Examination of the exponential wash-off function using experimental data

The standard exponential wash-off function has been widely adopted in urban stormwater quality models and tested in many studies of sediment wash-off experiments. There is neither sufficient information on the wash-off coefficients for other types of pollutant nor a good mathematical interpretation that accounts for the effects of surface features on wash-off processes. This study assesses the standard exponential wash-off function using experimental data on the wash-off of various types of pollutant. In-situ wash-off experiments using simulated rainfalls were conducted in typical urban areas with various landuse. Experimental data from the literature were also analysed to support the examination. Mass balance requirement was always applied as a criterion in the experimental data analysis and wash-off model examination. The experimental results indicate that wash-off processes of organic pollutants (BOD5), nitrate (NO3), total oil and grease (TOG), total petroleum hydrocarbon (TPH) and coliform stochastically comply with the standard exponential function. Highlights In-situ wash-off experiments were conducted for BOD5, NO3, TOG, TPH, coliforms and TSS. Wash-off coefficients of BOD5, NO3, TOG, TPH, coliforms and TSS are identified stochastically. Mass balance requirement is introduced to examine the mathematical interpretations of wash-off models. An improved exponential wash-off function with more input parameters is suggested.