Water Quality Analysis of Chlorpyrifos in The Patuxent River Lower and Middle Watersheds; Anne Arundel, Calvert, Charles, Prince George�s, and St. Mary�s Counties, Maryland

Water Quality Analyses (WQA) with EPA Concurrence of MDE's Findings: Patuxent River Lower

The Patuxent River is located on Maryland’s Western Shore and drains portions of Anne Arundel, Calvert, Charles, Howard, Montgomery, Prince George’s, and St. Mary’s Counties, Maryland.  The river has a length of approximately 71 kilometers (km) and a width ranging from 500 meters (m) upstream to 2.4 km at the mouth.  It flows to the south/southeast until it empties into the Chesapeake Bay.  The entire watershed covers an area of approximately 930 square miles.  The Patuxent River Lower watershed is located in Anne Arundel, Calvert, Charles, St. Mary’s, and Prince George’s Counties, and it drains an area of approximately 207,405 acres.  The Patuxent River Middle watershed is located in Anne Arundel, Calvert, and Prince George’s Counties, and it drains an area of approximately 66,478 acres. 

The Patuxent River Lower (basin code 02131101), located in Anne Arundel, Calvert, Charles, Prince George’s, and St. Mary’s Counties, Maryland, was identified on the State’s list of WQLSs as impaired by fecal coliform (1996, 1998, and 2004), nutrients (1996), sediments (1996), chlorpyrifos in the water column (2002), methylmercury (2002), and impacts to biological communities (2002 and 2004).  The Patuxent River Middle (basin code 02131102), located in Anne Arundel, Calvert, and Prince George’s Counties, Maryland, was identified on the State’s list of WQLSs as impaired by sediments (1996), nutrients (1996), chlorpyrifos in both the water column and underlying sediments (2002), and impacts to biological communities (2002 and 2004).  All fecal coliform listings are for tidal shellfish areas, whereas the biological listings are for nontidal waters, except for the 2004 Patuxent River Lower biological listing.  All other listings are for the tidal waterbody.

The report, available below, provides an analysis of recent monitoring data showing that the aquatic life criteria for chlorpyrifos are being attained in the Patuxent River Lower and Middle watersheds, and therefore the watersheds are not impaired by chlorpyrifos according to their designated uses and associated aquatic life criteria.  The analysis does not support the impairment listing for chlorpyrifos, and a TMDL is not necessary to meet the water quality standards for chlorpyrifos since criteria are currently being attained. 

Barring the receipt of contradictory data, the report will be used to support a chlorpyrifos listing change for both the Patuxent River Lower and Middle watersheds from Category 5 (waterbodies impaired by one or more pollutants requiring a TMDL) to Category 2 (Surface waters that are meeting some standards and have insufficient information to determine attainment of other standards), when the Maryland Department of the Environment (MDE) proposes the revision of Maryland’s 303(d) List for public review in the future.  The Patuxent River Middle listings for sediments, nutrients, and impacts to biological communities will be addressed separately at a future date.  Two TMDLs to address the bacteria listings for the sub-basins of the Patuxent River Lower watershed were approved by the EPA in 2005, a WQA to address the bacteria listing for the mainstem of the Patuxent River Lower was submitted to the EPA in 2006, and a TMDL to address the 2002 methylmercury listing for the Lake Lariat impoundment was approved in 2004.  The Patuxent River Lower listings for nutrients, sediments, and impacts to biological communities will be addressed separately at a future date.  

Although the waters of the Patuxent River Lower and Middle watersheds do not display signs of toxic impairments due to chlorpyrifos, the State reserves the right to require additional pollution controls in the watersheds if evidence suggests that chlorpyrifos from the basin is contributing to downstream water quality problems. 

Water Quality Analysis of Chlorpyrifos in The Patuxent River Lower and Middle Watersheds; Anne Arundel, Calvert, Charles, Prince George’s, and St. Mary’s Counties, Maryland

(Concurrence July 3, 2007)

EPA's Decision Letter EPA's Decision Letter

Main Report

WQA_pax_river_chlorpyrifos.pdf (575KB)

Comment Response Document

CRD_pax_river_chlorpyrifos.pdf (17KB)

 

Contact Information

Please direct questions or comments concerning this project to Maryland's TMDL Program at (410) 537-3572.