Organic matter as an indicator of groundwater calcium enrichment in paleo-channel area along middle reaches of the Yangtze River

ObjectiveAs an important source of drinking water, abnormal calcium (Ca) concentrations in groundwater could endanger human growth and lead to health risks. Previous research has primarily focused on how inorganic carbonate equilibrium processes involving atmospheric CO2 regulate groundwater Ca cycling. However, the important role of the organic carbon pool has been overlooked, and the microscopic mechanisms underlying the interactions between different components of organic matter (OM) and Ca remain elusive. MethodsThis study took the paleo-channel area along the middle reaches of the Yangtze River as the study area. Groundwater samples were analyzed using hydrochemical analysis, principal component analysis, parallel factor analysis (PARAFAC) of fluorescence excitation-emission matrix (EEM) spectroscopy, and fluorescence excitation-emission matrix regional integration. ResultsThe results demonstrated that groundwater Ca concentrations exhibited significant spatial heterogeneity, ranging from 111 to 213 mg/L, mainly concentrated in the meanders and oxbow lakes of the paleo-channel region. Greater OM content was observed in the groundwater with elevated Ca concentration, and the two exhibited similar spatial distribution patterns. In the study area, the EEM-PARAFAC model identified three major components in groundwater, including protein-like components (C1), microbial and terrestrial humic components (C2 and C3). ConclusionWith the increase of Ca concentration in groundwater, the content of protein-like components decreased while that of the humic-like components increased. Notably, the abundant organic matter buried in the paleo-channel area provides a strong reducing environment that favors the microbial degradation of protein-like organic matter. This process promotes the dissolution of Ca-bearing minerals, which contributes to the groundwater Ca enrichment. This research clarified the heterogeneous spatial distribution and occurrence environments of Ca in groundwater, characterized differences in organic matter composition among groundwater with different Ca concentrations, and revealed the mechanisms by which organic matter influenced the migration and enrichment of Ca in groundwater.