Consumers whose drinking water can be contaminated by the release of untreated wastewater after heavy rains face increased risk for gastrointestinal illness, according to a report. Many older cities such as Chicago have combined sewer systems — along with 772 other communities, primarily in the Northeast, Great Lakes and Pacific Northwest, serving a total of 40 million people. While some cities are building infrastructure to handle sewage and runoff separately, other regions with combined systems depend on reservoirs to provide extra capacity during extreme rainfalls.