Regional Water Table Research Articles

Paleoenvironment and vegetational history of a Middle Pennsylvanian intramontane peat swamp: an example from the Lower Radnice Coal, Kladno coalfield (Czech Republic)

AbstractThe depositional environment, hydrology and vegetational history of the Lower Radnice Coal (Duckmantian) in the Kladno coalfield was studied using sedimentary geology, coal petrology and paleobotanical/palynological methods. The peat accumulating wetland of the coal formed in a fluvial paleovalley approximately 15 km long and 2–5 km wide, bordered by basement paleohighs and landlocked in the interior of the central European Variscides. The peat swamp evolved on top of mud-dominated floodplain successions pedogenically modified to a vertic gleyed Protosol. Probably climatically controlled rising ground water table resulted in paludification that from downstream part gradually spread upstream. Most clastic load was deposited in the upper part of the valley, whereas only mud suspension was dispersed downstream throughout the vegetated swamp. The best conditions for peat accumulation were situated in the eastern part of the paleovalley, where up to 1.5 m thick coal with thin bands of impure coal and carbonaceous mudstone formed in an occasionally inundated rheotrophic system with peat accretion controlled by regional ground water table. The peat swamp was vegetated mainly by lepidodendrid lycopsids with Lepidodendron and Paralycopodites being dominant genera. Shrubby to ground cover vegetation was represented by medulosallean pteridosperms, small shrubby lycopsids, sphenopsids, and herbaceous ferns. Tree ferns were locally abundant, especially in mineral-rich substrates. The rheotrophic character of the peat swamp may indicate higher seasonality of the Variscan interior, compared to coastal areas in the North Variscan foreland with contemporaneous ombrotrophic peats. Modern equivalents of the Lower Radnice Coal swamp are inland planar tropical peat swamps in tributary paleovalleys of the Tasek Bera in peninsular Malaysia and central Congo basins. Graphical abstract Lower Radnice Coal peat swamp.

The Hydraulic Evolution of Groundwater-Fed Pit Lakes After Mine Closure.

Open pit mining frequently requires regional water tables to be lowered to access ore deposits. When mines close, dewatering ceases allowing the water table to recover. In arid and semi-arid mining regions, the developing pit lakes are predominantly fed by groundwater during this recovery phase and pit lakes develop first into "terminal sinks" for the surrounding groundwater system. With time, the re-establishment of regional hydraulic gradients can cause pit lakes to develop into throughflow systems, in which pit lake water outflows into adjacent aquifers. In this study, we use numerical groundwater modeling to aid process understanding of how regional hydraulic gradients, aquifer properties, net evaporation rates, and pit geometry determine the hydraulic evolution of groundwater-fed pit lakes. We find that before the recovery of the regional water table to its new equilibrium, pit lakes frequently transition to throughflow systems. Throughflow from pit lakes to downstream aquifers can develop within two decades following cessation of dewatering even under low hydraulic gradients (e.g., 5 × 10-4) or high net evaporation rates (e.g., 2.5 m/year). Pit lakes remain terminal sinks only under suitable combinations of high evaporation rates, low hydraulic gradients, and low hydraulic conductivities. In addition, we develop an approximate analytical solution for a rapid assessment of the hydraulic status of pit lakes under steady-state conditions. Understanding whether pit lakes remain terminal sinks or transition into throughflow systems largely determines the long-term water quality of pit lakes and downstream aquifers. This knowledge is fundamental for mine closure and planning post-mining land use.

The changing nature of groundwater in the global water cycle.

In recent decades, climate change and other anthropogenic activities have substantially affected groundwater systems worldwide. These impacts include changes in groundwater recharge, discharge, flow, storage, and distribution. Climate-induced shifts are evident in altered recharge rates, greater groundwater contribution to streamflow in glacierized catchments, and enhanced groundwater flow in permafrost areas. Direct anthropogenic changes include groundwater withdrawal and injection, regional flow regime modification, water table and storage alterations, and redistribution of embedded groundwater in foods globally. Notably, groundwater extraction contributes to sea level rise, increasing the risk of groundwater inundation in coastal areas. The role of groundwater in the global water cycle is becoming more dynamic and complex. Quantifying these changes is essential to ensure sustainable supply of fresh groundwater resources for people and ecosystems.

Factors and processes controlling hydrochemistry and evaporitic deposits in hypersaline wetland‐shallow lake systems

AbstractThree wetland‐shallow lakes were studied with the aim of analysing the processes and factors that control hydrochemistry and evaporite deposits in them in basins of the Pampean Plain, Argentina. In this sense, water balances and analysis of the water content of the lakes with normalized water difference index were made, and geologic‐geomorphological characteristics and groundwater flows were defined. Groundwater, surface water and evaporite samples were taken, along with sediment samples from wetland‐shallow lakes and surroundings. In situ pH, electrical conductivity and temperature, and laboratory chemical determinations of major ions of the water were measured. x‐ray diffraction (XRD) and scanning electron microscope (SEM)‐energy dispersive spectroscopy (EDS) analyses of evaporite and sediment samples were carried out. The results obtained allow the identification of two different systems. One of them is represented by Leubucó wetland‐lake basin, which is topographically higher and smaller than the other system, with Na‐HCO3 and Na‐Cl/SO4 groundwater inflows, with high concentrations of Ca2+ and Mg2+. In periods of high evaporation, the regional water table drops, and due to its topographic position, the lake dries. These determine the formation of thick layers of halite associated with magnesium salts on surface, and gypsum layers interbedded with clastic sediments. The other system involves de la Sal and Chasilauquen wetland‐lake basins, which have larger extensions. They receive Na‐HCO3 and Na‐Cl/SO4 groundwater inflow, but with low Mg2+ and Ca2+. Their lower topographic position determines that even during deficit periods, the water table intercepts the lakes surface and therefore, lakes have a shallow surface water level almost all the year, which is hypersaline, Na‐Cl type. These systems only have thin evaporite crusts, composed of halite and thenardite at the edges of the lakes, and the formation of Ca‐Mg sulfates is not observed. It is concluded that topographic position, basins size, geology, groundwater inflow and evaporation are the main processes and factors controlling hydrochemistry and evaporitic deposits.

Wetland Management in Recharge Regions of Regional Groundwater Flow Systems with Water Shortage, Nyírség Region, Hungary

Climate change and increasing human impacts are more emphasised in recharge regions, where the main flow direction is downward, resulting in negative water balance. Two wetlands located in the recharge position of regional groundwater flow systems were investigated in the Nyírség region, Hungary, as pilot areas for representing wetlands in similar hydraulic positions. Hydraulic data processes, chemical data evaluations, and numerical simulations revealed that the wetlands are fed via local flow systems, superimposing regional-scale recharge conditions in the area. The wetlands are discharge and flow-through types in connection with local flow systems. Nevertheless, in the case of significant regional water table decline—due to the high vulnerability of recharge areas to climate change—local flows are degraded, so they are not able to sustain the wetlands. To preserve the groundwater-dependent ecosystems in the areas, water retention at the local recharge areas of the wetlands may help in the mitigation of water level decline under present-day conditions. If the regional water table continues to decline, comprehensive water retention solutions are needed in the whole region. The results highlight that understanding the natural wetland–groundwater interactions at different scales is crucial for the preservation of wetlands and for successful water retention planning.

Potential Impacts of Okpuno Egbu Dumpsite on Groundwater Resources in Parts of Nnewi, Southeastern Nigeria

The study area, Nnewi is one of the most populated, commercial and industrialized cities in Anambra State. Geophysical data were collected from 15 vertical electrical sounding (VES) points and hydrogeochemical analysis of 15 groundwater samples collected from 15 locations were analyzed for their chemical composition and used to assess the impact of the dumpsite at Okpunoegbu on groundwater in the study area. The results of the vertical electrical sounding indicate that most of the aquiferous units fall within the fourth layer at most of the Relocations with an average resistivity of 1340.25Ωm, an average thickness of 41.75m and average depth of 51.25m. The range of the regional water table is from 25.41m to 99.42m. The Dar-Zarrouk parameters; longitudinal conductance and transverse resistance, s and t were determined from the geophysical data to be 0.007mho and 55966.9Ω respectively. The results of s indicate that the protective capacity of the overburden layers to underlying aquifer is low. which implies easy transmission of pollutants to the aquifers making them vulnerable to pollution. The results of water analysis show that the range of pH in the samples is between 5.73 to 6.95 indicating that 20% of the water samples are slightly acidic and 80% to alkaline. TDS, TSS and EC were found to be within the permissible limits of WHO and NSDWQ standard guidelines. COD and BOD were above the permissible limits indicating the water samples to be severely polluted. DO1 and DO5 range between 22.78 - 65.40 mg/L and 48-358.96 mg/L respectively. The major cations and anions were within the permissible limits.. However, the heavy metals range in samples were cadmium (0.04-0.37mg/L), chromium (0.01-0.10mg/L), mercury (0-0-07mg/L) and lead (0.1-0.93mg/L), exceed the permissible limits in some samples. The geospatial distribution maps of the heavy metals implicate the dumpsite and other sources as contributors of the increased concentrations of cadmium and lead. while indicating the dump as a major contributor of chromium and mercury to groundwater in the study area. The calculated quality indices, WQI and HMPI rate the groundwater as highly polluted and unsuitable for drinking, and the CD and HMEI as moderately to highly polluted, respectively. The identified hydrochemical facies classify the water as Ca-Cl and Ca-Mg –Cl – SO4 water type. The Cl – SO4 facie indicate end water usually with increased age found in the discharge areas. The Okpunoegbu dumpsite is an important source of groundwater contaminants and pollutants especially the heavy metals such as cadmium, mercury, chromium and lead. The protective capacity of the overburden is enhancing the vulnerability of the aquifers to pollution as they are rated as low. It is recommended that some remediation measures be carried out to restore the groundwater quality and prevent further pollution being initiated and appropriate water treatment be carried before use for drinking purposes.

Hydraulically Disconnected Rivers in the Highlands and Southern Riverine Plain of S.E Australia

The rivers of south-eastern Australia flow within a complex meander tract (Coonambidgal Formation) formed by phases of Quaternary stream activity. Pumping tests, hydrochemistry and groundwater monitoring of the Campaspe, Loddon and Murray River Valleys show that for significant parts of their courses, the rivers and their associated strip aquifers form a single integrated hydraulic unit perched above and disconnected from the regional water table by an underlying aquitard developed at the top of a varyingly thick and temporally dynamic vadose zone. Loss to the regional aquifer is not restricted to the riverbed but covers the entire width of the Coonambidgal Formation aquifer, which is one or two orders of magnitude greater. River-bed flux is not a measure of net river loss. Through diffusion and dispersion from the overlying saturated zone, aquitard enhancement or development is augmented by chemical processes active towards the top of the vadose zone. Unlike river-bed clogging, chemical clogging of aquifers is progressive and permanent. Post-European instability in the studied groundwater systems has seen catchment wide groundwater rises of up to 0.25 m/y. or 25 metre over the last century. Under the pre-existing norm of deeper water tables, disconnected streams would have been more numerous with the present aquitards being a legacy of that regime.

Plio-Pleistocene continental carbonates and silcretes in Wadi El-Natrun, NW Egypt: Their morphology and paleoenvironmental significance

To deduce the prevailing paleoenvironmental conditions during the deposition of the Plio-Pleistocene continental carbonate sequence at Wadi El-Natrun (NW Egypt), we examined and compared the macro- and micro-morphological, mineralogical and stable isotopic characteristics of calcretes, dolocretes, palustrine carbonates, and silcretes of the lower Pliocene Meikheimin and Plio-Pleistocene Qataji formations. Calcrete occurs in both formations and exhibits a variety of forms, including powdery, nodular, fracture-infill, massive and hard laminated crusts (hardpan). The calcretes developed in mixed vadose and phreatic diagenetic environments under an arid to semi-arid climate. Palustrine carbonates occur in the upper Qataji Formation and developed as nodular, desiccated, brecciated, mottled and pisoidal limestones that are interbedded with the calcretes. They exhibit varied micro-morphological features, including circum-granular and planar cracks, nodular and clotted fabrics, peloidal textures, infilling of burrows, root molds, and fenestral pores, which indicate modifications during subaerial exposure. The dolocrete facies in the upper Qataji Formation comprises laminated carbonate crusts. These include spheroidal dolomite and dolomicrite with Mg-rich clays that formed as primary precipitates in a continental setting. The silcretes consist of spheroidal, irregular opaline accumulations that formed as replacements of calcrete and dolocrete. These features reflect silicification in near-surface environments where there was an interplay between vadose and phreatic environments during deposition and early burial diagenesis. The stable isotopic data suggest deposition under warm, seasonally arid to semi-arid climatic conditions in an area dominated by C3 vegetation. The regional water table was higher in the early Pleistocene than during the early Pliocene but was subject to large fluctuations.

Partial Desalination of Saline Groundwater, including Flowback Water, to Produce Irrigation Water

Globally, more than 50 million ha of arable land is irrigated with saline water. The majority of this saline irrigation water is derived from saline groundwater. Global irrigation requirements may increase from 270 million ha in 2014 to about 750 million ha by 2050 as the global population increases to 9.1 billion people. The majority of this additional irrigation water is likely to come from saline groundwater sources. Desalination of irrigation water increases crop yield. A combination of high water volume requirements and low crop yields requires that, for widespread usage, the desalinated irrigation water product will require a delivery price of <USD 0.5 m3. This study considers five passive desalination routes (n-Fe0; n-Fe3O4; Fe0:Fe(b)@C0 polymer; n-Fe0:Fe(b) polymer; n-Fe(b) polymer) that may potentially achieve this goal: A common desalination mechanism is identified for the Fe0:Fe(b)@C0 polymer; n-Fe0:Fe(b) polymer; and n-Fe(b) polymer routes. The analysis establishes that the n-Fe(b) polymer route may be able to achieve (with a reaction time of 1 h) an 80% to 90% desalination of saline groundwater or flowback water (12.3 g NaCl L−1; EC = 17.6 dSm−1), to form partially desalinated irrigation water (1.2 to 2.4 g NaCl L−1; EC = 2 to 3.4 dSm−1) with an associated reduction in the sodium adsorption ratio (SAR) from 125 to between 1.2 and 2.5, for a potential material (n-Fe(b) polymer) treatment cost of <USD 0.01 m−3, after considering polymer reuse and recycle, but excluding all other plant and other operating costs. The examples demonstrate that the polymers can be used to create: (i) a desalinated stationary hydrodynamic plume, containing 47,123 m3 water (1 to 2.5 g NaCl L−1), within 157,080 m3 porous rock forming a confined, saline aquifer (18.59 g NaCl L−1), to act as a reservoir of desalinated water (96 m3 d−1) for irrigation, with the potential to produce >960 m3 d−1 as required; (ii) a desalinated, perched, stationary, shallow groundwater mound, located above the regional water table, containing >200 m3 of desalinated water.

Groundwater Temperature Modelling at the Water Table with a Simple Heat Conduction Model

This study aimed at the analysis and modelling of the groundwater temperature at the water table in different regions of Slovakia. In the first part, the analysis of the long-term trends of air and soil/ground temperature to a depth of 10 m is presented. The average annual soil/groundwater temperatures at different depths were the same but lower than the annual average air temperature by about 0.8 °C. The long-term trend analysis of the air temperature and soil temperature at a depth of up to 10 m in Slovakia showed that the air and soil/ground water temperature have risen by 0.6 and 0.5 °C, respectively, per decade over the past 30 years. The second part of the study aimed at modelling the daily groundwater temperatures at depths of 0.6–15 m below the surface. The simple groundwater temperature model was constructed based on a one-dimensional differential Fourier heat conduction equation. The given model can be used to estimate future groundwater temperature trends using regional air temperature projections calculated for different greenhouse gas emission scenarios.

Water mining and landscape: a study on three blocks (Gangajalghati, Mejhia and Saltora) of Bankura, West Bengal, India

Groundwater depletion occurs when the rate of groundwater extraction is higher than the rate of replenishment. It is unequivocal that the demand for fresh water is ever increasing. The study area lies in the semi-arid climatic zone and it's a solely agrarian society. The water scarcity leads to an impact on soil moisture and soil becomes drier during the dry months. The study is based on the changes in the regional water table which exhibit a relatively rapid response to acute scarcity of water. To specify the soil dryness gravimetric method has been used to study the selected soil samples. The groundwater withdrawal without estimating the water requirement at the regional level leads to soil dryness, and water stress situation to the agrarian economy, and it leaves a direct impact on the land. The perception of the farm owner, that ownership of farmland is synonymous with water mining right from their land leads to stress on the water table.

Hydrological and Isotopic Variability of Perched Wetlands on North Stradbroke Island (Minjerribah), Australia: Implications for Understanding the Effects of Past and Future Climate Change

Over the first two decades of the 21st century, many wetlands in eastern Australia exhibited declining water levels, causing concern for communities and environmental managers and raising questions about the roles of climate change and other human activity in these water level declines. In this context we examine the causes of water level variability in four wetlands on North Stradbroke Island (Minjerribah), in the humid subtropics of south-eastern Queensland, Australia, using a combination of hydrological and water isotope monitoring and modelling. North Stradbroke Island has a high concentration of wetlands perched above the regional water table, with cultural and ecological significance, and value for palaeoclimate research. From 2015 to 2019, wetland water depths decreased markedly at all sites, coinciding with increases in oxygen isotope ratios in surface waters. The data indicate that climate, specifically a decrease in precipitation relative to evaporation, was responsible for those declining water levels, and that groundwater extraction did not play a critical role. At two of the sites—both palustrine wetlands—declining surface water levels led to intermittent connectivity with the local perched aquifers. At the other two sites, which are both shallow lakes, the surface waters were constantly fed by perched groundwater. The hydrology of the two lakes was modelled using simple mass balance. However, in order to accurately model lake level change, it was necessary to vary catchment runoff and lake outflow via groundwater through time, highlighting complexity in projecting future hydrological change in these lakes. The long term resilience of these lakes depends on a combination of rainfall regime and the balance between catchment runoff and groundwater throughflow, the future of which is highly uncertain. As a consequence, continued efforts to project future hydroclimate and to model the complex hydrology of subtropical wetlands are essential.

Integrated geophysical investigation for understanding agriculturally induced landslides in southern Peru

We conducted near-surface geophysical surveys in and around the Majes I agricultural development (60 km west of Arequipa, Peru), where the nexus of geology and agriculture has increased landslide activity along the Majes–Siguas River Valley. Through DC resistivity, transient electromagnetics (TEM), and seismic surveys, we refined the understanding of local geology, characterized the agricultural impact on the local water table, and updated landslide modeling to help inform discussions on landslide mitigation strategies at Majes I and landslide prevention at the planned Majes II site. At the Majes I development, we identified an increase in water table and water saturation due to irrigation. At the planned Majes II site, which shares similar geology to Majes I, we interpret the regional water table that has yet to be affected by significant human development. We integrated these results into updated landslide modeling. Our modeling for Majes I suggests stable conditions prior to irrigation; as the water table rose from irrigation, landsliding began and evolved as a retrogressive failure that is now focused along the headscarp near critical infrastructure including the Carretera Panamericana (Pan-American Highway). Majes II is currently stable and irrigation management, such as drip versus flood techniques, must be supported. Soil ameliorants such as polymers and/or biochar should be encouraged to hold water near the roots to reduce the risk of landslide initiation. Combined this work shows the value of integrated hydrological and geophysical research for landslide management and optimized irrigation.

The Peculiar Hydrology of West-Central Florida’s Sandhill Wetlands, Ponds, and Lakes –Part 2: Hydrogeologic Controls

This study investigates hydrogeologic controls on a peculiar, poorly studied type of geographically isolated wetland in west-central Florida, USA, locally referred to as “sandhill” (or “xeric”) wetlands. Their peculiarity lies in their connectivity to a large regional aquifer, which controls their hydrology. Evidence for this connectivity was presented in Part I of this two-part series, but the mechanisms of their connectivity and ecohydrologic expression were yet undocumented. To make these determinations, we used geophysical, lithologic, hydrologic, and ecological data to construct hydrogeologic configurations for five distinct sandhill wetland types. From these configurations, two conceptual models were derived. The first model depicts the mechanisms by which sandhill wetlands connect to the regional aquifer. Three types of connectivity are proposed, each a function of the degree and depth of aquifer confinement: 1) direct-due to wetland embedment directly in the unconfined regional aquifer; 2) indirect-due to embedment in a surficial aquifer, where groundwater exchange with the regional aquifer occurs through breaches in the semi-confining unit; and 3) none-due to embedment in a surficial aquifer where groundwater exchange with the regional aquifer does not occur because the semi-confining unit is too deep. The second model conceptualizes sandhill wetland ecohydrologic expression as a function of depression geomorphology and the range of the regional water table. These models provide the basis of understanding of sandhill wetland, pond, and lake ecohydrology and can improve how natural resource managers classify, assess, and preserve these unique features, regionally and wherever wetlands of similar hydrogeologic control are found.

Facilitating Open Pit Mine Closure with Managed Aquifer Recharge.

Dewatering of open pit mines can lower the regional water table for distances of several kilometers from the pit. When the mine is closed, dewatering operations usually cease, and the water table near the pit begins to rise. If the pit is backfilled, the water table will eventually recover, but this recovery may take several hundred years. However, if the extracted water is re-injected into the subsurface, then this may accelerate recovery of the water table. We show that there is an optimal distance for re-injection, which is sufficiently far from the mine to minimize the amount of groundwater that flows back to the pit during mine operations (and hence necessitate additional pumping) but is still close enough to speed up the water table recovery post-mine closure. The optimal injection distance increases with the aquifer hydraulic diffusivity and the mine life (duration of dewatering and injection), and typically ranges between about two and nine times the radius of the mine pit. Where the mine pit is not backfilled, the relative reduction in drawdown due to injecting all the pumped water at the optimal distance is between approximately 10% and 50% after a recovery time equal to the mining period, increasing to 30% to 90% after a recovery time five times the mining period. The relative drawdown reduction due to managed aquifer recharge will be even greater for a pit which is backfilled when mining ceases.

Brasília municipal solid waste landfill: a case study on flow and slope stability

For geotechnical and environmental reasons, landfills are positioned above the regional water table and thus are formed in unsaturated conditions. This condition can be different if the drainage system and the rain regime of the site are such that they create a level of internal liquid in the landfill. During January and February 2019, excessive movements occurred in the slopes of the Brasília sanitary landfill. A geotechnical investigation indicated that the raised leachate level caused by the clogging of the drainage system contributed to the landfilled waste movements. The limit equilibrium analysis was used to predict the relationship between leachate level and slope stability. In order to understand the process that led to the rupture, flow and stability analysis by limit equilibrium were performed. The parameters associated with flow, water retention capacity, and shear strength were obtained based on literature evaluations. In addition, data from tests were used, which allowed to define more accurately the distribution of pore pressures of liquid that led to the failure. This study allowed to define the cause of failure and also to establish the role of the drainage system in maintaining the stability of the landfill. The studies indicated that although the gain of shear strength of landfill due to the unsaturated condition is negligible, the process of flow in unsaturated medium, associated with climatic aspects, are fundamental for a medium- and long-term analysis.

A rapid sediment pulse induced by glacial melting during the MIS 8/7e transition buried well‐developed karst in the Railton Valley, Tasmania, Australia

Recent catastrophic sinkhole collapse caused by regional water table lowering induced by quarrying has led to the partial exhumation and reactivation of a buried karst cave system in the floor of the Railton Valley in the lowlands of northern Tasmania, Australia, and widespread sinkhole development. Extensive silty sediments exposed in sinkholes, OSL dated to approximately 237 ka, are interpreted to be lacustrine deposits derived from rapidly deposited glacial outwash. The silty sediments have mineralogy consistent with derivation from a source in the upper Mersey catchment rather than locally and are interpreted to be the product of rapid melting of the Mersey Valley glacier during the MIS 8/7e transition. Thick Last Glacial alluvial fan and Holocene flood‐plain deposits mantle the lacustrine sediments. Exposures of glacial erratics and weathered till in streambeds provide further evidence that the valley was impacted by earlier Pleistocene glaciations.

Characteristics of inverted saturated zone under unclogged streams

Unclogged streams are widespread in nature and can generate dynamic inverted saturated zones (ISZ) underneath the streams above the regional water tables. This study is devoted to investigating the evolutional process of such an ISZ and its frontal movement. It defines a uniform saturation zone by moving the boundary of saturation flow from the inverted water table (IWT) to the end of the capillary saturation zone. Consequently, the conventional concept of specific yield, infiltration rate and governing equation at the ISZ frontal surface (IFS) must be revisited. Two new ISZ models are developed and quantified with great details with infiltration rate being constant and variable. The equations of hydraulic head profile development and the IFS are obtained. This study indicates that classical free surface equations, locating at IWT, is not self-consistent. The water balance requirement is not satisfied at IFS in the Green and Ampt (1911) model or the Polubarinova-Kochina (1962) model, causing physically unrealistic spatial variables. The increase of the IFS with time exhibits three different modes depending on the infiltration rate and the saturated hydraulic conductivity. The maximum depth of an IFS (corresponding to a depth with infiltration rate being stable) is linearly correlated with stream stage and air-entry head. The streambed infiltration flux decreases with time and appears to exhibit three modes similar to the IFS evolution. The findings of this study reconstruct the theoretical basis of the dynamic evolution process of stream-aquifer system and can help for better understanding of physical connotation of free surface equations and basic characteristics of ISZ evolution.

Responses of phreatophyte transpiration to falling water table in hyper-arid and arid regions, Northwest China

Quantitative assessment of the impact of groundwater depletion on phreatophytes in (hyper-) arid regions is key to sustainable groundwater management. However, a parsimonious model for predicting the response of phreatophytes to a decrease of the water table is lacking. A variable saturated flow model, HYDRUS-1D, was used to numerically assess the influences of depth to the water table (DWT) and mean annual precipitation (MAP) on transpiration of groundwater-dependent vegetation in (hyper-) arid regions of northwest China. An exponential relationship is found for the normalized transpiration (a ratio of transpiration at a certain DWT to transpiration at 1 m depth, Ta*) with increasing DWT, while a positive linear relationship is identified between Ta* and annual precipitation. Sensitivity analysis shows that the model is insensitive to parameters, such as saturated soil hydraulic conductivity and water stress parameters, indicated by an insignificant variation (less than 20% in most cases) under ± 50% changes of these parameters. Based on these two relationships, a universal model has been developed to predict the response of phreatophyte transpiration to groundwater drawdown for (hyper-) arid regions using MAP only. The estimated Ta* from the model is reasonable by comparing with published measured values.© 2021 China Geology Editorial Office.

Hydrogeological constraints for the genesis of the extreme lithium enrichment in the Salar de Atacama (NE Chile): A thermohaline flow modelling approach

The Salar de Atacama (SdA) is the largest Li reserve globally. The origin of Li, together with the rest of solutes, has been object of debate. Thus, rock weathering at low temperature, hydrothermal leaching or magmatic origin together with subsequent evaporation has been hypothesized. However, the extreme Li enrichment (>4000 mg/L) and the location of the Li-Mg-rich brines around the Salar Fault System (SFS) that crosses the nucleus of the SdA in half remain unexplained. The objective of this work is to define the thermohaline groundwater flow in the SdA basin to account for the genesis of its extreme Li enrichment.Thermohaline flow modelling has demonstrated the critical effect of the minimum hydraulic head (MHH) of the regional water table on the groundwater flow of salt flats. The MHH divides the basin into two isolated hydrodynamic systems and constitutes the endpoint towards which the most evaporated brines converge. The spatial mismatch between the locations of the Li-Mg-rich brines in the central-western zone of the nucleus (in the SFS) and the MHH in the easternmost zone of the nucleus discards recent evaporative concentration of the recharge water as the main mechanism of Li enrichment. Moreover, the persistence of a saline interface surrounding the nucleus at depth, regardless of the temperature gradient, also precludes lateral recharge (predominantly from the east) to ascend along the SFS.On the other hand, the computed thermohaline flow is compatible with the remobilization of buried layers of Li-Mg-enriched salts and/or clays by dilute recharge waters coming from the west or southwest of the basin. Here, the role of faults and density-driven flow is key to allow efficient downward and upward flow rates that favour the remobilization of Li and Mg.