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Soil Suitability/ Nitrate Loading Study Irerracon Manteca Farm Improvement ;- Manteca, California October 29, 2019 Terracon Project No. NA197073 2.4.2 Methods of Mitigation Mitigation may consist of the utilization of an advanced water treatment product, which are specifically designed to lower nitrates at point of discharge or other mitigation options. 3.0 CONCLUSIONS AND RECOMMENDATIONS In summary the site soils consisted of silty sands to 10+feet bgs. Percolation test boring B1 was excavated to a depth of 3.5 feet bgs and resulted in satisfactory percolation rates of 2.77 minutes per inch. Based on the favorable percolation results, the site soils appear suitable for a standard septic system. Depth to groundwater is anticipated at approximately 34.5 feet bgs with an anticipated northeasterly groundwater flow direction. Groundwater samples were collected from a faucet connected to the private groundwater well, located adjacent west of the classroom building. Nitrate as N was reported at 32 mg/L in groundwater sample GW-1 above the MCL of 10 mg/L-N or 140 mg/L nitrate above the MCL of 45 mg/L nitrate. Groundwater sample GW-2 reported below laboratory reporting limits of 0.010 ug/L for DBCP. The site is currently developed with two septic systems and is proposed to be developed with a third septic system. Septic System No. 1 (existing classroom building) nitrate balance is 24.1 mg/L-N above the MCL. Septic System No. 2 (existing office building)nitrate balance is 3.2 mg/L- N below the MCL; however, the combined nitrate loading balance for both Septic System No. 1 and No. 2 is 17.1 mg/L-N, above the MCL of 10 mg/L-N. Septic System No. 3 (proposed 15,000 SF agricultural building) nitrate loading balance is 15.3 mg/L-N which is anticipated to further increase nitrate in the aquifer above the MCL to 23.3 mg/L-N when combined with the existing classroom building. As the existing office building will be demolished when the proposed 15,000 SF agricultural building is developed,the office building was not included in the final nitrate loading calculation. In summary, the parcel appears suitable for the proposed development. The septic system design should accommodate the peak flows and follow the San Joaquin County OWTS standards where applicable. Elevated nitrates were identified in the private groundwater well above the MCL. If the EHD requires mitigation for the project, an advanced water treatment product may be considered a mitigation option. 4.0 LIMITATIONS As indicated, our study takes into account the subject property only with the sewage load applied over the property site area. The study does not take into account surrounding properties and/or lands. 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 Responsive ■ Resourceful ■ Reliable 19