Ascertaining aquatic ecological risks of urban stormwater discharges

Stormwater discharges are intermittent inputs into receiving waters. In overall, the wet-weather Row 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. (C) 1997 Elsevier Science Ltd.