Level for drinking water of 10 milligrams per liter. The limestone bedrock has features such as large fractures, caverns, and sinkholes that allow nitrate to move rapidly from the land surface to the ground water. In limestone areas, ground water from springs made up a large part of the water in streams. Thus, high nitrate concentrations were found in spring-fed streams.

Streams in agricultural areas underlain by limestone, like many streams in Lancaster County, Pa., and in the Great Valley of Virginia, West Virginia, Maryland, and Pennsylvania, have some of the highest concentrations of nutrients when compared to 18 other NAWQA studies across the country. Mill Creek in Lancaster County and Bachman Run in Lebanon County, Pa., were two streams studied in the Lower Susquehanna Basin that have high concentrations of nitrate. Overall, in limestone and non-limestone areas, 21 percent (58 of 274 wells) of the wells and 11 percent (31 of 272 stream samples) of the streams sampled had nitrate concentrations greater than 10 milligrams per liter.

Concentrations of nitrogen and phosphorus have changed over the past 10 to 20 years in the rivers where they flow into Chesapeake Bay. The amount of nutrients delivered to Chesapeake Bay is dependant upon the amount of rainfall. For example, 1996 was an exceptionally wet year and flood waters from these two rivers in 1996 carried large amounts of nutrients and suspended sediment to Chesapeake Bay. Therefore, the reported trends were adjusted for these types of variations in streamflow. Despite an estimated 44-percent increase in population in the Potomac River Basin from 1970 to 1990, total-phosphorus concentrations in the Potomac River at Washington, D.C., have decreased since 1979 and nitrogen concentrations have apparently stabilized. There are downward trends in both total-phosphorus and total-nitrogen concentrations for the Susquehanna River at Conowingo, Md. Different forms of
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Contact: Bruce Lindsey
blindsey@usgs.gov
(717) 730-6964
United States Geological Survey
16-Apr-1998


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