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Civil, Environmental . Et Wa ter Resources MEMORANDUM ENGINEERING CORP. TO: Kasey Foley, REHS Interim Director, San Joaquin Environmental Health Department FROM: Norman Hantzsche, PE, Questa Engineering Corporaty Ad. DATE: January 29, 2020 SUBJECT: OWTS Design Review for Truck Terminal, 25 E French Camp Road, French Camp PROJECT OVERVIEW The project is for the development of a Class 8 tractor-trailer vehicle parking facility with a shop, office and restroom. The subject property consists of a 15.71-acre parcel of which 4.48 acres will be developed for the project. The site is presently undeveloped, but was used historically for agriculture. Surrounding properties in the area consist of a mix of residential, commercial and industrial land uses. A new onsite wastewater system (OWTS) will be installed to accommodate wastewater flows from restroom use by employees and customers, up to an estimated maximum of about 43 people per day. Plans and supporting documentation for the OWTS are contained in the"Nitrate Loading Study, Soil Suitability Study"by Chesney Consulting, dated November 30, 2019. Surface soils on the site are reported to have a high clay content and slow permeability to a depth of about three feet; below that the soil transitions from sandy clay loam to loam at about 5 feet, and then to very permeable loamy sand at eight feet. Percolation testing demonstrated a rate of 83.3 min/inch at 36-inch depth and 4.8 min/inch at 96-inch depth. Estimated water table depth in the project area is 30 feet below grade, based on regional groundwater maps. The proposed OWTS consists of a standard 2,000-gallon septic tank with disposal to a 1,500 ft2 filter bed, followed by overflow to a 2-ft wide by 20-ft long by 8-ft deep leaching trench (sump) at the end. The design wastewater flow is estimated at 215 gpd, with a projected average annual wastewater generation of approximately 64,500 gallons per year. COMMENTS The Nitrate Loading Study by Chesney Consulting projects an estimated resultant groundwater nitrate—nitrogen concentration of 1.8 mg-N/L, well below the 10 mg-N/L drinking water standard. I agree with the methodology and calculations presented by Chesney with the exception of the value for rainfall recharge. The study states an assumption of 13 inches per year for average annual rainfall recharge (R value in the Hantzsche-Finnemore equation), based on the assumption that average recharge is equal to average annual rainfall. This is not universally true, since there will always be some amount of rainfall lost to runoff, evaporation from the 1 Box 70356, 1220 Brickyard Cove Rd.Suite 206 Pt.Richmond, CA 94807 T:510/236.6114 F:510/236.2423 E:Questa©QuestaEC.com