Water Quality Analyses (WQA) with EPA Concurrence of MDE's Findings: Potomac River Middle Tidal

The Potomac River Middle Tidal watershed is located in the central Maryland tidal region of the Potomac River.  It is located in Charles County and the river segment extends from Marshall Hall, MD downstream to Smith Point, MD.  The tidal segment of the Potomac River Middle Tidal differs from a true estuary in that there is little intrusion of salt from the lower Chesapeake Bay for the majority of the year; thus, there is neither longitudinal nor lateral distribution of salinity.  This atypical tidal exchange produces unusual salinity distributions within the Potomac River Middle Tidal basin.  Low salinity is primarily attributable to the heavy freshwater input from the upstream Potomac River and other tributaries which discharge directly to the Potomac River Middle Tidal basin.  The watershed area of the upstream Potomac River that drains to this watershed is approximately 12,540 square miles.

Potomac River Middle Tidal (basin code 02140102), located in Charles County, MD, was identified on the State’s list of water quality limited segments (WQLSs) as impaired by nutrients (1996 listing), suspended sediments (1996 listing), cadmium (Cd), (1996 listing), chromium (Cr), (1996 listing), copper (Cu), (1996 listing), lead (Pb) (1996 listing), polychlorinated biphenyls (PCBs), (2002 listing), and impacts to biological communities (2004 listing).   All impairments were listed for the tidal waters except for impacts to biological communities, which are listed for the tidal and non-tidal region.  Code of Maryland Regulations (COMAR) defines the tidal Potomac River upstream of Upper Cedar Point, MD which includes the Potomac River Middle Tidal, as a fresh waterbody.   This report provides an analysis of recent monitoring data, including hardness data, which shows that the aquatic life criteria for Cd, Cr, Cu, and Pb are being met in the Potomac River Middle Tidal.

The analyses support the conclusion that TMDLs for Cd, Cr, Cu, and Pb are not necessary to achieve water quality standards.  Barring the receipt of any contradictory data, this report will be used to support the Cd, Cr, Cu, and Pb listing change for the Potomac River Middle Tidal from Category 5 (“waterbodies impaired by one or more pollutants and requiring a TMDL”) to Category 2 (“surface waters that are meeting some standards and have insufficient information to determine attainment of other standards”) when MDE proposes the revision of Maryland’s 303(d) list for public review in the future.  The listings for nutrients, suspended sediments, PCBs, and impacts to biological communities will be addressed separately at a future date.  Although the waters of the Potomac River Middle Tidal watershed do not display signs of toxic impairments due to Cd, Cr, Cu, or Pb, the State reserves the right to require additional pollution controls in the Potomac River Middle Tidal watershed if evidence suggests that metals from the basin are contributing to downstream water quality problems.

Water Quality Analysis of Cadmium, Chromium, Copper, and Lead in the Potomac River Middle Tidal, Charles County, Maryland

(EPA Concurrence: June 6, 2006)

EPA's Decision Letter EPA's Decision Letter

Main Report 

WQA_potomac_river_middle_tidal_metals.pdf (382KB)

Comment Response Document

CRD_potomac_river_middle_tidal_metals.pdf (10KB)

Contact Information

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