Review on uncertainty of the first-flush phenomenon in diffuse pollution control

Abstract

Several definitions and criteria of the first-flush are being used to assess and control the nonpoint source (diffuse) pollution. The common consensus is that the first-flush is generally noticed in the small catchments (e.g. < 10 ha) of regular shapes. It is also considered that 80% of the pollution load can be captured by capturing 30% of the runoff volume, which is assumed to be due to first-flush of the storm event. However, such phenomenon is uncertain in the considerably large catchments, principally due to the dilution and delay in transport of the pollutant. This paper critically examines the ‘first-flush’ phenomenon in controlling diffuse pollution based on various studies conducted by the researchers. Based on the review, it can be inferred that the first-flush may be an effective criterion for sizing on-site treatment facilities for small catchments (with similar dimensions of length and width), from where the runoff is expected to reach the treatment facility (roughly) at the same time. However, for the large and elongated catchments with an area covering more than 10 ha, a huge volume of storm runoff needs to be captured that would make the treatment system large or less effective. As such, for the community and regional facilities; the presence of the first-flush needs to be confirmed based on the sampling and analysis of data collected from several storm events of different durations and intensities. Therefore, the use of the first-flush phenomenon should not be applied for the design of storm runoff treatment facility in large drainage system (say > 10 ha) without on-site monitoring being conducted. However, it is also recommended to come up with a simplistic approach of designing the best management practices (BMPs) to capture and treat certain depth of the initial storm runoff (e.g. first 20 mm), which will help reduce the effect of nonpoint source (NPS) pollution.