Optical Assessment of Dissolved Organic Carbon


Courtesy of Soil Science Society of America

Natural organic matter is present in all sources of drinking water. When chlorine is used for drinking-water treatment, a portion of the natural organic matter reacts to form halogenated compounds, referred to as disinfection by-products (DBPs), some of which are regulated by the USEPA due to their health effects.

The implementation of stricter drinking-water quality regulations for DBPs has highlighted the need to understand sources of organic matter to drinking-water intakes. Identifying sources of DBP precursors will enable utilities to implement effective, targeted watershed management strategies to reduce DBPs in their distribution systems. For many utilities, source water protection programs that lower the amount of DBP precursors entering water treatment plants may be more feasible and cost effective than implementing additional water treatment technologies.

Scientists at the U.S. Geological Survey (USGS) have been investigating ways to more easily assess the amount and character of dissolved organic carbon (DOC) in freshwater systems. A focus of their work is on the use of optical properties—absorbance and fluorescence—as proxies for DOC concentration, composition, source, and propensity to form DBPs. Optical measurements have the potential to be less expensive, faster, and more sensitive than laboratory chemical-based analyses.

In 2007 and 2008, USGS scientists conducted a study in collaboration with the Eugene Water and Electric Board to determine the main sources of DOC and DBP precursors to the McKenzie River, which provides drinking water to approximately 200,000 people in Oregon, USA. Water samples collected from the mainstem, tributaries, and reservoir outflows were analyzed for DOC concentration and DBP (trihalomethane and haloacetic acid) formation potential. In addition, the full spectrum of absorbance and fluorescence were measured to provide insight into DOM composition and assess whether optical properties are useful proxies for DOC and DBP precursor concentrations.

Results from this study were published in the November–December 2010 issue of the Journal of Environmental Quality. The researchers found that primary sources of DOC and DBP precursors to the McKenzie River originate from upstream terrestrial sources and are strongly linked to changes in hydrologic flow path. Although downstream tributaries had higher DOC concentrations and contained more reactive DOC, these
inputs comprised less than 5% of mainstem flows and thus had minimal impact on overall water quality. Optical measurements gave no indication that algal inputs within the reservoirs were significant during the period of study.

Although absorbance has previously been suggested as a proxy for DOC concentration, in this study there was interference in the absorbance spectra in downstream tributary samples. However, fluorescence data were very strongly correlated to DOC concentration and DBP formation potentials, suggesting it may be a useful proxy.
Findings from this study show that optical measurements—particularly fluorescence—have great potential to improve source water monitoring.

The recent development of commercially available field instruments that measure optical properties in situ can provide real-time, continuous assessment of water entering treatment plants. The authors say this type of data will help water utilities understand both short- and long-term trends in drinking-water quality, and thereby help facilitate successful and cost-effective treatment plant operations and watershed management strategies.

Adapted from Kraus, T.E.C., C.A. Anderson, K. Morgenstern, B.D. Downing, B.A. Pellerin, and B.A. Bergamaschi. 2010. Determining sources of dissolved organic carbon and disinfection by-product precursors to the McKenzie River, Oregon. J. Environ. Qual. 39:2100–2112.
View the full article online at www.agronomy.org/publications/jeq/tocs/39/6

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