Carbon balance model of groundwater-lake systems in arid and semi-arid areas and its application

A series of carbon-related hydrogeochemical processes occurring during groundwater runoff and recharge to the lake significantly impact the terrestrial carbon cycle. However, the migration and transformation of carbon in a complete groundwater-lake system has been reported rarely. In this paper, the Daihai Basin in Inner Mongolia was taken as an example and carbon balance equation of the groundwater-lake system was constructed by using the principle of mass conservation and hydrogeochemical simulation, thereby analyzing the migration and transformation mechanism of DIC in the process of groundwater runoff and recharge to the lake. The results showed that the increment of DIC in groundwater-lake system triggered by groundwater runoff and recharge to the lake in Daihai Basin was 4264.87 t/a (8.53 g/(m².a)), and the net carbon fixed from the soil and the atmosphere (as CO₂) was 5850.24 t/a (11.70 g/(m².a)). Specifically, the carbon absorbed from the soil during the recharge of atmospheric precipitation to groundwater via the vadose zone was 5607.65 t/a (12.41 g/(m².a)); the carbon absorbed from soil (or atmosphere) by water–rock interaction during groundwater runoff (as CO₂) was 529.55 t/a (1.17 g/(m².a)); the carbon absorbed from the atmosphere during the mixing process between groundwater and Daihai was 179.62 t/a (3.78 g/(m².a)); the carbon emitted to the atmosphere due to pressure changes during artificial exploitation of groundwater was 466.58 t/a (1.03 g/(m². a)). The CO₂ of the vadose zone is the main carbon source of groundwater, and the conversion of carbonate to bicarbonate during the mixing process between groundwater and lake contributes to the lake’s carbon sink function. The groundwater-lake system in arid and semi-arid areas has considerable carbon sequestration capacity and is an important carbon sink for terrestrial ecosystems.