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Soil Suitability/Nitrate Loading Study 1rerracon <br /> Vacant Commercial Property■ Stockton, California <br /> December 7, 2018 ■ Terracon Project No. NA187056 <br /> Table 6. <br /> Summary of Percolation Tests. <br /> Test No. Percolation Percolation Test Duration Comments <br /> Date Tested Test Rate Depth (feet) (minutes/inch) (minutes) (see notes) <br /> 131 (10/25/2018) 3.5 125 240 Unsatisfactory <br /> percolation rate <br /> 132 (11/01/2018) 25.0 125 240 Unsatisfactory <br /> percolation rate <br /> Note: A satisfactory percolation rate is 60 minutes per inch or less for shallow leach lines and 30 minutes per inch or less for septic <br /> pits per San Joaquin County Onsite Wastewater Treatment Systems Standards(EHD 42-014),January 2017. <br /> 1.6.7 Discussion of Percolation Test Results <br /> Based on the percolation test results, the shallow clay soils as well as the deeper clay/silt soils <br /> and cemented sandy soils demonstrated unsatisfactory percolation rates of 125 minutes per inch. <br /> 1.6.8 Discussion of Septic System Sizing <br /> In general, septic system construction should follow the San Joaquin County On-Site Wastewater <br /> Disposal Standards where applicable. The foregoing investigation and percolation test results <br /> indicate the site soils consist primarily of clay, cemented silty sand and sandy silt to 25 feet bgs. <br /> Based on the proposed size of the system (6,000 gpd)and unsatisfactory percolation test results, <br /> _Terracon recommends an engineered septic system. Systems in the area appear to be using ' <br /> septic tanks with 40 to 100 foot long leach lines and 25 foot deep seepage pits. Given the amount <br /> of daily effluent, we suggest discussing with the county on the possibility of utilizing deeper pits <br /> (greater than 25' deep), given the groundwater is 80 plus feet deep in the area and previous <br /> monitoring wells indicate sandy soils were encountered 30 to 40 feet deep. <br /> Refer to Section 1.3.2 for further discussion regarding the former on-site septic system sizing. <br /> 2.0 NITRATE LOADING STUDY <br /> Because of concern regarding nitrate leaching into groundwater from septic systems, a nitrate <br /> loading analysis is required by the EHD for the proposed development to determine the potential <br /> concentration of percolating effluent and rain fall over the long term. Where percolating effluent <br /> leaches into the ground, groundwater quality may be impacted to some degree should the effluent <br /> reach groundwater. <br /> The method utilized for the nitrate loading analysis is based on a simple mass balance formula, <br /> discussed in a published article by Hantzsche and Finnemore (1992), which provides a <br /> Responsive ■ Resourceful ■ Reliable 14 <br />