Hydrogeochemical Evolution And Carbon Sink Effects Driven By The Karst Water Circulation

The aquifers in the karst regions of southern China are highly heterogeneous,with well-developed conduits that make groundwater flow complex and responsive to climate change.These characteristics make it challenging to study the karst water cycle and hydrogeochemistry.Previous research on karst basin water circulation has focused mainly on the dynamic features of groundwater flow,without systematically analyzing the different stages of the"precipitation-surface water-groundwater transformation"process from multiple perspectives.The mechanisms of hydrochemical reactions and carbon sink effects caused by rainfall also require further investigation.The coupling response between groundwater flow and hydrochemistry can help to reveal the structure of the karst aquifer from multiple perspectives,which is of great theoretical and practical significance for understanding the complex and dynamic hydrogeological issues in karst areas.In addition,the karst carbon sink,as one of the environmental effects in the karst water cycle and hydrogeochemistry,is an important extension of practical application in karst hydrogeology research.This paper takes the upper reaches of the Qingjiang River basin in Hubei as an example.Firstly,the hydrochemical and chemical weathering characteristics of the study area were analyzed on a regional scale.Then,a systematic"source-to-sink"study was conducted to compare the hydrogeochemical processes and carbon sink effects in several typical karst water systems.Finally,high-resolution hydrological and high-frequency hydrochemical-isotope monitoring were used to explore the hydrogeochemical processes and the carbon sink effect driven by the water cycle at the process scale.The main conclusions and insights obtained are as follows:In the first part,a preliminary understanding of the material source,hydrochemical formation,and chemical weathering characteristics of the study area was gained through terrain and geomorphological surveys,hydrogeological surveys,rock and mineral analysis,hydrochemistry,and isotope monitoring.1.The upper reaches of the Qingjiang River basin had typical characteristics of a southern karst basin,such as terrain,meteorological conditions,and lithology.The Triassic strata were the most important aquifers in the study area,rich in gypsum and celestite minerals.In addition,the groundwater in this aquifer was abundant,with various conduits and karst groundwater systems of different scales.2.The main ions in karst water were Ca²⁺,Mg²⁺,HCO₃^-and SO₄^2-.The main hydrochemical types were Ca-HCO₃ type and Ca+Mg-HCO₃ type,while Ca-HCO₃+SO₄type mainly existed in the Triassic Jialingjiang Formation aquifer.Gypsum dissolution was one source of SO₄^2-ions.Carbonate weathering was the major cause of the hydrochemistry,while silicate rock and evaporite weathering also made some contributions.The variations in hydrodynamic conditions might affect the dissolution of evaporite minerals and silicate rock weathering.The water quality in the study area was generally good,indicating minimal human activity.However,surface water flowing through the town might be at potential risk of agricultural and urban pollution.3.Carbonate weathering involving carbonates was the dominant reaction in the upper reaches of the Qingjiang River basin,accounting for 76%of the total.Exogenous acids generated by human activity had a greater impact on surface water.The intensity of karst carbon sinks in the basin ranged from 0.05 to 131.07 t/km²·a,with an average value of 45.89 t/km²·a.The study showed that geological conditions affected the chemical weathering rates,and the dissolution of gypsum and celestite minerals enhanced carbonate weathering rates and may have improved the carbon sink effect of carbonate weathering in the short term.In the second part,discharge monitoring during rainfall and seasonal sampling of several karst water systems in the study area were carried out.These karst water systems were analyzed for their characteristics,and hydrogeochemical models were utilized to quantify the karst hydrogeochemical processes occurring along the groundwater flow path.The hydrogeochemical processes of the karst water systems were compared and analyzed for differences in karst carbon sink.1.The responses of the Qingjiangyuan,Tenglong Cave,and the Xiaoxi Underground River to the rainfall events were very rapid,usually within 6 hours.The response time is determined by the flow distance and medium.When the path is short and connectivity is good,the response is faster.The hydrological processes of these three karst water systems showed a’steep rise and slow fall’curve.When rainfall stopped,the discharge dropped immediately and experienced a long trailing period.This indicated that they all developed pipes and fissures.Differences in hydrochemistry between groundwater inlets and outlets suggested that water-rock interactions are influenced by rainfall and seasonal changes.Human activities had a limited impact on groundwater.2.Hydrogeochemical modeling results showed that carbonate weathering and CO₂dissolution were the main hydrogeochemical effects that occurred in the aquifer,and they were interrelated in the water cycle.Rainwater dissolves CO₂ in the soil during infiltration which promotes carbonate weathering.As more CO₂ dissolved,the dissolution of calcite and dolomite increased.The dissolution of minerals in groundwater depends on whether the solution is saturated and the dissolution rate.Calcite has a larger initial dissolution rate than dolomite.Therefore,in unsaturated groundwater,the dissolution of calcite is much larger than dolomite.As the dissolution of minerals in groundwater becomes saturated,the solubility of calcite decreases.In addition,the dissolution of gypsum can induce the common ion effect,which promotes dolomitization,and the influence of gypsum on hydrochemical evolution cannot be ignored.3.The results of karst carbon sink flux calculations for the three karst water systems showed that karst development and flow velocity had a significant impact on karst carbon sink effects.The Tenglong Cave had the highest degree of karst development,with the highest average flow and the largest total annual carbon sink at 38,150.62 t.The total carbon sink during the high water period and flat water period accounted for over 70%of the annual total.In order to accurately calculate the karst carbon sink flux,it is necessary to utilize high-resolution hydrological monitoring methods to study the karst carbon sink generated during rainfall processes.In the third part,the karst springs of Qingjiangyuan were selected as the research object.The characteristics of karst water systems during the rainfall were analyzed by high-frequency monitoring.The aim was to quantify the proportion of water released from different mediums and the sources of hydrochemical components.The hydrogeochemical evolution processes driven by the water cycle were revealed,and the effect of rainfall on karst carbon sinks was discussed.1.After rainfall,the conversion between surface water and groundwater was rapid,and the discharge of both increased quickly,while the conductivity and some ion concentrations decreased rapidly due to dilution.Surface water and groundwater showed different reactions.The discharge and hydrochemical reactions of surface water in the recharge zone were almost synchronous,with more dramatic changes and more sensitive reactions.In contrast,the reaction of groundwater was lagging behind,and its discharge and hydrochemical dynamics had a moderate buffering effect,indicating a high degree of heterogeneity of the medium.The attenuation stage of karst groundwater can be regarded as the superposition of water released from karst pipes,wide fissures,tiny fissures,and pore water.Generally,it showed two to three stages,with the proportion of released water being 21.4%,14.3%,and 64.2%for three stages,and 19.1%and 80.9%for two stages.2.The hydrogeochemical evolution driven by karst water circulation can be divided into three stages:pre-rainfall,early rainfall,and high discharge after rainfall.In the first stage,the groundwater and surface water were at low discharge and high ion concentration and were relatively stable,and the karstification intensity was relatively low.During the rainfall,the discharge and hydrochemical changes were drastic,and the concentrations of Ca²⁺,SO₄^2-,and Sr² ions increased slightly,which were related to the dissolution of gypsum and calcite,and water in the fissures was discharged preferentially.Minerals dissolved rapidly under strong hydraulic conditions,and the karstification intensity increased with the discharge within a limited range.When the threshold was exceeded,the dilution effect was greater than the karstification effect.Hydrochemical and isotopic data reflected the mixing of new recharge water and retained water.Pipes were the rapid runoff channel with strong water-rock interaction and short contact time.On the contrary,calcite,gypsum,and celestite dissolved sufficiently in the matrix flow.3.The study of karst carbon sinks based on rainfall events showed that rainfall was an important factor influencing karst carbon sinks.During the monitoring period in 2021,the karst carbon sink fluctuated in the range of 27.63～169.90 t/km²·a.The karst carbon sink caused by rainfall was large,accounting for about 75%of the total carbon sink for that year.Meanwhile,the surface and underground karst carbon sink during rainfall were similar,with maximums of 90.71 and 149.02 t/km²·a respectively.This highlights the importance of considering the impact of both surface and underground karst carbon sinks.