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r7ocompare this theoretical maximum concentration of 62 ppm NO3 with previous studies, data from theCastello/Linne Estates Nitrate Loading Study of water from the Maurland Manor well was reviewed. This well water analysis was found to be 60 ppm NO3. It was theorized that the nitrate concentration determined from the Maurland well was due to the following scenario: This well had not been operational for several months prior to being turned on to adequately purge the casing before sampling. At the time the well was shut off for excessive nitrate, the nitrate . concentration was 45 ppm in June 1998. When the sample was retrieved in September 1999, the nitrate level had increased to 60 ppm. The well log indicates perforations from 89 ft to 214 ft. It is theorized that nitrate "settled" on the top aquifer with the only influence on this concentration being an additional influx of nitrate, fluctuations in groundwater elevation, and the velocity of the groundwater flow. When the well was turned on, the cone of depression(drawdown curve) pulled in the accumulated nitrate into the well. It is theorized that since the Cole well has less upgrad.ient septic system influence than the Maurland well,the water table nitrate could be at a f lower concentration. It can be hypothesized that the observed nitrate concentrations within "Valpico Section" are coming into equilibrium. Sources contributing nitrate to the underlying groundwater include indigenous soil concentrations from decomposing organic matter, rainfall, upgradient agricultural fertilizer inputs, septic systems (particularly sumps and pits), and lawn/landscape fertilization. Factors decreasing the groundwater nitrate concentration include denitrification, groundwater movement (both vertical and horizontal), well pumping and hydraulics, and cleaner water recharge contributing to a dilution effect. Therefore, if each of these sources and attenuating factors could be quantified on a mass balance basis, it may be that nitrate input is now roughly equivalent to output, or attenuation. Given that agricultural irrigation recharge is the largest single contributor to groundwater nitrate concentrations based on my own observations, and since the surrounding land has been farmed for several decades, it is assumed that the nitrate concentrations in the underlying groundwater should be much higher than those observed, if the attenuating factors were not significant. This is what was observed with the soil chemical analyses. The nitrate concentration within the soil environment under the Cole's leachline revealed decreasing nitrate-nitrogen concentrations with depth, presumably due to denitrification. The denitrification potential is significant due to the comparatively high clay content of the indigenous soils, the higher soil pH, high soil moisture content and organic fraction content. NITRATE-NITROGEN IMPACT MITIGATION Given that the indigenous soils under the subject property are conducive for denitrification, the septic system for the new Cole residence on Parcel 1 should be installed to take advantage of these favorable soil characteristics. This can be accomplished by installing a dual septic system per San Joaquin County Sewage Standards Section 10.1 which states: "Estate type zoning (R=R) shall require a double leach field system with an alternating distribution box to be'installed at the time of construction."j It is proposed to install two separate leachfields: north of the new house and south of the new house. The south facing leachf eld will take advantage of the increased "drying effect" during the drier months of April through October. Leachline trenches should be installed as high in Page -8- VlaCfey Ag Research