DOI : 10. 1051/jp4 : 20030397
Morpho-tectonic
depression
interaction
and
vis-à-vis
and
trace
groundwater
early holocene land-ocean
metals
adsorption
contamination-a
in
sediment
study
case
from
the Bengal
delta, Bangladesh
A. A.
Khan,
M. A.
Hoque,
Ali1 and
M.
M. A.
Hasan
Depatiment of Geology, University of Dhaka, Dhaka 1000, Bangladesh
Chemistry Division, Atomic
Energy
Center, Dhaka 1000, Bangladesh
Abstract. The query mires the geoenviroiunental status
of the Bengal delta,
especially the physico-chemical
status of the
Holocene sediments have
developed potelltial groundwater aquifer system in the
Holocene sedimentation. Fluvio-deltaic
Bengal delta. Groundwater contamination
by arsenic bas emerged as one of the most serious health hazard. The
occurrence
Ti, Mo, Ni, Zn etc are restricted within the
and distribution of high arsenic as well as other trace metals viz., Co, Cu, Cr,
and occur in thé shallow aquifer ( —of70m)
transgressive system
morphotectonic depressions
tract. Moreover, arsenic hot
related with the
Holocene land-can
interface lobes. Thèse interface lobes are also susceptible
spots are found to be
to
other trace metals enrichment and subsequent sediment-water contamination. The sedimentary
structures and types of
deposition bear the signatures of transgressive phase those are intrinsically associated with tlie
trace metals enrichment.
The Bengal
delta, the mighty
largest load of the suspended matters those
gateway to the sea in the world carries the
adsorbed and/or redeposited in thé land — ocean
interface tlirough flocculation
and accumulation
processes. The adsorbed
and manganèse desolution.
elements are released from the sediments in reducing environment through iron
1. INTRODUCTION
Bangladesh
entirely
multiple
cover
(shallow,
typically
30 to 80m
aquifers
within the
highstand
high
of arsenic
thick
(upto
[2]. Highly
The
those
mixed
fine
to
thrown
insight
to the
and
source
Holocene
mostly
occur
sediments,
with the
deposits
above
are
nature,
causes
those
horizons,
and distribution
extent
associated
are
the Holocene
clay
peaty
coarse
2-] 8%, [3]), characterize the Holocene deposits. The
have
is blessed
country
deposits.
fluviodeltaic sediments [1]. Only the shallow
Pleistocene
with
definite
some
conditions.
conditions
paper deals with the geoenvironmental
metals
in the sediments
and in groundwater
of the
during
processes
entire
the Quaternary
arsenically contaminated
are
deposits
The
trace
2.
delta system. The
of
overlie
deposits
surface
contamination
geoenvironmental
aquifers
and unconformably
flooding
carbons
of prograding
&
deep)
Holocene
marine
organic
sediments
by
intermediate
the
delta
building
those
Holocene
are responsible
transgressive
for
unusual
concentration
and its
tract
of the
physico-chemical
activities.
BASIN SETTING
Bengal
rivers
delta,
one
[1]. Himalayas
of the largest depositional systems, formed
in
of sediments
sources
delta into
terminais
several
the noith
had
period
in
range
blocks
and the localized morphotectonic
subbains
been
the
with
domains
basin
otherhand, Sylhet tectonic
period
by
Indo-Burma
distinct
hydrodynamic
of the different morphotectonic block
sub-basins
Tertiary
and
delta
just after the filled
building
of the
those
major
in the northeast
marine
the
the Ganges, Brahmaputra-Jamuna, Meghna
setting
been
Madhupur
river
transgression
in the region
process
was very slow
of
the
Indo-Gangetic
depression
straddle
up
and
Barind
faults
and
as an
tract
inexhaustible
subdivide the
lineaments
courses. Nonetheless, these
in the
subsiding
acting
and
(figure 1). The
controlled the
is a rapidly
have
east
viz.,
Late
Quaternary
period
are the
subsided
[4].
On
the
in
the
blockin the Bengaa! basin [1]. Unti,'the
is activated
with
during the late-Quaternary
the
rise
of
sea
level
back in 18000 years and has been
This delta started to evolve rapidly in the Late Quaternary
Quaternary.
controlled by immense sediment discharge, tectonics and eustasy [1].
drains
of
(GBM)
river system combinedly
spectrum
The Ganges-Brahmaputra-Meghna
a wide
metamorphic,
magmatic and sedimentary rocks aged from Precambrian to Quaternary [5] and carries the largest
Tectonic subsidence superimposed onto the sediment
compaction and
matter in the world [6].
straddles a perennial depression to
accommodate
huge amount of sediments. During the
adjustment
Ganges-Brahmaputra-Meghna
time just after the filled
late-Quaternary
up of the Indo-Gangetic depression,
river system carries the largest sediment loads and suspended matters to the deltaic area. These sediments
silt and finer fragments of shale masses.
mainly consist of sand,
load ofsuspended
isostatic
Depressîon
Gans
Jamuna
g,aena)
epress
13mînd
a
tec.to
PU
ep-1ski
Vt-
3
2
2189
91
92
Figure 1 : Map showillg tlie generalized basin setting. Dots are arsenic contaminated wells (data source: [7]), thick black closures are
tlie collceptual positions of estuaries representing land-seawaterinterface lobes during tlie Holocene. Modified from [2].
3. DATA
ANALYSIS
AND
RESULT
3. 1 Sediment Chemistry
(NW Bangladesh)
Nawabganj
Eleven sediment
core samples from
were analyzed by XRF for bulk sediment
variations on average concentration
in the samples of
chemistry. Most of the trace metais show no significant
relationships
ofthe trace metals'Pearson's correlation
matrix were
different depths. To see the'inter-elemental
examined. A majority
of the trace metals (As,
Si, Ti, Zn, Rb, Ni, Co, Mo) shows a good correlation (r= 0.
45influence
of
Mn. This implies that the Fe-Mn minerais have
0. 80) with Fe &
a significant
on the accumulation
Silicon (Si) has a very good correlation (r=0. 60 —
90)0. with Fe, Mn, As, Ti, Zn, Rb, Ni,
trace metals. Moreover,
minerais.
Co, Mo suggesting that trace metals of Holocene sediments
are mainly associated with the Fe-Mn-Si
of the heavy metais
shows good to excellent positive correlation with Al,
Fe, Mn suggesting the
A majority
positive role of clay minerals
and Fe-Mn oxides/hydroxides in accumulating the
trace metals. The concentration
of trace metais in
aquifer
Holocene depressions
sediments from
contain
two major geomorphic units (Table i)
suggest that sedinaellts in the
and elevated regions.
more trace metals than that ofthe terrace
Table 1: Average O. xalate-extsactable constituents m sediments Fom varioussandy horizons across Bangladesh. Data from [7].
! Geomorphic
Unit
As mg/k/ %Fe AI mglcg
Area
! Morpho-Chaînai 2. 53
tectonic Laksluiiipur
2. 13
Depression West Latifpur 1. 7 0. 349
0. 516
0.461
Region
Purba
Fargilpur 0. 06 0. 0040.25
Terrace
Khitta
1329 50.6019
West Bilat Haripur 0.
3. 2 Groundwater
17
Mg mgig S04 mgakg
783
571
1299
850
501
549
30
72 7 14 0. 46 2 0.
61
P mg/kg Co mg/kg Zn mglkg Cu mglkg
59
197
2.4
1.72
4.75
4.01
151
4.28 4. 59 5. 67
6.34
4.27
31
0. 86
1.87
55 17 14 0. 5 1. 1 (Un
75
0. 047
214
109
0.28
chemistry
of the trace metals
tubewell water samples were analyzed by XRF. In most of the cases concentration
(As, Mn, Ni, Cu, Zn, Ti, Pb, Br, Rb, Mo, Cr, Sr) are very low to below detection limit in the deep tubewell but
standerd (O. Olppm). It
is observed that the
high in the shallow tubewell, although only
As exceed the WHO
picks
concentration
picks of the As, Ni, Cu, Zn, Cr, Ti, Br, Mo has a good coincidence with the concentration
Thirty
of Fe+Mn (figure-2).
54) with As and positive correlation (r=007-023)
It is seen that Fe has a good positive corrélation (r=0.
Cr, Sr but negative with Zn and Se. A better correlation is found of these
with Ni, Cu, Ti, Pb, Br, Rb, Mo,
metals with total concentration
of Fe &
Mn. This relation infers that these trace metals might be released from
Fe &
Mn oxihydroxides,
as it is claimed
[8] and amphoteric nature of Fe [9] facilitates
for the As by many workers [7]. Fe oxides have strong affinity for As
it to adsorb trace metals in the wide pH range and precipitates as
amorphous FeOOH in the neutral environment.
Ni has a very good correlation with Se, Ti, Pb, Br, Cr, Sr but
been released from other sources in addition to Fe &
Fe, implies that these metals have
Mn.
very low with As &
Arsenic (As) has a positive correlation with all the trace metals mentioned
Pb (r=0. 38), Sr (r=0. 47), Rb (r=0. 27). This implies
Mo (r=0. 77), Cr &
above and very good corrélation with
that Holocene transgressive
seawater
role in the enrichment of trace metals.
might have a significant
co-efficient
(r2=0. 16-0. 24) with the
National survey data [7] shows that Fe+Mn have higher correlation
(r2=0. 0004-0. 08) with As in the regions ofkhulna,
Sylhet and
total trace metals and low correlation co-efficient
Laxmipur-Faridpur.
shallow
aquifer
Data
[7] also
shows that alkali
of the morpho-tectonic
metals concentration.
This implies
earth
elements
(Mg,
Na,
Ca) concentration
depressions
and correspond to the regional
marine transgression
the simultaneousness of the
is high in the
extent of the higher trace
and trace metals adsorption
and/or redeposition.
4. DISCUSSION
4. 1 Sources of metals
in the sediments
of Holocene transgression
occurred along two main flow domains of the river system.
Land-ocean interaction
As a result, number of estuarine lobes have been developed (figure 1) within the transgressive domains [2]. The
lobes are the
sedimentation within these lobes bear the signature of tide dominated sedimentary features. These
interaction
regions of complex
between fluvial and marine processes that might have acted as geochemical
traps
grained sediments. The
mixing of continental
for the trace metals from the solution and bonded in the finer
marine saline water usually leads to floculation
and accumulation
fresh and
processes of the suspended
in the form of
sediments [10 ; 11]. In
such a slight saline environment,
iron and manganese co-precipitated
for aggregation.
amorphous FeOOH/MnOOH
coatings around the sediment particles
those act as the nuclei
This also prevent trace element desorption
by adsorption from solution. In the lower
site during the deposition
the adsorption
from
ion exchange
estuarine
of
the
site in addition
environment,
coated
adsorbed
particles
due
to acquiring fresh trace elements
trace metals can be desorbed from
reducing
to the depth dependent
environment.
4. 2 Solubility
Soiubility
and mobilization
of Fe-Mn minerals
of metals in groundwater
is directly proportiana ! ta the crystallinity. The amorphous Iran
goethite tao hématite with time [9]. Sediments
recrystalizes through lepidocrocites and
with FeOOH/MnOOH
coatings
contaminated
in the
aquifers were deposited
very
to the deposition of high organic
phase that would have lead
reciystallize FeOOH MnOOH.
rapidly
matter
during the Holocene
and finer sediments
transgressive-regressive
with
time
very short
to
Sample no
10
0.
1
2
0.01
0
0. oui
Fe+MnAsNie Cu Zn
|) (Cr O Ti O Br tW ~ Total Metals
Figure 2 : Coirelation
The
aquifer
of concentration picks of different
conducive
becomes
trace elements in groundwater
to the reducing
condition
from Nawabgang, NW Bangladesh-
due to the decomposition
of organic matter,
and overlying
finer sediments. The reducing
condition,
in tum, is responsible
for Fe-Mn
depth of the aquifer
the following equation
for
arsenic
oxides/hydroxide
dissolution.
This
by
can be visualized
as
" I/Fe2
HnAsn+ +30R. Hence the
MnOOHlFeOOH (HnAsn-)
+ H2O Ç : : > Mn2+IFe2++
trace metals can also be released
and mobilized
in the same
way
as arsenic
in the aquifers.
5. CONCLUSION
of
The morphotectonic depressions, geological signatures
sediment — groundwater geochemistry ofthe Quaternary
building
status
process and its physico-chemica]
transgressive-regressive system tracts and the
period lend support
of the intertidal-supratidal
to conclude
deposits
that
the Holocene delta
are responsible
The subsequent dissolution
of the aquifers
by arsenic.
of trace metals and the contamination
lobes
created within the estuarine
trace metals is due to the reducing environment
enrichment
of the
marine
for the
of the
transgression.
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