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MEMORANDUM Civil, Environmental Et Water Resources ENGINEERING CORP. TO: Michael Kith, REHS Program Coordinator, San Joaquin Environmental Health Department FROM: Norman Hantzsche, PE, Questa Engineering Corporation DATE: April 22, 2020 SUBJECT: OWTS Design Review for Nile Garden Elementary School, Manteca Project Overview and Initial Design The project is for modernization of the Nile Garden Elementary School located at 5700 East Nile Avenue in Manteca. The project includes the construction of a new multi-purpose building, kindergarten classroom building, and other improvements including upgrade and expansion of the onsite septic system (OWTS). The school has an existing student enrollment plus staff of 828, which is not projected to increase as a result of the project. The construction of the new OWTS facilities falls under the purview of the San Joaquin Environmental Health Department, which triggered the preparation of a Nitrate Loading Study (NLS) for County review. Questa was tasked with providing a third -party technical review of the NLS and OWTS design. Plans and supporting documentation for the OWTS were initially provided in: (1) engineering plans prepared by MCR Engineering (July 2019); and (2) "Soil Suitability - Nitrate Loading Study" (December 2019) by Condor Earth consultants. The subsurface conditions on the site consist of deep, unconsolidated alluvium. Surface soils in areas planned for the new/upgraded OWTS are reported to be well drained, silty sand/coarse loamy sands. Percolation tests at depths of 2 and 3 feet showed rates of 5.7 and 10 minutes per inch. Groundwater was reportedly encountered at a depth of 10 feet below ground surface in November 2018; it may rise higher by a few feet during other times of the year. Questa's initial review of the engineering plans and NLS raised questions about several issues, which were conveyed to the project engineer (MCR Engineering) in a conference call with Environmental Health on April 10, 2020. The main review points centered on the projected nitrate loading impact of the OWTS as indicated in the Condor Earth Nitrate Loading Study. Nitrate mass balance loading calculations by Condor projected a resultant concentration of 25.4 mg-N/L , which is significantly above the target limiting criterion of 10 mg-N!L contained in County OWTS requirements. Condor did not propose any specific nitrate mitigation measures, but did point out the limited significance of the impacts due to the surrounding agricultural land uses in the area. Box 70356, 1220. Brickyard Cove Rd. Suite 206 Pt. Richmond, CA 94807 T. 510/236.6114 F. 510/236.2423 E: Questa@QuestaEC.com