.7). The sand underneath the gravel surrounding the distribution pipe prevents the gravel from punching through the liner when it is placed. The gravel and pipes should be no farther than 5 ft from the edges of the bed and no farther than 10 ft apart (Figure 7.8). The pipes should be level or slope away from the inlet manifold at no more than a 2% grade. The ends should be capped. Stand pipes should penetrate the gravel at one or more strategic locations SO that water levels can be monitored, and to provide access in case the bed must be pumped out. Some designs call for a continuous layer of gravel over the bottom of the bed, but ignore them. There must be a sand wick extending to the bottom of the bed if the full reservoir capacity is to be used in the winter. If there is no ONSITE WASTEWATER DISPOSAL Figure 7.6 Pipe penetration of liner. capillary action pulling the liquid from the bottom of the bed, the bottom zone will be continuously saturated and will not be available for new storage when it is needed. After the stand pipes, gravel, and distribution pipes are placed (holes down), a layer of filter fabric or treated building paper should be placed on top of the gravel. Buy a roll of treated roofing felt and cut the roll in half to get two long strips 18 in. wide. Now add the sand. Provide a 2% or 3% slope from the center of the top to the edges to promote runoff (although most rainwater hitting the bed surface will soak right in). Also provide berms and channels to make sure that no surface runoff can possibly get to the LOW-IMPACT SYSTEMS Figure 7.7 ET distribution system. At this point, another caution is provided: over a period of six to nine months, the sand surface will subside. Depressions will form. Sometimes those depressions are deep enough to go below the saturated zone in the bed and, by definition, you get exposed sew- age. Plan to truck in another load of sand, and make sure it's ET system quality sand. This time, the contour should last, providing you can keep the dirt bikes from cutting dough