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ORIGINAL ARTICLE
Geochemical study of arsenic and other trace elementsin groundwater and sediments of the Old BrahmaputraRiver Plain, Bangladesh
Faruque Ahmed Æ M. Hawa Bibi Æ Hiroaki Ishiga ÆTakehiko Fukushima Æ Teruyuki Maruoka
Received: 29 January 2009 / Accepted: 29 July 2009 / Published online: 13 August 2009
� Springer-Verlag 2009
Abstract The geochemical study of groundwaters and
core sediments from the Old Brahmaputra plain of
Bangladesh was conducted to investigate the distribution of
arsenic and related trace elements. Groundwaters from tube
wells are characterized by pH of 6.4–7.4, dissolved oxygen
(DO) of 0.8–1.8 mg/l, Ca contents of 5–50 mg/l, and Fe
contents of 0.2–12.9 mg/l. Arsenic concentrations ranged
from 8 to 251 lg/l, with an average value of 63 lg/l.
A strong positive correlation exists between As and Fe
(r2 = 0.802; p = 0.001) concentrations in groundwater.
The stratigraphic sequences in the cores consist of yel-
lowish silty clays at top, passing downward into grayish to
yellowish clays and sands. The uppermost 3 m and lower
parts (from 13 to 31 m) of the core sediments are oxidized
(average oxidation reduction potential (ORP) ?170 and
?220 mV, respectively), and the ORP values gradually
become negative from 3 to 13 m depths (-35 to
-180 mV), indicating that anoxic conditions prevail in the
shallow aquifers of the Brahmaputra plain. Age determi-
nations suggest that clay horizons at *10 m depth were
deposited at around 2,000 and 5,000 years BP (14C ages)
during the transgressive phase of sea-level change. Ele-
vated concentrations of As, Pb, Zn, Cu, Ni, Cr, and V are
present in the silts and clays, probably due to adsorption
onto clay particles. Significant concentrations of As occur
in black peat and peaty sediments at depths between 9 and
13 m. A strong positive correlation between As and Fe was
found in the sediments, indicating As may be adsorbed
onto Fe oxides in aquifer sediments.
Keywords Arsenic � Groundwater � Sediment �Peat � Brahmaputra River Plain � Bangladesh
Introduction
High concentrations of naturally occurring arsenic have
been reported in groundwater from many regions, includ-
ing Bangladesh, India, Nepal, Thailand, China, Taiwan,
Vietnam, Chile, Hungary, and parts of the USA (Acharyya
et al. 1999; Smedley and Kinniburgh 2002; Garcia-Sanchez
and Alvarez-Ayuso 2003). Arsenic contamination of
groundwater is now a serious hazard among the many
environmental issues in Bangladesh. There has been
increasing concern over the safety of groundwater from
shallow aquifers in the Ganges, Brahmaputra, and Meghna
(GBM) Delta, where As levels exceed the World Health
Organization (WHO 2004) guideline of 10 and the 50 lg/l
limit adopted in Bangladesh.
Sediments act as both sources and sinks of As and other
toxic trace elements (Guern et al. 2003), and the fate of
trace metals is dependent on the biogeochemical transfor-
mations that occur in the sediments (Peltier et al. 2003).
The degradation of organic matter, present at high con-
centrations in most wetland sediments, drives the formation
of sediment redox gradients (Gaillard 1994). Arsenic is
widely distributed as a trace constituent in rocks, soils,
natural waters, and organisms, and it can be mobilized by
F. Ahmed (&) � T. Fukushima � T. Maruoka
Graduate School of Life and Environmental Sciences,
University of Tsukuba, 1-1-1 Tennoudai,
Tsukuba 305-8572, Japan
e-mail: [email protected]
M. H. Bibi
Department of Environmental Science and Technology,
Saitama University, 255 Shimo-okubo, Saitama 338-8570, Japan
H. Ishiga
Department of Geoscience, Shimane University,
Matsue 690-8504, Japan
123
Environ Earth Sci (2010) 60:1303–1316
DOI 10.1007/s12665-009-0270-7
weathering and microbial activity (Garcia-Sanchez and
Alvarez-Ayuso 2003). Moreover, redox conditions and the
availability of possible carrier phases also account for the
mobility and toxicity of As in natural environments (Fabian
et al. 2003). The mobility of As in the environment is
strongly affected by adsorption onto mineral surfaces
(mainly oxides and hydroxides of Fe, Mn, and Al; Manning
and Goldberg 1997). Sullivan and Aller (1996) also noted
that diagenetic distribution of As in the geologic environ-
ment may be controlled by the redox behavior of Fe
(oxy)hydroxide phases in sediments.
In recent years, many geochemical studies have been
directed at sediments to determine the extent of contami-
nation caused by As (Sullivan and Aller 1996; McArthur
et al. 2001; Ahmed et al. 2004). Studies have shown that
oxides and organic matter play an important role in ele-
mental distributions in the sediments. The geochemistry of
Holocene sediments is significant due to limited alteration
during burial, diagenesis, and tectonic deformation, and it
may provide a continuous record of environmental changes
(Ishiga et al. 2000). Geochemical analyses of the Holocene
alluvial sediments have thus been performed in several parts
of Bangladesh, including the Ganges and Meghna deltaic
regions, to elucidate the behavior of As in differing sedi-
mentary environments (Dhar et al. 1997; Yamazaki et al.
2000; Nickson et al. 2000; McArthur et al. 2001; Ahmed
et al. 2004; Akai et al. 2004; many others). However, these
studies have not considered the concentrations of As in the
groundwater and sediments of the Old Brahmaputra River
Plain, and as yet very little is known about the distributions
and sources of As and other trace elements in the
Brahmaputra sediments in Bangladesh.
The previous report of BGS and DPHE (2001) showed
that the groundwater of the Mymensingh district in the Old
Brahmaputra floodplain is also contaminated with As. Such
groundwater is used for drinking and extensive irrigation
by many households and thus constitutes a human health
risk. Therefore, a detailed study of the groundwater and
sediments is required to clarify the geological background
of As contamination in the Brahmaputra plain. Ground-
water samples were collected from the Mymensingh Sadar
county in Mymensingh district. Six sediment cores were
also collected from the county to determine their geo-
chemical compositions and distribution patterns, with
emphasis on the concentrations of As and other trace
metals in the sediments and their significance. The results
show that As concentrations are greater in the silts and
clays than in the sands.
Geologic outline of the study area
Bangladesh is a low-lying country at the head of the Bay of
Bengal and occupies most of the Bengal basin. Most of the
country consists of low alluvial, coastal, and deltaic plains
and lies within an elevation of 20 m above sea level. The
sediments are derived from three major rivers, the GBM,
which form the largest delta complex in the world. The
study sites have an area of approximately 100 km2 and are
located in the Old Brahmaputra floodplain and northern
end of the Madhupur Tract within the Mymensingh district
of northern Bangladesh (Fig. 1). The Old Brahmaputra
River flows through the district from northwest to south-
east, whereas the tributaries Sutia and Barera Rivers snake
(a) (b)Fig. 1 a, b Map of Bangladesh
and location of the study sites in
Mymensingh
1304 Environ Earth Sci (2010) 60:1303–1316
123
through the middle part of the study area. The sampling
area is characterized by a subtropical climate. Most of the
study area is rural, and agriculture is the principal eco-
nomic activity. Deep tube wells were constructed for
extensive irrigation and drinking water supply in the
region, but shallow hand pump waters are also used for
drinking water by many households.
Madhupur tract
The Madhupur tract is a tectonically uplifted area of Pleis-
tocene clays, known as the Madhupur Clay. This tract is high
in quartz and relatively low in feldspars and micas. On the
basis of reversed magnetic polarity, Monsur (1990) has
concluded that the Madhupur Clay was formed between
730,000 and 970,000 years ago. Most of the tract has been
dissected by valleys, and some of these valleys have been
partly filled with alluvium deposits, which include organic
mud layers up to 6,700 years of age, as established by radio-
carbon dating (Brammer 1996). The soils are well drained
and characterized by dark brown to brown topsoils and red-
dish-brown to yellowish-brown or yellowish-red subsoils.
Old Brahmaputra floodplain
The evolutionary history of the youngest sedimentary
sequence in the Ganges and Brahmaputra plains was dis-
closed by Umitsu (1993). At the time of the last glacial
maximum, 18,000 years ago, world sea level was about
130 m below its present level (BGS and DPHE 2001). The
unconsolidated sediments offered little resistance to ero-
sion by the ancient course of the Brahmaputra as it tried to
cut down to a lower base level. The sea level rose rapidly
between 18,000 and 8,000 years ago to approach its present
level. In the final stage, between 6,000 and 2,000 years
ago, the sea level was about 2–3 m higher than at present,
accounting for the slightly elevated Old Brahmaputra
floodplain (BGS and DPHE 2001). The floodplain is
comprised of alluvial sediments, mainly silt and clay, with
lesser fine- to medium-grained sand. Matured dark gray
soils are rich in organic matter.
Materials and methods
Analytical procedures for water samples
Sample collection and preservation
Groundwater samples were collected from 13 tube wells in
the Mymensingh Sadar county of the Brahmaputra plain
(Fig. 1). The waters were sampled from both shallow (12–
56 m) and deep aquifers (84–137 m). The tube wells were
pumped for 10 min before sampling to stabilize the tem-
perature and dissolved oxygen (DO) content. About 200 ml
of water were taken at each site, acidified with HNO3
(Cica-Reagent, Kanto Chemicals Co., Inc., Japan) to pH 2,
and stored in acid-prewashed high-density polyethylene
bottles (rinsed with 1% HNO3 followed by thorough rins-
ing with water) without headspace. The water samples
were stored in a cooler at 4�C immediately after collection
and analyzed within 2 weeks of collection. Locations and
depths of the tube wells were recorded at each site. The
water samples were not filtered because no particulates
were observed during collection or prior to analysis. This
strategy was also adopted because an accurate measure-
ment of the total amount of As present was sought and
because the groundwater from the tube wells is consumed
directly by the local villagers without any filtration
(McArthur et al. 2004; Nickson et al. 2005).
Field measurement of water quality
Water parameters including pH, electrical conductivity
(EC), and DO concentration were measured in the field
during sampling using a portable Horiba U-23 combined
instrument (Rikagaku kenkujo, Japan). Chemical oxygen
demand (COD; alkaline KMnO4 method), ammonia
(NH4?; Nessler Test Kit), phosphorus (PO4
3-; ammonium
molybdate tetrahydrate method), nitrate (NO3-; Nessler
Test Kit), and calcium contents (Ca2?; EDTA titrimetric
method) were examined in the field.
Geochemical analysis
Concentrations of total As and Fe in the water samples were
determined using an atomic absorption spectrophotometer
(AAS; Shimadzu AA-660G) with a graphite furnace atom-
izer (GFA-4B) according to standard procedures and using
commercial laboratory standards. The detection limit of the
AAS was 1 lg/l. The instrument was linearly calibrated
with custom single element standards (Kanto Chemicals
Co., Inc., Japan) for As and Fe, yielding r2 of 0.9986 and r2
of 0.9988, respectively. Calibration was made using a blank
solution (0.01 mol/l HNO3) and standard stock arsenic and
iron solutions (100 and 1000 mg/l, respectively). For
arsenic and iron analyses, groundwater samples were diluted
in 1.3 mol/1 HNO3 to bring the concentrations within the
range of the calibration. Samples with higher concentrations
were reanalyzed after further dilution.
Each sample was injected five to six times for each
element. Acceptance criteria for each run were \10%
variation in the consistency standard. Precision was B5%,
based on repeated measurements of standard solutions with
known concentrations. All analyses were made at Shimane
University, Japan.
Environ Earth Sci (2010) 60:1303–1316 1305
123
Analytical procedures for sediment samples
Sampling
Sediment cores were taken at six locations (My1, Parail
Village; My2, Najirabad Village; My3, Ghagra Village;
My4, Youth Training Center (YTC); My5, Bangladesh
Agricultural University (BAU) and My6, Mymensingh
City; Fig. 1), using the rotary drill percussion reverse cir-
culation (PRC) method. Depths of the cores ranged
between 21 and 31 m. About 200 g of wet sediments were
collected for each sample and packed in Ziploc� bags.
A total of 153 sediment samples were collected from the
cores, and ORP was measured with a Horiba U-23 ORP
meter. The samples were stored at 4�C in a cooler box for
transport to the laboratory. These cores were logged
and photographed, and the layers were visually classified
during sampling.
A total of 75 sediment samples were selected from
Cores My1, My2, and My6 for analysis. Approximately
50 g of each sediment sample were oven-dried at 110�C for
24 h. The dried samples were then ground for 20 min in an
automatic agate mortar and pestle grinder.
X-ray fluorescence analysis
Selected major oxide [Fe2O3* (total iron is expressed as
Fe2O3*), TiO2, CaO, and P2O5], total sulfur (TS), and trace
element (As, Pb, Zn, Cu, Ni, Cr, V, and Sr) concentrations
were determined by X-ray fluorescence (XRF) at Shimane
University using a RIX-2000 spectrometer (Rigaku Denki
Co. Ltd., Japan) equipped with a Rh-anode X-ray tube. All
analyses were made on pressed powder briquettes
(prepared using a force of 200 kN for 60 s) following the
method of Ogasawara (1987). Average errors for all ele-
ments are less than ±10% relative. Analytical results for
USGS standard SCo-1 (Cody Shale) were acceptable
compared to the proposed values of Potts et al. (1992).
Carbon dating
Conventional radiocarbon dating of two selected samples
from Cores My2 and My4 was carried out at the Geosci-
ence Laboratory of Nagoya University, Japan, using
accelerator mass spectrometry (AMS). Reference materials
of known ages were analyzed to verify the accuracy of the
results. The ages were calculated as 14C year BP, corrected
for 13C, and expressed at the ±1r level of analytical
confidence.
Results
Physico-chemical properties of the water samples
The pH values of groundwater were approximately neutral,
within a range of 6.4–7.4 (Table 1). Electrical conductivity
(EC) ranged from 303 to 550 lS/cm with an average of
370 lS/cm. COD values ranged between 1 and 20 mg/l,
but COD in two samples were under the detection limit.
DO values varied from 0.8 to 1.2 and 1.1 to 1.8 mg/l in the
shallow and deep aquifers, respectively. High DO values
may be related to pump aeration during sampling, as
reported by Nickson et al. (2000) for groundwater in
Bangladesh. The concentrations of DO suggest that the
waters from the deep tube wells were more oxic than those
Table 1 Groundwater quality of tube wells in Mymensingh, Bangladesh
Well
no.
Depth
code
Depth
(m)
pH EC
(lS/cm)
DO
(mg/l)
NH4?
(mg/l)
PO43-
(mg/l)
COD
(mg/l)
NO3-
(mg/l)
Ca2?
(mg/l)
FeT
(mg/l)
AsT
(lg/l)
Tw1 S 12 7.4 303 1.1 0.3 0.5 3 nd 50 5.8 57
Tw2 S 15 6.9 325 0.9 0.1 0.7 1 nd 50 4.9 9
Tw3 S 56 7.4 361 1.2 nd 0.2 2 nd 5 0.7 8
Tw4 S 55 6.4 319 1.2 nd 0.2 3 nd 30 1.7 12
Tw5 D 84 7.1 315 1.2 nd 0.3 nd nd nd 0.3 nd
Tw6 S 12 6.8 491 1.0 3.0 5.0 5 nd 50 10.8 159
Tw7 S 15 6.6 550 1.2 3.0 0.7 20 nd 50 12.9 251
Tw8 S 53 7.1 335 1.0 0.2 0.3 1 nd 50 4.3 18
Tw9 D 137 7.3 400 1.8 nd 0.6 nd nd 30 0.2 nd
Tw10 S 52 7.2 372 1.0 0.3 0.5 5 nd 50 7.0 26
Tw11 S 34 7.2 341 0.8 0.5 1.0 8 nd 50 5.6 27
Tw12 D 84 7.2 353 1.1 0.3 0.5 1 nd 15 0.7 nd
Tw13 D 137 7.4 340 1.1 nd 0.2 2 nd 25 0.3 nd
S shallow; D deep; nd not detected
1306 Environ Earth Sci (2010) 60:1303–1316
123
from the shallow wells. Ammonia concentrations in some
water samples from the shallow aquifers exceeded 3 mg/l.
Nitrate concentrations were not detected in any samples
from the deep and shallow wells. Phosphorus concentra-
tions varied from 0.2 to 5 mg/l and calcium ranged from 5
to 50 mg/l.
Arsenic and iron concentrations in groundwater
Dissolved As concentrations (as determined by AAS) of
shallow tube well waters in Mymensingh show significant
variation, ranging from 8 to 251 lg/l (12–56 m depths)
with an average 63 lg/l (Table 1). However, As con-
centrations were under the detection limit (1 lg/l) in all
deep aquifer samples Tw5, Tw9, Tw12, and Tw13 (84,
137, 84, and 137 m deep, respectively). Elevated As
contents of 251 lg/l (Tw7; 15 m) and 159 lg/l (Tw6;
12 m) were observed at Ghagra Village and 57 lg/l
(Tw1; 12 m) at Parail Village. Total As concentrations
in the remainder of the samples were moderately low
(8–27 lg/l).
Dissolved Fe concentrations show greater contrast
between the wells, ranging from 0.2 to 12.9 mg/l (Table 1).
The two highest Fe concentrations (12.9 and 10.8 mg/l)
were recorded in water samples from shallow tube wells
(Tw7 and Tw6, respectively). Relatively high Fe concen-
trations (7.0, 5.8, and 5.6 mg/l) were also found in samples
from depths of 52 m (Tw10), 12 m (Tw1), and 34 m
(Tw11) in the upper aquifer. The highest Fe concentration
(12.9 mg/l) was found in sample Tw7 from a 15-m tube
well, which also had the highest As content (251 lg/l).
Total Fe concentrations in all deep aquifer wells (84–
137 m depths) were relatively low, ranging from 0.2 to
0.7 mg/l (avg. 0.4 mg/l).
Sediment characteristics
Arsenic-affected areas in Bangladesh are mainly confined
to the Holocene alluvial aquifers at shallow and interme-
diate depths (BGS and DPHE 2001; Mukherjee and
Bhattacharya 2001), as reported by Acharyya et al. (2000)
for West Bengal, India. The Brahmaputra sediments in
Bangladesh are rich in quartz, mica (both muscovite and
biotite), feldspar, calcite, and dolomite (Brammer 1996;
BGS and DPHE 2001). Characteristics of the core sedi-
ments (My1, My2, My3, My4, My5, and My6) from sur-
face to depths of 21, 25, and 31 m are shown in Fig. 2. The
occurrence of peat and peaty clay layers, which are spread
widely in the Mymensingh region, are a possible boundary
between the Holocene and Pleistocene beds (Umitsu 1993;
Brammer 1996). Comparing the characteristics of the core
samples to Umitsu (1993) and Brammer (1996), the upper
parts of the cores (about 0–13 m) are supposed to be
Holocene sediments, and the lower parts of the cores (from
13 m depths) are Pleistocene sediments. The stratigraphic
sequence consists of silty clays and clays at top, passing
downward into grayish to yellowish clays and fine- to
medium-grained sands. The uppermost layers commonly
consist of yellowish silty clays. Color typically changes
from yellowish to gray and dark gray within the shallow
depths (0–13 m).
The fine-grained sediments prevalent in all six cores
within the shallow depths (0–13 m) are relatively rich in
organic matter, representing overbank facies. Clay layers
occur from 1 to 5 and 11 to 30 m in Core My1; from 2 to 5,
6 to 9, and 10.5 to 17 m in My2; from 2 to 6 and 10.5 to
21 m in My3; from 1 to 6, 7 to 10.5, 12 to 20, and 22 to
25 m in My4; from 2 to 7, 8 to 10, and 11.5 to 27 m in
My5; and 1 to 2, 5 to 8, and 13.5 to 20 m in My6. In Core
ORP, oxidation reduction potential
grayish clay
grayish fine sand
grayish silt
yellowish clay
bluish clay
yellowish silty clay
peat
grayish coarse sand
peaty clay
14C age 4315BP
reddish iron oxide
yellowish fine sand
black carbonate iron
yellowish silt
wood fragment
My4
4315BP
My5
+250
ORP+106(mV)
0
5
10
15
20
25
30
Depth(m) My1 My2
-152
+205
-150
My3
+138
-88
+210
Holocene
Madhupur Fm(Pleistocene)
My6Fig. 2 Columnar sections of
core sediments in Mymensingh
(My1, My2, My3, My4, My5,
and My6)
Environ Earth Sci (2010) 60:1303–1316 1307
123
My6, thick grayish silt sections were present at 2 and 10 m
below surface in the anoxic zone. The uppermost 3 m of
the sediments in all cores are oxidized (ORP from ?30 to
?220 mV), and ORP values gradually become negative
with increasing depth up to 13 m of the sediments (ORP
from –35 to –180 mV). These low ORP values indicate
anoxic conditions in the Holocene sediments. However,
ORP values become positive from 13 m depths (ORP from
?40 to ?260 mV), indicating the lower parts of the core
sediments were under oxic conditions.
Yellowish fine sands are more abundant in the aqui-
fers at depths of about 17–31 m in My2 and 20–31 m in
My4. Yellowish and bluish clays are abundant in other
core sediments of the Pleistocene bed. Black peat and
peaty clay layers occur at a depth of about 9–10.5 m in
My2 and My3, from 10.5 to 12 m in My4, from 11 to
11.5 m in My5, and at 13 to 13.5 m in My6. The sec-
tions immediately above and below the peat layers were
mainly grayish and yellowish clays, respectively. Oxide
and carbonate concretions rich in ferruginous minerals
(e.g., Fe-coated quartz and Fe oxide) were observed
mainly in the clay and silty layers. Exceptionally, wood
fragments were found in the gray silt sections in Core
My6.
Major and trace elements
Elemental compositions of the core sediments analyzed by
XRF are summarized in Table 2. Average upper conti-
nental crust (UCC) from Taylor and McLennan (1985) and
all lithotypes from Cores My1, My2, and My6 are included
in Tables 2 and 3, respectively, for comparison.
Table 2 The range, the mean value, and the standard deviation of the major and trace elements of Cores My1, My2, and My6 in Mymensingh
Core/area Trace elements (ppm) Major oxides and TS (wt%)
As Pb Zn Cu Ni Cr V Sr TiO2 Fe2O3 CaO P2O5 TS
My1 (n = 27)
Range 3–14 17–37 37–165 2–53 22–69 71–117 77–190 51–193 0.50–0.84 4.04–9.24 0.82–2.47 0.04–0.30 0.03–0.07
Mean 7 22 75 29 48 95 137 129 0.70 6.61 1.47 0.12 0.04
SD 3.1 4.0 28.2 12.2 13.7 13.2 34.2 40.7 0.08 1.69 0.46 0.06 0.01
My2 (n = 23)
Range 4–65 17–48 32–124 6–118 23–103 65–130 45–246 42–180 0.30–0.88 2.45–10.30 0.70–2.46 0.04–0.22 0.03–2.98
Mean 12 25 75 36 54 99 145 123 0.65 6.35 1.48 0.10 0.18
SD 12.7 7.9 30.5 27.2 23.9 15.5 58.9 47.1 0.18 2.47 0.50 0.05 0.61
My6 (n = 25)
Range 3–733 16–40 25–129 7–91 28–206 60–143 55–594 14–182 0.10–1.02 3.38–15.52 0.64–2.38 0.05–0.16 0.03–6.51
Mean 42 25 82 38 70 105 185 102 0.76 7.98 1.31 0.10 0.38
SD 144.9 5.7 28.0 18.1 34.1 17.1 95.5 46.0 0.21 2.36 0.48 0.03 1.32
UCC
Mean 2 20 71 25 20 35 60 350 0.50 5.00 4.20 0.16 na
UCC Upper continental crust (Taylor and McLennan 1985), na not available
Table 3 Concentrations of trace elements (mean ± SD) in all lithotypes from Cores My1, My2, and My6, Mymensingh
Core Lithology Trace elements (ppm)
As Pb Zn Cu Ni Cr V Sr
My1 Sand (n = 6) 4 ± 0.9 18 ± 0.8 48 ± 6.4 12 ± 5.4 29 ± 3.8 94 ± 12.6 96 ± 11.6 184 ± 8.6
S, Clay (n = 21) 8 ± 2.8 24 ± 3.7 83 ± 27.2 33 ± 8.6 53 ± 10.0 96 ± 13.6 149 ± 28.6 114 ± 31.2
My2 Sand (n = 7) 4 ± 0.7 19 ± 1.0 39 ± 7.1 9 ± 2.0 26 ± 2.5 87 ± 16.9 71 ± 27.4 164 ± 11.0
S, Clay (n = 14) 11 ± 3.2 27 ± 6.3 90 ± 22.7 43 ± 15.5 63 ± 13.9 104 ± 10.8 170 ± 29.2 110 ± 47.1
P, PC (n = 2) 46 ± 26.9 37 ± 15.1 96 ± 4.7 89 ± 41.0 94 ± 13.3 104 ± 24.9 224 ± 31.0 76 ± 20.9
My6 Sand (n = 5) 5 ± 1.3 21 ± 1.7 57 ± 20.9 17 ± 10.9 42 ± 11.8 88 ± 19.4 115 ± 49.8 110 ± 29.9
S, Clay (n = 18) 12 ± 4.8 26 ± 4.7 91 ± 22.4 42 ± 9.5 69 ± 14.2 108 ± 12.1 181 ± 31.1 104 ± 47.8
P, WF (n = 2) 411 ± 455.7 28 ± 17.0 66 ± 58.0 53 ± 53.8 157 ± 69.3 124 ± 26.6 397 ± 278.8 54 ± 56.6
S silt; P peat; PC peaty clay; WF wood fragment
1308 Environ Earth Sci (2010) 60:1303–1316
123
The sediments in Core My1 contain less As (avg. 7 ppm;
range 3–14 ppm), Pb (avg. 22 ppm; range 17–37 ppm), Cu
(avg. 29 ppm; range 2–53 ppm), Ni (avg. 48 ppm; range 22–
69 ppm), Cr (avg. 95 ppm; range 71–117 ppm), and V (avg.
137 ppm; range 77–190 ppm) than in either My2 or My6
(Table 2). The highest concentrations of Zn and Sr (165 and
193 ppm) and the highest average value of Sr (129 ppm)
occur in My1. CaO and P2O5 contents are elevated, in the
range of 0.82–2.47 and 0.04–0.30 wt% (avg. 1.47 and
0.12 wt%), respectively. Ca and Sr have similar behaviors in
sediments, and therefore, both CaO and Sr show surface
enrichment in the core samples. Sediments in this core contain
the least TS (ranging from 0.03 to 0.07 wt%; avg. 0.04 wt%)
among the three cores.
On average the sediments in Core My2 contain 12 ppm As
(range 4–65 ppm), 75 ppm Zn (range 32–124 ppm), 54 ppm
Ni (range 23–103 ppm), and 99 ppm Cr (range 65–130 ppm).
Average concentrations of Pb and Cu are similar to those in
My6. However, the highest concentrations of Pb (48 ppm) and
Cu (118 ppm) were found in this core sample. Black peat
sample My2-9 had the highest As content (65 ppm), along
with the highest CaO (2.46 wt%) and TS (2.98 wt%) at 9–
9.5 m depth. The concentrations of TiO2 and P2O5 in My2
range from 0.30 to 0.88 and 0.04 to 0.22 wt%, respectively.
CaO contents vary between 0.70 and 2.46 wt% in My2 and
yield the highest average value of 1.48 wt%. Abundances of
Fe2O3 and TS show significant variation in My2, with ranges
of 2.45–10.30 and 0.03–2.98 wt%, respectively.
Abundances of trace metals in the sediments of Core My6
are higher than in those from My1 and My2. The highest
average values of As (42 ppm), Pb (25 ppm), Zn (82 ppm),
Cu (38 ppm), Ni (70 ppm), and Cr (105 ppm) were all
recorded in sediments from this core. Vanadium abundances
range between 55–594 ppm, and the average (185 ppm) is
also highest among the three sample sets. Anomalously high
As (733 ppm), Ni (206 ppm), and V (594 ppm) concentra-
tions were found in a wood fragment sample (My6-11;
Table 2). Black peat sample My6-15 also had a higher con-
centration of As (89 ppm) at 13–13.5 m depth. Sediments in
this core contain the lowest Sr (avg. 102 ppm; range 14–
182 ppm) among the three cores. Overall concentrations of
TiO2, CaO, and P2O5 in the sediments range from 0.10 to 1.02,
0.64 to 2.38, and 0.05 to 0.16 wt%, respectively. Average
concentrations of P2O5 are almost identical in all cores. Fe2O3
and TS contents vary considerably, with ranges of 3.38–15.52
and 0.03–6.51 wt%, respectively. The highest Fe2O3 and TS
values (15.52 and 6.51 wt%) were observed in the same wood
fragment sample (My6-11) at a depth of 10.8 m.
Sediment dating
Organic-matter-rich black clays were used for dating. The
conventional radiocarbon ages were 1985 ± 30 years BP
at 9–9.5 m in Core My2 and 4315 ± 35 years BP at 10.5–
11 m in My4. As Umitsu (1985, 1993) described, the
radiocarbon ages of the peat layers in the Ganges plain
(5 m depth) and Sylhet basin (9 m depth) of Bangladesh
were dated 3230 ± 110 and 4180 ± 120 years BP,
respectively, showing similar ages to those of this study.
Discussion
Characteristics of As release in groundwater
Relationships between (a) As and Fe, (b) As and COD, and (c)
Fe and COD in groundwater are shown in Fig. 3. A strong
positive relationship exists between As and Fe (r2 = 0.802;
p = 0.001; Fig. 3a) in the water samples. This correlation is
consistent with many reports from the Ganges and Meghna
regions (e.g., Anawar et al. 2003; Tareq et al. 2003). Arsenic is
also positively correlated with COD (r2 = 0.704; p = 0.004;
Fig. 3b). A moderate positive relationship was found between
Fe and COD (r2 = 0.567; p = 0.007; Fig. 3c) in groundwa-
ters from the Mymensingh Sadar.
In this study, the highest As value observed (251 lg/l)
was associated with the highest COD (20 mg/l), indicating
decomposition of organic matter in the shallow aquifer.
The combination of these conditions with the highest
Fe contents (12.9 mg/l) supports reduction of Fe
(oxy)hydroxides (Nickson et al. 1998, 2000; Zheng et al.
2004), because organic matter plays an indispensable role
in the release of As (McArthur et al. 2001, 2004).
As and Fe distributions with well depth
Concentrations of both As and Fe in the shallow wells are
above the WHO (10 lg/l and 0.3 mg/l, respectively) and
Bangladesh limits (50 lg/l and 1 mg/l, respectively) for
drinking water. No linear relationship exists between well
depth and As and Fe contents in the waters, but concen-
trations of both elements clearly tend to be lower at greater
depth (Fig. 4). Additional As and Fe data from the
respective areas (Mymensingh Sadar; BGS and DPHE
2001; n = 11) are also shown in Fig. 4 for comparison.
Concentrations are broadly similar to those determined in
this study. Maximum As concentrations occur at depths
between 12 and 40 m, whereas samples deeper than 60 m
are arsenic-poor (\10 lg/l) and are below the WHO limit
for potable water. Moreover, the overall decrease in As
concentration with increasing aquifer depth is consistent
with other studies (Acharyya et al. 1999; Chowdhury et al.
1999). Arsenic and Fe abundances in the waters of the
upper aquifer vary considerably (8–251 lg/l and 0.7–
12.9 mg/l, respectively). These variations can be attributed
to varying redox states at shallow depths within the aquifer.
Environ Earth Sci (2010) 60:1303–1316 1309
123
All shallow tube wells of the aquifer contain high-As
concentrations, but waters from deep tube wells are As-free or
As-poor. Bhattacharyya et al. (2003) also reported that As
concentrations tend to be greatest in the intensively exploited
shallow aquifer (10–50 m), whereas waters from deeper
aquifers up to 150 m tend to have much lower As contents.
This contrast in As concentrations between groundwater from
shallow and deep aquifers can be explained by resorption of
As onto residual FeOOH (McArthur et al. 2004) and by the
nature of the aquifer sediments (Zheng et al. 2004).
Inter-element relationship in core samples
Table 4 shows the correlation matrix for elements in the
sediments. Strong positive relationships were observed
between the concentrations of Fe2O3 and As, Pb, Zn, Cu,
Ni, V, and TiO2, and between V and As, Pb, Zn, Cu, Ni,
and TiO2 in Core My1 (Table 4). Non-significant correla-
tions for Sr, CaO, and P2O5 suggest a different source for
these elements in the sediments. In Core My2, Fe2O3
concentrations are positively correlated with As, Pb, Zn,
Cu, Ni, Cr, V, and TiO2 (Table 4). Significant associations
of V with Pb, Zn, Cu, Ni, and Cr were observed in the
sediments. Negative or poor relationships for Sr, CaO, and
P2O5 reflect a differing control for these elements. Strong
positive relationships of Fe2O3 with As, Ni, Cr, and V, of
TS with As, Ni, V, and Fe2O3, and of V with As, Ni, and Cr
were observed in the sediments of Core My6. Conversely,
concentrations of Sr, CaO, and P2O5 again show no rela-
tionships, indicating different behavior of these elements.
(a) (b)
Fig. 4 a, b Relationship
between well depth and As and
Fe contents in Mymensingh
groundwater. Filled symbols are
the data from this study; open
symbols are from BGS and
DPHE (2001). The WHO and
Bangladesh As (10 and 50 lg/l,
respectively) and Fe (0.3 and
1 mg/l, respectively) for potable
water are indicated by the
dashed lines
(a)
(c)
(b)
Fig. 3 a–c Relationships
between Fe, COD, and As, and
COD and Fe in groundwater,
Mymensingh
1310 Environ Earth Sci (2010) 60:1303–1316
123
Relationships among the geochemical data show that
metallic elements are strongly or positively correlated with
total Fe (Table 4), suggesting that Fe2O3 may exert a major
role in controlling the metal concentrations in the core
sediments, as discussed by Singh et al. (2005) for river
sediments in India. The strong positive correlation matrices
of a suite of metals (As, Pb, Zn, Cu, and Ni in My1; Pb, Zn,
Cu, Ni, and Cr in My2; As, Ni, and Cr in My6) with V
Table 4 Correlations between the elements in sediments of three cores
As Pb Zn Cu Ni Cr V Sr TiO2 Fe2O3 CaO P2O5 TS
Core My1 (n = 27)
As 1.00 0.47 0.28 0.48 0.69 0.34 0.65 -0.82 0.55 0.69 -0.74 -0.28 0.04
Pb 1.00 0.85 0.79 0.74 0.37 0.70 -0.51 0.59 0.67 -0.62 -0.32 0.35
Zn 1.00 0.84 0.72 0.38 0.68 -0.23 0.45 0.70 -0.36 0.03 0.26
Cu 1.00 0.86 0.43 0.89 -0.54 0.78 0.84 -0.62 -0.19 0.09
Ni 1.00 0.48 0.91 -0. 70 0.75 0.90 -0.75 -0.31 0.11
Cr 1.00 0.63 -0.18 0.64 0.50 -0.12 -0.18 0.21
V 1.00 -0.69 0.86 0.95 -0.69 -0.30 0.03
Sr 1.00 -0.66 -0.69 0.96 0.51 0.17
TiO2 1.00 0.72 -0.62 -0.38 0.10
Fe2O3 1.00 -0.71 -0.22 -0.06
CaO 1.00 0.58 0.16
P2O5 1.00 -0.02
TS 1.00
Core My2 (n = 23)
As 1.00 0.42 0.39 0.56 0.61 0.11 0.55 -0.50 -0.08 0.42 0.21 0.08 0.93
Pb 1.00 0.68 0.95 0.90 0.65 0.77 -0.44 0.41 0.31 -0.36 -0.23 0.11
Zn 1.00 0.75 0.80 0.53 0.83 -0.27 0.57 0.79 -0.08 0.15 0.15
Cu 1.00 0.95 0.63 0.85 -0.46 0.43 0.43 -0.22 -0.09 0.26
Ni 1.00 0.64 0.92 -0.60 0.54 0.58 -0.29 -0.17 0.33
Cr 1.00 0.71 -0.32 0.64 0.41 -0.28 -0.09 -0.14
V 1.00 -0.66 0.71 0.79 -0.32 -0.10 0.26
Sr 1.00 -0.55 -0.50 0.69 0.50 -0.31
TiO2 1.00 0.63 -0.58 -0.14 -0.32
Fe2O3 1.00 -0.18 -0.01 0.25
CaO 1.00 0.70 0.41
P2O5 1.00 0.22
TS 1.00
Core My6 (n = 25)
As 1.00 -0.27 -0.40 -0.18 0.86 0.48 0.90 -0.42 -0.67 0.69 0.04 -0.11 0.99
Pb 1.00 0.48 0.80 0.10 0.28 -0.05 -0.19 0.37 0.12 -0.26 -0.55 -0.21
Zn 1.00 0.78 -0.01 0.26 -0.17 0.43 0.43 0.09 0.26 -0.08 -0.36
Cu 1.00 0.28 0.41 0.07 0.03 0.35 0.25 0.06 -0.40 -0.09
Ni 1.00 0.75 0.92 -0.40 -0.40 0.78 0.01 -0.30 0.87
Cr 1.00 0.75 -0.39 0.14 0.76 -0.22 -0.31 0.46
V 1.00 -0.55 -0.31 0.90 -0.14 -0.27 0.88
Sr 1.00 -0.07 -0.51 0.83 0.59 -0.39
TiO2 1.00 -0.04 -0.37 -0.15 -0.70
Fe2O3 1.00 -0.15 -0.27 0.67
CaO 1.00 0.61 0.09
P2O5 1.00 -0.12
TS 1.00
Bold text highlights strong correlations
Environ Earth Sci (2010) 60:1303–1316 1311
123
suggest the possibility of formation of complexes with
organic matter. This is consistent with the study conducted
by Tribovillard et al. (1994) of sediments in the United
Kingdom. The negative or poor correlations of Sr, CaO,
and P2O5 in all cores reflect a different control for these
elements. Strong positive correlations between Sr and CaO
were found in all cores and are most likely related to their
similar geochemical behavior. With some exceptions,
inter-relationships between As, Pb, Zn, Cu, Ni, and Cr
show that all are significantly correlated with each other,
indicating a common source or a similar enrichment
mechanism in the sediments.
Vertical distributions of major and trace elements
The vertical distribution of As in Core My1 is similar
to that of Fe2O3, although from 15 m downward the
distributions vary (Fig. 5). Abundances of As, Pb, Zn, Cu,
Ni, Cr, and Fe2O3 in the silts (0–0.5 m) and clays (0.5–5
and 11–30 m) are greater than those in the sands (5–11 and
30–30.5 m; Table 3). In contrast, CaO and Sr contents tend
to have greater concentrations in sands than in the clays.
A suite of trace metals (Pb, Zn, Cu, Ni, and Cr) are enri-
ched in the bluish clays at a depth of 15 m, possibly due to
the migration of soluble metals to this depth and adsorption
onto clay particles. These particles have a high capacity for
adsorbing or retaining trace metals, as found by Yan et al.
(2000) in a clay-rich aquitard sequence in Saskatchewan,
Canada, and by Peltier et al. (2003) in Dead Stick pond
sediments in Chicago, IL, USA.
The vertical profile of As in Core My2 is similar to that
of iron, though from 16 m upward it shows a less consis-
tent trend (Fig. 6). Concentrations of As, Pb, Zn, Cu, and
Ni show peaks at 4–5 m, 9–10 m, and 13–16 m (Fig. 6).
Fig. 5 Vertical distribution of
As, Fe2O3, Pb, Zn, Cu, Ni, Sr,
and CaO in Core My1,
Mymensingh
Fig. 6 Vertical distribution of
As, Fe2O3, Pb, Zn, Cu, Ni, Sr,
and CaO in Core My2,
Mymensingh
1312 Environ Earth Sci (2010) 60:1303–1316
123
The trends of these elements are similar to Fe, which shows
peak concentrations at the same depths. The contents of
these trace metals are thus probably controlled by the
presence of Fe oxides (Preda and Cox 2002), as Fe
(oxy)hydroxides have high affinity for trace metals (Tessier
et al. 1994). Trace metals (As, Pb, Zn, Cu, Ni, and Cr) are
enriched in the silts, clays, peat, and peaty clay at 0–0.5
and 2–17 m relative to the sands at 0.5–2 and 17–31 m
(Fig. 6). Padmalal et al. (1997) and Singh et al. (2005)
reported trace metal-enrichment in fine-grained clays, as
they possess higher surface areas than coarser grains. CaO
and Sr show similar trends in Core My2.
Moderate trends in the depth-wise variation of trace
elements are seen in Core My6 (Fig. 7). Arsenic and Fe
exhibit almost similar vertical distributions, with strong
spikes in abundances at 10.8 and 13 m (Fig. 7). The
highest concentrations of TS were also found at the same
depths. This association suggests co-precipitation of these
elements with Fe sulfide minerals or adsorption on to Fe
sulfides in an anoxic environment, as proposed by Huerta-
Dıaz et al. (1998) for lake sediments in Canada and by
Ayyamperumal et al. (2006) for river sediments on the
southeast coast of India. Arsenic is present in the crystal
structure of many sulfide minerals as a substitute for S. As
the geochemistry of As follows closely that of S, the
greatest concentrations of the element tend to occur in
sulfide minerals, of which pyrite is the most abundant
(Smedley and Kinniburgh 2002). The vertical distributions
of Ni, Cr, and V also show distinct peaks at 10.8 m and Pb,
Cu, and Ni at 13 m (Fig. 7). These elements have a strong
affinity for organic matter, Fe oxides and clay minerals
(Tessier et al. 1994; Tribovillard et al. 1994; Singh et al.
2005). Therefore, the vertical profiles of Pb, Cu, Ni, Cr,
and V show similarities throughout the core. The concen-
tration of Sr decreases sharply at 10.8 m depth. From the
bottom of the core to the surface, CaO and Sr increase
irregularly upward and reach their highest values
(2.38 wt% and 182 ppm) in the uppermost sample. The
sandy character of the sediments reduces their adsorptive
properties, leading to a reduction of metal levels in sedi-
ments from the My6 core (Fig. 7).
In this study, coincident peaks for some trace metals
(Pb, Zn, Cu, Ni, Cr) at approximately the same depth (My1,
15 m; My2, 9–16 m; My6, 10–13 m) suggest the influence
of post-depositional effects, such as reduction of sulfides,
precipitation of metallic sulfides under anoxic conditions,
or re-precipitation of trace metals on Fe (oxy)hydroxide
coatings (Millward and Moore 1982). Moreover, because
there are no major industries in the study areas and no
known natural point sources for metal enrichment in the
sediments, post-depositional contamination of the cores
from anthropogenic pollution is improbable. Therefore, the
trace metals present are likely incorporated in the clay
matrix (e.g., from weathering of bedrock) and in the fine-
grained sediments and have been influenced by post-
depositional diagenetic remobilization.
Characteristics of As in sediments
In this study, the clays in Cores My1, My2, and My6
contain 5–14, 6–15, and 7–20 ppm As, respectively, while
contents in the sands (3–6, 4–6, and 3–6 ppm, respectively)
are one-half to one-third lower. These results are compa-
rable to data for samples from the Ganges Delta in Jessore
region (4–18 ppm in clays; 3–7 ppm in sands; Yamazaki
et al. 2003). The Jessore region is one of the most
As-contaminated areas of Bangladesh. This contrasting
distribution of As between clays and sands in the
Ganges–Meghna–Brahmaputra (GMB) River system is
also consistent with other reports (e.g., Yamazaki et al.
Fig. 7 Vertical distribution of
As, Fe2O3, Cu, Ni, Cr, V, Sr,
and CaO in Core My6,
Mymensingh
Environ Earth Sci (2010) 60:1303–1316 1313
123
2000; Anawar et al. 2002). Arsenic may be scavenged by
clay minerals (Yan et al. 2000), as they have high surface
areas and negative surface charge (Padmalal et al. 1997;
Singh et al. 2005).
The core sediments in the Mymensingh region are sig-
nificantly enriched in As (Tables 2, 3) compared with the
UCC (avg. 2 ppm As from Taylor and McLennan 1985;
avg. 5 ppm As from Rudnick 2005). Excluding wood
fragment, the maximum As abundances observed in Cores
My2 and My6 (Table 2) are 10–20 times higher than the
baseline concentrations of As in sediments (3–10 ppm;
Smedley and Kinniburgh 2002). These elevated As con-
centrations in the core sediments are most probably related
to diagenetic redistribution controlled by the redox
behavior of Fe (oxy)hydroxide phases (Sullivan and Aller
1996). McArthur et al. (2001) and Ravenscroft et al. (2001)
reported that the presence of arsenic-bearer phases, i.e.,
Fe (oxy)hydroxides, is one of the major hosts of As in
the Ganges sediments. Moreover, re-adsorption onto or
co-precipitation with Fe oxides and hydroxides and sub-
stitution in Fe sulfide minerals could also contribute to As
levels in these sediments, as suggested by the strong cor-
relation between As and Fe2O3 in Cores My1 (r = 0.69)
and My6 (r = 0.69) and between As and TS in My2
(r = 0.93) and My6 (r = 0.99; Table 4).
The existence of peat bands at around 9–13 m depth
(ORP -60 to -170 mV) in the study areas represents a
reducing and marshy wetland in an anoxic environment. In
the tropics, peat evolves in situ by accumulation of fallen
trees, leaves, and branches (Markgraf 1989). Ishiga et al.
(2000) reported that the fluvial sediments supplied by the
Ganges are rich in organic matter, derived from higher
plants that grew in a humid and warm climate. Such natural
organic matter (peat and peaty sediments, which spread
widely in the Bengal lowland after ca. 5,000 years BP;
Umitsu 1993) is one of the potential sources of As con-
tamination of groundwater in Bangladesh (Ishiga et al.
2000; Yamazaki et al. 2000).
In this study, the black peat in My2 and peat and wood
fragment in My6 have the highest abundances of As
(65 ppm in My2; 89 and 733 ppm in My6, respectively),
TS (2.98 wt%; 1.76 and 6.51 wt%, respectively) and Fe2O3
(9.29 wt%; 8.80 and 15.52 wt%, respectively). Decom-
posed wood fragments (ORP -110 mV; at 10.8 m in My6)
as a source of carbon and organic matter in silts with
detectable odor reflect simultaneous fermentation. Arsenic
might have been deposited onto fine-grained organic-rich
sediments that were preferentially deposited under low
energy conditions in the Ganges and Meghna floodplain
during the Mid-Holocene sea-level rise (Acharyya et al.
2000). Appearance of these peat bands in the Holocene
sediments accelerated the supply of organic matter, as
evidenced by higher COD value (about 20 mg/l; Table 1)
in Mymensingh groundwater. High concentrations of
NH4? (about 3 mg/l; Table 1) in shallow tube wells are
likely related to the burial of peat. This result is consistent
with the study conducted by McArthur et al. (2001) of
sediments in the Ganges regions. Peat occurs extensively
beneath the As-affected areas of Samta and Deuli villages
in southwestern Bangladesh, and peat contains high con-
centrations of As (50–260 ppm), within which microbial
degradation is high (Ishiga et al. 2000; Yamazaki et al.
2000). Akai et al. (2003) also reported that As was dis-
solved into water when reducing conditions were achieved
by microbial activity. Thus, the peat is the driver for
reduction of Fe (oxy)hydroxides in the Old Brahmaputra
plain, as observed by McArthur et al. (2004) in the Ganges
plain. Widespread occurrence of peat and peaty sediments,
normally rich in As, has possibly enriched the transgressing
seawater (Umitsu 1993) and is inferred to be one of the
main sources of As in the Bengal basin.
Conclusions
Our present results show that groundwaters from the
shallow aquifers in the Mymensingh region are character-
ized by approximately neutral pH, moderate COD, and
high concentrations of As and Fe. Arsenic concentrations
in groundwaters are highly variable (8–251 lg/l), and
many exceed the maximum permissible limits of the WHO
(10 lg/l) and Bangladesh (50 lg/l) for drinking water.
Strong positive correlation between As and Fe in the water
samples suggests that dissolved As may be adsorbed to Fe
(oxy)hydroxides.
The cores in the study areas pass downward from yel-
lowish silty clays into grayish to yellowish clays and sands.
The shallow aquifer sediments of the Brahmaputra plain
are in reducing conditions, as indicated by negative ORP
values in the upper part core samples. Abundances of As
and other trace elements (Pb, Zn, Cu, Ni, Cr, and V) and
Fe2O3 are greater in silts and clays compared to those in the
sands. Anomalously high concentrations of As, V, Fe2O3,
and TS were found in a blackish wood fragment sample at
11 m depth. Concentrations of these elements are also
higher in black peat and peaty sediments at depths between
9 and 13 m, possibly because these consist of blackish very
fine clays rich in organic matter. Arsenic and other trace
metal contents are strongly correlated with Fe2O3,
suggesting that Fe oxides play a role in controlling
abundances.
This study demonstrates that the shallow aquifers of the
Holocene sediments in the Old Brahmaputra plain are
affected by As-enrichment, whereas the aquifers in the
Pleistocene sediments are low in As. High-As and low-As
groundwaters are produced from the Holocene (anoxic) and
1314 Environ Earth Sci (2010) 60:1303–1316
123
Pleistocene (oxic) aquifers, respectively. Since groundwa-
ter is used as a source of drinking water in Bangladesh,
routine monitoring of As levels in the waters from wells
must be performed to avoid high-As concentrations in
water.
Acknowledgments The authors thank Professor Yoshikazu Sampei
and Professor Yoshihiro Sawada of Shimane University for access to
the AAS and XRF facilities, and Professor M. A. Sattar of Bangladesh
Agricultural University, Mymensingh, Bangladesh, for his coopera-
tion in sampling. We thank the Mymensingh City Authority for their
valuable support during sample collection. The radiocarbon dating
was carried out at Nagoya University, Japan.
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