Section 303(d) of the federal Clean Water Act (CWA) and the U.S. Environmental Protection Agencys (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 listed on the Integrated Report of Surface Water Quality in Maryland (Integrated Report), the State is to either establish a Total Maximum Daily Load (TMDL) of the specified substance that the waterbody can receive without violating water quality standards, or demonstrate via a Water Quality Analysis (WQA) that water quality standards are being met (CFR 2010). In 2002, the State began listing biological impairments on the Integrated Report. Maryland Department of the Environment (MDE) has developed a biological assessment methodology to support the determination of proper category placement for 8-digit watershed listings. The Potomac River Washington County watershed (basin code 02140501) was identified on the Integrated Report under Category 5 as impaired by nutrients and suspended sediments (1996), methylmercury (2002), evidence of biological impacts (2002), and polychlorinated biphenyls (PCBs) in fish tissue (2008) (MDE 2008a). All impairments are listed for non-tidal streams with the exception of the 2002 listing for methylmercury in the Potomac River Dam 4 impoundment. The 2008 Integrated Report specified that the designated use impaired by nutrients and sediment is Aquatic Life and Wildlife. The 1996 suspended sediment listing was refined in the 2008 Integrated Report to a listing for total suspended solids. Similarly, the 1996 nutrients listing was refined in the 2008 Integrated Report, and phosphorus was identified as the specific impairing substance. Consequently, for the purpose of this report the terms nutrients and phosphorus will be used interchangeably. The 2008 Integrated Report also restricted the impairment for biological impacts to 1st through 4th order streams in the watershed. The listings for sediments, impacts to biological communities, methylmercury, and PCBs in fish tissue will be addressed separately at a future date.