Anwar Zahid

Megacity pumping and preferential flow threaten groundwater quality.

Many of the worlds megacities depend on groundwater from geologically complex aquifers that are over-exploited and threatened by contamination. Here, using the example of Dhaka, Bangladesh, we illustrate how interactions between aquifer heterogeneity and groundwater exploitation jeopardize groundwater resources regionally. Groundwater pumping in Dhaka has caused large-scale drawdown that extends into outlying areas where arsenic-contaminated shallow groundwater is pervasive and has potential to migrate downward. We evaluate the vulnerability of deep, low-arsenic groundwater with groundwater models that incorporate geostatistical simulations of aquifer heterogeneity. Simulations show that preferential flow through stratigraphy typical of fluvio-deltaic aquifers could contaminate deep (>150 m) groundwater within a decade, nearly a century faster than predicted through homogeneous models calibrated to the same data. The most critical fast flowpaths cannot be predicted by simplified models or identified by standard measurements. Such complex vulnerability beyond city limits could become a limiting factor for megacity groundwater supplies in aquifers worldwide.

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.

Terrestrial water load and groundwater fluctuation in the Bengal Basin

Groundwater-level fluctuations represent hydraulic responses to changes in groundwater storage due to aquifer recharge and drainage as well as to changes in stress that include water mass loading and unloading above the aquifer surface. The latter ‘poroelastic’ response of confined aquifers is a well-established phenomenon which has been demonstrated in diverse hydrogeological environments but is frequently ignored in assessments of groundwater resources. Here we present high-frequency groundwater measurements over a twelve-month period from the tropical, fluvio-deltaic Bengal Aquifer System (BAS), the largest aquifer in south Asia. The groundwater level fluctuations are dominated by the aquifer poroelastic response to changes in terrestrial water loading by processes acting over periods ranging from hours to months; the effects of groundwater flow are subordinate. Our measurements represent the first direct, quantitative identification of loading effects on groundwater levels in the BAS. Our analysis highlights the potential limitations of hydrogeological analyses which ignore loading effects in this environment. We also demonstrate the potential for employing poroelastic responses in the BAS and across other tropical fluvio-deltaic regions as a direct, in-situ measure of changes in terrestrial water storage to complement analyses from the Gravity and Climate Experiment (GRACE) mission but at much higher resolution.

Security of deep groundwater in the coastal Bengal Basin revealed by tracers

Uncertainty persists regarding the vulnerability of deep groundwater across Asias megadeltas. In the coastal Bengal Basin aquifer system, shallow groundwater ( 80 million people. Here we report new radiocarbon evidence from a network of nine dedicated, multi‐level monitoring wells which indicates residence times of between 103 and 104 years for groundwater at depths >150 m. Modern groundwater detected in some deep abstraction wells using anthropogenic tracers (SF6, CFCs) is attributed to short‐circuiting of shallow groundwater within wells. Age‐depth profiles and hydrochemical data in monitoring wells confirm the regional resilience of deep groundwater to ingress of shallow contaminated groundwater. Our results are consistent with high regional anisotropy in the aquifer and support continued use of deep groundwater though the potential for leakage of shallow contaminated groundwater in deep abstraction wells requires careful monitoring.

Monitoring the Coastal Groundwater of Bangladesh

The low-lying topography and geographic position makes Bangladesh very vulnerable regarding the anticipated impacts of climate change that likely affects nearly all sectors of socio-economic life, where water sector is the most vulnerable and sensitive among them and country’s coastal areas are at the greatest risk. The primary source of fresh water in the coastal belt is groundwater. But very few studies have been conducted to assess the influence of climate change on this resource. There is also high vulnerability to contamination with salinity due to mixing of pre-existing fresh and saline groundwater accelerated by irrigation pumping and vertical infiltration of salt water from periodic storm surge flooding. Changing climate and population stress might affect various components like recharge, discharge, storage, and water quality. The volume of water which is retained in the top soil is required for agriculture and has a role on the process of evaporation, recharge of groundwater, and innovation of runoff. Tidal saltwater wedge because of rising sea levels would cause to encroach further upstream in rivers, resulting salinity intrusion in aquifers. To know the evidence of changes in the events of hydrologic cycle including groundwater quality and storage is very important in order to adapt with the climate change impacts. The principal source of the irrigation water in Bangladesh is groundwater since decades and is one of the major factors making the nation almost self-dependent in crop production. Bangladesh being a very low-elevated country, where main part of the landform in the coast is up to 2–3 m above mean sea level, sea-level rise can cause increased intrusion of saline water both in surface water and in groundwater system. Inadequate safe water for irrigation and water supply will create more stress on fresh water. Therefore, it is important to map fresh water—saline water distribution in coastal aquifers with other important physico-chemical parameters and predict future changes in this environment due to both climate change and anthropogenic stresses. Considering this, a monitoring network has been established by Bangladesh Water Development Board in 19 coastal districts with monitoring wells at variable aquifer depths down to 350 m. The objective of the project, under the support of Bangladesh Climate Change Trust, is to establish a long-term monitoring network to assess and monitor coastal water resources both on quantitative and on qualitative aspects. This paper focuses on to determine the potential of available fresh water resources and distribution of salinity in aquifers both spatially and vertically, mainly based on the monitoring data, for sustainable long-term use of very scarce fresh water in the region.

Model Impact of Climate Change on the Groundwater Flow and Salinity Encroachment in the Coastal Areas of Bangladesh

The coastal population of Bangladesh has already been suffering from the salinity encroachment both in groundwater and surface water regime. Reduced river discharge, coastal surges, shrimp farming, lowering of groundwater table due to dry season irrigation in a large part of the country accelerate the rate of saline water distribution. In addition, sea level rise, due to the impact of climate change, may contribute to salinity encroachment on coastal freshwater resources, particularly in the shallow alluvial aquifers. Though the groundwater table is within 2–5 m below ground surface, availability of fresh and safe water in the coast is very limited in upper aquifers because of the arsenic contamination and water salinity. For the coastal population, deep (>250 m) tube wells are the main source of drinking water and irrigation water supply is mostly restricted to surface water including rainwater. In monsoon, freshwater pockets are available at the shallow depth (<8 m) from seasonal precipitation but mostly turn to brackish condition during dry period. Therefore, assessment and monitoring of development stress and probable impact of climate change on freshwater resource are utmost important. The main purpose of the study is to assess the impact of climate change and development stresses on the availability of fresh water resources in the coastal area. For that purpose, integrated hydrological model has been prepared describing the subsurface condition both the saturated and unsaturated zone together with the influence of various water components of the hydrological cycle. Groundwater salinity models are developed to simulate salinity transport in the sea, river and through the porous medium of aquifer for a range of existing and possible future conditions. It has been seen from the simulation result of the model that under climate change condition during the month of March and April, the salinity level is highest for all river system within the study area and significant during period from December to June. The climate change scenario illustrates that the groundwater level increases within the range of 0.6–0.8 m under climate change scenario. Movement of salinity is found insignificant from the river to the aquifer. Major rivers in the south central coast there is a considerable interaction between surface water and groundwater due to the tidal effect. On the other hand, there is negligible interaction between the aquifer and adjacent river in the south-eastern coastal plain.