FIGURE 1-1 The Allegheny, Monongahela, and Ohio Rivers (the “Three Rivers”) in Allegheny County in southwestern Pennsylvania; shaded areas includes Allegheny County communities serviced by ALCOSAN in addition to the City of Pittsburgh.

Untreated human waste, stormwater, and agricultural runoff may spread parasitic protozoa (e.g., Giardia, Cryptosporidium), enteric bacteria and viruses, and other waterborne contaminants (NRC, 2004). Such microorganisms and contaminants are public health threats particularly to children, the elderly, the immunocompromised, and other sensitive populations (Balbus et al., 2000; NRC, 2001, 2004).

For approximately the last quarter of a century, tests of water quality by the U.S. Geological Survey (USGS), the Ohio River Valley Water Sanitation Commission (ORSANCO), and other governmental, nongovernmental, and university groups have found that fecal coliform levels (bacterial indicators of fecal contamination) have repeatedly been in violation of water quality standards at certain monitoring stations on the Monongahela and Allegheny Rivers, especially during wet weather events (WSIP, 2002; see Chapter 3). Furthermore, over the last decade, the Allegheny County Health Department has issued warnings on significant numbers of days (roughly 30 to 50 days per year during the May through December recreation season) to avoid bodily contact with the water in large portions of the rivers. During dry weather, water in the main stem rivers meets recreational guidelines for indicator microorganisms. However, water in many tributaries remains contaminated by indicator organisms and pathogens even in dry weather. Dry weather sewage treatment system problems (e.g., failing on-site sewage treatment and distribution systems [OSTDSs, or “septic systems”], malfunctioning package plants), agricultural practices, and natural sources may contribute to these dry weather

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