Groundwater arsenic in Bengal Delta
Plains - Testing of Hypotheses
Prosun Bhattacharya1, Andre Sracek2
and Gunnar Jacks1
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.
1. Definition of problem
The regional problem of arsenic contaminated groundwater from the vast
tract of alluvial aquifers in Bengal Delta Plains is known to have affected
a population of about 38 million in West Bengal and another 40 million
in different districts of Bangladesh (ACIC, 1998). Need of water for domestic
as well as irrigation purposes had triggered rapid development of groundwater
resources during the last two decades. 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. There are serious health problems
related to the consumption of groundwater with extremely high arsenic concentrations
(up to 3700 ug/L) in groundwater alluvial aquifers in West Bengal, India
and in Bangladesh over a prolonged period of time has manifested in adverse
health effects among the rural and semi-urban population population
(Goriar et al., 1984; Chakraborty et al., 1987; Guha
Mazumder et al., 1988; Das et al., 1996, Dhar et al.,
Two principal hypotheses about the origin of arsenic have been put forward
(Bagla and Kaiser, 1996):
- the arsenic derived from the oxidation of As-rich pyrite in the shallow
aquifer as a result of lowering of water table due to overabstraction of
ground water for irrigation (group from Jadavpur University, Calcutta);
- the arsenic derived by desorption from ferric hydroxide minerals present
as coatings in the aquifer sediments under reducing conditions (group from
Royal Institute of Technology in Stockholm).
The first hypothesis assumes a significant quantity of sulfide minerals
in the aquifer sediments. However, the arsenic-rich sediments had origin
in the Rajmahal hills North of the Ganga Delta Plains and preservation
of sulfide minerals during long term river transport is unlikely. This
means that if there are sulfide minerals present in the Ganga sediments,
then they are of authigenic origin and were formed in fine grained deltaic
sediments under reducing conditions.
2. Behaviour and possible sinks for
Recent investigation in the Chakdaha block of Nadia district (Larsson
and Liess, 1997) indicated very low concentrations of sulfate (in the range
1-6 mg/L). The oxidation of As-rich pyrite by the oxygen dissolved in water
can be described as
FeS2-As(s) + 7/2O2 + H2O = Fe2+
+ 2SO42- + 2H+ + As(aq)
Thus, dissolution of 1 mol of pyrite produces 2 moles of sulfate. The
amount of arsenic produced would depend on concentration of arsenic in
This means that very low sulfate concentrations are contradictory to
the hypothesis about the oxidation of pyrite. There
are several potential sinks for sulfate. First of them is the precipitation
of gypsum, CaSO4.2H2O, which can be described as
Ca2+ + 2SO42- + 2H2O = CaSO4.2H2O
However, gypsum is very soluble mineral and its precipitation requires
high concentrations of Ca2+ and SO42-.
Concentrations of Ca2+ in water samples from Chakdaha were in
the range from 60 to 120 mg/L (Jana, 1998, personal communication). Much
higher concentrations of Ca2+ would be required to bring about
the precipitation of sulfate and thus, this process can be excluded as
a possible sink for sulfate.
Other possible sinks for sulfate are iron minerals like Fe2+
mineral melanterite, FeSO4.7H2O, and Fe3+
mineral jarosite, KFe3(SO4)2(OH)6
. Both minerals are quite exotic and are generally associated with acid
mine drainage (Alpers et al., 1994). Furthermore, jarosite is stable
only under low pH conditions (pH<3.0). Precipitation of both minerals
require extremely high concentrations of iron and sulfate. This means that
also this possibility is extremely unlikely.
Sulfate reduction thus remains as the principal possibility of sulfate
removal. The process can be expressed as
2CH2O + SO42- = H2S + 2HCO3-
There is a formation of HS- instead of H2S under
higher pH conditions (pH>7.0). The reaction requires the presence of
organic matter and sulfate reduction bacteria. Sulfate reduction takes
place in very reduced environment, where O2, NO3-,
Mn4+ and Fe3+ have already been reduced. The Eh values
at which sulfate reduction begins are about -180 mV at neutral pH region
(Stumm and Morgan, 1981). Limited Eh data from Chakdaha presented by Larsson
and Liess, (1997), are in the range from +23 to -175 mV at neutral pH.
There was no trend of the Eh values with depth. These results are higher
than the values typical for the reduction of sulfate, but the situation
is complicated by the fact that measurements was done on samples from pumped
domestic wells with long screens. Thus, several distinct redox zones may
have been mixed and the Eh values are not representative.
Other criteria for sulfate reduction are:
- decrease of sulfate concentration along flowpath combined with decrease
of dissolved organic matter (DOC).
2) enrichment of residual sulfate in 34S and negative correlation
between 34S and sulfate concentration;
- increase of alkalinity combined with depletion of dissolved inorganic
matter (DIC) in 13C (original 13C is diluted by lighter
carbon coming from organic matter with 13C values from -25 to
-30 per mil.); however, the behaviour can be masked by enrichment of DIC
in 13C during methanogenesis, with produces residual DIC very
enriched in 13C.
None of the criterions presented above has however been checked so far.
3. Mineralogical considerations
One of consequences of pyrite oxidation under neutral pH conditions
is the formation of ferric hydroxide rims on pyrite grains (Nicholson et
al., 1990). The reaction including the oxidation of Fe2+
can be expressed as
Fe2+ + 1/4O2 + 2.5 H2O = Fe(OH)3
These rims represent secondary Fe(OH)3 formed after diagenesis
of sediments and they are located on the surface of pyrite. On the other
hand, the Fe(OH)3 formed during diagenesis form coatings on
grains of sand and are independent from pyrite. The mineralogical relation
between pyrite and ferric hydroxide can not be determined by sequential
dissolution because the method is not able to distinguish between primary
and secondary ferric hydroxide. It can be done by mineralogical investigation
(Jambor, 1994), under scanning electron microscope (SEM). The SEM can be
also used to determine if pyrite oxidation takes place, (Blowes and Jambor,
1990). In the positive case there should be dissolution pits on the surface
of pyrite. Thus, well-preserved samples of solid material from the aquifer
obtained by coring are necessary.
· Investigation of sulfate behaviour
plays a significant role in testing of hypothesis about the origin of arsenic
in ground water in West Bengal and Bangladesh. Concentrations of sulfate
at the Chakdaha site are very low (less than 10 mg/L) and there does not
seem to be any mineral phase controlling sulfate concentration in ground
· Redox conditions do not seem to
be reducing enough for reduction of sulfate, but redox data are still limited.
Another complication is related to the sampling of pumped domestic wells
with large screen intervals with resulting mixing of ground water from
several redox horizons. Thus, the possibility of sulfate reduction and
redox conditions using several redox indicators and 34S and
13C isotopes requires more investigation. However, this can
not be done by sampling of pumped domestic wells. Instead, several piezometric
nests with piezometers screened at different depth are necessary. The investigation
of unsaturated zone by suction lysimeters can also be very helpful for
investigation of sulfate and arsenic behaviour.
· Mineralogical investigation under
scanning electron microscope (SEM) focused on the mineralogical relation
between pyrite and ferric hydroxide can distinguish between primary ferric
hydroxide formed during diagenesis and secondary ferric hydroxide formed
during the oxidation of pyrite. If pyrite dissolution takes place, then
there should be signs of dissolution on pyrite surface. The investigation
can be combined with sequential dissolution of the aquifer sediments, which
is not able to determine the origin of ferric hydroxide separately.
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