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Since septic effluent is composed predominately of ammonium(NH4) ions,the pH, along with the CEC, are significant influences on ammonium molecules. The pH of the soil is alkaline with a pH of 8.0 which may inhibit ammonium formation and stability, and may create a small propensity for ammonia(NH3) formation. Cation Exchange Capacity measures the ability of the soil to theoretically trap and hold ammonium molecules. The clayey surface and subsurface soils (with gravel removed)have a comparatively high CEC which may increase the retention of ammonium molecules for microbial uptake and suppression of nitrification(nitrate formation). In cation exchange, a positively charged ion within the clay complex is replaced and released by another type of ion. Ionic exchange,through its effects on nutrient availability and acidity, has a tremendous effect on biological and chemical transformations. The significance of cation exchange becomes important when the chemistry of septic effluent is analyzed. Nitrogen in septic tank effluent consists of approximately 75%N in the ammonium ion(NH4') form and 25%N in the organic form. If the ammonium ion is bound and eventually utilized,nitrification cannot occur. Nitrification is an aerobic reaction accomplished predominately by autotrophic bacteria which convert ammonium (NH4') to nitrite (NO2) and subsequently to nitrate (NO3). Therefore, the clay soil fraction at the leachline depth of 42 inches should provide nitrate formation mitigation. There is a thick clay layer referred to as the Corcoran Clay found throughout most of California's Central Valley. This clay layer "pinches off' and is relatively thin as it approaches the Diablo Mountain Range. From previous studies near the base of the Range,the Corcoran clay layer appears to extend from 151 ft to 197 ft. This is significantly "thinner" than its width toward the interior of the Central Valley. Water from aquifers under the Corcoran clay typically have nitrate contamination less severe than aquifers above the clay layer, suggesting the layer attenuates the downward migration of nitrate. C. PERCOLATION TEST RESULTS The percolation tests were conducted on July 3, 2003 under U.S.E.P.A. and San Joaquin County Environmental Health guidelines. A shallow(42 inches) and a deep (25 feet)perc test were done in the proposed location of the disposal area. According to the Environmental Health Department, percolation tests may be one hour in duration if the first six inches of water seeps away in less than 30 minutes after the 24-hour saturation period. This was certainly the case for both the shallow and deep perc test borings;however, it was decided to conduct a full four hour test to produce saturated soil conditions that would represent "worst-case scenario" conditions. As indicated on the attached FIELD PERCOLATION TESTING REPORTS, the perc rates for the two test borings reveal excellent perc rates. These rates are considered comparatively rapid and may be attributed to the aforementioned gravel mixture and gravel stratum within the shallow and deep test borings. The test results are summarized below: �L�_ Page -3- Chesney Cortsultir:g