Optical nitrate sensors refine Mississippi River nutrient tracking

By on September 4, 2013
A Mississippi River nitrate sensor package at the Baton Rouge station. (Credit: USGS)

Although the Mississippi River has borne the products of human society for many centuries, recent history has seen the introduction of a new, environmentally detrimental passenger among its currents: nitrate runoff.

When heavy rainfall and flooding wash nitrate-rich fertilizer and manure from agricultural lands into streams, tributaries or the Mississippi itself, major water quality issues can arise. Chief among these is the Gulf of Mexico’s hypoxic dead zone, a low-oxygen area that this year grew to the size of Connecticut. The U.S. Geological Survey is using advanced optical sensors that can better detect nitrate in the Mississippi and its tributaries, and help identify the sources of the runoff.

These optical sensors “use the absorbance of UV light by nitrate” to detect concentrations of runoff in water, according to Brian Pellerin, a USGS research soil scientist. Older sensors, he said, rely on the precise mixture of chemical reagents, or the use of ion-selective electrodes. The former method takes time to work, while the latter, despite being relatively inexpensive, is “just not that accurate.”

“Our job is to provide this data and make it available for decision makers,” Pellerin said. In order To do this, the technology must be both accurate and timely. The new sensors transmit data every 15 minutes to 3 hours. Located at 36 points along the Mississippi and several of its tributaries, the real-time capabilities of these sensors allow scientists to measure the pulse of nitrate concentration in comparison to streamflow with unprecedented precision.

The new sensors offer a “much better sense as to where the real high [nitrate] loads are coming, when they’re coming, and what happens in a drought year or in a flood year,” Pellerin said. In the past, this sort of information would have been predicted using USGS historical data and a model. Data-modeling, however, is unsuitable for certain purposes such as determining nitrate levels in drinking water sources. Such circumstances call for the real-time measurements that only advanced sensors can provide.

Although optical sensors are only recently being used to monitor the nitrate pulse in the Mississippi River Basin, the technology is not unheard of.

“These optical sensors have been around for a while,” Pellerin said. They were “largely used either for wastewater… or they were in the ocean.”

As more accurate data is available to the USGS and policy makers, strategies for nutrient management can be approached with higher certainty. Whether or not these strategies will be optional or voluntary for agricultural professionals is a big political issue, according to Pellerin.

Regardless of politics, the USGS hopes the optical sensors will assist in the reduction of nitrate runoff, and the diminishment of the Gulf’s dead zone.

“It’s a pretty exciting time for these sensors,” Pellerin said. “We’re witnessing a revolutionary time in how we do science.”

Image: A Mississippi River sensor package at the Baton Rouge station. (Credit: USGS)

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