Probabilistic Approach to the Estimation of Urban Stormwater Pollution Loads on Receiving Waters

Abstract

In recent years, the issue of receiving water protection from pollution by urban stormwater discharges has gained in importance. This paper examines the basic processes and functions behind urban stormwater pollutant delivery into surface waters and develops a set of tools that allow the estimation of pollutant load dynamics on receiving waters and the generation of statistics of pollutant concentration in stormwater runoff and in the receiving water mixing zone. In particular, the group of expressions developed in this paper allows the calculation of runoff parameters (volume, discharge rate and pollutant load) on an event average basis for an unregulated catchment. Using Monte Carlo simulation techniques, the runoff pollutant concentration probability distribution (as event averages) are obtained. Merging these runoff statistics with the stream parameters allows the receiving water pollutant concentration characteristics to be obtained as well as the probability of exceeding threshold pollutant concentrations in the mixing zone of a stream. The simulation can be performed with different levels of complexity with respect to catchment hydrologic representations and pollutant load functions. As a result, the magnitude of influence of urban runoff on a surface water body can be determined, pollutants of concern can be identified, and certain remedial measures recommended. The probabilistic approach allows for more rational and refined assessments of surface water quality. As opposed to the calculation of pollutant concentration in the mixing zone based on average values and extreme flow statistics, probability-based calculations yield complete probability distributions of pollutant concentrations in the stream and the probability (frequency) of exceeding the limiting pollutant concentration. This work concentrates on approaches to chemical criteria violation control in smaller scale receiving waters; e.g., lowdischarge rivers and creeks as this type of receiving waters is the most common and the most vulnerable to pollution from stormwater discharges.