Understanding the inorganic carbon transport and carbon dioxide evasion in groundwater with multiple sulfate sources during different seasons using isotope records

The geochemical cycle of carbon under anthropogenic activities controls the transport of carbon between surface sediments, hydrosphere and atmosphere, especially for atmospheric and marine carbon dioxide. Currently, the anthropogenically-induced carbon cycle in groundwater is an important part of the cycle in inland waters and cannot be underestimated as a geochemical process affecting the global carbon cycle. Therefore, there is a need to conduct deep research using isotopes in which the isotopic mixing and fractionation response to the carbon cycle can be assessed. The results showed that the δ³⁴S-SO₄²⁻ and δ¹⁸O-SO₄²⁻ values in groundwater inherited the isotopic characteristics of four sulfate sources and shifted from acid mine drainage (AMD) derived from pyrite oxidation to atmospheric sulfates and agricultural fertilizers as well as domestic sewage along the groundwater flow path. In particular, the evolution of sulfate sources was accompanied by changes in the δ¹³C-DIC values in different seasons. During the dry season, the Rayleigh function described a possibility of enriched δ¹³C-DIC values in an AMD-affected aquifer, where a dropping water table or decreasing water content in unsaturated zones provided favorable acid buffering conditions. Bicarbonate neutralized protons to CO₂ that was in turn spilled over the water surface. During the wet season, the depleted δ¹³C-DIC values in the AMD-affected aquifer reflected another possibility of the carbon cycle in which a rising water table caused a similar closed system to some extent and slowed down the replenishment of bicarbonate so that δ¹³C-DIC almost recorded the δ¹³C of soil CO₂. In spite of the same AMD source, their influences on the carbon cycle in the dry and wet seasons were CO₂ evasion and deficient CO₂ uptake, respectively. By contrast, regardless of the season, the carbon cycle gradually shifted towards a net consumption of CO₂ in the aquifer with atmospheric and agricultural sulfates as well as domestic sewage, and δ¹³C-DIC inherited the characteristic of HCO₃⁻, indicating that these sources in anthropogenically-induced carbon processes hardly involved in isotopic fractionation of δ¹³C-DIC.