ASCERTAINING AQUATIC ECOLOGICAL RISKS OF URBAN STORMWATER DISCHARGES

Stormwater discharges are intermittent inputs into receiving waters. I n overall, the wet-weather Row periods in the receiving waters resulti ng from stormwater flows may represent only a fraction of the annual f low. In most humid regions the probability of wet weather overflow fro m urban storm sewers is between 4 and 8% and is less for combined sewe r overflows. A methodology which enables one, using probabilistic prin ciples, 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 togethe r to approximate the synergic effects. It can be used for ranking poll utants in wet-weather discharges as well for ranking the discharges th emselves. A reciprocal of the risk is the recurrence interval between the occurrences of such damages. The methodology was documented by cal culating risks of four metals in an urban stream. (C) 1997 Elsevier Sc ience Ltd.