Mechanism of Groundwater Impacting on Carbon Cycling in Small Closed Lakes in Arid and Semi-arid Areas

Small closed lakes, prevalent in arid-semi and arid regions, strongly rely on groundwater replenishment. Hydrochemical interactions at their interface induce carbon migration and transformation. However, little research addresses the mechanism behind carbon dynamics in these lakes during groundwater replenishment. This study, exemplified by Chaidamu Nor Lake in the Hulunbuir Grassland, employed mass conservation and hydrogeochemical simulation to model the carbon balance in the groundwater-lake mixing zone. The results reveal that groundwater recharge prompts the conversion of dissolved carbonates (CO₃²⁻, NaCO₃⁻, MgCO₃) in lake water to bicarbonates (HCO₃⁻, NaHCO₃⁻, MgHCO₃⁺), accentuating carbon sink effects during mixing. Groundwater input introduces 52.5 t/a of dissolved inorganic carbon (DIC) into Chaidamu Nor Lake, with 2.2 t/a of DIC removed via CaCO3 precipitation at the groundwater-lake interface. Calcite precipitation during mixing dissolves CO2 into the solution and absorbs 51.2 t/a (46.6 g/(m₂·a)) of atmospheric carbon. Groundwater recharge leads to a net DIC increase of 101.5 t/a (92.3 g/(m₂·a)) in Chaidamu Nor Lake. The mixing of groundwater and alkaline lakes exhibits significant carbon sequestration capacity and proves to be a significant carbon sink in terrestrial ecosystems.