Wg Burgess

Arsenic poisoning of Bangladesh groundwater

In Bangladesh and West Bengal, alluvial Ganges aquifers used for public water supply are polluted with naturally occurring arsenic, which adversely affects the health of millions of people. Here we show that the arsenic derives from the reductive dissolution of arsenic-rich iron oxyhydroxides, which in turn are derived from weathering of base-metal sulphides. This finding means it should now be possible, by sedimentological study of the Ganges alluvial sediments, to guide the placement of new water wells so they will be free of arsenic.

Mechanism of arsenic release to groundwater, Bangladesh and West Bengal

In some areas of Bangladesh and West Bengal, concentrations of As in groundwater exceed guide concentrations, set internationally and nationally at 10 to 50 m gl ˇ1 and may reach levels in the mg l ˇ1 range. The As derives from reductive dissolution of Fe oxyhydroxide and release of its sorbed As. The Fe oxyhydroxide exists in the aquifer as dispersed phases, such as coatings on sedimentary grains. Recalculated to pure FeOOH, As concentrations in this phase reach 517 ppm. Reduction of the Fe is driven by microbial metabolism of sedimentary organic matter, which is present in concentrations as high as 6% C. Arsenic released by oxidation of pyrite, as water levels are drawn down and air enters the aquifer, contributes negligibly to the problem of As pollution. Identification of the mechanism of As release to groundwater helps to provide a framework to guide the placement of new water wells so that they will have acceptable concentrations of As. # 2000 Elsevier Science Ltd. All rights reserved.

Constraints on sustainable development of arsenic-bearing aquifers in southern Bangladesh. Part 1: A conceptual model of arsenic in the aquifer

Abstract Arsenic is widespread in groundwater of the Holocene alluvial aquifers in southern Bangladesh, yet its concentration is highly variable spatially and with depth. A conceptual model of arsenic in the aquifer is proposed, as a basis for addressing questions concerning sustainability of groundwater development. Patterns and profiles of arsenic distribution in the aquifer have been determined at Meherpur in western Bangladesh, over an area of 15 km2 and a depth range of 15–225 m. The hydrochemical and hydraulic environments of arsenic occurrence have been established. The conceptual model incorporates the conditions of arsenic release to groundwater, the depth distribution of the arsenic source, likely sedimentological controls on the lateral discontinuity of the arsenic source, and the hydraulic regime imposed by pumping from the hydrogeologically leaky, multi-layered aquifer. Reducing conditions, conducive to arsenic release from sedimentary iron oxyhydroxides, are widespread. The arsenic source occurs at a distinct horizon at a depth of about 20m, but is laterally discontinuous. The catchments of shallow, hand-pumped tubewells (HTWs) are limited in extent by vertical leakage. Arsenic concentration in water pumped from tubewells depends on the depth separation between the HTW screen and the arsenic source, the overlap between the HTW catchment and the arsenic source layer, and the duration of pumping. Implications are drawn for treatment, tubewell location and design, monitoring, and predictive modelling.

Hydrochemical associations and depth profiles of arsenic and fluoride in Quaternary loess aquifers of northern Argentina

Abstract Arsenic and fluoride in groundwater from Quaternary loess deposits in Argentina pose major health concerns. Common sources for arsenic and fluoride have been suggested but the processes of mobilization are disputed, and distributions in groundwater are largely unresolved at a sample density >1/50 km2. At Los Pereyras in Tucuman Province, northern Argentina, we have evaluated distributions and hydrochemical associations of arsenic and fluoride with a sample density of 0.75 per km2 over an area of 75 km2, to a depth of 230 m. Groundwater in the loess is oxic and alkaline. Fluoride is restricted to the upper 20 m of the Quaternary loess, where it reaches 8.3 mg/l. Arsenic has a vertical layering consistent with that of fluoride, ranging from 20 to 760 μg/l in the upper 20 m and 58-163 μg/l below this. There are two sources of arsenic, one unrelated to the fluoride source. Positive correlations between arsenic and fluoride with pH, but not with alkalinity, support desorption from iron oxyhydroxides as the likely mechanism of release to groundwater for arsenic and fluoride, rather than the weathering of silicate minerals. Stratigraphic and/or palaeohydrological controls may explain the observed depth distributions within the loess aquifer.

Preliminary observations on the release of arsenic to groundwater in the presence of hydrocarbon contaminants in UK aquifers

Abstract Degradation of contaminant hydrocarbons in groundwater by microbially mediated oxidation, linked to the reduction of electron acceptors, is fundamental to the strategy of ‘monitored natural attenuation’ (MNA) for oxidizable hydrocarbons, which is increasingly being adopted at polluted aquifer sites throughout Europe and North America. Commonly, oxygen is depleted and following the reduction of nitrate, solid-phase Fe oxides become the dominant electron acceptors. Arsenic, associated with Fe and Mn oxides in soils and sediments, may therefore be mobilized to groundwater and pose an additional threat to environmental receptors. In a pilot study of three aquifers in England, we have examined the extent to which arsenic is released to groundwater under Fe(III)-reducing conditions imposed by contaminant hydrocarbons. Results show that arsenic is locally mobilized in the Chalk to < 10 μg/l, in Quaternary gravels to 70 μg/l and in the Triassic sandstones to 160 μg/l. At the Chalk and Quaternary gravels sites arsenic mobilization is demonstrably linked to reduction of Fe- and Mn-oxides. This is not so at the Triassic sandstone site, where release of arsenic is related to elevated bicarbonate alkalinity. Redox-driven arsenic mobilization at other Triassic sandstone locations is possible. Further work is required on the solid-phase sources of arsenic in the aquifers, and to relate the hydrochemical observations to groundwater hydraulic conditions.

Estimating transmissivity from surface resistivity soundings: an example from the Thames Gravels

Abstract Surface electrical soundings can be used to extrapolate pumping test results over an area. Using the Dar Zarrouk parameters, a constant can be found which will simply translate resistivity to transmissivity, provided that either clay content or porosity is the primary control on resistivity and permeability. The applicability of the technique is tested on the gravel aquifer at Desborough Island, UK.

Constraints on sustainable development of arsenic-bearing aquifers in southern Bangladesh. Part 2: Preliminary models of arsenic variability in pumped groundwater

Abstract Numerical models of groundwater flow and arsenic transport to tubewells in southern Bangladesh have been developed, based on a conceptual model derived from field observations. The catchment of a single hand-pumped tubewell (HTW) is incorporated within a model domain 8110m2 in area and 60m thick. Three tubewell specifications represent typical Bangladesh HTW designs. Constant-concentration cells act as a single-layered arsenic source, arranged to represent the observed depth distribution of arsenic in the aquifer and the range of possible patterns of overlap between HTW catchments and discontinuous zones of arsenic release from sediment to groundwater. A variety of sorption regimes is simulated, and sensitivity to sorption is illustrated. Boundary conditions are modified to simulate the effects of deep production wells. The models reproduce the observed scale and range of arsenic concentration in groundwater pumped from HTWs, and demonstrate likely long-term trends. Breakthrough of arsenic to HTWs may occur a few years after the start of pumping, but at many tubewells the concentration of arsenic could continue to rise significantly over tens to hundreds of years. Spatial distributions and depth profiles of arsenic in groundwater from tubewells should be viewed as transient in the long term. These preliminary models allow implications for the sustainability of the shallow alluvial aquifer to be quantified provisionally. The mechanisms and scale of sorption of arsenic by the aquifer sediments remain as significant uncertainties.

Trends in Arsenic concentration at tubewells in Bangladesh: conceptual models, numerical models and monitoring proxies.

Groundwater across much of central and southern Bangladesh contains As at concentrations many times the World Health Organization recommended limit for drinking water, 10 μg/l. Approximately 70% of shallow hand-pumped tubewells (HTWs) comply with the Bangladesh national limit for drinking water, 50 μg/l. But will the As concentration in HTW discharge change with time? Attention is turning to the use of deeper tubewells (DTWs). Fewer than 1% of DTWs exceed the 50 μg/l As limit. For how long will DTWs remain As-safe? Prospects for sustainable development of groundwater either by shallow tubewells or by deep tubewells can only be judged by addressing the questions: will As concentration in tubewell discharge change with time; if so by how much, and how quickly? This chapter describes models developed to anticipate trends in As concentration at HTWs and DTWs in the alluvial aquifers, and assesses the field evidence for changing concentration over time.

Hydrogeological typologies of the Indo-Gangetic basin alluvial aquifer, South Asia

The Indo-Gangetic aquifer is one of the world’s most important transboundary water resources, and the most heavily exploited aquifer in the world. To better understand the aquifer system, typologies have been characterized for the aquifer, which integrate existing datasets across the Indo-Gangetic catchment basin at a transboundary scale for the first time, and provide an alternative conceptualization of this aquifer system. Traditionally considered and mapped as a single homogenous aquifer of comparable aquifer properties and groundwater resource at a transboundary scale, the typologies illuminate significant spatial differences in recharge, permeability, storage, and groundwater chemistry across the aquifer system at this transboundary scale. These changes are shown to be systematic, concurrent with large-scale changes in sedimentology of the Pleistocene and Holocene alluvial aquifer, climate, and recent irrigation practices. Seven typologies of the aquifer are presented, each having a distinct set of challenges and opportunities for groundwater development and a different resilience to abstraction and climate change. The seven typologies are: (1) the piedmont margin, (2) the Upper Indus and Upper-Mid Ganges, (3) the Lower Ganges and Mid Brahmaputra, (4) the fluvially influenced deltaic area of the Bengal Basin, (5) the Middle Indus and Upper Ganges, (6) the Lower Indus, and (7) the marine-influenced deltaic areas.RésuméL’aquifère de l’Indus et du Gange est une des ressources en eau transfrontalière la plus importante au monde et un des aquifères le plus exploité au monde. Pour mieux comprendre le système aquifère, des typologies ont été caractérisées pour cet aquifère ; elles intègrent pour la première fois un jeu de données disponibles sur l’ensemble du bassin de l’Indus et du Gange à une échelle transfrontalière, et apportent une conceptualisation alternative de ce système aquifère. Traditionnellement considéré et cartographié comme un simple aquifère homogène aux propriétés aquifères similaires et comme une ressource d’eau souterraine à l’échelle transfrontalière, les typologies mettent en évidence des différences significatives spatiales de la recharge, de la perméabilité, de la capacité de stockage et de la chimie des eaux souterraines sur l’ensemble du système aquifère à une échelle transfrontalière. Ces changements sont systématiques coïncidant aux changements à large échelle de la sédimentologie de l’aquifère alluvial du Pléistocène et de l’Holocène, du climat et des pratiques récentes d’irrigation. Sept typologies de l’aquifère sont présentées, chacune ayant un ensemble distinct de défis et d’opportunités pour le développement des eaux souterraines et une résilience différente à l’exploitation et au changement climatique. Les sept typologies sont: (1) la marge de piedmont, (2) Le haut Indus et le Ganges moyen supérieur, (3) le Ganges inférieur et le Brahmapoutre moyen, (4) la zone deltaïque du bassin du Bengale sous influence fluviale, (5) l’Indus moyen et le Ganges supérieur, (6) l’Indus inférieur, et (7) la zone deltaïque sous influence marine.ResumenEl acuífero Indo-Gangético es uno de los recursos hídricos transfronterizos más importantes y el acuífero más explotado del mundo. Para comprender mejor el sistema acuífero, por primera vez se han caracterizado tipologías para el acuífero, integrando los conjuntos de datos existentes a través de la cuenca hidrográfica Indo-Ganges a una escala transfronteriza, y proporcionando una conceptualización alternativa de este sistema acuífero. Tradicionalmente consideradas y cartografiadas como un solo acuífero homogéneo de propiedades acuíferas y recursos de agua subterránea comparables a escala transfronteriza, las tipologías iluminan diferencias espaciales significativas en la recarga, permeabilidad, almacenamiento y química del agua subterránea a través del sistema acuífero a esta escala transfronteriza. Estos cambios son mostrados por ser sistemáticos, coincidentes con cambios en gran escala en la sedimentología del acuífero aluvial del Pleistoceno y del Holoceno, en el clima y en las prácticas recientes de riego. Se presentan siete tipologías del acuífero, cada una con un conjunto distinto de desafíos y oportunidades para el desarrollo del agua subterránea y una diferente resiliencia a la extracción y al cambio climático. Las siete tipologías son: (1) el margen del piedemonte, (2) el Indus superior y el Ganges superior-medio, (3) el Ganges inferior y el Brahmaputra medio, (4) el área deltaica fluvialmente influenciada de la cuenca de Bengala, (5) el Ganges superior, (6) el Indo Inferior, y (7) las áreas deltaicas con influencia marina.摘要印度-恒河含水层是世界上最重要的跨界水资源之一,也是世界上开采量最大的含水层。为了更好地了解含水层系统,结合现有的印度-恒河流域盆地数据,第一次在跨界尺度上对含水层的类型进行了特征描述,提供了这个含水层系统供选择的概念模型。传统上,被认为并被绘制为可比较含水层特性的单一均质含水层及跨界尺度上地下水资源,类型学阐明了这个跨界尺度上含水层系统补给、透水率、储存量和地下水化学上的重要空间差别。这些变化显示与更新世和全新世冲积含水层、气候和最近灌溉实践沉积学上的大尺度变化是系统性的、也是一致的。展示了含水层的七个类型,每个类型都具有一套独特的地下水开发的挑战和机会以及针对抽水和气候变化的不同恢复力。七个类型为:(1)山前边缘;(2)印度河上游及恒河中上游;(3)恒河下游及雅鲁藏布江中游;(4)孟加拉流域洪积影响的三角洲地区;(5)印度河中游及恒河上游;(6)印度河下游;(7)海相影响的三角洲地区。ResumoO aquífero do Indo-Gangético é um dos recursos hídricos transfronteiriços mais importantes do mundo, e o mais explorado. Para melhor entender o sistema aquífero, tipologias foram caracterizadas para o aquífero, que integram pela primeira vez bancos de dados existentes sobre a bacia de abastecimento Indo-Gangética em uma escala transfornteiriça, e fornecem uma conceptualização alternativa desse sistema aquífero. Tradicionalmente considerado e mapeado como um aquífero homogêneo de propriedades aquíferas comparáveis e recursos subterrâneos em escala transfronteiriça, as tipologias elucidam diferenças espaciais significantes na recarga, permeabilidade, armazenamento, e química das águas subterrâneas pelo aquífero nessa escala transfronteiriça. Essas mudanças aparentam ser sistemáticas, concorrentes com mudanças em larga escala na sedimentologia do aquífero aluvial do Pleistoceno e Holoceno, clima e práticas recentes de irrigação. Sete tipologias do aquífero são apresentadas, cada uma tendo conjuntos de desafios e oportunidades distintos para o desenvolvimento das águas subterrâneas e uma resiliência diferente nas mudanças de extração e clima. As sete tipologias são: (1) a margem piemonte, (2) O Alto Indo e o Alto-médio Ganges, (3) O Baixo Ganges e o Médio Brahmaputra, (4) a área deltaica influenciada fluvialmente da Bacia de Bengala, (5) o Médio Indo e o Alto Ganges, (6) o Baixo Indo, e (7) as áreas deltaicas com influencia marinha.

Modelling the influence of solution-enhanced conduits on catchment-scale contaminant transport in the Hertfordshire Chalk aquifer

Abstract Catchment-scale (>40 km2) contamination of the Chalk aquifer of Hertfordshire by bromate, emanating from a disused industrial site north of St Albans, represents the largest occurrence of point-source groundwater contamination in the UK. The influences of ‘double porosity’ diffusive exchange and rapid transport along solution-enhanced conduits complicate predictive modelling of contaminant transport to threatened public supply wells. Tracer testing indicates that solution-enhanced flow routes exist beyond the surface distribution of dissolution features in Hertfordshire, more extensively than previously thought. A quantitative conceptual understanding of this flow system has been incorporated into a spatially distributed equivalent porous media representation in MODFLOW and MT3D-MS. The calibrated model reproduces essential features of the aquifer system, including heads and flows, seasonal responses, and the timing and spatial distribution of observed tracer breakthroughs in the solution-enhanced aquifer, but does not fully capture the magnitude and form of tracer and bromate advance. Due to the influence of local solution enhancement and matrix effects, detailed breakthroughs at receptors cannot be resolved at the coarse grid scale. However, the model is able to simulate general trends.