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This oxidation of iron creates rust. When organic matter and an anaerobic environment are present within the soil environment, denitrification may be promoted. Denitrification is the conversion of nitrate to gaseous products and the primary mode by which nitrate concentrations are reduced. Important soil parameters for nitrate loading assessment are the organic matter content, pH, cation exchange capacity (CEC), clay content percentage, soluble salts, and the nitrate-nitrogen content. From the soil types encountered, the organic matter content in these soils is low, contributing to a low estimated nitrogen release (ENR). This suggests a very small potential for future nitrate-nitrogen loading. Since septic effluent is composed predominately of ammonium (NH4) ions, the pH, along with the CEC, have significant influence on ammonium molecules. The CEC measures the ability of the soil to theoretically trap and hold ammonium molecules. Although the shallow soils did not demonstrate acceptable percolative capacity during the four- hour pere tests, nevertheless, a small percentage of septic effluent will infiltrate and percolate within the leachline trenches. Clay soils have a higher CEC than other soil types and the encountered silty, clayey soil strata possess a comparatively high CEC. fhis suggests there is a greater potential for the retention of ammonium molecules for microbial uptake and suppression of nitrification (nitrate formation). Cations (positively charged ions) such as NH4+, K`, Ca" and Mg'' are removed from solution by the clay soil fraction because the clay particles possess a negative charge. 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 as the ammonium (NH4') ion form and 25% N in the organic form. if the ammonium ion is bound and eventually used, nitrification cannot occur. Nitrification is an aerobic reaction accomplished predominately by autotrophic bacteria which convert ammonium (Nf'4') to nitrite (NO,) and subsequently to nitrate (NO ). There is a thick clay laver found between 100 and 300 feet, referred to as the Corcoran Clay and underlies most of California's Central Valley. It has been demonstrated through well water chemistry that nitrate attenuation can occur because of this stratum. This is due to its thickness and 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. C. PERCOLATION TEST RESULTS Percolation tests were conducted on all four of the proposed parcels under U.S.E.P.A. and San Joaquin County Environmental Health guidelines. A 42-inch deep test boring and a 25-foot test 3