Photo credit: François Birgand“Right now, incomplete or infrequent water quality data can give people an inaccurate picture of what’s happening – and making decisions based on inaccurate data can be risky,” says Dr. François Birgand, an assistant professor of biological and agricultural engineering at NC State and co-author of a paper describing the work. “Our approach will help people get more detailed data more often, giving them the whole story and allowing them to make informed decisions.”
In addition to its utility for natural resource managers, the technique will also allow researchers to develop more sophisticated models that address water quality questions. For example, the researchers are using data they collected using the new technique to determine the extent to which fertilizer runoff contributes to water pollution in specific water bodies and the role of wetlands in mitigating the effect of the runoff.
The researchers used existing technology called “UV-Vis” spectrometers, which are devices that measure the wavelengths of light absorbed by water to collect water quality data. The upside to these devices is that they can collect data as often as every 15 seconds, and over long periods of time. This is far more frequent than is possible with conventional water sampling and lab analysis techniques. The downside is that they are designed to monitor only a handful of key water quality parameters: nitrates, dissolved organic carbon and turbidity – or how clear the water is.
But the NC State research team developed a technique that uses a suite of algorithms to significantly expand the amount of information that can be retrieved from the spectroscopy data collected by UV-Vis devices. Specifically, the new technique allows researchers to get information on the levels of organic nitrogen, phosphates, total phosphorus, and salinity of the water. This water quality data can offer key insights to a host of questions, including questions about nutrient pollution.
The researchers tested the new technique in a restored brackish marsh that experiences approximately 70 centimeters of tidal variation – and a salinity that can vary from freshwater to saltwater within minutes when the tide turns.
“We found that the automated results using our technique were comparable to the results we obtained by testing water samples in the lab,” Birgand says. “So we gain a lot in terms of monitoring frequency, without sacrificing accuracy.”
The paper, “Using in situ ultraviolet-visual spectroscopy to measure nitrogen, carbon, phosphorus, and suspended solids concentrations at a high frequency in a brackish tidal marsh,” is published online in Limnology and Oceanography: Methods. Lead author is former NC State Ph.D. student Randall Etheridge. Co-authors include Birgand; Dr. Jason Osborne, an associate professor of statistics at NC State; Dr. Christopher Osburn, an assistant professor of marine, earth and atmospheric sciences at NC State; Dr. Michael Burchell, an associate professor of biological and agricultural engineering at NC State; and Justin Irving of s::can Measuring Systems.
The work was supported by National Science Foundation grant DGE-0750733, U.S. Environmental Protection Agency grant EPA 2871, and the North Carolina Water Resources Research Institute.