Geochemical modelling of the speciation, transport, dispersal and fate of metal contaminants in water systems in the vicinity of tailings storage facilities
Date
2016
Authors
Grover, Bronwyn Patricia Camden
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Abstract
Gold mining of the Witwatersrand Basin reefs has been responsible for the rise
of Johannesburg as an economic centre of South Africa. While mining
provided a base for business and infrastructure development for the region, it
has also generated social and environmental problems for the country. Tailings
storage facilities (TSFs), a common sighting around Johannesburg and across
the entire basin, have been built to contain the processed waste following
extraction of gold from the pyrite containing quartzite ore. When the fine
grained waste is exposed to atmospheric conditions, oxidation of remnant
sulphides occurs resulting in acidic, metal rich and sulphate rich plumes that
enter the environment through surface and groundwater systems.
This thesis sought to better understand the release, transport, dispersal and fate
of metals emanating from TSFs and their remnant footprints on the
Witwatersrand. These metals included aluminium, copper, chromium, iron,
manganese, nickel and uranium and are known to be toxic to humans
depending on their concentration and speciation. Traditionally, analytical
methods have been employed in studies focussing on the characterisation of
some of these processes in the region. While these studies have generally
conducted quantitative assessment of the extent of pollution, little
comprehensive interrogation and fingerprinting of the processes that are
influential in determining the potential risk posed by metals has been done.
This has largely been due to the shortcomings of analytical methods to
determine these. To this end, this research has employed geochemical
modelling to complement the traditional analytical methods.
The approach to study the release of metals from TSFs involved assessment of
the partitioning of metals within tailings and their potential release using batch
and sequential extraction methods. Processes of metal release within the
tailings were simulated through geochemical modelling (using the PHREEQC
and Geochemist’s Workbench codes). The simulations were based on the
percolation of rainwater through these layers and the changes in its chemistry
along the path. The potential seepage of this plume along the path was then
correlated to observed efflorescent mineral crusts that are temporary sinks for
metals and are a common feature in the vicinity of the tailings and water bodies
such as ponds and streams. The potential impact of the mineral crusts on the
water chemistry of receiving water systems following their dissolution was
assessed using forward geochemical modelling. The transport of the metals in
groundwater was also studied. This involved simulations of the transition in
chemistry of a plume from a TSF along an aquifer of known composition. This
was based on a 1-D reactive transport model constructed using information
from sequential extraction work on the aquifer rock (to identify the key
minerals to consider) and site data (mainly flow rates) from previous studies.
The processes occurring in the removal of metals from acid mine drainage
(AMD) through a permanent sink in the form of a pump-and-treat plant in the
Central Goldfield of the basin were simulated using PHREEQC.
The findings from the research showed that two different plumes were
produced from an abandoned TSF as a result of rainwater percolation, notably
a plume produced from the dissolution of secondary salts formed in the
oxidised layer and a sulphuric acid rich plume in the unoxidised layer. These
differences were apparent in the geochemical composition of the mineral crusts
collected on the walls of tailings dumps and from a pond into which the plumes
were draining. On dissolution, mineral crusts were found to produce acidic
solutions with crusts containing predominantly Fe producing pH values below
3. The simulated dissolution of various types of mineral crusts gave insight into
the impact of minerals present in the smallest amount. This showed that the
bulk mineralogy as determined by analytical techniques such as PXRD and
remote sensing could not be used with confidence to deduce the impact of the
mineral crusts on receiving water bodies.
The characteristics of surface plumes released from tailings TSF were
compared to other water systems in the area around Soweto, with
complementary interpretation conducted using chemometric methods. From
principal component analysis (PCA), surface water systems were found to form
distinct groups largely influenced by mineral solubility, alkalinity and
dissolved oxygen content.
The 1-D reactive transport simulations involved acidic, metal and sulphate rich
water ingressing the aquifer (below the TSF). Several scenarios were modelled
including simulations with different dolomite contents; allowing for surface
complexation and the presence of cation exchange surfaces. At a point 500 m
from the water ingress in the dolomite rich aquifer, Fe and Mn were largely
precipitated out (as confirmed by sequential extraction results on the aquifer
rock) while the sulphate concentration was reduced by almost half. On the
other hand, Ca concentrations were conservative largely because of continuous
dissolution of dolomite and precipitation of gypsum along the flow path.
The simulations of the high density sludge treatment plant involved forward
modelling of the treatment process with the sludge responsible for the removal
of trace metals from the incoming acid mine drainage. The model can be of use
for cost and process optimisation at the facility.
This research has had notable outputs in the form of publications; models on
metal release, transport and attenuation; and models on pump-and-treat
processes. These will form an important repository of information and for
benchmarking any further studies related to AMD.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2016.
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Citation
Grover, Bronwyn Patricia Camden (2016) Geochemical modelling of the speciation, transport, dispersal and fate of metal contaminants in water systems in the vicinity of tailings storage facilities, University of Witwatersrand, Johannesburg, <http://wiredspace.wits.ac.za/handle/10539/21078>