25th International Meeting on Organic Geochemistry IMOG 2011

P-139
Role and nature of organic matter in the mobilisation of arsenic
in shallow reducing aquifers
Wafa. M. Al Lawati 1 , Jiin-Shuh Jean 2 , Ming-Kuo Lee 3 , Thomas. R. Kulp 4 , Michael Berg 5 ,
Elisabeth Eiche 6 , Athanasios Rizoulis 1 , Jon Lloyd 1 , David Polya 1 , Bart. E. van Dongen 1
1 School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester,
United Kingdom, 2 Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan (Province
of China), 3 Department of Geology and Geography, Auburn University, Auburn, United States of America,
4 U.S. Geological Survey, Water Resources Division, Menlo Park, United States of America, 5 Eawag, Swiss
Federal Institute of Aquatic Science and Technology, Dubendorf, Switzerland, 6 Institute of Mineralogy and
Geochemistry, Universitat Karlsruhe, Karlsruhe, Germany (corresponding
author:Wafa.AlLawati@postgrad.manchester.ac.uk)
Over a hundred million people across the globe are
chronically exposed to high concentrations (>10 ppb)
of the geogenic poison, As, in shallow reducing
groundwaters utilised as drinking water. The detailed
mechanisms by which As is mobilized into the waters
of these aquifers is still a matter of debate. However,
it is generally believed that microbes play a key role
through the reductive dissolution of As-bearing Fe(III)
hydrated oxides and/or the reduction of sorbed As(V).
Electron donors such as organic matter (OM) are
required to drive this process, yet As release rates are
often poorly correlated to total OM – this may in part
reflect the varying bioavailabilities of different types of
OM [1,2], the importance of other electron donors or
the confounding influence of other, notably
mineralogical or microbiological, processes. The role
and nature of OM in As mobilisation therefore remains
rather unambiguous, perhaps in part due to the
limited number of studies to date. An improvement in
understanding of this link is required to better inform
theories of long-term anthropogenically driven secular
increases in groundwater As in these aquifers [3].
In this project, we further investigate the correlation
between As release from aquifers and the OM
present. To date, all aquifer sediments analysed,
including new sediments from well known As hotspots
in Vietnam, West Bengal and Taiwan, revealed inputs
from comparable multiple sources including
contributions from naturally occurring petroleum
seeping into the shallow aquifer sediments from
deeper thermally mature source rocks. However,
comparison between core-sites with contrasting As
concentrations (e.g. < 10 ppb and up to 600 ppb) in
the groundwater indicated no noticeable differences in
biomarker distribution patterns, suggesting that As
mobilisation is not associated with a specific organic
source at these sites.
Microcosm experiments were used to determine the
correlation between different sources of OM and As
mobilisation. No correlation between any OM source
and the amounts of As(III) released could be
observed (Fig.1). This suggests that As mobilisation
might not be exclusively associated with a specific
type of OM but could be different from site to site.
Further more, microcosms using OM-lean aquifer
sediments (TOC < 0.02%) showed that As can even
be mobilised in the absence of substantial amounts of
bioavailable organics [4] suggesting that other
electron donors need to be considered as well.
Combined, these studies show that the precise
mechanism of As mobilisation in a variety of
sediments is still unknown but at best, a complex
interplay between microbiology, mineralogy and
geochemistry at the nano-scale should be expected.
Fig.1. Correlations between (a)TOC, HMW (b)nalkanes,
(c)n-alkanoic acids and (d)n-alkanols and the
% As(III) released in microcosm experiments using
sediments from a Taiwanese As hot spot aquifer.
References:
[1] Rowland, H.A.L. et al. J. Environ. Qual. 38, 1598-
1607 (2009)
[2] van Dongen, B.E. et al. Appl. Geochem. 23, 3047-
3058 (2008)
[3] Polya, D.A. and Charlet, L. Nature Geosc. 2, 383-
384 (2009)
[4] Hery, M. et al. Geobiol. 8, 155-168 (2010)
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