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MEMORANDUM <br />Civil, <br />Environmental <br />Et Water <br />Resources <br />ENGINEERING CORP. <br />TO: Kasey Foley, REHS <br />Interim Director, San Joaquin Environmental Health Department <br />FROM: Norman Hantzsche, PE, Questa Engineering Cotporatioo Y5 <br />DATE: February 3, 2020 <br />SUBJECT: OWTS Design Review for Robertson Ready Mix, S Harlan Road, French Camp <br />PROJECT OVERVIEW <br />The project is for the installation of restroom facilities to replace portable toilets at the Robertson <br />Ready Mix facility on Harlan Road in the town of French Camp. The project site improvements <br />also include onsite stormwater retention/percolation facilities. The subject property consists of <br />an 11.07 -acre parcel of which 4.82 acres are encompassed in the project. A new onsite <br />wastewater system (OWTS) will be installed to accommodate wastewater flows from restroom <br />use by onsite employees and truck drivers, up to an estimated maximum of 20 people per day. <br />Plans and supporting documentation for the OWTS are contained in the "Nitrate Loading Study, <br />Soil Suitability Study" by Chesney Consulting, dated November December 27, 2019. Surface <br />soils on the site are reported to be very permeable sandy loam to about 4 -ft depth, with <br />increasing clay content and slower percolation at deeper depths. Percolation testing demonstrated <br />a rate of 5.6 min/inch at 36 -inch depth and 62.5 min/inch at 96 -inch depth. Estimated water table <br />depth in the project area is 28-29 feet below grade, based on regional groundwater maps. The <br />proposed OWTS consists of a standard 2,000 -gallon septic tank with disposal to a standard <br />leachline system, including three 36 -inch deep by 85 -ft long trenches (255 lineal feet total). The <br />design wastewater flow is estimated at about 100 gpd, with a projected average annual <br />wastewater generation of approximately 23,000 gallons per year (0.071 acre-feet/year). <br />Eke) 0101 D)►I K9 <br />Following the Hantzsche-Finnemore methodology, the Nitrate Loading Study by Chesney <br />Consulting projects an estimated resultant groundwater nitrate—nitrogen concentration of 0.67 <br />mg-N/L due to the proposed OWTS, well below the 10 mg-N/L drinking water standard. The <br />value for average annual rainfall -recharge (R) used in the analysis was developed through an <br />explicit accounting of impervious and pervious surfaces, resulting in an estimated annual <br />recharge volume of 5.70 acre-feet over the 4.82 acre site. I agree with the approach and <br />calculations, and concur with the conclusion that the nitrate loading impacts will be negligible <br />(<1.0 mg-N/L). No further analysis or response is required. I recommend acceptance of the <br />Nitrate Loading Study as submitted. <br />Box 70356, 1220 Brickyard Cove Rd. Suite 206 Pt. Richmond, CA 94807 T.• 510/236.6114 F: 510/236.2423 E: Questa®QuestaEC.com <br />