Changes in spectroscopic and molecular weight characteristics of dissolved organic matter in a river during a storm event

Changes in dissolved organic matter (DOM) characteristics were investigated during a storm event in the Kyungan River using UV-visible, fluorescence spectroscopy, resin fractionation, and size exclusion chromatography (SEC). Water samples were collected at nine sampling times to reflect a variation of the river water level. A dramatic increase was observed for chemical oxygen demand (COD) versus biochemical oxygen demand, suggesting that non-biodegradable organic components may be more contained in the organic matters driven by the storm. Specific UV absorbance values increased from 2.15 to 3.16 L/mgC-m, reaching the maximum level at the highest water level. The storm runoff resulted in the reduction of protein-like fluorescence (PLF), the increase of fulvic-like and humic-like fluorescence for the synchronous fluorescence spectra of DOM. Weight-average molecular weight (MW_w) values increased from 1,100 to 1,510 Da due to the increment of high MW fractions in the SEC chromatograms. Overall changes in DOM composition may be explained by the inflow of soil-derived DOM from the upstream basins brought by the storm. The humification index (HIX) exhibited a positive correlation with MW_w values, suggesting that HIX may be suggested to a prediction descriptor for DOM MW during the storm event. PLF presented a negative correlation with DOM MW, suggesting that protein-like fluorescent compounds are associated with low MW components in the river. More input of humic substances by the storm runoff appears to shift DOM into a higher MW value as revealed by a positive correlation between MW_w and hydrophobic fraction.