『Abstract
Mineral and aqueous geochemical data are combined with a conceptual
groundwater flow model, to establish the origin and fate of iron,
aluminium and manganese in the groundwater system of a small back-barrier
island. The flow model domain consists of an unconfined island
fresh groundwater lens overlying a semi-confined hypersaline aquifer.
The two aquifers are separated by a discontinuous, clay-rich aquitard
and both contain diffusion governed variable density flow fields.
High concentrations of dissolved iron and manganese are associated
with brackish to hypersaline groundwater, although there is no
systematic relationship with salinity. Calculation of S2-/SO42- and Fe2+/Fe3+
redox couples and the results of thermodynamic modelling show
that redox disequilibrium in the groundwater is widespread. Groundwater
samples containing aqueous sulphide and ferric iron complexes
are supersaturated with respect to pyrite, goethite and haematite
but the prevailing state of redox disequilibrium controls mineral
dissolution and precipitation. Aqueous iron in the deeper regions
of both aquifers is derived from the dissolution of iron oxide-hydroxides
in lateritic palaeosols controlled by seasonal fluctuations in
groundwater redox state. Aqueous manganese is potentially derived
from the dissolution of ilmenite and amorphous oxide-hydroxides.
The oxidation of iron sulphides contributes to the aqueous iron
concentration and sulphuric acid production in the shallow groundwater.
The solubility of aluminium is also limited by this process, governed
by acidity regulation. a significant proportion of aqueous iron
is transmitted from the semi-confined to the overlying unconfined
aquifer through discontinuities in the aquitard layer. movement
of metals in solution outside the island groundwater system is
restricted by the presence of diffusion boundaries within variable
density transition zones.
Keywords: Iron oxide-hydroxides; Density dependent flow; Baroclinic;
Barotropic; Redox couples 』
1. Introduction
2. Geology and hydrogeology
3. Methods
4. Results
4.1. Palaeosol mineralogy and geochemistry
4.1.1. Mesozoic lateritic bedrock palaeosol
4.1.2. Quaternary lateritic palaeosol
4.1.3. Quaternary humate-rich podzollic palaeosol
4.2. Groundwater chemistry
4.2.1. Aquifer 1 - unconfined
4.2.2. Aquifer 2 - semi-confined
4.3. Mineral phase equilibria and solubility
5. Discussion
5.1. Mineral geochemistry
5.2. Lithological and density dependent flow constraints on solute
transport
5.3. Synthesis
6. Conclusions
Acknowledgements
References