Inorganic carbon migration and transformation in groundwater evaporation discharge area.

During groundwater evaporation discharge, a series of carbon-related water-rock interactions potentially impact the terrestrial carbon cycle significantly. However, the migration and transformation of carbon in groundwater evaporation discharge area remain inadequately understood. Using the Tumochuan Plain in Inner Mongolia as a case study, this paper constructs a carbon balance equation for groundwater evaporation discharge area by employing mass balance principles and hydrogeochemical simulation methods, thereby analyzing the mechanisms of carbon diversion during groundwater evaporation. The result showed that evaporation discharge area of Tumochuan Plain was a 'carbon sink'. Carbon emission rate to atmosphere in study area was 7.35 g/(m·a), while carbon fixation rate by calcite precipitation and dissolved inorganic carbon (DIC) into groundwater was 37.15 g/(m·a). The precipitation of calcite and the dissolution of dolomite were the main water-rock interactions controlling the migration and transformation of DIC. The carbon absorbed by dolomite dissolution reached 21,698.02 t/a (30.56 g/(m·a)), offsetting a significant portion of the CO emitted during calcite precipitation. In addition, the calcium released by the dissolution of dolomite and anorthite effectively promoted the precipitation of calcite, which was the primary factor for groundwater to become a carbon sink in this area.