Announced in November, a U.S. Geological Survey (USGS) article published in Environmental Science & Technology describes how advanced optical sensor technology on small streams and large rivers throughout the Mississippi River basin are helping to accurately track nitrate pulses to the Gulf of Mexico. USGS is using the sensors to collect nitrate concentration data every hour. The sensors are improving the ability to track where nitrate pulses are coming from and forecast when they will arrive at the Gulf. Further, the hourly data make it easier to detect changes in nitrate levels related to basin management and to track progress toward the goal of reducing the size of the dead zone in the Gulf.

“High frequency data from these sensors has revealed considerable variability in nitrate concentrations in small rivers and streams,” said Brian Pellerin, USGS researcher. “However, we were surprised to see nitrate concentrations vary by as much as 20% in a week in a river as large as the Mississippi River without similar changes in streamflows.”

Rapid changes are very easy to miss with traditional water-quality monitoring approaches. However, hourly information on nitrate levels improves the accuracy and reduces the uncertainty in estimating nitrate loads to the Gulf of Mexico, especially during drought and flood years.

This high frequency data also provides new insights into timing and magnitude of nitrate flushing from soils during wet and dry conditions. For instance, the high frequency data revealed high nitrate concentrations during the spring and early summer of both 2013 and 2014 following the drought of 2012.

The USGS, in cooperation with numerous local, state, and other federal agencies, currently operates more than 100 real-time nitrate sensors across the Nation. Real-time nitrate monitoring is supported by the USGS National Stream Quality Accounting Network, Cooperative Water Program, and the National Water-Quality Assessment Program.