NATURAL ARSENIC IN GROUNDWATER: OCCURRENCE, REMEDIATION AND MANAGEMENT - CHAPTER 4 doc

Mineralogical characteristics of the Meghna floodplain sediments and
arsenic enrichment in groundwater
A.M. Sikder & M.H. Khan
Arsenic Research Group [BD], Banani, Dhaka, Bangladesh
M.A. Hasan & K.M. Ahmed
Department of Geology, University of Dhaka, Dhaka, Bangladesh
ABSTRACT: The study attempts to investigate the mineralogical composition of the fluvial
deposits forming aquifers in order to understand the behavior of the solid phases in relation to
arsenic enrichment in groundwater in the ‘Meghna floodplain’of the Bengal Basin. The major elem-
ents in the sediment samples along with its total arsenic content were determined by XRF spec-
trophotometer. Leachable arsenic in the sediments samples from the zone of oxidation (3–4 m
depth) under atmospheric condition at pH 5.5 is insignificant, though the samples contain rela-
tively high amount of total arsenic. Jarosite (KFe
3
[SO
4
]
2
[OH]
6
) is identified in a number of sam-
ples and alunite (KAl
3
[SO
4
]
2
[OH]
6
) is also observed in the lower part of the sequence. Carbonate
minerals are found to exist throughout the entire sequence in the form of siderite (FeCO

3
),
dolomite [CaMg(CO
3
)
2
] and rhodocrosite (MnCO
3
). The present mineralogical and chemical
analyses of sub-surface sediment samples of the studied area reveals that oxidation due to the fluc-
tuation of the groundwater level do not contribute significant amount to the arsenic release to
groundwater. The strong correlation between TOC content and total arsenic indicates that organic
matters might have played an important role in the mobilization of arsenic to the groundwater of
the Meghna floodplain of the Bengal Basin.
1 INTRODUCTION
Arsenic enrichment in groundwater at shallow depths in the Ganges-Brahmaputra-Meghna
(GBM) delta have been considered as an environmental catastrophe of the current time (Dhar
et al. 1997, Bhattacharya et al. 1997, BGS & DPHE 2001, Bhattacharya et al. 2002, Chakraborty
et al. 2002, Ahmed et al. 2004). Arsenic occurrences in GBM delta have been reported as a geogenic
event whereas the release mechanism into groundwater has been described as reductive dissol-
ution or desportion of arsenic bound to iron oxyhydroxide (Bhattacharya et al. 1997, Nickson et al.
2000, McArthur et al. 2001, Anawar et al. 2002, Smedely & Kinniburgh 2002, Tareq et al. 2003,
Akai et al. 2004, Zheng et al. 2004). However, there are some gaps in the precise understanding
regarding the state of arsenic in the sediments and the processes that trigger its release and mobil-
ization into groundwater (Mallik & Rajagopal 1996, Acharyya et al. 1999, Yamazaki et al. 2000,
Dowling et al. 2002, Harvey et al. 2003).
The recent sediments of the basin exhibit a thick succession of fluviatile sediments deposited
by the GBM river systems and characterized by complete or truncated cycles of fining upward
sequences dominated by coarse to fine sand, silt and clay (Bhattacharya et al. 1997). In general,
the GBM sediments consist of about 50–65% quartz, 7–15% feldspar, 7–20% lithic fragments,

5–15% mica with insignificant presence of opaque minerals. The persistence of sulfide group of
minerals is quite unusual in the sediments of warm and humid climate, although authigenic pyrite
along with corroded iron oxyhydroxides noticed in the fine clastic sediments (Ahmed et al. 2001).
31
Natural Arsenic in Groundwater: Occurrence, Remediation and Management –
Bundschuh, Bhattacharya and Chandrasekharam (eds)
© 2005, Taylor & Francis Group, London, ISBN 04 1536 700 X
Copyright © 2005 Taylor & Francis Group plc, London, UK
Though some mineralogical analyses of the recent sediments of GBM delta have carried out by
different investigators, but attempts have not been made to characterize the opaque phases of min-
erals, in particular.
The present study aims to look into the mineralogy of the sediments including characterization
of the opaque phase of the minerals and its relation to release mechanism under different geo-
logical and geochemical environments.
2 METHOD OF STUDY
An exploratory boring with a modified ‘split spoon’ system was conducted in the ‘Meghna flood-
plain’ to collect continuous and intact undisturbed sediments samples from the whole sedimentary
sequences from 3 to 40 meter covering the modal depths for yielding water through hand tubewells
of the present study area (Fig. 1). The mineralogical investigations coupled with chemical analy-
sis of the collected sediment samples were expected to provide useful evidence regarding the state
and release mechanism of arsenic.
The major elements and the content of total arsenic (As) of the bulk sediment samples have
been determined by X-ray fluorescence (XRF) spectrophotometer. A small number of samples
with high arsenic content were also analyzed with Instrumental Neutron Activation Analysis
(INAA) to validate the results of the XRF analysis.
To determine the amount of leachable arsenic from the sediments in the atmospheric condition,
1 gm of sediment sample was dispersed in 100 ml de-ionized water in a beaker and stirred with a
magnetic stirrer for 3 hours. The pH of the mixture was measured and maintained to 5.5 by sodium
hydroxide as per requirement. Then the homogeneous mixture was filtered through a 45-micron
filter using a vacuum pump and the arsenic content was determined with Hydride Generation Atomic

32
Figure 1. Map of the study area showing geomorphology and location of the drilled sampling hole.
Copyright © 2005 Taylor & Francis Group plc, London, UK
Absorption Spectrophotometer (HG-AAS). The samples from shale intervals were selected to
determine the total organic carbon (TOC) content with TOC analyzer using combustion method.
Standard procedures were followed for mineralogical analysis of the clay (Ͼ2␮m) fraction with
XRD. Special emphasis was given in the preparation of the heavy mineral fractions for XRD
analysis to avoid the interference of the major rock forming silicates and the clay minerals. The
samples were thoroughly washed and decanted until the clay particles were completely removed.
Then selected washed samples were chosen for heavy mineral fraction separation based on the dif-
ferences in density of the minerals by Heavy Liquids Method (Mullar 1966) for XRD analysis.
Samples were prepared in order to obtain free minor constituents (i.e. oxides, hydroxides and sul-
fides group of minerals) of the sediments for accurate determination of possible minerals con-
taining solid phase arsenic.
3 RESULTS
3.1 Lithofacies analysis
The entire lithologic succession was examined from the cored sediment samples (Fig. 2). The zone
of oxidation was observed to extend up to a depth of about 6m from the surface as evidenced by
the yellowish-brown color of the sediments. The preservation of few inches thick black patches
within the zone of oxidation testifies the presence of high amount of organic matter in the shal-
lower part of the studied sequence. Ferruginous concretions with concentric nature have been
observed in the zone of oxidation with decreasing downward tendency in development. These con-
cretions hint at the presence of the microbial activity (i.e. chelation) in the ‘zone of oxidation’.
The predominant lithology of the studied sequence was alteration of sand and shale. The light
gray sands were fine to medium grained with abundance of mica. The sand beds were character-
ized by laminations, cross-laminations with occasional trough cross-laminations and clay drapes.
A medium to coarse-grained micaceous yellowish brown thickly bedded sand sequence was
encountered between 21 and 30 m. Clay drapes were also observed within this sandy sequence.
33
Figure 2. Depth profiles of total arsenic content, leachable arsenic at pH 5.5, total organic carbon (TOC) in

the studied sedimentary sequence.
Copyright © 2005 Taylor & Francis Group plc, London, UK
Thick layers of dark gray shale with sand lenticles were observed in the studied sequence at num-
ber of depths.
A 5 cm thick peat layer was encountered at a depth of 38m. The sediments were also character-
ized by the occurrence of decomposed and partially decomposed plant debris. Organic matter was
also found to concentrate within the dark the laminations.
The studied section exhibited cycles of fining upward sequences (Fig. 2) which can be inter-
preted as floodplain deposit with strong tidal influence based on the presence of clay drapes and
sand lenticels through out the sequence. The coarser sandy horizons in the middle and lower part
of the sequence can also be interpreted as the product of high-energy deposition based on the
textural and sedimentological characteristics (i.e. flood surge).
3.2 TOC of the sediments
TOC content of sediments were considered to play a significant role in the arsenic enrichment
processes. Besides the occurrence of patches of organic matter in the zone of oxidation and the
peat layer in the lower part of the sequence, the studied sediments contain relatively high amount
of TOC in the range of 0.3 to 1.01%. The maximum concentrations of TOC were found at the peat
layer where it reaches to 45.7% (Table 1). Concentrations of organic matter in different layer were
even visible with naked eye.
3.3 Major elements
Oxides of the major elements of the selected samples were determined to find out the possible link
of arsenic and other elements in the sedimentary sequence (Table 1). Determination of the total
34
Table 1. Distribution of the major elements, TOC and As in the sediments.
Depth SiO
2
As* TOC
(m) (%) Al
2
O

3
Fe
2
O
3
TiO
2
MgO CaO Na
2
OK
2
O MnO P
2
O
5
(mg/L) (%)
4.3 62.58 16.01 6.53 0.73 2.28 1.17 1.52 3.13 0.10 0.12 46 0.34
6.1 60.81 16.09 6.79 0.79 2.70 1.44 1.45 3.19 0.12 0.10 50 0.74
8.3 66.15 14.40 6.61 0.71 2.47 1.71 1.69 2.96 0.10 0.10 22 0.81
10.1 73.24 10.92 3.91 0.52 1.50 2.28 1.93 2.22 0.07 0.12 11 0.38
10.1 61.93 16.16 7.01 0.80 2.73 1.84 1.59 3.06 0.11 0.11 51 0.65
13.5 65.33 11.37 7.76 1.26 2.71 5.76 1.60 1.44 0.18 0.39 7 0.33
17.8 69.42 12.37 5.30 0.56 2.00 1.92 1.88 2.65 0.08 0.12 18 0.46
17.8 48.74 14.86 14.85 1.13 5.36 1.14 1.05 3.26 0.22 0.15 80 1.49
20.5 70.03 11.95 4.88 0.67 1.74 2.43 1.90 2.39 0.09 0.14 15 0.44
21.5 62.15 15.96 6.96 0.81 2.65 1.73 1.56 3.00 0.12 0.12 55 0.58
21.5 68.15 12.77 5.25 0.61 2.02 2.02 1.97 2.69 0.08 0.12 21 0.49
26.7 60.61 14.95 7.17 0.83 2.87 1.97 1.66 3.00 0.12 0.12 39 0.72
26.7 73.11 11.23 3.91 0.52 1.38 2.06 1.98 2.41 0.06 0.10 11 0.39
27.6 66.73 12.43 6.60 0.74 2.40 1.24 1.44 1.99 0.13 0.11 69 0.57

27.6 72.15 11.46 5.00 0.75 1.64 2.46 1.81 2.19 0.09 0.18 14 0.37
27.9 60.66 16.81 7.18 0.79 2.72 1.28 1.32 3.05 0.12 0.11 70 0.60
27.9 56.92 18.31 7.73 0.68 2.83 1.17 1.31 3.26 0.14 0.12 61 0.69
33.1 55.68 13.42 7.31 0.82 2.53 0.69 1.12 3.26 0.10 0.10 109 1.01
35.3 56.18 18.39 7.83 0.81 2.92 0.99 1.20 3.38 0.11 0.10 83 0.70
37.1 69.75 11.63 5.11 0.66 1.69 2.22 1.86 2.40 0.09 0.14 13 n.a**
37.1 58.71 16.81 7.17 0.80 2.84 1.32 1.42 3.27 0.12 0.10 59 0.73
38.0 15.05 0.54 0.49 0.05 0.17 0.22 0.21 0.12 Ͻ0.01 0.03 175 45.7
39.3 66.50 10.34 5.35 0.68 1.94 1.36 1.63 2.64 0.07 0.10 37 0.48
* Concentration of leachable arsenic from the sediments; ** Not analyzed.
Copyright © 2005 Taylor & Francis Group plc, London, UK
arsenic content of the sediments and the understanding of its release mechanism were the main
focus of the study. The data were plotted against depth (Fig. 3) to see whether there is any correl-
ation of distribution between the major elements and the arsenic content in the studied sedimen-
tary sequence.
3.4 Determination of leachable arsenic
The determination of the amount of leachable arsenic from sediments of different depths is very
important to understand the release mechanism from sediments to water. Sediments from differ-
ent depths were selected to determine the amount of arsenic released at a controlled condition of
pH 5.5 under the atmospheric condition. In selection of the samples emphasis was given to ‘zone
of oxidation’ (up to a depth of about 6m) and the zone where most of the hand tube wells in the
area were screened (i.e. most abstracted shallow aquifer). The results of leaching tests are pres-
ented in Table 2. The depth profiles of leachable arsenic along with lithology, total arsenic and
total organic carbon are shown in Figure 2.
3.5 Mineralogical analyses
The present study was primarily designed to find out the mineralogical aspects of the sediments in
order to understand the behavior of the solid phase minerals in relation to the arsenic enrichment
in the groundwater at shallow depths.
3.5.1 Normative calculation of mineralogical composition
Normative calculations were carried out to determine the modal mineralogical composition of the

sediment samples using a computer program ‘SEDNORM’ (Cohen & Ward 1991). The calculated
composition of minerals is plotted against depth to develop a theoretical basis to formulate the
strategy of further analysis.
3.5.2 Clay mineralogy
The relative abundance of the clay minerals in the clay samples were determined based on the rela-
tive intensity of the strongest peak of the constituent minerals in XRD traces (see discussions below).
35
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
-45
-40
-35

-30
-25
-20
-15
-10
-5
0
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
050100 150 200
Total As (mg/kg)
Depth (meter)
051015 20 25 30
Fe
2
O
3
%
0.00 1.00 2.00 3.00 4.00
MnO
2
%

0.00 5.00 10.00
CaO%
Figure 3. Depth profile of elements showing the relationship with total arsenic content of sediments.
Copyright © 2005 Taylor & Francis Group plc, London, UK
3.5.3 XRD of heavy mineral fraction
The environmental sensitive minerals were identified by removing the major constituents of the
sediments (i.e. clay and silicates group of minerals) in order to avoid their interference in the
analysis. The main concern of the analyses was to find the forms in which metals and metalloids
move through the hydrosphere. Thus attention was given in the identification of minerals that do
not show up very often in routine mineralogical analysis. The significant minerals identified from
the XRD traces are listed in the Table 3.
4 DISCUSSION AND CONCLUSIONS
The plots of relevant oxides (Fig. 3) revealed that among the major elements, arsenic showed good
correlation only with iron oxide and manganese oxide (Fe
2
O
3
and MnO). The total arsenic content
36
Table 2. Results of leaching test of the sediments samples.
Depth Arsenic in sediments Arsenic in leachate
(m) (mg/kg) (mg/L)
3.1 60* Leaching does not occur
3.4 84* Leaching does not occur
4.0 19.8* Leaching does not occur
6.4 49.7* 0.071
8.3 2.48 0.252
10.4 62.10* 0.103
16.3 2.35 0.112
17.2 0.72 0.14

23.0 2.02 0.124
23.9 2.78 0.069
24.5 3.50 0.099
24.8 3.54 0.216
25.1 3.64 Leaching does not occur
25.8 0.74 Leaching does not occur
28.2 63.1* Leaching does not occur
33.7 109 0.085
35.9 83 0.12
38.0 175 Leaching does not occur
39.2 37 Leaching does not occur
*I
NAA
.
Table 3. Results of XRD analysis of heavy mineral fractions (specific gravity Ͼ 2.9).
Sample no. Depth (m) Weight (%) Significant minerals
1 4.6 2.11 Jarosite
2 5.2 1.93 Ankerite, Magnetite, Dolomite
3 9.2 4.26 Pyrolusite, Magnetite, Amphiboles, Kaolinite
4 12.9 3.15 Pyrolusite, Magnetite, Amphiboles, Siderite
5 15.0 5.68 Pyrolusite, Amphiboles
6 19.0 5.45 Magnetite, Amphiboles, Kaolinite
7 20.8 3.27 Magnetite, Amphiboles, Kaolinite
8 23.0 4.27 Jarosite, Pyrolusite, Amphiboles, Dolomite
9 24.2 3.76 Pyrolusite. Magnetite, Amphiboles
10 25.1 4.99 Alunite, Pyrolusite, Amphiboles
11 32.8 3.32 Jarosite, Amphiboles, Kaolinite
12 35.9 1.68 Pyrolusite, Amphiboles
Copyright © 2005 Taylor & Francis Group plc, London, UK
was moderately high in the samples of shallower depth including those from the ‘zone of oxida-

tion’. Highest arsenic content was observed (109mg/kg) in the sample from 33 m corresponding
with a shale sequence.
Potassium oxide (K
2
O) and magnesium oxide (MgO) also showed well conformity with the
total arsenic content of the sediments (Table 1). These conformities were strong in the deeper parts
than the shallower part of the sequence. This was probably due to the weathering effects in the
shallower depth. But the sequences in the deeper parts, not affected by the weathering process,
showed a good correlation among potassium (K), magnesium (Mg) and arsenic (As) and that was
probably due to the distribution of those elements from the source of sediments. The most com-
mon K bearing minerals in sediments is muscovite (K
2
Al
4
[Si
6
Al
2
O
20
] (OH)
4
]) whereas the source
of Mg is generally the ferromagnesian minerals (i.e. amphiboles and biotite
) and to a lesser extent
magnesite (MgCO
3
).
Manganese oxide (MnO), titanium oxide (TiO
2

), sodium oxide (Na
2
O) and calcium oxide
(CaO) do not show any correlation in the distribution with Arsenic content of the sequence (Table
1 and Fig. 3).
The samples covering the ‘zone of oxidation’ ranging in depth from 3 to 4 m did not leach any
arsenic in the solution at pH 5.5, although XRF analysis of bulk samples showed the presence rela-
tively high amount of total arsenic in the same samples. Significant amount of leaching of arsenic
occurred from the coarser sequences ranging in depth from 8 to 17 m that corresponded with shal-
low aquifer of the study area. The modal and mean depth of depletion occurred at a depth of 25 m.
The shale samples of 25 to 28 and 38 to 39 m depths also did not leach any arsenic in the labora-
tory experiments at pH 5.5 although the total arsenic contents of the corresponding samples were
also very high.
The clay minerals (2–4 ␮m in size) are considered as the most sensitive indicator of the geo-
chemical environment and weathering processes. Clay minerals can also bind elements in their
interlayer and the binding capacity of clays directly depends on the development and presence of
certain type of clay minerals (e.g. halloysite and mixed-layer clays) (Paquet & Clauer 1997). The
formation of the clay minerals in the shallower depth is scanty, which corresponds with high activ-
ity zone of weathering. But the studied sediment samples are found rich in mica. The presence of
mica in the clay fraction is also strikingly very high.
The increase of the kaolinite content with depth indicates the increase of acidity and decrease
of dissolved silica in the pore fluid. Absence of smectite throughout the sequence and weak pres-
ence of non-swelling illite-smectite as revealed from the glycol solvated XRD traces of the sam-
ples indicate weak acidic and low silica groundwater conditions.
The presence of kaolinite throughout the sequence with few occurrences of illite-smectite
mixed layer clays hints that the studied sequence suffered less pervasive hydrolysis. Kaolinite is
usually observed in depths where acidity is high and dissolved silica is low. So the increase in the
amount of kaolinite with depth probably is due to the increase of acidity of the pore fluids with
depth (Fig. 4).
Jarosite was identified in more than one sample. Jarosite (KFe

3
[SO
4
]
2
[OH]
6
) is a rather widely
distributed mineral in oxidized zone and frequently occurs as a weathering product of pyrite (FeS
2
)
in certain sediments. Jarosite is not very stable in water and yields iron hydroxides on hydrolysis
and therefore supposed to be a rare mineral in sediments of warm humid climate (Batekhtin 1981).
Alunite (KAl
3
[SO
4
]
2
[OH]
6
) was also observed in the lower part of the studied sequence. Similar
occurrences of Alunite in little deeper depth had also been reported from other areas of the coun-
try (Fig. 5).
Magnetite (Fe
2
O
3
) is observed in most of the samples except from those of the ‘oxidizing zone’
and that implies, magnetite were present in the entire sequence probably right form the source

(Fig. 6). Pyrolusite (MnO
2
) was also identified in most of the samples as the end product of alter-
ation in the zone of low degree of oxidation (Batekhtin 1981, Deer et al. 1967).
The expected mineral association of pyrolusite in sedimentary environment is goethite, hematite,
lepidocrosite, magnetite, calcite and quartz. Goethite may also be present in the sand sequence
but probably not identified in XRD traces due to the low resolution of the tool and sensitivity of
37
Copyright © 2005 Taylor & Francis Group plc, London, UK
chosen radiation. A mineral is possible to identify in XRD if it forms at least 5% of the bulk com-
position. Also the source of radiation (CuK␣) used in the present analysis was not suitable for the
identification of the iron oxide minerals (Batekhtin 1981, Deer et al. 1967).
Carbonate minerals were observed throughout the sequence in the form of ankerite (Ca, Fe, Mg,
Mn (CO
3
)
2
), siderite (FeCO
3
), dolomite (CaMg(CO
3
)
2
) and rhodocrosite (MnCO
3
). The intensity
of occurrence of the carbonates is higher in the shallower depth. The presence of carbonates in the
zone of oxidation could have an intimate relationship with chelation of the metals due to the micro-
bial activity, which also correspond with the observation of the high arsenic content in samples
38

Figure 4. XRD trace of oriented clay fraction (Ͼ2 ␮m) showing the variations of the development of clay
minerals with depth.
Figure 5. XRD traces showing the typical peaks of jarosite and alunite identified in the studied sediments.
Copyright © 2005 Taylor & Francis Group plc, London, UK
from the shallower depth but without any leaching of arsenic. Chelated metals tend to be very
tightly bound thus more insoluble than the other solid phases of metals (Paquet & Clauer 1997).
The depth profile of TOC and the arsenic content exhibits quite well conformity. Especially the
arsenic content of the peat sample (145 mg/kg) is very significant and provides an important clue
for the future investigations.
Almost all the areas of the Meghna floodplain go under water during the monsoon and water
remains over the land for at least for 3 months. Strict demarcation of the aerobic (oxidizing) and
anaerobic (reducing) zoning appeared incompatible at the studied area to some extent, as the oxi-
dizing zone change to reducing zone for a certain period in every year. Although it appears from
the present chemical and mineralogical analysis of sub-surface sediments samples of the study
area that oxidation due to the fluctuation of the groundwater level do not contribute any signifi-
cant amount of arsenic to groundwater.
In consideration of the present geomorphic and groundwater scenario the detection of jarosite and
alunite in the recent sedimentary sequence of GBM is very unusual. But the presence of peat, iron
stained sand and dispersed organic matter throughout the studied sequence supports that the geo-
chemical environment favorable for the formation of jarosite and alunite might have prevailed in the
Meghna Plain due to frequent and strong tidal incursions in a costal and lagoonal setting (Breemen
1975, Breemen & Pons 1978, Kazutak 1992). Weak acidic groundwater condition with less perva-
sive hydrolysis in the unweathered section of the studied sequence as revealed from the clay miner-
alogical analysis support to some extent the presence of Jarosite in the ‘zone of oxidation’. However
the explanation of the presence of Jarosite in the reducing environment in deeper part of the sequence
other than the preservation of the weathering products of geological past due to rapid subsidence
demands further investigations. Lack of advanced analytical facilities limited the study to a large
extent. Detailed sediment logical and pore water investigations are essential to draw definite conclu-
sions about the solid phase arsenic and subsequent mobilization to groundwater.
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Figure 6. XRD trace showing the major peaks of magnetite and pyrolusite in the studied sediments.
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