A mechanistic model for estimating bacteria levels in stormwater ponds

Abstract

This paper presents a three-dimensional CFD based hydro-environmental model that simulates fate and transport of bacteria in water bodies. The model numerically solves unsteady incompressible Reynolds-Averaged Navier-Stokes equations on a structured grid. Free-floating and particle-attached bacteria were modelled separately regarding both fate and transport. Therefore, a sediment transport model was integrated into the main model in order to model particle-attached bacteria transport. In addition, Volume of Fluid approach was implemented to capture the water surface movements. Wind effect was also considered in the modelling using shear stress on the water surface. Since stormwater reuse is the source of some public health concerns, a stormwater pond was chosen as the test case for the model. The model was applied to simulate the distribution of bacterial indicator organisms in the Inverness Stormwater Pond in Calgary, Alberta, which is a large T-shaped pond with several inlets and outlets. The bacteria distribution in the pond was simulated for three rain events that occurred in the area. In six locations of the pond the modelled bacteria distribution was compared to collected data using non-dimensional bacteria concentrations. The comparison showed good agreement and indicated that the middle of the pond, close to the surface had the lowest levels of bacteria and thus, was considered the optimal location for withdrawal for reusing pond water. Furthermore, planting a tree barrier on the north bank of the West wing of the pond was shown to mitigate bacteria transport away from the inlets into the pond body and substantially decrease the risk of contamination at the optimal water withdrawal location.