Genesis of arseniferous groundwater
in the alluvial aquifers of Bengal Delta Plains and strategies for low-cost
Prosun Bhattacharya1, Maria Larsson1,
Andrea Leiss1, Gunnar Jacks1, Andre Sracek2
and Debashis Chatterjee3
1Division of Land and Water Resources, Royal
Institute of Technology, S-100 44 STOCKHOLM, Sweden
2Department of Geology, University of Laval,
Ste Foy, Quebec G1K 7P4, Canada.
3Department of Chemistry, University of
Kalyani, Kalyani-741 235, West Bengal, India.
The occurrence, origin and mobility of arsenic in natural
waters has received significant attention in recent years. Mobilization
of arsenic in groundwater is governed by the geochemical processes involving
leaching of continental rocks as well as sediments. Anthropogenic inputs
particularly due to the application and use of arsenical wood preservatives
(Bhattacharya et al. 1996, 1998) as well as pesticides could also lead
to significant emission of arsenic in groundwater, especially under anoxic
conditions. The risk for arsenic contamination in groundwater is therefore
higher than in surface waters. Arsenic contaminated groundwater as the
primary source of drinking water in several areas has particularly accentuated
the problem because of the effects of arsenic exposure on human health.
Presence of arsenic in concentrations above the maximum
permissible limit (> 50 mg/L) in groundwater
from the alluvial aquifers has been reported from several parts of the
world such as USA, Argentina, Chile, New Zealand, Taiwan, India as well
as in Bangladesh (Ahmed et al. 1997; Bhattacharya et al.,
1996; 1997 and references therein). The commonly existing As-species in
groundwater are in the form of As(V) as arsenate and As(III) as
arsenite, the later being more mobile and toxic for living organisms.
Methylation of inorganic arsenic to monomethyl- and dimethylarsenic acids
is coupled to the biological activity in water.
The problem of arsenic contamination in groundwater in
the vast tract of alluvial aquifers in Bengal Delta Plains is a subject
of global concern and known to have affected a population of about 38 million
in West Bengal and another 40 million in different districts of Bangladesh
(ACIC, 1998). During the last two decades, need of water for domestic as
well as irrigation purposes prompted development of groundwater resources.
Such overdraft of groundwater could be envisaged as one of the key factors
responsible for the spreading of arsenic epidemic in this part of the world.
Consumption of groundwater with high-As content over a prolonged period
of time has manifested in adverse health effects among the population such
as arsenical dermatosis, hyperkeratosis and several other symptoms
of arsenicosis (Goriar et al., 1984; Chakraborty et al.,
1987; Guha Mazumder et al., 1988; Das et al., 1996). The
rural and semi-urban areas are affected to a greater extent because the
groundwater is used to a major extent as a source of drinking water.
The Bengal Delta Plains is characterized by a thick succession
of fluviatile sediments pertaining to Quaternary age. The arseniferous
belts located in the Upper Delta Plains (UDP) are mostly characterized
by complete or truncated cycles of fining upward sequences dominated by
course to medium sand, fine sand, silt and clay sediments. Geochemical
and hydrogeological characteristics of these alluvial sediments influence
the mobility of arsenic in groundwater, but the source of arsenic in these
sediments is dependent on the geology of the source terrain. Interaction
of the aqueous phase with the different mineral phases of the aquifer sediments
play a predominant role in controlling the retention and/or mobility of
As under different redox conditions within the subsurface environment.
Chemical processes as adsorption-desorption, precipitation-dissolution
of unstable As minerals, organic content, biological activity are known
to control the redox conditions within the aquifers (Robertson, 1986; Bhattacharya
et al., 1997). The present contribution is therefore aimed to highlight
the need for an integrated research in order to understand the mechanisms
governing the mobilization of arsenic from the sedimentary aquifers. Characterization
of sedimentary fills of the basin should be an integral part of any research
engagement in order to understand the phenomenon of remobilization of As.
The role of secondary surface-reactive mineral phases such as hydrous ferric
oxides (HFO) and hydrous aluminum oxides (HAO) are especially important
as they act as the potential adsorbents of the inorganic arsenic phases.
The adsorption characteristics of HFO as well as HAO could be easily manipulated
due to changes in the redox conditions of the sedimentary environment thereby
facilitating mobilization of As under the modified redox conditions during
groundwater development. The land use pattern is another factor which may
create physico-chemical conditions favoring the mobilization of As.
The data on As-chemistry of groundwater samples from several
pumped wells in Nadia District indicate arsenic concentrations in the range
of 100-300 mg/L. Predominance of As(V) species
is evident from the consistent ratios of As(III)/As(V) less than 1. Analytical
data on the sediment samples from the bore hole sites in the region reveal
significant variations in the content of total As within the aquifers at
different depths. The sandy aquifers at depths of 27-63 m indicate concentration
in the range of 40-55 mg/kg. The silty-clayey and clayey sediments at intermediate
depths indicate high Astot content (133 mg/kg). The deeper aquifers
at depths 70-122 m reveal Astot contents of 44-61 mg/kg while
the clayey sediments underlying these sandy aquifers are characterized
by lower Astot contents (ca. 20 mg/kg). The concentrations of
Fe, Al and Mn show similar trends. Similar pattern of variation is noted
for the other trace elements like Mo, V and Cr although their concentrations
are lower. Sequential leaching batch experiments for the sediments using
deionized water and 0.01M NaHCO3 have indicated that the amount
of leachable As in these sediments range between 116-383 mg/L,
similar to the As concentrations in groundwater.
Studies on selective extraction of these sediments using
oxalate and pyrophosphate media has been carried out to understand the
relationship of As with the secondary Fe, Al and Mn phases as well as with
the organically bound As. Oxalate extraction of these sediments reveal
that HFO as the predominant fraction (Feox=264-1238 mg/kg) as
compared to HAO as secondary minerals in these sediments (Alox=27-294
mg/kg). Oxalate extractable Mnox (54-325 mg/kg) is however very
low in these sediments. The clayey sediments at depths however indicate
presence of both Feox (983 mg/kg) and Alox (294 mg/kg)
fractions and complimented by high Siox (229 mg/kg) indicate
presence of secondary aluminosilicates. Asox (30 mg/kg) fraction
is also comparatively high in these sediments. The amount of pyrophosphate
extractable Fe, Al, Mn and As are significantly low, suggesting that bulk
of the secondary Fe, Al and Mn phases are inorganic in nature.
The results of our investigations reveal that these surface
reactive secondary Fe and Al phases play an important role in adsorbing
the bulk of As in the sedimentary aquifers in the Bengal Delta Plains.
These Fe- and/or Al-phases are characterized by variable surface charge,
negative at higher pH and positive at lower pH. At lower pH, these surface
reactive phases attain net positive charge leading to significant adsorption
of As(V) species. The occurrence of As in groundwater is a process driven
by the changing redox conditions where the arsenic phases are selectively
desorbed as a response to the reduction of Fe3+ phases to soluble
Fe2+ species. High-As occurrences concomitant with the increased
Fe contents in groundwater supports this hypothesis. It can be conjectured
that part of the As in the groundwater is quantitatively related to the
release of As phases mainly as As(V) form adsorbed on the surface reactive
Fe-oxides and hydroxides. Although the geological sources of As in the
alluvial sediments of the Bengal Delta plains could be proved unequivocally,
more detailed research is needed to characterize the chemistry of the aquifer
materials in order to understand the water-solid-phase reactions operating
in conjunction groundwater development.
Another important aspect of research was to highlight
the need to develop low-cost geochemical techniques for the removal of
As suitable for application in the developing countries. Laterite, a local
raw material ubiquitously found in India has been tested as an adsorbent
for for arsenic using the groundwater samples collected from the village
Ghetugachi in Nadia district through a series of laboratory investigations
(Larsson and Leiss, 1997). The laterite was found to have an efficiency
to adsorb 50-90 percent of arsenic depending mainly on the natural variability
of the laterite and the variations in groundwater chemistry.
Long term groundwater management strategies must be adopted
to prevent mobilization of arsenic to the groundwater by regulating the
land use pattern in the region. Laterite can be used as a small-scale alternative
for the people drinking water from arsenic affected aquifers.
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