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[51 LOCAL GROUNDWATER AND SURFACE WATER INFORMATION The groundwater, or the zone of saturation is located at approximately 56 ft below grade according to a well contractor who recently drilled a new domestic on-site well. An observation hole could not be located on the top of the well casing, so the exact static water level could not be determined with a water level meter. There was no evidence of a perched water table condition during our exploratory boring procedures and there is no potential impact to surface waters, primarily irrigation waters in the area. Analysis of water drawn from a domestic well adjacent to the north of the existing residence on Parcel 1, illustrates a medium-high concentration of nitrate at 38 ppm NO3. The pesticide DBCP was not detected (ND). 161 CONCLUSIONS The percolation rates for the subject property indicate that a septic system with an 8,000 to 12,000 square foot leachfield may be used for new residential structures on parcels 2, 3 and 4. The final square foot area will be dependent upon the number of bedrooms each structure will have. Permit Applications for additional leachline within the study area would indicate a larger leachfield should be installed. In addition, we are also recommending sumps (or pits) be installed to ensure adequate management of septic effluent, particularly on four or more bedroom houses. If a high number of people will occupy a residential structure, for example six people in a three- bedroom house, then sumps should definitely be installed. A 50% standby replacement area must also be incorporated into the design of the septic system on each parcel. The referenced leachfield areas and sumps or pits will help ensure stability and guard against leachfield failure. A failure would primarily result from the formation of a "biomat" or "clogging mat" on the wetted soil where the effluent enters the soil. 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. This type of biological activity will probably prevail due to the sandy/silty nature of the leachfield area soil. With respect to the protection of the underlying groundwater from pathogens and nitrates (NO,), the depth to groundwater (-56 ft) can be considered a medium depth. The analytical results for ~ nitrate concentration of 38 ppm suggest there has been impact to the underlying groundwater. 11 1 J Rik- 001 � lity &.trot J.Pm ,on