soils is that often the topsoil is more permeable than the subsoil, and by elevating the absorption trenches or bed above the topsoil, this more permeable layer can be used for disposal. Another reason is 106 ONSITE WASTEWATER DISPOSAL Filter material Fine soil Topsoil Gravel Broken sod Figure 6.6 Cross section of a mound. that slowly permeable soils tend to smear easily during construc- tion, resulting in relatively impermeable sidewalls and bottom sur- faces. Mound construction does not require excavations in the natu- ral soil. Some evaporation from the mound surface and uptake of water by plants can be expected, although the mound's functioning does not depend on this. Design Sand mounds need not be eyesores. They can be designed to fit into the natural landscape and can add interest to the topography. Once the basic necessities of the mound are satisfied, use your imagination to finish the landscaping. The design parameters that follow generally reflect those recommended by the University of Wisconsin School of Engineering. The first step in the proper design of a sand mound is to select a proper site. Location of the mound is not restricted by the necessity for gravity flow. All sand mounds use pressure distribution from dosing chambers or, in rare circumstances, dosing siphons. But there are site restrictions, nevertheless. Mounds should not be located in swales or depressions where CONVENTIONAL SYSTEM MODIFICATIONS Table 6.1 Recommended Mound Interior Absorption System Configurations Natural Soil Percolation Rate Slope (%) (min per in.) Greater than 15 bed or trench unsuitable 60-120 trench not advisable unsuitable Greater than 120 unsuitable unsuitable unsuitable rainfall runoff will collect. Mounds located at the top of slight rises will maximize the spread of the effluent in all directions beneath the soil. Long narrow mounds on gentle slopes appear to work very well. Mounds exposed to sun and wind will dispose of some of the effluent through evaporation and plant transpiration and therefore reduce the