The migration and transformation processes of dissolved organic matter in rainwater- drip water- phreatic water of a typical karst spring catchment, in South China

The dissolved organic matter (DOM) migration and transformation in karst aquifer are of great significance for geochemical processes but remain poorly understood. In this study, we selected the Guilin Yaji Karst Experimental Site as a natural laboratory and collected rainwater (RW), drip water (DW), and phreatic water (PW) from September 2019 to September 2021 to study the DOM migration and transformation in a typical bare karst spring catchment. Hydrochemical and stable hydrogen and oxygen isotopes were applied to study the water cycle of the aquifer. Absorbance and fluorescence spectroscopy were used to characterize the DOM quality. Three-dimensional fluorescence combined with fluorescence regional integration was used for identifying the DOM compositions. The RW DOM had a weak humification degree and strong terrestrial characteristics. It was affected by the monsoon and showed higher humification in the dry season than in the wet season. The higher rainfall intensity had a dilution effect on the DOM fluorescence components levels when the rain intensity was>10 mm/day (not the threshold value). Continuous rain events led to the continuous dilution effect on RW DOM in the dry season, but such a phenomenon was not obvious in the wet season. The DW DOM inherited weak humification characteristic that was close to RW DOM but was altered by the hydrogeology conditions and plant root and microbial activities intensities of the upper vadose zone. The higher humification degree of PW DOM may be related to the higher degree of microbial processing caused by the longer groundwater residence time. The humic and hydrophobic degree of DOM for fissure PW was weaker than that for conduit PW because fissure media provides a superior condition for DOM to adsorb to the mineral surface. In the wet season, when the preferential channel of the aquifer was activated, the PW DOM showed similar characteristics to RW DOM, indicating a significant hydrologic effect on the PW DOM. Our results provide a valuable reference to advance understanding of DOM behavior in karst aquifer and highlight the importance of hydrogeologic conditions and hydrologic processes on DOM migration and transformation.