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January 21, 2021 BGG Job No. G130.09 NITRATE LOADING STUDY Due to concerns of nitrate leaching into groundwater from septic systems, a nitrate loading analysis was required by the San Joaquin County Environmental Health Department (SJC EHD) for approval of the proposed Zinc House Winery septic system. The intent of the study is to estimate the potential concentration of percolating effluent produced by the proposed septic system for an extended period. Where percolating effluent leaches into the ground, groundwater quality may be impacted to a degree, should the effluent reach the groundwater table. The method utilized for the nitrate loading analysis is based on a simple and convenient mass balance formula, discussed in a published article by Hantzsche and Finnemore (1992), predicting groundwater Nitrate -Nitrogen impacts. The H&F equation is a conservative approximation of groundwater nitrate -nitrogen concentration resulting from the combined effect of on-site sewage disposal systems and rainfall precipitation. The H&F equation and method are appropriate for estimating longterm effects on groundwater quality; it is not intended for use in predicting seasonal fluctuations (H&F, 1992). The method considers the development area, soil denitrification factor, rainfall, rainfall nitrate concentration, waste effluent quantity, and waste effluent nitrate concentration from onsite septic systems. Mass Transport Soil Properties Field percolation test results indicate that the surface soils are moderately permeable. The site soils will permit some infiltration of water through the surface soils. The longer the effluent remains near the surface, the more biologic treatment may occur. If permeable soils exist to the depth of the ground water aquifer, it is possible that any remaining surface contaminants may be transported to the saturated zone of the upper aquifer. Nitrate Sources Septic systems have been identified to be a significant source of nitrate impact to groundwater. Dairy farming and orchard fertilization in sandy soils is also a significant source of nitrate contamination in shallow or sandy aquifers in San Joaquin County. Rainfall Contribution An estimate for rainfall quantity and quality was required for this nitrate loading analysis. The H&F method utilizes the "average recharge rate of rainfall" for calculating potential nitrate impacts to the aquifer. Some portion of total rainfall will dissipate by evapotranspiration, the sum of evaporation and plant transpiration, hence reducing the amount of rainfall percolation into the aquifer and percolating effluent, especially in summer months. Rainfall recharge (deep percolation of rain) is a significant factor in the nitrate loading estimate; however, data on rainfall recharge rates is not widely available. For H&F's study conducted in the Chico area, a recharge rate was calculated to be 75 percent of the average rainfall. Since the Chico area is similar geologically to San Joaquin County region, an estimated recharge rainfall for this study was calculated based upon 75 percent of annual rainfall for the project area. BAEZ GEOTECHNICAL GROUP Soil Suitability Study and Nitrate Loading Study- Valdovinos Residence Page 6