The study treats the numerical simulation of transport of road salt to surface water bodies, with special focus on dilution and detention processes. The simulations are restricted to chloride as the main water quality parameter. The transport of chloride from the road environment to a surface water body can occur on different transport paths: - Input into the surface water body directly from the technical drainage system, - Transport in the subsurface resulting from direct infiltration (e.g. road verge, retention or treatment basins), - Transport in the subsurface resulting from drift and diffuse infiltration. All three transport paths are considered in the study. The simulations do not represent real cases, but typical and generalized situations. The results concerning subsurface transport show remarkable dilution and damping effects. It can be shown that a yearly average chloride concentration in the surface water (resulting from road salt) can be estimated reasonably well using a simple mass balance. Seasonal variations in the concentrations are small in most cases. Therefore, simple mass balancing can be used in most cases instead of numerically modelling chloride transport in the subsurface. The simulations concerning the transport in the technical drainage system are used to identify more or less sensitive parameters within the wide range of different existing input parameters. For example, it can be shown that the rainfall regime plays a minor role for the computed results. In contrast,, the amount of road salt and the maximum concentration in the road runoff, as expected, have an important impact. Furthermore, the possibilities of dilution and detention of road salt within different basin types as part of the drainage system are investigated. The results indicate that the design of the basin (volume, geometry, flow damping, permanent basin fill levels, occurrence of density stratification) have a significant influence on the computed mass flow to the surface water body. The results of the both models (subsurface and technical drainage system) gave an input into a water body. This was evaluated at a characteristic point in the water body. In most cases this will be the downstream gauge of a catchment (e.g. a gauche). The superposition between flow and concentration of the water body the subsurface and the technical drainage system were computed under the assumption of complete mixing. Regarding a (possibly partial) water body catchment a mixing of flow and concentration from the upstream reach with groundwater was computed in a first step. This did result in a discharge and a background concentration at the downstream gauge of the water body. In a second step the mixing at this point with the input from the technical drainage system was considered. In addition to the computed inputs from the subsurface and technical drainage system the flow and the background concentration of the water body represent boundary conditions. To achieve a general as possible approach these boundary conditions are treated in a matrix, which gives multiple combination of the both values. (Author/publisher)
Abstract