Laserfiche WebLink
10 1 P10 Lei 'IMPIOUTl <br />ENGINEERING CORP. <br />TO: Kasey Foley, REHS <br />Interim Director, San Joaquin Environmental Health Department <br />FROM: Norman Hantzsche, PE, Questa Engineering Corporation F <br />DATE: March 4, 2020 <br />SUBJECT: OWTS Design Review for New Residence, 7707 S Ash St, French Camp <br />PROJECT OVERVIEW <br />The project is for the development of a new 2 -bedroom residence on a 0.23 -acre lot in French <br />Camp. Plans and supporting documentation for the OWTS are contained in the "Nitrate Loading <br />Study, Soil Suitability Study" by Chesney Consulting, dated December 20, 2019. Surface soils <br />on the site are reported to be fine sandy loam with good percolation. Percolation testing <br />demonstrated a rates of 13.9 min/inch at 36 -inch depth and 7.8 min/inch at 96 -inch depth. <br />Estimated water table depth in the project area is 35 to 45 feet below grade, based on regional <br />groundwater maps. The proposed OWTS consists of a standard 1,200 -gallon septic tank with <br />gravity disposal to two leachfield filter beds, each 7' wide by 37' long. The 100% reserve area <br />will require the use of a pump system. <br />COMMENTS <br />The Nitrate Loading Study by Chesney Consulting projects an estimated resultant <br />groundwater nitrate—nitrogen concentration of 9.9 mg-N/L, utilizing the Hantzsche- <br />Finnemore equation. However, the analysis includes a calculation error in converting the <br />annual recharge from gallons to acre-feet. The total recharge estimate of 55,096 gallons is <br />equal to 0.17 ac -ft, rather than 1.27 ac -ft as indicated by calculations shown on page 4. This <br />has a significant effect on the calculation of the resultant N value. Substituting the corrected <br />value of 0.17 ac -ft for recharge gives a projected resultant value of 33 mg-N/L, well in excess <br />of the drinking water criterion of 10 mg-N/L. <br />2. The nitrate loading analysis uses a value of 300 gpd for the wastewater flow, which is an <br />appropriate factor for OWTS design, but overestimates the long-term average wastewater <br />flow for nitrate loading analysis. I recommend using an average wastewater flow value of <br />150 gpd for the proposed 2 -bedroom house. Substituting 150 gpd for wastewater flow gives a <br />resultant nitrogen concentration of nearly 25 mg-N/L, still well above the 10 mg-N/L <br />criterion. <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 />