-0375 •:• logelodi.com March 19, 2019 Project Number: 1847 Linda Turkatte, Director San Joaquin County Environmental Health Department 1868 E. Hazelton Ave. Stockton, CA 95205 Regarding: Second Addendum to Soil Suitability/Nitrate Loading Study Jhamat Property 5310 Las Positas Cir., Stockton, CA Ms. Turkatte: Based on our meeting yesterday, we have prepared this addendum to the above Nitrate Loading Study. You indicated that the nitrate loading calculation for the Site should be re-evaluated using precipitation and evapotranspiration data obtained from the California Irrigation Management Information System for the past five years. As in the previous report and addendum 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. 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. For the project Site, the local percolation and evapotranspiration rates are presented in Plates 1 and 2. The Site is located between the CIMIS Stations at Holt (station 248) and Staten Island (station 242); because the rain isocontours run to the northeast in this part of San Joaquin County, taking both stations into account will yield a value more representative of the Morada area. During the past five-year period, data from the Holt station were only available for 2017 and 2018; based on the calculation shown, the estimated rate of recharge from percolating waters for the Holt station is determined to be 3.72 inches. A statistical error of 25% is associated with this value, as calculated on Plate 1. Data for the Staten Island station were available for the past three years; Environmental Engineering Geology Geophysics