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FFapproximately ick clay layer referred to as the Corcoran Clay found throughout most of California's ey. This clay layer extends under the subject site from approximately 120 feet to 200 feet below grade. It has been demonstrated from deep wells which continue layer that there has been nitrate attenuation under this clay stratum. This is due to itsd slow permeability. Aquifers below the clay layer are recharged in part from other areas, such as the Coastal Range to the west and Sierra Nevada Range to the east where nitrate influences are less pronounced. 2. SUBSURFACE SOIL HYDRAULIC CONDUCTIVITY As referenced on the Log of Boring, the subsurface soils consist predominately of a sandy silt/silty sand. The hydraulic conductivity (K) of the vadose zone (subsurface soils) determines the downward flow characteristics of septic effluent and surface waters (e.g., rainfall and domestic irrigation). To quantify this parameter,the Shallow-Well Pump-In Test Method (Soil Science Society of America) was employed. The test was conducted near the center of Parcel 1. The deep soil boring for soil classification was used for this test. A constant level of water was ` supplied to the boring,to a height of 9 ft. The rate at which water flowed into the soil (Q) was determined to be 6,179 cc/hr. Soil characteristics of the soil profile do not vary significantly the entire length of the test boring, so an average hydraulic conductivity is obtained by this method. .- Test conditions were established where the distance between the water level in the hole and the restrictive hardpan layer is equal to the distance between the water level and the bottom of the boring. Therefore, the equation to determine the hydraulic conductivity in cm/hr is as follows: K= 3 In h/r II h(h+2T„) Where: h=9 ft(274 cm) r=0.21 ft(6.4 cm) Q =6,179 cc/hr T„= 9 ft (274 cm) Therefore, K=0.1 cm/hr These test results indicate the hydraulic conductivity of the subsoil strata possess an average permeability considering the sand content and demonstrates the silt and clay fractions have an influence on subsurface permeability. The hydraulic conductivity test results indicate it would theoretically take a nitrate molecule approximately 8,534 hrs or 356 days to travel from near the soil surface to the water table, --28 ft below grade. This hydraulic conductivity is also accounting for the hydraulic barrier properties of the hardpan layer. 4 Va!!ey Ag Research