Despite efforts to reduce nitrate levels in the Mississippi River Basin, concentrations and transport at eight major study sites did not consistently decline from 1980-2008. These results are based on a new scientific model developed by the USGS that takes into account variation in river flows in order to gain an accurate understanding of long term trends. The results of the new USGS study are published in the journal Environmental Science and Technology.
“While conservation practices may have decreased nitrate levels in some portions of the basin, we aren't seeing widespread effects at larger scales,” said Lori Sprague, USGS hydrologist and lead author on the report. “Applying this new model to decades of USGS water quality data allows us to distinguish between the effects of natural changes in precipitation and streamflow and the effects of purposeful changes in the management of nitrate in the basin.”
Excessive nutrients like nitrate in the Mississippi River Basin contribute to hypoxia, or dead zones, in the Gulf of Mexico. The dead zones are the result of too little oxygen to support most marine life in bottom and near-bottom water. State and federal partners serving on the Mississippi River Gulf of Mexico Watershed Nutrient Task Force are striving to decrease nutrients transported to the Gulf to reduce the size of the hypoxic zone to less than 5,000 square kilometers (about 2,000 square miles) by 2015.
For this new study, the USGS analyzed data from eight study sites taken between 1980-2008, including 3,368 individual water-quality samples and 110,732 individual daily streamflow values.
These results reflect the cumulative changes over time in nitrate sources and conservation practices throughout the Mississippi River Basin and highlight the need for comprehensive nutrient management strategies that will reduce nutrients in both streams and groundwater.
The new method developed for analyzing these trends was critical for understanding these changes and will be used for future analysis of nutrient monitoring data collected in the basin.
“When we analyze long-term nutrient trends for the Mississippi River or other rivers, it’s important that we consider flow variations, because water quality can change greatly from year to year due to precipitation and runoff,” said Robert Hirsch, USGS research hydrologist who led the development of the new method. “This new method enables us to remove this source of variation from data and provides greater insight into the effects of conservation practices.”
The full Environmental Science and Technology article and additional information about the eight sites can be found online.
The same method of analysis was also applied to nutrient inputs to the Chesapeake Bay Watershed in a 2010 USGS report. For results of that study, visit the USGS News Release: A New Understanding of 31 Years of Chesapeake Bay Nutrient Trends.
Source: US Geological Survey