Clifford I. Voss

Evaluation of the sustainability of deep groundwater as an arsenic-safe resource in the Bengal Basin.

Tens of millions of people in the Bengal Basin region of Bangladesh and India drink groundwater containing unsafe concentrations of arsenic. This high-arsenic groundwater is produced from shallow (<100 m) depths by domestic and irrigation wells in the Bengal Basin aquifer system. The government of Bangladesh has begun to install wells to depths of >150 m where groundwater arsenic concentrations are nearly uniformly low, and many more wells are needed, however, the sustainability of deep, arsenic-safe groundwater has not been previously assessed. Deeper pumping could induce downward migration of dissolved arsenic, permanently destroying the deep resource. Here, it is shown, through quantitative, large-scale hydrogeologic analysis and simulation of the entire basin, that the deeper part of the aquifer system may provide a sustainable source of arsenic-safe water if its utilization is limited to domestic supply. Simulations provide two explanations for this result: deep domestic pumping only slightly perturbs the deep groundwater flow system, and substantial shallow pumping for irrigation forms a hydraulic barrier that protects deeper resources from shallow arsenic sources. Additional analysis indicates that this simple management approach could provide arsenic-safe drinking water to >90% of the arsenic-impacted region over a 1,000-year timescale. This insight may assist water-resources managers in alleviating one of the worlds largest groundwater contamination problems.

Groundwater lens dynamics of Atoll Islands

A variable-density groundwater model is used to analyze the effects of various controls on the size of the freshwater lens, the structure of the transition zone, and the propagation of tidal fluctuations in a two-layer atoll island groundwater system. Modeling results indicate that mixing of fresh water and saltwater occurs primarily as a result of oscillating vertical flow due to tidal fluctuations and depends to a lesser extent on transverse dispersion along the dominantly horizontal recharge-discharge path of flow. The controls on the amount of mixing are: (1) the accumulated vertical distance, which increases with tidal range and is restricted by vertical permeabilities, and (2) vertical longitudinal dispersion. Comparison of cross-sectional simulations of atoll islands using nontidal and tidal models shows that the nontidal model must use artificially high values of transverse dispersivity to compensate for the lack of tidally driven, vertical mixing processes. Although the tidal model has high computational requirements, it can be used to calibrate vertical permeabilities and is best suited for problems dealing with groundwater resource evaluations, hydrologic events, and hydrologic processes. The limitations of the nontidal model are that it cannot be used for calibration of vertical permeabilities and will not realistically simulate those cases in which transition zones are thick or recharge low.

Analysis of an anisotropic coastal aquifer system using variable-density flow and solute transport simulation

Abstract The groundwater system in southern Oahu, Hawaii consists of a thick, areally extensive freshwater lens overlying a zone of transition to a thick saltwater body. This system is analyzed in cross section with a variable-density groundwater flow and solute transport model on a regional scale. The simulation is difficult, because the coastal aquifer system has a saltwater transition zone that is broadly dispersed near the discharge area, but is very sharply defined inland. Steady-state simulation analysis of the transition zone in the layered basalt aquifer of southern Oahu indicates that a small transverse dispersivity is characteristic of horizontal regional flow. Further, in this system flow is generally parallel to isochlors and steady-state behavior is insensitive to the longitudinal dispersivity. Parameter analysis identifies that only six parameters control the complex hydraulics of the system: horizontal and vertical hydraulic conductivity of the basalt aquifer; hydraulic conductivity of the confining “caprock” layer; leakance below the caprock; specific yield; and aquifer matrix compressibility. The best-fitting models indicate the horizontal hydraulic conductivity is significantly greater than the vertical hydraulic conductivity. These models give values for specific yield and aquifer compressibility which imply a considerable degree of compressive storage in the water table aquifer.

Climate change impacts on the temperature and magnitude of groundwater discharge from shallow, unconfined aquifers

Cold groundwater discharge to streams and rivers can provide critical thermal refuge for threatened salmonids and other aquatic species during warm summer periods. Climate change may influence groundwater temperature and flow rates, which may in turn impact riverine ecosystems. This study evaluates the potential impact of climate change on the timing, magnitude, and temperature of groundwater discharge from small, unconfined aquifers that undergo seasonal freezing and thawing. Seven downscaled climate scenarios for 2046–2065 were utilized to drive surficial water and energy balance models (HELP3 and ForHyM2) to obtain future projections for daily ground surface temperature and groundwater recharge. These future surface conditions were then applied as boundary conditions to drive subsurface simulations of variably saturated groundwater flow and energy transport. The subsurface simulations were performed with the U.S. Geological Survey finite element model SUTRA that was recently modified to include the dynamic freeze-thaw process. The SUTRA simulations indicate a potential rise in the magnitude (up to 34%) and temperature (up to 3.6°C) of groundwater discharge to the adjacent river during the summer months due to projected increases in air temperature and precipitation. The thermal response of groundwater to climate change is shown to be strongly dependent on the aquifer dimensions. Thus, the simulations demonstrate that the thermal sensitivity of aquifers and baseflow-dominated streams to decadal climate change may be more complex than previously thought. Furthermore, the results indicate that the probability of exceeding critical temperature thresholds within groundwater-sourced thermal refugia may significantly increase under the most extreme climate scenarios.

Permafrost thaw in a nested groundwater-flow system

Groundwater flow in cold regions containing permafrost accelerates climate-warming-driven thaw and changes thaw patterns. Simulation analyses of groundwater flow and heat transport with freeze/thaw in typical cold-regions terrain with nested flow indicate that early thaw rate is particularly enhanced by flow, the time when adverse environmental impacts of climate-warming-induced permafrost loss may be severest. For the slowest climate-warming rate predicted by the Intergovernmental Panel on Climate Change (IPCC), once significant groundwater flow begins, thick permafrost layers can vanish in several hundred years, but survive over 1,000 years where flow is minimal. Large-scale thaw depends mostly on the balance of heat advection and conduction in the supra-permafrost zone. Surface-water bodies underlain by open taliks allow slow sub-permafrost flow, with lesser influence on regional thaw. Advection dominance over conduction depends on permeability and topography. Groundwater flow around permafrost and flow through permafrost impact thaw differently; the latter enhances early thaw rate. Air-temperature seasonality also increases early thaw. Hydrogeologic heterogeneity and topography strongly affect thaw rates/patterns. Permafrost controls the groundwater/surface-water-geomorphology system; hence, prediction and mitigation of impacts of thaw on ecology, chemical exports and infrastructure require improved hydrogeology/permafrost characterization and understanding.RésuméL’écoulement souterrain de l’eau dans les régions froides présentant un pergélisol accélère le dégel généré par le réchauffement climatique et change le modèle du dégel. Les simulations d’écoulement souterrain et de transport de chaleur avec gel/dégel dans les régions froides typiques et écoulement en réseau, indiquent que le taux de dégel précoce s’accroît particulièrement du fait de l’écoulement, à un moment où les impacts environnementaux défavorables d’une disparition du permafrost induite par le réchauffement climatique peuvent être les plus sévères. Pour le taux de réchauffement climatique le plus bas prévu par la Commission Intergouvernementale sur le Changement Climatique (IPCC), une fois qu’un écoulement souterrain significatif débute, les niveaux de pergélisol épais peuvent disparaître en plusieurs centaines d’années, mais survivre plus de 1000 ans quand l’écoulement est minimal. Le dégel à grande échelle dépend principalement du bilan d’advection de la chaleur et de la conduction dans la zone supra-pergélisol. Les masses d’eau à la base des taliks ouverts permettent un écoulement infra-pergélisol lent, avec une influence moindre sur le dégel régional. La prépondérance de l’advection sur la conduction dépend de la perméabilité et de la topographie. L’écoulement souterrain autour du pergélisol et l’écoulement à travers le pergélisol impactent le dégel différemment; le dernier type d’écoulement accroît le taux de dégel de façon précoce. La saisonnalité air-température augmente aussi le dégel précoce. L’hétérogénéité de l’hydrogéologie et la topographie affectent fortement caractéristiques et taux du dégel. Le pergélisol contrôle le système eau souterraine-eau de surface-géomorphologie ; par suite, la prévision et l’atténuation des impacts du dégel sur l’écologie, sur les mobilisations chimiques et sur le substrat nécessitent une caractérisation et une meilleure compréhension de l’hydrogéologie liée au pergélisol.ResumenEl flujo de agua subterránea en regiones frías que contienen un permafrost acelera el calentamiento climático impulsado por el descongelamiento y cambios en los patrones de descongelamiento. Los análisis de simulación del flujo de agua subterránea y del transporte de calor con congelamiento / descongelamiento en terrenos típicos de regiones frías con flujos jerarquizados indican que la tasa de deshielo temprana es particularmente realzada por el flujo y que la época en que los impactos ambientales adversos del calentamiento climático inducido por la pérdida de permafrost puede ser más severa. Para las tasas más lentas del calentamiento climático predichas por el Intergovernmental Panel on Climate Change (IPCC), una vez que un flujo significativo de agua subterránea comienza, las gruesas capas de permafrost pueden desaparecer en varios cientos de años, pero sobreviven más de 1000 años donde el flujo es mínimo. El descongelamiento a gran escala depende principalmente del balance de la advección y conducción de calor en la zona supra—permafrost. Los cuerpos de agua superficial sustentados por taliks abiertos permiten mostrar el lento flujo sub-permafrost, con una menor influencia en el descongelamiento regional. El dominio de la advección sobre la conducción depende de la permeabilidad y la topografía. El flujo de agua subterránea alrededor del permafrost y el flujo a través del permafrost impactan en forma diferente en el descongelamiento; este último aumenta la tasa de descongelamiento temprano. Estacionalmente la temperatura del aire también incrementa el deshielo temprano. La heterogeneidad hidrogeológica y la topografía afectan fuertemente las tasas / patrones de deshielo. El permafrost controla el sistema geomorfología—agua superficial—agua subterránea; por lo tanto la predicción y mitigación de los impactos del descongelamiento sobre la ecología, las exportaciones de productos químicos y la infraestructura requiere una mejor caracterización y entendimiento de la hidrogeología / permafrost.摘要具有永久冻土的寒区地下水流使气候变暖驱使的融化加速,并改变融化类型。具有嵌入水流的典型寒区地下水流及冻结/融化的热传导模拟分析表明早期消融速率主要由水流增强的,气候变暖导致的永久冻土消失引起的环境负效应的时间是严格的。根据联合国政府间气候变化专门委员会(IPCC)预计的最慢气候变暖速率,一旦明显的地下水流形成,厚的永久冻土层在几百年内会消失,但是在最小水流时可存在千年以上。大规模的消融更依赖于上永久冻土地带的热对流及传导平衡。下伏有开放融区的地表水体可以允许缓慢的地下永久冻土水流,这对区域消融影响较小。强于传导的对流优势取决于渗透性和地形。永久冻土附近的地下水流以及通过永久冻土的水流对消融的不同影响,后者增加早期消融速率。空气温度的季节性同样增强了早期消融。永久冻土控制地下水-地表水-地貌系统;所以消融对生态、化学输出以及基础设施的影响预测及减缓需要提高对水文地质/永久冻土的描述和理解。ResumoO fluxo de água subterrânea em regiões frias contendo permafrost acelera a fusão motivada por aquecimento climático e altera os padrões de fusão. A análise por simulação do escoamento de água subterrânea e de transporte de calor com congelamento/fusão em terrenos de regiões frias típicas com escoamento encaixado indicam que a taxa de fusão precoce é particularmente ampliada pelo fluxo, num momento em que os impactos ambientais adversos da perda de permafrost induzida pelo aquecimento climático podem ser mais severos. Para a mais baixa taxa de aquecimento climático prevista pelo Painel Intergovernamental sobre Mudanças Climáticas (IPCC), logo que se inicie um significativo fluxo subterrâneo, as camadas espessas de permafrost podem desaparecer em algumas centenas de anos, mas sobrevivem para cima de 1000 anos quando o fluxo é mínimo. A fusão em larga escala depende principalmente do balanço entre adveção e condução de calor na zona superior do permafrost. As massas de água superficial sobrepostas a taliks abertos permitem um fluxo lento na zona inferior do permafrost, com menor influência na fusão regional. O domínio da adveção sobre a condução depende da permeabilidade e da topografia. O fluxo de água subterrânea em torno do permafrost e o fluxo através do permafrost afetam a fusão de modo diverso; este último ampliando as taxas de fusão precoce. A heterogeneidade hidrogeológica e a topografia afetam fortemente as taxas/padrões de fusão. O permafrost controla o sistema geomorfológico-água subterrânea-água superficial; em consequência, a predição e a mitigação de impactos da fusão na ecologia, na exportação de químicos e nas infraestruturas requerem uma melhorada caracterização e compreensão da hidrogeologia/permafrost.

Sampling design for groundwater solute transport: Tests of methods and analysis of Cape Cod tracer test data

Tests of a one-dimensional sampling design methodology on measurements of bromide concentration collected during the natural gradient tracer test conducted by the U.S. Geological Survey on Cape Cod, Massachusetts, demonstrate its efficacy for field studies of solute transport in groundwater and the utility of one-dimensional analysis. The methodology was applied to design of sparse two-dimensional networks of fully screened wells typical of those often used in engineering practice. In one-dimensional analysis, designs consist of the downstream distances to rows of wells oriented perpendicular to the groundwater flow direction and the timing of sampling to be carried out on each row. The power of a sampling design is measured by its effectiveness in simultaneously meeting objectives of model discrimination, parameter estimation, and cost minimization. One-dimensional models of solute transport, differing in processes affecting the solute and assumptions about the structure of the flow field, were considered for description of tracer cloud migration. When fitting each model using nonlinear regression, additive and multiplicative error forms were allowed for the residuals which consist of both random and model errors. The one-dimensional single-layer model of a nonreactive solute with multiplicative error was judged to be the best of those tested. Results show the efficacy of the methodology in designing sparse but powerful sampling networks. Designs that sample five rows of wells at five or fewer times in any given row performed as well for model discrimination as the full set of samples taken up to eight times in a given row from as many as 89 rows. Also, designs for parameter estimation judged to be good by the methodology were as effective in reducing the variance of parameter estimates as arbitrary designs with many more samples. Results further showed that estimates of velocity and longitudinal dispersivity in one-dimensional models based on data from only five rows of fully screened wells each sampled five or fewer times were practically equivalent to values determined from moments analysis of the complete three-dimensional set of 29,285 samples taken during 16 sampling times.

New permafrost is forming around shrinking Arctic lakes, but will it last?

Widespread lake shrinkage in cold regions has been linked to climate warming and permafrost thaw. Permafrost aggradation, however, has been observed within the margins of recently receded lakes, in seeming contradiction of climate warming. Here permafrost aggradation dynamics are examined at Twelvemile Lake, a retreating lake in interior Alaska. Observations reveal patches of recently formed permafrost within the dried lake margin, colocated with discrete bands of willow shrub. We test ecological succession, which alters shading, infiltration, and heat transport, as the driver of aggradation using numerical simulation of variably saturated groundwater flow and heat transport with phase change (i.e., freeze-thaw). Simulations support permafrost development under current climatic conditions, but only when net effects of vegetation on soil conditions are incorporated, thus pointing to the role of ecological succession. Furthermore, model results indicate that permafrost aggradation is transitory with further climate warming, as new permafrost thaws within seven decades.

Solute transport with equilibrium aqueous complexation and either sorption or ion exchange: Simulation methodology and applications

Abstract Methodologies that account for specific types of chemical reactions in the simulation of solute transport can be developed so they are compatible with solution algorithms employed in existing transport codes. This enables the simulation of reactive transport in complex multidimensional flow regimes, and provides a means for existing codes to account for some of the fundamental chemical processes that occur among transported solutes. Two equilibrium-controlled reaction systems demonstrate a methodology for accommodating chemical interaction into models of solute transport. One system involves the sorption of a given chemical species, as well as two aqueous complexations in which the sorbing species is a participant. The other reaction set involves binary ion exchange coupled with aqueous complexation involving one of the exchanging species. The methodology accommodates these reaction systems through the addition of nonlinear terms to the transport equations for the sorbing species. Example simulation results show (1) the effect equilibrium chemical parameters have on the spatial distributions of concentration for complexing solutes; (2) that an interrelationship exists between mechanical dispersion and the various reaction processes; (3) that dispersive parameters of the porous media cannot be determined from reactive concentration distributions unless the reaction is accounted for or the influence of the reaction is negligible; (4) how the concentration of a chemical species may be significantly affected by its participation in an aqueous complex with a second species which also sorbs; and (5) that these coupled chemical processes influencing reactive transport can be demonstrated in two-dimensional flow regimes.

The transboundary non-renewable Nubian Aquifer System of Chad, Egypt, Libya and Sudan: classical groundwater questions and parsimonious hydrogeologic analysis and modeling

Parsimonious groundwater modeling provides insight into hydrogeologic functioning of the Nubian Aquifer System (NAS), the world’s largest non-renewable groundwater system (belonging to Chad, Egypt, Libya, and Sudan). Classical groundwater-resource issues exist (magnitude and lateral extent of drawdown near pumping centers) with joint international management questions regarding transboundary drawdown. Much of NAS is thick, containing a large volume of high-quality groundwater, but receives insignificant recharge, so water-resource availability is time-limited. Informative aquifer data are lacking regarding large-scale response, providing only local-scale information near pumps. Proxy data provide primary underpinning for understanding regional response: Holocene water-table decline from the previous pluvial period, after thousands of years, results in current oasis/sabkha locations where the water table still intersects the ground. Depletion is found to be controlled by two regional parameters, hydraulic diffusivity and vertical anisotropy of permeability. Secondary data that provide insight are drawdowns near pumps and isotope-groundwater ages (million-year-old groundwaters in Egypt). The resultant strong simply structured three-dimensional model representation captures the essence of NAS regional groundwater-flow behavior. Model forecasts inform resource management that transboundary drawdown will likely be minimal—a nonissue—whereas drawdown within pumping centers may become excessive, requiring alternative extraction schemes; correspondingly, significant water-table drawdown may occur in pumping centers co-located with oases, causing oasis loss and environmental impacts.RésuméUne modélisation simplifiée de la nappe donne un aperçu du fonctionnement hydrogéologique du Système Aquifère Nubien, le plus grand aquifère non alimenté du monde (Tchad, Egypte, Lybie, Soudan). Les problèmes classiques liés à la ressource en eau souterraine existent (amplitude du rabattement et rayon d’influence des stations de pompage), avec des questions de gestion internationale coordonnée des rabattements transfrontaliers. Le Système Aquifère Nubien est épais dans sa plus grande partie, il contient une nappe de grande qualité mais reçoit une recharge insignifiante, de sorte que la disponibilité de la ressource en eau est limitée dans le temps. Les données sur l’aquifère modélisé font défaut pour ce qui est de la réponse à grande échelle, elles fournissent seulement des informations autour des pompages. Des données transposées fournissent une base essentielle pour la compréhension de la réponse régionale. L’abaissement du niveau de la nappe holocène depuis la fin de la période pluvieuse précédente, il y a des milliers d’années, conduit aux oasis et sebkha actuelles, lieux où la surface de la nappe recoupe la surface du sol. La baisse est contrôlée par deux paramètres régionaux, la diffusivité hydraulique et l’anisotropie verticale de la perméabilité. Les données secondaires qui fournissent un aperçu sur le fonctionnement sont les rabattements près des stations de pompage et l’âge isotopique des eaux souterraines (un million d’années en Egypte). Le modèle tridimensionnel résultant, structuré de façon fortement simplifiée cerne l’essentiel du comportement de l’écoulement souterrain régional du Système Aquifère Nubien. Le modèle prévisionnel informe le gestionnaire de la ressource que le rabattement transfrontalier sera vraisemblablement minimal—un non problème—quoique le rabattement autour des stations de pompage puisse devenir excessif, nécessitant des schémas alternatifs de prélèvement et qu’un rabattement significatif puisse se produire autour des stations de pompages localisées dans les oasis, provoquant leur ruine et des impacts environnementaux.ResumenUn escaso modelado del agua subterránea proporciona una visión del funcionamiento hidrogeológico del Sistema Acuífero Nubio (NAS), el mayor sistema de agua subterránea no renovable del mundo (en Chad, Egipto, Libia, Sudan). Existen temas clásicos del recurso de agua subterránea (magnitud y extensión lateral del descenso del nivel de agua próximo a centros de bombeo) con cuestiones de manejo internacional conjunto en relación con el descenso del nivel de agua transnacional. Mucho del NAS de gran espesor, contiene agua subterránea de alta calidad, pero recibe una recarga insignificante, de manera que la disponibilidad del recurso agua está limitada en el tiempo. Se carece de datos informativos en relación a su respuesta a gran escala, proporcionándose sólo información a escala local cerca de los bombeos. Los datos de proximidad proporcionan un soporte primario para entender la respuesta regional. El descenso del nivel freático Holoceno a partir del período lluvioso previo, después de miles de años, resulta en localizaciones actuales de oasis/sabkha donde el nivel freático todavía intersecta la superficie del terreno. El descenso del nivel de agua es controlado por dos parámetros regionales, la difusividad hidráulica y la anisotropía vertical de la permeabilidad. Datos secundarios que proporcionan un conocimiento son los descensos del nivel del agua cerca de los bombeos y las edades isotópicas del agua subterránea (aguas subterráneas de un millón de años en Egipto). La representación resultante, fuerte y simplemente estructurada del modelo tridimensional captura la esencia del comportamiento del flujo regional de agua subterránea del NAS. Los pronósticos del modelo aportan al manejo del recurso ya que el descenso del nivel de agua transnacional será probablemente mínimo—no problemático—mientras que el descenso dentro de los centros de bombeo pueden convertirse en excesivo, requiriendo esquemas de extracción alternativos, y un descenso significativo del nivel freático puede ocurrir en los centros de bombeo localizados en coincidencia con los oasis, causando la pérdida de oasis e impactos ambientales.摘要的地下水模拟论述了世界上最 大的不可更新的地下水 系统--(乍得、埃及、利比亚和苏丹的)努比亚含水层系统的水文地质情况。典型的地下水资源问题与有关跨国界抽水的联合的国际管理问题共存。大部分努比亚含水层系统很 厚,包含高质量的地下水,但是接受很少的补给,因此水资源可用量有 时间上的限制。有关大规模响应方面的含水层资料匮乏,只有抽水井附近的局部 规模的信息。替代性资料为了解区域 响应提供了主要支持。从前一个洪积期全新世水位下降,几千年后导致形成了目前的绿洲/盐沼,这里的水位仍然贯穿地面。消耗受两个 区域参数 控制,水利扩散系数和渗透的垂直各向异性。次级资料为水泵附近的抽水资料及同位素地下水年龄(埃及百万年的地下水)。合成、牢固、简单结构的三维模型获得了努比亚含水层系统区域 地下水 水流特性。模型预测结果告知了资源管理部门,跨国界水位下降可能很小-不是个问题—而抽水中心 分内的下降可能过度,需要采取更替抽水的 方案,重大的 水位下降可能出现在抽水中心—与绿洲在同一位置,引起绿洲萎缩和 环境影响。ResumoA modelação parcimoniosa de águas subterrâneas fornece perceções sobre o funcionamento hidrogeológico do Sistema Aquífero Núbio (SAN), o maior sistema de água subterrânea não renovável do mundo (Chade, Egito, Líbia, Sudão). Neste sistema existem os problemas clássicos dos recursos hídricos subterrâneos (magnitude e extensão lateral do rebaixamento nas proximidades de centros de bombeamento), associadas a questões de gestão internacionais que dizem respeito a rebaixamentos transfronteiriços. Grande parte do SAN é de grande espessura, contendo águas subterrâneas de alta qualidade, mas recebe uma recarga insignificante, apresentando portanto uma disponibilidade hídrica temporalmente limitada. Há falta de dados que permitam prever respostas do aquífero a grande escala, providenciando apenas informações a escala local, na vizinhança dos pontos de bombeamento. Os dados de proximidade às captações não permitem a avaliação da resposta regional: o rebaixamento do nível freático do Holocénico a partir do período pluvioso anterior, após milhares de anos, resulta na localização atual dos oásis/sabkha, onde o nível freático ainda interseta a superfície. O grande exaurimento é controlado por dois parâmetros regionais, a difusividade hidráulica e a anisotropia da permeabilidade vertical. Os dados secundários que fornecem informações são os rebaixamentos perto dos pontos de bombeamento e as idades das águas subterrâneas estimadas por métodos isotópicos (da ordem dos milhões de anos no Egito). A forte representação da estrutura simples do modelo tridimensional capta o fundamental do comportamento do fluxo regional de águas subterrâneas do SAN. Previsões do modelo de gestão de recursos informam que os rebaixamentos transfronteiriços serão provavelmente mínimos—um não problema—enquanto os rebaixamentos na área de influência dos centros de bombeamento podem tornar-se excessivos, necessitando de esquemas de extração alternativos, e significativos nos centros de bombeamento co-localizados em zonas de oásis, causando perdas e impactes ambientais nesses oásis.

Effects of clay dispersion on aquifer storage and recovery in coastal aquifers

Cyclic injection, storage, and withdrawal of freshwater in brackish aquifers is a form of aquifer storage and recovery (ASR) that can beneficially supplement water supplies in coastal areas. A 1970s field experiment in Norfolk, Virginia, showed that clay dispersion in the unconsolidated sedimentary aquifer occurred because of cation exchange on clay minerals as freshwater displaced brackish formation water. Migration of interstitial clay particles clogged pores, reduced permeability, and decreased recovery efficiency, but a calcium preflush was found to reduce clay dispersion and lead to a higher recovery efficiency. Column experiments were performed in this study to quantify the relations between permeability changes and clay mineralogy, clay content, and initial water salinity. The results of these experiments indicate that dispersion of montmorillonite clay is a primary contributor to formation damage. The reduction in permeability by clay dispersion may be expressed as a linear function of chloride content. Incorporating these simple functions into a radial, cross-sectional, variable-density, ground-water flow and transport model yielded a satisfactory simulation of the Norfolk field test – and represented an improvement over the model that ignored changes in permeability. This type of model offers a useful planning and design tool for ASR operations in coastal clastic aquifer systems.