Minimizing Contaminant Transport to Groundwater From Rapid Infiltration Basins: Model Evaluation of Soil Heterogeneity

Rapid Infiltration Basin Systems (RIBS) use the controlled application of treated wastewater to soil to remove constituents in the wastewater before recharging groundwater. Wastewater is usually enriched in nitrogen and phosphorus compounds, and if RIBS are not designed or operated appropriately, these constituents may contaminate groundwater.  Vegetation and soil development under RIBS may cause organic and fine-grained mineral matter to accumulate on the soil surface, altering soil porosity and permeability, and interfering with soil aeration. These processes affect the treatment performance of RIBS.  

In this study, numerical modeling is used to investigate effects of soil development under vegetation for different soil types and environmental conditions on RIBS performance. Soil developed under vegetation is modeled by defining a layer having lower permeability. TOUGH2/ iTOUGH2, a general-purpose numerical simulation program for multi-phase fluid flow in porous media, is used for modeling fluid movement. TOUGHREACT is used for modeling nitrogen fate and transport.

To investigate complex surface and subsurface flow patterns caused by non-uniform flooding during RIBS operation, we performed numerical simulations of coupled overland flow within the RIBS and subsurface flow beneath it. Effects of soil heterogeneity on nutrient removal are studied by defining spatially distributed scaling factors for intrinsic permeability.

Flow and transport simulations indicate that discharging wastewater in less permeable soils results in more water spreading over the basin and a larger saturated area under the basin which is favorable for denitrification and nitrogen removal. Nitrate loading to groundwater is greater for RIBS underlain by high-permeability media. Development of a fine-grained soil in RIBS constructed on highly permeable sands reduces nitrate loading to the groundwater.