Water Quality Analyses (WQA) with EPA Concurrence of MDE's Findings: Gwynns Falls

The Gwynns Falls watershed is located in the Patapsco River Basin within Maryland.  The watershed encompasses 41,710 acres (61 square miles) in Baltimore City and Baltimore County, Maryland.  The headwaters of the Gwynns Falls begin in Glyndon, Maryland and flows southeast until its confluence with the Middle Branch of the Patapsco River near downtown Baltimore.  Five major tributaries of the Gwynns Falls, listed north to south, include:  Red Run, Horsehead Branch, Scotts Level Branch, Dead Run, and Maidens Choice Creek.

Gwynns Falls (assessment unit ID: MD-02130905) was identified on the State of Maryland’s Integrated Report as impaired by nutrients, sediments (1996 listings), bacteria (fecal coliform), and impacts to biological communities (2002 listings).  The designated uses for Gwynns Falls are as follows:  Gwynns Falls and tributaries above Reisterstown Road, and Red Run and its tributaries – Use III (Nontidal Cold Water); Dead Run and tributaries – Use IV (Recreational Trout Waters); and all remaining waters – Use I (Water Contact Recreation, and Protection of Nontidal Warmwater Aquatic Life).  See Code of Maryland Regulations (COMAR) & (5)(e).   A TMDL was completed in 2006 to address the bacteria listing. The 1996 nutrients listing was refined in the 2008 Integrated Report and phosphorus was identified as the specific impairing substance.  Consequently, for the purpose of this report the terms nutrients and phosphorus will be used interchangeably.  A TMDL for sediment is scheduled to be submitted to the EPA in 2009, and the listing for impacts to biological communities will be addressed separately at a future date. 

A data solicitation for information pertaining to pollutants, including nutrients, in the Gwynns Falls basin (as part of a data solicitation for the Patapsco River basin) was conducted by MDE in November 2007, and all readily available data from the past five years have been considered.  Currently, there are no specific numeric criteria for nutrients in Maryland’s water quality standards.  Nutrients typically do not have a direct impact on aquatic life; rather, they mediate impacts through excessive algal growth leading to low dissolved oxygen.  Therefore, the evaluation of potentially eutrophic conditions due to nutrient over-enrichment will be based on whether nutrient-related parameters (i.e., dissolved oxygen levels and chlorophyll a concentrations) are found to impair designated uses in the Gwynns Falls (in this case, protection of aquatic life and wildlife, fishing, and swimming).

Recently, MDE developed a biological stressor identification (BSID) methodology to identify the most probable cause(s) of the existing biological impairments in Maryland 8-digit watersheds based on the suite of available physical, chemical, and land use data.  The BSID analysis for the Gwynns Falls indicates inorganic pollutants, ammonia toxicity, and flow/sediment stressors are associated with impacts to biological communities; these findings will be addressed separately.  The BSID analysis did not identify any nutrient stressors present and/or nutrient stressors showing a significant association with degraded biological conditions.  The results of the BSID study, combined with the analysis of recent water quality data presented in the report below, indicate that the Gwynns Falls is not being impaired by nutrients.  The WQA, available below, supports the conclusion that a TMDL for nutrients is not necessary to achieve water quality standards in the Gwynns Falls. 

Although the waters of the Gwynns Falls do not display signs of eutrophication, the State reserves the right to require future controls if evidence suggests that nutrients from the basin are contributing to downstream water quality problems.  In December 2007, EPA approved TMDLs of nitrogen and phosphorus for the Baltimore Harbor.  The Gwynns Falls watershed is located upstream of the Baltimore Harbor and drains into the Harbor’s tidal waters.  Although the amount of nutrients entering the Gwynns Falls is not causing localized impairments, it is contributing to the eutrophication of the downstream tidal waters of the Harbor.  Therefore, the TMDL for the Baltimore Harbor requires nutrient reductions in the Gwynns Falls necessary to meet water quality standards in the Harbor.  On the same principle, additional reductions may also be required by the forthcoming Chesapeake Bay TMDL, currently under development and due to be established by EPA by the end of 2010.

Barring the receipt of contradictory data, the report will be used to support a revision of the phosphorus listing for the Gwynns Falls watershed, from Category 5 (“waterbody is impaired, does not attain the water quality standard, and a TMDL is required”) to Category 2 (“waterbodies meeting some [in this case nutrients-related] water quality standards, but with insufficient data to assess all impairments”) when MDE proposes the revision of the Integrated Report.

Water Quality Analysis of Eutrophication for the Gwynns Falls Watershed in Baltimore County and Baltimore City, Maryland

(Concurrence March 15, 2010)

EPA's Decision Letter

EPA's Decision Letter

Main Report

WQA_Gwynns_Falls_eutro.pdf (706KB)

Comment Response Document

CRD_Gwynns_Falls_eutro.pdf (12KB)

Additional Supporting Information

Biological Stressor Identification Gwynns Falls Watershed Report

BSID Gwynns_Falls.pdf (707KB)

Biological Stressor Identification Methodology Report

BSID Methodology.pdf (459KB)


Contact Information

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