Influence mechanism of groundwater on the carbon cycle in alkaline lakes

[Display omitted] •The recharge of groundwater leads to the separation of a large number of DIC from the water in the form of CO2 and CaCO3 in alkaline lakes.•Ca2+ concentration in groundwater is the key factor affecting the carbon cycle of alkaline lakes.•The precipitation of calcium carbonate is the main hydrochemical action to control the migration and transformation of DIC after the mixing of groundwater and alkaline lake water.•In the hydrochemical analysis of groundwater recharge to lakes, the mixing ratio of the two water bodies can be expressed by the porosity of the aquifer. The discharge of groundwater to the lake is a mixed process of two different kinds of water, and the hydrochemical action at the contact interface of them may lead to the migration and transformation of carbon. However, the input of carbon from groundwater to surface water is usually measured by the product of dissolved carbon (DC) concentration and water volume at present, and there are few reports on the physicochemical reactions in the mixing process. In this paper, taking Daihai Lake as an example, the principle of mass conservation and hydrogeochemical simulation were used to construct the carbon balance equation of the groundwater and lake water mixing zone, and the mechanism of carbon fixation and emission in the mixing process was analyzed. The results show that the precipitation of calcium carbonate was the main hydrochemical reaction controlling the migration and transformation of dissolved inorganic carbon (DIC) after mixing groundwater with alkaline lake water. A large amount of HCO3– in the lake was transformed into CaCO3 and CO2 under the action of groundwater. The total amount of DIC discharged by groundwater to Daihai Lake was 770 t/a. Due to the water chemical reaction at the interface between groundwater and lake water, the carbon removed by CaCO3 precipitation and CO2 emission was 645 t/a respectively. The mixture of the two kinds of water resulted in a net reduction of 520 t/a of DIC in Daihai Lake. Ca2+ concentration in groundwater was the main factor affecting DIC migration and transformation in the mixing process of the groundwater and lake water. With the increase of Ca2+ concentration in groundwater, more carbon in the form of HCO3– would be separated from lake water. This study has important significance for revealing the impact of groundwater on the lake carbon cycle and the role of groundwater in the global carbon cycle.