Weathering Of Carbonate Rocks Research Articles

Characteristics of an Inorganic Carbon Sink Influenced by Agricultural Activities in the Karst Peak Cluster Depression of Southern China (Guancun)

Land use in karst areas affects soil properties, impacting carbon sinks. Accurate estimation of carbon sink flux in karst areas through zoning and classification is crucial for understanding global carbon cycling and climate change. The peak cluster depression is the largest continuous karst landform region in southern China, with the depressions primarily covered by farmland and influenced by agricultural activities. This study focused on the Guancun Underground River Basin, a typical peak cluster depression basin, where sampling and analysis were conducted during the agricultural period of 2021–2022. Using hydrochemical analysis and isotopic methods, the results indicated that: (1) The primary hydrochemical type in the Guancun Underground River Basin is HCO₃-Ca, with hydrochemical composition mainly controlled by carbonate rock weathering. (2) The primary sources of Cl−, SO₄2−, and NO₃− are agricultural activities, with agriculture contributing 0.68 mmol/L to dissolved inorganic carbon (DIC), accounting for about 13.86%, as confirmed by ion concentration analysis and isotope verification. (3) The size of the depression area is proportional to the contribution of agricultural activities to DIC, while also being influenced by dilution effects. A comparison was made regarding the contribution of other land use types to DIC. The impact of land use on DIC in karst processes should not be overlooked, and zoning and classification assessments of carbon sink flux under different influencing factors contribute to carbon peaking and carbon neutrality goals.

Rare earth elements in clay-sized fractions: Implications for weathering fingerprint from parent materials to soils

Rare earth elements (REEs) have gained attention as tracers of pedogenic processes over the last few decades. Clay-sized fractions (CSFs, < 2 μm) may play a crucial role in hosting REEs. To better understand the pedochemical signals of REEs in clay-sized phases, such as iron oxides and phyllosilicates, we analyzed REE speciation in CSFs of carbonate rocks (limestone), clastic rocks (sandstone and shale), and their derived saprolites. Our results quantified the REE content (41.2–144.6 mg kg−1) and speciation in CSFs of parent materials, revealing that REEs were primarily hosted in amorphous iron oxides (Feox1), followed by crystalline iron oxides (Feox2) and phyllosilicates. Compared to parent materials, saprolites derived from carbonate rocks exhibited more than a two-fold enrichment of REEs in major clay-sized phases, confirming the role of CSFs as REE sinks during carbonate rock weathering. Furthermore, the initial REE patterns of CSFs of carbonate rocks underwent alteration throughout the weathering process, likely due to water–mineral interactions. Our findings suggest that REEs in CSFs record the weathering fingerprint for soils derived from carbonate rocks, while they are indicative of provenance for soils originating from clastic rocks.

Irrigation impact on water quality and aquifer vulnerability in Kala Oya basin, Sri Lanka

The impact of precipitation and irrigation return flow on water quality and groundwater vulnerability was investigated in the Kala Oya basin, Sri Lanka using the Weighted Arithmetic Water Quality Index, DRASTIC Index, and isotope hydrology. Groundwater and surface water were analyzed for water quality parameters and stable isotopes (18O and 2H) in dry and wet seasons. In both seasons, neutral to alkaline water was observed. Electrical conductivity (EC) and major ions were higher in shallow (EC: 718.3 ± 323.8 μS/cm; Ca2+: 49.54 ± 32.99 mg/L) and deep groundwater (EC: 708.1 ± 321.5 μS/cm; Ca2+: 37.06 ± 11.21 mg/L) than in surface water (EC: 371.7 ± 154.4 μS/cm; Ca2+: 29.66 ± 11.64 mg/L), while similar values of EC and major ions were reported in both shallow and deep groundwater samples. Ca–Mg–HCO3 was the dominant groundwater type which was derived from silicate and carbonate rock weathering. Observations in the wet season revealed δ2H ranged from −47.97 to −16.71‰ and δ18O ranged from −7.56 to −2.87‰, while in the dry season, δ2H ranged from −44.90 to −14.06‰, and δ18O ranged from −7.69 to −2.08‰. The enriched δ18O and δ2H groundwater values were mainly the result of progressive evaporative concentration of the heavier isotopes before water infiltration and groundwater recharge. The solute concentrations were enriched in groundwater by agricultural irrigation returns and associated evaporation indicating the influence of irrigation return flow on groundwater. It is suggested that agricultural practices increase groundwater vulnerability, which recommends special attention to long-term groundwater quality and quantity monitoring, hydrological modelling, land-use planning, and the development of sustainable policies in the basin.

Chemical characteristics and health risk evaluation of natural waters in the Du River Source National Nature Reserve: A case study in Zhushan County, Hubei Province, China

To study the distribution of trace elements in natural water of the Du River Source National Nature Reserve and to assess the water quality and health risks, Zhushan County in Hubei Province was selected as the study area. Element content in 361 natural water samples collected from Zhushan County were measured by ICP-MS, ICP-OES, and HG-AFS. The main anions and cations present in water samples from Zhushan County are Ca2+ and HCO3−. The water chemistry is predominantly influenced by the weathering of carbonate rocks. The water samples with high content of selenium (Se) (0∼82.9 μg/L, mean 4.6 μg/L) in natural water in Zhushan County are mainly distributed in the northern part of Zhushan. The strontium (Sr) content of 49.6% of the water samples (0.001–2.177 mg/L, mean 0.234 mg/L) reached the criteria of natural mineral water for drinking in China (Sr ≥ 0.2 mg/L), which is distributed throughout the county. The high content of metasilicic acid (H2SiO3) (0.026–35.910 mg/L, mean 12.598 mg/L) and zinc (Zn) (0∼407.218 μg/L, mean 12.406 μg/L) are concentrated in northern Zhushan County. 99.7% water samples were freshwater and 98.9% meet the criteria of “good” water quality. All of the natural water samples have low health risk and low heavy metal pollution. 6.1% water samples meet the criteria of Se-type mineral water, while 45.4% meet the criteria of Sr-type mineral water, and 4.4% water samples meet the criteria of “low sodium, high Se, and high Sr” mineral water. Zhushan County has the potential for Se-type mineral water and Sr-type mineral water development. The findings of this study hold immense significance for the public health implications of drinking water in Du River Source, thereby offering valuable insights for effective water resources management.

Insights into biogeochemistry and hot spots distribution characteristics of redox-sensitive elements in the hyporheic zone: Transformation mechanisms and contributing factors

Biogeochemical hot spots play a crucial role in the cycling and transport of redox-sensitive elements (RSEs) in the hyporheic zone (HZ). However, the transformation mechanisms of RSEs and patterns of RSEs hot spots in the HZ remain poorly understood. In this study, hydrochemistry and multi-isotope (N/C/S/O) datasets were collected to investigate the transformation mechanisms of RSEs, and explore the distribution characteristics of RSEs transformation hot spots. The results showed that spatial variability in key drivers was evident, while temporal change in RSEs concentration was not significant, except for dissolved organic carbon. Bacterial sulfate reduction (BSR) was the primary biogeochemical process for sulfate and occurred throughout the area. Ammonium enrichment was mainly caused by the mineralization of nitrogenous organic matter and anthropogenic inputs, with adsorption serving as the primary attenuation mechanism. Carbon dynamics were influenced by various biogeochemical processes, with dissolved organic carbon mainly derived from C3 plants and dissolved inorganic carbon from weathering of carbonate rocks and decomposition of organic matter. The peak contribution of dissolved organic carbon decomposition to the DIC pool was 46.44 %. The concentration thresholds for the ammonium enrichment and BSR hot spots were identified as 1.5 mg/L and 8.84 mg/L, respectively. The distribution pattern of RSEs hot spots was closely related to the hydrogeological conditions. Our findings reveal the complex evolution mechanisms and hot spots distribution characteristics of RSEs in the HZ, providing a basis for the safe utilization and protection of groundwater resources.

Effects of rainfall on the karst-related carbon cycle due to carbonate rock weathering induced by H2SO4 and/or HNO3

To determine the mechanism underlying the effect of rainfall on the karst-related carbon cycle while controlled by that of H2SO4 and/or HNO3, high-resolution monitoring was conducted in Lanhuagou Spring, which is a typical epikarst spring in southwest China. During the monitoring period, the signals from major elements and stable isotopes showed that the CO2 generated by H2SO4 and/or HNO3 entirely escaped from the water. Therefore, the CO2 consumption induced by H2CO3 and CO2 emission induced by H2SO4 and/or HNO3 jointly affected the karst-related carbon cycle. The CO2 consumption induced by H2CO3 dropped following rainfall events and recovered gradually thereafter, which was only affected by the retention time of infiltrating water (dilution effect of rainfall). However, the CO2 emission induced by H2SO4 and/or HNO3 had showed a downward trend from the first rainfall event (R1) to the end of monitoring. The effects of rainfall on CO2 emission induced by H2SO4 and/or HNO3 were related to both the dilution effect and the removal of H2SO4 and/or HNO3. Although the average CO2 consumption induced by H2CO3 dropped from 2.8 mmol/L before rainfall to 2.21 mmol/L thereafter (decreased by 21.07 %), the average CO2 emission induced by H2SO4 and/or HNO3 dropped from 1.29 to 0.49 mmol/L (decreased by 62.02 %); the range of CO2 emission induced by H2SO4 and/or HNO3 decline (0.8 mmol/L) was larger than that of CO2 consumption induced by H2CO3 (0.59 mmol/L), resulting in higher net CO2 consumption (CO2 consumption induced by H2CO3 minus CO2 emission induced by H2SO4 and/or HNO3) after rainfall than before rainfall in the monitoring period. Therefore, in the model used to predict karst-related carbon flux caused by global climate change, it is necessary to consider the impact of rainfall on CO2 emission induced by H2SO4 and/or HNO3. Importantly, the net CO2 consumption rather than CO2 consumption induced by H2CO3, as well as runoff should be considered as two multipliers when carbonate weathering is interfered by H2SO4 and/or HNO3.

Hydrochemistry and Irrigation Quality of High-Altitude Lakes: A Case Study of the Ramaroshan Lake Complex, Nepal Himalayas

The Ramaroshan Lake Complex (RLC) in Sudurpaschim Province, Nepal, is a Himalayan lake cluster that holds significant ecological, economic, religious, and esthetic importance. This study aimed to provide a comprehensive characterization of the hydrochemical properties of water within the RLC and assess its suitability for irrigation purposes. A total of 38 water samples were collected from seven different lakes of the complex. The physicochemical parameters and major ions were then analyzed. The water samples from the RLC were alkaline, and based on total hardness, they ranged from soft to moderately hard categories. The presence of major ions included the following: Ca2+ &gt; Na+ &gt; Mg2+ &gt; K+ &gt; Fe3+ &gt; NH4+ and HCO3− &gt; Cl− &gt; SO42− &gt; NO3− &gt; PO43−. The alkaline earth metals (Ca2+ and Mg2+) dominated the alkali metals (Na+ and K+) and weak acids (HCO3−) dominated the strong acids (Cl− and SO42−). The dominant hydrochemical facies of the lake water was a Ca-HCO3 type indicating a calcium carbonate type of lithology. Carbonate rock weathering was the most dominant process in influencing the hydrochemistry of the water. A high ratio of (Ca2++ Mg2+)/Tz+ and a lower ratio of (Na+ + K+)/Tz+ revealed the dominance of Ca2+ and Mg2+ resulting from carbonate weathering, with little contribution from silicate weathering. Different irrigation indices revealed the suitability of the RLC water for irrigation. The insights derived from this study are pivotal in safeguarding water quality and bolstering sustainability efforts. The study also furnishes foundational data crucial to an array of stakeholders including researchers and policymakers and significantly contributes to advancing water management strategies and fostering ecosystem conservation in the Himalayan freshwater lakes, particularly in the face of the overarching challenge posed by global climate change.

Patterns of distribution of eluvia soils in the oil region of Tatarstan

The article is devoted to the study of eluvial deposits in the southeastern part of the Republic of Tatarstan. As a result of river erosion, a highly dissected and rugged terrain has formed here. Its characteristic elements are erosion-denudation watersheds separated by narrow river valleys and terraced slopes. According to the results of the research, it was revealed that the differences in the structure of weathering profiles in carbonate and terrigenous soil massifs are due to the structural and mineralogical features of rocks that were formed at the stage of sedimentation and subsequent diagenesis. In the central part, on the site of the upper plateau, the rocks underwent mainly physical weathering processes, on the site of the middle plateau – chemical weathering processes, on the site of the lower plateau – physical and chemical weathering processes. Such selectivity in the manifestation of hypergenic processes is due to the predominant lithotypes of sedimentary rocks composing the surfaces of the upper, middle and lower plateaus of the studied territory. Based on the conducted research, the zoning of the studied territory was performed and an engineering and geological map was built. The results obtained made it possible to understand the mechanism of weathering of carbonate and terrigenous rocks, as well as to assess the potential change in their physical and mechanical properties during the construction and operation of buildings and structures.

Research progress on algal biokarst and its implications for carbonate rock dissolution and weathering

Research progress on algal biokarst and its implications for carbonate rock dissolution and weathering

Concentration, speciation, and fractionation of rare earth elements in alluvial soils in contiguous karst landform, southwestern China

Karst is a unique landform formed by strong chemical weathering of carbonate rocks. This study investigated the concentration and speciation of rare earth elements and yttrium (REY) in alluvial soils and REY fractionation from carbonate rocks and alluvial soils in the world's largest contiguous karst landform in southwestern China. The results showed that the REY of alluvial soils ranged from 73.77 to 1251.56 μg⋅g−1, with a median value of 230.30 μg⋅g−1, which was higher than that of alluvial soils across China (178 μg⋅g−1) and the upper continental crust (168 μg⋅g−1), revealing a high background of REY in these karst areas. Meanwhile, the average REY in alluvial soils (244.96 μg⋅g−1) was close to 14 times that in carbonate rocks (17.61 μg⋅g−1) in the study area, indicating that REY was significantly enriched in the pedogenesis. Correlation analysis showed that the REY concentration was not significantly related to any single factor (i.e., Fe2O3, MnO, Al2O3, TOC, pH, or CIX). BCR speciation analysis revealed a declining trend in the proportion of REY across the residual (F4, 73.21 %), reducible (F2, 19.40 %), oxidizable (F3, 6.89 %), and weakly acid-extracted (F1, 0.50 %) phases. Additionally, significant REY fractionations from carbonate rocks to alluvial soils were observed. The bedrock-normalized REY pattern revealed an enrichment of light rare-earth elements (LREE) and middle rare-earth elements (MREE), a depletion of heavy rare-earth elements and yttrium (HREY), and positive Ce and Gd anomalies. Secondary mineral phases that formed during weathering—such as clay minerals, oxyhydroxides, phosphates, and organic matter—played an important role in the retention and fractionation of the REY in the soils. Besides these natural factors, anthropogenic inputs also contributed to the REY fractionation, as indicated by the slight positive Gd anomaly in the karst alluvial soils. Furthermore, the differences of δCe and δEu in the soils developed from three types of pedogenic rocks—including carbonate, basalt, and granite—were discussed. The different δEu anomalies could help to identify the parent rocks of alluvial soils.

Hydrochemical Characteristics and Control Factors of Surface Water in Kuaize River Basin at the Upper Pearl River

The Kuaize River is a small typical karst watershed in the source area of the Pearl River as well as an important coal mining area in Eastern Yunnan with a fragile ecological environment. Strengthening the research on the water environment in the region plays an important role in supporting the comprehensive management of the ecological environment and water resources in the source region of the Pearl River. Through the systematic collection of surface water, karst groundwater, and mine water samples, mathematical statistics analysis, correlation analysis, ion ratio analysis, absolute principal component scores multiple linear regression(APCS-MLR), and other methods were used to study the characteristics of hydrochemical evolution and control factors in Kuaize River Basin. The results showed that the average pH value of surface water in Kuaize River Basin was 7.8, which was weakly alkaline. The main cations were Ca2+ and Na+, showing the characteristics of Ca2+>Na+>Mg2+>K+. The main anions were HCO3- and SO42-, showing the characteristics of HCO3->SO42->NO3->Cl-. The variation coefficients of Na+, SO42-, and NO3- in surface water were high, showing strong spatial variability. The water chemical type of the trunk stream was mainly HCO3-Ca, whereas the water chemical type of the tributary was relatively complex, mainly HCO3-Ca, HCO3-Ca·Na, and HCO3·SO4-Ca·Na. The chemical composition of surface water was mainly affected by rock weathering, cation exchange, and human activities. Ca2+, Mg2+, Na+, and HCO3- in surface water mainly came from the weathering of carbonate rock and silicate rock; SO42- mainly came from the oxidation of sulfide, such as pyrite in coal seams; K+, Cl-, and NO3- mainly came from domestic sewage and agricultural activities. The APCS-MLR receptor model analysis results showed that the surface water in the Kuaize River Basin was mainly affected by sulfide oxidation, carbonate weathering, weathering of silicate rock in mine water, domestic sewage, agricultural activities, and unknown sources. In general, the contribution rate of human activities such as mining, domestic sewage, and agricultural activities to the surface water reached 47.17%, indicating that human activities were the key driving factor of surface water chemistry in the Kuaize River Basin.

Hydrochemical characteristics and salt intrusion in groundwater of the lower Chao Phraya river basin: Insights from stable isotopes and hydrochemical analysis

Saltwater intrusion is a significant hydrogeological challenge in coastal regions. This study investigates salt intrusion and hydrogeological processes in the Lower Chao Phraya River basin using stable isotopes and hydrochemical analysis. One Hundred groundwater wells sampled from different aquifers were analyzed for major ions spanning from 2010 to 2021, as well as stable isotopes specifically for the samples in the year 2021. The results reveal four distinct groundwater types: Ca(HCO3)2, NaHCO3, NaCl, and CaSO4, based on their predominant cations and anions. Shallow groundwater, which represents the discharge zone affected by salt intrusion, halite dissolution, and cation exchange reactions, exhibits a significant concentration of NaCl. Water-rock interactions play a crucial role in shaping groundwater chemistry, with carbonate dissolution and precipitation dominating the Ca(HCO3)2 type and silicate weathering influencing the CaSO4 type. The NaHCO3 type is associated with ion exchange and seawater intrusion. Silicate and carbonate rock weathering are the dominant processes in the region. Stable isotope analysis indicates that groundwater recharge primarily comes from rainfall and surface water, with limited contributions from deep sources. Overall, this study provides valuable insights into the hydrochemical characteristics and complex water-rock interactions within the groundwater system of the Chao Phraya River basin. As a result, it holds significant potential for informing effective groundwater management practices and facilitating resource planning in the region. Furthermore, the findings and methodologies presented here offer valuable lessons for addressing saltwater intrusion and hydrogeological challenges in other coastal regions worldwide.

Possible Effects of Changes in Carbonate Concentration and River Flow Rate on Photochemical Reactions in Temperate Aquatic Environments.

In temperate environments, climate change could affect water pH by inducing enhanced dissolution of CaSO4 followed by biological sulphate reduction, with the potential to basify water due to H+ consumption. At the same time, increased atmospheric CO2 could enhance weathering of carbonate rocks (e.g., dolomite) and increase the total concentration of dissolved carbonate species. Both processes enhance phototransformation by the carbonate radical (CO3•-), as shown for the non-steroidal anti-inflammatory drug paracetamol, provided that the dissolved organic carbon of water does not undergo important fluctuations. Climate change could also affect hydrology, and prolonged drought periods might considerably decrease flow rates in rivers. This is a substantial problem because wastewater pollutants become less diluted and, as a result, can exert more harmful effects due to increased concentrations. At the same time, in low-flow conditions, water is also shallower and its flow velocity is decreased. Photochemical reactions become faster because shallow water is efficiently illuminated by sunlight, and they also have more time to occur because water takes longer to cover the same river stretch. As a result, photodegradation of contaminants is enhanced, which offsets lower dilution but only at a sufficient distance from the wastewater outlet; this is because photoreactions need time (which translates into space for a flowing river) to attenuate pollution.

Hydrochemical Characteristics and Controlling Factors in Wudu River Basin of Guizhou Province

TheWudu River is a typical mining-type watershed in the karst mountainous area of western Guizhou Province. Based on the collection of the main stream, tributaries, spring water, and mine water samples in Wudu River Basin, the hydrochemical characteristics and control factors of Wudu River Basin were studied using Gibbs diagram, Piper diagram, and mathematical statistics analysis, and the solute contribution rate of different sources was calculated. The results revealed that the pH value of the water in the Wudu River Basin ranged from 7.87 to 8.52, with an average of 8.14. The TDS values ranged from 135 to 243 mg·L-1, with an average of 191.7 mg·L-1. The major cations in natural river and spring water were Ca2+ and Mg2+, the major anion was HCO3-, and the hydrochemical type was HCO3-Ca. However, owing to the influence of mining activities, the major cations in some tributaries were Ca2+ and Na+, and the hydrochemical types transitioned to HCO3·SO4-Ca and HCO3·SO4-Ca·Na. The ion components of river water in Wudu River Basin were affected by mine water discharge and cation exchange, carbonate rock weathering, silicate rock weathering, and agricultural fertilization. The high concentration of SO42- and Na+in mine water was the primary source of SO42- and Na+in the tributaries of the Wudu River. The method for calculating chemical material balance showed that the contribution rate of carbonate rock weathering ranged from 44.12% to 86.92%, with an average of 74.32%. The contribution rate of mining activities ranged from 3.28% to 37.07%, with an average of 11.61%. Carbonate rock weathering was the main controlling factor of hydrochemical components in the Wudu River Basin; meanwhile, mining activities also had a certain impact on river water chemistry but they showed spatial heterogeneity. The average contribution rates of atmospheric precipitation, silicate rock weathering, agricultural activities, and domestic sewage were 3.75%, 4.67%, 2.85%, and 2.81%, respectively, which had a limited impact on the hydrochemical components of the basin.

Sulfur disproportionating microbial communities in a dynamic, microoxic-sulfidic karst system.

Biogeochemical sulfur cycling in sulfidic karst systems is largely driven by abiotic and biological sulfide oxidation, but the fate of elemental sulfur (S0 ) that accumulates in these systems is not well understood. The Frasassi Cave system (Italy) is intersected by a sulfidic aquifer that mixes with small quantities of oxygen-rich meteoric water, creating Proterozoic-like conditions and supporting a prolific ecosystem driven by sulfur-based chemolithoautotrophy. To better understand the cycling of S0 in this environment, we examined the geochemistry and microbiology of sediments underlying widespread sulfide-oxidizing mats dominated by Beggiatoa. Sediment populations were dominated by uncultivated relatives of sulfur cycling chemolithoautotrophs related to Sulfurovum, Halothiobacillus, Thiofaba, Thiovirga, Thiobacillus, and Desulfocapsa, as well as diverse uncultivated anaerobic heterotrophs affiliated with Bacteroidota, Anaerolineaceae, Lentimicrobiaceae, and Prolixibacteraceae. Desulfocapsa and Sulfurovum populations accounted for 12%-26% of sediment 16S rRNA amplicon sequences and were closely related to isolates which carry out autotrophic S0 disproportionation in pure culture. Gibbs energy (∆Gr ) calculations revealed that S0 disproportionation under in situ conditions is energy yielding. Microsensor profiles through the mat-sediment interface showed that Beggiatoa mats consume dissolved sulfide and oxygen, but a net increase in acidity was only observed in the sediments below. Together, these findings suggest that disproportionation is an important sink for S0 generated by microbial sulfide oxidation in this oxygen-limited system and may contribute to the weathering of carbonate rocks and sediments in sulfur-rich environments.

Hydrological characteristics and available water storage of typical karst soil in SW China under different soil–rock structures

Soil hydrological characteristics are influenced by factors such as parent rock weathering, human activities, and soil texture. However, the influence of the complex soil–rock structures and heterogeneous soil types on soil hydrological characteristics resulting from the weathering of carbonate rocks into soils on slopes in the karst region of SW China is not fully understood. The relationships between zonal and nonzonal soil hydrological characteristic differences, land uses, and soil–rock structures were analyzed using a typical watershed in the SW China karst region. In this study, (1) the difference between zonal and nonzonal soil hydrological characteristics is significant. For infiltration capacity (Ks), yellow soil (19.50 ∼ 1058.00 cm·d-1) < limestone soil (34.50 ∼ 2364.00 cm·d-1), while for soil available water storage, limestone soil on the dolomitic slope (43.26 mm) > yellow soil (20.16 ∼ 35.25 mm) > limestone soil on the limestone slope (17.73 ∼ 34.72 mm). (2) Land-use practices and soil–rock structures have long affected the hydrological characteristics of soil in karst. (3) The bare bedrock on carbonate slopes leads to a reduction in the total amount of soil per unit area on the slope, which compresses the space for vegetation growth and reduces the total amount of water provided by the soil for vegetation growth per unit area, which confirms one of the reasons for the low plant biomass in karst. These results suggest that the utilization of soil water in karst areas should consider the weights of soil type, lithology, and soil–rock structures.

Geochemistry of Weathering Cover and the Main Influencing Factors in Karst Area of Guilin, Southwest China

The chemical weathering of carbonate rocks is the main form of interaction between earth spheres in the karst critical zone. The karst weathering cover, which is composed by residua from carbonate rocks weathering, contains important information about the climate environment and material cycle. We present the chemical composition of weathering covers in karst area of Guilin, Guangxi province, analyze their weathering process and strength, and compare them with the other weathering covers in other karst area in China, including Yunnan, Guizhou, Hunan, and Qinghai Tibet Plateau. The results showed: (1) the chemical composition of Guilin weathering covers were similar to that of carbonate weathering covers in other areas of China, and had the common characteristics of uniform distribution of chemical composition in the profile and obvious enrichment of Fe, Al and trace elements. During the formation of carbonate weathering cover and the residua, the rapid dissolution of Ca and Mg had an important impact on the migration and enrichment of other elements. (2) The chemical index of alteration (CIA) of carbonate weathering covers in Guilin and other karst areas of China was much larger than that in the upper crust (UCC) (60.13), which showed strong chemical weathering characteristics of the humid and hot climate. (3) The weathering process of carbonate rock was different from that of silicate and loess. In the early stage of carbonate rock weathering, soluble components (calcite and dolomite) had been rapidly dissolved and leached. All the carbonate weathering residua was measured to be in the stage of K2O weathering and Al2O3 increasing. Therefore, the weathering degree of carbonate weathering cover was mainly determined by the leaching of K2O and the increase of Al2O3. As a result, there was no correlation between CIA and Na/K (molar ratio), but was a significant negative correlation between CIA and K/Al (molar ratio) in the carbonate weathering cover. (4) The CIA values of weathering residua and carbonate weathering cover in southern China were negatively correlated with latitude, reflecting the influence of climate factors. From the influence of lithology, the chemical compositions of parent rock can affect the CIA of weathering cover, and the content of insoluble matter in carbonate rock was negatively correlated with CIA. From the influence of topography, the CIA value of weathering residua decreased from the high to the low position and from the shallow to the deep part of the profile.

Combining hydrochemistry and 13C analysis to reveal the sources and contributions of dissolved inorganic carbon in the groundwater of coal mining areas, in East China.

East China is a highly aggregated coal-grain composite area where coal mining and agricultural production activities are both flourishing. At present, the geochemical characteristics of dissolved inorganic carbon (DIC) in groundwater in coal mining areas are still unclear. This study combined hydrochemical and carbon isotope methods to explore the sources and factors influencing DIC in the groundwater of different active areas in coal mining areas. Moreover, the 13C isotope method was used to calculate the contribution rates of various sources to DIC in groundwater. The results showed that the hydrochemical types of groundwater were HCO3-Ca·Na and HCO3-Na. The main water‒rock interactions were silicate and carbonate rock weathering. Agricultural areas were mainly affected by the participation of HNO3 produced by chemical fertilizer in the weathering of carbonate rocks. Soil CO2 and carbonate rock weathering were the major sources of DIC in the groundwater. Groundwater in residential areas was primarily affected by CO2 from the degradation of organic matter from anthropogenic inputs. Sulfate produced by gypsum dissolution, coal gangue accumulation leaching and mine drainage participated in carbonate weathering under acidic conditions, which was an important factor controlling the DIC and isotopic composition of groundwater in coal production areas. The contribution rates of groundwater carbonate weathering to groundwater DIC in agricultural areas and coal production areas ranged from 57.46 to 66.18% and from 54.29 to 62.16%, respectively. In residential areas, the contribution rates of soil CO2 to groundwater DIC ranged from 51.48 to 61.84%. The results will help clarify the sources and circulation of DIC in groundwater under the influence of anthropogenic activities and provide a theoretical reference for water resource management.

Geochemical behaviors of uranium and thorium during weathering and pedogenesis of carbonate rock: constraint from their speciation.

During weathering and pedogenesis of carbonate rock with poor-uranium (U) and thorium (Th), U and Th present the characteristics of strong leaching (especially U) and significant residual enrichment, the cause of which is still unclear. In this paper, a weathering profile developed by dolomite in karst area of Guizhou province in southwest China was selected, which showed zonation characteristics of bedrock (Y), powdery rock (Yf), and soil layer (T1 to T12) from the bottom to up. Through the determination of the occurrence speciation of U and Th in Y and weathering profile, combined with mineralogical, geochemical characteristics, and element mass balance calculation, the constraints of U and Th speciation on the geochemical behavior of U and Th during the weathering of carbonate rock were revealed. The results proved that U and Th in Y preferentially existed in acid insoluble phase, for example, the contents of U and Th in Y were 0.90 mg·kg-1 and 0.28 mg·kg-1, respectively, while those in acid insoluble matter were 2.34 mg·kg-1 and 2.57 mg·kg-1, respectively, but because the mass percentage of acid insoluble matter was extremely low (0.95%), the mass percentages of U and Th in the acid soluble phase in the whole rock were absolutely superior (96% of U and 86% Th). The U and Th in the acid soluble phase of Y were mainly adsorbed on the crystal surface of carbonate minerals or existed in the cement, and the U and Th in the carbonate lattice only accounted for a small proportion. From Y to Yf with the initial dissolution, U and Th released from the surface of carbonate minerals and cements were in carbonate-rich alkaline environment, and these portions of U and Th were leached out, resulting in strong loss of U and Th in the Yf (the loss rates are 83% of U and 65% of Th, respectively). From the Yf to the overlying soil layer T1, the carbonate components were completely dissolved, and the U and Th released from the carbonate lattice showed different behaviors, where U was completely leached and Th tended to stay in the weathered residue. Thus, in the soil layer T1 formed by Y or Yf , the residual U was the inheritance of the U in the acid insoluble phase of Y; For Th, it not only inherited the Th of acid insoluble phase of Y, but also superimposed the Th from carbonate lattice in Y. On the other hand, during the evolution process from Y to Yf and to soil layer T1, with the dissolution of carbonate, the acid insoluble phase also showed a significant tendency of chemical weathering. However, the U and Th in the Y acid insoluble phase were not leached with the decomposition of the acid insoluble phase but were redistributed among the residual phases. For the geochemical behaviors of U and Th in the evolution of soil profile (T1~T12), they were subjected to the occurrence speciation of U and Th in T1 and the change of U and Th occurrence speciation with the upward direction of soil profile. The U and Th released from the carrier minerals were mainly redistributed among the residual solid phases, which weakened the intensity of their further loss. This study deepens the understanding of the geochemical behavior of radionuclides in karst environment and provides reference for the treatment of radioactive pollution in karst areas.

A large and overlooked Cd source in karst areas: The migration and origin of Cd during soil formation and erosion

There is increasing concern regarding the substantial enrichment of Cd during the weathering of carbonate rocks and subsequent risks posed to the ecological environment and food security in karst areas. However, the incomplete understanding of Cd migration mechanisms and material sources restricts soil pollution control and land management. This study investigated the migration regulation of Cd during soil formation and erosion in karst areas. The results demonstrate that soil Cd concentration and bioavailability are both significantly higher in alluvium compared with those in eluvium. This increase is primarily attributed to the chemical migration of active Cd, rather than the mechanical migration of inactive Cd. Additionally, we analyzed the Cd isotopic characteristics of rock and soil samples. The isotopic composition of the alluvial soil was −0.18 ‰ ± 0.01 ‰, which is obviously heavier than the δ114/110Cd value of the eluvium (−0.78 ‰ ± 0.06 ‰). The Cd isotopic fingerprint revealed that the active Cd in the alluvium of the study profile was probably derived from the corrosion of carbonate rocks rather than by eluviation of the eluvium. Moreover, Cd tends to occur in soluble mineral components of carbonate rocks rather than in residues, which suggests that carbonate weathering has a great potential to release active Cd into the environment. It is estimated that the Cd release flux caused by carbonate weathering is 5.28 g Cd km−2 yr−1, accounting for 9.30 % of the anthropogenic Cd flux. Therefore, the corrosion of carbonate rocks is a substantial natural Cd source and poses significant potential risks to the ecological environment. It is suggested that the contribution of Cd from natural sources should be considered during ecological risk assessments and studies of the global Cd geochemical cycle.