WaterSubject

An understanding of pollutant characteristics on impervious surfaces is essential to estimate pollutant washoff characteristics and to design methods to minimise the impacts of pollutants on the environment. This paper presents data on surface pollutant characteristics on an urban road surface in Melbourne, Australia, from samples collected over a 36 day period. The data indicate that buildup over the dry days occurs relatively quickly after a rain event, but slows down after several days as redistribution occurs. The surface pollutant also becomes finer over the dry days as it is disintegrated. The washoff of surface pollutant is dependent on the rainfall and runoff characteristics, but the results here show that common storms only remove a small proportion of the total surface pollutant load. The data also show that street sweeping may have an adverse impact on pollutant washoff because the street sweeper releases the finer material but only removes some of them, making the fine sediment available for washoff by the next storm. The data also show that most of the nutrients are attached to the finer sediments, and to effectively reduce nutrient loads in particulates, treatment facilities must be able to remove the finer particles (down to 50 ?m for TP and down to 10 ?m for TN), and not just the total sediment or suspended solid load.Stormwater pollution is a major problem in urban areas. The loadings and concentrations of suspended solids, nutrients and other contaminants are much higher in urban stormwater runoff than in runoff from unimpaired and rural areas. Estimates of urban stormwater pollutant loads are required to assess the impact of stormwater pollution on drainage waterways and receiving waters and to design methods for minimising the impacts. An inaccurate representation of non-point source pollution can lead to the design of undersized and ineffective measures, or oversized measures with excessive capital costs and maintenance requirements. It is also important to have a clear understanding of the amount of pollutants associated with different particle size ranges so that the treatment facilities can be effectively designed to target the most polluted sediment sizes.

Stormwater quality modelling started in early 1970s and many models have been proposed. The models typically view stormwater pollution as a two-stage process, pollutant buildup and pollutant washoff. Buildup is the accumulation of pollutants on the catchment surface during dry periods and washoff is the removal of the pollutants by rainfall and runoff. The pollutant availability on the catchment surface (buildup) is typically estimated as a linear, exponential, power or Michaelis–Menton function of the number of dry days since the previous storm event. Pollutant washoff is commonly modelled as an exponential decay of the available surface pollutant load, with rainfall intensity, rainfall volume, runoff rate or runoff volume used as the explanatory variable(s) to describe the decay.

There have been numerous studies on pollutant washoff. Although the pollutant availability on the catchment surface is an important variable used in most existing washoff models, there have been few studies on pollutant buildup since the detailed experimental pollutant accumulation study. The only conclusion that can be drawn from these limited studies is that surface pollutant load increases with antecedent dry period. However, the small data sets and large scatter in the data makes the form of the relationship hard to determine. There is also doubt that the assumption that buildup starts from zero after a rain event is realistic. For example, through an experiment and through a modelling study, show that storm events typically remove only a small proportion of the overall surface pollutant load.

There are therefore two alternative views to the pollutant accumulation process. The first view is that adopted in most event water quality models, where the surface pollutant load builds up from zero over the antecedent dry days. Most of the available load is then washed off during storm events. The second view is that a storm event removes only a small amount of the surface pollutant load and buildup occurs relatively quickly to return the surface pollutant load back to the level before the storm within several days. This and the redistribution of pollutants would result in a catchment surface having a similar amount of pollutant load most of the time. The suggestion that the surface pollutant load remains largely the same has important implications because it precludes the need of a detailed simulation of buildup in water quality models.

The surface pollutant load data collected in this experimental study on an urban road surface indicate that pollutant buildup occurs over the dry days. The pollutant accumulation on the road surface occurs relatively quickly after a rain event, but slows down after several days as redistribution occurs. The surface pollutant also becomes finer over the dry days as it is disintegrated by traffic and other factors. The washoff of surface pollutant is dependent on the rainfall and runoff characteristics, but the results here show that common storms only remove a small proportion of the total surface pollutant load. The results also suggest that the rainfall and runoff disintegrates and dissolves more surface pollutant that they can actually remove.

The data also show that street sweeping may have an adverse impact on pollutant washoff. The street sweeper releases the finer sediment but does not have enough suction to remove them, making the fine sediment available for washoff by the next storm.

The analyses here indicate that although more than half of the surface pollutant are coarser than 300 ?m, <15% of the total TP and TN are attached to particle sizes >300 ?m. This suggests that to effectively reduce nutrient loads in particulates, treatment facilities must be able to remove the finer particles (down to 50 ?m for TP and down to 10 ?m for TN), and not just the total sediment or suspended solid load.