t than the natural surrounding ground level. The distribution pipe should be covered with 2 in. of gravel and a soil infiltration barrrier such as untreated building paper. The backfill can be the soil from the excavation, provided that that soil is not less dense than the natural topsoil. If you backfill and cap the excavation with sand, while the surrounding topsoil is compact loam, rainfall will run into the excavation and overload the system. It is a very good idea to rototill the soil for 10 ft on either side of the excavation SO that when the cap is placed, the backfill and the natural soil will mix and rainfall will seep straight down instead of running along the natural ground surface under the fill and flowing into the disposal excavation. Operation and Maintenance In addition to normal system operation and maintenance, the con- dition of the soil caps should be monitored from time to time. A good way to prevent their erosion is to plant grass on them and repair any damage occurring to the surface. SAND MOUNDS Application In many areas where there is extremely porous subsoil, slowly permeable soil, a high ground water table, or shallow soil above bedrock or other relatively impermeable layer, an elevated drain CONVENTIONAL SYSTEM MODIFICATIONS Sand Mound Septic Wet Well Dosing Chamber Figure 6.5 Sand mound system components. field cannot be raised high enough to provide adequate separation distance between the disposal excavation and the limiting soil con- dition. In these areas, use of a sand mound should be considered. A sand mound is basically a pile of medium textured sand placed over the natural topsoil, into which a shallow drain field is excavated. However, details of design are more complex and critical than any system SO far discussed, and these should be thoroughly under- stood before installation is attempted. Even slight deviations from the procedures described here could result in premature failure of the mound. Therefore, the consequences of any modifications to these procedures should be w