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may not impede the downward movement to the extent a sand strata would based upon the phenomenon of impedance of water movement ftorn a finer grained soil encountering a coarser grained soil. Also, there will be greater microbiological activity within this underlying clayey stratum which will have a greater potential for the destruction of bacteria and viruses. C. PERCOLATION TEST RESULTS The percolation test was conducted on August 28, 2001 under U.S.E.P.A. and San Joaquin County Environmental Health guidelines, with two variations as described below. One shallow' perc test boring of 42 inches in depth was prepared in the location of the future leachfield (See Site Plan). This test depth was stipulated by the Environmental Health Department and represents the maximum depth of the soil/effluent interface. The test boring was drilled on , August 27. After drilling, the boring was twice pre-saturated to 90% of the test hole volume with clear water. After approximately 14 hrs, all the water had seeped away. This pre-saturation procedure was to induce maximum soil saturation in the soil around the test boring. Although test procedures call for a one hour test in sandy soils, the decision was made to conduct a four- hour test, again to ensure maximum soil saturation under "worst-case" scenario conditions. As illustrated on the attached FIELD PERCOLATION TESTING REPORT, the perc rate for the leachfield area indicates a rapid percolation, as was expected due to the sandy soil structure. The I percolation rate did not slow over the test intervals, revealing the downward movement of water was never impeded and the soil never became increasingly saturated. i D. GROUNDWATER INFORMATION Groundwater depths and elevations are illustrated on maps published by the San Joaquin County Flood Control and Water Conservation District. However, due to the tremendous variability in groundwater elevations in the Tracy area, groundwater lines are not drawn for this area. The last map denoting lines of equal elevation for the Tracy area was published in 1992 and show the general groundwater flow to be in a northeasterly direction, toward the San Joaquin River. Therefore, the groundwater under the project site is almost assuredly flowing toward the San Joaquin River. However, the influence of the River on the groundwater hydraulics can certainly change the directional flow of the groundwater. It is highly probable that there has been dramatic fluctuations in groundwater elevations under I the subject property over the years. The critical issue regarding the groundwater elevation is that there must be sufficient distance between the bottom of a leachline trench and the highest anticipated depth to groundwater for sufficient treatment of bacteria and viruses to occur. This is generally accepted to be five feet. 4 va!!ey Ag research