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Civil, Environmental . £t Water Resources MEMORANDUM ENGINEERING CORP. TO: Kasey Foley,REHS Interim Director, San Joaquin Environmental Health Department FROM: Norman Hantzsche, PE, Questa Engineering Corporati DATE: February 3,2020 SUBJECT: OWTS Design Review for Robertson Ready Mix, S Harlan Road,French Camp PROJECT OVERVIEW The project is for the installation of restroom facilities to replace portable toilets at the Robertson Ready Mix facility on Harlan Road in the town of French Camp. The project site improvements also include onsite stormwater retention/percolation facilities. The subject property consists of an 11.07-acre parcel of which 4.82 acres are encompassed in the project.A new onsite wastewater system (OWTS)will be installed to accommodate wastewater flows from restroom use by onsite employees and truck drivers,up to an estimated maximum of 20 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 December 27,2019. Surface soils on the site are reported to be very permeable sandy loam to about 4-ft depth, with increasing clay content and slower percolation at deeper depths. Percolation testing demonstrated a rate of 5.6 min/inch at 36-inch depth and 62.5 min/inch at 96-inch depth. Estimated water table depth in the project area is 28-29 feet below grade,based on regional groundwater maps. The proposed OWTS consists of a standard 2,000-gallon septic tank with disposal to a standard leachline system, including three 36-inch deep by 85-ft long trenches (255 lineal feet total). The design wastewater flow is estimated at about 100 gpd, with a projected average annual wastewater generation of approximately 23,000 gallons per year(0.071 acre-feet/year). COMMENTS Following the Hantzsche-Finnemore methodology,the Nitrate Loading Study by Chesney Consulting projects an estimated resultant groundwater nitrate—nitrogen concentration of 0.67 mg-N/L due to the proposed OWTS,well below the 10 mg-N/L drinking water standard. The value for average annual rainfall-recharge (R)used in the analysis was developed through an explicit accounting of impervious and pervious surfaces,resulting in an estimated annual recharge volume of 5.70 acre-feet over the 4.82 acre site. I agree with the approach and calculations,and concur with the conclusion that the nitrate loading impacts will be negligible (<1.0 mg-N/L).No further analysis or response is required. I recommend acceptance of the Nitrate Loading Study as submitted. 1 Box 70356, 1220 Brickyard Cove Rd.Suite 206 Pt.Richmond,CA 94807 T.510/236.6114 f:510/236.2423 E:QuestaQa QuestaEC.com