Seasonal changes of dissolved CO2 and its linkage with optical characteristics of DOM in groundwater in agricultural areas

Groundwater contains significant quantities of dissolved CO2, which are crucial contributors to the global carbon cycle. Dissolved organic matter (DOM) functions as a principal carbon and energy source for microbial communities within groundwater ecosystems, yet the relationship between dissolved CO2 and optical characteristics of DOM remains ambiguous. Here, we delineate the variations in seasonal and vertical distribution of dissolved CO2 concentrations and DOM optical characteristics, and assess their interconnections in the groundwater of an agricultural region. The results showed that the average concentration of dissolved CO2 in groundwater was 11,300 ± 5,788 ppm, with the highest levels observed in summer and the lowest in winter. Additionally, shallow wells exhibited higher concentrations of dissolved CO2 compared to deep wells. The mass balance calculation suggested that 40,092 t of CO2 are released annually due to the degassing of supersaturated CO2 from groundwater extraction. Three-dimensional fluorescence with parallel factor analysis (EEM-PARAFAC) identified five components of DOM in groundwater: Components C1, C2, C3 and C4 are humic-like substances, and C5 is a tryptophan-like substance associated with protein structures. The analysis of compositional features and fluorescence indices revealed that DOM is predominantly derived from microbial sources, with mixed contributions from terrestrial sources in groundwater. Principal component analysis (PCA) showed that first two axes accounted for 74.7 % of the variance in the seasonal variation of DOM optical characteristics and dissolved CO2 concentration, which reveals that DOM decomposition contributes to CO2 saturation in groundwater. However, a larger proportion of CO2 may be derived from leaching in the vadose zone during the rainy season or irrigation periods in shallow aquifer. This study enhanced the understanding of carbon dynamics within agricultural groundwater.