Hydrogeochemical patterns,processes and mass transfers during aquifer storage and recovery (ASR) in an anoxic sandy aquifer

The hydrogeochemical processes that took place during an aquifer storage and recovery (ASR) trial in a confined anoxic sandy aquifer (Herten, the Netherlands) were identified and quantified, using observation wells at 0.1, 8 and 25 m distance from the ASR well. Oxic drinking water was injected in 14 ASR cycles in the period 2000–2009. The main reactions consisted of the oxidation of pyrite, sedimentary organic matter, and (adsorbed) Fe(II) and Mn(II) in all aquifer layers (A–D), whereas the dissolution of carbonates (Mg-calcite and Mn-siderite) occurred mainly in aquifer layer D. Extinction of the mobilization of SO₄, Fe(II), Mn(II), As, Co, Ni, Ca and total inorganic C pointed at pyrite and calcite leaching in layer A, whereas reactions with Mn-siderite in layer D did not show a significant extinction over time. Iron(II) and Mn(II) removal during recovery was demonstrated by particle tracking and pointed at sorption to neoformed ferrihydrite. Part of the oxidants was removed by neoformed organic material in the ASR proximal zone (0 – ca. 5 m) where micro-organisms grow during injection and die away when storage exceeds about 1 month. Anoxic conditions during storage led to increased concentrations for a.o. Fe(II), Mn(II) and NH₄ as noted for the first 50–200 m³ of abstracted water during the recovery phase. With a mass balance approach the water–sediment reactions and leaching rate of the reactive solid phases were quantified. Leaching of pyrite and calcite reached completion at up to 8 m distance in layer A, but not in layer D. The mass balance approach moreover showed that Mn-siderite in layer D was probably responsible for the Mn(II) exceedances of the drinking water standard (0.9 μmol/L) in the recovered water. Leaching of the Mn-siderite up to 8 m from the ASR well would take 1600 more pore volumes of drinking water injection (on top of the realized 460).     

Application of multiple-point geostatistics on modelling groundwater flow and transport in a cross-bedded aquifer (Belgium)

Sedimentological processes often result in complex three-dimensional subsurface heterogeneity of hydrogeological parameter values. Variogram-based stochastic approaches are often not able to describe heterogeneity in such complex geological environments. This work shows how multiple-point geostatistics can be applied in a realistic hydrogeological application to determine the impact of complex geological heterogeneity on groundwater flow and transport. The approach is applied to a real aquifer in Belgium that exhibits a complex sedimentary heterogeneity and anisotropy. A training image is constructed based on geological and hydrogeological field data. Multiple-point statistics are borrowed from this training image to simulate hydrofacies occurrence, while intrafacies permeability variability is simulated using conventional variogram-based geostatistical methods. The simulated hydraulic conductivity realizations are used as input to a groundwater flow and transport model to investigate the effect of small-scale sedimentary heterogeneity on contaminant plume migration. Results show that small-scale sedimentary heterogeneity has a significant effect on contaminant transport in the studied aquifer. The uncertainty on the spatial facies distribution and intrafacies hydraulic conductivity distribution results in a significant uncertainty on the calculated concentration distribution. Comparison with standard variogram-based techniques shows that multiple-point geostatistics allow better reproduction of irregularly shaped low-permeability clay drapes that influence solute transport.     

Geochemical controls on sediment reactivity and buffering processes in a heterogeneous aquifer

The injection and recovery of oxic water into deep anoxic aquifers may help to alleviate short- and long-term imbalance between freshwater supply and demand. The extent and structure of physical and geochemical heterogeneity of the aquifer will impact the water quality evolution during injection, storage and recovery. Water–sediment interactions within the most permeable parts of the aquifer, where the bulk of the injectant will penetrate, may dominate, however, water quality may also be impacted by interactions within the finer-grained, less permeable but potentially highly reactive media. In this study, the heterogeneity of the reductive capacity of an aquifer selected for water reuse projects was characterised, the amount, type and reactivity of the sedimentary reductants present determined, and the relationship between reductive capacity and sedimentary lithologies quantified. The average potential reductive capacities (PRC_TOT), based on total organic C and pyrite concentrations of the sediment, were quantified for sands (382 μmol O₂ g⁻¹), clays (1522 μmol O₂ g⁻¹), and silts (1957 μmol O₂ g⁻¹). Twenty-seven samples, spanning the three different lithologies, were then incubated for 50 days and the measured reductive capacities (MRC) determined for the sands (29.2 μmol O₂ g⁻¹), silts (136 μmol O₂ g⁻¹), and clays (143 μmol O₂ g⁻¹). On average, the MRC were 10% of the PRC_TOT. The main consumers of O₂ were pyrite (20–100%), sedimentary organic matter (SOM; 3–56%), siderite (3–28%) and Fe(II)-aluminosilicates (8–55%). The incubation data plus hydrogeochemical modelling, indicated that pH-buffering was controlled firstly by dissolution of trace level carbonates, followed by dissolution of feldspars. Zinc, Co, Ni, Cd and Pb were readily mobilized during incubation.     

Geochemical processes in mill tailings deposits: modelling of groundwater composition

A general model is presented for geochemical processes occurring in the unsaturated zone of a carbonate-depleted, pyritic tailings deposit. Quantification of slow geochemical reactions, using published, empirical rate laws from small-scale experiments on monomineralic samples, and geochemical equilibrium reactions successfully reproduced the relative rates of field processes in the case study, Impoundment 1 in Kristineberg. Reproduction of absolute rates was achieved by scaling down all laboratory-derived mineral weathering rates by two orders of magnitude. The sensitivity of the modelled groundwater composition and pH to rates of pH-buffering processes and redox reactions indicated that inclusion and accurate quantification of all dominant geochemical processes on the field scale is necessary for reliable prediction of groundwater composition and pH.     

Geochemical processes controlling fluoride-bearing groundwater in the granitic aquifer of a semi-arid region

The aim of the present study is to identify the geochemical processes responsible for higher fluoride (F^–) content in the groundwater of the Yellareddigudem watershed located in Nalgonda district, Andhra Pradesh. The basement rocks in the study area comprise mainly of granites (pink and grey varieties), which contain F-bearing minerals (fluorite, biotite and hornblende). The results of the study area suggest that the groundwater is characterized by Na⁺: HCO facies. The F^– content varies from 0.42 to 7.50 mg/L. In about 68% of the collected groundwater samples, the concentration of F^– exceeds the national drinking water quality limit of 1.5 mg/L. The weathering of the granitic rocks causes the release of Na⁺ and HCO ions, which increase the solubility of ions. Ion exchange between Na+ and Ca²⁺, and precipitation of CaCO₃ reduce the activity of Ca²⁺. This favours dissolution of CaF₂ from the F-bearing minerals present in the host rocks, leading to a higher concentration of F^– in the groundwater. The study further suggests that the spatial variation in the F^– content appears to be caused by difference in the relative occurrence of F-bearing minerals, the degree of rockweathering and fracturing, the residence time of water in the aquifer materials and the associated geochemical processes. The study emphasizes the need for appropriate management measures to mitigate the effect of higher F groundwater on human health.     

Contribution of geophysical surveys to groundwater modelling of a porous aquifer in semiarid Niger: An overview

Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling.     

Reactive transport modelling of groundwater-bentonite interaction: Effects on exchangeable cations in an alternative buffer material in-situ test

Bentonite clays are regarded a promising material for engineered barrier systems for the encapsulation of hazardous wastes because of their low hydraulic permeability, swelling potential, ability to self-seal cracks in contact with water and their high sorption potential. SKB (Svensk Kärnbränslehantering) has been conducting long term field scale experiments on potential buffer materials at the Äspö Hard Rock Laboratory for radioactive waste disposal in Sweden.The Alternative Buffer Material (ABM) test examined buffer properties of eleven different clay materials under the influence of groundwater and at temperatures reaching up to 135 °C, replicating the heat pulse after waste emplacement. Clay materials were emplaced into holes drilled in fractured granite as compacted rings around a central heater element and subsequently brought into contact with groundwater for 880 days. After test termination, and against expectations, all clay materials were found to have undergone large scale alterations in the cation exchange population. A reactive-diffusive transport model was developed to aid the interpretation of the observed large-scale porewater chemistry changes. It was found, that the interaction between Äspö groundwater and the clay blocks, together with the geochemical nature of the clays (Na vs Ca-dominated clays) exerted the strongest control on the porewater chemistry. A pronounced exchange of Na by Ca was observed and simulated, driven by large Ca concentrations in the contacting groundwater. The model was able to link the porewater alterations to the fracture network in the deposition hole. The speed of alterations was in turn linked to high diffusion coefficients under the applied temperatures, which facilitated the propagation of hydrochemical changes into the clays. With diffusion coefficients increased by up to one order of magnitude at the maximum temperatures, the study was able to demonstrate the importance of considering temperature-dependant diffusion in understanding and predicting geochemical alterations of engineered barriers systems after relatively short exposure times following waste emplacement.     

The transition of a freshwater karst aquifer to an anoxic marine system

Jewfish Sink is located in the shallow seagrass flats of the Gulf of Mexico in west central Florida. Jewfish Sink was a submarine spring until the drought of 1961–1962 when it ceased flowing. Today, the sink is an anaerobic marine basin and provides the opportunity to study the implications of saltwater intrusion in coastal karstic areas. The biogeochemistry of Jewfish Sink was studied from summer 2001 through spring 2004. A distinct feature of the sink is the uniform cold temperature (16–17°C) of the deeper anoxic water that does not match groundwater found nearshore or onshore (22–24°C). There are four zones within the sink: oxic zone, transition zone, upper anoxic zone, and anoxic bottom water. The anoxic bottom water does not mix with water from above but may be linked to deep Gulf shelf water through ancient aquifer conduits. The other three zones vary seasonally in oxygen, salinity, and temperature because of limited mixing in the winter due to cooding and sinking of surface water. The walls of the anoxic zones have characteristic microbial mats that are found in other sulfidic karstic features in the area. Bacterial activity appears to be carbon, limited in the anoxic zones where sulfate reduction appears to be the major metabolic process. The reduction of sulfate to sulfide appears to be driven by irregular influexes of organic matter including macroalgae, horseshoe crabs, and stingrays that become entrapped within the sink. Bacterial activity in the oxic zones appears to be phosphate limited. Although the system is partially isolated from the overlying marine ecosystem, organic input from above drives the bacterial anaerobic ecosystem, resulting in a sulfide pump. In this model, sulfide percolates up through the karst and removes oxygen from the overlying sediment, which has likely caused changes in the shallow benthic ecosystem. Jewfish Sink appears to be part of an extensive anoxic subterranean estuary that extends under parts of at least three coastal counties in Florida and serve as a model for the effects of rising sea levels or aquifer mining.     

Modeling of transient groundwater flow,pollutant transport,and biodegradation in an aquifer with large hydraulic head variations

Industrially sourced dense non-aqueous phase liquids (DNAPLs) contaminated an alluvial aquifer in France decades ago. The location(s) and nature of the pollution source zone(s) were unknown, and the dissolved concentrations of volatile organic compounds in the monitoring wells varied greatly with time. The aquifer was in hydraulic equilibrium with an artificial canal whose water level was highly variable (up to 5 m). These variations propagated into the aquifer, causing changes in the groundwater flow direction; a transient numerical model of flow and solute transport showed that they correlate with the concentration variations because the changes in the flow direction resulted in the contaminant plume shifting. The transient hydrogeological numerical model was built, taking into account solvent biodegradation with first-order chain, since biodegradation has a significant influence on the pollutant concentration evolution. The model parameterization confirms the position of the source zones among the potential troughs in the bedrock where DNAPLs could have accumulated. The groundwater model was successfully calibrated to reproduce the observed concentration variations over several years and allowed a rapid validation of the hypotheses on the functioning of the polluted system.     

地源热泵系统地下水热量运移阶段特性模拟研究

含水介质中地下水热量运移过程模拟是地源热泵系统这类新型浅层地温能利用工程科学论证的重要内容。通过建立饱和含水介质中热量运移对流-弥散数学模型,模拟了拟建地源热泵系统理想的对井抽-灌运行特定水流及热源条件下地下水热量运移过程。结果表明热泵系统完整运行周期(一年)内地下水热量运移过程具有显著的"阶段性"特征,"夏、冬抽-灌输运"和"春、秋停运蓄存"两阶段取决于不同的作用因素。从数学和概念模型两方面讨论了地下水流作用对热量输运过程的决定性影响,说明不同时期抽、灌井互换运行是解决拟定工作模式下热(冷)能浪费问题的理想途径。     

Mobile sediment in an urbanizing karst aquifer: implications for contaminant transport

 Here we investigate geochemical characteristics of sediment in different compartments of a karst aquifer and demonstrate that mobile sediments in a karst aquifer can exhibit a wide range of properties affecting their contaminant transport potential. Sediment samples were collected from surface streams, sinkholes, caves, wells, and springs of a karst aquifer (the Barton Springs portion of the Edwards (Balcones Fault Zone) Aquifer, Central Texas) and their mineralogy, grain-size distribution, organic carbon content, and specific surface area analyzed. Statistical analysis of the sediments separated the sampling sites into three distinct groups: (1) streambeds, sinkholes, and small springs; (2) wells; and (3) caves. Sediments from the primary discharge spring were a mix of these three groups. High organic carbon content and high specific surface area gives some sediments an increased potential to transport contaminants; the volume of these sediments is likely to increase with continued urbanization of the watershed. Received: 13 April 1998 · Accepted: 6 October 1998     

两种方法在地下水位估值中的应用

对于许多区域水资源问题,用数值方法进行潜水水流模拟时,需要给出每个节点上地下水位值.本文首先简单介绍了趋势面方法,然后着重阐述了泛克里格方法的基本原理及它们在地下水位估值中的应用,通过比较两种方法的计算结果可以得出泛克里格方法是进行地下水位估值的空间最优估计方法.     

Three-dimensional finite element modelling for consolidation due to groundwater withdrawal in a desaturating anisotropic aquifer system

A poroelastic numerical model is presented to evaluate three-dimensional consolidation due to groundwater withdrawal from desaturating anisotropic porous media. This numerical model is developed based on the fully coupled governing equations for groundwater flow in deforming variably saturated porous media and the Galerkin finite element method. Two different cases of unsaturated aquifers are simulated for the purpose of comparison: a cross-anisotropic soil aquifer, and a corresponding isotropic soil aquifer composed of a geometrically averaged equivalent material. The numerical simulation results show that the anisotropy has a significant effect on the shapes of three-dimensional hydraulic head distribution and displacement vector fields. Such an effect of anisotropy is caused by the uneven partitioning of the hydraulic pumping stress between the vertical and horizontal directions in both groundwater flow field and solid skeleton deformation field. Copyright © 1999 John Wiley & Sons, Ltd.     

Reactive transport processes occurring during nuclear glass alteration in presence of magnetite

In this study, we have investigated and clarified the processes occurring during the alteration of SON68 glass – the reference nuclear glass for the waste arising from reprocessing of spent fuel from light water reactors – at 50 °C in Callovo-Oxfordian clay groundwater in presence of magnetite. Magnetite is known to be one of the iron corrosion products expected to be present in the vicinity of glass in geological disposal conditions. The effects of the amount of magnetite relative to the glass surface and the transport of aqueous species during glass alteration were studied. A first series of experiments was focused on the effect of various magnetite amounts by mixing and altering glass and magnetite powders. In a second series of experiments, magnetite was separated from the glass by a diffusive barrier in order to slow down the transport of aqueous species. Glass alteration kinetics were analyzed and solids were characterized by a multiscale approach using Raman Spectroscopy, Scanning and Transmission Electron Microscopy, Energy-Dispersive X-ray and Scanning Transmission X-ray Microscopy coupled with Fe L_2,3-edge and C K-edge NEXAFS.It appears that glass alteration increases with the amount of magnetite and that the transport of aqueous species is a key parameter. Several processes have been identified such as (i) the silica sorption on the magnetite surface, (ii) the precipitation of Fe-silicates in the vicinity of the glass (iii) the precipitation of SiO₂ on the magnetite surface, (iv) the incorporation of Fe within the alteration layer. Process (iv) was not frequently observed, suggesting local variations in geochemical conditions. Moreover, this process is strongly influenced by the transport of aqueous species as indicated by the morphology and composition of the alteration layers. Indeed, when glass and magnetite are homogeneously mixed, the glass alteration layer consists of a gel enriched in Fe having the same Fe(II)/Fe(III) ratio as in magnetite. When both materials are separated by a diffusive barrier, the glass alteration layer consists of a porous gel (not enriched in iron) in presence of a mixture of Fe-silicates with Fe having the same valence as in magnetite, rare-earth precipitates and phyllosilicates. These results suggest that Fe incorporation within the alteration layer changes depending on the distance and the time required for dissolved Fe originating from the magnetite to reach the glass.     

Numerical simulation of groundwater flow and contamination transport in Nahavand plain aquifer,west of Iran

Numerical simulation of groundwater flow used for the estimation of hydraulic and hydrologic parameters which is an important tool for the management of aquifers. This study presents the results of a mathematical model developed for the simulation of groundwater flow in Nahavand plain aquifer in the southwest Hamadan province. For this purpose Groundwater Modeling Software (GMS) was used which supports the MODFLOW-2000 code. After gathering required data such as the hydrological, hydrogeological and topography maps, a 3D hydrogeological model of plain was constructed with borehole and surface elevation data. Then MODFLOW was used for simulation of flow. After initial simulation of the flow, the model was calibrated in steady state with trial-and-error and parameter estimation methods the observed head of groundwater table monitoring data of 1997. Results of calibration show that error between observed head and computed head is in allowable range. Also results of computed head with model show that groundwater flow is in the direction of the dominate slope (southeast to northwest). Finally MODPATH code which simulates advective transport of particles was used for estimation of flow path and source of contaminants.     

The processes affecting oxytetracycline contamination of groundwater in a phreatic aquifer underlying industrial fish ponds in Israel

Oxytetracycline (OTC) is an antibiotic that is regularly added to fish ponds used in industrial- scale fish-farming along the Mediterranean coast of Israel. It is not found in unpolluted water. The ponds are constructed upon porous sands that are also the host rock of the local aquifer. These ponds leak. Monitoring for OTC was carried out in a fish pond and three surrounding water wells that exploit the local aquifer. The local groundwater is composed of the native groundwater and the leakage additions. Two wells, P-204 P-205 are used to insure that the water levels in the ponds are maintained. A frequently pumping well, M-2, is located closest to the main source of OTC input at pond B. It is used to supply drinking water and OTC was found in it. This contaminant is related to leakage from the nearby fish pond. OTC is generally assumed to be immobile and effectively removed by soil and sediments. In this case, continuous leakage of the pond has created preferential saturated flow paths that bring the contaminants directly to groundwater, bypassing the immobilizing sorption processes that usually take place in the unsaturated zone. In the absence of such conduits OTC is effectively excluded from reaching other producing wells.     

Modelling the vulnerability of groundwater to contamination in an unconfined alluvial aquifer in Pakistan

The area of Thal Doab is located in the Indus Basin and is underlain by a thick alluvial aquifer called the Thal Doab aquifer (TDA). The TDA is undergone intense hydrological stress owing to rapid population growth and excessive groundwater use for livestock and irrigated agricultural land uses. The potential impact of these land uses on groundwater quality was assessed using a DRASTIC model in a Geographic Information System environment. Seven DRASTIC thematic maps were developed at fixed scale and then combined into a groundwater vulnerability map. The resultant vulnerability index values were grouped into four zones as low, moderate, high and very high. The study has established that 76% of the land area that is underlain by the TDA has a high to very high vulnerability to groundwater contamination mainly because of a thin soil profile, a shallow water table and the presence of soils and sediments with high hydraulic conductivity values. In addition, only 2 and 22% of the total area lie in low and moderate vulnerability zones, respectively. The outcomes of this study can be used to improve the sustainability of the groundwater resource through proper land-use management.