Big Piney Reservoir is an impoundment in the Youghiogheny River Watershed (basin code 05-02-02-04) in Garrett County, Maryland. Big Piney Reservoir was identified on the State of Marylands draft 2002 list of Water Quality Limited Segments (303(d) list) as impaired by mercury contamination, based on data for mercury concentrations in fish tissue. Concentrations in the water are well below the threshold for concern in regard to drinking water. The Maryland water quality standards Surface Water Use Designation (Code of Maryland Regulations (COMAR 26.08.02.07)) for Big Piney Reservoir is Use III-P Natural Trout Waters and Public Water Supply. The Maryland Department of the Environments (MDE) current public fish consumption advisory to eat limited amounts of fish from Big Piney Reservoir is not supportive of the recreational fishing use. Therefore, this document proposes to establish a Total Maximum Daily Load (TMDL) for mercury in Big Piney Reservoir. The methodology used to compute this mercury TMDL consists of two broad steps. The first step is to determine a maximum Allowable Ambient Water Column Concentration (AAWCC) of mercury in the water column that ensures the bioaccumulation of the total mercury by fish will remain below a maximum fish tissue concentration. The second step is to determine a maximum allowable load that is consistent with the maximum water column concentration. The resultant TMDL includes a Load Allocation (LA), a Waste Load Allocation, a margin of safety (MOS), and a Future Allocation (FA). The TMDL methodology considers all sources, including direct atmospheric deposition to the surface of the lake, nonpoint source contributions from the watershed, and point source contributions.