Since the first identified Cryptosporidium outbreaks occurred in the 1980s and the massive 1993 Milwaukee, WI outbreak affected more than 400,000 people, the concern over the public health risks linked to protozoan pathogens Cryptosporidium and Giardia has grown. Cryptosporidium and Giardia, found in streams, rivers, groundwater, and soil, form hardy, disinfection-resistant oocysts and cysts. Both organisms are recognized causative agents of gastrointestinal illnesses linked to the consumption of contaminated surface or groundwater. This study, the first in a planned series to estimate the urban contribution to the total Cryptosporidium and Giardia receiving-water loads, focused on combined sewer overflow (CSO). CSOs are discharges of mixed untreated sewage and stormwater released directly into receiving waters during rainfall. This engineered relief is necessary to accommodate hydraulic strain when the combined rain and sanitary flows exceed the system capacity. Limited comprehensive data are available assessing the CSO discharge contribution as a source of these two pathogens. Works by States et al. and Gibson et al. each found Cryptosporidium and much greater Giardia concentrations in CSOs draining parts of Pittsburgh, PA. This project estimated the relative detection frequency and concentration of Cryptosporidium and Giardia in CSO. Analytical results were obtained using a modification of Method 1623, originally developed for much cleaner environmental samples. These data are useful for drinking water treatment plants located downstream of CSOs. It is also significant in determining the potential concentrations of parasites at treatment plant intakes and for assessing health risks for water contact and fishing activities. Commonly monitored indicator organisms (total coliform, fecal coliform, E. coli, Enterococcus, and fecal streptococcus), endospores, and selected physical and chemical parameters were analyzed to further describe the samples. CSO from urban areas was not found to be a significant contributor of Cryptosporidium, however, it was found to be a Giardia source.

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