A Method for Quantifying the Role of Carbonate Acid, Sulfuric Acid and Nitric Acid in Carbonate Weathering After Modifying the Effect of Evaporite in Qingjiang Karst Catchment

Qingjiang river is the second largest tributary of the Yangtze River in Hubei province, it’s also a typical karst catchment. Eighty-two important groundwater samples were collected during high and low water period of 2019. The results show that: (1) The major hydrochemistry types are Ca+Mg-HCO 3 and Ca-HCO 3 , indicate that carbonate weathering is the main source of groundwater chemistry; (2) The results of inverse hydrochemical modeling show that there are two kinds of groundwater-carbonate rock interactions. One is co-dissolution of calcite and dolomite, the other is dedolomitization, and thereinto, dedolomitization is widespread in dolomite aquifers. Furthermore, gypsum has a tendency to dissolve in each aquifer, and the common ion effect of Ca 2+ caused by gypsum dissolution promotes dedolomitization. The modeling results suggest that major elements have a good traceability effect on the material source of groundwater. (3) The chemical weathering of carbonate rock is mainly affected by carbonic acid, sulfuric acid and nitric acid. After modifying the impact of evaporite and atmospheric input, the calculations show that the contribution of carbonic acid involved in carbonate weathering is 70.9% (high water period) and 70.0% (low water period). Through statistics of karst springs discharge and contribution of acid involved in carbonate weathering, the two are in a positive relationship. The result can reect the laws of sulfuric acid and nitric acid under the hydrodynamic condition in different seasons. Therefore, the carbonate weathering should be carefully evaluated in karst areas which have abundant groundwater and the role of groundwater in carbonate weathering is worthy of further study.


Introduction
Chemical weathering causes the transfer of chemical elements from rock to water, increasing solute of water (Zhang et al. 2016), at the same time chemical weathering consumes/releases CO 2 to the atmosphere (Gaillardet et al. 1999;Moon et al. 2009). Therefore, carbonate weathering plays an important role in geological carbon sink, which affects global climate change (Gaillardet et al. 1999;Kump et al. 2000;Maher et al. 2014). Geological carbon sink mainly includes silicate weathering and carbonate weathering (Hartmann et al. 2009;Liu et al. 2011). The main reason why carbonate weathering is neglected in global carbon cycle is its instability (Torres et al. 2014;Xie et al. 2021). Because carbonate weathering can quickly consume CO 2 from the atmosphere, and it will release back to atmosphere through carbonate minerals precipitation, this process only lasts for centuries (Martin. 2017;Sun et al. 2021). However, dissolved inorganic carbon (DIC) converts into organic carbon by photosynthesis. This process can keep carbon remaining stable for a long time, thus making carbonate weathering become non-negligible carbon sink over geological timescale Sun et al. 2021).
The intensity of chemical weathering depends on many factors, but a complex and changeable actual situation may lead to misestimate the in uence of each factor (Moon et al. 2007;. Lithology is an important factor that affecting chemical weathering, while different minerals have its own weathering resistance that determines the strength of chemical weathering. For example, the chemical weathering rate of carbonate rock is 12 times and 9.2 times faster than sandstone and granite, respectively (Meybeck. 1987). The chemical weathering rate of the river which owing through steep terrain areas is signi cantly higher than other rivers (Zhang et al. 2016). Meanwhile, climate and topographic are usually positively correlated with weathering intensity (Zhang et al. 2019). Furthermore, aquatic organisms convert bicarbonate into organic carbon during non-ood season, this process contributes 15.5% of carbonate weathering , so this factor should be considered when estimating the carbon sink of carbonate weathering. While carbonate weathering by exogenous acid (mainly includes sulfuric acid and nitric acid) plays an important role in areas where human activities have a greater impact (Li et al. 2011;Yu et al. 2015). Because exogenous acid produced by human activities participates in carbonate weathering that will release CO 2 to the atmosphere. Thus, calculating the contribution of sulfuric acid and nitric acid involved in carbonate weathering is an effective method to evaluate the weathering of exogenous acid (Li et al. 2011;Xie et al. 2021).
River solute derives from rock weathering, soil leaching and atmospheric input, and it is a comprehensive re ection of groundwater, rainwater, soil water and biological effects (Soulsby et al. 2007;Liu et al. 2021). There are many studies of chemical weathering intensity on catchment scale (Han et al. 2004;Wang et al. 2012;Sun et al. 2021). However, few people pay attention to carbonate weathering under the in uence of evaporite mineral, such as gypsum. The Yangtze Platform was in a hot and dry climate when the karst aquifer was formed in Central China, seawater evaporated and crystallized into carbonate and evaporite minerals  Qingjiang river is a typical karst catchment in the middle reaches of the Yangtze River, and it is also a National Key Ecological Function Zone (Sun et al. 2016;Jiang et al. 2017). The study area includes formations from Sinian to Triassic, which restricts carbonate rock and evaporite in the study of hydrogeochemical process. In this study, Qingjiang catchment is taken as an example to establish the hydrogeochemical background of each karst aquifer for reference. The contribution of carbon acid, sulfuric acid and nitric acid involved in carbonate weathering has been calculated after modifying by evaporite. This study expects to provide scienti c basis for chemical weathering where evaporite existence.
The aims of this research are to: (1) establish the hydrogeochemical background of the karst aquifers distributed in the Yangtze Platform in Central China, (2) calculate transfer amount of mineral in karst aquifers, (3) discover the differences and mechanisms of hydrogeochemical processes in different hydroperiod, (4) estimate the contribution of carbon acid, sulfuric acid and nitric acid involved in carbonate weathering after modifying the in uence of evaporite.

Study Area
found that there is strong correlation between the chemical weathering rate and runoff (Kump et al. 2000), thus groundwater sampling in this study avoids the occurrence of precipitation event in a week. In order to accurately obtain the hydrochemical information of groundwater, the portable water quality parameter analyzer (AR8011 and PH828, Smart Sensor Company) was used to measure the groundwater temperature (T), pH and electrical conductivity (EC), with a resolution of 0.1°C and 0.01 pH unit, respectively.
Before collecting groundwater samples, rinse the polyethylene (HDPE) bottle with groundwater 3 times, make sure that no bubbles remain in the bottle during storage. All groundwater samples were ltered through 0.45μm Acetate Fiber membrane lters, and then placed in 50ml HDPE bottles. The bottles for cation analysis were acidi ed to pH less than 2 with ultra-puri ed HNO 3 , and samples were sealed with para lm and stored at 4°C in refrigerator before laboratory analysis.
Major cation (Ca 2+ Mg 2+ Na + K + ) were analyzed by ICP-OES (iCAP7600, Thermo Fisher Scienti c, USA), anions (SO 4 2-NO 3 -Cl -) were analyzed by ion chromatograph (IC-2200, Thermo Fisher Scienti c, USA), with a resolution of 0.01 mg/L for all ions. HCO 3 was measured by the potentiometric titrimetric method, each sample was titrated three times, and the average error was < 5%. All sample analyses were completed in the Geological Survey Experimental Center of China University of Geosciences, Wuhan. The analysis results were checked by calculating the normalized inorganic charge balance (NICB=(TZ + -TZ -)/(TZ + +TZ -)), where TZ + =2Ca 2+ +2Mg 2+ +Na + +K + , TZ -=HCO 3 -+2SO 4 2-+NO 3 -+Cl -, the unit of TZ + and TZis meq·L −1 . The NICB of all samples were within ±5%, except for the sample with a NICB of 8%, which collected from Dragon Cave in Enshi. The mean value of NICB was 2%, suggested that the analytical uncertainty in this study was relatively small, and the contribution of organic dissolution was not signi cant in the charge balance (Yu et al. 2015;Huang et al. 2021 Thereinto, the hydrochemical type of groundwater from Dragon Cave in Enshi was Ca-HCO 3 in high water period and changed to Ca-HCO 3 +SO 4 type in low water period, which is inferred to be caused by human pollution. The Piper trilinear diagram can objectively re ect the impact of different material sources (Piper. 1944;Vasić et al. 2021). Groundwater in Qingjiang catchment is characterized by high concentration of Ca 2+ , Mg 2+ and HCO 3 -, indicates that carbonate weathering is dominant. According to the projection position in Fig.2, the samples distributed between limestone weathering end-member and dolomite weathering end-member. Moreover, the proportion of "SO 4 2-+ Cl -" is less than 40%, with only one sample reach 54.9% in low water period, and the source of SO 4 2and Clmay relate to sulfate mineral or human pollution (Frank et al. 2019). Meanwhile, the proportion of "Na + K" and "Cl"" is less than 10%, suggests that human pollution and evaporite minerals such as halite and sylvite have little impact on groundwater, and SO 4 2is more likely to derive from gypsum. Therefore, carbonate weathering is the main impact factor of groundwater chemistry, chemical weathering of evaporite is the secondary factor, and silicate weathering has less effect.

Source identi cation of major ions
The main source of ions in groundwater includes atmospheric input, chemical weathering of rock and human input (Gibbs. 1970;Han et al. 2004 Evaporite distributed in western regions of the study area, and gypsum was easily formed during the diagenesis of evaporite (Li. 1988 a limited impact on groundwater. Generally, NO 3 derives from urban and agricultural pollution input, so its concentration can reveal the impact of human activities on hydrochemistry (Agrawal et al. 1999;Jeong. 2001). Therefore, the result shows that groundwater in the study area affected by human pollution is relatively small.

Saturation Index
The saturation index (SI) is a method of material source analysis with a higher accuracy than ion ratio relationship, it can indicate whether minerals are dissolved or precipitated in groundwater (Wen et al. 2012;Mohamed et al. 2019). It is generally recognized that SI is in equilibrium within the range of 0±0.5 due to the errors of ions analysis and ion activity calculation (Ma. 2016). Based on the relationship between groundwater chemistry and SI, it can be inferred which kinds of mineral provide material source for groundwater (Stradioto et al. 2020). The saturation index of calcite ( Fig.6a) (Santhanam et al. 2021). While the relationship between the saturation index and corresponding ion concentration in low water period is better than high water period. Because in different hydroperiod, seasonal change may lead to variations of groundwater circulation, which cause differences in water-rock interaction (Liu et al. 2007;Marco et al. 2013). The dynamic variations of groundwater level are more intense in high water period due to abundant recharge. Therefore, groundwater in karst aquifer ows faster (Frank et al. 2019), and water-rock interaction time is shorter. However, recharge reduces in low water period, groundwater alternates between conduit and matrix is slow, and groundwater lixiviation is more su cient.

Inverse geochemical modelling
Geological background is the dominant factor of groundwater chemistry (Romanov et al. 2003;Liu et al. 2020). In order to study mineral transfer and its seasonal variation between water and rock, inverse geochemical modeling is used to simulate karst aquifers in different hydroperiod. Six representative simulation ow paths are selected: Flow path 1 is in the Triassic aquifer, starting with the Daye limestone aquifer and passes through the Jialingjiang gypsum-bearing dolomite aquifer; Flow path 2 is in the Jialingjiang aquifer; Flow path 3 is in the Daye aquifer; Flow path 4 is in the Permian limestone aquifer; Flow path 5 is in the Ordovician aquifer; Flow path 6 is in the Cambrian dolomite aquifer.
According to the geological background of the study area and saturation index analysis results, the determination of "possible mineral phases" in the inverse geochemical modeling are calcite, dolomite, gypsum and halite. Meanwhile, the karst groundwater system is in open state, and CO 2 dissolves in groundwater and participates in rock weathering, therefore, CO 2 is also the "possible mineral phase".
Along the direction of groundwater ow in different aquifer, the starting point and ending point are selected to establish the inverse geochemical modeling, and the calculation results of mineral transfer are shown in Fig.7.
The modeling results show that calcite, dolomite and CO 2 are the main mineral phases, and dissolution of dolomite reaches 33.39×10 -5 mol·L −1 in Flow path 6. As for gypsum and halite, mineral transfer amounts are relatively small, dissolution of gypsum in Flow path 2 is only 5.375×10 -5 mol·L −1 , which is the highest among all ow paths. Furthermore, the transfer amount of calcite, dolomite and CO 2 are larger than 0 in Flow path 2, 3 and 4, indicates that the dissolution of calcite, dolomite and CO 2 occurs in these ow paths. Thereinto, the dissolution of calcite in Flow path 3 and 4 is larger than dolomite, while the dissolution of calcite in Flow path 2 is less than dolomite. Because Flow path 2 ows through dolomite aquifer, Flow path 3 and 4 are in limestone aquifers. In addition, the dissolution of dolomite is greater than the precipitation of calcite in Flow path 1, 2, 5 and 6, which ows through dolomite formations. Therefore, the lithology of aquifers plays an important role in Qingjiang catchment, carbonate minerals transfer amount is larger than other minerals in karst aquifers, and it also determines the transfer amount of calcite and dolomite.
Generally, the phenomenon that dolomite and gypsum dissolves as well as calcite precipitates is called dedolomitization (Baumann et al. 2019). Among all ow paths, the transfers amount of calcite in Flow path 1, 5 and 6 is less than 0 in both high and low water period, indicates that precipitation occurs in these ow paths. At the same time, the transfer amount of dolomite, gypsum, and CO 2 is larger than 0, which shows that dissolution occurs in these ow paths. When the gypsum dissolves, the concentration of Ca 2+ and SO 4 2in groundwater increases, and the common ion effect of Ca 2+ occurs. Under the in uence of common ion effect, calcite precipitates, which promotes the dissolution of dolomite and leads to the increase of Ca 2+ , Mg 2+ and SO 4 2in groundwater (Wu et al. 2020).

Hydrogeochemical Process
According to the modeling results, the water-rock interactions in Qingjiang catchment include chemical weathering of carbonate rock and evaporite. Carbonate weathering is manifested in two forms, one is the co-dissolution of calcite and dolomite, the other is dedolomitization. The hydrogeochemical process occurs in Flow path 3 and 4 is the co-dissolution of calcite and dolomite, the reaction equations are However, the ow path of an underground river in dolomite aquifer shows an opposite characteristic of carbonate minerals. The starting point and ending point of Flow path 2 are the entrance and discharge of the largest underground river in Qingjiang catchment, and Flow path 1 is located in the recharge area of the underground river. As the results show, dedolomitization occurs in Flow path 1, and the transfer amount of carbonate minerals during high water period is larger than low water period. Whereas, the codissolution of calcite and dolomite occurs in Flow path 2, and the transfer amount of carbonate minerals is larger in low water period. Because Flow path 2 is in the conduit of the underground river, which has a faster ow, and carbonate minerals are likely to dissolve. Whereas, more groundwater with su cient lixiviation enters the conduit in low water period. Under the circumstance of strong groundwater hydrodynamic, soluble minerals continue to dissolve and transport with groundwater, results in codissolution of calcite and dolomite (Bullen et al. 1998). Furthermore, the role of hydrodynamics change still has not drawn enough attention in previous studies of hydrogeochemical process in karst area.
The modeling results re ect the characteristics of water-rock interaction in karst aquifer. Groundwater is composed of rapid ow in conduit in high water period, while ssure ow and matrix ow were collected to conduit in low water period, with a su cient water-rock interaction. Therefore, the transfer amount of calcite, dolomite and CO 2 varies with the seasons, which is higher in low water period.
The dissolution of evaporite, karsti cation and heavy rain can promote the dedolomitization process (Nader et al. 2008). However, dedolomitization cannot be simply considered that dolomite dissolution and calcite precipitation proceed at the same time (Evamy. 1967). In the dedolomitization reaction experiment involved in gypsum, no calcite was found in the dolomite dissolution residues, indicates that the dedolomitization is not simultaneous reaction of dolomite and calcite, but Mg continuously precipitates rst, then dolomite is rich in Ca gradually, nally turns into calcite (Deng et al 1993). The lithology of Jialingjiang formations in the study area is carbonate rock and evaporite. When Jialingjiang formations deposited, the paleoclimatic was hot and arid, and the South China Marine Transgression provided su cient seawater. As seawater evaporated and concentrated, calcite, dolomite, gypsum, and halite were precipitated successively (Zhong et al. 2020 (Ayora et al. 2001).
In the study area, gypsum plays a secondary important mineral role in hydrogeochemical processes and has a tendency to dissolve in each aquifer. Especially for Jialingjiang aquifer, gypsum dissolution is much higher than other aquifers, and the dissolution of gypsum in Flow path 1 is 11.49 μmol·L −1 (high water period) and 24.49 μmol·L −1 (low water period). As for the Flow path 2 is 21.72 μmol·L −1 (high water period) and 53.75 μmol·L −1 (low water period). Although the level of gypsum is low in aquifer, it still affects the chemical composition of groundwater, because the weathering rate of evaporite and carbonate rock is 80 and 12 times faster than that of silicate rock (Meybeck. 1987). The inverse geochemical modeling results show that gypsum participates in the chemical evolution of groundwater and cannot be neglected.
4.5. Quantify the impact of carbonic acid, sulfuric and nitric acids in carbonate weathering after modifying by evaporite and atmospheric input Generally, research of carbonate weathering is chosen in the catchment that the in uence of evaporite is slight, because except for carbonate mineral, gypsum dissolution and atmospheric input provide more Ca 2+ for groundwater. Therefore, this study attempts to calculate the contribution of sulfuric acid and nitric acid after modi ed by evaporite. It is assumed that all Ca 2+ , Mg 2+ and HCO 3 in groundwater are derived from carbonate weathering, and thereinto k 1 mol·L −1 carbonic acid, k 2 mol/L nitric acid, and k 3 mol/L sulfuric acid. Based on the conservation of mass, carbonate weathering involved in carbonic acid, sulfuric acid and nitric acid can be described as Eq. (4): (k 1 +k 2 +2k 3 )Ca x Mg (1-x) CO 3 + k 1 H 2 CO 3 + k 2 HNO 3 + k 3 H 2 SO 4 →(k 1 +k 2 +2k 3 )xCa 2+ + (k 1 +k 2 +2k 3 )(1-x)Mg 2+ + (2k 1 +k 2 +2k 3 )HCO 3 -+ k 2 NO 3 -+ k 3 SO 4  2 +2k 3 ). Furthermore, carbonate weathering by sulfuric acid and nitric acid cannot be calculated separately, thus the contribution of carbonate weathering by sulfuric acid and nitric acid can be calculated by (k 2 +2k 3 )/(k 1 +k 2 +2k 3 ) (Xie et al. 2021).
Qingjiang catchment is located in the western interior of Hubei Province, so the in uence of the ocean is neglected. Groundwater recharge mainly comes from atmospheric input, therefore, ions derive from atmosphere can be calculated by referring to the concentration ratio of Cland other ions of rainwater (Moon et al. 2007). The atmospheric input correction is calculated as Eq. (7) dissolution. Inverse geochemical modeling is used to calculate the transfer amount of gypsum from atmosphere to each aquifer, and the average value is selected for the Sinian aquifer that has not been simulated, and the calculated results are presented in Table 1.
The contribution of carbonate weathering by carbonic acid, sulfate acid and nitric acid is shown in Fig.8. The contribution of carbonate weathering by carbonic acid varies from 34.6% to 89.9% in high water period, with an average of 70.9%, and from 31.3% to 91.8%, with an average of 70.0% in low water period. While the contribution of carbonate weathering by sulfate acid and nitric acid varies from 10.2% to 65.4%, with an average of 29.1% in high water period, and from 8.2% to 68.7%, with an average of 30.0% in low water period, respectively. However, in low water period, the contribution of sulfate acid and nitric acid in Enshi Dragon Cave is 100%, and garbage pollution event has been reported, so statistics eliminate this sample.
According to the test results, the concentration of nitrate in rainwater is 1.18 mg·L −1 , while the average concentration of nitrate in groundwater is 11.91 mg·L −1 , indicating that atmospheric input is less. Karst depressions and trough valleys are widely distributed in the recharged area of Qingjiang catchment (Wang et al. 1995). Flat karst tough valley is often used as an agricultural land, agricultural e uent with higher NO 3 concentration transports underground through the ponor and provides material source for nitric acid (Kačaroğlu. 1999). Due to the cascaded hydropower project in mainstream, the impact of fossil fuel pollution is slight (Sun et al. 2016), and no acid deposition events have been reported in the study area. Therefore, it is believed that SO 4 2mainly derived from gypsum dissolution, which is consistent with the previous analysis.
The results show that carbonate weathering is mainly controlled by carbonic acid, while the contribution of sulfate acid and nitric acid in low water period is slightly higher than high water period. Except for HCO 3 -, the conductivity and major ion concentration of groundwater in low water period are higher, indicating that the groundwater is affected by the dilution effect of rainwater in high water period. It may cause a higher contribution of sulfate acid and nitric acid in carbonate weathering during low water period. However, the results of this study are contrary to Wang (2021)'s research in Yangtze River basin, which shows that sulfate acid and nitric acid have a greater in uence on carbonate weathering in Qingjiang river during high water period. The contribution of carbonate weathering involved in carbonic acid is about 55% (high water period) and 60% (low water period) in Wang (2021)'s research. Sulfate acid and nitric acid input from river is signi cantly greater than groundwater, but this study shows that sulfate acid and nitric acid input from groundwater cannot be neglected. This view provides new ideas for the chemical weathering of carbonate rocks.
According to the statistics of calculated results and groundwater discharge (Fig.9), there are 52 karst springs having a discharge less than 50 L·s -1 . For these springs, the contribution of sulfate acid and nitric acid to carbonate weathering shows little difference, with a mean value of 31.9% and 31.8% in high and low water period, respectively. There are 29 karst springs and underground rivers with a discharge larger than 50 L·s -1 in low water period, with an average value of 39.0% in high water period, but only 35.3% in high water period. Thereinto, 17 samples have a greater contribution of sulfate acid and nitric acid involved in carbonate weathering during low water period. It is notable that in the aquifers with larger groundwater discharge, there is a positive correlation between spring discharge and the contribution of sulfate acid and nitric acid to carbonate weathering. This means that karst springs and underground rivers with strong underground runoff are more susceptible to the sulfate acid and nitric acid. Due to the dual effects of conduit ow and matrix ow in karst groundwater, the difference of aquifer medium is an important factor that affects carbonate weathering. Due to less recharge, groundwater is mainly composed of matrix ow with a longer water-rock interaction in low water period, which has higher concentration of NO 3 and SO 4 2-. In order to accurately estimates carbonate weathering by sulfate acid and nitric acid in karst area, the impact of groundwater should be considered, and its role is worthy of further study.

Conclusions
According to the hydrochemical analysis of karst groundwater in Qingjiang Catchment, the dissolution of calcite, dolomite, gypsum and halite provides major ions for groundwater. Carbonate weathering is the main source of groundwater, followed by evaporite weathering and atmospheric input. There are two types of water-rock reactions occurred in the aquifer. One is co-dissolution of calcite and dolomite, which shows that the dissolution of calcite is larger than dolomite in limestone aquifer, while dolomite aquifer is the opposite. The other is dedolomitization, which is widespread in dolomite aquifers, manifested by dolomite and gypsum dissolution, as well as calcite precipitation. As a secondary mineral, the dissolution of gypsum causes common ion effect of Ca 2+ and promotes dedolomitization.
In the calculation of carbonate weathering after modifying by evaporite and atmospheric input, the contribution of carbonate weathering by carbonic acid varied from34.6% to 89.9%, with an average of 70.9% in high water period, and varied from 31.3% to 91.8%, with an average of 70.0% in low water period, respectively. Statistics show that there is a positive correlation between karst springs discharge and carbonate weathering. The contribution of sulfate acid and nitric acid involved in carbonate weathering is higher in the aquifers with a larger groundwater discharge. Furthermore, the contribution of sulfate acid and nitric acid is higher in low water period than high water period. Because karst aquifer has a characteristic of special medium, groundwater recharge is abundant in high water period and mainly composed of conduit ow, which has a rapid water-rock interaction. However, groundwater recharge is less in low water period and mainly composed of matrix ow with longer water-rock interaction. Therefore, the role of sulfuric acid and nitric acid in karst area with abundant groundwater resources should not be ignored and worthy of further study.

Declarations
Triassic formations, 10 Permian formations, 11 Ordovician formations, 12 Cambrian formations, 13 Sinian formations, 14 Study area Figure 2 Piper diagram of the groundwater in Qingjiang river catchment Inverse geochemical modeling results of groundwater along ow path (unit is 10*-5 mol·L−1). A positive number indicates that the mineral is dissolved, and a negative number indicates that the mineral is precipitated.
Page 24/25 Figure 8 The contribution of carbonic acid, sulfate acid and nitric acid to carbonate weathering in high water period (a) and low water period (b)