Sediment color tool for targeting arsenic-safe aquifers for the installation of shallow drinking water tubewells.

In rural Bangladesh, drinking water supply mostly comes from shallow hand tubewells installed manually by the local drillers, the main driving force in tubewell installation. This study was aimed at developing a sediment color tool on the basis of local driller's perception of sediment color, arsenic (As) concentration of tubewell waters and respective color of aquifer sediments. Laboratory analysis of 521 groundwater samples collected from 144 wells during 2009 to 2011 indicate that As concentrations in groundwater were generally higher in the black colored sediments with an average of 239 µg/L. All 39 wells producing water from red sediments provide safe water following the Bangladesh drinking water standard for As (50 µg/L) where mean and median values were less than the WHO guideline value of 10 µg/L. Observations for off-white sediments were also quite similar. White sediments were rare and seemed to be less important for well installations at shallow depths. A total of 2240 sediment samples were collected at intervals of 1.5m down to depths of 100 m at 15 locations spread over a 410 km(2) area in Matlab, Bangladesh and compared with the Munsell Color Chart with the purpose of direct comparison of sediment color in a consistent manner. All samples were assigned with Munsell Color and Munsell Code, which eventually led to identify 60 color shade varieties which were narrowed to four colors (black, white, off-white and red) as perceived and used by the local drillers. During the process of color grouping, participatory approach was considered taking the opinions of local drillers, technicians, and geologists into account. This simplified sediment color tool can be used conveniently during shallow tubewell installation and thus shows the potential for educating local drillers to target safe aquifers on the basis of the color characteristics of the sediments.

Temporal and seasonal variability of arsenic in drinking water wells in Matlab, southeastern Bangladesh: a preliminary evaluation on the basis of a 4 year study.

Temporal and seasonal variability of As concentrations in groundwater were evaluated in As-affected areas of Matlab, southeastern Bangladesh. Groundwater samples from 61 randomly selected tubewells were analyzed for As concentrations over a period of three years and four months (from July 2002 to November 2005) and monitored seasonally (three times a year). The mean As concentrations in the sampled tubewells decreased from 153 to 123 µg/L during July 2002 to November 2005. Such changes were pronounced in tubewells with As concentration >50 µg/L than those with As concentrations <50 µg/L. Similarly, individual wells revealed temporal variability, for example some wells indicated a decreasing trend, while some other wells indicated stable As concentration during the monitoring period. The mean As concentrations were significantly higher in Matlab North compared with Matlab South. The spatial variations in the mean As concentrations may be due to the differences in local geological conditions and groundwater flow patterns. The variations in mean As concentrations were also observed in shallow (<40 m) and deep (>40 m) wells. However, to adequately evaluate temporal and seasonal variability of As concentration, it is imperative to monitor As concentrations in tubewells over a longer period of time. Such long-term monitoring will provide important information for the assessment of human health risk and the sustainability of safe drinking water supplies.

Groundwater chemistry and arsenic mobilization in the Holocene flood plains in south-central Bangladesh.

A comparative study of arsenic enrichment in the Bengal Delta (BD) was carried out in three alluvial aquifers in south-central Bangladesh. Investigated sites included Sonargaon in Narayanganj, Chandina in Comilla and Sirajdikhan in Munshiganj districts. At all sites samples from different depths were collected, and water chemistry and redox status vs. depth trends were determined. The concentrations of DOC and HCO(3)(-) were highest at Sirajdikhan site, while at the Sonargaon and Chandina sites the concentrations were lower. On the contrary, the NH(4)(+) concentration was high at the Chandina site as compared to the other sites. There was a good match between dissolved As and Fe at the Sirajdikhan and Sonargaon sites, but not at the Chandina site. The dissolved aqueous concentration of Mn was low at the Chandina site, which suggested that the Mn(IV) redox buffering step was missing. Speciation modeling indicated a possibility of siderite precipitation at all sites, but precipitation of rhodochrosite only at the Sonargaon and Sirajdikhan sites. At the Sirajdikhan site, the log P(CO2) values were very high (-1.37), which revealed the production of CO(2) in redox processes. Principal component analysis (PCA) indicated an impact of sea water and redox status of different samples. These results suggest that the dissolved As is de-coupled from dissolved Mn because when released, As is re-adsorbed onto the Fe(III) minerals in solid phase, as well as from dissolved Fe when precipitation of Fe(II) minerals controls the aqueous concentrations of Fe. In addition, several other concurrent redox processes may exert kinetic constraints depending on refractory characteristics of Fe(III) minerals.

Geochemical characterisation of shallow aquifer sediments of Matlab Upazila, Southeastern Bangladesh - implications for targeting low-As aquifers.

High arsenic (As) concentrations in groundwater pose a serious threat to the health of millions of people in Bangladesh. Reductive dissolution of Fe(III)-oxyhydroxides and release of its adsorbed As is considered to be the principal mechanism responsible for mobilisation of As. The distribution of As is extremely heterogeneous both laterally and vertically. Groundwater abstracted from oxidised reddish sediments, in contrast to greyish reducing sediments, contains significantly lower amount of dissolved arsenic and can be a source of safe water. In order to study the sustainability of that mitigation option, this study describes the lithofacies and genesis of the sediments within 60 m depth and establishes a relationship between aqueous and solid phase geochemistry. Oxalate extractable Fe and Mn contents are higher in the reduced unit than in the oxidised unit, where Fe and Mn are present in more crystalline mineral phases. Equilibrium modelling of saturation indices suggest that the concentrations of dissolved Fe, Mn and PO(4)(3-)-tot in groundwater is influenced by secondary mineral phases in addition to redox processes. Simulating As(III) adsorption on hydroferric oxides using the Diffuse Layer Model and analytical data gave realistic concentrations of dissolved and adsorbed As(III) for the reducing aquifer and we speculate that the presence of high PO(4)(3-)-tot in combination with reductive dissolution results in the high-As groundwater. The study confirms high mobility of As in reducing aquifers with typically dark colour of sediments found in previous studies and thus validates the approach for location of wells used by local drillers based on sediment colour. A more systematic and standardised colour description and similar studies at more locations are necessary for wider application of the approach.

Hydrogeochemical comparison and effects of overlapping redox zones on groundwater arsenic near the Western (Bhagirathi sub-basin, India) and Eastern (Meghna sub-basin, Bangladesh) margins of the Bengal Basin.

Although arsenic (As) contamination of groundwater in the Bengal Basin has received wide attention over the past decade, comparative studies of hydrogeochemistry in geologically different sub-basins within the basin have been lacking. Groundwater samples were collected from sub-basins in the western margin (River Bhagirathi sub-basin, Nadia, India; 90 samples) and eastern margin (River Meghna sub-basin; Brahmanbaria, Bangladesh; 35 samples) of the Bengal Basin. Groundwater in the western site (Nadia) has mostly Ca-HCO(3) water while that in the eastern site (Brahmanbaria) is much more variable consisting of at least six different facies. The two sites show differences in major and minor solute trends indicating varying pathways of hydrogeochemical evolution However, both sites have similar reducing, postoxic environments (p(e): +5 to -2) with high concentrations of dissolved organic carbon, indicating dominantly metal-reducing processes and similarity in As mobilization mechanism. The trends of various redox-sensitive solutes (e.g. As, CH(4), Fe, Mn, NO(3)(-), NH(4)(+), SO(4)(2-)) indicate overlapping redox zones, leading to partial redox equilibrium conditions where As, once liberated from source minerals, would tend to remain in solution because of the complex interplay among the electron acceptors.

Targeting low-arsenic aquifers in Matlab Upazila, Southeastern Bangladesh.

Groundwater with high concentration of geogenic arsenic (As) occurs extensively in the Holocene alluvial aquifers of Bangladesh. Local drillers in Matlab Upazilla are constructing deeper tubewells than in the recent past, primarily because of low concentrations of dissolved Fe and As. Locally a thick layer of black to grey sediments overlies an oxidised unit of yellowish-grey to reddish-brown sediments. The correlation between the colour of both units and the groundwater redox conditions was investigated to provide an easy tool for targeting low-arsenic groundwater. Based on the sediment colour at the screen depths described by local drillers, 40 domestic shallow tubewells were selected for water sampling. Four colours were used to describe the sediments: black, white, off-white (buff) and red. Generally, the groundwater was anoxic and the As concentrations ranged from less than 5.2 to 355 microg/L. Water derived from the black sediment is characterized by relatively higher concentrations of dissolved NH(4)(+), DOC, Fe, P, As and by low Mn and SO(4)(2-) concentrations. The off-white and red sediments had high concentration of Mn and low NH(4)(+), DOC, Fe, P and As concentrations. The water abstracted from the black sediments indicated the most reducing environment, followed by white, off-white and red respectively. Three boreholes verified the driller's perception of the subsurface lithologic conditions. Discrepancies between the driller's and the research team description of the sediment colours were insignificant. This study shows that sediment colour is a reliable indicator of high and low-As concentrations and can be used by local drillers to target low-arsenic groundwater.