Mound systems require at least 2 feet of permeable soil to finish treating and safely dispose of sand-treated wastewater. Ohio State University Extension Bulletin 813 recommends a 2-foot-deep soil layer between the soil surface and a limiting soil condition. A soil limiting condition includes ground or perched water tables, hard, unfractured bedrock, dense glacial till, compacted zones, dense clays, pans such as fragipans, sand and gravel layers, and fractured rock.
Do not confuse mound systems designed for wastewater treatment and disposal through soil absorption with systems sometimes called ET mounds. ET stands for evapotranspiration. For ET systems some small mounds of soil or fill material are constructed to accept septic tank effluent. The designers and builder mistakenly believe that trees planted on the top of these mounds of soil will take up the water year-round and the wastewater will simply go away. In Ohio, however, we are blessed with abundant rainfall. More water, in fact, falls to the earth as precipitation in Ohio than is transpired by plants. That is why Ohio has beautiful streams and wetlands. These evapotranspiration systems are quickly overloaded in Ohio, leaching out on the lot and creating a lush green spot and eventually a soft, wet, smelly area in the lawn. To keep this from happening, drains are sometimes installed around the ET system, carrying the overflow of untreated wastewater with its pollutants to a nearby ditch or stream by a buried drainage pipe. One sure sign of how these systems are overwhelmed with water is when the trees quickly die. Trees need aerobic soil to live, and they die in soil saturated with water.
Table 2 lists by name the soil series with properties that are in the range of soil depths and permeabilities considered suitable for mound systems. The 168 soil series are present in 25.4% of Ohio's land area. Figure 6 shows where in Ohio these soil series are distributed. Remember, these soils series are used to describe soils over a range of depths. As shown in Figure 7 one of the soils listed ranges from 1.5 feet to 5 feet of depth above a limiting condition. Some of these depth differences are natural and some are the result of erosion and human activities. It is necessary to check the soil depth to a limiting condition in a soil pit before designing, building, or approving a mound system.