Environmental context. Dissolved organic matter (DOM) is part of the global carbon cycle, ecologically and geochemically active, and costly to remove in water treatment. Spectroscopic monitoring at a single wavelength provides some indication of DOM concentration, but variations in optical properties mean that accurate determinations currently rely on slow and costly laboratory methods. We show that for water samples containing non-anthropogenic DOM, ultraviolet absorbance at two wavelengths can quantify DOM rapidly, cheaply and accurately, and also indicate its quality.

Abstract. The precise simulation of ultraviolet absorption by 23 contrasting surface-water DOM samples was achieved with a model based on two components, one absorbing light strongly (A) and the other weakly (B). The parameterised model can be used to predict [DOC] in water samples simply from absorbance values at two wavelengths, while information on DOM quality is provided by the calculated fractionation into A and B. The model was tested by predicting [DOC] for a separate dataset obtained by combining results for 12 samples each from surface waters in the UK, Canada and the USA, and from UK groundwaters. A close correlation (R² = 0.997) was obtained, with only slight underestimation of the true [DOC]. The proportions of components A and B varied considerably among the sites, which explains why precise prediction of [DOC] from absorbance data at a single wavelength was not possible. When the model was applied to samples collected from river locations in a heterogeneous UK catchment with areas of industry and high human population, [DOC] was underestimated in many cases, which may indicate the presence of non-absorbing pollutant DOM.