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| Titel |
Potential of using arsenic-safe aquifers as sustainable drinking water sources in arsenic-affected areas of Bengal basin, India and Bangladesh |
| VerfasserIn |
Prosun Bhattacharya, Abhijit Mukherjee, Ashis Biswas, Mohammed Hossain, Mattias von Brömssen |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250135970
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| Publikation (Nr.) |
 EGU/EGU2016-16907.pdf |
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| Zusammenfassung |
| Naturally occurring arsenic (As) in Holocene aquifers in Bengal basin (India and
Bangladesh) have undermined a long success of supplying the population with
safe drinking water. Several studies have shown that many of the tested mitigation
options have not been well accepted by the people. Instead, local drillers target
presumed safe groundwater on the basis of the colour of the sediments. The overall
objective of the study has thus been focused on assessing the potential for local drillers
to target As safe groundwater. The specific objectives have been to validate the
correlation between aquifer sediment colours and groundwater chemical composition,
characterize aqueous and solid phase geochemistry and dynamics of As mobility
and to assess the risk for cross-contamination of As between aquifers in areas of
southeastern Bangladesh and West Bengal. Drillings to a depth of 60 m revealed
two distinct hydrostratigraphic units, a strongly reducing aquifer unit with black
to grey sediments overlying a patchy sequence of weathered and oxidised white,
yellowish-grey to reddish-brown sediment. The aquifers are separated by an impervious clay
unit. The reducing aquifer is characterized by high concentrations of dissolved
As, DOC, Fe and PO43−-tot. On the other hand, the off-white and red sediments
contain relatively higher concentrations of Mn and SO42− and low As. Groundwater
chemistry correlates well with the colours of the aquifer sediments. Geochemical
investigations indicate that secondary mineral phases control dissolved concentrations of
Mn, Fe and PO43−-tot. Dissolved As is influenced by the amount of Hfo, pH and
PO43−-tot as a competing ion. Laboratory studies suggest that oxidised sediments have
a higher capacity to absorb As. Monitoring of hydraulic heads and groundwater
modelling illustrate a complex aquifer system with three aquifers to a depth of 250 m.
Groundwater modelling studies illustrate two groundwater flowsystems: i) a deeper regional
predominantly horizontal flow system, and ii) a number of shallow local flow systems. It
was confirmed that groundwater irrigation, locally, affects the hydraulic heads at
deeper depths. The aquifer system is however fully recharged during the monsoon.
Groundwater abstraction for drinking water purposes in rural areas poses little threat for
cross-contamination. Installing irrigation- or high capacity drinking water supply wells at
deeper depths is however strongly discouraged and assessing sustainability of targeted
low-As aquifers remain a main concern. Delineation of safe aquifer(s) that can be
targeted by cheap drilling technology for tubewell (TW) installation becomes highly
imperative to ensure access to safe and sustainable drinking water sources for the
As-affected population in Bengal Basin. In order to replicate the salient outcomes of
the Matlab study results, an investigation was carried out in West Bengal, India
covering an area of ∼100 km2 to investigate the potentiality of brown sand aquifers
(BSA) as a safe drinking water source which is currently being practiced in the
area for safe tubewell installation. The results revealed salient hydrogeochemical
contrasts within the sedimentary sequence designated as shallow grey sand aquifers
(GSA) and the brown sand aquifers (BSA) within shallow depth (< 70 m). These
two sand groups with all possible variability in the colour shades were analogous
to the reducing and the oxidized sequences as delineated aquifers based on the
sediment color as perceived by the local driller in Matlab. Although the major ion
compositions indicated close similarity, the redox conditions were markedly different in
groundwater abstracted from the two group of aquifers. The redox condition in the BSA
is delineated to be Mn oxy-hydroxide reducing, not sufficiently lowered for As
mobilization into groundwater. In contrast, lower Eh in groundwater of GSA, along
with the enrichments of NH4+, PO43−, Fe and As reflect reductive dissolution
of Fe-oxyhydroxide coupled to microbially mediated oxidation of organic matter
as the prevailing redox process causing As mobilization into groundwater of this
aquifer type. The outcomes of the study has thus established a scientific knowledge
linking relationship between the colour of aquifer sediments, redox conditions and
hydrogeochemical parameters that provides unique opportunity for the local drillers in
rural communities to target As-safe aquifers for well installations in Bengal basin. |
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