Final TMDLs Approved by EPA: Prettyboy Reservoir

Prettyboy Reservoir is an impoundment located near Beckleysville and Hereford in Northwestern Baltimore County, Maryland.  The impoundment is owned by the Baltimore City Department of Public Works and is situated in the Gunpowder River watershed.  Upstream watershed usages include an unnamed park surrounding the reservoir, fishing, boat launching facilities and fish stocking.  Downstream watershed usage includes a water supply to the City of Baltimore via Loch Raven Reservoir.  The inflow to Prettyboy Reservoir is primarily via the Gunpowder River.     

Based on mercury data in fish tissue from a subset of lakes across the State, the Maryland Department of Environment (MDE) announced a statewide fish consumption advisory for lakes in 2002.  This advisory has been established statewide as a precautionary measure because the primary source of mercury is understood to be atmospheric deposition, which is widely dispersed.  Based on additional fish tissue data, Maryland has verified that Prettyboy Reservoir is impaired due to mercury in fish tissue. 

Section 303(d) of the federal CWA and EPA’s implementing regulations direct each state to identify and list waters, known as water quality limited segments (WQLSs), in which current required controls of a specific substance are inadequate to achieve water quality standards. The CWA requires the development of a Total Maximum Daily Load (TMDL) for all impaired waters on their Section 303(d) list.  A TMDL reflects the maximum pollutant loading of an impairing substance a waterbody can receive and still meet water quality standards.  A TMDL can be expressed in mass per time, toxicity or any other appropriate measure (40 CFR 130.2(i)).  A TMDL must take into account seasonal variations, critical conditions and a margin of safety (MOS), to allow for uncertainty.  Maryland’s 2002 proposed 303(d) list prepared by MDE lists Prettyboy Reservoir as impaired for mercury (watershed 02130806). This TMDL was developed in response to the determination of impairment.   A nutrient TMDL was completed in 2006.  

TMDL of Mercury for Prettyboy Reservoir, Baltimore County, Maryland

(Approved on August 16, 2004)

EPA's Decision Letter

EPA's Decision Letter

Main Report


Comment Response Document



Contact Information

Please direct questions or comments concerning this project to Maryland's TMDL Program at (410) 537-3818.