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.

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.

L'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.

El 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.

O 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.

VULNERABILITY OF LOW-ARSENIC AQUIFERS TO MUNICIPAL PUMPING IN BANGLADESH.

Sandy aquifers deposited >12,000 years ago, some as shallow as 30 m, have provided a reliable supply of low-arsenic (As) drinking water in rural Bangladesh. This study concerns the potential risk of contaminating these aquifers in areas surrounding the city of Dhaka where hydraulic heads in aquifers >150 m deep have dropped by 70 m in a few decades due to municipal pumping. Water levels measured continuously from 2012 to 2014 in 12 deep (>150m), 3 intermediate (90-150 m) and 6 shallow (<90 m) community wells, 1 shallow private well, and 1 river piezometer show that the resulting drawdown cone extends 15-35 km east of Dhaka. Water levels in 4 low-As community wells within the 62-147 m depth range closest to Dhaka were inaccessible by suction for up to a third of the year. Lateral hydraulic gradients in the deep aquifer system ranged from 1.7×10-4 to 3.7×10-4 indicating flow towards Dhaka throughout 2012-2014. Vertical recharge on the edge of the drawdown cone was estimated at 0.21±0.06 m/yr. The data suggest that continued municipal pumping in Dhaka could eventually contaminate some relatively shallow community wells.

Quaternary stratigraphy, sediment characteristics and geochemistry of arsenic-contaminated alluvial aquifers in the Ganges-Brahmaputra floodplain in central Bangladesh.

This study focuses on the Quaternary stratigraphy, sediment composition, mineralogy, and geochemistry of arsenic (As)-contaminated alluvial aquifers in the Ganges-Brahmaputra floodplain in the central Bangladesh. Arsenic concentrations in 85 tubewells in Manikganj area, 70 km northwest of Dhaka City, range from 0.25 microg/L to 191 microg/L with a mean concentration of 33 microg/L. Groundwater is mainly Ca-HCO(3) type with high concentrations of dissolved As, Fe, and Mn, but low level of SO(4). The uppermost aquifer occurs between 10 m and 80 m below the surface that has a mean arsenic concentration of 35 microg/L. Deeper aquifer (>100 m depth) has a mean arsenic concentration of 18 microg/L. Sediments in the upper aquifer are mostly gray to dark-gray, whereas sediments in the deep aquifer are mostly yellowing-gray to brown. Quartz, feldspar, mica, hornblende, garnet, kyanite, tourmaline, magnetite, ilmenite are the major minerals in sediments from both aquifers. Biotite and potassium feldspar are dominant in shallow aquifer, although plagioclase feldspar and garnet are abundant in deep aquifer sediments. Sediment composition suggests a mixed provenance with sediment supplies from both orogenic belts and cratons. High arsenic concentrations in sediments are found within the upper 50 m in drilled core samples. Statistical analysis shows that As, Fe, Mn, Ca, and P are strongly correlated in sediments. Concentrations of Cd, Cu, Ni, Zn, and Bi also show strong correlations with arsenic in the Manikganj sediment cores. Authigenic goethite concretions, possibly formed by bacteria, are found in the shallow sediments, which contain arsenic of a concentration as high as 8.8 mg/kg. High arsenic concentrations in aquifers are associated with fine-grained sediments that were derived mostly from the recycled orogens and relatively rapidly deposited mainly by meandering channels during the Early to Middle Holocene rising sea-level conditions.

Temporal variability of groundwater chemistry in shallow and deep aquifers of Araihazar, Bangladesh.

Samples were collected every 2-4 weeks from a set of 37 monitoring wells over a period of 2-3 years in Araihazar, Bangladesh, to evaluate the temporal variability of groundwater composition for As and other constituents. The monitoring wells are grouped in 6 nests and span the 5-91 m depth range. Concentrations of As, Ca, Fe, K, Mg, Mn, Na, P, and S were measured by high-resolution ICPMS with a precision of 5% or better; concentrations of Cl were measured by ion chromatography. In shallow wells <30 m deep, As and P concentrations generally varied by <30%, whereas concentrations of the major ions (Na, K, Mg, Ca and Cl) and the redox-sensitive elements (Fe, Mn, and S) varied over time by up to +/-90%. In wells tapping the deeper aquifers >30 m often below clay layers concentrations of groundwater As were much lower and varied by <10%. The concentrations of major cations also varied by <10% in these deep aquifers. In contrast, the concentration of redox-sensitive constituents Fe, S, and Mn in deep aquifers varied by up to 97% over time. Thus, strong decoupling between variations in As and Fe concentrations is evident in groundwaters from shallow and deep aquifers. Comparison of the time series data with groundwater ages determined by (3)H/(3)He and (14)C dating shows that large seasonal or inter-annual variations in major cation and chloride concentrations are restricted to shallow aquifers and groundwater recharged <5 years ago. There is no corresponding change in As concentrations despite having significant variations of redox sensitive constituents in these very young waters. This is attributed to chemical buffering due to rapid equilibrium between solute and solid As. At two sites where the As content of groundwater in existing shallow wells averages 102 microg/L (range: <5 to 648 microg/L; n=118) and 272 microg/L (range: 10 to 485 microg/L; n=65), respectively, a systematic long-term decline in As concentrations lends support to the notion that flushing may slowly deplete an aquifer of As. Shallow aquifer water with >5 years (3)H/(3)He age show a constant As:P molar ratio of 9.6 over time, suggesting common mechanisms of mobilization.

Community wells to mitigate the arsenic crisis in Bangladesh.

OBJECTIVE: To monitor the effectiveness of deep community wells in reducing exposure to elevated levels of arsenic in groundwater pumped from shallower aquifers. METHODS: Six community wells ranging in depth from 60 m to 140 m were installed in villages where very few of the wells already present produced safe water. By means of flow meters and interviews with villagers carrying water from the community wells, a study was made of the extent to which these were used during one year. The results were compared with household and well data obtained during a previous survey in the same area. FINDINGS: The mean arsenic concentration in water pumped from wells already in use in the villages where the community wells, were installed was 180 +/- 140 micrograms/l (n = 956). Monthly sampling for 4-11 months showed that arsenic levels in groundwater from five of the six newly installed wells were consistently within the WHO guideline value of 10 micrograms/l for drinking-water. One of these wells met the Bangladesh standard of 50 micrograms/l arsenic but failed to meet the WHO guideline values for manganese and uranium in drinking-water. The community wells were very popular. Many women walked hundreds of metres each day to fetch water from them. On average, 2200 litres were hand-pumped daily from each community well, regardless of the season. CONCLUSION: A single community well can meet the needs of some 500 people residing within a radius of 150 m of it in a densely populated village. Properly monitored community wells should become more prominent in campaigns to reduce arsenic exposure in Bangladesh. Between 8000 and 10,000 deep community wells are needed to provide safe water for the four to five million people living in the most severely affected parts of the country.