SR0087014_SSNL - Laserfiche WebLink

Home Browse Search

July 31, 2023 BGG No. G130.09D SUMMARY AND CONCLUSIONS The soil conditions encountered near the proposed septic system and leach field appear to be suitable as designed. A satisfactory soil percolation rate of 10.0 minutes per inch was recorded from a percolation test conducted in the proposed septic system area,in accordance with SJCEHD OWTS Standards 2017. The depth to groundwater from nearby well observations is reported to be approximately 60 to 70 feet bgs,fluctuating seasonally. Groundwater flow direction in the general site region is toward the south-southeast. Based on the anticipated depth of the leach field to be less than 5 feet deep, allowing for groundwater table separation of more than 50 feet,modifications from a standard septic system should not be required. Based on the proposed residential site improvements and the studies discussed herein, the septic system and leach field is expected to contribute a theoretical long-term buildup of nitrate in the aquifer of approximately 10.54 mg/L-N. This level is above the US EPA drinking water recommendation of 10 mg/L-N.However,it is our opinion that additional considerations should be taken into account regarding the factors used in the nitrate loading calculations, which may demonstrate a lower nitrate loading factor than the theoretical value calculated. One consideration is the nitrogen concentration of rain. Hantzsche and Finnemore listed 1 mg/L-N as the nitrogen concentration of rainwater for their calculation,and we have frequently utilized this value for the nitrogen loading calculation as well. However, recent sampling during rainwater events in the general site area have resulted in measured nitration concentrations of rainwater averaging approximately 0.2 mg/L-N. Additional considerations are the theoretical versus actual liquid waste volume usage for the proposed residence and the current use of the neighboring properties. As required by the current California Building Code, all plumbing fixtures in the proposed residence will be required to meet low flow volume requirements, including sinks, showers, and toilets. Thus, though an estimated value of 250 gpd has been utilized as the effluent flow rate, the actual usage of the proposed residence may be 50 to 75%lower than the theoretical volume used for the calculation. In addition, as described earlier in this report,the site is surrounded by undeveloped agricultural properties in use predominantly for orchard tree agriculture. These properties are not discharging waste, thus alleviating the potential for sites like the subject property to contribute to an excess buildup of nitrate in the aquifer. Based on these additional considerations, it is our opinion that a conservatively amended Nitrate Loading calculation would result in an estimated long-term buildup of nitrate in the aquifer of approximately 9 mg/L-N,below the US EPA drinking water recommended maximum. Hence,based on these considerations, it is our opinion that the site is suitable for the use of a standard septic system utilizing leach lines. We recommend that the proposed septic tank size be increased to 2,500-gallon capacity to promote additional denitrification and mitigate excessive long-term buildup of Nitrate-Nitrogen in the groundwater. BAEZ GEOTECHNICAL GROUP Soil Suitability Study&Nitrate Loading Study-Becerra Residence,18163 S.Carrolton Road Page 9