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11655 S UNION ROAD Project No. 2020-00095 Page 17 <br />For the site, the total daily toilet use contributing wastewater quantity, considering the <br />existing and planned residential dwellings is 350 gallons per day. The average total nitrogen <br />concentration of domestic wastewater is normally considered to be 20-85 mg/L-N with 35 <br />to 45 mg/L-N being typical (Crites and Tchobanoglous, 1998). Based on EHD request with <br />previous studies, it is estimated that residential domestic wastewater concentration will be <br />approximately 35 mg/L-N. <br />5.4.2 Denitrification Estimate <br />Denitrification is an important process which occurs within the leach field whereby certain <br />bacteria utilize the oxygen in the nitrate (NO3) and release nitrogen gas (N2) to the <br />atmosphere. Denitrification is expressed as a percentage, and the normally accepted range <br />is 10 percent to 35 percent. Soils with higher clay content, moist soil conditions, high pH, <br />and organic material denitrify by about 35 percent, and soils with high sand content and fast <br />percolation rates are estimated to denitrify at a lower rate, about 10 percent to 15 percent. <br />Based on the fine sandy loam soil type for the subject property, denitrification is estimated <br />to be 25 percent. <br />5.5 Nitrate Loading <br />A nitrate loading scenario was performed to determine the impact from the proposed <br />project. The scenario utilizes the variables discussed previously which represent the <br />estimated conditions and uses associated with this project. The following table presents a <br />summary of the nitrate loading calculation for the parcel. <br />Table 4. <br />Summary of Nitrate Mass Balance Analysis. <br />Parcel Result <br />(Nc) * <br />(mg/L-N) <br />Acres <br />(A) <br />Average Daily Flow <br />(Q) <br />(gpd) <br />Effluent <br />Concen- <br />tration (Nw) <br />(mg/L-N) <br />Leachfield <br />Denitrifi- <br />cation (d) <br />Deep Percolation <br />of Rainfall (R) <br />(Plate 7) (in.) <br />1 18.4 0.77 350 35.00 25% 2.74 <br />* Maximum contaminant level (MCL) for nitrate in drinking water is 10 mg/L-N. <br />5.6 Impact of Nitrate Loading to Nearby Water Sources <br />The nitrate maximum contaminant level (MCL) set by the US EPA is 45 mg/L-NO3 or <br />10 mg/L-N. The domestic well reported groundwater at the site was 15 mg/L-N. As <br />discussed above, the depth to groundwater is approximately 35 feet below ground surface <br />at the site. Although the soil is a fine sandy loam, the Veritas soil reported for the area is <br />known to have a hard pan layer from approximately 50 to 70 inches below ground surface; <br />based on professional experience with Veritas soil in the nearby area and confirmed by <br />review of a well completion log within the Veritas soil layer located on French Camp, the site <br />likely has a hardpan layer which would further protect the groundwater. <br />Petralogix Engineering, Inc. <br />26675 Bruella Road, Galt, Ca 95632 <br />(209)-400-5729 <br />www.petralogix.com • I I