Arsenic in shallow groundwater of Bangladesh: investigations from three different physiographic settings

Occurrences of arsenic (As) in the Bengal Basin of Bangladesh show close relationships with depositional environments and sediment textures. Hydrochemical data from three sites with varying physiography and sedimentation history show marked variations in redox status and dissolved As concentrations. Arsenic concentration in groundwater of the Ganges Flood Plain (GFP) is characteristically low, where high Mn concentrations indicate redox buffering by reduction of Mn(IV)-oxyhydroxides. Low DOC, \( {\text{HCO}}^{ - }_{3} \), \( {\text{NH}}^{ + }_{4} \) and high \( {\text{NO}}^{ - }_{3} \) and \( {\text{SO}}^{{2 - }}_{4} \) concentrations reflect an elevated redox status in GFP aquifers. In contrast, As concentration in the Ganges Delta Plain (GDP) is very high along with high Fe and low Mn. In the Meghna Flood Plain (MFP), moderate to high As and Fe concentrations and low Mn are detected. Degradation of organic matter probably drives redox reactions in the aquifers, particularly in MFP and GDP, thereby mobilising dissolved As. Speciation calculations indicate supersaturation with respect to siderite and vivianite in the groundwater samples at MFP and GDP, but groundwater in the GFP wells is generally supersaturated with respect to rhodochrosite. Values of log P_CO2 at MFP and GDP sites are generally higher than at the GFP site. This is consistent with Mn(IV)-redox buffering suggested at the GFP site compared to Fe(III)-redox buffering at MFP and GDP sites.     

Hydrology of part of the Oramiriukwa River basin,southeast of Owerri,Imo State,Nigeria

The Oramiriukwa River is within the sandy coastal plain strata of the Benin formation (Miocene–Recent). The base flow is very high ranging from 79.13–98.56%, which is caused by the excellent hydraulic interconnection between the river and the adjacent unconfined aquifer. Recharge rates are high, estimated to range from 1.8×10¹²–2.5×10¹² m³/year. Coastal sands are medium-to-coarse grained, moderately-to-poorly sorted, angular to subangular, with lenses of clay and clayey fine-grained sands. The coastal sands and clay lenses form aquifer and aquitard systems, which are unconfined to semi-confined. Groundwater recharge potential is high. Runoff from precipitation is low. Groundwater and surface water are fairly acidic; pH ranges from 5.5–6.1 (groundwater) and 5.8–6.5 (surface water), and hardness is generally low. Chemical analysis and percentage sodium show that groundwater and surface water are somewhat potable after some pH modification of the surface water. The waters are good for agricultural use, especially for irrigation and poultry water supply. However, pollution from landfill leachate is serious. Electronic Publication     

Arsenic toxicity of groundwater in parts of the Bengal basin in India and Bangladesh: the role of Quaternary stratigraphy and Holocene sea-level fluctuation

 Arsenic toxicity in groundwater in the Ganges delta and some low-lying areas in the Bengal basin is confined to middle Holocene sediments. Dissected terraces and highlands of Pleistocene and early Holocene deposits are free of such problems. Arsenic-rich pyrite or other arsenic minerals are rare or absent in the affected sediments. Arsenic appears to occur adsorbed on iron hydroxide-coated sand grains and clay minerals and is transported in soluble form and co-precipitated with, or is scavenged by, Fe(III) and Mn(IV) in the sediments. It became preferentially entrapped in fine-grained and organic-rich sediments during mid-Holocene sea-level rises in deltaic and some low-lying areas of the Bengal basin. It was liberated subsequently under reducing conditions and mediated further by microbial action. Intensive extraction of groundwater for irrigation and application of phosphate fertilizer possibly triggered the recent release of arsenic to groundwater. This practice has induced groundwater flow, mobilizing phosphate derived from fertilizer, as well as from decayed organic matter, which has promoted the growth of sediment biota and aided the further release of arsenic. However, the environment is not sufficiently reducing to mobilize iron and arsenic in groundwater in the Ganges floodplains upstream of Rajmahal. Thus, arsenic toxicity in the groundwater of the Bengal basin is caused by its natural setting, but also appears to be triggered by recent anthropogenic activities. Received: 23 August 1999 · Accepted: 16 November 1999     

Arsenic contamination of groundwater: Mitigation strategies and policies

Contamination of groundwater by arsenic from natural geochemical sources is at present a most serious challenge in the planning of large-scale use of groundwater for drinking and other purposes. Recent improvements in detection limits of analytical instruments are allowing the correlation of health impacts such as cancer with large concentrations of arsenic in groundwater. However, there are at present no known large-scale technological solutions for the millions of people—mostly rural—who are potentially affected in developing countries. An overall framework of combating natural resource degradation is combined with case studies from Chile, Mexico, Bangladesh and elsewhere to arrive at a set of strategic recommendations for the global, national and local dimensions of the arsenic crisis. The main recommendations include: the need for flexibility in the elaboration of any arsenic mitigation strategy, the improvement and large-scale use of low-cost and participatory groundwater quality testing techniques, the need to maintain consistent use of key lessons learned worldwide in water supply and sanitation and to integrate arsenic as just one other factor in providing a sustainable water supply, and the following of distinct but communicable tracks between arsenic-related developments and enhanced, long-term, sustainable water supplies.

Dryland salinity processes within the discharge zone of a local groundwater system,southeastern Australia

Detailed study of a localised saline discharge zone in southeastern Australia shows that the salinisation is mostly due to the shallow water table (<1–2 m from the surface). Direct evaporation, particularly in summer, leads to extremely high soil–water salinities at the surface, even though the underlying groundwater is moderately fresh. Groundwater discharge is localised at a break of slope, where the water table intersects the surface, and where the transition from permeable sands to clay-rich sediments inhibits lateral groundwater flow. Higher salt concentrations build up in the clays because of the long residence times during which soil-waters are exposed to evapotranspiration and the reduced potential for salts to be flushed from the sediments. As a result the saline discharge area does not correspond to the part of the site with the largest salt store. Results of the study demonstrate that for dryland salinisation to occur, the groundwater beneath the discharge zone need not be saline, and the presence of a large salt store does not necessarily lead to problems of dryland salinisation if, as in the clay-rich sediments at the site, the salt lies below the pasture root zone. Furthermore, mobilisation of salt stores within low permeability sediments by rising groundwater may be minor.

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Résumé L’étude détaillée d’une zone localisée d’émergence salée au sud-est de l’Australie montre que la salinisation est surtout due à la nappe phréatique (<1–2 m sous la surface). L’évaporation directe, particulièrement durant l’été, conduit à des salinités de l’eau du sol extrêmement élevées à la surface, même si l’eau souterraine sous-jacente est modérément douce. L’émergence de l’eau souterraine est localisée à la rupture de pente, là où la nappe phréatique rencontre la surface du sol et où la transition entre sables perméables et sédiments riches en argiles inhibe les écoulements d’eau souterraine latéraux. Les plus fortes concentrations en sel s’accumulent dans les argiles du fait de temps de résidence élevés, durant lesquels les eaux du sol sont exposées à l’évapotranspiration et à un lessivage réduit des sédiments. Il en résulte que l’aire d’émergence des eaux salées ne correspond pas à la partie du site rencontrant la réserve de sel la plus importante. Les résultats de l’étude démontrent que pour que la salinisation d’une zone aride devienne effective, l’eau souterraine sous la zone ne doit pas être forcément salée, et la présence d’une zone étendue de réserve de sel ne conduit pas forcément à des problèmes de salinisation de zones arides si, comme dans les sédiments argileux du site, le sel ne repose pas sous la zone de pature. De plus, la mobilisation des réserves de sel dans les sédiments peu perméables par la montée du niveau de l’eau souterraine devrait être mineure.

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Resumen El estudio detallado de una zona local de descarga salina en el sudeste de Australia, muestra que la salinización es principalmente debido a un nivel freático poco profundo (<1–2 m de la superficie). La evaporación Directa, particularmente en verano, conlleva a salinidades de suelo-agua sumamente altas en la superficie, aunque el agua subterránea subyacente es moderadamente dulce. La descarga de Agua subterránea se localiza en una interrupción de la ladera, dónde el nivel freático intercepta la superficie, y donde la transición de las arenas permeables a los sedimentos ricos en arcilla inhibe el flujo lateral del agua subterránea. Las concentraciones de sal más altas se forman en las arcillas debido a los tiempos de residencia largos durante los cuales se exponen el conjunto suelo-agua a la evapotranspiración y también por el potencial reducido para las sales de ser expulsadas de los sedimentos. Como resultado el área de la descarga salina no corresponde a la parte del sitio con el contenido de sal más grande. Los resultados del estudio demuestran que para que ocurra la salinización en terrenos secos, el agua subterránea bajo la de zona de descarga no necesita ser salina, y que la presencia de un almacenamiento de sal grande, no necesariamente lleva a los problemas de salinización en terrenos secos si, como en los sedimentos ricos en arcilla del sitio, la sal yace debajo de la zona de raíz de la pastura. Además, la movilidad de depósitos de sal dentro de los sedimentos de permeabilidad baja pueden ser menores, por causa del agua subterránea ascendente.

     

Near surface lithology and spatial variation of arsenic in the shallow groundwater: southeastern Bangladesh

This study investigated the relationship between near-surface lithology and the spatial variability of As concentrations using sediment grain-size analysis and electromagnetic induction survey in the southeast Bangladesh. It has been observed that the aquifers overlain by finer sediments have higher concentrations of As in groundwater, whereas As concentrations are remarkably low in aquifers having permeable sandy materials or thinner silt/clay layer at the surface. The near-surface lithology acts as a controlling factor for spatial distributions of groundwater As within the very shallow depths (<15 m). Shallow alluvial aquifers can provide low-As drinking water in many areas of the country when tube wells are properly installed after investigation of the overlying near-surface sediment attributes and hydraulic properties.     

Dissolved fluoride in the Lower Ganges-Brahmaputra-Meghna River system in the Bengal Basin, Bangladesh

The dissolved fluoride (F⁻) in the Lower Ganges-Brahmaputra-Meghna (GBM) river system, Bengal basin, Bangladesh, was studied during 1991–1993 to determine its distribution and source in the basin, and its annual flux to the Bay of Bengal. The concentration of dissolved F⁻ varied between 2 and 11 μmol l⁻¹ with statistically significant variations both spatially and temporally in the basin. Such variations are attributable to the geology of the individual subbasins (Ganges, Brahmaputra and Meghna), dilution by rainwater during monsoon and groundwater contribution to the river systems during dry season. Correlation coefficients among F⁻ and major cations and anions suggest diverse inorganic processes responsible for regulating the concentration of F⁻ in these river systems. However, fluorite seems to be one of the major sources of dissolved F⁻. The concentration of F⁻ in the Lower GBM river system is low compared to the rivers draining Deccan Plateau and arid regions of the subcontinent, for example, Yamuna and its tributaries. However, it is within the range of most of the other Peninsular and Himalayan rivers. The GBM system contributes about 115×10³ tonnes year⁻¹ of dissolved F⁻ into the Bay of Bengal, and thus accounts for about 3% of the global F⁻ flux to the oceans annually. Received: 19 May 1999 · Accepted: 11 October 1999     

Geostatistical analysis of arsenic concentration in groundwater in Bangladesh using disjunctive kriging

The National Hydrochemical Survey of Bangladesh sampled the water from 3,534 tube wells for arsenic throughout most of Bangladesh. It showed that 27% of the shallow tube wells (less than 150 m deep) and 1% of the deep tube wells (more than 150 m deep) exceeded the Bangladesh standard for arsenic in drinking water (50 µg L^–1). Statistical analyses revealed the main characteristics of the arsenic distribution. Concentrations ranged from less than the detection limit (0.5 µg L^–1), to as much as 1,600 µg L^–1, though with a very skewed distribution, and with spatial dependence extending to some 180 km. Disjunctive kriging was used to estimate concentrations of arsenic in the shallow ground water and to map the probability that the national limit for arsenic in drinking water was exceeded for most of the country (the Chittagong Hill Tracts and the southern coastal region were excluded). A clear regional pattern was identified, with large probabilities in the south of the country and small probabilities in much of the north including the Pleistocene Tracts. Using these probabilities, it was estimated that approximately 35 million people are exposed to arsenic concentrations in groundwater exceeding 50 µg L^–1 and 57 million people are exposed to concentrations exceeding 10 µg L^–1 (the WHO guideline value).     

Distribution of arsenic and other trace elements in the Holocene sediments of the Meghna River Delta, Bangladesh

Geochemical study of the Holocene sediments of the Meghna River Delta, Chandpur, Bangladesh was conducted to investigate the distribution of arsenic and related trace and major elements. The work carried out includes analyses of core sediments and provenance study by rare earth element (REE) analysis. Results showed that the cores pass downward from silty clays and clays into fine to medium sands. The uppermost 3 m of the core sediments are oxidized [average oxidation reduction potential (ORP) + 230 mV], and the ORP values gradually become negative with depths (−45 to −170 mV), indicating anoxic conditions prevail in the Meghna sediments. The REE patterns of all lithotypes in the study areas are similar and are comparable to the average upper continental crust. Arsenic and other trace elements (Pb, Zn, Cu, Ni, and Cr) have greater concentrations in the silts and clays compared to those in the sands. Positive correlation between As and Fe was found in the sediments, indicating As may be adsorbed on Fe oxides in aquifer sediments.     

Preliminary evidence of a link between surface soil properties and the arsenic content of shallow groundwater in Bangladesh

The extremely heterogeneous distribution of As in Bangladesh groundwater has hampered efforts to identify with certainty the mechanisms that lead to extensive mobilization of this metalloid in reducing aquifers. We show here on the basis of a high-resolution transect of soil and aquifer properties collected in Araihazar, Bangladesh, that revealing tractable associations between As concentrations in shallow (< 20 m) groundwater with other geological, hydrological, and geochemical features requires a lateral sampling resolution of 10–100 m. Variations in the electromagnetic conductivity of surface soils (5–40 mS/m) within a 500 m × 200 m area are documented with 560 EM31 measurements. The results are compared with a detailed section of groundwater As concentrations (5–150 μg/L) and other aquifer properties obtained with a simple sampling device, “the needle-sampler”, that builds on the local drilling technology. By invoking complementary observations obtained in the same area and in other regions of Bangladesh, we postulate that local groundwater recharge throughout permeable sandy soils plays a major role in regulating the As content of shallow aquifers by diluting the flux of As released from reducing sediments.     

Naturally occurring arsenic in groundwater and identification of the geochemical sources in the Duero Cenozoic Basin, Spain

Arsenic concentrations surpassing potability limit of 10 μg/L in the groundwater supplies of an extensive area in the Duero Cenozoic Basin (central Spain) have been detected and the main sources of arsenic identified. Arsenic in 514 samples of groundwater, having mean values of 40.8 μg/L, is natural in origin. Geochemical analysis of 553 rock samples, assaying arsenic mean values of 23 mg/kg, was performed. Spatial coincidence between the arsenic anomaly in groundwater and the arsenic lithogeochemical distribution recorded in the Middle Miocene clayey organic-rich Zaratan facies illustrates that the rocks of this unit are the main source of arsenic in groundwater. The ferricretes associated to the Late Cretaceous–Middle Miocene siliciclastics also constitute a potential arsenic source. Mineralogical study has identified the presence of arsenic in iron oxides, authigenic pyrite, manganese oxides, inherited titanium–iron oxides, phyllosilicates and organomineral compounds. Arsenic mobilization to groundwater corresponds to arsenic desorption from iron and manganese oxides and from organic matter.     

Spatial relationship of groundwater arsenic distribution with regional topography and water-table fluctuations in the shallow aquifers in Bangladesh

The present study has examined the relationship of groundwater arsenic (As) levels in alluvial aquifers with topographic elevation, slope, and groundwater level on a large basinal-scale using high-resolution (90 m × 90 m) Shuttle Radar Topography Mission (SRTM) digital elevation model and water-table data in Bangladesh. Results show that high As (>50 μg/l) tubewells are located in low-lying areas, where mean surface elevation is approximately 10 m. Similarly, high As concentrations are found within extremely low slopes (<0.7°) in the country. Groundwater elevation (weekly measured by Bangladesh Water Development Board) was mapped using water-table data from 950 shallow (depth <100 m) piezometers distributed over the entire country. The minimum, maximum and mean groundwater elevation maps for 2003 were generated using Universal Kriging interpolation method. High As tubewells are located mainly in the Ganges–Brahmaputra–Meghna delta, Sylhet Trough, and recent floodplains, where groundwater elevation in shallow aquifers is low with a mean value of 4.5 m above the Public Works Datum (PWD) level. Extremely low groundwater gradients (0.01–0.001 m/km) within the GBM delta complex hinder groundwater flow and cause slow flushing of aquifers. Low elevation and gentle slope favor accumulation of finer sediments, As-carrying iron-oxyhydroxide minerals, and abundant organic matter within floodplains and alluvial deposits. At low horizontal hydraulic gradients and under reducing conditions, As is released in groundwater by microbial activity, causing widespread contamination in the low-lying deltaic and floodplain areas, where As is being recycled with time due to complex biogeochemical processes.     

Texture and mineralogy of sediments from the Ganges-Brahmaputra-Meghna river system in the Bengal Basin, Bangladesh and their environmental implications

 The Bengal basin, Bangladesh, represents one of the most densely populated recent floodplains of the world. The sediment flux through the basin is one of the highest on a global scale. A significant portion of this sediment load find its sink in the basin itself because of its lower elevation and frequent flooding. The textural, mineralogical and chemical nature of the sediments thus have an important bearing on the environmental quality of the basin as well as for the Bay of Bengal. The sediment load of the Ganges-Brahmaputra-Meghna (GBM) river system consists exclusively of fine sand, silt and clay at their lower reaches within the Bengal basin, Bangladesh, and is deposited under uniformly fluctuating, unidirectional energy conditions. The sediments have a close simitarity in grain size with the sediments of the surrounding floodplain. The mineral assemblage is dominated by quartz and feldspars. Illite and kaolinite are the major clay minerals, and occur in almost equal proportion in bed sediments. The heavy mineral assemblage is dominated by unstable minerals which are mostly derived from high-rank metamorphic rocks. The characteristic smaller grain-size, i.e. having large surface-to-mass ratios, and the mineralogy of sediments suggests that they are susceptible to large chemical adsorptive reactions and thus could serve as a potential trap for contaminants. However, the sediments of the GBM river system in the Bengal basin, Bangladesh, shows lower concentration of Pb, Hg and As, and a marginally higher value for Cd as compared to that of standard shale. Considering population density and extensive agricultural practice in the basin, the sediments can in the long run become contaminated. Received: 9 November 1994 · Accepted: 18 June 1996     

Estimation of regional-scale groundwater flow properties in the Bengal Basin of India and Bangladesh

Quantitative evaluation of management strategies for long-term supply of safe groundwater for drinking from the Bengal Basin aquifer (India and Bangladesh) requires estimation of the large-scale hydrogeologic properties that control flow. The Basin consists of a stratified, heterogeneous sequence of sediments with aquitards that may separate aquifers locally, but evidence does not support existence of regional confining units. Considered at a large scale, the Basin may be aptly described as a single aquifer with higher horizontal than vertical hydraulic conductivity. Though data are sparse, estimation of regional-scale aquifer properties is possible from three existing data types: hydraulic heads, ¹⁴C concentrations, and driller logs. Estimation is carried out with inverse groundwater modeling using measured heads, by model calibration using estimated water ages based on ¹⁴C, and by statistical analysis of driller logs. Similar estimates of hydraulic conductivities result from all three data types; a resulting typical value of vertical anisotropy (ratio of horizontal to vertical conductivity) is 10⁴. The vertical anisotropy estimate is supported by simulation of flow through geostatistical fields consistent with driller log data. The high estimated value of vertical anisotropy in hydraulic conductivity indicates that even disconnected aquitards, if numerous, can strongly control the equivalent hydraulic parameters of an aquifer system.     

Controls on groundwater flow in the Bengal Basin of India and Bangladesh: regional modeling analysis

Groundwater for domestic and irrigation purposes is produced primarily from shallow parts of the Bengal Basin aquifer system (India and Bangladesh), which contains high concentrations of dissolved arsenic (exceeding worldwide drinking water standards), though deeper groundwater is generally low in arsenic. An essential first step for determining sustainable management of the deep groundwater resource is identification of hydrogeologic controls on flow and quantification of basin-scale groundwater flow patterns. Results from groundwater modeling, in which the Bengal Basin aquifer system is represented as a single aquifer with higher horizontal than vertical hydraulic conductivity, indicate that this anisotropy is the primary hydrogeologic control on the natural flowpath lengths. Despite extremely low hydraulic gradients due to minimal topographic relief, anisotropy implies large-scale (tens to hundreds of kilometers) flow at depth. Other hydrogeologic factors, including lateral and vertical changes in hydraulic conductivity, have minor effects on overall flow patterns. However, because natural hydraulic gradients are low, the impact of pumping on groundwater flow is overwhelming; modeling indicates that pumping has substantially changed the shallow groundwater budget and flowpaths from predevelopment conditions.     

Recharge mechanism and hydrochemistry evaluation of groundwater in the Nuaimeh area, Jordan, using environmental isotope techniques

The relationship between the stable isotopic and chemical composition of precipitation and groundwater was studied in the Nuaimeh area of the Ajloun Highlands in Jordan. The isotopic composition values of precipitation and groundwater are almost identical. The spatial variation of stable isotopes in precipitation is mainly due to the effect of seasonal temperature, altitude and amount. The groundwater reveals identical variation in isotopic composition to the precipitation due to direct recharge and the karstic nature of the outcropping Turonian aquifer. Tritium levels in wells are high and their content is similar to the weighted mean value of tritium content in precipitation, indicating local recharge and low residence time. The ¹⁴C activity in the tritiated groundwater is found to be about half of the ¹⁴C activity of precipitation in the region. A geochemical evolution through dissolution of carbonate by water–carbonate rock interactions reduced the atmospheric ¹⁴C activity from 114 to 61 pmc in the groundwater. A ¹⁴C of around 61 pmc and 7.6 TU values are considered the initial concentration for the recharge in the shallow carbonate aquifer in the Yarmouk Basin. The large fluctuation of water level in observation wells during the rainy season indicates the sensitivity and direct response of the aquifer to the recharge. The chemical composition of the groundwater (Ca²⁺–HCO₃^–) gives emphasis to the short duration of water–rock interaction and indicates dissolution of the carbonate aquifer. The elevated concentrations of Cl^– and NO₃^– in groundwater are attributed to anthropogenic sources.

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Resumen Fue estudiada la relación entre la composición isotópica estable y la composición química, tanto de la precipitación como del agua subterránea, en el área de Nuaimeh en las montañas de Ajloun, en Jordania. Los valores de la composición isotópica de la precipitación y del agua subterránea son casi idénticos. La variación espacial de los isótopos estables en la precipitación, es debida principalmente al efecto de la temperatura estacional, a la altura y a la cantidad. El agua subterránea muestra una variación idéntica a la precipitación en cuanto a la composición isotópica, debido a la recarga directa y a la naturaleza cársica del acuífero Turoniano aflorante. Los niveles de tritio en los pozos son altos y su contenido es similar al valor medio ponderado del contenido de tritio en la precipitación, indicando una recarga local y un tiempo de residencia corto. Se ha encontrado que la actividad de ¹⁴C en el agua subterránea tritiada, es alrededor de la mitad de la actividad del ¹⁴C en la precipitación para la región. La evolución geoquímica ocurrida a partir de la disolución del carbonato, por las interacciones roca carbonatada–agua, redujeron la actividad atmosférica del ¹⁴C desde 114 en porcentaje de carbono moderno (pcm) hasta 61 pcm en el agua subterránea. Los valores del ¹⁴C cercanos a 61 pcm y de 7.6 unidades de tritio, se han considerado como las concentraciones originales para la recarga en el acuífero carbonatado somero de la Cuenca de Yarmouk. Una gran fluctuación en el nivel de agua de los pozos de observación, durante la estación lluviosa, indica la sensibilidad y la respuesta directa del acuífero frente a la recarga. La composición química del agua subterránea (Ca²⁺–HCO₃^–), enfatiza en la corta duración de la interacción de agua–roca, e indica disolución del acuífero carbonatado. La concentración elevada de Cl^– y NO₃^– en el agua subterránea, se atribuye a fuentes antropogénicas.

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Résumé Dans la région montagneuse dAjloun de Jordanie on a étudié la relation entre la composition chimique et isotopique des précipitations et des eaux souterraines. La composition isotopique est presque identique dans précipitations et les eaux souterraines. La variation spatiale de la teneur en isotopes stables dans les précipitations est déterminée en principal par les variations saisonnière de la température, laltitude ainsi que par la quantité des précipitations. A cause de la recharge directe et de la nature karstique des affleurements de laquifère dage touronienne, les eaux souterraines présentent la même composition isotopique que les précipitations. La teneur en ³H mesurée dans les forages présente des valeurs élevées, proches de la valeur moyenne des précipitations, ce quindique une recharge locale et un temps court de résidence. On a déterminé pour lactivité de ¹⁴C une valeur proche de la moitié trouvée dans les précipitations. Lévolution géochimique par dissolution des carbonates pendant linteraction entre leau et les roche a diminué lactivité du ¹⁴C, de la valeur de 114 pcm en atmosphère à 61 pcm dans les eaux souterraines. Dans laquifère calcaire de surface de bassin Yarmouk, on a considéré comme de concentrations initiales, les valeurs de 61 pcm pour ¹⁴C et 7.2 UT pour ³H. Les grandes fluctuations des niveaux des eaux souterraines observées dans les forages pendant les saisons pluvieux montrent la sensibilité ainsi que la réponse directe de laquifère au recharge. La composition chimique des eaux souterraines (Ca²⁺–HCO₃^–) montre de plus le temps court de linteraction entre leau et la roche, en indiquant aussi la dissolution de laquifère calcaire. Les taux élevées de la concentration en Cl^– et NO₃^– dans les eaux souterraines ont été attribuées aux sources humaines.

     

Arsenic in groundwater,Quaternary sediments,and suspended river sediments from the Middle Gangetic Plain,India: distribution,field relations,and geomorphological setting

A groundwater arsenic (As) survey in Mirzapur, Varanasi, Ghazipur, Ballia, Buxar, Ara, Patna, and Vaishali districts of UP and Bihar shows that people from these districts are drinking As-contaminated groundwater (max. 1,300 μg/l). About 66 % of tubewells from Buxar to Mirzapur areas and 89 % of tubewells from Patna to Ballia areas have As?>?10 μg/l (WHO guideline). Moreover, 36 % of tubewells from Buxar to Mirzapur areas and 50 % of tubewells from Patna to Ballia areas have As above 50 μg/l. Most of the As-affected villages are located close to abandoned or present meander channels of the Ganga River. In contrast, tubewells located in Mirzapur, Chunar, Varanasi, Saidpur, Ghazipur, Muhammadabad, Ballia, Buxar, Ara, Chhapra, Patna, and Hazipur towns are As-safe in groundwater because of their positions on the Pleistocene Older Alluvium upland surfaces. The iron (Fe) content in tubewell water samples varies from 0.1 to 12.93 mg/l. About 77 % As-contaminated tubewells are located within the depth of 21 to 40 m in the Holocene Newer Alluvium aquifers. The potential source of As in sediments carried through the rivers from the Himalayas. Maximum As concentrations in the Older and Newer Alluvium sediments are 13.73 and 30.91 mg/kg, respectively. The Himalayas rivers, i.e. Yamuna, Ganga, Gomati, Ghaghara, Gondak, Buri Gandak, and Kosi rivers carrying suspended sediments have high content of As (max. 10.59 mg/kg).     

Regional hydrostratigraphy and groundwater flow modeling in the arsenic-affected areas of the western Bengal basin, West Bengal, India

The first documented interpretation of the regional-scale hydrostratigraphy and groundwater flow is presented for a ～21,000-km² area of the arsenic-affected districts of West Bengal [Murshidabad, Nadia, North 24 Parganas and South 24 Parganas (including Calcutta)], India. A hydrostratigraphic model demonstrates the presence of a continuous, semi-confined sand aquifer underlain by a thick clay aquitard. The aquifer thickens toward the east and south. In the south, discontinuous clay layers locally divide the near-surface aquifer into several deeper, laterally connected, confined aquifers. Eight 22-layer model scenarios of regional groundwater flow were developed based on the observed topography, seasonal conditions, and inferred hydrostratigraphy. The models suggest the existence of seasonally variable, regional, north–south flow across the basin prior to the onset of extensive pumping in the 1970s. Pumping has severely distorted the flow pattern, inducing high vertical hydraulic gradients across wide cones of depression. Pumping has also increased total recharge (including irrigational return flow), inflow from rivers, and sea water intrusion. Consequently, downward flow of arsenic contaminated shallow groundwater appears to have resulted in contamination of previously safe aquifers by a combination of mechanical mixing and changes in chemical equilibrium.     

Delineating low-arsenic groundwater environments in the Bengal Aquifer System, Bangladesh

Studies within the As-affected Bengal Basin have indicated that low-As groundwater can be found in a variety of geological and geomorphological settings. The hydrogeological environments that host low-As groundwater may be interpreted within a geological framework determined by the Quaternary evolution of the Bengal Aquifer System (BAS). This provides the basis for delineating the position and extent of shallow low-As groundwater, low-As groundwater in oxidised ‘red-bed’ sediments, and deep low-As groundwater. Data available on a national scale allow a preliminary delineation of these low-As groundwater environments across Bangladesh, based on empirical associations of low-As groundwater occurrences with topography, water table elevation, surface sediment lithology, geology and the screen depth of deep wells in low-As zones.     

Effect of agricultural land use on the chemistry of groundwater from basaltic aquifers, Jeju Island, South Korea

Geochemical processes were identified as controlling factors of groundwater chemistry, including chemical weathering, salinization from seawater and dry sea-salt deposition, nitrate contamination, and rainfall recharge. These geochemical processes were identified using principal component analysis of major element chemistry of groundwater from basaltic aquifers in Jeju Island, South Korea, a volcanic island with intense agricultural activities. The contribution of the geochemical processes to groundwater chemistry was quantified by a simple mass-balance approach. The geochemical effects due to seawater were considered based on Cl contributions, whereas the effects due to natural chemical weathering were based on alkalinity. Nitrogenous fertilizers, and especially the associated nitrification processes, appear to significantly affect groundwater chemistry. A strong correlation was observed between Na, Mg, Ca, SO₄ and Cl, and nitrate concentrations in groundwater. Correspondingly, the total major cations, Cl, and SO₄ in groundwater were assessed to estimate relative effect of N-fertilizer use on groundwater chemistry. Cl originates more from nitrate sources than from seawater, whereas SO₄ originates mostly from rainwater. N-fertilizer use has shown the greatest effect on groundwater chemistry, particularly when nitrate concentrations exceed 6–7 mg/L NO₃–N. Nitrate contamination significantly affects groundwater quality and 18% of groundwater samples have contamination-dominated chemistry.