Because the stream samples primarily were collected when the flow was low and dominated by ground water (base flow), the occurrence and trends of phosphorus and other forms of nitrogen such as ammonia are difficult to determine from the NAWQA data. These nutrients generally are associated with storm runoff and would not be well characterized by an analysis of base-flow data. Therefore, data from an ongoing study of the Susquehanna River (Langland and others, 1998) was used to show trends for these nutrients. The study showed that the concentration of total nitrogen in the Susquehanna River at Conowingo Dam, the river's inflow to the Chesapeake Bay, decreased in the 1985-96 time period. The concentration of nitrate (one component of total nitrogen) remained unchanged during this period.
The downward trends in total nitrogen (see map and table at right) are probably the result of large decreases in concentrations of ammonia and organic nitrogen--other components of total nitrogen. The decreases in concentrations of ammonia and organic nitrogen, and subsequent decreases in total nitrogen, are attributed to improvements in sewage-treatment plants and implementation of agricultural best-management practices. The specific environmental circumstances that would explain the lack of change in nitrate concentration during a time of downward trends in total nitrogen could be related to the nitrate in streams that originates in ground water or to other nonpoint sources. Further study would be needed to determine the causes of these opposing trends.
Studies of phosphorus show that trends in concentration have decreased throughout the basin. These trends are attributed to a ban on phosphate detergents as well as improvements in sewage-treatment plants and implementation of agricultural practices that decrease surface runoff.
A study of nutrient concentrations in the Susquehanna River Basin from 1985 to 1996 indicated downward trends in concentrations of total nitrogen and phosphorous (Langland and others, 1998). These trends are attributed to improvements in sewage-treatment plants, a ban on phosphate detergents, and implementation of agricultural best-management practices. Nitrate, a component of total nitrogen, showed an upward trend at Lewisburg and no change at any other sites. Constant or increasing nitrate concentrations may be related to flow from ground water and nonpoint sources. (Map modified from U.S.Environmental Protection Agency, 1997).
|Site location||Percentage change in nutrient concentrations 1985-96 minimum/trend/maximum|
|Total phosphorus||Total nitrogen||Nitrate-nitrogen1|
|Susquehanna River at Danville||-41/-27/-10||-37/-31/-23||No change|
|West Branch Susquehanna River at Lewisburg||No change||-24/-15/-4||8/18/29|
|Juanita River at Newport||-59/-51/-41||-31/-26/-21||No change|
|Susquehanna River at Marietta2||-47/-35/-20||-37/-30/-22||No change|
|Conestoga River at Conestoga||-32/-20/-6||-23/-19/-14||No change|
|Susquehanna River at Conowingo Dam||-62/-53/-42||-25/-18/-10||No change|
1The trend shown is for total nitrate plus nitrite nitrogen,
which is a close approximation to the trend in nitrate-nitrogen.
2The trend for Marietta is based on data from 1987 to 1996.
The table above shows the trends in concentrations calculated by Langland and others (1998), illustrating the magnitude of the trends shown on the map as a percentage. The bold number on the table is the trend from the statistical model used to detect changes in concentration over time (Cohn and others, 1992). The maximum and minimum results illustrate the range of possible trends. Negative numbers indicate downward trends. The term "no change" is used in cases where it is uncertain whether the trend is upward or downward.