The effects of rainfall on groundwater hydrogeochemistry and chemical weathering

Due to the special hydrogeological conditions in karst areas, groundwater responds quickly to rainfall. The covariation of ion concentrations and spring discharge can help better understand the hydrogeochemical process of groundwater occurring in the heterogeneous karst aquifers. In this study, high-resolution monitoring of groundwater discharge, hydrochemistry, and stable isotopes was conducted at the Qingjiangyuan (QJY), a spring of the Qingjiang watershed in Hubei, China. The purpose is to investigate the changes in hydrogeochemical processes and chemical weathering under the influence of rainfall. The dynamics of spring discharge indicate the presence of pipelines and fissures of different sizes. According to the spring discharge attenuation curves, there are at least three medium types in the aquifer, which account for 45.7%, 34.2%, and 20.1% of the total groundwater. Pearson correlation analysis shows that the main sources of the solute in the QJY are carbonate minerals (mainly calcite and dolomite), evaporites (mainly gypsum and sylvite), celestite, and strontianite. Anthropogenic activities have less impact on groundwater solutes. Although carbonate minerals dominate the hydrochemistry, the changes in hydrogeochemical behavior caused by rainfall may come from gypsum, which is supported by the ion concentrations. At the early rainfall stage, Ca2+ concentration increased from 42.9 to 45.6 mg/L, followed by the SO42- from 15.2 to 16.6 mg/L. When the discharge increased to the maximum (2320 L/s), Ca2+ and SO42- showed opposite trends, decreased to 39.7 mg/L and 10.4 mg/L, respectively. The results also suggest that carbonate rocks and evaporites have important roles in hydrochemistry. The contributions of these three end-members were quantified based on the law of mass conservation. The proportions of carbonate weathering and evaporite weathering were 83.4% (85.2-80.3%) and 11.6% ( 6.9-18.0%), respectively, and rain was 5.0% (0.1-10.4%). These results were integrated into a hydrogeological conceptual model that explains the hydrogeochemical processes, including rock weathering, piston, and dilution effects caused by rainfall. The proposed conceptual model helps to improve the understanding of hydrogeochemical processes and chemical weathering in karst areas.