Zone Water Research Articles

Exclusion-zone water inside and outside of plant xylem vessels

The fourth phase of water has garnered increased attention within the scientific community due to its distinct properties that differentiate it from regular water. This unique state seems to arise potentially from a liquid crystalline structure, which has been observed near various hydrophilic surfaces to possess the capability of excluding microspheres. Consequently, it has been labeled as exclusion zone (EZ) water. When in contact with hydrophilic surfaces, water could exhibit the ability to form organized layers of EZ water. In this study, we investigated the quick buildup of EZ water exposed to xylem vessels of four vegetable plants: cabbage, celery, asparagus, and pumpkin. Among them, pumpkin vessels showed larger EZs, up to 240 ± 56 μm in width. The width of EZ water found near the xylem vessels of the other plants ranged from 133 ± 22 to 142 ± 20 μm. EZ water generally excludes a wide range of particles, including polystyrene microspheres with various surface modifications, as well as silica microspheres. This implies that the formation of EZ water is not an artificial result of using specific microsphere types but rather demonstrates EZ’s ability to exclude particles regardless of their composition. Inside single xylem vessels of the pumpkin, we could observe the dynamics of EZ buildup, growing from the inside edge of the vessel toward the center. The relationship between vessel diameter, vessel length, and salt concentration on EZ generation inside the xylem vessel was also explored. The results showed that EZ water can build up both inside and outside the xylem vessels. Our findings suggest that EZ generation inside xylem vessels is associated with water flow, likely driven by a proton gradient. Further research is warranted to elucidate the role of EZ water in the physiology of living plants, particularly considering the limitations of the current experiments conducted on cut-out xylem vessel samples.

Climate change impact on groundwater resources in sandbar aquifers in southern Baltic coast

Shallow coastal aquifers are vulnerable hydrosystems controlled by many factors, related to climate, seawater-freshwater interactions and human activity. Given on-going climate change, sea level rise and increasing human impact, it is especially true for groundwater resources situated in sandbars. We developed numerical models of unsaturated zone water flow for two sandbars in northern Poland: the Vistula Spit and the Hel Spit using HYDRUS-1D. The simulations were performed for three types of land use: pine forest, grass cover and bare soil, for 2024–2100 based on weather data and sea level rise forecasts for two emissions scenarios (RCP 4.5 and RCP 8.5). The results present prognosis of groundwater recharge, water table level and water content changeability in near-term (2023–2040), mid-term (2041–2060), and long-term period (2081–2100). Expected sea level rise and decreasing hydraulic gradient of the sandbar aquifers will probably cause in-land movement of the freshwater–saltwater interface, leading to significant decrease or complete salinization of groundwater resources. The study shows that holistic monitoring including groundwater level and salinization, sea level rise, and metheorological data (precipitation amount and variability, temperature) is crucial for sustainable management of vulnerable aquifers located in sandbars.

Ocean deoxygenation dampens resistance of diatoms to ocean acidification in darkness

Respiratory activity in the oceans is declining due to the expansion of hypoxic zones and progressive deoxygenation, posing threats to marine organisms along with impacts of concurrent ocean acidification. Therefore, understanding the combined impacts of reduced pO2 and elevated pCO2 on marine primary producers is of considerable significance. Here, to simulate diatoms’ sinking into the aphotic zone of turbid coastal water, we exposed the diatoms Thalassiosira pseudonana and Thalassiosira weissflogii in darkness at 20°C to different levels of pO2 and pCO2 conditions for ~3 weeks, and monitored their biomass density, photosynthetic activity and dark respiration, and examined their recovery upon subsequent exposure to light at 20°C, simulating surface water conditions. Along with decreased cell abundance and size, measured photosynthetic capacity and dark respiration rates in these two diatoms both gradually decreased during the prolonged darkness. Reduced pO2 alone did not negatively affect the photosynthetic machinery in both the dark-survived diatom, and enhanced their subsequent recovery upon light exposure. Nevertheless, the combination of the elevated pCO2 and reduced pO2 (equivalent to hypoxia) led to the biomass loss by about 90% in T. pseudonana, and delayed the recovery of both species upon subsequent exposure to light, though it did not reduce the cell concentration of T. weissflogii during the elongated darkness. Our results suggest that reduced O2 availability diminishes the abilities of the diatoms to cope with the acidic stress associated with ocean acidification, and the expansion of hypoxic waters could delay the photosynthetic recovery of coastal diatoms when they are transported upwards through mixing from dark layers to sunlit waters.

Partial melt in mesoscale upper mantle upwellings beneath ocean basins

How tectonic plate motions are coupled with mantle flows remains an open question. Quasi-periodic 2000 km wavelength undulations aligned with absolute plate motion in the Pacific and Indian Ocean basins, observed in gravity and seafloor topography and coinciding with seismically imaged low shear velocity fingers in the upper mantle, suggest the presence of meso‑scale convection below the lithosphere. However, the correspondence of sub-lithospheric mantle mass excess, seafloor lows and slow upper mantle seismic velocities cannot be explained by temperature variations alone. Here we introduce a simplified system of bi-dimensional convective cells of width ∼1000 km, extending from the base of the lithosphere through the extended mantle transition zone (down to 1000 km depth), at least partly driven from below. From mass balance considerations in a viscous Earth, we show that the density excess required in the hot upwelling limbs may reflect the formation of stable dense lenses of dehydration-induced partial melt atop the 410 km discontinuity, and upward entrainment of a small fraction of quasi-buoyant partially molten and recrystallized material across the upper mantle. Our model provides an explanation for the thin low shear velocity layer detected intermittently above the 410 km discontinuity in some parts of ocean basins away from subducted slabs, and supports the presence of water in the transition zone.

Evaluating the significance of wetland representation in isotope-enabled distributed hydrologic modeling in mesoscale Precambrian shield watersheds

Evaluation of regional-scale models against both streamflow and stable isotopes of water is a relatively new and evolving area of distributed hydrological modeling. While applications of isoWATFLOOD, an isotope-enabled distributed hydrological model are increasing, evaluation of the impacts of model structural features, such as wetland connectivity, on simulation of runoff components is still limited. Wetlands are important water bodies that impact streamflow generation and its spatial variation in Precambrian shield watersheds in parts of Canada and elsewhere. This study investigates the efficiency of isoWATFLOOD in simulating streamflow and instream δ18O and the effects of model percent connected wetlands (%CW) in a mesoscale (∼12,000 km2) Precambrian Shield watershed in Northeastern Ontario, Canada. Model calibration and validation are performed using daily streamflow for five hydrometric gauging stations and biweekly to monthly instream δ18O from 11 river sampling locations from across the watershed. Five model versions with different %CW are generated, calibrated, and validated separately. Results show all models can simulate the timing and pattern of streamflow and δ18O. %CW impacts simulated streamflow during both calibration and validation periods, calibrated parameter sets and simulated internal hydrological processes. Model performance varies spatially with no single value of %CW providing best performance across sub-catchments with 40 % CW providing best mean streamflow simulations in calibration. Increasing %CW (10 % to 40 %) results in higher contribution of lower soil zone water to streamflow that markedly improves baseflow simulation. This study builds on recent isotope-enabled hydrologic simulation capacity of isoWATFLOOD, contributing new examination of model representation of wetland connectivity, a critical component of Precambrian Shield watersheds and evidence of how stable isotopes can contribute to addressing the challenge of equifinality. Results suggest the need to develop model capability for representation of spatial variation in wetland connectivity to recognize this important aspect of heterogeneity in watersheds influencing hydrologic function.

Study on Water and Salt Transport Characteristics of Sunflowers under Different Irrigation Amounts in the Yellow River Irrigation Area

The control of irrigation volume is of significant importance in arid regions of northwest China. Particularly, it has a crucial impact on the salinization of shallow groundwater areas. In 2022 and 2023, field experiments were conducted to test three distinct under-membrane irrigation treatments. These treatments were assigned water quotas of HW (27 mm), MW (22.5 mm), and LW (18 mm). The HYDRUS-2D model was integrated with a field experiment to accurately simulate the dynamic fluctuations of soil water and salt in the sunflower root zone. The model’s performance was assessed and verified using real-field data from 2022 and 2023, and the simulation results closely matched the measured values. This research also used stable hydroxide isotopes to assess the water supply from various soil layers at different time intervals in sunflower plants. The results indicated that the three different levels of irrigation applied under the membrane had a significant impact on soil water content. Specifically, there was a significant difference in soil water content at a depth of 0–40 cm (p < 0.05), while there was little effect on the water content at a depth of 40–60 cm (p > 0.05). After irrigation, the average salt content in the top 0–20 cm of soil decreased by 7.0% compared to the medium and low irrigation levels, and by 10.8% compared to the medium irrigation level. Additionally, the medium irrigation level resulted in a 10.8% decrease in salt content compared to the low irrigation level, and a 4.1% decrease compared to the medium irrigation level. During the same period, the soil salinity levels at depths of 0–20 cm, 20–40 cm, 40–60 cm, and 60–100 cm in the area outside the membrane were measured to be 2.7~4.8 g·kg−1, 2.8~4.0 g·kg−1, 2.7~3.4 g·kg−1, and 1.7~2.6 g·kg−1, respectively. These levels decreased by 13.1~55.5%, 0.7~42.8%, −0.4~16.2%, and −72.7~7.5%, respectively. Following irrigation, the HW treatment mostly absorbed water in the 0–40 cm soil layer, while the MW and LW treatments absorbed water in both the 0–40 cm and 60–80 cm soil levels. The results indicated that the most optimal drip irrigation method beneath the membrane in this location was achieved when the amount of water applied was between 25–30 mm. This method demonstrated a combination of water conservation, high crop yield, and effective salt suppression.

Molecular dynamics study on roles of surface mixed hydrate in the CH4/CO2 replacement mechanisms

The CH4/CO2 replacement method is a promising approach for the exploitation of natural gas hydrates, offering dual benefits of extracting CH4 and sequestering CO2. This technique not only facilitates the extraction process but also mitigates the greenhouse effect and maintains geological stability, thereby contributing to sustainable energy utilization and environmental preservation. Currently, there are multiple viewpoints about the replacement mechanism. Furthermore, mixed hydrates can form on the surface during the replacement process. The unclear mechanism of CH4/CO2 replacement and the obstruction of mass transfer caused by the mixed hydrate impede its practical applicability. Clarifying the effects of surface mixed hydrate in replacement mechanisms can offer valuable insights for identifying optimal operating conditions in practical exploitation applications. Therefore, the present study aims to reveal the significant roles of surface mixed hydrates in the replacement mechanism by employing the molecular dynamics simulation methodology. The results showed that under the mechanism of cage complete rupture, the surface mixed hydrate decomposes rapidly to form a thickening water zone. Additionally, each layer of the hydrate cage structure undergoes a process from deformation or partial rupture to sudden complete disintegration, which is concomitant with an abrupt change of potential energy. Under the mechanism of cage incomplete rupture, the surface mixed hydrates hinder mass transfer and decelerate the reaction rate. A similar situation arises when the number of the surface mixed hydrate layers is increased. Therefore, the key to distinguishing the replacement mechanism lies in whether the environmental conditions are sufficient to break the potential energy barrier of the surface mixed hydrate and disrupt its cage structure.

Effects of different irrigation methods and mulching on yield, growth and water use efficiency of strawberry

ABSTRACT A greenhouse experiment was conducted from 2015–2016 to 2017–2018 on an Aeric Haplaquept to evaluate the effect of irrigation (surface irrigation at furrows at irrigation water: cumulative pan evaporation = 1, drip irrigation to meet 100%, 80% and 60% of actual crop evapotranspiration demand), and mulch (nonwoven jute agro textile, biodegradable plastic film, rice straw and no mulch) regimes on strawberry. Average soil water storage in root zone (86 mm) and fraction of potential plant available water (61%) was highest in jute agrotextile mulched and drip irrigated at 80% actual evapotranspiration. This combination of mulch and irrigation regime increased belowground (root length density, 7.15 × 103 mm−3) and above ground (leaf area index, 9.4) crop growth and emerged as the best option with three- and four-times higher yield (31.1 Mg ha −1) and water use efficiency (23.1 kg m−3), respectively than surface irrigated unmulched strawberry. Results suggest that these findings be incorporated into packages and practices of strawberry cultivation in the moist tropical sub-humid region of Bengal basin and other similar agroecological regions of South Asia.

Underwater dissimilar friction stir lap welding of aluminum alloy and CFRTP

With the rapid development of the transportation and electronic equipment industries, the demand for robust joining technology for metal/polymer has become imperative, especially in the pursuit of lightweight solutions. Underwater welding is one of the methods to reduce the formation of holes and other welding defects, which is conducive to improve the quality and strength of joints. In this study, the underwater friction stir lap welding (UFSLW) of 6061-T6 Al alloy and carbon fiber reinforced thermoplastic (CFRTP) joints is investigated. The welds are evaluated by optical microscope (OM) image, scanning electron microscope (SEM), and energy dispersive X-ray spectrometer (EDS) of the fracture, tensile, and hardness analysis. Research reveals that the presence of cracks and tunnel defects similar to those in air FSLW (AFSLW) is reduced in the underwater welding process. From the microscopic result analysis, the rapid dissipation of frictional heat and substrate viscosity in the cooling medium of circulating water in the stirring zone reduces the interaction between Al alloy and molten CFRTP, which leads to the rapid transformation of the plasticized material into a solid state and reduces the formation of thick stacked layer. In addition, finer Al alloy fragments are obtained in the welding zone due to the fast cooling rate. Ultimately, the maximum tensile strength of 32.12 MPa is achieved, which is 10.76% higher than that of the same welding parameters in air. Based on UFSLW technology, the joint strength of this study is 69% higher than that of Al alloy and polymer joints, which demonstrates enormous application potential and opens up new prospects for the joining technology of dissimilar materials.

Evaluation of the Impact of Climate Variability on the Soil-Water Characteristics Curve

The hydro-mechanical behavior of unsaturated soil, particularly expansive soil, is influenced significantly by cyclic wetting and drying. Understanding the soil parameters is crucial when evaluating the performance of infrastructures constructed on expansive clay. As a result of extreme rainfall events, highway slopes containing highly expansive Yazoo clay in Mississippi, U.S., become vulnerable to volume change. The phenomenon creates perched water zones within the slopes and poses a risk of slope failure. The soil-water characteristic curve (SWCC) defines the relationship between water content and soil suction, which can be obtained from different laboratory procedures. However, conventional laboratory methods have some limitations. To address this, various analytical and predictive models have been developed, but they can only offer estimates based on soil characteristics and lack seasonal variations occurring in field conditions. Studying seasonal SWCC through field measurements can help understand soil responses to changing moisture conditions. The current study utilized field data from six highway slopes in Mississippi and classified the data into different seasons: spring, summer, and fall. After obtaining van Genuchten parameters from the fitted curve for each season, the finite element method was applied to evaluate the parameters for accurate numerical analysis of infrastructures containing expansive clay. The study observed the variations in flow parameters with seasonal change that cannot be achieved when data from only one season is considered. The findings underscore the importance of field instrumentation data for developing SWCC and the significance of seasonal flow parameters in infrastructure design.

Bottom‐bounce ray angle subspace detector in the shadow zone of deep water

AbstractPassive source detection is a challenging problem in the shadow zone, where the sound is contributed primarily by bottom‐bounce rays. The conventional beamforming detector (CBFD), which utilises the sound energy from a single direction, suffers potential significant performance degradation in the multipath‐signal scenario. The matched field detector (MFD) offers optimal performance by exploiting full‐wave field characteristics but is limited due to its reliance on prior ocean environmental knowledge. The authors demonstrate that the incident sound on a near‐surface vertical line array in the shadow zone can be approximated as a coherent sum of two plane waves that share a symmetric arrival angle about the horizontal. This leads to the signal subspace depending only on the arrival angle, which the authors call the bottom‐bounce ray angle subspace (BRAS). With generalised likelihood ratio test theory, the authors further derive the BRAS detector (BRASD). It can utilise the full signal energy under the premise of weak environmental knowledge requirements and is generally superior to the CBFD. Simulation results in a typical deep‐water channel under different source‐position and array configuration conditions demonstrate the effectiveness of the BRASD and suggest that it can even offer a performance comparable to that of the MFD under specific conditions.

Wastewater irrigation beneath the water table: analytical model of crop contamination risks

Wastewater irrigation alleviates freshwater scarcity. However, conventional (near)surface irrigation techniques directly expose crops to contaminants. Irrigating wastewater into shallow phreatic zones to raise the water table enhances groundwater evapotranspiration, while using the vadose zone as a bioreactor that attenuates contaminants through dilution, adsorption, and biodegradation. Nevertheless, contaminants may spread across the groundwater, soil, and vegetation. In this study, we focus on the crop contamination risks, and derive a simple analytical model to estimate crop solute uptake. Although crops are not directly exposed to the irrigated wastewater, contaminants (and nutrients) may spread to the root zone. Results show that crop contamination is primarily determined by the root zone water balance, and by solute dispersion and biogeochemical reaction parameters. The model contributes towards identifying hydrogeologically and climatically suitable locations for phreatic zone wastewater irrigation, determining acceptable levels of irrigation water quality, and evaluating crop contamination hazards against the fertigative value of wastewater.

An experimental study of alcohol injection to mitigate water blockage in commingled layered reservoirs

This study explores the effectiveness of alcohol injection as a stimulation treatment to mitigate water blockage in the vicinity of the wellbore. Over the years, water blockage has emerged as a recurring challenge within the oil and gas industry, leading to diminished well productivity.In a commingled reservoir with multiple layers, when a producing well is shut-in for well intervention or workover, water may encroach from water zones into the oil-bearing formations through the wellbore due to pressure differences between the layers. Water encroachment can have a significant impact on the production of a well. It can reduce the oil production rate, increase the water cut, and shorten the well's lifespan. Therefore, it is essential to take steps to mitigate the water encroachment effect in commingled reservoirs. This study explores the use of alcohol injection to reduce transition time and remediate formation damage in commingled oil reservoirs. Limited studies have shown that alcohol has the potential to enhance fluid relative Permeability in gas condensate reservoirs and reduce condensate banking near the wellbore. However, this approach has not yet been tested in commingled oil reservoirs.A series of core flooding tests were conducted using Berea sandstone cores and a Texan light crude oil sample. The core flood setup was designed to represent the water blockage condition in the formation. Two different alcohols were tested: isopropyl alcohol (IPA) and Methanol. Experimental results showed that mutual solvents significantly reduce water blockage and shorten the transition period. However, asphaltene precipitation may become a problem with increasing the molecular weight of injected alcohol.

Modeling the Seepage-induced Damage in Water-Rich Fault Zones at Tunnel Faces by Double Point Material Point Methods

During tunnel construction, crossing water-rich fault fracture zones leads to water influx hazards at the tunnel face. Excavation exposes the fault, causing disturbances and changes in the hydraulic gradient. Thus, the tunnel face becomes a low hydraulic head surface. The naturally high permeability of the fracture zone provides a pathway for groundwater flow, resulting in water seepage along the fracture zone and subsequent water influx at the tunnel face. To investigate this interaction, a two-phase double-point material point method was employed aimed at developing a coupled fluid-solid numerical model for excavating submarine fault tunnels. The model incorporates solid-liquid phase interaction, including rock mass permeability, water viscosity, and tunnel geometry. It analyzes the changes in the liquid-phase flow velocity, pore water pressure, and particle trajectories in the fractured zone ahead of the working face after the fracture zone is exposed, revealing the dynamic evolution of water influx following excavation. Additionally, this study discusses the impact of the solid-liquid phase permeability coefficients on the range of water influx hazards. The research findings demonstrate that the two-phase double-point material point method effectively captures the seepage and water influx processes, offering valuable insights into the mechanisms of sudden water influx in tunnel engineering.

Centrifuge Modeling of the Monotonic Capacity of Offshore Ring Anchors in Sand

The development of offshore wind technology has become a feasible solution to meet the increasing demands for clean and renewable energy. The United States has a total of 4250GW offshore wind energy potential; however, 65% of it is in deep water zones (Lopez et al., 2022) where wind turbines with fixed foundations typically are economically and technically unfeasible. In those situations, floating turbines supported by subsea anchors are a more competitive solution. Based on previous studies, ring anchors can be more material-efficient than piles and caissons because they require less material. Ring anchors also perform better than drag anchors due to their greater embedment depth. To further understand the behavior of ring anchors in saturated sand, a series of centrifuge load tests were performed at the University of California Davis Center for Geotechnical Modeling (CGM) at an acceleration of 70g. This test series investigated the effect of the anchor embedment depth and loading angle on the monotonic loading behavior. The ring anchor models were embedded in dense saturated sand, and then connected to an actuator using taut steel wire ropes. Sensors were used to measure the line tension, displacement, and inclination. The results indicate that the ring anchors mobilize greater capacities as their embedment depth is increased and when they are loaded at angles close to the horizontal direction, while vertical loading leads to the smallest capacity. The anchor displacement during the tests deviated slightly from the loading direction, showing a horizontal deviation at the earlier stages of the tests and a vertical one after the peak load. Furthermore, soil disturbance induced by the anchor installation was found to have a strong effect on the vertical capacity of the ring anchors. Overall, this study provides valuable information regarding the monotonic loading behavior of ring anchors which can aid in their future field deployment.

Numerical modeling for representation of wave-induced currents. Application to the port of Ilo-Peru

The objective of this article is to represent the wave-induced currents in the areas near the port of Ilo, with the use of SMC software and a self-developed program in Matlab. In the present study, the parabolic RefDif, for a monochromatic wave model was used for the wave module. The numerical model developed in Matlab allows representing the hydrodynamics of waves and induced currents based on the Navier Stokes equations and mass conservation, in differential form with two spatial dimensions (x, y) and one temporal (t). This model was developed taking into consideration the variables averaged in the vertical z axis and variable draft, considering the phenomenon of protrusion and refraction that the waves suffer when approaching the coastal zone of shallow waters. The model developed in Matlab was validated for the simplified case of straight beaches

Investigating land use changes and trends of hydro morphological indicators on the area and volume of the Ovan Lake's water zone based on the time series of Landsat data

Investigating land use changes and trends of hydro morphological indicators on the area and volume of the Ovan Lake's water zone based on the time series of Landsat data

Elevational Patterns of Forest Evapotranspiration and Its Sensitivity to Climatic Variation in Dryland Mountains

Elevational climatic heterogeneity, complex terrains, and varying subsurface properties affect the sensitivity of evapotranspiration (ET) in dryland mountain forests to hydrometeorological changes. However, the elevational distribution of ET sensitivity and its major influencing factors remain poorly understood. This study focused on the mid-altitude zone (1000–3500 m) forests in the Chinese Western Tianshan Mountains and assessed ET sensitivity to multiple climate variables, including precipitation (P) and potential evapotranspiration (PET), from 2000 to 2020. To evaluate the multi-year mean and trends in ET sensitivity, multi-source remote sensing data and regional survey data were analyzed using Spearman’s correlation coefficient, the sliding window method, and Kendall’s test. Furthermore, the relative importance of environmental variables (topography, geology, soil, and vegetation) was investigated. P and PET showed no significant trends, while ET exhibited a significant increasing trend (5.81 mm/yr, p < 0.01), particularly at elevations above 2000 m. Most forests (93.5%) showed a positive sensitivity of ET to P, and 70.0% showed a positive sensitivity of ET to PET, mainly at elevations of 1500–2500 m. Additionally, the trend in ET sensitivity to P decreased with an increasing elevation, with 64.5% showing a positive trend. Meanwhile, the trend in ET sensitivity to PET increased with elevation, with 88.1% showing a positive trend. Notably, 53.2% of the forests showed increasing ET sensitivity trends to both P and PET, primarily at elevations of 2000–3000 m with a mean normalized difference vegetation index (NDVI) of 0.56. Geological factors, particularly the hydrological properties of weathered bedrock, contributed the most (~47%) to mean sensitivity. However, geological and vegetative factors, including the NDVI and root zone water availability, were the main contributors (35% each) to the sensitivity. This study highlights the elevation-dependent sensitivity of dryland mountain forests to hydrothermal changes, with higher-elevation forests (>2000 m) being more sensitive to global warming.

Iron transformation and hydroxyl radical production during mixing surface water and groundwater in the riparian zone: Effects of bicarbonate and surface water-groundwater ratio

The interaction between surface water and groundwater in the riparian zone significantly influences iron (Fe) transformation. This study characterized the Fe mineral composition of sediments collected from the riparian zones upstream and downstream of the Xinglong Dam of Han River, China. The coexistence of divalent (Fe–CO3) and Fe(III) minerals in the sediments suggested varying redox conditions of the riparian zone. Furthermore, the transformation processes of dissolved Fe(II) under different bicarbonate concentrations and surface water-groundwater ratios during the interaction between surface water and groundwater were investigated by laboratory batch experiments to understand the formation of Fe–CO3 and Fe(III) minerals in the riparian zone. The results demonstrated that elevated bicarbonate concentrations (0–20 mM) and surface water ratios promoted the transformation rate and amount of dissolved Fe(II) to solids. The predominant minerals of the formed solid were lepidocrocite, goethite, and Fe–CO3, according to the results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. The rising bicarbonate concentration facilitated Fe–CO3 synthesis. The Fe(III) mineral (goethite) was more likely to develop when dissolved oxygen (DO) and bicarbonate concentrations were high. The generated hydroxyl radical (•OH) during dissolved Fe(II) oxidation was also increased with elevated HCO3− concentration and surface water ratio, indicating higher oxidative potential in the riparian zone with higher bicarbonate and surface water ratio. These findings offer insights into the Fe transformation in redox-fluctuating zones.

Laboratory and numerical simulations of infiltration process and solute transport in karst vadose zone

Groundwater in karst systems is a vital resource, which is often highly vulnerable to contaminants that infiltrate through the vadose zone. Understanding the processes of water and contaminant infiltration in the vadose zone is extremely important for protecting and managing karst water resources. A laboratory-scale physical model based on a typical conceptual model of the karst vadose zone was constructed to characterize the influence of the rainfall intensity and internal structure on the infiltration process and solute transport in the karst vadose zone under controlled conditions. Five factors, including the rainfall intensity, the surface slope, the degree of karstification, the thickness of the transfer zone, and the existence of epikarst, were considered in laboratory experiments. A corresponding lumped-parameter model was subsequently developed to further analyze and investigate the water and solute infiltration processes in the laboratory scale physical model. Recession- and breakthrough curves generated from the lab experiments and the water and solute loss from each reservoir generated from the simulation results were analyzed. The downward trend of breakthrough curves' peak value and the variation of solute loss's proportion in three rainfall events demonstrate that both the increase in thickness and karstification degree will intensify the buffer effect of the karst vadose zone, and this buffer effect is mainly affected by the slow flow system which can be inferred from the increasing percentage of cumulative discharge of slow reservoir (qS). With increasing karstification degree, the recession coefficient (α) exhibits opposite trends depending on the existence of epikarst or not, indicating the karstification degree has a more significant effect on hydraulic conductivity (K) when an epikarst is present while affecting effective porosity (φ) more noticeably in the absence of epikarst. The rainfall intensity affects the results of both experimental and numerical simulation most significantly. The percentage of discharge and solutes flowing from the slow reservoir (qS and msoil_S) have negative correlations with rainfall intensity, suggesting a greater influence on fast flow with increasing rainfall intensity.