 load on the soil below. There should be a minimum of about 2 ft of natural soil below the mound to accept the effluent from the mound. With shallower soil, there is a greater chance that effluent will come to the surface, and there is less treatment of the effluent. Mound design can accommo- date a level site or a sloping site. Mounds built on slopes must follow the contours of those slopes. Mounds should not be used where the slope exceeds 15%. Where the natural soil percolation rate is slower than 60 to 90 min per in., the maximum slope advisa- ble is about 6%. Mounds should not be used where the percolation rate is slower than 120 min per in. Percolation rates between 60 and 120 min per in. dictate that trenches be used in the mounds to promote optimum distribution over the basal area of the mound. In soils where the percolation rate is faster than 60 min per in., beds can be used to reduce the overall mound length. Where the soil is reasonably permeable and on sloping ground, beds can be used, but they should become narrower as the slope increases. For slopes greater than 10% or so, trenches should be used. These restrictions are summarized in Table 6.1, which is an interpretation of Univer- sity of Wisconsin recommendations. Finally, don't forget about the setback distances to wells, houses, and SO on, as mentioned in Chapter 3. In addition to proper site selection, there are four basic steps in mound design: ONSITE WASTEWATER DISPOSAL Table 6.2 Application Rates for Various Mound Fills Application Rate Soil Type (gpd/ft2 Gravel unsuitable Coarse sand, medium sand Fine sand, loamy sand Sandy loam, loam Silt loam and finer unsuitable 1. calculating the design flow 2. designing the trenches or bed inside the mound 3. calculating the mound dimensions 4. designing the distribution system Flow to the mound is found by multiplying the number of people who will be using the system by 75 gal per person per day. As with conventional disposal systems, it is assumed that there will be two persons per bedroom in the ho