Chemistry and source of the dissolved inorganic carbon of sediments porewater in Huixian karst wetland

The ecological environment of karst wetland is highly fragile, and both natural processes and human activities have led to the gradual deterioration of wetland water quality. However, the influence on the evolution of carbon remains unclear. To identify the chemical characteristics of porewater and its impact on inorganic carbon evolution, three boreholes (HX-04, HX-05, and HX-06) were drilled in October 2023 with a depth of 60−80 cm in Mudong lake and Baxian lake in Huixian karst wetland. Lake water samples overlying the borehole were collected. Porewater was extracted from 21 core samples using a squeezing method, and subsequently porewater chemistry and various isotopes (²H, ¹⁸O, ³⁴S_so4, and ¹³C_DIC) were analyzed comprehensively. The results reveal that the porewater of wetland sediments is classified as HCO₃—Ca and SO₄•HCO₃—Ca type water, with chemical components primarily controlled by dissolution of carbonate rocks and calcareous silicate rocks. In the degraded wetland, porewater has enriched sulfate with a content of 71.4 to 363.2 mg/L, and the negative δ³⁴S value indicates that the sulfate originates from the oxidation of pyrite in the northern strata of the study area. The dissolved inorganic carbon content of wetland porewater is high, and δ¹³C_DIC is significantly enriched, with an average value of −4.7‰. The dissolved inorganic carbon in the low-sulfate porewater of Mudong Lake primarily derives from methanogenesis of wetland and recharge of karst groundwater with DIC from carbonate rock weathering by carbonic. The input of exogenous sulfuric acid inhibits methanogenesis, and thus the enriched δ¹³C_DIC value in high-sulfate porewater suggests a mixture of carbonate rocks weathering both by carbonic acid and sulfuric acid. During the period of elevated water levels, there was no evidence of nitrate contamination in the porewater. The chemical composition and sources of DIC in porewater are influenced by the diverse karst subsystems. This study can provide theoretical basis for the restoration of Huixian karst wetland, rational utilization of water resources, and further research on carbon cycle.