 distribution system, and absorption system. It is calculated in the normal way: 75 gal per person per day, and two persons per bedroom. Figure 6.17, based on data generated at the University of North Carolina, shows maximum allowable loading rates for soils with vari- ous percolation rates. The figure is valid only for this particular system design. The mean of four or more carefully performed perco- lation tests should provide a reliable basis for the loading rate deter- ONSITE WASTEWATER DISPOSAL mination. Be sure that the percolation rates conform to what you would expect from looking at the soil structure and texture. The total surface area needed for the disposal of the wastewater is calculated by dividing the daily design flow by the loading rate determined from Figure 6.17. This is not the required trench bottom area but the total amount of ground surface area necessary. If you want to use this system as an irrigation system, calculate the area needed as described for the flood irrigation system as well, and use the larger area of the two. Optimally, the absorption area should have a slope of no greater than 6%. This shallow subsurface system design has been made to work on slopes greater than 30%, but such conditions require care- ful engineering and meticulous installation (by hand). Interceptor drains are often necessary to redirect effluent moving downslope. The site should not have a topography that would collect rainwater. Sometimes, diversion channels or berms can be used to correct topographical inadequacies. If irrigation is intended, level segments should be available. The surface area to be used as a drain field should be no wider than 140 ft, the maximum length of the distribution laterals. If there is a contour to this area, the trenches should be laid out along the contour SO that each trench bottom will be level. Multiple trenches should be constructed parallel to each other along a contour line and dug 5 ft apart (center to center) over the whole surface of the absorp- tion field area (Fig