Influence of upstream land use on dissolved organic matter and trihalomethane formation potential in watersheds for two different seasons

Different land uses of upstream catchments may affect the quantity and the quality of dissolved organic matter (DOM) in watersheds, but the influence may differ by season. In this study, we examined concentrations and selected spectroscopic properties of DOM and the propensity to form trihalomethanes (THMs) for 19 different middle-sized watersheds across the Han River basin in Korea. Sampling was conducted for non-storm events during pre-monsoon (May) and monsoon seasons (July). The anthropogenic land uses including agricultural and residential areas occupied 2.3 to 49.4 % of the upstream catchments of the watersheds. Non-aromatic, labile, and less condensed DOM structures were more abundant in the monsoon season. Parallel factor analysis (PARAFAC) modeling with fluorescence data demonstrated that a combination of three different fluorescence components could explain the seasonal and the spatial distributions of DOM characteristics. Terrestrial humic-like fluorescence was the most abundant component for all the DOM samples, while protein-like fluorescence became more pronounced for the monsoon season. THM concentrations did not differ between the two seasons. Observed seasonal differences in the concentrations and the characteristics of DOM suggested a greater contribution of groundwater to the streams in watersheds in the monsoon versus the pre-monsoon season. Significant correlations among anthropogenic land use, microbial humic-like fluorescence, and the propensity to form THMs were found only for the pre-monsoon season. Principal component analysis (PCA) demonstrated that, regardless of the season, anthropogenic land uses increased the concentrations of DOM and nutrients but that their effects on the DOM properties were not evident for the monsoon season.