Throughout the past several years there has been an increasing concern to reduce the pollutants within urban stormwater discharges, particularly combined sewer overflows (CSOs). The majority of the research on CSOs has focused on physical, chemical, and some biological components. This preliminary study investigated the occurrence of two pathogenic protozoa, Cryptosporidium and Giardia, in an urban stream during dry weather compared to their occurrence in a combined sewer overflow (CSO) end-of-pipe discharge to determine the loading potential and the potential human health impacts. Cryptosporidium oocysts and Giardia cysts were commonly observed in the urban stream during 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 weather conditions discharged Cryptosporidium and Giardia at high levels, 250-40,000 oocysts/100L and 9,000-283,000 cysts/100L respectively. This preliminary study suggests that CSOs may significantly contribute to the load of Cryptosporidium and Giardia in ambient waters and source waters utilized for recreational use and potable water. However, further investigation will be needed to determine and characterize the full effect of this apparent loading source.

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