Estimating long term acceptance rates for on-site wastewater systems in Georgia

Summary

To provide a more scientific basis for estimating long term acceptance rates for on-site wastewater systems(OWSs) or septic systems in Georgia, we developed a new method based on soil saturated hydraulic conductivity.

Situation

Studies have shown that the wastewater infiltration rate in drainfield trenches declines over time due to the formation of a dense biomat at the soil-trench interface that impedes infiltration and causes ponding in the trench. An estimate of the final steady wastewater infiltration rate is needed to specify the trench bottom area required for onsite wastewater systems (OWSs). This is known as the Long Term Acceptance Rate (LTAR). Currently, Georgia uses an empirical approach to estimate LTAR based on long-term experience in Georgia and other states with conventional (gravel) OWSs in a relatively small number of common soil classes. However, it is difficult to predict LTAR for less common soil classes and for modified OWSs (such as chamber and particle systems).

Response

We used a two-dimensional computer model (HYDRUS) to determine the steady flux through the trench bottom for 12 USDA soil textural classes with a fully developed biomat under normal operating conditions. We also tested how well a simple equation developed by Bouma (1975) estimated the bottom flux. With a modification to account for unsaturated flow within the biomat, the Bouma equation produced very similar estimates of trench bottom flux for all soil textural classes to those found using HYDRUS. To estimate the LTAR, we took 50% of the steady trench bottom flux as a safety factor. We also developed an Excel spreadsheet that uses the modified Bouma equation to calculate LTAR for the 12 USDA soil textural classes.

Impact

We worked with a soils subcommittee to develop a proposal to be presented to the Georgia Department of Human Resources (DHR) Technical Review Committee to convert from the current method of estimating LTAR to our method using the spreadsheet. For any soil with a measured saturated hydraulic conductivity, an estimate of LTAR can be made. The effect of different OWS pre-treatment systems and architecture can be incorporated by assuming different biomat properties for these systems. The spreadsheet along with a guidance document is posted on our website at www. cropsoil.uga.edu/soilphysics/publications.html.

State Issue

Conservation & Management of Natural Resources

Details

• Year: 2008
• Geographic Scope: State
• County: Clarke
• Program Areas:
  • Agriculture & Natural Resources