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w The three existing seepage pits accommodate the excess septic effluent produced during peak water-use periods. Two 36 inch diameter seepage pits completed to 25 feet bgs and backfilled with rock each provide an additional 400 gallon capacity for septic effluent during peak-use periods. A third rock filled seepage pit that is 42 inches in diameter and 25 feet deep provides an additional 540 gallon capacity. The estimated total infiltration capacity of the three seepage pits is approximately 1 ,300 gallons. Assuming that no infiltration were to occur for one week, the three seepage pits provide assimilative capacity for the wastewater produced by four people for an entire week (for calculations, see Appendix F). The additional water storage capacity supplied by the seepage pits compensates for the slow water infiltration of the soils where the leach lines are located. As discussed in Section 4, a sandy soil layer is encountered at approximately 10 to 15 feet bgs. Although the infiltration rate in this sand layer was not measured, it is expected to be significantly higher than in the overlying clay soils. The additional storage capacity of the seepage pits combined with the rapid infiltration rate of the sandy subsoil layer provides excess capacity for a four bedroom residence. 6.1 .2 Nitrogen Loading The actual nitrogen loading from the existing residence was not measured for this septic system evaluation. Therefore, nitrogen loading estimates are based on information published in established references including "Design Manual for Onsite Wastewater Treatment and Disposal Systems (EPA, 1980). Nitrogen loading from the existing septic system will be evaluated in two ways. First, average values for nitrogen concentrations in septic tank influent and effluent will be considered relative to site characteristics (cation exchange capacity, soil type, depth to groundwater, etc.) and expected attenuation factors for nitrogen in soil. The factors affecting nitrogen movement through soils include ammonia volatilization, adsorption of ammonium to soils, mineralization of organic nitrogen, and nitrification and denitrification rates. For a general discussion of these processes, see Appendix G. For this property, Figure 3 summarizes the nitrogen transformations from a typical household and indicates approximate concentrations for various nitrogen species (e.g. organic nitrogen, nitrate, ammonia, etc.) in the unsaturated zone. Figure 3 suggests that the typical household located on clayey soils similar to those at the property will produce wastewater containing some nitrate. However, with the clayey soils present at the SAC 125.12 8 DAMES & MOORE