Section 303(d) of the federal Clean Water Act (CWA) and the U.S. Environmental Protection Agency's (EPA) implementing regulations direct each state to identify and list waters, known as water quality limited segments (WQLSs), in which current required controls of a specified substance are inadequate to achieve water quality standards. This list of impaired waters is commonly referred to as the 303(d) List. For each WQLS, the State is to either establish a Total Maximum Daily Load (TMDL) for the specified substance that the waterbody can receive without violating water quality standards, or demonstrate that water quality standards are being met (CFR 2006a). The Patuxent River Lower (basin code 02131101), located in Anne Arundel, Calvert, Charles, Prince Georges, and St. Marys Counties, Maryland, was identified on the States list of WQLSs as impaired by fecal coliform (1996, 1998, and 2004), nutrients (1996), sediments (1996), chlorpyrifos in the water column (2002), methylmercury (2002), and impacts to biological communities (2002 and 2004) (MDE 2006a). The Patuxent River Middle (basin code 02131102), located in Anne Arundel, Calvert, and Prince Georges Counties, Maryland, was identified on the States list of WQLSs as impaired by sediments (1996), nutrients (1996), chlorpyrifos in both the water column and underlying sediments (2002), and impacts to biological communities (2002 and 2004) (MDE 2006a). All fecal coliform listings are for tidal shellfish areas, whereas the biological listings are for nontidal waters, except for the 2004 Patuxent River Lower biological listing. Chlorpyrifos is a pesticide used primarily in agricultural applications, particularly on grain, cotton, field, fruit, nut, and vegetable crops, that has been in production since the 1960s. It is moderately toxic to humans and highly toxic to aquatic organisms, but it does not bioaccumulate.