Controlled Laboratory Experiments Investigating the Influence of Shallow Groundwater Tables on Runoff through Vegetative Filter Strips

Abstract

Vegetative filter strips (VFS) are implemented adjacent to surface water bodies with the purpose of slowing down runoff and allowing for infiltration and settling of particles. However, there is currently limited data demonstrating how VFS perform under shallow water table conditions. Future versions of VFS design models are being constructed for this case and therefore a need exists to derive data to validate the model. The objectives of this research were to simulate a VFS in a controlled laboratory setting to quantify the influence of shallow groundwater tables on runoff. A steel box (100 cm wide, 200 cm long and 70 cm deep) was used to simulate surface runoff through a VFS. A silt loam soil was packed in the box and planted with Bermuda grass. Five experiments were performed by controlling the groundwater table including a free drainage case, a series of static water table cases, and a dynamic water table case. Precipitation was not considered in these experiments. Water was evenly distributed onto the upslope end of the VFS at a steady rate and outflow measured at the downslope end. The influence of the water table position on the outflow rate and infiltration was determined. As expected, results indicated that the higher the groundwater table the less infiltration occurred. A water table near the ground surface (1 to 2 cm) for this silt loam soil was required to limit infiltration such that the inflow and outflow rates were approximately equivalent. The dynamic water table case demonstrated that the water table can rapidly increase near the soil surface due to infiltration in the VFS. In cases with shallow groundwater, less infiltration will result in less removal of contaminants in the runoff and therefore greater quantities of contaminants reaching nearby surface water bodies. The experimental data will be critical for validating future VFS design models that will consider the performance of VFS in areas of shallow groundwater.