Water Table Impacts on Wastewater Storage around Onsite Drainfield Trenches: Evaluation by Model Simulation

Abstract

Core Ideas Water storage volume is important for loading rates of on‐site wastewater systems. HYDRUS (2D/3D) was used to model the slope of the effluent surface away from a trench. Modeled slopes confirm field observations across conductivity classes and soils. Arkansas uses a unique system for determining effluent loading rates for onsite wastewater systems based on wastewater storage in unsaturated soil above a seasonal water table. Evaluation of the soil volume available for storage relies on estimates of the slope of the effluent plume away from the trench, which has rarely been evaluated in the field. The objectives of this study were (i) to evaluate the simulated effluent slope for select soils in different hydraulic conductivity classes and with different seasonal water table depths, and (ii) to illustrate the utility of using model simulations to evaluate regulatory parameters established with limited measured data. The geometry of the effluent surface was evaluated by simulation of wastewater input and effluent movement with the HYDRUS (2D/3D) two‐dimensional model. Nine soils mapped in Arkansas and in varying hydraulic conductivity classes were used. For each soil, initial seasonal water tables were established at target depths of ∼40, 60, and 100 cm. Limited lateral spread of effluent resulted in effluent slopes that increased proportionately to water‐table depth. Averaged across water‐table depths, the mean slope of the effluent surface was 16, 15, and 29% for soils in the high, moderate, and low hydraulic conductivity classes, respectively. Model‐simulated slopes for soils in moderate and low conductivity classes were lower than those currently assumed for calculation of effluent storage volume. The differences were small, however, and regulatory storage volumes result in appropriately conservative loading rates that minimize the risk of failure due to effluent surfacing in the drainfield area.