Microbial Fate and Transport in a Seasonally Saturated North Carolina Coastal Plain Soil.

Insufficient separation distance between the bottom of septic trenches and the water table (WT) may allow untreated wastewater to reach groundwater.  The objectives of this study were to evaluate: (1) fate of Escherichia coli at various WT depths and durations of saturation, (2) bacterial transport in unsaturated soil, (3) the efficiency of treatment in saturated conditions by seasonal high water table (SHWT), and (4) the effect of resting on the efficiency of soil treatment of wastewater. Columns containing Norfolk loamy sand (Typic Kandiudults) B horizon material were constructed.  Water tables were established at 30, 45, and 60 cm below the surface.  Two hundred mL of an artificial wastewater inoculated with approximately 10⁶ colony forming units (CFU) of E. coli were applied daily on top of each column.  A sample was collected from the top of the WT and analyzed for E. coli.  After microbes had broken through, the WT in 30-cm columns was lowered to 60 cm below the surface, and the experiment was continued 58 d.  The WT was then brought to the surface to represent SHWT and monitored for another 55 d.  Subsequently, no wastewater was applied for 30 d followed by 30 d of application and monitoring.   Results show that 30 cm of separation distance was insufficient for treating E. coli because bacterial concentrations in outflow solution were higher than 45- and 60-cm outflow.  Lowering the WT resulted in reduced microbial count. Less effective treatment was observed at SHWT, where there was no treatment in 30-cm outflow samples, but some treatment in 45- and 60-cm sampling ports.  Lower numbers of microbes were detected following the 30-day rest period.  The currently required 30-cm separation between trench bottom and SHWT in North Carolina may not be sufficient for treatment of bacteria in loamy sand soils.