Total Maximum Daily Load of Sediment in the Patuxent River Upper Watershed, Anne Arundel, Howard and Prince George's Counties, Maryland.

摘要

This document, upon approval by the U.S. Environmental Protection Agency (EPA), establishes a Total Maximum Daily Load (TMDL) for sediment in the Patuxent River Upper watershed (basin number 02131104) (2008 Integrated Report of Surface Water Quality in Maryland Assessment Unit ID: MD-02131104). Section 303(d) of the federal Clean Water Act (CWA) and the EPAs implementing regulations direct each state to identify and list waters, known as water quality limited segments (WQLSs), in which current required controls of a specified substance are inadequate to achieve water quality standards. For each WQLS, the State is required to either establish a TMDL of the specified substance that the waterbody can receive without violating water quality standards, or demonstrate that water quality standards are being met (CFR 2009b). The Maryland Department of the Environment (MDE) has identified the waters of the Patuxent River Upper watershed on the States 2008 Integrated Report as impaired by nutrients nitrogen and phosphorus (1996), sediment (1996), bacteria (2002 and 2008), methylmercury - Cash Lake (2004), and impacts to biological communities (2006) (MDE 2008a). The designated use of the Patuxent River Upper mainstem and its tributaries is Use I (Water Contact Recreation and Protection of Aquatic Life) (COMAR 2009 a,b). The TMDL established herein by MDE will address the 1996 sediments listing, for which a data solicitation was conducted, and all readily available data from the past five years have been considered. A Water Quality Analysis (WQA) for eutrophication to address the nutrients (nitrogen and phosphorus) listing was approved by EPA in 2007, and the watershed was delisted for bacteria in 2002 (relisted in 2008 mainstem only from Old Queen Anne's Bridge Road to the rivers confluence with the Little Patuxent River). A methylmercury TMDL for Cash Lake is scheduled to be submitted to the EPA in 2010. In the 2012 Integrated Report, the listing for impacts to biological communities will include the results of a stressor identification analysis. The Patuxent River Upper watershed aquatic life assessment scores, consisting of the Benthic Index of Biotic Integrity (BIBI) and Fish Index of Biotic Integrity (FIBI), indicate that the biological metrics for the watershed exhibit a significant negative deviation from reference conditions based on Marylands biocriteria listing methodology. The biocriteria listing methodology assesses the condition of Marylands 8-digit (MD 8- digit) watersheds by measuring the percentage of sites, translated into watershed stream miles, that are assessed as having BIBI and/or FIBI scores significantly lower than 3.0 (on a scale of 1 to 5), and then calculating whether this percentage differs significantly from reference conditions (i.e., unimpaired watershed: <10% of stream miles differ from reference conditions) (Roth et al. 2005; MDE 2008a). The objective of the TMDL established herein is to ensure that watershed sediment loads are at a level to support the Use I designation for the Patuxent River Upper watershed, and more specifically, at a level to support aquatic life.