Water Quality Analyses (WQA) with EPA Concurrence of MDE's Findings: Rocky Gorge Reservoir

The Rocky Gorge Reservoir watershed is located in the Patuxent River sub-basin of the Chesapeake Bay watershed, within Maryland.  The reservoir’s watershed drains 84,480 acres of Howard, Montgomery, and a small portion of Prince George’s Counties.  The T. Howard Duckett Dam was completed on the Patuxent River in 1952, creating Rocky Gorge Reservoir, which is owned by the Washington Suburban Sanitary Commission (WSSC).  Water supply intakes in the reservoir feed the WSSC’s Patuxent Water Filtration Plant near Burtonsville, MD, which provides drinking water to Howard and Montgomery Counties.  The reservoir is primarily fed by the Patuxent River, a portion of which is impounded by the Brighton Dam to form the Triadelphia Reservoir, and by Hawlings River to the northwest. 

Maryland’s water quality standards specify that all surface waters of the State shall be protected for water contact recreation, fishing, and protection of aquatic life and wildlife.  The specific Maryland Surface Water Use Designation in the Code of Maryland Regulations (COMAR) for Rocky Gorge Reservoir is Use I-P (Water Contact Recreation, Protection of Aquatic Life, and Public Water Supply).  The Maryland Department of the Environment (MDE) has identified Rocky Gorge Reservoir on the State’s 2012 Integrated Report of Surface Water Quality as impaired for impacts to biological communities in 1st- through 4th-order streams in the watershed (2004), mercury in fish tissue (2010) and nutrients (1998).

The WQA presented below addresses the 2010 mercury in fish tissue listing for Rocky Gorge Reservoir, for which a data solicitation was conducted, and all readily available data from the past five years have been considered. A TMDL addressing the 1998 listing for nutrients in Rocky Gorge Reservoir was approved by the EPA in 2008. The listing for impacts to biological communities will be addressed at a future date.

Although Rocky Gorge Reservoir does not display signs of an impairment due to mercury in fish tissue, the State reserves the right to require future controls if evidence suggests that mercury from the reservoir is contributing to increases of methylmercury in fish tissue downstream. Barring the receipt of contradictory data, this report will be used to support the revision of the Integrated Report listing for mercury in fish tissue in Rocky Gorge Reservoir from Category 5 (“waterbody is impaired, does not attain the water quality standard, and a TMDL is required”) to Category 2 (“waterbody is meeting some [in this case mercury in fish tissue related] water quality standards, but with insufficient data to assess all impairments”).  

Water Quality Analysis of Mercury in Fish Tissue in Rocky Gorge Reservoir in Howard, Montgomery, and Prince George’s Counties, Maryland 

(Approved on February 5, 2015)

EPA's Concurrence Letter EPA's Concurrence Letter
Main Report

Rocky_Gorge_Reservoir_Mercury_WQA_final.pdf (1836KB)

Comment Response Document

Rocky_Gorge_Reservoir_Mercury_CRD_final.pdf (20KB)


Contact Information

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