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4.3 GEOLOGY AND SOILS Wastewater Disposal Feasibility Impact 4.3-1 Without the benefit of adequate field testing, the installation and use of individual residential septic systems in easements as presently planned and identified by the applicant could result in system failures, localized saturation and bacteriological contamination problems. Individual septic tank-leachfield systems are the proposed method of sewage disposal for the proposed project. They would be constructed in private sewer easements located in common areas nearby home site parcels, on the golf course and in open space areas. Septic systems are regulated by San Joaquin Local Health District under County Ordinance Code Section 5-3088. Feasibility foron-siteseptic systems is based mainly on soil depth and texture, percolation rates, groundwater separation, amount of available land, and setbacks to water courses and wells. Ground slope, because of the gentleness of slope on the project area, is not a constraint. As part of the EIR study, six percolation tests (P-1 through P-6)were conducted on six different proposed sewage disposal easement sites (Table 4.3-1). Tests were conducted between 22 and 42 inches in accordance with the procedures contained in the EPA Design Manual for On-Site Wastewater Treatment and Disposal Systems. Tests were not conducted in the hardpan layers encountered at 24-inches in P-1, 36-inches in P-4, and 56-inches in P-5. They were conducted in the "dry" season; the results would not necessarily be the same if they had been performed during wet winter months. This is because some soils (e.g., clayey soils as observed in P-2) swell and drain much slower under wet conditions. Also, groundwater, either perched on top of hardpans and claypans or as static groundwater, could produce slower percolation rates during winter months. The percolation test results are provided in Table 4.3-1, and show good to excellent rates in all test holes. Based upon percolation rates and soils information,the project site was divided into four classes of soil suitability. A description of each of the four suitability classes is given in Table 4.3-2;the areas of the site in each class are mapped in Figure 4.3-2. As can be seen from Figure 4.3-2, significant portions of the project area are poorly suited for on-site septic systems. Comparing the proposed 28 septic system easement sites with the rating of soils/septic system suitability reveals the following: • Seven of the proposed disposal sites are located in areas considered to have no limitations or moderate limitations for septic systems. • Eighteen of the proposed septic system easements are in areas having severe constraints, due mainly to the very shallow depth of soil over hardpan. 4.3-4