Abstract

The extent to which stormwater pollutant loads are affected by build up and wash off of pollutants is widely debated. Physical, chemical, biological, and sociological processes controlling pollutant build up and wash off are exceedingly complex and difficult to predict. Not only do these effects vary between locations but they vary between constituents as well. Representations of build up and wash off in estimating storm water pollutant loads is usually limited to emulating the physical processes. Various stormwater management models such as SWMM (Huber and Dickenson, 1988) and P8 (Walker, 1990) use deterministic models of build up and wash off to aid in storm water quality assessments. SWMM users are referred to a variety of studies on build up to aid in the selection f empirical parameters in build up function. However, calibration of necessary parameters may be extremely difficult.

Empirical relationships can provide site specific information on the build up and wash off of catchment pollutants. From an empirical viewpoint there is evidence to suggest that build up and wash off are important factors in determining annual and seasonal loads in certain areas. For instance, some regional stormwater monitoring programs have reported that event mean concentrations decrease as a rainy season progresses (Montoya, 1989). Conclusions have been drawn from these observations that build up (and subsequent wash off) is occurring. Very few stormwater monitoring programs have measured actual build up and wash off specifically. However, most programs do measure event mean concentration and total runoff volume. Simultaneous observations of concentration and volume allow quantification of event mass which is a direct measure of event "wash off'. Significant build up and wash off effects can be identified empirically through multiple linear regression of event mass versus serial rainfall statistics including cumulative rainfall to date, days since last storm, and event rainfall volume. Knowledge of how runoff mass is related to serial rainfall parameters can be used to reflect build up and wash off in continuous simulation models thereby producing more accurate estimates of time series of runoff event pollutant mass.

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