Combined sewer overflows: A source of Cryptosporidium and Giardia?

Throughout the past several years there has been an increasing concern to r educe the pollutants within urban stormwater discharges, particularly combi ned sewer overflows (CSOs). The majority of the research on CSOs has focuse d on physical, chemical, and some biological components. This preliminary s tudy investigated the occurrence of two pathogenic protozoa, Cryprosporidiu m and Giardia, in an urban stream during dry weather compared to their occu rrence in a combined sewer overflow (CSO) end-of-pipe discharge to determin e the loading potential and the potential human health impacts. Cryptospori dium oocysts and Giardia cysts were commonly observed in the urban stream d uring dry weather conditions, with concentrations of 5-105 oocysts/100L and 13-6,579 cysts/100L respectively. The CSO end-of-pipe samples during wet w eather conditions discharged Cryptosporidium and Giardia at high levels, 25 0-40,000 oocysts/100L and 9,000-283,000 cysts/100L respectively. This preli minary study suggests that CSOs may significantly contribute to the load of Cryprosporidium and Giardia in ambient waters and source waters utilized f or recreational use and potable water. However, further investigation will be needed to determine and characterize the full effect of this apparent lo ading source. (C) 1998 Published by Elsevier Science Ltd on behalf of the I AWQ. All rights reserved.