In many areas of Pennsylvania the soils have layers, defined as limiting zones, within the soil profile that are less than 60 inches deep. According to the Pennsylvania on-lot sewage regulations, a limiting zone is bedrock, an impervious soil layer, a high water table, a seasonal high water table or a layer with insufficient fines to properly treat the effluent. In soils with limiting zones within the top 60 inches, it is not possible to provide 48 inches of suitable soil between the bottom of an in-ground absorption area and the top of the limiting zone.
To provide for on-lot treatment of wastewater in soils with between 20 and 60 inches of suitable soil available above the limiting zone, Pennsylvania has approved the use of elevated sand mounds. The elevated sand mound, a constructed mound of sandy fill material placed on top of the 20 to 60 inches of natural or prevailing soil, has been shown to provide excellent treatment of effluent. Elevated sand mounds are limited to sites having a maximum of 15 percent slopes. In addition, the Perc Rate must be between 3 and 180 minutes per inch. Proper siting, design, construction and maintenance of mound treatment systems are key to their proper functioning.