|Breen K J, - USGS|
|Gavin A J, - USGA|
Submitted to: Chesapeake Research Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 26, 1994
Publication Date: N/A
Interpretive Summary: Concern over the health of the Chesapeake Bay has focused on the delivery of nonpoint source pollutants from tributaries draining the Bay watershed. While the plant nutrients nitrogen and phosphorus have received major attention, and numerical targets have been set for their reduction, toxic substances continue to impair the Bay. Atrazine a widely used herbicide of fcorn is on the list of "toxics of concern" for the Bay. Atrazine has been measured in surface and groundwater of many parts of the country associated with intensive agriculture. The Susquehanna River Basin (27,100 sq. mi.) is the largest watershed draining to the Bay, encompasses areas of intensive agriculture and contributes the largest volume of freshwater of any Bay tributary. This study was undertaken to provide a snapshot of atrazine concentrations and loads in tributaries of the Lower Susquehanna River. Measurements were made in areas of differing land use, differing aquifer types and landforms throughout the Lower Susquehanna Basin. Concentrations ranged from undetectable to 1.05 ug Atrazine/l. All measured concentrations were well below EPA guidelines. Median atrazine concentration, load and discharge were lowest for watersheds situated on shale and sandstone. The mass of atrazine discharged into Susquehanna (load) was low mainly because less water came out of the shale and sandstone watersheds and not because concentrations were so much different. Atrazine concentration and load in the Susquehanna itself showed a steady increase as it passed through southern Pennsylvania and northernmost Maryland.
Technical Abstract: During June 14-21, 1993, a study to describe the occurrence and yields of triazine herbicides in streams of the Lower Susquehanna River Basin in Pennsylvania and Maryland was conducted as part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Program. Concentrations of triazine herbicide residues were determined by immunoassay analyses and reported as atrazine concentrations. Triazine concentration, stream discharge, and drainage-area data were used to estimate yields of triazine herbicides for 43 individual surface-water sites including the Susquehanna River at Danville, Harrisburg, Marietta, and the basin outflow at Conowingo Dam. Yields of triazine herbicides also were determined for 6 major tributaries and for 33 subbasins that represent the Piedmont Physiographic Province and the Appalachian Mountain and Great Valley Sections of the Ridge and Valley Physiographic Province. Detectable concentrations of triazine herbicides, ranging from 0.1 to 1.0 ug/L (micrograms per liter) were measured in water samples from 39 of the 43 surface-water sites. Nondetections were in samples from two sites that represent forested subbasins, the Susquehanna River at Danville and the West Branch Susquehanna River at Lewisburg. Median concentrations of triazines ranged from 0.45 to 0.55 ug/L for subsets of samples representing the Ridge and Valley and Piedmont Physiographic Provinces in limestone and noncarbonate bedrock settings. Loads and yields of triazine herbicides are smaller for the Ridge and Valley than for the Piedmont. A ranking on the basis of instantaneous load of the major tributaries sampled during this study of stream base flow is: Conestoga River > Swatara Creek > Muddy Creek > Pequea Creek > East Mahantango Creek > West Branch Susquehanna River.