Search Results (9)

In this paper, the generation of inorganic carbon (mainly HCO₃⁻) in a karst system (soil-limestone system) under karst soil and non-karst soil conditions was investigated using two Zn²⁺ concentrations and water flow rates. The results showed that (1) the dynamic equilibrium state of the chemical weathering of limestone is altered by Zn²⁺, which is the primary cause of the change in HCO₃⁻ in soil-limestone systems; (2) ion exchange and adsorption are the primary characteristics of Zn²⁺ depletion under 1 mg/L ZnCl₂ settings, whereas Zn²⁺ under 50 mg/L ZnCl₂ conditions created two new solid phases (Zn₅(OH)₆(CO₃)₂, ZnCO₃) in the soil-limestone system; (3) the dissolution rate of limestone increases with the water flow rate, which facilitates the dissolution process; (4) the notable difference in ion release between non-karst and karst soil conditions could potentially be attributed to variations in the mineral composition, specific surface area, and particle size of the two soil types; (5) the combination of SEM, XPS, FT-IR, and XRD microstructure observation methods reveals that when limestone is exposed to a high flow rate (1.23 mL/min) and a high concentration (50 mg/L) of ZnCl₂, it experiences obvious dissolution and surface precipitation phenomena, as well as a significant change in HCO₃⁻ content. Full article

The distributional characteristics of microorganisms in karst cave ecosystems have been widely studied. However, in such a dark, humid, and oligotrophic habitat, studies on the differences in carbon-sequestering bacteria in multiple habitats are limited. Therefore, to learn the distribution characteristics of carbon-sequestering colonies in cave habitats and their correlation with habitat factors (e.g., pH, Ca²⁺, Mg²⁺, etc.), samples from five cave habitats (weathered rock walls, underground river water, drips, sediments, and air) were collected from the twilight and dark zones of Shiziyan Cave (CO₂ concentration 5385 ppm). The results of high-throughput sequencing and statistical analyses showed that there were significant differences in the distribution of communities in different habitats, with higher abundance in sediments habitat and underground river water habitat, and the dominant phyla of Pseudomonadota (30.53%) and Cyanobacteria (75.11%) in these two habitats. The microbial diversity of the carbon-sequestering microbial community was higher in sediments than in underground river water. The pH, and Ca²⁺, ${\mathrm{SO}}_4^{2-}$, and ${\mathrm{NO}}_3^{-}$ concentrations can alter the diversity of carbon-sequestering microbes, thereby affecting carbon cycling in caves. Carbon metabolism analyses suggest that microbes in the habitat can cooperate and coexist by participating in different carbon metabolic pathways. These results expanded our understanding of carbon-sequestering microbial communities in cave systems and their responses to the environment. Full article

This study investigates the processes leading to karst development in the southeastern part of Riyadh city extending up to Al Kharj. Numerous solution features such as sinkholes, collapsed dolines, and solution caverns are common in the area. The role of water in the development of the karst features was investigated using an integrated geological and hydrochemical approach. Geological investigations included the petrographic analysis of rock samples collected from zones of intense karstification with special emphasis on mineral dissolution. The study showed that the Sulaiy Formation is commonly fractured, brecciated, foliated, and contains numerous cavities, vugs, and openings. These features have formed by mineral dissolution by circulating groundwater, which has removed anhydrite beds from the underlying Arab–Hith sequence. Karstification likely started from the tectonically weak zones when there was more groundwater recharge. Studies show that during the early to mid-Holocene period, the climate in the Arabian Peninsula was humid, promoting groundwater recharge and subsequent mineral dissolution, though the process of karstification must have started much earlier. Hydrochemical findings reveal that mineral dissolution (halite and calcium sulfate) is the main process affecting groundwater chemistry. The Piper plot revealed two main hydrochemical facies: the (Ca²⁺ + Mg²⁺)–(Cl+ SO₄²⁻) Type (Type A) and the (Na⁺ + K⁺)–(SO₄²⁻ + Cl) Type (Type B). Most of the samples belong to Type B, typical of groundwater facies affected by dissolution of halite and anhydrite mineral. The absence of the (Ca²⁺ + Mg²⁺)–(CO₃²⁻ + HCO₃⁻) type of groundwater facies indicates a lack of recent groundwater recharge and the removal of carbonate minerals from the system through precipitation, as evidenced by the saturation indices. Plots of the major ionic pairs (cations vs. anions) in groundwater indicate strong halite and gypsum/anhydrite dissolution. Of the three carbonate minerals, calcite has the highest average saturation index followed by aragonite and dolomite. This suggests significant past rock–water interaction leading to carbonate dissolution. Presently, any additional calcium or carbonate ions introduced into the water lead to calcite precipitation. The study indicates that the process of karst development may not be active today. Currently, groundwater chemistry is mainly influenced by rock–water interaction leading to gypsum/anhydrite dissolution, which has resulted in a high concentration of Na⁺, Ca²⁺, Cl⁻ and SO₄²⁻ ions in groundwater. The dissolution of gypsum and halite from the Hith Formation weakens the structural integrity of the overlying Sulaiy Formation, creating large underground cavities. These cavities increase the risk of roof collapse, leading to cover-collapse sinkholes as the roof becomes too thin to support the weight above. Full article

Bicarbonate ions (HCO₃⁻) are abundant in karst water with poor lead (Pb) utilization and biodegradation. This study aimed to investigate the mechanism of HCO₃⁻ on the Pb removal efficiency and uptake ability of Pontederia crassipes (a widespread hydrophyte in the karst area) from karst water. The Pb concentration, Pontederia crassipes morphology, and functional group were detected. As the HCO₃⁻ molarity in karst water increased (3, 4, and 5 mmol/L), the removal of Pb increased (85.31%, 93.28%, and 95.16%), whereas the bioconcentration amount of Pb decreased (573, 501, and 301 mg/kg), mainly due to the insoluble PbCO₃ and Pb (OH)₂. The Pb bioconcentration factor was the highest (15,564) at 4 mmol/L HCO₃⁻ due to the maximum strength of cation exchange and cell wall protein C=O. High HCO₃⁻ molarities changed the variety of positive ions of cation exchange (HCO₃⁻ ≤ 4 mmol/L: Na, K, and Mg; HCO₃⁻ > 4 mmol/L: Mg and K), and relieved the breaking of roots, stomatal closure, and vascular system shrinking. Moreover, high HCO₃⁻ molarities diminished the C≡C oxidation, enlarged the displacement of SO₄²⁻ and C-O, and stimulated the methyl transfer reaction and the bonding between -CH₃ and Pb. Full article

Dissolved CO₂ in karst water is the key driving force of karstification. Replenishment of CO₂ concentrations in karst water occurs by meteoric water that percolates through the vadose zone, where CO₂ produced from microbial activity is dissolved. CO₂ can thus be transported with the percolating water or in the gas phase due to ventilation in karst systems. We measured seasonally fluctuating CO₂ concentrations in the air of a karst cave and their influence on aqueous CO₂ concentrations in different depths of a stagnant water column. The observed data were compared to numerical simulations. The data give evidence that density-driven enhanced dissolution of gaseous CO₂ at the karst water table is the driving force for a fast increase of aqueous CO₂ during periods of high gaseous concentrations in the cave, whereas during periods of lower gaseous concentrations, the decline of aqueous CO₂ is limited to shallow water depths in the order of 1 m. This is significant because density-driven CO₂ dissolution has not been previously considered relevant for karst hydrology in the literature. Attempts at reproducing the measured aqueous CO₂ concentrations with numerical modeling revealed challenges related to computational demands, discretization, and the high sensitivity of the processes to tiny density gradients. Full article

Hydrological variations hold a significant influence over the water chemistry in the karst systems within the critical zone. In this context, the Baget Catchment (BC) was monitored at high-resolution over 2 hydrological years. The high-frequency survey at the outlet of BC displayed multiple hydrochemical patterns in response to hydrological variations, mixing water sources, and biogeochemical processes. Among the major elements, sulfate exhibited the widest relative variation during flooding and showed a significant dilution, whereas calcium and bicarbonate revealed a chemostatic behavior as a result of carbonate dissolution in the karst. Hysteretic analysis evidenced the control of different hydrological reservoirs over the stream transport processes of dissolved elements. The concentration of dissolved organic carbon increased during the hydrograph rising limb and was controlled by surface runoff. The relationships between Ca²⁺ and HCO₃⁻ concentrations and stream discharge exhibited hysteresis patterns with counterclockwise loops, unlike all other elements, due to the carbonate weathering by biogenic CO₂-rich water. Finally, high-frequency sampling during storm events improved the understanding of the factors controlling the hydrochemical dynamic of the Baget stream water. The relative contributions of the karst and epikarst zones, of rainwater, as well as the role of different biogeochemical processes and the hydrological conditions were highlighted. Full article

Monitoring of various naturally present substances or physical properties of the water, commonly called environmental tracers, can provide valuable insight in characteristics of groundwater flow systems and intrinsic processes. Analysis of individual tracer data most often gives an ambiguous interpretation, but employment of multiple diverse tracers can greatly increase interpretation reliability. We monitored multiple natural tracers including spring water electrical conductivity, temperature, loads of major anions and cations, stable isotopes of water, and total organic carbon together with discharge dynamics on Krbavica springs located in the Croatian part of Dinaric Karst region. We also monitored dissolved oxygen concentration as an indicator of “excess air” dynamics in the spring water, which is a very rarely recognized and not properly understood phenomenon in the karst groundwater studies. Analysis of the monitoring data revealed main characteristics of the karst system, among which following can be emphasized: (1) oxygen concentration together with conductivity and temperature (parameters monitored with high temporal resolution) were strongly related to discharge dynamics, while seasonal patterns were absent; (2) supersaturation with oxygen confirmed presence of “excess air”, most pronounced in high water conditions following the hydrograph peaks, indicating prevalence of closed flow conditions within the system; (3) electrical conductivity showed “anomalous” gradual decrease during the hydrograph recessions, attributed both to CO₂ dynamics and mobilization of water from tiny fissures during high recharge conditions; and (4) stable isotope measurements confirmed good mixing of water within the system with mean residence time of a few years. Simultaneous monitoring of diverse tracer dynamics enabled detailed characterization of the karst system without excessive ambiguity. Full article

It is important to investigate temporal variations of water chemistry for the purpose of improving water quality in karst groundwater systems. Groundwater samples were collected daily under various land uses of Guiyang. Major ions and stable carbon isotope composition of dissolved inorganic carbon (δ¹³C_DIC) were analyzed to understand the biogeochemical processes. The water chemistry was dominated by Ca²⁺, Mg²⁺, HCO₃^-, and SO₄^2-, which mainly derived from the dissolution of carbonate rocks (limestone and dolomite) and oxidation of sulfide. The groundwater was defined as of the HCO₃-Ca Mg and HCO₃·SO₄-Ca·Mg type, according to its hydrochemical characteristics. Results suggested that hydrochemical concentrations changed quickly, in response to rainfall events. The fast response revealed that karst groundwater was easily impacted by rainfall and anthropogenic inputs according to temporal variation of water chemistry. The distribution of DIC (dissolved inorganic carbon) and δ¹³C_DIC showed that DIC is mainly sourced from soil CO₂(g) influx and carbonate dissolution. δ¹³C_DIC and major ions ratios suggested that carbonate minerals were dissolved by H₂SO₄ at groundwater in wooded area, contributing an important source for DIC due to the slight enrichment of heavy δ¹³C_DIC. More negative δ¹³C_DIC values were observed after rainfall reflected the fact that soil CO₂(g) and organic carbon oxidation influxes accounted for a large share during DIC formation. Various δ¹³C_DIC and hydrochemical patterns were observed under various land use and human activity conditions. Meanwhile, relative high nitrate loads were found in groundwater after rainfall, suggesting high anthropogenic inputs following rainwater as having side effects on water quality. This study suggests that water chemistry and isotopic proof provide a better understanding of water quality and carbon dynamics responding to rainfall events in the karst groundwater systems. Full article

Exploring the hydrogeochemical processes of karst groundwater has significant meaning for protecting local groundwater systems in semi-arid areas. Taking a typical semi-arid karst groundwater system—the Liulin spring area—as the research region, hydrogeochemical processes from rainfall infiltration to formation of higher total dissolved solids (TDS) water were studied, applying a mass balance model and the prediction of water chemical components in the focus area was explored. The results showed that hydrogeochemical processes dominating chemical components of karst groundwater included lixiviation, cation exchange and mixture. Calcite dissolved during rainfall infiltration processes in recharge area and saturated, then precipitated along the whole flow path. CO₂ dissolved significantly along with rainfall infiltration process and outgassed in discharge area and stagnant area. The dissolution of dolomite, gypsum and halite accompanied entire flow path and maximum dissolution load occurred in stagnant area. Mg-Na or Ca-Na exchange prevailed along flow path but exchange types depended on ionic concentration. The mixture between surface water and karst groundwater took place in surface water leakage belt in recharge and discharge area and mixture ratio for surface water ranged from 40% to 70%. TDS of the Liulin springs will increase with decreasing surface water leakage. Conversely, TDS of karst groundwater near Henggou area will decrease accompanied by the continuous discharge of the Henggou artesian well. Full article