Ascertaining aquatic ecological risks of urban stormwater discharges

Stormwater discharges are intermittent inputs into receiving waters. In overall, the wet-weather flow periods in the receiving waters resulting from stormwater flows may represent only a fraction of the annual flow. In most humid regions the probability of wet weather overflow from urban storm sewers is between 4 and 8% and is less for combined sewer overflows. A methodology which enables one, using probabilistic principles, to calculate risk of wet-weather dicharges to aquatic ecology (aquatic life) is presented. Risk is a joint probability that a genus will be acutely affected by the concentration. It is a dimensionless, additive number, i.e. risks by several pollutants can be added together to approximate the synergic effects. It can be used for ranking pollutants in wet-weather discharges as well for ranking the discharges themselves. A reciprocal of the risk is the recurrence interval between the occurrences of such damages. The methodology was documented by calculating risks of four metals in an urban stream.