Lake Lariat is an impoundment in the Patuxent River Watershed (basin 02-13-11-10) in southern Calvert County, Maryland. Lake Lariat was identified on the State of Marylands draft 2002 list of Water Quality Limited Segments (303(d) list) (submitted October 4, 2002) as impaired by mercury contamination, based on data for mercury concentrations in fish tissue. Mercury concentrations in the water are well below the threshold for concern in regard for drinking water. The Maryland water quality standards Surface Water Use Designation (Code of Maryland Regulations (COMAR 26.08.02.02)) for Lake Lariat is Use I-P Water Contact Recreation, Protection of Aquatic Life and Public Water Supply. The Maryland Department of the Environments (MDE) current public fish consumption advisory to eat limited amounts of fish from Lake Lariat is not supportive of the recreational fishing use. Therefore, this document proposes to establish a Total Maximum Daily Load (TMDL) for mercury in Lake Lariat. 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 based on safe human consumption levels. The second step is to determine a maximum allowable load that is consistent with the maximum water column concentration. The resultant TMDL includes a Waste Load Allocation (WLA), a Load Allocation (LA), a Future Allocation (FA) and a margin of Safety (MOS). The TMDL methodology considers all sources, including direct atmospheric deposition to the surface of the lake, nonpoint source contributions from the watershed, and any existing point source contributions.