Geoscience Reference
In-Depth Information
sulfide, the concentration of dissolved As is decreased. Speciation of As is affected
by its sorption on surrounding minerals. The presence of amorphous iron oxides,
for example, favors strong As adsorption, but this adsorption capacity is controlled
by pH and redox conditions (Smedley
2008
).
The anthropogenically induced pattern of groundwater arsenic contamination in
Bangladesh and West Bengal (India) has been the object of numerous studies (e.g.,
Nickson et al.
2000
; Islam et al.
2004
; Harvey et al.
2002
,
2006
; Polizotto et al.
2006
;
Neumann et al.
2010
). The arsenic originates from sedimentary units within the
aquifers that contain small amounts of sedimentary pyrite, which is known to
associate arsenic (Nickson et al.
2000
). The most accepted explanation for the
arsenic contamination of groundwater beneath the Ganges Plain is that it derives
from reductive dissolution of arsenic-rich Fe(III) oxyhydroxides in the sediment.
The reductive mobilization of arsine is catalyzed by biological activity and presence
of electron donors in the sediments, which in turn are derived from delivery of
surface organic carbon into subsurface communities (Islam et al.
2004
). Addition-
ally, mobilization of sorbed arsenic may also impact arsine release and availability.
The fluctuation of the water table, as a result of rainfall and pumping sequences for
irrigation, favors cycles of oxidized and anoxic periods. Under anoxic conditions,
iron oxides dissolve and arsenic is released into the water phase. The concentration
of arsenic in the aquifer, up to 100 m depth, ranges between 0.01 and 10 lM over
vertical and horizontal distances (Harvey et al.
2006
and references therein).
Release of natural solid phase arsenic into groundwater has been attributed to
various mechanisms, including (a) oxidative or reductive degradation of arsenic-
bearing solids, in competition with ligand displacement by phosphate, (b) reduc-
tive dissolution of Fe(III) oxides with concomitant arsenic release, (c) arsenic
release via redox cycling in surface soils/sediments and transported into the sandy
aquifer, and (d) microbially mediated oxidation of organic carbon, inducing
release of arsenic from sediments. Arsenic occurrence in groundwater is strongly
influenced by anthropogenically induced processes where groundwater irrigated
rice fields and constructed ponds (both rich in fertilizers, dissolved organic carbon,
and other agrochemicals) contribute most of the recharge to the aquifer.
Neumann et al. (
2010
) studied the origin of dissolved arsenic under rice cul-
tivation in the Ganges Delta (Bangladesh) and observed that recharge from ponds
carries degradable organic carbon into the shallow aquifer and that groundwater
pumping and irrigation of pond water increase the depth of the drawn water where
dissolved arsenic concentration is greatest. An example of arsenic concentration in
seven well clusters within a 0.03 km
2
area of rice cultivation is depicted in
Fig.
17.16
a, in comparison with the depth profiles of two conservative signatures,
chloride and methane (Figs.
17.16
b,c). Chemical signatures measured at the depth
of the peak arsenic concentration are matched by those measured in the pond
surface and sediment pore water. The groundwater residence time in the aquifer—
which under current practices is decades or centuries (Harvey et al.
2002
)—may
be altered under different management schemes. However, the change in
groundwater chemistry by added arsenic remains irreversible on a human lifetime
scale.
