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DOM concentration and quality from UV absorbance

Professor Ed Tipping, Centre for Ecology & Hydrology, Lancaster

Friday 25 March 2011, 1300-1400
LEC Training Room 1

Centre for Sustainable Water Management Seminar

Dissolved organic matter (DOM) is a ubiquitous collection of components in surface, soil and ground waters, comprising the partial decomposition products of living material, chiefly plants and algae, but also derived from agricultural, industrial and domestic human activities. Differences in source material and rates of decomposition, and physical fractionation processes, generate substantial heterogeneity in DOM concentrations and properties.

Routine methods for the accurate determination of DOM in different samples are based on the measurement of DOC, usually after conversion to CO2. This requires the return of samples to the laboratory, and is both time-consuming and fairly expensive. Optical absorbance at a single wavelength in the UV-visible range usually correlates strongly with [DOC], and is therefore often used to monitor natural DOM. However, differences in DOM properties among waters, and temporally within a given water, mean that this can only be an approximate method. Indeed, variation in the ratio of absorbance to [DOC] is widely used to characterise the quality of DOM, notably through the specific UV absorbance at 254 nm (SUVA).

We have developed a model that considers UV-absorbing DOM to comprise two components (A and B). Component A absorbs UV light more strongly, especially at longer wavelengths (> 300 nm). The two components give different spectral shapes, and this can be exploited to calculate their contributions to the total DOM in a given sample. By also taking into account "UV-invisible" DOM in the sample (~1 mg DOC L-1), it is possible to calculate [DOC] from measurements at two wavelengths. The same calculation also yields the fraction of each component, a measure of DOM quality.

We have parameterised the model with data for c.1600 surface water samples collected in Canada, United Kingdom, and USA. The procedure permits rapid and cheap determination of [DOC], and has promise for continuous monitoring.