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lierracon Soil Suitability / Nitrate Loading Study Morada Produce —Ag Processing Building Stockton, California May 14, 2021 Terracon Project No. NA217014 Based on the percolation tests results, the two deep borings achieved a passing percolation rate of 30 minutes per inch or less for seepage pits. The San Joaquin County Onsite Wastewater Treatment Systems Standards (OWTS) Table 1.10.2 requires a minimum 20 foot separation between the bottom of the dispersal system to groundwater. Depth to groundwater was reported at 134 feet bgs minus the 25 foot deep seepage pit; therefore, a 109 foot soil column is anticipated between the bottom of the seepage pits and groundwater. 1.6.7 Discussion of Percolation Test Results Based on the percolation tests results, Borings B1 and B2 achieved satisfactory percolation rates of 3.12 and 5.0 minutes per inch, respectively, suitable for seepage pits. 1.6.8 Discussion of Septic System Sizing In general, septic system construction should follow the San Joaquin County On-Site Wastewater Disposal Standards (OWTS) where applicable. 2.0 NITRATE LOADING STUDY Because of concern regarding nitrate leaching into groundwater from septic systems, a nitrate loading analysis is required by the EHD for the proposed development to determine the potential concentration of percolating effluent and rain fall over the long term. Where percolating effluent leaches into the ground, groundwater quality may be impacted to some degree should the effluent reach groundwater. The method utilized for the nitrate loading analysis is based on a simple mass balance formula, discussed in a published article by Hantzsche and Finnemore (1992), which provides a convenient and simplified method to estimate long-term groundwater nitrate impacts on an area- wide basis. Hantzsche and Finnemore found that over a long period of time, water quality in the upper saturated groundwater zone closely approximates the quality of percolating recharge waters. They believe that their method is a good planning tool and provides useful information to planners and local officials but with some real limitations. In their article published in Ground Water, Hantzsche and Finnemore mention that "...the equation ignores dispersion, lateral flow, and mixing with groundwater flow from up-gradient areas. These processes would generally contribute to additional reduction of nitrate-nitrogen concentrations in groundwater to the extent that the nitrate-nitrogen concentration of groundwater flow from up-gradient areas is lower." The Hantzsche and Finnemore approach is "...a conservative (worst case) first approximation of groundwater nitrate-nitrogen concentration resulting from the combined effect of on-site sewage disposal systems and precipitation. This (the equation and approach) is for estimation of long- Responsive • Resourceful • Reliable 13