Experimental and Numerical Analyses of Perched Groundwater Mounds Below Septic Systems

Abstract

AbstractFor purification, effluent must move through unsaturated soil for an adequate time and distance before contacting the saturated zone. Formation of perched groundwater mounds may limit the purification processes. The dynamics of groundwater mounds associated with septic systems must be understood in order to reduce the potential for contamination from septic systems. Previous models for perched mounds have not been verified with in situ measurements. A septic tank filter field system consisting of a triple line source was installed in a Captina silt loam (Typic Fragiudult). The soil has a fragipan at the −0.7 m depth with a wavy, irregular boundary. Tensiometers were used to monitor the temporal and spatial distributions of soilwater potential. The results revealed that steady‐state soil water conditions existed during spring and fall. Soil water pressure potentials at unmeasured locations were kriged from in situ data with quadratic drift removed using universal kriging. The kriged pressure potentials were contoured resulting in equipotential lines, with the zero equipotential line representing the water table. The Laplace equation in two dimensions, including a source term, appropriate for these flow conditions, was solved using a Galerkin finite element technique. Contours of equal pressure potential predicted by the model for various flow conditions were obtained. The two‐dimensional simulation model was verified with the in situ data by overlaying the contour graphs of the kriged values with the model's predicted values. The effect of highly variable saturated hydraulic conductivity on the model's prediction was also investigated. The finite element model adequately predicted the depth of the water table as well as the range in equipotential lines and their locations when based on the mean saturated hydraulic conductivity. Therefore, the finite element model can be considered acceptable for modeling a perched water table below septic systems under steady‐state conditions.