Journal of Hydrology 444–445 (2012) 209–220
Contents lists available at SciVerse ScienceDirect
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h y d r o l
14
Department of Earth Sciences, University College London, Gower Street, London WC1 E6BT, United Kingdom a r t i c l e i n f o
Article history:
Received 17 December 2011
Received in revised form 18 March 2012
Accepted 11 April 2012
Available online 21 April 2012
This manuscript was handled by Philippe
Baveye, Editor-in-Chief, with the assistance of Philippe Négrel, Associate Editor
Keywords:
Carbon-14
Bangladesh
Deep groundwater
Sustainability
Hydraulic anisotropy
Recharge s u m m a r y
Environmental isotopes and
14
C dating were applied to estimate ages and recharge sources of deep groundwater ( P 150 m bgl) in south-east Bangladesh. With one exception, deep groundwater is shown to have been recharged more recently than 10 Ka (range 3–9 Ka, mean 7.6 Ka), under climatic conditions indicated by d
18
O and d
2
H as similar to the present day. Groundwater age distributions have been used to infer the scale of aquifer hydraulic anisotropy. 2D groundwater flow modelling is able to reproduce the observed vertical profiles of groundwater age, as determined at two locations, when the aquifer is assigned a permeability anisotropy ( K x
/ K z
) of at least 10
3
. Under these conditions, deep groundwater originates as recharge in the hill regions at the eastern boundary of the basin. Recharge rates estimated from the groundwater ages are close to an estimate of the current rate of deep groundwater abstraction. Cautious development and careful monitoring are therefore necessary, as excessive deep groundwater pumping could draw dissolved arsenic from the shallow levels of the Bengal Aquifer System (BAS) and contaminate the deep groundwater resource.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Isotopic methods of groundwater dating have made it possible
to estimate mean ages of groundwater pumped from wells ( Bethke and Johnson, 2008; Kazemi et al., 2008 ). Carbon-14 is a widely
used radiometric dating technique for groundwater because of the almost ubiquitous presence of dissolved inorganic carbon
(DIC). This technique exploits the decay of 14 C from the onset of recharge to estimate groundwater age at locations along the flowpath. The 14 C dating method is not without problems, though many
of them have been addressed to minimise uncertainties ( Fontes and Garnier, 1979; Plummer et al., 1994; Aravena et al., 1995;
Clark and Fritz, 1997; Buckau et al., 2000; Geyh, 2000; Mokrik et al., 2008; Coetsiers and Walraevens, 2009; Cartwright, 2010 ).
In principle, combining groundwater age dates with isotopic characterisation and interpretation of chemical composition provides important information on the timing, rate and sources of recharge
( cf.
Maduabuchi et al., 2006 ), which in most cases guide assess-
ment of groundwater sustainability.
The Bengal Aquifer System (BAS) is a basin-wide alluvial aquifer system supported by the Bengal Basin (
Michael and Voss, 2009b;
Burgess et al., 2010; Hoque et al., 2011
), which straddles Bangla-
⇑
Corresponding author. Tel.: +44 (0)20 7679 7871.
E-mail address: m.hoque@ucl.ac.uk
(M.A. Hoque).
0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jhydrol.2012.04.022
desh and West Bengal of India. In both countries rural water use is almost entirely dependent on groundwater, the great majority of rural inhabitants employing hand-operated tubewells (HTW) to pump groundwater from depths between ca 5 and 350 m below ground level (bgl). In addition, the economy of the region is centred on agriculture, also reliant on groundwater for irrigation. Although groundwater is readily available at shallow depth (<50 mbgl), since the discovery of the widespread occurrence of excessive dissolved arsenic (As) in shallow groundwater (
Das et al., 1994; Bhattacharya et al., 1997; Dhar et al., 1997; Nickson et al., 1998; DPHE/BGS,
2001 ) attention has focused on the chemical quality of the ground-
water. Evidence of the insignificant presence of As at >150 mbgl
(
Bhattacharya et al., 1997; DPHE/BGS, 2001 ) suggests deep ground-
water as a potential source of ‘As-safe’ groundwater in the region.
This empirical observation has led to the accelerating installation of deep (>150 m) tubewells to avoid groundwater As in the affected areas, mostly in the southern part of Bangladesh and West
Bengal (
Ahmed et al., 2006 ) with little regard to the sustainability
of deep groundwater pumping. In addition to the problem of As, lowering of the water table in some parts of BAS renders shallow
(<100 m) HTWs inoperative, particularly in the northern region during the driest time of the year (
Ali et al., 2012; Shamsudduha et al., 2011 ); this has also encouraged an increase in deep ground-
water pumping. It is matter of record that use of deep groundwater through installation of deep tube-wells has increased over the past two decades, and several hundred thousand deep wells have

210 already been installed across Bangladesh as a whole (
JICA, 2006
to establish the sustainable limit of deep groundwater abstraction
(
Mukherjee et al., 2007b; Michael and Voss, 2008; Hoque, 2010;
Radloff et al., 2011
data.
M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220
DPHE/DFID/
). Basin scale and regional groundwater flow modelling
) remains poorly constrained due to lack of field
The strata in the BAS are such that nearly all boreholes intersect multiple layers of sand and silt–clay materials of limited lateral extent. These silt–clay materials impose a K x
/ K z which would be expected to have a great influence on the ground-
water flow pattern ( Ababou, 1996; Zijl, 1999; Tóth, 2009; Hoque,
2010
). The anisotropy is important as an element in the security of the deep groundwater in BAS against invasion of As from shallow levels (
Michael and Voss, 2008; Burgess et al., 2010
). High anisotropy values have been determined for hydraulic representa-
tion of the aquifer at basin scale ( Michael and Voss, 2009b
). In this paper we present new 14 C ages for deep groundwater in SE Bangladesh, including two vertical profiles to >300 m depth, which provide an opportunity for independent assessment of the anisotropy by reference to simple 2D model representations of groundwater flow. These profiles are consistent with a hydraulic anisotropy ( K x
/ K z
) of at least 1000. Our results have important
implications for the ‘flow path’ security of deep groundwater ( Michael and Voss, 2008; Burgess et al., 2010
), the timing and rate of recharge, and hence the sustainability of the deep groundwater resource. Our study also provides new data on the chemistry and isotopic nature of the deep groundwater.
2. Hydrogeological setting hydraulic anisotropy interbedded silt–clay layers of limited lateral extent within the
aquifer system ( Fig. 1
). The analyses show that the aquifer is dominantly sandy, but inspection of logs reveals one or multiple silt– clay unit(s) in every log record. Individual silt–clay layers are variable in thicknesses and depth of occurrence, making lateral correlation inappropriate, but together these silt–clay layers impose a hydraulic anisotropy on the aquifer which plays a vital role in determining patterns of groundwater flow within the basin (
Michael and Voss, 2009a; Hoque, 2010
). Inverse groundwater modelling and statistical analysis of drillers’ logs for the BAS suggest 10 4 as a value for vertical anisotropy at basin scale (
Michael and Voss,
2009b
).
In the recent decades the natural groundwater dynamics of the shallow groundwater component of BAS has been greatly affected by the intensive abstraction related to irrigation, industrial, urban
and drinking purposes ( Bhuiyan, 1984; Harvey et al., 2006; Hoque et al., 2007; Shamsudduha et al., 2011
).
3. Methods
3.1. Locations
A total of 18 groundwater samples were collected from domestic and monitoring wells of between 15 and 336 m depth in the south-eastern region of Bangladesh between the Meghna River and the eastern boundary of the basin (
Fig. 1
,
Table 1
). Thirteen samples were of deep groundwater, from depths between 150 and 336 m. Five samples were from shallower depths between
15 and 135 m. At two locations, Titas and Kachua (
Fig. 1
), depth profiles were established, of 3 and 4 sampling levels respectively.
The Bengal Basin is bounded by hilly areas and mountainous geological shield terrain in the north, west and east, and to the south the basin is open to the Bay of Bengal. From the southern boundary the ground gradually rises with a gradient of approximately 1E-6 to 4E-4 to the basin margins. A tropical monsoon climate, with a hot-humid and rainy summer and a dry winter dominates the basin (e.g.,
Sanderson and Ahmed, 1979 ). Across
most of the region annual rainfall is more than 1500 mm, and areas near the hills in the east and northeast receive more than
4000 mm. Most of the rainfall occurs during the monsoon period
(June–September) with only a small amount in winter (November–February). Groundwater recharge, from rainfall and to some extent from surface water bodies, is transient as a response to
groundwater withdrawal ( Shamsudduha et al., 2011 ).
A thick sequence of Pliocene–Holocene alluvial and deltaic sediments is present in the basin. The full thickness of the Late Cretaceous–Recent sedimentary sequence reaches to more than 20 km in the SE shore-face, from a few 100 m on the basin margin, a palaeo-continental shelf in the northwest (
Alam, 1989
). Sedimenta-
tion in the basin started during the Late Cretaceous ( Alam, 1989
), with a fluvio-deltaic landscape evolving during the Quaternary
(
Umitsu, 1993
) which also characterises the present-day basin topography. Detailed descriptions of the basin-scale stratigraphy and sedimentation history can be found in
Reimann (1993), Goodbred and Kuehl (2000), Alam et al. (2003), and Uddin and Lundberg
(2004)
. The present-day aquifer-system occupies the uppermost few hundred metres of the sedimentary sequence deposited since
Mio–Pliocene time. This, the Bengal Aquifer System (BAS), overlies a basin-wide marine clay (Upper Marine Shale – UMS) of Mio–Pliocene age at 1200–2000 m below the ground surface (
Burgess et al., 2010 ). The BAS is made up of unconsolidated Plio–Pleisto-
cene–Holocene sediments which host a number of regional aquifers that are hydraulically connected on a basin-wide scale.
Hydrostratigraphical analysis (
Mukherjee et al., 2007b; Hoque,
2010
) indicates thick accumulations of fine to coarse sands with
3.2. Sampling
Sampling was completed in January 2008. With well screen lengths of 1–3 m, samples collected are treated as representing a point sample. Intake depths were measured using a manual probe.
A hand-held Garmin GPS receiver (with WGS 84 datum) was used to geo-reference the sampled wells.
A standard, systematically applied sampling procedure was followed. Wells were purged for 30–50 min (at a rate 20–25 L/min) depending on the depth of the well, prior to collection of samples.
The required purging volume was aimed to be a withdrawal of three borehole volumes of water. In every case a flow-cell for monitoring Electrical Conductivity (EC), field measured redox potential
(ORP) and pH was installed half-way through the well-purging. In all cases stability in the flow-cell readings was achieved, after which time two 30 mL water samples were collected through
0.25
l m polycarbonate filters along with water samples for stable isotope analysis and 14 C activity analysis. Those for trace element
(including arsenic) and cation determination were acidified to pH 2 with ultrapure nitric acid on site. Unfiltered water samples were collected and stored in 20 mL glass bottles with rubber-lined metal caps for the separate analysis of for
13
C/
12
C and
14
18 O/ 16 O and 2 H/ 1 H. Samples
C activity determination were collected in thickwalled plastic bottles, according to HCO
3 content. To each water sample 0.1–0.2 mL of 1% Sodium Azide solution was added to prevent biological activity. Groundwater samples remained exposed for 1 min during sampling, before storage in an ice box at the site and subsequently in a freezer.
Field parameters (ORP, pH and EC) were measured onsite prior to sample collection, using a composite portable instrument, HI
9828 Multiparameter by Hana Instruments Ltd. (Bedfordshire,
UK), calibrated every day at the first time of sampling. Alkalinity
(i.e., HCO
3
) was determined on site by colorimetry and titration with 1.4N H
2
SO
4 solution.

M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220 211
Fig. 1.
The study area in SE Bangladesh and its hydrostratigraphical pattern.
In the left panel: Groundwater sample locations are shown as square symbols. The blocked squares indicate Titas (T) and Kachua (K), where depth profiles were established. Drillers’ log locations are shown as as light-grey dots. The solid yellow east–west line indicates alignment of the 2D groundwater flow model (see
Fig. 11 ). The heavy dashed yellow line indicates the basin margin in the east.
Right panel: five hydrostratigraphical sections are shown along the lines a–e indicated on the left panel. The sections are aligned from west to east (a–d), and from north to south (e). Hydrostratigraphical modelling was performed using Rockworks
Ò
14 with a 2000 2000 5 m
3 grid.
All the samples were returned to London for hydrochemical and stable isotope analysis. Samples collected for radiocarbon activity analysis were submitted to University of Toronto in Canada from
Bangladesh. Analyses were completed by June 2008.
3.3. Laboratory methods
Hydrochemical analyses were carried out at the Wolfson Laboratory for Environmental Geochemistry at University College London. The anions Cl and SO
2
4 were determined using a Dionex
DX-120 ion-chromatograph with an IonPac As14 column. The precision and accuracy of analyses were tested by running duplicate analyses on selected samples. For the two samples with
EC > 3000 l S/cm, SO
4 may be semi-quantitative as inappropriate dilution is used otherwise Cl values run beyond the capacity of the IC column.
The cations (Ca 2+ , Mg 2+ , Na + and K + ) and trace elements (Fe, Mn,
As, Sr) were determined by ICP-AES. Following each run of nine samples, certified standards, SLRS-4 (National Research Council,
Canada) and GRUMO 3A (VKI, Denmak) and synthetic multi-element chemical standards were run and background correction was made. Relative percent difference among the duplicate runs was within ±10%.
Stable isotope analysis was carried out at the British Geological
Survey (BGS) isotope lab by Dr. W.G. Darling using standard preparation techniques followed by isotope ratio measurement on VG-
Micromass 602E or Optima mass spectrometers. All data are expressed in ‰ with respect to Vienna Standard Mean Ocean Water
(VSMOW) on the delta scale: d ¼ ½ð R sample
= R standard
Þ 1 10 3 where R sample is 18 O/ 16 O or 2 H/ 1 H ratio of the sample, and R standard is the corresponding ratio in VSMOW. Analytical precision is ±0.2
‰ for d
18
O and ±2 ‰ for d
2
H (W.G. Darling, personal communication).
Radiocarbon activity and d
13 C were determined by the IsoTrace lab of University of Toronto and University of Waterloo, Canada by
Accelerator Mass Spectrometry (AMS) ( http://www.physics.utoronto.ca/~isotrace/ accessed on 10th July 2009).
4. Results
Field measurements and laboratory analyses are listed in
Table 1 .
4.1. Groundwater chemistry
The hydrochemistry of groundwater samples collected in the region (
Table 1
) is consistent with previous descriptions of hydrochemical patterns in the BAS (
Nickson et al., 2000; Mukherjee et al.,
2008 ). With one exception, As is restricted to the shallow level of

212 M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220 the aquifer which also exhibits more variation in ORP and HCO
3
( Fig. 2
). The field measured redox potential (ORP) generally shows negative values throughout the depth profile but varies between
95 and +62 mV (median 37, n = 13) in deep groundwater and between 117 and +227 (median 67, n = 5) in shallow groundwa-
ter ( Fig. 2 ).
Hydrochemical type ranges from Ca–HCO
3 ranging between 70 and 4940 to Na–Cl, with EC l S/cm, strongly correlated
( r = 0.96, n = 18) with Na and Cl, but with no apparent structure spatially or with depth. There is a trend of decreasing molar ratio of Na to Cl with increasing EC but no evident relationship between
(Na + Cl)/HCO
3 and distance from the eastern basin margin (
Fig. 3 ).
Calcium (Ca) has a strong correlation with Sr ( r = 0.95, n = 13) but neither shows a linear trend with respect to the basin margin.
A number of parameters e.g., HCO
3
, As, Fe, Mn are sensitive to redox state. Arsenic (As), and HCO
3 show a similar pattern of variability with depth (
Fig. 2
). Arsenic concentration in the sampled deep wells is insignificant (below detection limit, 2 l g/L); only one deep groundwater (NN09D) contains As (53
WHO drinking water guideline (10 l l g/L) above the g/L) and the Bangladesh drinking water standard (50 l g/L).
4.2. Stable Isotopes
All the groundwater samples fall close to and sub-parallel to the world meteoric line (WML) (
Craig, 1961
) ( Fig. 4 ). The
d
2
H vs.
d
18
O fields of deep and shallow groundwater are overlapping. Deep groundwater has d
18 O 2.74
‰ to 5.46
‰ and d
2 H 12.3
‰ to
33.1
‰ with d
2
H = 7.45
d
18
O + 7.70 ( n = 13, R
2
= 0.93); shallower groundwater has d
18 O 2.75
‰ to 5.18
‰ and d
2 H 18.9
‰ to
32.9
‰ .
( d
18
There is a suggestion of a less depleted isotopic composition
O 2.75 and d
2 H 18.9) for groundwater at <50 m in the Titas profile (
Fig. 5
), but no systematic trend in isotopic composition with depth is seen in the Kachua profile.
4.3. Radiocarbon and d
13 C
The 14
C activity of DIC, ranging from 2.53 to 121 pMC ( n = 18), generally decreases with depth and also west from the eastern basin margin (
Fig. 6
). At depth >150 mbgl, 14 C activity ranges from
2.53 to 81.8 pMC ( n = 13) with highest activity at 185 mbgl depth found at the eastern margin of the basin along the piedmont and lowest activity at 203 mbgl depth found close to the coast. Median
14 C activity and mean ± standard deviation, respectively, of the thirteen deep groundwater samples were 25.1 and 25.4 ± 19 pMC.
The d
13 C value of DIC in the deep groundwater samples ranges between 6.36
‰ and 38.6
‰ , with a median value 13.1
‰ and with no systematic trend with depth (
Fig. 5 and 6 ). The water sam-
ple from well JB10D has a highly depleted d
13 C value of 38.6
‰ ; all other groundwater samples are less depleted than 21 ‰ .
4.4. Radiocarbon dating of groundwater
Here a basic correction has been made for initial ondly, dilution of
14
C values. Sec-
14 C by interaction with aquifer carbonate, presumed to be dead to 14 C, is reflected in the d
13 C of DIC and we used d
13 C to account for this dilution.
Knowing the 14 C activity of DIC in recharge is a prerequisite of groundwater age estimation. Many empirical approaches have been applied to estimate this initial activity (
Geyh, 2000 ). Here,
the relationship 3 H vs.
14 C is used to estimate the initial 14 C activity in DIC following
Verhagen et al. (1974)
. Available 3 H and 14 C data
from across the Bengal basin reported by earlier workers ( Aggarwal et al., 2000; DPHE/BGS, 2001; Zheng et al., 2005
) were complied to derive the initial activity of 14 C by extrapolation of those

M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220 213
Fig. 2.
Vertical distribution of the field measured redox potential (ORP), Electrical Conductivity (EC), arsenic (As) and HCO
3 concentration in groundwater from the study area.
Fig. 3.
Scatter plots illustrating chemical relationships for the deep (>150 m) groundwaters: (a) molar (Na + Cl)/HCO
3
Na/Cl vs .
EC; (c) Sr vs. Ca, (d) Ca and Sr vs .
distance from eastern basin margin.
vs .
distance from the eastern basin margin, (b) molar

214 M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220
Fig. 4.
d
18
O vs .
d
2
H of groundwater in the study area. The regression line is shown for the deep (>150 m) groundwaters. WML: the wold meteoric water line of
Craig
(1961)
.
assumed that initial 14 C activity was 100 pMC and no dilution occurs other than decay; this is the uncorrected apparent age (UC). In the second case, an initial value for
14
C activity was taken as 87 pMC, as derived from the 3 H/ 14 C plot, but with no dilution other than decay; this is called the corrected age, C
1
. In the third case
(equation 1, below), the estimated value of initial activities is applied and dilution of 14 C is accounted for by reference to the d
13 C values; this is called the corrected age, C
2
( Table 2 ). No corrected
C
2 ages was calculated for sample JB10D which exhibits strongly depleted d
13 C indicative of methanogenesis. In Eq. (1) measured values for
( ‰
14 C and d
13
) and the half-life of
C are indicated as 14 C m
(pMC) and
14 C is taken as 5730 years.
d
13 C m
14 C age ð yrs Þ
¼ 8267 ln 87 =
14 C m d 14 C m
= 25 ð 1 Þ
Corrected C
2 age estimates for the deep groundwater are with one exception younger than 10,000 years BP (before present). The mean age is 7 Ka. The groundwater uncorrected ages increase with depth, but the trend ( R 2 = 0.02, n = 9) becomes insignificant for the corrected C
2 ages (
Fig. 9
). Furthermore, despite no prominent spatial pattern of groundwater age distribution, a general trend of increasing groundwater age is evident westward from the basin margin (
Fig. 9
).
data describing a consistent linear trend ( Fig. 7
). The estimate for initial activity of 87 pMC is close to the fixed correction value of
85 pMC as proposed by
Vogel and Ehhalt (1963)
.
The starting condition for d
13 C in DIC in the vadose zone is taken as 25 ‰ after
Harvey et al., (2002) ; consistent with the dominance
of ‘C3 type’ plants ( d
13 C range 23 ‰ to 30 ‰ ) in the Bengal Basin at 7–8 Ka BP (before present) (
Sarkar et al., 2009
). Influences on d
13 C beyond the vadose zone (principally by dissolution of carbonate minerals and isotopic exchange) also influence the 14 C activity measured in groundwater.
Carbonate minerals are present at 2.1–6.9% in sediments throughout the aquifer sequence in Bangladesh, mainly as siderite
(FeCO
3
) and ankerite [Ca(Fe, Mg, Mn,)(CO
3
) 2
] ( Hasan, 2008
).
Weathering of carbonates in BAS as reported previously (
Dowling et al., 2003 ) is indicated by the strong correlation between Ca
and Sr ( Fig. 2 ) and by the correspondence of high (less negative)
d
13 C values with high HCO
3 concentrations (
Coetsiers and Walraevens, 2009
and
Fig. 8
). Based on the sedimentation rate and eustatic cyclicity in the region
Burgess et al. (2010)
estimated the ages of deeper (> 70 m) aquifer sediments in BAS to be 20 Ka to 1 Ma. We have therefore taken the aquifer carbonates to be dead to 14 C. The simplest isotopic mixing model utilises the difference in d
13 C of
DIC from primary recharge and the admixed DIC from solid phase aquifer carbonates.
Three groundwater ages have been determined from the measured 14
C activities of each sample ( Table 2 ). In the first case it is
5. Discussion
5.1. Groundwater ages
Activities of 14 C in DIC from groundwater at >150 mbgl have previously been found to be in the range of 7.6–77.5 pMC with mean ± standard deviation 30.51 ± 20.79 ( n
= 13) ( Aggarwal et al.,
2000; DPHE/BGS, 2001 ) in Bangladesh, and around 20 pMC in West
Bengal ( cf.
DPHE/BGS, 2001
). Our results are consistent with these previous measurements. The corrected C
2 ages of groundwater in the study area at depths >150 mbgl, at <10 Ka, are also within the range of previous estimates of 2–12 Ka (
DPHE/BGS (2001)
, close to the travel times derived from groundwater flow modelling
at basin scale of 11 Ka ( Michael and Voss, 2009a ), and consistent
with an inference of ages >1000 years from 4 He accumulation measurements (
Dowling et al., 2003
).
A consistently reducing condition throughout the depth profile
(e.g.,
Mukherjee et al., 2008 ) within BAS is indicated by negative
values of ORP, with two exceptions. Shallow groundwater contains excessive levels of As, associated with the reductive dissolution of
FeOOH ( Nickson et al., 1998
). Similar reductive dissolution of FeO-
OH is found to be associated with a high content of Fe in the deeper
Fig. 5.
Depth variation of groundwater isotopic composition and lithology (sand in light grey shade, silt–clay in blue shade) at Titas (DT11S, DT11M, and DT11D) and Kachua
(KHOP1, KHRP2, KHOP4, and KHOP5) sites.

M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220 215
Fig. 6.
(a)
14
C activities and d
13
C in groundwater vs .
distance from the eastern basin margin; (b)
14
C activities vs .
depth; (c) d
13
C vs .
14
C activity; (d) d
13
C vs .
depth.
Fig. 7.
3
H vs .
14
C activity, indicating an initial
14
C activity of 87 pMC. Samples with
>0.5 TU and >87 pMC found to contain a post-nuclear bomb component. Data from
DPHE/BGS (2001), Aggarwal et al., (2000)
and
Zheng et al., (2005) .
Fig. 8.
DIC d
13 elevated HCO
3
C vs .
HCO
3 concentration, demonstrating a correspondence between content and enrichment in
13
C. The regression line disregards the outlier possibly influenced by methanogenesis and illustrates the isotopic effect of carbonate mineral dissolution.
groundwater without excessive As (
Halim et al., 2010
). The reduction of FeOOH driven by microbial metabolism of organic matter contributes to HCO
3
Elevated HCO
3 in groundwater (
Chapelle and Lovley, 1992 ).
concentration (e.g. >200 mg/L at CL02D, FG01D,
DB04D, KHOP5) might indicate additional contribution through microbial metabolism of FeOOH ( cf.
McArthur et al., 2001 ) and
the necessity of further correction considering the contribution of
Solid Organic Carbon (SOC) to 14 C (
Aravena et al., 1995
). Five groundwater samples reported here have d
13 C values 20 ‰ or lighter, a possible indication of organic contribution to DIC pool, but these do not have elevated HCO
3
.
Harvey et al. (2002)
have shown at shallow depth <100 m in central Bangladesh that radiocarbon ages of DOC are as old (3–5 Ka) as aquifer sediments, while
DIC-derived ages for groundwater may be much younger. They suggested that DIC is therefore not the oxidative product of the older DOC. This observation has not been tested using DOC-derived age estimates from other parts of the basin, but if true, organic matter oxidative dilutions of groundwater 14 C in DIC would not be significant, and so have not been included in the corrected C
2 ages presented here.
At CL02D, for which the highest corrected C
2 tion is made, 23 Ka, the stable isotopes d
2 H and d age determina-
18 O do not show the signature of the cooler climate which might be expected at this time (see
Fig. 10 ). The possibility that mixing gives rise to
an anomalous result at this location has been considered. Mixing of seawater (Na/Cl = 0.86) with freshwater (Na/Cl > 1) may give rise to Na/Cl molar ratio close to unity (
Vengosh and Rosenthal,
1994 ). A molar ratio close to unity (
cf
Fig. 2
) has been used to

216 M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220
DT11S
DT11M
CU07L
KHOP4
DS08D
JB10D
LC06D
CL02D
FG01D
NN09D
DB04D
BJ03D
DT11D
CG05D
KHOP5
KH98D
KHOP1
KHOP2
Table 2
Estimated groundwater ages. Ages are rounded to the nearest decade. nd: not determined (see text).
Field ID Uncorrected age,
UC (yrs)
Modern
10,750
Modern
3170
6540
21,900
12,270
30,400
13,990
10,940
15,480
11,830
11,010
1660
11,440
12,610
8550
12,110
Corrected age,
C
1
(yrs)
Modern
9600
Modern
2020
5390
20,750
11,120
29,250
12,840
9790
14,330
10,680
9860
510
10,290
11,460
7400
10,960
Corrected age,
C
2
(yrs)
Modern
4670
Modern
Modern
2390 nd
Modern
22,800
3030
4130
8940
4380
7530
Modern
7610
4240
5960
5610 suggest mixing of freshwater with seawater intruded or en-
trapped throughout southern Bangladesh (
anomaly.
Ravenscroft and McArthur, 2004 ). There is however, no strong evidence of a marine
component in the range and trend of the groundwater d
2 H and d
18 O in the study area (
Fig. 4
). The impact of mixing on the current estimation of groundwater ages is therefore not considered likely, and the isotopic character of sample CL02D remains an
In general, geochemical modification of groundwater occurs progressively along flow-paths following recharge; the longer the flow-path and the travel-time, the greater the hydrochemical mat-
uration ( Tóth, 2009 ). The general hydrochemical pattern of
groundwater in the present study region does not conform to such a systematically linear spatial pattern (
Fig. 2
a) but a general trend of increasing age with distance from the eastern basin margin is
evident ( Fig. 9
b). Deeper groundwater in the BAS is generally arsenic-free (<10 l
g/L) ( Bhattacharya et al., 1997; Burgess et al.,
2010 ), but one deep groundwater in the present study (NN09D)
at 222 m depth is estimated 4.1 Ka old and contains 53 l g/L As.
This may represent remnant As incompletely flushed under previous sea-level low stands (
Burgess et al., 2010 , which includes other
examples) but this explanation needs further investigation.
5.2. Groundwater age and test of hydraulic anisotropy
Basin scale modelling ( Michael and Voss, 2009b
) indicates that groundwater flow is strongly influenced by hydraulic anisotropy imposed by discontinuous low permeability layers interbedded within the aquifer. We used simple modelling to use the observed profiles of groundwater ages as an independent test of aquifer anisotropy. Three 2D groundwater-flow models were constructed
( Fig. 10
) using MODFLOW-2000 ( Harbaugh et al., 2000
) and MOD-
PATH ( Pollock, 1994 ) along an W–E section (
Fig. 1 ). Lateral and ba-
sal boundaries of the models were considered as no-flow. The base of the model was taken as the upper surface of the UMS. The top boundary was assigned a fixed head, to provide topographicallydriven gravitational groundwater flow. The natural condition with no abstraction was considered. Topographic elevation differences
Fig. 9.
Spatial and vertical distribution of groundwater age estimates. (a) Spatial distribution of estimated (C the eastern basin margin. The linear regression line indicates y = 227 x 2396, R
2
2
) ages in plan view; (b) groundwater (C
= 0.46; (c) vertical distribution of groundwater (C
2
) ages.
2
) ages vs .
distance from

M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220 217
Fig. 10.
2D groundwater models on W–E alignment (indicated on
Fig. 1
a–c illustrate the vertical distribution of travel time to a centrally located position for three values of hydraulic anisotropy ( K x
/ K z
= 10, 100, and 1000); (d) model simulation of travel times and groundwater (C
2
) age estimates at Kachua and Titas.
in the model were taken from SRTM elevation data (
EROS, 2002 ).
Effective horizontal hydraulic conductivity ( K x lue 35 m/d with hydraulic anisotropy ( K x
/ K z
) was assigned a va-
) sequentially 10 1 , 10 2 , and 10 3
. Modelling results ( Fig. 10
) demonstrate how aquifer anisotropy controls the pattern of groundwater flow and travel times, and indicates that a high value of anisotropy, at least 10 3 , is required to replicate the observed groundwater ages. This result further validates the estimates of anisotropy in aquifer effective hydraulic conductivity, K x
/ K z
, of
Michael and Voss (2009b)
which was derived from lithological analysis and groundwater flow modelling. Our modelling emphasises that deep groundwater is recharged from the eastern basin margin under natural flow conditions, leading to groundwater age within the basin that is (almost) invariant with depth. Though a general increase in groundwater age is evident in a westerly direction away from the eastern basin margin (
Fig. 9 b), 3D models would be required to
replicate groundwater age more closely. The simple 2D models representing the aquifer as homogeneous and anisotropic do not capture the full extent of heterogeneities in three dimensions and hence must underestimate the time scale of groundwater circulation. The vertical heterogeneity, imposed by silt–clay layers of limited lateral extent, also compartmentalises groundwater flow velocity within the aquifer (
Hoque, 2010
). Nevertheless, the models illustrate how topography coupled with aquifer anisotropy ensures that groundwater at <70–100 mbgl in BAS is part of a shallow flow-system, while groundwater at >150 m is part of the deep

218 M.A. Hoque, W.G. Burgess / Journal of Hydrology 444–445 (2012) 209–220 regional flow-system. Shallow groundwater circulation takes place over a time interval less than hundreds of years, while deeper groundwater circulation in the study region occurs over thousands of years.
5.3. Recharge and sustainability
Shallow groundwater generally has high 14 C activity (up to 120 pMC) indicating modern recharge, in comparison to deep groundwater which with one exception has 14 C activity <45.3 pMC and ages determined as 1000s years BP. The overlapping fields and similar ranges of d
2 H and d
18 O for shallow and deep groundwater nevertheless suggest that all groundwater in the region (to maximum sampling depth of 336 m) was recharged under climatic conditions similar to the present day, and this is consistent with previous
findings ( Mukherjee et al., 2007a; Hasan et al., 2009 ).
The relationship of d
2 H– d
18
O and groundwater age ( Fig. 11 ) may
suggest a subtle difference in isotopic composition between older
(>10 ka) and younger (<10 ka) groundwater, when considering uncorrected age. However, the isotopic content would be expected to be lighter under a cooler climate at >10 Ka. Corrected C
2 ages indicate (with one exception) that groundwater is in fact younger than 10 Ka and that shallow and deep groundwater having similar isotopic compositions were recharged under similar climatic conditions. The isotopic character of groundwater therefore supports our interpretation that the actual age of groundwater in the region
is younger than the uncorrected age. An earlier study ( Aggarwal et al., 2000
) stated that the stable isotopic signature of deeper water (at depths >150 m) is different ( d
18
O ranging between 3 and 6 ‰ ) from that of shallower water, suggesting palaeo-recharge during a different climatic regime between 3 and 20 Ka ago. We interpret the groundwater age at all levels to be younger than 10 Ka.
The slope of d
18 O vs.
d
2 H reflects, approximately, the equilibrium fractionation associated with hydrogen and oxygen, and the intercept reflects the kinetic fractionation at the source regions
( Murad and Krishnamurthy, 2008 ). Deviations of slope and inter-
cept give useful information regarding secondary processes related to surface water-ground water interaction. Deep groundwater in the study region has a slope similar to the WML, 7.45, indicating limited or no enrichment of rain water prior to infiltration, whereas shallow groundwater has a lower slope suggestive of some evaporation. Groundwater flow modelling indicates that deep groundwater is associated with a regional flow system and is recharged from the basin margin hilly areas, where less evaporation prior to infiltration would be expected.
Increasingly, attention is drawn to recharge of deep groundwater as a constraint on its long-term sustainability as a source of water, yet recharge estimations are non-existent. Under the flow regime indicated by the modelling, the deep groundwater ages can be used in a first estimate of the average recharge rate to the deeper regions of BAS in south east Bangladesh, R , over the time period represented by the age determinations. Estimating the spatially integrated groundwater velocity of deep groundwater away from its recharge region ( v ) from
Fig. 9
b as 4.4 m/yr, Darcy’s Law can be applied to infer the long term average recharge rate over the outcrop area upgradient of the measurement points, following
Scanlon et al. (2002) , using:
R ¼ v nA = S ð 2 Þ where n is effective porosity, A is the cross-sectional area of aquifer orthogonal to deep groundwater flow and S is the surface area of the region of recharge.
In estimating A we take 100 m depth as the upper limit of deep groundwater flow (from the 2D modelling section
Fig. 10 c), 350 m
as the lower limit of observed groundwater flow (our deepest sample is from 336 m depth), and 130 km as the length of the basin margin boundary orthogonal to our study region. Hence our recharge estimate refers to a depth interval of 250 m between 100 and 350 m, and a cross-section area perpendicular to flow ( A ) of
3.3E7 m 2 . We take the width of the outcrop as 25 km, and hence the surface area of recharge ( S ) as 3.3E9 m 2 . For an effective porosity of 15%, this yields an average recharge rate of 6.6 mm/yr.
This estimate of recharge is comparable with a basin-scale mod-
el estimate of 5–17 mm/yr based on flow modelling ( Michael and
Voss, 2009a
) and 10 mm/yr from the
4
He accumulation rate ( Dowling et al., 2002
) for deep groundwater.
The current rate of deep groundwater abstraction may be estimated from population data (
BBS, 2004
). Our study area contains six districts with an area of 12,700 km 2 , and a population estimated to be 1.68E7 in 2011 (assuming 1.5% growth rate from
2001). We take per capita consumption to be 60 L per day ( Milton et al., 2006
) and estimate that 20% of the population currently rely on deep groundwater. This is equivalent to a current usage of
6 mm/yr over the study area.
The similarity of the estimated deep groundwater recharge rates and current abstraction rates should be an incentive for more detailed field data acquisition, monitoring, and model representation of BAS, to properly address the vulnerability of deep groundwater in BAS to over abstraction and contamination by As from shallow levels. The hydraulic response of BAS to this estimate of deep groundwater abstraction should be assessed in 3D models, developed with consideration to the aquifer heterogeneities, in order to formulate guidelines for the sustainable management of the resource.
Acknowledgements
Fig. 11.
d
2
H and d
18
O vs .
14
C age for groundwater: (a) uncorrected age, UC; (b) corrected age, C
2
. Note 10 Ka is indicated.
M.A.H. gratefully acknowledges support (BDCS-2006-37) from the Commonwealth Scholarship Commission. We are grateful to
Professor K.M. Ahmed, University of Dhaka, for his support in

Bangladesh and to Iqbal Mahmood for his assistance during fieldwork. We are grateful to the Department for Public Heath Engineering (DPHE), Government of Bangladesh for provision of drillers’ log records. We thank two anonymous reviewers for helpful comments which led to improvements in the paper.
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