Impact of spring flooding on DOM characterization in a small watershed of the Hun River, China

Potential water pollution derived from melting snow is a growing concern to watershed management in northern climates. For most small watersheds in the Hun River Basin, substantial portions of the annual load of pollutants such as organic matter, ammonia, phosphorus and metals come from snowmelt and early spring runoff. Thus, the annual spring flood is a real threat to the trophic state and water quality of Hun River. By three-dimensional fluorescence technology, the composition and characteristics of dissolved organic matter (DOM) in early spring runoff of a small watershed (the Baitapu River) were analyzed, and the correlation with land use was investigated. The result showed that compared with NH₃-N and TP, DOM was not the major contributor to water quality deterioration in spring flooding, but its composition still presented significant differences. Compared with that in medium flow period, the abundance of a soluble microbial byproduct-like material in the spring-flood period decreased from 28.6 to 15.8 %, while those of tyrosine-like material and humic-like material increased from 36.0 and 1.8 % to 44.4 and 10.7 %, respectively. DOM in spring flooding had a relatively higher aromaticity degree, a greater humification level, and a larger molecular weight. Meanwhile, DOM from urban areas contained more tyrosine-like material. For the Baitapu River, urban areas were the major DOM source in both the medium flow period (from dispersed point sources) and the spring flood period (from a wastewater treatment plant). Therefore, DOM composition can be used as an effective indicator for tracking pollution sources.