Vadose Research Articles

Influence of Liquid Splitting Behavior at Intersections on Infiltration Dynamics in an Unsaturated Fracture Network

AbstractUnderstanding the macro‐scale flow characteristics in the fractured vadose zone is of great importance for subsurface hydrological and environmental applications. Here we develop an idealized fracture network model composed of a series of linked intersections, aiming to reveal the roles of local fluid flow, storage and splitting behaviors at intersections in controlling macroscopic unsaturated flow in a fracture network. By setting different local flow rules, unsaturated flow dynamics and structure in networks are systematically investigated. Numerical simulations suggest that avalanche infiltration mode, that is, sudden release of a large amount of water from the network, emerges spontaneously, and is modulated by the local splitting behavior. In terms of water infiltration structure, we find that uneven liquid splitting at intersections inevitably leads to preferential flow in networks. The preferential paths strongly rely on the competition between gravitational and capillary forces, relating to network structure and water saturation. When flow is dominated by gravity, preferential paths extend along large‐aperture routes; conversely, when flow is dominated by capillarity, the small‐aperture paths are preferred. In terms of infiltration dynamics, we show that the power spectral density of the water saturation time series in the network follows a power law with an exponent of −2 for all simulations with different structural parameters and local flow rules, suggesting a universal self‐organized criticality behavior for unsaturated flow in fractured rocks. The improved understanding from this study may shed new light on the complex flow dynamics for unsaturated flow in fractured media.

Modeling moisture infusion in ceramic using Richards equation: Experimental and analytical validations, and exploration of three time-dependent wetting scenarios

Spontaneous imbibition of water into ceramic tile is modeled using the well-known Richards equation (RE) employed for modeling unsaturated flow in the vadose zone. The Brooks and Corey model for relative permeability and Van Genuchten model for capillary pressure is employed to generate the final form of RE that predicts moisture migration in porous ceramic. COMSOL is employed to numerically solve RE in a cuboid, block like ceramic geometry. The simulation results, obtained using the mesh optimized through a grid independence test, are validated in three different ways. First, the simulation predictions are compared with the experimental results obtained from a neutron microscope imaging study of wetting of a ceramic block. Good qualitative and quantitative matches were obtained. Later, the simulation predictions for a 1-D moisture migration situation were compared with two different published analytical solutions, and once again, good agreements were observed. After the code validation, simulations for moisture migration through evolving saturation plots were carried for three different cases of the ceramic-block top getting wetted by falling drops: a single droplet, two droplets with a time gap, and continuous droplets. Evolution of saturation plots were studied where the third case was observed to cause the maximum water infiltration into ceramic.

Groundwater Vulnerability Assessment and Protection Strategy in the Coastal Area of China: A GIS-Based DRASTIC Model Approach

Groundwater vulnerability reflects the risk level of groundwater contamination and its self-repairing ability, as well as its sustainability for use. Therefore, it provides significant scientific support for implementing measures to prevent groundwater contamination, especially in coastal areas. In this study, considering the lithology of vadose in valley plains and the extent of karst subsidence areas, a GIS-based DRASTIC model was employed to assess groundwater vulnerability in Tangshan City, a coastal area in China. The assessment results were presented and mapped using GIS, based on a comprehensive evaluation of seven parameters, including “Depth of groundwater, Vertical net recharge, Aquifer thickness, Soil media, Topography, Impact of vadose zone, and Hydraulic conductivity”. The identified groundwater vulnerability zones included the highest, higher, moderate, low vulnerability those four zones, which accounted for 4%, 53%, 25%, and 18%, respectively. In addition, according to the results of field investigation, the karst subsidence area and the mined-out coastal area were directly classified as the highest vulnerable areas and covered 1.463 km2; more attention is required here in subsequent groundwater protection processes and strategies. Finally, the groundwater pollution index was used to validate the groundwater vulnerability distribution results, and these two were in high agreement, with an R2 coefficient of 0.961. The study is crucial for the rational utilization and protection of water resources in Tangshan City.

Effects of Sodium Selenite on the Rhizosphere Environment, Growth, and Physiological Traits of Oilseed Rape (Brassica napus L.)

Soil selenium (Se) speciation characteristics and their influence on the Se enrichment pattern and physiological characteristics of oilseed rape are poorly understood. We investigated dynamic changes in rhizosphere soil physicochemical properties, Se uptake and partitioning, biomass, and physiological indices in oilseed rape under five exogenous Se condition levels (0, 1.0, 2.5, 5.0, and 10.0 mg kg−1 Se in sodium selenite) using soil cultivation experiments. The rhizosphere pH and dissolved organic carbon in the soil solution were higher than those of the non-rhizosphere soil solution. The total Se, water-soluble Se, exchangeable Se, and organic Se contents in soils, as well as rapeseed root/leaf Se contents, significantly increased with increasing exogenous Se. Under 2.5 mg kg−1 Se, the biomass of rapeseed roots and leaves increased at the sixth week (82% and 58%) and eighth week (48% and 32%), respectively, reaching the highest level. Applications of 5.0 mg kg−1 Se at 6 and 8 weeks significantly increased the glutathione peroxidase activity (49%/82%), and decreased malondialdehyde content (23%/39%). Canonical correlation and ridge regression analyses showed that Se in the rapeseed roots/leaves significantly and positively correlated with water-soluble Se, exchangeable Se, and organic Se in rhizosphere soil. Overall, moderate-concentration Se soil application benefited oilseed rape growth (optimum = 2.5 mg kg−1 Se). Our findings reveal the response of oilseed rape to soil Se application based on plant growth and physiological traits, rhizospheric soil solution properties, and Se speciation transformation.

Sensitivity analysis of the hyperparameters of an ensemble Kalman filter application on a semi-distributed hydrological model for streamflow forecasting

Data Assimilation (DA) is an important step for improving prediction accuracy and real-time correction of hydrological models for operational forecasting purposes. The Ensemble Kalman filter (EnKF) is one of the most popular techniques used to address the issues of updating the model states and parameters by creating a novel set of initial conditions in real-time. This study aims at identifying optimal seasonal EnKF parametrizations to reduce the uncertainty associated with the initial conditions in a semi-distributed hydrological model of a snow-dominated catchment in Canada. Sensitivity analysis is performed to evaluate the effects of the EnKF individual hyperparameters (temperature, precipitation and inflow uncertainties) and the updating of three state variables (vadose zone, saturated zone and snowpack) on the skill of short-term (up to 9 days lead-time) forecasts. Results show that the performance of the forecasts is sensitive to the individual hyperparameters and particularly so to the temperature uncertainty, which varies between seasons. Additionally, the forecast skill is related to the choice of the state variables to be updated depending on the season. The vadose zone state variable displays higher importance and sensitivity than the other states, and the results indicate that all state variables should not be systematically updated on this catchment and with the implemented hydrological model. Finally, combining the best hyperparameter values with the optimal combination of state variables to update, insight is provided on the success of the data assimilation scheme that is evaluated on a rolling horizon using a set of seasonal rules to gain forecast performance over the span of multiple years.

A Novel Analytical Solution for Ponded Infiltration With Consideration of a Developing Saturated Zone

AbstractPonding at the soil surface exerts profound impacts on infiltration. However, the effects of ponding depth on infiltration, especially the development of a saturated zone below the soil surface, have yet to be considered in present infiltration models. A new general Green‐Ampt model solution (GAMS) was derived for a one‐dimensional vertical infiltration problem under a uniform initial moisture distribution with ponding on its surface. An expression was included in the new solution for simulating the saturated layer developed below the soil surface as long as the pressure head at the surface is sufficiently high to saturate the soil. The GAMS simulates the infiltration processes closer to the numerical solution by HYDRUS‐1D than the traditional and the recently improved Green‐Ampt model. Moreover, an inversion method to improve the estimates of soil hydraulic parameters from one‐dimensional vertical infiltration experiments that is based on the GAMS was suggested. The effect of ponding depth (hp), initial soil moisture content, soil texture, and hydraulic soil properties (saturated hydraulic conductivity Ks, water‐entry suction hd and shape coefficient n of soil water retention curve) in the saturated zone was also evaluated. The results indicate that the saturated zone length increased at a comparable rate with the unsaturated wetted zone length during infiltration. Generally, a larger saturated zone was found for soils with higher initial soil moisture contents, coarser texture, higher Ks values, greater n, and lower −hd. Our findings reveal that including the saturated zone in the infiltration model yields a better estimate of the soil hydraulic parameters. The proposed GAMS model can improve irrigation design and rainfall‐runoff simulations.

Quantifying the relative contributions of different flood generating mechanisms to floods across CONUS

The occurrence of flooding in a catchment is attributable to various mechanisms. Several researchers have used linear models to quantify the relative contributions of different flood generating mechanisms. However, the use of linear models for this quantification may not be suitable given the complex and nonlinear processes from climate to floods. In this study, we used nonlinear machine learning models with interpretability methods to quantify the contributions of multiple flood generating mechanisms to floods across the continental United States (CONUS). Four flood generating mechanisms were considered comprehensively, which varied in terms of the influence of antecedent soil moisture condition or snowmelt on flood generation. These mechanisms involved three common mechanisms (rainfall, rainfall excess, and snowmelt/rain on snow) and one complex mechanism (effective precipitation) that accounts for snowmelt/rain on snow lost to unsaturated soil zone. Consistent with previous studies, we observed that rainfall alone (i.e., without considering other hydrometeorological processes) is seldom the dominant mechanism for flood generation. Floods in most catchments are largely attributable to rainfall excess and snowmelt. Unlike previous studies, we noted that in the midwest and northeastern CONUS, snowmelt/rain on snow upon saturated soil was the greatest contributor to local flood generation. This finding highlights that both snowmelt and soil moisture condition affect flood generation in the midwest and northeastern CONUS, instead of only snowmelt as has been reported previously. Overall, this study provides new insights into flood generating mechanisms in CONUS and demonstrates the potential of applying interpretable machine learning to examine flood generating mechanisms.

Assessment of Metalaxyl migration through vadose zone of alluvial sandy soil using column experiment and HYDRUS numerical modeling

Contemporary research on pesticides/fungicides as potential sources of groundwater contamination, including their migration pathways, especially in the Western Bengal basin (WBB), is scarce. The present research intends to study the vulnerability of groundwater towards pollution from metalaxyl. Metalaxyl is a fungicide added anthropogenically to the sandy soil of WBB for the cultivation of crops like tomatoes, potatoes and mustard. The study explores the mechanics of metalaxyl adsorption in soil and its migration to the associated groundwater system. Chemical analyses show high concentrations of metalaxyl within groundwater (472.9 μg/L, maximum amount) from the study area (Nadia district of WBB). The groundwater ubiquity score of metalaxyl (4.6) depicts that it is very much prone to leach through the sandy soils of WBB to the underlying groundwater system. The results of column leaching experiments and their congruence to the findings of numerical modelling study using HYDRUS software confirm the fact. The adsorptive resilience of the studied soils towards metalaxyl is insignificant (soils of North Chandmari (S1) =0.1087 mg/g, Ghoragacha (S2) =0.21 mg/g, and Khaldarpara (S3) =1.771 mg/g). However, the presence of excess iron concentration may enhance the adsorptive capacity of the soil toward Metalaxyl, thereby limiting its migration toward the zone of saturation.

Hydrogen and oxygen isotope signal transmission in rainfall, soil water, and cave drip water in Liangfeng Cave, Southwest China

δD and δ18O isotope signatures were widely used as tracers to investigate recharge processes of rainfall transfer into caves in the vadose zones of karst regions. The present research systematically monitored rainfall, soil water, and drip water at the Liangfeng Cave in Guilin City, China, from January 2020 to September 2022, as these aspects remain poorly quantified. The δD and δ18O compositions of rainfall were depleted during the rainy season and enriched during the dry season. The local meteoric water line (LMWL) was described as δD = 7.98δ18O + 11.52. The dry season is mainly characterized by resident soil water, with little mobile soil water, whereas the primary source of recharged drip water is stored bedrock water on the top of the cave. During the rainy season, resident and mobile soil water exchanged with each other, resulting in homogenous δD and δ18O compositions across different soil depths and indicating a lack of ecohydrological separation; however, δD and δ18O signature in drip water may differ from the original observed in rainfall, suggesting that the residence time affected the response time of the drip water to rain. Moreover, mixed stored older water in the overlying bedrock was the primary drip water recharge source each season; thus, drip water's isotope amplitude values were more depleted than rainfall's. Distinct flow paths also created differences in the lag time and amplitude at each drip site. During high-intensity rain, the isotopic signals were rapidly transmitted by preferential flow at different soil depths and via drip water. The δD and δ18O signals in the drip water showed significant depleted excursions several months after high-intensity rainfall. These findings indicate that ecohydrological separation did not occur under any circumstances within the study area, and care should be taken when interpreting significant depleted excursions of δD and δ18O signals in drip water during the summer monsoon or the amount of rainfall in stalagmites across seasonal or interannual scales.

A glimpse of the paleome in endolithic microbial communities

BackgroundThe terrestrial subsurface is home to a significant proportion of the Earth’s microbial biomass. Our understanding about terrestrial subsurface microbiomes is almost exclusively derived from groundwater and porous sediments mainly by using 16S rRNA gene surveys. To obtain more insights about biomass of consolidated rocks and the metabolic status of endolithic microbiomes, we investigated interbedded limestone and mudstone from the vadose zone, fractured aquifers, and deep aquitards.ResultsBy adapting methods from microbial archaeology and paleogenomics, we could recover sufficient DNA for downstream metagenomic analysis from seven rock specimens independent of porosity, lithology, and depth. Based on the extracted DNA, we estimated between 2.81 and 4.25 × 105 cells × g−1 rock. Analyzing DNA damage patterns revealed paleome signatures (genetic records of past microbial communities) for three rock specimens, all obtained from the vadose zone. DNA obtained from deep aquitards isolated from surface input was not affected by DNA decay indicating that water saturation and not flow is controlling subsurface microbial survival. Decoding the taxonomy and functional potential of paleome communities revealed increased abundances for sequences affiliated with chemolithoautotrophs and taxa such as Cand. Rokubacteria. We also found a broader metabolic potential in terms of aromatic hydrocarbon breakdown, suggesting a preferred utilization of sedimentary organic matter in the past.ConclusionsOur study suggests that limestones function as archives for genetic records of past microbial communities including those sensitive to environmental stress at modern times, due to their specific conditions facilitating long-term DNA preservation.5FvsXWM5isfqTEMW9i73VYVideo

Revealing the infiltration process and retention mechanisms of surface applied free DNA tracer through soil under flood irrigation

It has been recently demonstrated that free DNA tracers have the potential in tracing water flow and contaminant transport through the vadose zone. However, whether the free DNA tracer can be used in flood irrigation area to track water flow and solute/contaminant transport is still unclear. To reveal the infiltration process and retention mechanisms of surface applied free DNA tracer through soil under flood irrigation, we tested the fate and transport behavior of surface applied free DNA tracers through packed saturated sandy soil columns with a 10 cm water head mimicking flood irrigation. From the experimental breakthrough curves and by fitting a two-site kinetic sorption model (R2 = 0.83–0.91 and NSE = 0.79–0.89), adsorption/desorption rates could be obtained and tracer retention profiles could be simulated. Together these results revealed that 1) the adsorption of free DNA was dominantly to clay particles in the soil, which took up 1.96 % by volume, but took up >97.5 % by surface area and densely cover the surface of sand particles; and 2) at a pore water pH of 8.0, excluding the 4.9 % passing through and 3.1 % degradation amount, the main retention mechanisms in the experimental soil were ligand exchange (42.0 %), Van der Waals interactions (mainly hydrogen bonds), electrostatic forces and straining (together 44.7 %), and cation bridge (5.3 %). To our knowledge, this study is the first to quantify the contribution of each of the main retention mechanisms of free synthetic DNA tracers passing through soil. Our findings could facilitate the application of free DNA tracer to trace vadose zone water flow and solute/contaminant transport under flood irrigation and other infiltration conditions.

The Nitrate Fate Tool: A Decision Support System for the Assessment of the Groundwater Vulnerability to Nitrate in Support of Sustainable Development Goals

This article introduces a new web-based and freely accessible tool, the Nitrate Fate tool (NFt), for the assessment of groundwater vulnerability to nitrate pollution in a variety of pedoclimatic conditions. The contamination of water resources by nitrate, in fact, represents a growing and persistent global environmental problem, and the utilization of practical tools to assist personnel working in the agricultural sector is key for mitigating the impact on land use, while maintaining farmers’ incomes. The (NFt) has been developed and integrated into the geospatial decision support system, LandSupport, as a way to support multiple stakeholders in conducting the so-called what-if scenario analysis (e.g., what would happen to the crop production if I substitute a quote of inorganic fertilizer with the same quote of an organic one?). The tool couples a state-of-art crop-growth model—which simulates crop growth dynamics, the nitrogen and carbon cycles—with a novel transfer function model in order to assess the transport of nitrate through the unsaturated zone to the groundwater table. Within the LandSupport platform, the results are shown both as coloured maps and as cumulative charts representing the travel times and the concentrations of root leachate to groundwater table depths. This work details the tool’s rationale, the coupling of the models, and their implementation. Moreover, this article shows examples of applications supporting several public authorities and end-users, underlining that, by combining all of the information on soils, groundwater table depths, management and climates, it is possible to obtain a comprehensive understanding of nitrogen transport dynamics. Two case studies are presented: the Piana del Sele and the eastern plain of Naples, both located in the Campania region of Italy. The results of the tool’s applications reveal significant groundwater vulnerability in both plains, mainly due to the shallow groundwater table depths, resulting in remarkably fast mean nitrate travel times ranging from 0 to 6 years. Finally, the tool provides a reproducible and replicable solution, and future implementation is foreseen for additional case studies all over the world.

Groundwater Quality and Suitability Assessment for Irrigation Using Hydrogeochemical Characteristics and Pollution Indices: A Case Study of North Al-Quwayiyah Governorate, Central Saudi Arabia

The present work focuses on a comprehensive hydrochemical assessment of groundwater within a shallow aquifer located in the central region of Saudi Arabia. This aquifer serves as the principal source of groundwater supply for agricultural irrigation purposes. A total of twelve groundwater samples were systematically collected and subjected to thorough analysis to determine various physicochemical parameters. These parameters encompassed electrical conductivity (EC), pH, temperature, total dissolved solids (TDS), as well as concentrations of major ions. Analysis of major ion analysis was employed to elucidate the chemical attributes of groundwater within the research area. This approach facilitated an understanding of the geochemical evolution and the potential suitability of the groundwater for agriculture irrigation. The recorded range of electrical conductivity (EC) for the groundwater in this region falls between 3283 and 11,000 µS/cm, with an average value of 6709.4 µS/cm. The pH levels of the groundwater exhibited a slightly alkaline nature, ranging from 7.8 to 8.6. All sampled wells exhibited brackish water conditions (TDS > 1000 mg/L) based on TDS concentrations. The findings of this investigation demonstrated that the average ion concentration followed the order: Na+ > Ca+ > Mg2+ > K+ and Cl− > SO42− > HCO3− > NO32−. Applying Piper’s classification system, the groundwater samples were classified into two predominant categories: Ca-Cl and mix-Ca-Mg-Cl. The predominance of Ca2+ and Mg2+ over alkalis indicated mixed groundwater facies influenced by processes of reverse ion exchange and extensive interactions between water and rock formations. The distinct chemical characteristics of the groundwater were attributed to a combination of factors, including the percolation of irrigation-returned water, chemical transformations within the vadose zone, and extensive interactions with subsurface lithology. Despite major ion concentrations surpassing the permissible limits outlined by the World Health Organization (WHO) for drinking water, the assessment of quality indices was conducted to ascertain the suitability of the groundwater for irrigation purposes. These quality indices encompassed the permeability index (PI), magnesium hazard (MH), Kelly’s ratio (KR), sodium-adsorption ratio (SAR), residual sodium carbonate (RSC), sodium percentage (Na%), and magnesium ratio (MR). A meticulous evaluation of groundwater quality for agricultural utilization indicated that all sampled groundwater sources were deemed suitable for irrigation purposes.

Sulfite Poses a Risk of Hexavalent Chromium Rebound in Vadose Zone: A Challenge of the Stability of CrxFe1-x(OH)3.

Cr(VI) rebound is the primary risk associated with the reduction remediation of Cr(VI)-contaminated soil. The potential impact of sulfites, which can be produced by microbial activities or originate from sulfur-containing remediation agents, on the Cr(VI) rebound in the vadose zone has been overlooked. When sulfites are present, the stability of CrxFe1-x(OH)3 is compromised and significantly inferior to that of Cr(OH)3, as demonstrated in this paper. First, Fe acts as a catalyst for the conversion of adsorbed sulfite to SO4·-, which subsequently triggers the oxidation of Cr(III) and results in the rebound of Cr(VI). The heterogeneous catalysis by Fe on the surface of CrxFe1-x(OH)3 plays a predominant role, contributing to 78% of the actual oxidation of Cr(III) among all employed catalytic processes. The presence of ambient Cl- can exacerbate the rebound effect of Cr(VI) by promoting the generation of HOCl. Furthermore, a portion of released Cr(VI) was reduced to Cr(III) by dissolved sulfite in the presence of dissolved Fe as a catalyst, thereby increasing the dissolution and migration risk associated with CrxFe1-x(OH)3. Hence, the presence of sulfites results in a significant increase in the Cr(VI) rebound and Cr(III) release from CrxFe1-x(OH)3. This challenges the conventional understanding of the stability of CrxFe1-x(OH)3.

Upper-plate conduits linked to plate boundary that hosts slow earthquakes

In shallow subduction zones, fluid behavior impacts various geodynamic processes capable of regulating slip behaviors and forming mud volcanoes. However, evidence of structures that control the fluid transfer within an overriding plate is limited and the physical properties at the source faults of slow earthquakes are poorly understood. Here we present high-resolution seismic velocity models and reflection images of the Hyuga-nada area, Japan, where the Kyushu-Palau ridge subducts. We image distinct kilometer-wide columns in the upper plate with reduced velocities that extend vertically from the seafloor down to 10–13 km depth. We interpret the low-velocity columns as damaged zones caused by seamount subduction and suggest that they serve as conduits, facilitating vertical fluid migration from the plate boundary. The lateral variation in upper-plate velocity and seismic reflectivity along the plate boundary correlates with the distribution of slow earthquakes, indicating that the upper-plate drainage system controls the complex pattern of seismic slip at subduction faults.

A comparative study of DNAPL migration and transformation in confined and unconfined groundwater systems

To explore the migration and transformation process of dense non-aqueous liquid (DNAPL) pollutants’ multiphase flow, specifically nitrobenzene (NB), in confined groundwater (CG) versus unconfined groundwater (UG), a two-dimensional sandbox experimental device was designed and constructed. This involved constructing a vadose zone-UG- aquitard-CG structure, which was then subjected to different scenarios. Real-time analysis and numerical simulation methods were established and employed, with a particular focus on the detailed investigation results of actual contaminated site. The study found that when the same amount of NB was injected, the special structure of the CG layer resulted in a more pronounced reverse diffusion of NB in both the dissolved and NAPL phases. This was especially true for the dissolved phase, which was more likely to diffuse reversely. Meanwhile, CG did not directly interact with the vadose zone, and there was no loss of gas phase NB after the leakage in CG. As a result, higher concentrations of dissolved phase NB were generated, leading to the emergence of a larger area of NB contaminant plumes with CG flow. Importantly, the simulation study of the actual site and the laboratory experimental results were found to be validated, further validating the conclusion that direct leakage of NB into CG results in a higher concentration and larger area of dissolved phase contaminant plume, causing more serious pollution to the groundwater environment.

Environmental Assessment of Soil and Groundwater Pollution by BTEX Leaching in Valencia Region (Spain)

The impact of hydrocarbon spills in the unsaturated zone is a significant environmental concern, particularly in locations where contamination arises from leaks in underground fuel storage tanks (USTs). This paper presents the outcomes achieved through the utilization of VLEACH, a finite-difference numerical model, to assess the concentrations of organic contaminants in the solid, liquid, and gas phases within the vadose zone. Additionally, it evaluates the mass transfer of pollutants to the aquifer as part of an environmental assessment for the placement of a forthcoming service station. The study encompasses an analysis of 18 scenarios under realistic conditions based on actual field data. These scenarios were constructed, taking into account various factors, including the nature of the leak (one-time or permanent), the depth of the phreatic level, and the soil conditions and properties. The results highlight the potential environmental consequences of a permanent leak as compared to those resulting from a specific accident. The findings further emphasize the substantial influence of soil moisture on transport phenomena within the vadose zone. Variations in soil moisture significantly alter hydraulic conductivity, impact magnitudes, transport velocities, and even the predominant transport mechanism. Consequently, precise delineation of soil moisture becomes a crucial parameter in such simulations. Additionally, it has been observed that each component of BTEX (benzene, toluene, ethylbenzene, and xylene) experiences varying transport velocities through the vadose zone. Benzene, having a greater propensity to concentrate in the liquid and gas phases, exhibits the swiftest movement through the vadose zone. The detection of benzene in aquifers can serve as an indicator of the potential future arrival of the remaining BTEX compounds.

Assessing Terrestrial Water Storage Variations in Southern Spain Using Rainfall Estimates and GRACE Data

This paper investigates the relationship between rainfall, groundwater and Gravity Recovery and Climate Experiment (GRACE) data to generate regional-scale estimates of terrestrial water storage variations in the Andalucía region of southern Spain. These estimates can provide information on groundwater depletion (caused by periods of low rainfall or droughts) and groundwater recovery. The spatial distribution of groundwater bodies in southern Spain is complex and current in situ groundwater monitoring methods are deficient, particularly in terms of obtaining representative samples and in implementing and maintaining groundwater monitoring networks. The alternative approach proposed here is to investigate the relationship between precipitation time series and changes in the terrestrial water storage estimated from GRACE observations. The results were validated against the estimated fluctuation in regional groundwater. The maximum correlation between the mean groundwater level and the GRACE observations is 0.69 and this occurs at a lag of one month because the variation in gravity is immediate, but rainfall water requires around one month to travel across the vadose zone before it reaches the groundwater table. Using graphical methods of accumulated deviations from the mean, we show that, in general, groundwater storage follows the smooth, multi-year trends of terrestrial water storage but with less short-term trends; the same is true of rainfall, for which the local trends are more pronounced. There is hysteresis-like behaviour in the variations in terrestrial water storage and in the variations of groundwater. In practical terms, this study shows that, despite the abnormal dryness of the Iberian Peninsula during the 2004–2010 drought, the depleted groundwater storage in Andalucía recovered almost to its pre-drought level by 2016. In addition, groundwater storage and terrestrial water storage show very similar trends but with a delay in the groundwater trend.

Numerical simulation for strip-island freshwater lenses involving a precipitation-fed freshwater table

The freshwater table is the upper boundary of the entire freshwater lens and is very important for understanding the formation of freshwater lenses and for managing fresh groundwater on islands. However, little attention has been given to the position of the freshwater table, as it is difficult to visually distinguish it from the unsaturated zone in physical models. Under the natural conditions, the process of rainfall infiltration into soils involves both pore-water flow and air flow in soil voids, and thus, the airflow induced by infiltration has implications for groundwater recharge and formation of freshwater lenses. Therefore, research on the precipitation-fed freshwater table in an oceanic island should consider the water-air two-phase flow induced by rainwater infiltration. This study focuses on simulating the steady-state position of the freshwater table using TOUGH2/EOS7, a model that considers liquid and gas two-phase flow and brine, water and air three-component in unsaturated-saturated porous media, and verifying it by comparing the numerical results with the analytical solutions. The numerical results delineated the entire geometry of island freshwater lenses that involve a freshwater table replenished by precipitation, which were then compared with the Dupuit-Ghyben-Herzberg (DGH) analytical solutions that ignore the outflow face and with Van der Veer's analytical solutions that consider the outflow face. Reasonable fits between the numerical results and the DGH analytical solutions were obtained, with the exception that the lens obtained by the DGH analytical model is smaller in size than that simulated by the numerical model for various cases; a better agreement was found between the results from the numerical model and Van der Veer's analytical solutions for the homogeneous island case. The agreements between the results from the proposed numerical model and the DGH and Van der Veer's analytical models verified the proposed numerical model. The numerical modeling method investigated here enhanced the numerical simulation techniques for studying groundwater movements and the interaction of freshwater and saltwater on oceanic islands and can provide a reference for modeling more complicated island groundwater patterns.

The development of laterite weathering profiles as a function of rainfall and time: A geophysical approach

AbstractLaterite weathering profile (LWP) thicknesses are functions of precipitation rate and time, but their exact dependence on them is uncertain. We investigate LWP development on ground surfaces in Hawai'i that are neither aggrading nor eroding across substrates from 0.01 to 4 Ma and rainfall rates of <250 to >3000 mm/a. The Hawaiian Islands provide an excellent opportunity for LWP studies across climates and over millions of years on a single rock type, basalt. LWP weathering rates are usually determined by geochemical approaches. We present a geophysical method once bedrock ages and precipitation rates are known. We employed multichannel analysis of surface waves and horizontal‐to‐vertical spectral ratio methods. Results indicate that >70% of the variability in LWP thickness is due to precipitation and bedrock age. The remainder is attributed to measurement uncertainty and heterogeneity in the permeability of basalt. LWPs develop by two paths. Dry (<1000 mm/a) areas have a negative water balance with evapotranspiration exceeding rainfall. LWPs thicken until they reach a steady state where the storage capacity of the saprolite precludes the percolation of water into subjacent lava. In wetter areas, downslope interflow produces thick laterite wedges near coastlines that migrate upslope over ~1 Ma. Subsequently, they thicken and reach a steady state where precipitation cannot deliver water through the vadose zone. LWPs' thickness (T) increases as a function of time (t) and precipitation (P) according to the expression (95% confidence): . The first partial derivative, or weathering rate, is . Weathering rates decrease with time and are strong functions of t and P. These expressions add considerable insight into the rates and processes driving weathering at large scales and can also be solved for t, giving a rough estimate of the time of landscape formation of eroded surfaces.