Saturated Zone Research Articles

A case study of canal seepage quantification using gain/loss method and electrical resistivity tomography in an intensively managed water resource system in the Treasure Valley, Idaho, United States

Monitoring groundwater-surface water (GW-SW) interactions is essential for effectively managing the available water resources in semi-arid and arid environments. The focus of this study was to quantify how much SW is being exchanged with the shallow GW aquifer in the Treasure Valley (TV), Idaho, USA. Previous water budgets estimated regional canal seepage without incorporating canal variability and flow measurement uncertainty. To address this, we applied both direct (gain/loss) and indirect (electrical resistivity tomography (ERT)) techniques. Direct seepage measurements were taken on canals capturing a range of different characteristics in the TV. The discrete measurements were then used to estimate the total seepage anticipated to be lost from these canals during the irrigation season. Our findings showed high seepage variability across the canals. The ERT inversion approach was utilized before and after the irrigation season by applying an advanced inversion scheme to better constrain the canal seepage spatial variability and uncertainty by quantifying changes in the saturated zone with 2D-ERT results. Temporal changes in the subsurface resistivity were observed due to the lateral flow from the nearby surface canal during the irrigation season. The combination of these approaches improves our understanding of SW-GW interactions in intensively managed irrigation systems.

Evaluation of the Effects of Rainwater Infiltration on Slope Instability Mechanisms

Mass movements can be caused by factors from different categories, such as geological factors and climate change. From a geological point of view, the soil profile and the geotechnical properties of the materials are crucial in influencing slope instability. From a climate change perspective, rainfall intensity is one of the main triggers of mass movements. Studies related to rainfall infiltration focus on saturated slope zones; therefore, areas of slope stability with infiltration in the unsaturated zone present large gaps. The Brazilian government environmental diagnostics company, the Mineral Resources Research Company (CPRM), identified the municipality of Areia/PB as a danger zone. The region has landslides that occur mostly during the rainy season. Such events lead to the presumption that rainwater infiltration is responsible for the failure of the municipality’s slopes. Thus, the studies proposed in this research aim to determine the influence of precipitation on the stability of the slopes present in the region. The results show that antecedent precipitation has a greater influence on stability, indicating that daily precipitation alone cannot be used as a determinant for landslides. It was concluded that the role of precipitation in slope stability will vary for different locations, with varying surface conditions, variable tropical rainfall, or different microclimatic conditions.

Using hydrochemistry and stable isotopes to character the karst water environment in a cave site, South China

For cave sites located within the seasonal fluctuation zone of groundwater, obtaining a comprehensive understanding of the hydrogeological conditions and hydrochemical environment is of utmost importance in assessing the preservation of cultural deposits. However, due to the heterogeneity of karst development, the hydrochemical environment of the overburden karst water-bearing unit may exhibit complex variations. In this study, the characteristics of the water environment in a foot cave system and its surrounding aquifer were investigated at the Zengpiyan site, employing hydrochemistry, hydrogen and oxygen isotope analysis, as well as sulfate isotope analysis, in conjunction with the hydrogeological background. Based on the exposure of the karst cave and the groundwater flow conditions, the water-bearing medium was categorized into five types. The aquifer exhibited either an oxidation or reduction condition, with a distinct "dissolved oxygen hole" observed in the saturated zone of the cave site. Ion concentration analysis revealed that the groundwater was influenced by the prior accumulation of cinders in the upstream region, leading to sulfate contamination within the foot cave system. However, during the groundwater movement from upstream to downstream, the attenuation rate of pollutants displayed significant variations. Notably, the sulfate content was unusually low within the site cave. Hydrogen and oxygen isotopes provided insights into the differing circulation and movement velocities of groundwater within the local environment. Additionally, sulfate isotopes confirmed the sources of sulfate and the occurrence of bacterial sulfate reduction within the site cave. Consequently, an environmental zoning classification was established based on these analyses. Although the concentration of dissolved ions appears low in the anoxic area, it does not imply a diminished environmental risk. Under reducing conditions, these pollutants can be converted into more aggressive gases, posing a substantial threat to the preservation environment of cave sites. Therefore, sufficient attention should be given to this aspect in order to ensure adequate protection.

Utilizing geophysical methods for assessing groundwater resources in the Dijil River Catchment, Northwestern Ethiopia

This study utilizes geophysical methods to assess groundwater resources in the Dijil River catchment near Debremarkos Town, Northwestern Ethiopia. Recent alluvial deposits and volcanic rocks of varying ages characterize the area. The aim is to map subsurface formations and evaluate groundwater potential. Two hundred twenty-eight magnetic data were collected, mainly oriented in NE-SW, NW-SE, E-W, and N-S directions, and twenty-four Vertical Electrical Sounding data utilizing the Schlumberger configuration. The results of the magnetic data reveal lineaments in different directions in the study area. Most of the Geoelectric sections show three layers, of which the second or the third layers are aquifers having minimum and maximum resistivity of 6 and 155 Ohm.m, respectively, and average resistivity range of 13–50 Ohm.m with a thickness ranging 24–200m. The layers represent alluvial deposits, highly weathered and fractured basalt (major aquifer zone), and moderately to slightly weathered and fractured basalt. Shallow and deeper low resistivity horizons, indicating groundwater saturation zones, are visible. The integrated geophysical survey aligns well with available borehole data.

Epikarst Controls of Runoff Composition in Subterranean Stream After Rainstorm Events

ABSTRACTEpikarst plays a critical role in karst water circulation, however, its studies have often been limited to small springs or basins, ignoring its importance in larger, highly karstified subterranean streams. This study focused on the Longlingong (LLG) subterranean stream system, where the discharge and electrical conductivity (EC) of the subterranean stream outlet for 2.5 years, and found that 85% of all rainstorm events monitored during this period showed a pattern of increasing and then decreasing EC. High‐resolution hydrochemistry monitoring was conducted at the outlet during a rainstorm event when this response pattern occurred. Simultaneously, epikarst water (EW), saturated zone fissure karst water (FW), and surface water (PW) from this system were collected as three recharge sources for hydrochemical analyses based on the results of the field investigations, and end‐member mixing analyses were performed using the EMMA‐MIX model. These combined methods can accurately characterise the response of the subterranean stream system to rainstorm events, assess the contribution and response of individual recharge sources throughout the rainstorm, and reveal the mechanisms involved. The results indicate that the increase in EC is primarily driven by changes in HCO3− concentration resulting from the rapid discharge of EW through sinkholes and shafts. EW contributes 28.7% of the rain‐induced subterranean stream runoff, exceeding FW. The epikarst exhibits a rapid response to rainstorms, as evidenced by a remarkable 681% increase in EW discharge following a rainstorm event. Flood peaks in the subterranean stream are mainly composed of PW (44%) and EW (43.6%). This study highlights the key role of the epikarst as a karst groundwater reservoir. The sensitivity of the epikarst to rainstorms, particularly its role in facilitating rapid piston‐like migration of EW during initial runoff, highlights its significant influence on discharge and hydrochemistry. This research contributes to a deeper understanding of the function of epikarst within highly karstified subterranean streams.

Effect of crack depth on the initiation and propagation of crack-induced sliding in a paleosol area on a loess slope: Three-dimensional investigation based on model testing and laser scanning

On the Chinese Loess Plateau (CLP), crack-induced sliding in paleosols triggers retrogressive slope collapse, compromising the long-term stability of loess slopes. Establishing a quantitative relationship between crack depth and slide size is key to elucidating the progressive evolution of crack-induced sliding in paleosols; however, research on this topic is scarce. In this study, a model test was conducted on a paleosol slope subjected to five drying–rainfall cycles. Three-dimensional (3D) laser scanning technology was employed to accurately quantify the variations in crack depth and slide size, and water content sensors were embedded to monitor the preferential flow changes induced by these cracks. Results reveal that crack depth plays a pivotal role in initiating and controlling the propagation of crack-induced sliding in paleosols by influencing the preferential flow capacity through cracks. In the slide initiation stage, a critical crack depth capable of triggering sliding by inducing a sufficiently significant preferential flow and large saturation zones was identified. The depth at which sliding occurs can be expressed as a variable dependent on crack depth based on the single triggering effect of cracking on sliding. Once a crack-induced slide is initiated, the combined action of prior sliding events and current crack propagation accelerates the subsequent sliding in terms of increased size and varying spatial locations by influencing the preferential flow capacity and sliding force values. A comprehensive empirical formula was established to characterize the depth of crack-induced slides, considering the dominant influence of crack depth and slope gradient on sliding development at this stage of slide evolution. Our findings emphasize the pivotal role of crack depth in triggering progressive crack-induced sliding in paleosols, thereby providing valuable insights for soil conservation in paleosol areas on loess slopes on the CLP.

Estimating permeability of rock fracture based on geometrical aperture using geoelectrical monitoring

The permeability of rock fracture is essential for fluid flow and storage stability in subsurface engineering and geological fluid resource development. Estimating this permeability using geoelectrical monitoring offers a promising approach. However, quantitatively interpreting the relationship between the resistivity (ρ) and permeability (k) in saturation zone is limited by the scarcity of rock-physical models for rock fracture. In this study, the three-dimensional (3-D) fractures, including six groups of fractal dimensions, 25 fracture apertures and six contact areas, were generated using the Weierstrass function method. The resistivity and the k-ρ relationship in these 3-D fractures were investigated through experiments and numerical simulations. Then, the effects of aperture, fractal dimension, and contact area on resistivity were examined. The results show that the resistivity is dominated by contact area when the aperture is below a critical fracture aperture, and by fracture aperture when it is above the critical fracture aperture. Additionally, higher fractal dimensions are found to increase the critical fracture aperture. An empirical formula for describing the k-ρ relationship is established. This study demonstrates that geoelectrical monitoring has potential as a long-term technology for evaluating rock fracture permeability in subsurface geoengineering projects. Once fracture resistivity is monitored in advance, the empirical formula can be efficiently applied in the field for rapid permeability estimation.

Electrical resistivity imaging of crude oil contaminant in coastal soils – A laboratory sandbox study

Characterizing the subsurface distribution of crude oil after a spill in a coastal environment is challenging due to variations in the soil and fluid properties. In situ sampling is limited in capturing the lateral and vertical migration of the crude oil within the vadose and saturated zones. This study presents a laboratory sandbox framework used to assess the effectiveness of electrical resistivity imaging for investigating the spatiotemporal distribution of crude oil in coastal sandy soils. A sandbox with dimensions L = 240 cm, W = 60 cm, and H = 60 cm was constructed using a 10 mm plexiglass and filled to a 40 cm height with 2 mm medium to fine-grained sand. At each stage of the experiment, 20 kg of sand was mixed with 1 l of water to create moist sand, after which the mixture was flushed over 12 h to remove suspended fine particles. Both saturated and unsaturated conditions were simulated by setting the water table at 10 cm and draining a fully saturated system overnight. Two liters of crude oil were spilled and monitored for 30 h. A surface array of 98 electrodes, with a unit electrode spacing of 2 cm, was installed along two transects 12 cm apart. Resistivity measurements were collected using a dipole-dipole array before, during, and after the simulated crude oil spill. The time-lapse electrical resistivity results revealed an initial gravity-induced vertical migration under both saturated and unsaturated conditions; over time, lateral migration of crude oil became apparent. In the saturated zone, there was a noticeable reduction in the percentage difference in resistivity from 700 % to 400 % after 24 h, depicting a spatial and temporal redistribution of the crude oil attributed to variation in pore geometry. This highlights the sensitivity of electrical resistivity measurements to subtle but measurable anisotropy in the distribution of soil pores. Overall, electrical resistivity proved successful in imaging the non-ideal behavior of crude oil pollutants and the associated spatial changes in the pore-size distribution of subsurface sediments.

Environmental fate and transport of PFAS in wastewater treatment plant effluent discharged to rapid infiltration basins

Fate and transport of per- and polyfluoroalkyl substances (PFAS) in wastewater treatment plant (WWTP) effluent discharged to rapid infiltration basins (RIBs) is investigated using data from 26 WWTPs in Michigan, USA. PFAS were found to accumulate in groundwater downgradient from RIBs with median groundwater-effluent enrichment factors for ten commonly detected, terminal-form perfluoroalkyl acids (PFAAs) ranging from 1.3 to 5.2. Maximum contaminant levels for drinking water were exceeded in groundwater at all WWTPs with available PFAS data. Numerical models of unsaturated fluid flow and PFAS transport honoring RIB site properties, such as median vertical separation distance to the water table and a realistic range of area-normalized effluent fluxes, show long-chain PFAS undergo significant delays from air-water interface (AWI) adsorption, requiring up to 15 times longer to reach maximum mass flux to the saturated zone under low-flux conditions, where AWI area is 2.5 times greater. Short-chain PFAS commonly detected in effluent are only minimally affected by AWI adsorption and show little to no attenuation under high-flux conditions. The nonlinear inverse relationship between water content and AWI area highlights the important role of AWI adsorption in modulating unsaturated transport of long-chain PFAS to underlying groundwater due to the broad range of flux rates applied to RIB systems.

Characteristics and Indicative Significance of Groundwater Stable Isotopes in the Loess Plateau at the Regional Scale

Regional groundwater recharge is a critical scientific issue for sustainable groundwater resource development and management. However, spatial variations in groundwater recharge in the Loess Plateau （LP） remain poorly understood. To fill this knowledge gap, a systematic sampling campaign and stable isotope analysis were carried out for groundwater （shallow aquifer） in 13 major catchments during July 2019. The main objectives of this study were： ① to understandthe spatial distribution and influencing factors of stable isotopes in groundwater and ② to reveal the groundwater recharge sources and pathways and their spatial variations, combined with the precipitation stable isotope datasets. Stable isotopes in groundwater had poor spatial variations at the regional scale； however, they became isotopically depleted with the increase in annual average precipitation on the catchment scale （r = -0.87）. Compared with the stable isotope of precipitation, stable isotopes of groundwater were generally depleted and were similar to the precipitation of the rainy season （July-September）. These together indicated that there was pronounced seasonality of groundwater recharge, and the main recharge period was the rainy season. In particular, the recharge seasonality index （δP/G） was closely related to the catchment's average annual precipitation （r = -0.77） and leaf area index （r = -0.63）. In addition, groundwater lc-excess was generally negative, with the catchment-mean value ranging from -4.3‰ to -0.7‰. Hydrologically, this indicated that groundwater recharge pathways （ratio of matrix flow vs. preferential flow） were different among these catchments, which should be quantitatively determined by combining the saturated zone （groundwater） and the unsaturated zone （soil） in future work. Our findings can improve the understanding of groundwater recharge in LP and provide a scientific basis for sustainable management of groundwater resources at the regional scale.

Characterising the discharge of hillslope karstic aquifers from hydrodynamic and physicochemical data (Sierra Seca, SE Spain)

Groundwater flow has been investigated in the Sierra Seca, Spain. Maximum recharge to the central core of the mountain occurs at high elevations, which provides recharge to two overlapping karst aquifers constituting a groundwater storage zone at a lower elevation break in slope. Both karst aquifers are associated with three springs arising from the upper part of the permeable formations. The climate is characterized by long and intense periods of drought and short periods of rainfall, which trigger discharges from the springs. Spring flow recession curve analysis, cross-correlation and monitoring of groundwater temperature and electrical conductivity have demonstrated contrasts observed in the hydrodynamic and physicochemical response of the three springs during flood events. One spring records floods with narrow and short peaks of high discharge accompanied by sharp drops in temperature and electrical conductivity. Another spring records floods with somewhat wider peaks and sharp increases in temperature and electrical conductivity (piston effect), whereas the third spring shows great consistency in all monitored characteristics. It is concluded that the absence of a piston effect in the spring with the highest flow rates is due to the contribution of rapidly circulating water that is expelled by semi-active karst networks (overflow) before reaching the saturated zone, which does not occur in the other springs due to the absence of overflow hydrological pathways. The most regular spring owes its functioning to the contribution of infiltrated water in the bed of an upstream riverbed, which explains this regularity.

Leaching behavior and release mechanism of pollutants from different depths in a phosphogypsum stockpile

Phosphogypsum (PG), a byproduct during the production of phosphoric acid and phosphate fertilizers, is predominantly stockpiled with a height greater than hundreds of meters. In this study, the leaching behavior of pollutants from PG stored at different depths was systematically investigated through batch tests, column tests, and geochemical modeling. PG samples were collected at different depths within a range of 48 m from a large-scale PG stack in China. The results showed that the pH, electrical conductivity, and elemental concentration of the leachate exhibited spatial variability in terms of the depth distribution, with evident bottom enrichment effects for metals and soluble salts. The pH-dependent leaching tests investigated the impact of pH variations on the solubility of various elements in PG, with a specific focus on elements precipitation occurring within the natural pH range. The geochemical modeling of leaching tests conducted by PHREEQC enabled the identification of the dominant phases controlling the solubilization of the elements, as well as the dynamic process of changes in element forms and concentrations with pH variation. Column leaching tests reveal the differences in pollutant properties between the unsaturated and saturated zones within the PG stack and categorize the leaching mechanisms of elements into three models including dissolution, diffusion, and wash-off. This study aims to reveal the leaching characteristics of PG at different depths, so as to provide a data foundation for the design of liner system, leachate management strategies, and remediation of heavy metal pollution of PG stack sites.

A framework and generic models for quantifying surface environmental impact of VOCs emissions from the complex fractured rocks

To analyze the surface cumulative mass of VOCs from residual sources in dual-media fractured rocks and assess environmental health risks, complex 3D numerical models were constructed. These models comprehensively considered fracture-rock interactions, density-driven effects, and surface pressure fluctuations. The investigation identified the key control factors affecting surface cumulative mass, including the fracture aperture, pollutant source location, fracture density, and so on. Additionally, a regression-based general surrogate model was established using the obtained representative dataset. According to U.S. EPA's Respiratory Inhalation Reference Concentrations, the cumulative mass of CH2ClCHCl2 in one day for one-third of the model exceeds the concentration limit. Benzene and TCE concentrations reached 29 and 740 times the reference limits, significantly impacting air quality and health. Surrogate model analysis showed that in the worst-case scenario, 1 min's surface cumulative mass could cause Benzene concentrations to exceed the limit by 57 times. The implications of the study lies in reminding us that even after groundwater remediation in the saturated zone, residual VOCs in the capillary zones can still significantly impact surface environmental health risks. This investigation also presents an effective framework that integrates complex, time-consuming numerical modeling with simple, efficient statistical modeling to predict concerned variables and their uncertainties. This study provides a reference basis for the control of environmental pollution pertaining to VOCs volatilization from buried capillary zones at specific depths.

A self-sustaining effect induced by iron sulfide generation and reuse in pyrite-woodchip mixotrophic bioretention systems: An experimental and modeling study

Dual electron donor bioretention systems have emerged as a popular strategy to enhance dissolved nitrogen removal from stormwater runoff. Pyrite-woodchip mixotrophic bioretention systems showed a promoted and stabilized removal of dissolved nutrients under complex rainfall conditions, but the sulfate reduction process that can induce iron sulfide generation and reuse was overlooked. In this study, experiments and models were applied to investigate the effects of filler configuration and dissolved organic carbon (DOC) dissolution rate on treatment performance and iron sulfide generation in pyrite-woodchip bioretention systems. Key parameters govern that DOC dissolution and microbe-mediated processes were obtained by experiments. The water quality models that integrate one-dimensional constant flow, sorption and microbial transformation kinetics were used to predict the performance of bioretention systems. Results showed that the mixotrophic bioretention system with woodchip mixed in the vadose zone and pyrite in the saturated zone achieves a better performance in both nitrogen removal efficiency and by-product control. Comparably, woodchip and pyrite mixed in the saturated zone could encounter a high secondary pollution risk. The sensitivity coefficients of oxic/anoxic DOC dissolution rates to total nitrogen removal are 0.36 and -2.43 respectively. Iron sulfide generation was affected by DOC distribution and the competition between heterotrophic denitrifiers, autotrophic denitrifiers, and sulfate-reducing bacteria (SRB). DOC accumulation has an antagonistic effect on iron production and sulfate reduction. Extra DOC accumulation favors sulfate reduction while high DOC concentration inhibits pyrite-based denitrification and reduces Fe(III) production. The recycling of iron sulfide can improve the robustness and sustainability of bioretention systems.

Characterizing preferential infiltration of loess using geostatistical electrical resistivity tomography

Preferential infiltration is prevalent in loess and is pivotal in disasters such as landslides. However, the inherent multi-scale heterogeneity of loess makes traditional in-situ monitoring techniques challenging for characterizing the preferential infiltration process. This study employed the Geostatistical Electrical Resistivity Tomography (GERT) to examine the spatial distribution of mass water content (ωs) in loess strata to understand the preferential infiltration processes in loess. This study improved stimulus-response data quality and used GERT to characterize the spatial-temporal distribution of electrical conductivity (σ) and estimate ωs through the σ-ωs relationship. This study indicates that the surface loess layer has higher ωs than deeper layers, rapidly declining as depth increases. Under preferential infiltration, early-stage water forms a spherical saturated zone, transforming into an ellipsoid as it descends. Using equivalent homogeneous parameters as prior information effectively improved σ estimation. This study found a 15.9% error in the total change in water content based on the GERT survey compared to the known amount of injected water. It explores the possible impacts of the uncertainty of the unknown relationship between σ and ωs, its spatial variability, and the accuracy of the survey on the error. The formation of an electric double-layer structure within the loess as the saturation increases, which reduces the sensitivity of GERT to changes in water content, is likely another cause. Finally, these areas are essential for advancing future studies on applying geophysical tools to accurately estimate water distributions in loess formations.

Numerical solution for the interior electric displacement of the moving DBY‐PS model for semi‐permeable cracked piezoelectric material

AbstractIn this study, we analysed a moving crack at the interface of an infinitely long piezoelectric bilayer using the Dugdale–Barenblatt yield (DBY) model and the polarisation saturation (PS) model. To model the moving crack problem, a Yoffe‐type crack moves at a constant subsonic speed on the interface of an infinitely long piezoelectric bilayer. The crack faces are assumed to be semi‐permeable, and at the boundary of the bilayer, in‐plane electrical and out‐of‐plane mechanical stresses are applied. Due to the application of electro‐mechanical loads, cracks propagate, mechanical yielding zones and electric saturation zones are developed. To arrest the crack from further propagation, mechanical yield stress and saturation electric displacement are applied at the developed zones. To address this problem analytically and numerically, the mixed boundary value problem is transformed into a set of coupled Fredholm integral equations (FIEs) of the second kind using the Fourier transform and the Copson method. The closed‐form analytical expressions for the length of the electrical saturation zone (ESZ), whether longer, shorter or equal to the mechanical yielding zone (MYZ), show dependence on external electro‐mechanical loads under semi‐permeable crack conditions. The algorithm to solve the electric crack condition parameter (ECCP) has been defined using numerical discretization and the bisection method. Illustrative examples demonstrate the proposed technique's effectiveness and suitability for Yoffe‐type moving cracks. The numerical results show the convergence of the ECCP. Furthermore, the numerical results show how mechanical and electrical zone lengths and energy release rate (ERR) are affected by electrical and mechanical loads, strip thickness and crack velocity. In addition, the size of the mechanical yielding zone is consistently promoted by electrical load, while the promotion or prevention of the electrical saturation zone by mechanical load depends on the relative sizes of the nonlinear zones.

Microbial community structure and function in a sandy soil column under dynamic hydrologic regimes

The capillary fringe (CF) exhibits steep chemical and redox gradients, representing a critical reactive interface in the subsurface. The spatial extent of the CF is temporally dynamic because of water table and soil moisture fluctuations. To simulate the spatio-temporal responses of the CF’s microbial community structure and activity, a 91 cm long sandy soil column experiment was conducted by imposing three different sequential hydrologic regimes: a stable water table separating the unsaturated and saturated zones (static regime), periodic pulse infiltration events with a constant water table position (infiltration regime), and water table fluctuations under three drainage-imbibition cycles of 6, 12, and 18 days (fluctuating regime). The investigation focused on the microbially-mediated turnover of nitrogen (N) and carbon (C). Geochemical profiles in the soil column were monitored continuously, while microbial DNA was extracted for 16S rDNA sequencing at the start, middle and end of the experiment. The observed microbial community compositions corresponded to five categories: (1) initial soil, (2) near-surface and (3) CF and saturated soil under the infiltration regime, (4) unsaturated and (5) saturated soil in the fluctuating regime. Based on co-occurrence network analysis, microbial physiologies matched the predominant geochemical conditions, and relatively stable community structures prevailed throughout the experiment. NO3- and NH4+ were more available to the microbes in the fluctuating regime’s alternating oxic/anoxic zone, leading to a more flexible network. The prevalent microbial metabolic pathways predicted using FAPROTAX were mainly associated with N and C metabolisms and significantly correlated with the taxonomic compositions. However, the dominant microbial groups were shared among all the soil samples. The limited spatial differences in microbial community structure under the fluctuating regime were likely due to a more pronounced role of microbial activity than microbial diversity, and similar drying-wetting conditions between the soil column and the filed site. Internally consistent geochemical and microbial depth stratifications were identified by (1) the redox potential and nitrogen species distributions, (2) spatial moment analysis of NO3- and NH4+ concentrations, (3) microbial community composition and (4) function. The five imposed hydrologic regimes generated distinct and coupled geochemical and microbial signatures. These signatures resulted from variations in the soil microbial community’s activity, rather than diversity or abundance.

The impact of lenses on the seepage failure of tailings dam.

The presence of lenses such as tailings slurry, frozen soil, and saturated zones disrupts the continuity of tailings dams and their normal seepage patterns, elevating the seepage line of the dam body and significantly impacting local stability. This study, to investigate how lenses affect the stability and failure mechanisms of tailings dams, employs numerical simulation and physical models and constructs a model of the tailings dam, incorporating tailings clay lens and void lens, to investigate variations in hydraulic gradients, seepage velocities, seepage flow, pore water pressure, and the patterns of seepage failure. This research reveals that the tailings clay lens within the dam body increases the hydraulic gradient in its vicinity due to its low permeability and raises the phreatic line. As the tailings clay lens approaches the dam body, the phreatic line tends to escape along the upper part of the lens towards the dam surface. In addition, the void lens could lead to a more pronounced seepage gradient along its path on the dam surface, with a liquefaction beneath it. As the void lens nears the toe of the slope, the dam failure mode transitions from a step-like progressive failure to an arch-shaped settlement failure along the void lens.

Short high-accuracy tritium data time series for assessing groundwater mean transit times in the vadose and saturated zones of the Luxembourg Sandstone aquifer

Abstract. Among the manifold of environmental tracers at hand, tritium is the only one that can give information on groundwater age within the timescale of 100 years for the entire flow system, i.e., unsaturated and saturated. However, while in the Southern Hemisphere, a single water sample is sufficient for tritium-based young groundwater dating, several tritium measurements spanning multiple years are still needed in the Northern Hemisphere to disentangle the natural cosmogenic tritium input from that caused by the atmospheric thermonuclear weapons tests mainly carried out in the early 1960s. Although it is advised to focus tritium dating on sites where long chronicles of tritium data are available, in this study we tested the potential for short high-accuracy tritium data series (∼4 years) to date groundwater from 35 springs draining the Luxembourg Sandstone aquifer (central western Europe). We determined groundwater mean transit times using the lumped-parameter model approach in a Monte Carlo uncertainty estimation framework to provide uncertainty ranges inherent to the low number of tritium data at hand and their related analytical errors. Our results show that unambiguous groundwater mean transit time assessments cannot be determined solely based on such recent short tritium time series, given that several ranges of mean transit times appeared theoretically possible. Nonetheless we succeeded in discriminating groundwater mean transit times in the vadose and saturated zones of the aquifer through a stepwise decision process guided with several supplementary data. The mean transit time required for water to cross the vadose zone was estimated to be between 0.5±0.5 and 8.1±1.2 years depending on the spring, while for water to flow through the saturated zone, it varied from 5.7±2.4 to 18.9±4.6 years (median ± half of the 5–95 percentile range). Our findings are consistent with both the tritium measurements of individual springs and the hydrogeological context of the study area. We specifically corroborated the dating results using the known hydrogeological properties of the Luxembourg Sandstone aquifer, the hydrochemistry of the studied springs, and their discharge dynamics. When translated into water velocities (which average ∼12 and ∼170 m yr−1 for the vadose and the saturated zones, respectively), the tritium dating results mirrored the horizontal–vertical anisotropy of the aquifer's hydraulic properties caused by the bedded character of the Luxembourg Sandstone. In addition to improving our understanding of water transit times in the Luxembourg Sandstone aquifer, this study demonstrates how it is currently possible to use short tritium time series to date young groundwater bodies at new sites in central Europe.

Big Disaster from Small Watershed: Insights into the Failure and Disaster-Causing Mechanism of a Debris Flow on 25 September 2021 in Tianquan, China

The occurrence of debris flow events in small-scale watersheds with dense vegetation in mountainous areas that result in significant loss of life and missing individuals challenges our understanding and expertise in investigating and preventing these disasters. This has raised concerns about the occurrence of large debris flow disasters from small watersheds. This study focused on a catastrophic debris flow that took place in Longtou Gully (0.45 km2) in Tianquan County, Ya’an City on 25 September 2021, which resulted in 14 deaths and missing individuals. Through comprehensive field investigations, high-precision remote sensing data analyses, and numerical simulations, we analyzed the triggering mechanisms and dynamic processes of this event. Our results indicate that the convergence hollow at the channel head exhibited higher hydraulic conditions during rainfall compared to gentle slopes and convex terrains, leading to the instability of colluvial soil due to the expansion of the saturated zone near the soil–bedrock interface. The entrainment of material eroded from the channel resulted in an approximately 4.7 times increase in volume, and the channel scarp with a height of about 200 m amplified the destructive power of the debris flow. We emphasize the need to take seriously the possibility of catastrophic debris flows in small-scale watersheds, with colluvial deposits in hollows at the channel head under vegetation cover that serve as precursor material sources, and the presence of channel scarps formed by changes in the incision rate of the main river, which is common in the small watershed on both sides. This study provides insights for risk assessment of debris flows in small-scale catchments with dense vegetation cover in mountainous areas, highlighting the importance of vigilance in addressing disasters in small-scale catchments, particularly in regions with increasing human–environment conflicts.