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Soil Suitability/ Nitrate Loading Study Addendum Irerracon Arriaga Property Stockton, California August 31, 2020 Terracon Project No. NA207046 water was inadvertently added to the boring due to equipment error. Boring B4 demonstrated a percolation rate of 125 minutes per inch. Terracon anticipates both boring B2 and B4 would have demonstrated similar percolation rates based on the similar clayey soil conditions present at 3.33 to 3.5 feet bgs and suggests the percolation rate at boring B4 of 125 minutes per is more representative of the clayey soil conditions encountered on the site. Based on Terracon's 2019 NLS (Terracon project No. NA187059), depth to groundwater was anticipated to be greater than 97 feet bgs. Nitrate as N (5.9 mg/L)was identified in a groundwater sample collected from the onsite domestic below the MCL of 10 mg/L-N. A second groundwater sample reported below laboratory detection limits for DBCP. The average deep percolation of rain for Stockton area was estimated to be 2.64 inches per year. The nitrate concentration of rainfall is estimated to be 1 mg/L-N. The estimated potential average daily flow for the existing and proposed residence was 560-gallons of effluent per day. Based on the clay soils anticipated, a denitrification factor of 35 percent was selected. Nitrate levels for the existing two bedroom residence was calculated to be 12.6 mg/L-N and the proposed two bedroom mobile home was calculated to be 12.6 mg/L-N for a combined total of 25.2 mg/L-N, when an average two person occupancy was calculated per bedroom. In February 2020, the EHD approved a revised calculation method for long term average discharge of waste water, adjustment to the evapotranspiration rate, and net rainfall recharge; therefore, Terracon revised our 2019 NLS (Terracon project No. NA187059). Based on the revised nitrate loading study, the average net rainfall recharge for Stockton is revised to 8.35 inches/year. The nitrate concentration of rainfall is estimated to be 1 mg/L-N. The revised long term average discharge rate for the existing and proposed residence is 300-gallons of effluent per day. The existing two bedroom residence is anticipated to be 4.5 milligrams per liter mg/L-N which is below the 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. Therefore, mitigation for the project appears unwarranted. In summary,the parcel appears suitable for the proposed development. The septic system design should accommodate peak flows and follow the San Joaquin County OWTS standards where applicable. 040 Responsive ■ Resourceful ■ Reliable 16