In order to protect water resources from contamination, it is necessary to assess the waterborne spreading of contaminants, both from all existing sources of pollution and from potential new sources (as a consequence of, for example, management changes and accidents or incidents that lead to discharge of pollutants). In this paper we adopt an approach that has previously been mostly used to quantify solute transport in soil and groundwater systems. Here, we develop the approach and apply it at the catchment scale for the estimation of waterborne contaminant spreading and the related risk for contamination of sensitive water environments within and downstream from a catchment area.
The proposed approach consists of two calculation steps: (1) calculation and mapping of travel times and travel time distributions for flowing water, and waterborne non-reactive substances that directly follow the movement of the water, from identified or potential sources of pollution to sensitive downstream waters, and (2) the previously estimated travel time distributions are used for calculation and mapping of the transport of specific reactive pollutants with the water, and of the related risks for contamination of sensitive water environments.
In the two calculation steps, random variability of transport properties and processes is represented statistically, while a scenario analysis is used to account for uncertainty about present or future conditions that cannot be quantified in statistical terms (e.g. uncertainties related to the characterisation of the sources of pollution, the water flow, and the transport of substances with the water). Through this approach one can relatively easily assess how large the uncertainties are, and identify those intervals of transport conditions and substance-specific mass attenuation properties for which the uncertainties are of great practical significance for water management in a catchment area, as well as the remaining intervals for which they are not of such significance. In cases where the uncertainties are found to be of minor importance from a management point of view, the scenario analysis provides a relatively robust basis on which to make decisions concerning, for example, urban planning, emergency preparedness and the appropriate measures to reduce the spreading of contaminants. Also, by clearly identifying the cases for which the uncertainties are of great practical significance, one obtains a rational basis on which to assess the value of further investigations in order to reduce the uncertainties.