Final TMDLs Approved by EPA
Seneca Creek is a free flowing creek that originates near the city of Damascus in the northwest portion of Montgomery County, Maryland. The creek flows 27 miles in a southerly direction through the municipalities of Germantown and Gaithersburg, until it empties into the nontidal Potomac River near the town of Seneca. The watershed is located in the Middle Potomac River sub-basin of the Chesapeake Bay watershed. It is the largest watershed located entirely within Montgomery County, covering approximately 129 square miles. Three large tributary systems flow into the creek’s mainstem: Little Seneca Creek, Great Seneca Creek, and Dry Seneca Creek. There are no “high quality”, or Tier II, stream segments (Benthic Index of Biotic Integrity (BIBI) and Fish Index of Biotic Integrity (FIBI) aquatic life assessment scores > 4 (scale 1 – 5)) located within the watershed requiring the implementation of Maryland’s antidegradation policy. Also, approximately 0.3% of the watershed is covered by water (i.e., streams, ponds, etc.). The total population in the Seneca Creek watershed is approximately 216,518.
The Maryland Department of the Environment (MDE) has identified the waters of the Seneca Creek watershed on the State’s 2010 Integrated Report as impaired by sediments (1996, Clopper Lake - 1998), nutrients – phosphorus (1996, Clopper Lake - 1998, and Little Seneca Lake - 1998), chlorides (2010), ammonia (2010), and impacts to biological communities (2002). The designated use of the Seneca Creek mainstem and its tributaries is Use I-P (Water Contact Recreation, Protection of Aquatic Life, and Public Water Supply), except for 1) Little Seneca Creek and its tributaries, from the outlet of Little Seneca Lake to the stream’s confluence with Bucklodge Branch, and Wildcat Branch and its tributaries, which are designated as Use III-P (Nontidal Coldwater and Public Water Supply), and 2) Little Seneca Creek and its tributaries upstream of Little Seneca Lake, which are designated as Use IV-P (Recreational Trout Waters and Public Water Supply).
The TMDL, available below, addresses the 1996 sediments listing, for which a data solicitation was conducted, and all readily available data from the past five years were considered. The TMDLs set the maximum load limit for sediments, and provides load allocations to point and nonpoint sources. A Water Quality Analysis (WQA) for eutrophication to address the MD 8-digit watershed nutrients/phosphorus listing was approved by the EPA in 2009. A TMDL of phosphorus and sediments for Clopper Lake was approved by the EPA in 2002, and a WQA for eutrophication to address the Little Seneca Lake nutrients/phosphorus listing was approved by the EPA in 2006. The general listing for impacts to biological communities was removed due to a stressor identification analysis completed in 2009, and as a result, the 2010 Integrated Report now identifies chlorides, ammonia, and sediments as specific stressors impairing aquatic life.