Hydrochemistry Characteristics and Estimation of the Dissolved Inorganic Carbon Flux in the Caohai Lake Wetland Catchment of Guizhou Province

Abstract

The Caohai Lake wetland in Weining County, Guizhou Province, is the largest karst wetland in China, and karst groundwater is an essential source of recharge and material. This study collected groundwater from the Caohai Lake catchment during the wet season, dry season, and flat season, and the main ion components of the groundwater were analyzed. The hydrochemistry characteristics and main ion sources of the groundwater were revealed, and the preliminary estimates of the karst carbon sink flux in the Caohai Lake catchment were calculated based on the solute load method. The results indicated that the water chemistry of the groundwater in the Caohai Lake catchment is primarily the HCO3-Ca type, and individual groundwater points in the southeast are the SO4-Ca type during the flat season. The dominant ions in the groundwater are Ca2+, Mg2+, and HCO3-. Seasonally, the average concentrations of SO42- and Mg2+ decreased in the order of flat season>dry season>wet season, while the average concentration of NO3- decreased in the order of wet season>dry season>flat season, and the seasonal variation of the other ions were not significant. Spatially, the concentrations of Ca2+ and HCO3- in the groundwater in the northern part of Caohai Lake, of K+, Na+, and Cl- in the southwest, and of Mg2+, NO3-, and SO42- in the southeast were relatively high. Ca2+, Mg2+, and HCO3- in the groundwater were primarily controlled by the dissolution of carbonate rocks, and carbonic acid, sulfuric acid and nitric acid were all involved in the weathering of carbonate rocks in the catchment. Most of the NO3- in the groundwater was affected mostly by the soil nitrogen and domestic sewage, while NO3- in the southeast groundwater was primarily affected by agricultural activities. The sources of K+, Na+, Cl-, and SO42- exhibited no obvious temporal and spatial differences, and the K+, Na+, and Cl- were mainly affected by human activities, while SO42- was mostly derived from the dissolution of sulfur compounds in the formation. The studied area has a high proportion of allogenic acids (sulfuric acid and nitric acid) participating in the weathering of carbonate rocks, which is the highest in the flat season and the lowest in wet season. After deducting the proportion of allogenic acids participating in the weathering of carbonate rocks, the average inorganic carbon flux in the Caohai Lake catchment is preliminarily estimated to be approximately 181.5 t·a-1, and the average karst groundwater transport of HCO3- to Caohai Lake is approximately 1144.1 t·a-1.