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r�� nitrate producing areas. The pesticides DBCP and EDB were not detected(ND). Considering the intense, although relatively recent, rice production in this part of the County, a higher nitrate impact concentration would have been expected from the well analysis. The nitrate concentration illustrates there are significant clay layers in addition to the hardpan layer that have probably attenuated the downward migration of nitrate molecules. [5] CONCLUSIONS The percolation rates obtained for the subject parcels indicate considerable variability in soil structure at the test depths from approximately 5 to 10 feet below grade. From the observed high clay content shallow soils, it was determined that shallow perc tests would certainly fail at the typical leachline depth of 42 inches. Results of the percolation testing can only be assumed to be representative of the subsurface conditions encountered in the area of testing. Therefore, a prospective buyer of any of the subject parcels may wish to conduct percolation testing in the exact location of a seepage pit or have expert opinions made during septic system emplacement. We are recommending for all proposed structures that leachlines be placed as high in elevation as allowable by the Health Department.This is normally 6 to 12 inches. Shallow leachline emplacement will allow evapotranspiration to take place through both evaporation and plant life transpiration during favorable months of the year. The leachfield(s) should consist of 8,000-12,000 square feet which will ultimately depend on the size of the structure and/or the number of bedrooms. It is also recommended to install seepage pits in place of sumps to take advantage of greater infiltrative surface area and underlying sand strata. The number of seepage pits will also depend on the size and/or number of bedrooms. If there will be several people inhabiting the residence, particularly children, a septic system should be oversized to accommodate additional wastewater flows. A 50% standby replacement area must also be incorporated into the design of any septic system. All related septic system structures and any domestic well must comply with the published "distance" requirements from property borders, roads, irrigation fields and laterals, etc. The recommended leachfield areas and seepage pits will help ensure stability and guard against septic system failure. A failure would primarily result from the advanced formation of a "biomat" or "clogging mat" on the wetted soil/effluent interface. This mat is composed mostly of aerobic and anaerobic bacteria and bacterial products consisting of a slime of polyuronides and polysaccharides. If the soil/effluent interface is well aerated, a variety of soil microfauna such as nematodes and protozoa may digest the clogging mat bacteria. Since the subsurface soils consist of a heavier clay material, anaerobic conditions will probably prevail which may hasten leachfield failure. In this scenario, the seepage pits then manage most of the effluent, although seepage pits also develop biomat formation. Halley Ag Research