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33503 S Koster Road Project No. 2021-00073 P a g e | 9 septic tank to mitigate potential buildup of nitrate by increasing retention and digestion time within the tank to improve leach field conditions is recommended. While our study has shown that nitrate concentrations within groundwater will not likely buildup to levels above background based on the onsite use, it should be recognized that San Joaquin County EHD may require long-term nitrate monitoring (in the form of annual well sampling) or a septic system capable of mitigating and reducing nitrate. 7.0 EXISTING AND PROPOSED SEPTIC TANK USAGE 7.1 Existing On-Site Septic Tank Usage, Permits and Repair History Two permits available for review related specifically to the site septic system. Permit No. 87-2400, issued in 1987 is an application for the installation of a new 1200-gallon capacity septic tank and 2 leach lines with seepage pits to 12 feet below ground surface. One repair/addition permit (Permit No. SR0026157) reported for the site in 2001 indicates a total of 12 infiltrators were installed in the leach field system. The permits are available for review in Appendix B. 7.2 Expected Maximum Septic Tank Usage for Site Expected maximum septic tank usage for the Site is to accommodate the current three- bedroom, single-family residence, and the proposed two-bedroom, two-bath, accessory dwelling-unit. 7.3 General Area Septic Tank Usage, Permit, and Repair History In an attempt to determine the septic system density for the vicinity of the site, our office reviewed a recent aerial photograph available from GoogleEarthTM. We counted the clusters within approximately one square mile of the site, with the assumption each building cluster has one septic system. Using this method, we determined there are approximately 50 septic systems per square mile in the immediate vicinity of the site. This is a low number of septic systems, with the majority of these systems located scattered within large tracts of agricultural land. The EHD septic permit files were obtained from the San Joaquin Environmental Health Department (EHD) Online EHD Program Records6 in an effort to obtain copies of permits for any septic tanks located within the general area of the subject property. Copies of the septic search permits for surrounding parcels have been included in Appendix D of this report. The records are summarized in the following table: 6 https://lfweb.sjgov.org/WebLink/Browse.aspx?dbid=1&cr=1 accessed September 9, 2019