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Soil Suitability/ Nitrate Loading Study Addendum Irerracon Arriaga Property Stockton, California August 31, 2020 Terracon Project No. NA207046 Result Average Effluent Leachfield Net Rainfall Land Use (Nc)* Acres Daily Flow Concentration Denitrification Recharge (mg/L-N) (A) (Q)(gpd) (Nw) (d) (R)(in/year) (mg/L-N) Existing 2 Bedroom Residence+ 7.1 1.25 300 35 35% 8.35 Proposed 2 Bedroom Mobile Home Maximum contaminant level(MCL)for nitrate in drinking water is 10 mg/L-N. Based on the revised nitrate loading calculation, the existing two bedroom residence is anticipated to be 4.5 milligrams per liter(mg/L) nitrate (N) below the EPA maximum contaminant level (MCL) of 10 mg/L-N. The existing residence and proposed two bedroom mobile home is anticipated to be 7.1 mg/L-N also below the MCL of 10.0 mg/L-N. 2.3.6 Impact of Nitrate Loading to Nearby Water Sources Refer to Terracon's prior NLS report dated April 26, 2019 (Terracon Project No. NA187059). 2.4 Discussion of Nitrate Loading Results 2.4.1 Total Nitrate Impact Based on the method and assumptions described, the result of the nitrate loading balance (7.1 mg/L N for both residences) indicates that the development on the parcel is not anticipated to cause a buildup of nitrates in the aquifer above the MCL of 10.0 mg/L-N. 2.4.2 Methods of Mitigation Based on the nitrate loading results, mitigation measures appear unwarranted for the project. 3.0 CONCLUSIONS AND RECOMMENDATIONS The site is currently developed with an existing two bedroom residence, well and septic system and is proposed to be developed with a second two bedroom mobile home and additional septic system. There were no septic permits available for the site address; therefore, the size and design of the existing septic system was unknown. Based on the San Joaquin County OWTS, the site is located in a clay area of the county where leach lines and seepage pits are permitted. Septic systems in the area appear to utilize 65 to 100-foot long leach lines with or without 25 foot deep seepage pits at the end of each leach line. The maximum daily flow rate for the existing residence and proposed residence is 560 gpd. Terracon conducted two shallow percolation test borings (62 and 64) in the vicinity of the proposed leachfield. Onsite soils consisted primarily of clay between approximately 36 and 42 inches bgs. Boring B2 demonstrated a percolation rate of 35.7 minutes per inch; however, excess me Responsive ■ Resourceftul ■ Keliable 15