New technique allows frequent water quality monitoring for suite of pollutants

[March 08, 2014]  RALEIGH, N.C. — Researchers from North Carolina State University have developed a new technique that uses existing technology to allow researchers and natural resource managers to collect significantly more information on water quality to better inform policy decisions.

"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 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.

To collect water quality data, the researchers used existing technology called "UV-Vis" spectrometers, which are devices that measure the wavelengths of light absorbed by water. 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 are 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.

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A copy of the study abstract follows:

"Using in situ ultraviolet-visual spectroscopy to measure nitrogen, carbon, phosphorus, and suspended solids concentrations at a high frequency in a brackish tidal marsh"

Authors: Randall Etheridge, François Birgand, Jason A. Osborne, Christopher L. Osburn, and Michael R. Burchell II, North Carolina State University; Justin Irving, s::can Measuring Systems

Published: online March 2014, Limnology and Oceanography: Methods

DOI: 10.4319/lom.2014.12.10

Abstract: The collection of high frequency water quality data are key to making the next leap in hydrological and biogeochemical sciences. Commercially available in situ ultraviolet-visual (UV-Vis) spectrometers make possible the long-term collection of absorption spectra multiple times per hour. This technology has proven useful for measuring nitrate, dissolved organic carbon, and total suspended solids in many environments, but has not been tested in tidal marsh conditions where upstream freshwater mixes with estuarine waters, resulting in rapid changes in concentrations and salinity. These three parameters encompass only a portion of the nutrients that are of interest in these systems. To test the potential of spectroscopy to measure these and other nutrient concentrations, spectrometers were installed in a constructed brackish tidal marsh and absorbance spectra were compared to lab analyses for coinciding discrete samples. Variable selection techniques, including partial least squares regression, lasso regression, and stepwise regression, were used to develop models with which nitrate, total kjeldahl nitrogen, dissolved organic carbon, phosphate, total phosphorus, total suspended solids, and salinity in brackish marsh waters can be predicted from UV-Vis spectrometer measurements. Significant relationships between the absorption spectra and the laboratory measured concentrations were observed for all of the parameters. Phosphate and total phosphorus were the only nutrients which had R² values less than 0.86 for their best calibrations. This study shows the potential to collect multiple water quality parameters at a high frequency in brackish waters using in situ spectrometers and gives the tools to replicate this analysis in all environments.

[Text from news release received from North Carolina State]