Final TMDLs Approved by EPA: Loch Raven Reservoir

Loch Raven Reservoir is an impoundment located near Timonium in Baltimore County, Maryland.  The impoundment lies on the Gunpowder River.  Prettyboy Reservoir lies in the Loch Raven Reservoir watershed and drains into Loch Raven Reservoir.  The Prettyboy watershed comprises approximately 26% of the area of the Loch Raven watershed.  The City of Baltimore Department of Public Works owns both water bodies and uses them in a system to provide a major source of public water to the Baltimore metropolitan area.   The Loch Raven Dam was constructed in 1923 and modified in 1986.

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 this year.  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 Loch Raven 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 the Loch Raven Reseroir as impaired for mercury (watershed 02130805). This TMDL was developed in response to the determination of impairment.    A water quality analysis for heavy metals was completed in 2003.  Nutrient and sediment TMDLs were completed in 2006. 

TMDL of Mercury for Loch Raven 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.