Year Established: 2019 Start Date: 2019-05-31 End Date: 2020-05-30
Total Federal Funds: $20,000 Total Non-Federal Funds: $41,210
Principal Investigators: John R. Buchanan
Abstract: The primary objective of this project is to develop a comprehensive understanding of how effluent moves in the soil when using subsurface drip irrigation systems to disperse domestic wastewater in clayey soils within the Central Basin Physiographic Region. The Central Basin (sometimes called the Nashville Basin) includes Davidson County and the many bedroom communities that surround Nashville. There is a great demand for new housing in this area; however, the wastewater infrastructure is very limited. Instead of building new sewer systems that would feed into regional wastewater treatment plants, many housing developments (subdivisions) in this region have constructed decentralized wastewater management systems. Within the subdivision, each home has a septic tank for primary treatment. Small diameter, pressurized sewers collect and transport the septic tank effluent to a small treatment system that provides secondary/tertiary level treatment. Once treated, the effluent is land-applied by subsurface drip irrigation within the subdivision. Using the Tennessee Department of Environment and Conservationâ€™s (TDEC) Dataviewer (TDEC, 2018), it was estimated that TDEC-Division of Water Resources has issued 174 operating permits to decentralized systems that utilize subsurface drip dispersal systems in the Central Basin. Most of these systems serve residential developments; however, a few of these systems serve schools, churches, and small businesses. The Central Basin is known for having shallow soils that are generally unsuitable for traditional septic systems. By using a decentralized strategy, more homes can be built in areas with unsuitable soils. This, of course, assumes that a sufficient area of suitable soils can be found close by to accept the treated effluent. In other words, there still has to be suitable soils, but the soil can be located separately from the homes. A suitable site should have sufficient soil depth to provide additional treatment and be capable of deep drainage so that the effluent will move into the subsurface. In an attempt to quantify a suitable soil, the Tennessee Department of Environment and Conservation, Division of Water Resources (TDEC-DWR) requires soil evaluators to determine the depth, texture, and structure of the soil within potential effluent dispersal areas. Once evaluated, the results are compared to a table published in TDECâ€™s design guidelines (TDEC, 2016) and a hydraulic loading rate (HLR) is determined for that site. A typical HLR is 8 liters per day per square meter (0.2 gallons per day per square foot - gpd/ft2). So, for example, if a housing development is estimated to produce 114,000 L of wastewater per day (30,000 gpd), then 14,000 m2 (150,000 ft2) is needed for the dispersal area. At issue is that clay and silty clay soils, that have weak blocky structure (within the top 51 cm or 20 inches), are not allowed for subsurface drip dispersal. Soils that match this description are common in the Central Basin and this regulatory restriction has limited new house construction. Clay and silty clay soils with weak blocky structures are recognized as having low permeability. As such, the conservative approach taken by TDEC-DWR is fully justified. However, these soils have some permeability and can percolate effluent at a low rate. Research is needed to determine an appropriate HLR for these soils.