Groundwater mounding in the vadose zone from on-site wastewater systems: Analytical and numerical tools

On-site wastewater systems (OWS) provide a sustainable method for disposing of residential wastewater. Large "cluster" OWS systems are now frequently being used to serve multiple-unit housing developments. Groundwater perching, mounding, and lateral flow can occur when the effluent application rate to an infiltration area exceeds the soil's capacity to infiltrate the wastewater. This study addresses mounding of wastewater on a low hydraulic conductivity (K) layer in the vadose zone, and focuses on modeling tools available to design systems so that the potential for the occurrence of surface or side-slope breakout is minimized. In particular, we demonstrate how an existing analytical model developed by Khan (1976) can be used for evaluating the mounding problem. The robustness and limitations of the analytical solution due to certain constraining assumptions are evaluated for a few realistic scenarios by comparing it to results of a numerical model that is not constrained by the assumptions. The height and extent of mounding depends on both the hydraulic conductivity of the soil below the infiltration field, as well as the K value of the low-permeability layer. The numerical modeling demonstrates that the analytical solution is more accurate for the case of a thick low-K layer, and for conditions that represent a deep water table, because these conditions are more closely aligned with the conceptual model that the analytical solution derivation is based on. The analytical solution is less accurate for cases of a shallow water table, or a thin low-K layer. Mounding is smaller for thin low-K layers and for deep water tables. In general, it is shown that the analytical model is a conservative predictor for mounding for several reasons. First, it does not consider the capacity of the vadose zone to conduct infiltrating water laterally. Second, nonuniformities in a low-permeability layer (e.g., breaks in the layer) tend to reduce the mounding compared to the uniform layer assumed by the analytical solution. Third, anisotropic soils, where the vertical K is less than the horizontal K (a typical case), exhibit a smaller mounding height and lateral extent of mounding than predicted by the analytical solution. Measurement or accurate estimation of hydraulic conductivity values for subsurface soil layers is necessary for both analytical and numerical models if an accurate assessment of mounding is desired.