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Soil Suitability 1 Nitrate Loading Study Irerracon W hWow Property ■ Stockton, CA November 20, 2019 F Terracon Project No. NA197078 1.6.7 Discussion of Percolation Test Resuits Based on the percolation tests, boring B1 (3.5 feet bgs) failed to demonstrate a satisfactory percolation rate due to the clay soils present at shallow depths. Borings B4 (16 Feet bgs). B5 (18 feet bgs), and B6 (7 Feet bgs)demonstrated a relatively steady percolation rate between 125 and 250 minutes per inch throughout the testing period with the exception of one or two zero readings which resulted in a zero percolation rate. Boring B3 was tested twice once at 25 feet bgs which failed to percolate sufficiently due to very dense soils at depth. A sand lense was encountered at approximately 21 feet bgs in boring 133; therefore, Terracon backfilled the boring with approximately 4 feet of gravel and conducted a second percolation test at 21 feet bgs which demonstrated in a passing percolation rate of 27.7 minutes per inch. There was a question as to which percolation rate the EHD would accept; however, on November 7, 2019, Terracon was informed by Mr. Michael Kith that the passing percolation rate at boring 83 was acceptable. 1.6.8 discussion of Septic System Sizing In general, septic system construction should follow the San Joaquin County On-Site Wastewater Treatment Standards (OWTS) where applicable. The color coded map of San Joaquin County available at the EHD indicates the site is located in a clay zone where leach lines with pits are permitted. Refer to the OWTS Table 9.7.1 — Standard OWTS Design Requirements (page 28 of 40) for septic tank and leach line sizing recommendations. 2.0 NITRATE LOADING STUDY Because of concern regarding nitrate leaching into groundwater from septic systems, a nitrate loading analysis is required by the EHD for the proposed development to determine the potential concentration of percolating effluent and rain fall over the long term. Where percolating effluent leaches into the ground, groundwater quality may be impacted to some degree should the effluent reach groundwater. The method utilized for the nitrate loading analysis is based on a simple mass balance formula, discussed in a published article by Hantzsche and Finnemore (1992), which provides a convenient and simplified method to estimate long-term groundwater nitrate impacts on an area- wide basis. Hantzsche and Finnemore found that over a long period of time, water quality in the upper saturated groundwater zone closely approximates the quality of percolating recharge waters. They believe that their method is a good planning too[and provides useful information to planners and local officials but with some real limitations. In their article published in Ground Water, Hantzsche and Finnemore mention that "_..the equation ignores dispersion, lateral flow, and mixing with groundwater flow from up-gradient areas. These processes would generally contribute to additional reduction of nitrate-nitrogen concentrations in groundwater to the extent that the nitrate-nitrogen concentration of groundwater flow from up-gradient areas is lower." Responsive ■ Resourceful a Reliable 13