Trends in arsenic concentration at tubewells in Bangladesh: conceptual models, numerical models, and monitoring proxies
W. Burgess1, K.M. Ahmed2, A. Carruthers1, H. Cheetham1, J. Cobbing1, 3, M. Cuthbert1, 4, S. Mather1, E. McCarthy1, 5
Available online 1 November 2007.
Abstract
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.
Article Outline
2.1. Introduction
Arsenic occurs widely in groundwater of the alluvial aquifers of the southern part of the Bengal Basin in West Bengal and Bangladesh, beneath the floodplains of the rivers Ganges, Brahmaputra, and Meghna ( [Ray, 1996] , [Kinniburgh and Smedley, 2001] and [Ravenscroft et al., 2005] ). Ninety-five per cent of the population in this region relies on groundwater for drinking and domestic use. In addition, groundwater is widely used for irrigation. Most tubewells in Bangladesh have been installed since the early to mid-1980s, but appreciation of the basin-wide scale of the As problem dates from the mid-1990s (Kinniburgh and Smedley, 2001). The scale and extent of As occurrence in groundwater across the southern and eastern regions of the Bengal Basin have been documented in a national survey of Bangladesh (ibid.), in which it was estimated that 27% of tubewells used for domestic water supply contain As above the Bangladesh national limit of 50 μg/l, and 47% exceed the World Health Organization recommended limit of 10 μg/l. Long-term monitoring records do not exist, but faced with this grave situation, possible changes in As concentration in tubewell discharge water should be anticipated (Burgess et al., 2003). The concerns of water supply management and the public health issues of As in groundwater-based drinking water and irrigation water all require consideration of how As concentration at tubewells used for groundwater extraction may change over time. This chapter considers the likely future trends in As concentration at hand-pumped tubewells (HTWs), most of which tap shallow levels of the Bengal Basin aquifer, and at deep tubewells (DTWs) fitted with motorized pumps for irrigation and for municipal public water supply, which tap deeper levels of the aquifer.
2.2. The hydrogeological context of As occurrence
Depth profiles of sediment hydraulic conductivity from western and central Bangladesh illustrate the hydrostratigraphy of the Bengal Basin aquifer and the hydrogeological context of As occurrence (Fig. 2-1). An uppermost layer of silty clay, a few metres in thickness, forms an upper aquitard across the floodplain regions. Beneath this, to a depth of approximately 160 m, lies a fining-upwards sequence of coarse- to medium-grained sands with inter-bedded layers of silty clay. This sequence constitutes the multi-layered, hydrogeologically leaky, shallow aquifer of the floodplain regions. Extensive occurrence of As in groundwater is largely restricted to this shallow aquifer. It is here, between the depths of 10 and 100 m below ground level (bgl), that the highest As concentrations occur, commonly in the order of 1000 μg/l, and the greatest spatial variability is observed. There are instances of HTWs with <1 μg/l As being situated less than 100 m from adjacent HTWs with >500 μg/l As. Arsenic is present at a lower and more consistent concentration in groundwater from deeper levels of the aquifer. In the national groundwater survey of Bangladesh (ibid.), fewer than 1% of tubewells deeper than 150 m exceeded 50 μg/l.
Full-size image (36K)Fig. 2-1.
The Bengal Basin aquifer system and associated depth distributions of As (filled circles) in groundwater, illustrated for Meherpur, western Bangladesh (top), and Magura, central-southern Bangladesh (bottom). Hydraulic conductivity data (solid line) from Cheetham (2000); As data from [022] and [007]
The Bengal Basin aquifer system and associated depth distributions of As (filled circles) in groundwater, illustrated for Meherpur, western Bangladesh (top), and Magura, central-southern Bangladesh (bottom). Hydraulic conductivity data (solid line) from Cheetham (2000); As data from [022] and [007]
A deep, silty-clay aquitard separates the shallow floodplain aquifer from a deeper aquifer in some places, as at Magura in south-central Bangladesh (Fig. 2-1) and at Kulna in the southern coastal regions, but this aquitard is thinner and discontinuous in some places, as at Meherpur in western Bangladesh (Fig. 2-1), and is absent in others, as at Manikganj in central Bangladesh (GRG, 2002). The lateral continuity of the aquitard at the base of the shallow aquifer is one of the main points of uncertainty concerning the likely sustainability of the deeper aquifer as an As-safe groundwater source. A fuller description of the hydrogeological setting of the As occurrence is given by Ravenscroft et al. (2005).
The floodplain deposits terminate against in lying blocks of older, Plio-Pleistocene sediments that form slightly elevated tracts of land in south-central and north-western Bangladesh, the Madhupur Tract and the Barind Tract, respectively. The confined sand aquifers of these tracts are sedimentologically similar to the shallow aquifer sediments of the floodplain regions, but their long history of flushing by meteoric water under oxidizing conditions during the Pleistocene has produced a very different hydrochemical environment ( [Davies, 1995] and [Ravenscroft et al., 2005] ), and As is absent.
2.2.1. A conceptual model of As in the aquifer
A conceptual model of As occurrence and movement in the aquifer has been developed (Burgess et al., 2002a) from observations of the hydrochemical context, detailed patterns of As spatial and depth distributions, and consideration of the hydraulic characteristics of the aquifer. Arsenic is released to groundwater by desorption from and dissolution of iron (Fe) oxyhydroxide within the alluvial sediments, under the chemically reducing conditions induced by microbial metabolism of organic carbon (Nickson et al., 2000). The mass of As in the solid phase is much greater than in solution, and the rate of release to groundwater is sufficient to maintain a concentration of 500 μg/l, locally exceeding 1000 μg/l, in porewater in the source zone (Burgess et al., 2002a). The principal zone of As release to groundwater is relatively shallow, within the top 100 m but variable across the basin: within 25 m of the ground surface at Chaumohani, south-east Bangladesh (Mather, 1999), 45 m at Meherpur, western Bangladesh (Perrin, 1998), 75 m at Magura, south-central Bangladesh (Carruthers, 2000), and Lalpur, West Bengal (McCarthy, 2001), and 100 m at Manikganj (see Fig. 2-2 for locations, and Fig. 2-1 for illustration of the profiles at Meherpur and Magura). The highest concentrations of As in groundwater are found in tubewells at these depths. The greatest range of As concentration is also observed here, with extreme spatial variability. Groundwater pumped from depths greater than 100 m has a considerably lower and more limited range of As concentration.
Full-size image (32K)Fig. 2-2.
Location of village-scale As surveys. Lp = Lalpur (West Bengal, India), and Mp = Meherpur, Mg = Magura, Mk = Manikganj, Ch = Chaumohani (Bangladesh)
Location of village-scale As surveys. Lp = Lalpur (West Bengal, India), and Mp = Meherpur, Mg = Magura, Mk = Manikganj, Ch = Chaumohani (Bangladesh)
The Fe oxyhydroxide source, and the organic carbon which drives the arsenic release, are both concentrated in the finer floodplain deposits. These are cut through by an array of coarse channel sands. The As release zone is therefore depth-specific and laterally discontinuous. In places, the spatial distribution of As-rich groundwater suggests sedimentological control on the As source (Burgess et al., 2000), although there is insufficient sedimentological data to test these proposals. The same sedimentological characteristics lead to the aquifer itself being multi-layered and hydrogeologically leaky. Under these conditions, vertical leakage restricts the catchments of HTWs to a radius of only a few metres. The lateral discontinuity of the As release zone is on a scale comparable with the extent of the HTW catchments, thus accounting for the observed patterns of spatial variability (Burgess et al., 2002a). Higher-yielding DTWs have catchments of a few tens to a few hundreds of metres radius, but these DTWs also ultimately draw water from the shallow regions of the aquifer. Tubewells tapping the deeper aquifer are therefore vulnerable to contamination in time where the intermediate aquitard is absent or discontinuous, albeit to a lesser extent on account of the considerably greater dilution within their larger catchments.
The potential for a range of overlap configurations between the discontinuous As release zone and the limited HTW catchments is the basis of the conceptual model, explaining the observed variability in As concentration. Shallow HTWs with screens at the level of the As source may draw directly on groundwater with the highest As concentrations, with minimal dilution, immediately following installation and the start of pumping. In contrast, a shallow HTW with its catchment isolated from the As release zone may draw directly on As-free groundwater, and remain free of As for its entire lifetime. Partial overlap between the As release zone and the HTW catchment results in dilution, and an intermediate As concentration in the tubewell discharge, developing after a period of pumping As-free water. Hence, the greatest variability in As concentration is observed in the discharge of HTWs with screens at shallow levels, generally shallower than 75 m ( [Burgess et al., 2000] and [Kinniburgh and Smedley, 2001] ). The much larger catchments of deep production and irrigation tubewells are more likely to incorporate As release zones, but will also lead to more dilution by As-free groundwater, and possibly a long period of As-free discharge before the arrival of As at the depth of the tubewell screen. This may explain the occasional occurrence of As in groundwater from deep tubewells at more consistent, lower, yet in places still with appreciable concentrations ( [013] and [SOES/DCH, 2000] ).
2.3. Predicting As in tubewell discharge
Numerical models have been applied, following the conceptual scheme described above, to reproduce the observed patterns of As occurrence at tubewells and to explore possible future trends in the context of the hydraulic regime imposed by pumping from a multi-layered, leaky aquifer ( [009] and [Cuthbert et al., 2002] ). The numerical modelling packages MODFLOW (McDonald and Harbaugh, 1984) and MT3D (Zheng, 1992) have been used to simulate groundwater flow and As transport in individual, representative catchments for HTWs and DTWs. The steady-state groundwater flow regime imposed by pumping from the tubewell is first simulated using MODFLOW. Within this steady-state flow regime, solute transport is modelled using MT3D, for a maximum duration of 50 years. The application of a steady-state flow field is justified because the flow regime around a pumping tubewell stabilizes rapidly in the highly permeable, hydrogeologically leaky aquifers of the Bengal Basin (Herbert et al., 1989). The results, however, provide a quantitative indication of As concentration in tubewell discharge increasing over a period of many years. Diurnal and seasonal variations are not represented in the models.
The As source is represented as a constant concentration boundary condition imposed at a series of cells in the model. This assumes that the release of As from the source area is faster than the flux of water across it, and that the source is sufficient to maintain a constant porewater As concentration. Depletion of the source is therefore not considered. This is justified on account of the large excess of As present in the sediments compared to the water (Cuthbert et al., 2002).
Parameter values used in the models have been selected in order for the results to be representative, but there are no monitoring data sufficient for use in calibration; so the models are indicative rather than being calibrated simulations valid for predictive modelling of specific DTWs. Results demonstrate the likely response of the aquifer to pumping, in terms of the As concentration in discharge at tubewells in the long term.
2.3.1. Modelling As at shallow HTWs
Models of representative shallow HTWs have been developed using a study in western Bangladesh ( [006] , [022] and [Burgess et al., 2002a] ) for the characterization of permeability profiles and the distribution of As with depth in the aquifer. Full details of the modelling are given by Cuthbert et al. (2002). Arsenic concentration in HTW discharge water (Fig. 2-3) is a function of the depth separation between the HTW screen and the level of As release, the extent of As release zones within the HTW catchment, and the duration of pumping. The conceptual model indicates that As concentration should be expected to increase with time at HTWs incorporating regions of As release within their catchments. The numerical models show that breakthrough of As to the shallow HTWs may be delayed by a few years after the start of pumping (e.g. HTW 2C in Fig. 2-3), implying that many tubewells that are initially considered free of As may become affected some years later. The time scale of the model predictions is however very sensitive to the scale of sorption of As, which controls the retardation of As as it moves through the aquifer sediments towards the tubewell screen (ibid.). Non-linear (Freundlich) isotherm parameters (Kf: 3.1, n: 0.18) were used, representing conditions of neutral pH, available Fe concentration in the sediment of 50 mg/kg, and low phosphate concentration (0.008 mg P/l), following Kinniburgh and Smedley (2001). However, the mechanisms and scale of As sorption are poorly understood for the sediments and the specific hydrochemical environment of the Bengal Basin aquifers (Carruthers, 2004). The HTW modelling results are therefore indicative only. However, they suggest that it may be common for As in HTW discharge to increase over decades or longer, as a result of pumping. Considering the density and age of HTW installations (typically 10 HTWs per km2 and 0–20 years, respectively), it is possible that detailed time-specific surveys might capture the full range of As concentrations in time. If so, the maximum As concentrations in the future will not exceed the highest concentrations currently observed on a local scale. For surveys in western, central, and eastern Bangladesh incorporating 5–10 HTWs per km2, maximum concentrations of 890 μg/l (Burgess et al., 2002a), 350 μg/l (Cobbing, 2000), and 980 μg/l (Mather, 1999) respectively, have been observed.
Full-size image (20K)Fig. 2-3.
Variations in As content of HTW discharge – model results for HTWs screened 10 m below As source; HTW catchments overlap As source layer by 100% (Well A), 50% (Well B), and 10% (Well C). After Cuthbert et al. (2002)
Variations in As content of HTW discharge – model results for HTWs screened 10 m below As source; HTW catchments overlap As source layer by 100% (Well A), 50% (Well B), and 10% (Well C). After Cuthbert et al. (2002)
2.3.2. Modelling As at DTWs
Models of As movement in a DTW catchment have been used to represent the case where no aquitard fully protects the deeper, As-free aquifer from the shallow groundwater system. The models therefore represent the deeper levels of the shallow aquifer, at depths of about 150 m bgl, as well as the deeper aquifer itself at depths greater than 200 m in places where the aquitard is absent or partially absent, as is thought may be the case over extensive parts of central Bangladesh. In other respects, the models are similar to those developed for the shallow HTW catchments, modified to incorporate the greater extent of the DTW catchments, parameterized according to observed profiles of permeability to depths of 250 m (Fig. 2-1), and employing a simpler description of the As source zone. Full details of the modelling are given by Cheetham (2000). Following the conceptual model, DTW catchments are assumed sufficiently extensive to be insensitive to small-scale lateral discontinuity of the zones of As release within the upper levels of the shallow aquifer. Therefore, the DTW models consider only one source geometry for the As release zone, which coincides in extent with the DTW catchment itself.
A summary of the DTW model descriptions is given in Table 2-1. The base case model (model B) simulates a DTW screened at 230–250 m, discharging 1600 m3/d, with a porewater As concentration maintained at 500 μg/L throughout the release zone at 20 m depth, a low-permeability layer at 100–110 m, and no sorption. In model D, sorption is applied using isotherm parameters following Kinniburgh and Smedley (2001). In model E, the low-permeability layer is partly replaced by material with hydraulic properties of the deeper aquifer. Model F simulates the effect of an As source within a clay layer at 200 m depth. While there is no evidence for a deep As source zone, it is a possibility which a precautionary approach should not discount. The trend of As concentration in the base case DTW model is illustrated in Fig. 2-4. Simulated As arrival times and concentrations in the DTW discharge are summarized in Table 2-2.
Table 2-1. A summary description of DTW models
| All models | Screen: 230–250 m; pumping: 1600 m3/day; catchment radius: 550 m; recharge: 630 mm/year; permeability: see Fig. 2-1; Kx/Kz: 10; porosity: 15–40%; S: 10−4; Sy: 0.5–30%; dispersivity: 10 m; 29 layers |
|---|---|
| B, base case | Arsenic source: 500 μg/l at 20 m, no sorption |
| D, sorption | As (V) as Freundlich, Kf: 3.1, n: 0.18; As (III) as Langmuir, KL: 1.72, sm: 13.3 |
| E, discontinuous aquitard | Two-thirds aquitard |
| F, deep source | Arsenic source: 65 μg/l at 195 m |
Full-size image (17K)Fig. 2-4.
Variations in As content of DTW discharge – model results for base case DTW (model B in Table 2-1)
Variations in As content of DTW discharge – model results for base case DTW (model B in Table 2-1)
Table 2-2. A summary of modelled As arrival times and concentrations after 50 years of pumping
| Model | Arrival time for 1 μg/l (years) | Arrival time for 50 μg/l (years) | Arsenic at 50 years (μg/l) |
|---|---|---|---|
| B, base case | 35 | – | 9 |
| D, sorption | – | – | 0 |
| E, discontinuous aquitard | 15 | 30 | 213 |
| F, deep source | <1 | – | 34 |
In the base case, As breaks through to the DTW screen at a concentration of 1 μg/l only after 35 years of pumping, and at 50 years the concentration is still below 10 μg/l. For the case of a partially absent aquitard, As breaks through to the DTW screen at 1 μg/l after 15 years, with 50 μg/l being exceeded after 30 years. For the case including sorption, there is no breakthrough of As within the 50-year modelling timespan. Observed As trends in shallow HTWs ( [Burgess et al., 2002b] and [Burgess et al., 2002a] nd below) suggest that retardation may not be as effective as indicated by the available sorption data. Treating As as a conservative (unsorbed) solute represents the worst case situation for any given As source distribution. It also represents an alternative mechanism of As mobilization, that of As release by organic acids moving from organic-rich, possibly peaty, horizons in the sediment column (McArthur et al., 2004). Under this scenario, organic acids are the mobile species, with As being mobilized as the organics induce reductive dissolution/desorption. This might happen across a range of depths, possibly close to the tubewell screen, and be unrelated to particular As concentrations in the sediment column, so that the possible influence of As sorption could be minimized. Even under this scenario, however, there would be some opportunity for As sorption as it moves towards the tubewell screen. Sorption is very poorly constrained by the available experimental data, but some retardation would be expected to occur, and it may be considerable.
The models suggest that appearance of As in groundwater discharge at DTWs deeper than 200 m after less than 15 years of pumping is only possible if the aquitard is absent, and if sorption is negligible, or if deeper As sources are present. Shallower DTWs, completed close to the base of the shallow aquifer, may be affected in less than 10 years, but only if sorption is less than expected, or if deeper As sources are present (GRG, 2002). If these specific hydrogeological conditions are discounted, a trend of increasing As concentrations above the background value at DTWs in less than 10 years suggests the leakage of As-bearing water from shallow levels behind poorly grouted casing. This does not discount the possible presence of As at background levels, probably in the order of 10 μg/l. The maximum eventual As concentration at DTWs will be considerably diluted with respect to the highest concentrations currently observed at shallow HTWs, which are generally between 100 and 1000 μg/l in southern Bangladesh.
2.4. Evidence for changing As concentration at tubewells
To examine the model predictions, in the absence of long-term strategic monitoring data, evidence for changes in As concentration at tubewells may be sought from:
(1) the proxy relationship between As concentration and tubewell age in the National Hydrochemical Survey of Bangladesh (Kinniburgh and Smedley, 2001); (2) the proxy relationships between As concentration and tubewell age in detailed village-scale surveys; (3) opportunistic time-series from individual tubewells; (4) oxygen and hydrogen isotopic characterization of groundwater in the aquifer.
2.4.1. Arsenic concentration and tubewell age in Bangladesh
Results of the National Hydrochemical Survey of Bangladesh by the British Geological Survey and the Bangladesh Department of Public Health Engineering (Kinniburgh and Smedley, 2001) indicate that the percentage of tubewells shallower than 150 m with As in excess of 50 μg/l, the national drinking water limit, increases consistently with time (Fig. 2-5). Fifty-six per cent of these tubewells installed before 1970 exceed 50 μg/l, compared to 21% of tubewells installed since 1995. This is indirect evidence at a regional scale that As concentration in tubewell discharge increases with time. Despite the large data set, incorporating results from 3216 tubewells at a sampling density of 1 tubewell per 37 km2 in a survey of scale 106 km2, this interpretation may be compromised by the fact that in recent years there has been more emphasis on groundwater development in northern and western Bangladesh, areas of lower groundwater As concentration. This bias is less likely to be an issue at a local scale.
Full-size image (11K)Fig. 2-5.
Trend of increasing As content with tubewell age in Bangladesh, illustrated from a tabulated summary of results consolidated into 5-year groups by Kinniburgh and Smedley (2001)
Trend of increasing As content with tubewell age in Bangladesh, illustrated from a tabulated summary of results consolidated into 5-year groups by Kinniburgh and Smedley (2001)
2.4.2. Arsenic concentration and tubewell age at village scale
Detailed surveys have been carried out under the London–Dhaka-Arsenic-in-Groundwater Programme, incorporating 267 HTWs at five villages across the Bengal Basin at Meherpur, Chaumohani, Magura, Lalpur, and Manikganj (Fig. 2-2). Each survey covers an area of 10–20 km2 at a spatial density of 5–10 tubewells per km2, with an average of 53 sampling points per village.
The complexity of the source distribution masks any simple relationship between tubewell age and absolute As concentration (Fig. 2-6 illustrates the data from Meherpur and Manikganj). In relation to the frequency of exceedance of the 50 μg/l limit in tubewells of different ages, a clearer trend emerges (Fig. 2-7 illustrates the trends at Meherpur and Lalpur). In the case of Meherpur, less than 50% of those tubewells shallower than 150 m and installed in the preceding 5 years had an As content greater than 50 μg/l at the time of sampling. In contrast, all the tubewells installed more than 15 years prior to sampling exceeded the limit.
Full-size image (25K)Fig. 2-6.
Tubewell age vs As concentration: Meherpur, Bangladesh (top), and Manikganj, Bangladesh (bottom)
Tubewell age vs As concentration: Meherpur, Bangladesh (top), and Manikganj, Bangladesh (bottom)
Full-size image (21K)Fig. 2-7.
Tubewell age (individual years) vs percentage of tubewells exceeding 50 μg/l: Meherpur, Bangladesh (top), and Lalpur, West Bengal (bottom)
Tubewell age (individual years) vs percentage of tubewells exceeding 50 μg/l: Meherpur, Bangladesh (top), and Lalpur, West Bengal (bottom)
These results may themselves be biased on account of the fewer data relating to older tubewells. To address this, the data have been consolidated into 5-year age intervals as for the Bangladesh national data set (Fig. 2-8). The Manikganj data indicate no evident trend. Four of the five village-scale surveys do however show trends consistent with the national Bangladesh data set, indicating a tendency for As concentration at HTWs to rise over the past 25 years, at a local scale as well as for the regional scale. This is a proxy indication of a general trend of As concentration rising with the duration of pumping at individual tubewells.
Full-size image (16K)Fig. 2-8.
Tubewell age (consolidated year groups) vs percentage of tubewells exceeding 50 μg/l: village surveys of West Bengal and Bangladesh. Data have been consolidated into 5-year age intervals, with expanded age intervals to incorporate a minimum of 10 HTW results where necessary
Tubewell age (consolidated year groups) vs percentage of tubewells exceeding 50 μg/l: village surveys of West Bengal and Bangladesh. Data have been consolidated into 5-year age intervals, with expanded age intervals to incorporate a minimum of 10 HTW results where necessary
2.4.3. Time-series monitoring of As concentration
There has been no long-term monitoring of the quality of water from shallow HTWs, which are predominantly privately owned. However, over the past few years, there has been an expansion of centralized public water supply through the installation of DTWs in some towns (DPHE, 1996), and some repeat sampling has been undertaken. A number of the DTWs, originally free of As, have become affected after some months of operation. Limited data available (Fig. 2-9) from Meherpur, Magura, and Manikganj suggest that some DTWs may be vulnerable to contamination by downward leakage of groundwater carrying As from shallower levels of the aquifer.
Full-size image (17K)Fig. 2-9.
Arsenic concentration vs time since tubewell installation, for DTWs from Meherpur (Mp), Magura (Mg), and Manikganj (Mk), Bangladesh
Arsenic concentration vs time since tubewell installation, for DTWs from Meherpur (Mp), Magura (Mg), and Manikganj (Mk), Bangladesh
2.4.4. Isotopic indication of vertical leakage
There is a trend towards lighter isotopic composition with depth below about 100 m for groundwater at Meherpur and Manikganj in Bangladesh (Fig. 2-10). Groundwater δ18O decreases by more than 2 per mil between the depths of 50 and 250 m, with the isotopically heaviest groundwater between 20 and 75 m (Meherpur) and at 25 m (Manikganj), coincident with the depth range of As release. Environmental isotopes may therefore provide a basis for tracing vertical groundwater movement in the aquifer. Arsenic derived from the source region at shallow depth, breaking through in vertical leakage to a deep tubewell, should be accompanied, and preceded, by a δ18O signature in the water which is characteristic of the shallower levels in the aquifer, i.e. with heavier δ18O. A linear relationship might therefore be sought between As content and δ18O at the DTWs.
The limited available data (Fig. 2-11) show that for tubewells deeper than 100 m at both Meherpur and Manikganj, the isotopically heaviest groundwater also contains As at highest concentrations, consistent with the expectation of downward leakage of isotopically heavier, As-rich groundwater from shallow levels. Elevated As in isotopically enriched groundwater would more likely become evident at DTWs of greater age and higher discharge. At Meherpur, the single anomalous occurrence of elevated As from greater than 200 m is in groundwater that is also isotopically heavy for this depth, pumped from an irrigation tubewell that has been in regular use since 1974.
2.5. Discussion
The conceptual model of As in the Holocene aquifer of the Bengal Basin (Burgess et al., 2002a) implies that a general trend of increasing As concentration with time should be anticipated at tubewells throughout the region. Relationships between the proportion of HTWs exceeding 50 μg/l As and the time since tubewell installation offer a proxy measure of the increase in As with duration of pumping over the past 35 years. The relationship is positive in the data set of the National Hydrochemical Survey of Bangladesh for tubewells to a maximum depth of 150 m, and also in four out of five detailed village-scale surveys across the Bengal Basin. This suggests a general trend of increasing As concentration in HTWs. Many, but not all, shallow HTWs currently free of As might be expected to become affected in future years, emphasizing the need for vigilance and a continuing programme of monitoring.
Numerical models of shallow HTWs estimate the time for As transport to tubewell screens placed only 25 m below the As source layer to be decades or longer (Cuthbert et al., 2002). The models of As breakthrough to HTW screens are no more than indicative, because of the uncertainty in the scale of sorption, but it appears from the field evidence that As may be more mobile in the aquifer than is consistent with available estimates of sorption based on laboratory batch experiments. The potential mobility of As raises concern for the security of deeper tubewells (DTWs) which are being considered as a source of As-safe water for domestic supply. Available data from a small number of DTWs across the basin suggest that As has broken through to depths of 100 to 220 m at tubewells operating for between 10 and 26 years. Preliminary data on groundwater isotopic character are consistent with the view that DTWs will draw heavily on leakage from shallower levels in the aquifer, but it cannot be ruled out that leakage is occurring behind poorly grouted casing rather than as vertical flow through the aquifer.
For DTWs deeper than 200 m, the models suggest that As will only appear in less than 15 years if the aquitard is absent, as at Manikganj, and if sorption is negligible, or if deeper As sources are present. Shallower DTWs completed close to the base of the shallow aquifer may be affected in less than 10 years under the same conditions. Where these specific hydrogeological conditions do not exist, observations of increasing As concentrations above the background value at DTWs in less than 10 years suggest the leakage of As-bearing water from shallow levels behind poorly grouted casing, as may be occurring at Meherpur. This does not discount the possible presence of As at background levels in the deeper aquifer, probably in the order of 10 μg/l.
Arsenic breakthrough to a deep (>220 m) HTW would be slower than to a DTW, approximately in proportion to the discharge rates. Where there are layers or discontinuous lenses of silty clays, with low permeability, below the As source, modelling suggests that deep HTWs should remain effectively free of As (i.e. <10 μg/l) for the foreseeable future.
2.6. Future directions
Collectively, these indications of a general trend of rising As concentration with duration of pumping for the relatively shallow HTWs and the reasonable expectation of rising trends under some hydrogeological conditions at DTWs used for distributed public water supply, demand serious attention. There are important implications for tubewell location and design, monitoring, water treatment and for estimates of health impacts.
The sustainability of groundwater development is directly related to the maintenance of ‘As-safe’ tubewell discharge ( [Burgess et al., 2002a] and [Ravenscroft et al., 2005] ). The possibility of As breakthrough to DTWs, even for those taking groundwater from the deep aquifer, emphasizes the need for caution in developing the deeper groundwater resources. In relation to treatment, the efficiency of some current and emerging methodologies is in part a function of initial As concentration (Atkins, 2001). Furthermore, estimates of dose–response relationships and predictions of future health impacts are crucially dependant on assumptions regarding past, and future, As concentrations (Yu et al., 2001). Epidemiological evaluations based on time-specific surveys of groundwater As concentration may overestimate the historic As dose, thereby underestimating the dose–response relationship, leading to an underestimate of future impacts. Prediction of future impacts may be further underestimated if the likely rise in As concentration at tubewells is unaccounted for.
Acknowledgements
This work is an output of the London–Dhaka-Arsenic-in-Groundwater Programme. The Natural Environment Research Council (UK) provided an Advanced Course Studentship and overseas fieldwork allowance to Mark Cuthbert. Minerex Environmental Ltd provided financial support for Eileen McCarthy. We thank the Department of Public Health Engineering, Government of Bangladesh, the Public Health Engineering Department of West Bengal, and Dr. Debashis Chatterjee of Kalyani University for assistance with fieldwork and provision of data. Chemical analysis was carried out by the Robens Institute for Public & Environmental Health at Surrey University, and by Tony Osborn at the Wolfson Geochemistry Laboratory, UCL. We thank Dr George Darling, British Geological Survey, for the isotope measurements and for helpful discussions, and Peter Ravenscroft, Arcadis International, for advice on the hydrogeology of Bangladesh in general, and on the As problem in particular. The numerical models of groundwater flow and reactive solute transport used the MODFLOW and MT3D codes of the United States Geological Survey. We are grateful for financial support from the DPHE of the Government of Bangladesh, and UNICEF, for work at Manikganj District Town, and for funding from the NERC, and the UCL Graduate School in support of individual projects under the London–Dhaka-Arsenic-in-Groundwater Programme.
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