Section 303(d) of the federal Clean Water Act (CWA) and the U.S. Environmental Protection Agencys (USEPA) 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. A water quality standard is the combination of a designated use for a particular body of water and the water quality criteria designed to protect that use. For each WQLS listed on the Integrated Report of Surface Water Quality in Maryland, 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. The Maryland Department of the Environment (MDE) has identified the non-tidal waters of the Potomac River Montgomery County (MD basin number 02140202) in Marylands Integrated Report as impaired by nutrients, sediments (1996 listings); impacts to biological communitiesnon-tidal waters (2006 listing); and polychlorinated biphenyls (PCBs) in fish tissues non-tidal waters (2006 listing). The 1996 nutrients listing was refined in the 2008 Integrated Report and phosphorus was identified as the specific impairing substance. Similarly, the 1996 suspended sediment listing was refined in the 2008 Integrated Report to a listing for total suspended solids. In 2002, the State began listing biological impairments on the Integrated Report. The current MDE biological assessment methodology assesses and lists only at the Maryland 8-digit watershed scale, which maintains consistency with how other listings on the Integrated Report are made, how TMDLs are developed, and how implementation is targeted. The listing methodology assesses the condition of Maryland 8-digit watersheds with multiple impacted sites by measuring the percentage of stream miles that have an Index of Biotic Integrity (IBI) score less than 3, and calculating whether this is significantly different from a reference condition watershed (i.e., healthy stream, <10% stream miles degraded). The Maryland Surface Water Use Designation in the Code of Maryland Regulations (COMAR) for the waters of the Potomac River Montgomery County is Use I-P (Water Contact Recreation, Protection of Nontidal Warmwater Aquatic Life, and Public Water Supply) (COMAR 2010 a,b,c). The Potomac River Montgomery County watershed is not attaining its designated use of protection of aquatic life because of biological impairments. As an indicator of designated use attainment, MDE uses Benthic and Fish Indices of Biotic Integrity (BIBI/FIBI) developed by the Maryland Department of Natural Resources Maryland Biological Stream Survey (MDDNR MBSS). The current listings for biological impairments represent degraded biological conditions for which the stressors, or causes, are unknown. The MDE Science Services Administration (SSA) has developed a biological stressor identification (BSID) analysis that uses a case-controlled, risk-based approach to systematically and objectively.