Hydrochemistry, 13C isotopic composition (δ13CDIC) of dissolved inorganic carbon (DIC) and multidimensional fluorescence spectroscopy of dissolved organic matter (DOM) were investigated to understand carbonate dissolution (CD) dynamics in a lacustrine aquifer of Jianhan Plain, China. The ubiquitous electron donor (i.e., DOC), high concentrations of reduced species (Fe2+, NH4+) present in groundwater and the significant biological signature indices (i.e., FI and BIX) of DOM indicate that bio-redox reactions have extensively occurred in the groundwater. Groundwater with the DOM characteristics of lower proportion of humic-like component C1 (refractoriness), higher protein-like component C3 (lability), smaller structured index A253/A203 (simple structure) and larger spectrum slop S275-295 (small molecular weight) had the lower isotope δ13CDIC, which suggests the DOM biodegradability favors the isotopic fractionation that promises the depleted δ13CDIC and the biogenic CO2 (BCO) from degradation of organic matter could substantially impact CD, considering the existing high concentration of free-CO2 up to 10-1.4 atm.
The δ13CDIC from the dissolution of BCO and carbonate was estimated to be at -16.4‰ based on the assumption of a close groundwater system. Thus the present δ13CDIC values of groundwater ranging from −14.24‰ to −1.69‰ fall within the range of -16.4‰—0‰ which is bounded by the estimated value and by that of marine carbonate carbon. Therefore, the CD were considered to be dominantly induced by BCO as well as non-CO2 protons (NCP) as that by the latter produces identical 13C isotope composition to marine carbonate (0‰) and thus low δ13CDIC could be balanced by the enriched end member which allows the consistency of the present δ13CDIC range with the estimated.
The observed opposite relationship of δ13CDIC with SICaCO3 (r=0.62，p<0.001) indicates that the groundwater samples depleted in 13CDIC had the larger calcite saturations while those enriched in 13CDIC had relatively lower saturations. This information suggests that the DIC in the groundwater with larger carbonate saturations was more likely produced by BCO-induce CD while those with the smaller was derived more by NCP-driven CD. The positive correlation of the fluorescence intensity (Fmax) of C1 with δ13CDIC (r=0.81，p<0.001) indicated that in groundwater with high concentrations of the stable component, the refractory property was against degradation and the groundwater exhibited higher 13CDIC signature that can be contributed with NCP. On the contrary, the groundwater with low concentrations of stable organic component showed the exhaustion of the labile fraction of DOM and thus featured more depleted 13CDIC from biogenic organic carbon.
The results suggest that DOM with different reactive properties would differentially suffer from biodegradation leading to the consumption of labile DOM and the 13CDIC depletion of groundwater, corresponding to BCO-induced CD, while the survival of refractory DOM and the 13CDIC enrichment, seeing more NCP-driven CD.