 Abstract | The successful management of solid wastes arising from the combustion of low-rank
coal for electricity generation presents significant engineering and environmental
challenges. The power stations in the Latrobe Valley region of Victoria, Australia, have
long recognised the need for improved long term understanding of ash disposal. This
thesis presents the work undertaken in evaluating the mechanisms which lead to the
transport of solutes from ash disposal and develops a methodology to quantify their
potential long term impacts on groundwaters beneath a disposal site. The Loy Yang
power station is used as a case study.
A detailed literature review is presented on the mechanisms involved in the leaching of
solutes from ash disposal. In general, the release of solutes is well understood and is
related to the dissolution of more soluble minerals in the ash and advective transport
through pore waters as leachate, although the exact controls for trace elements is less
well documented. The proportions of particular solutes and/or trace elements is site
specific. For the Latrobe Valley, however, there remains little research undertaken on
the behaviour of Loy Yang ash, especially aged or leached ash excavated from a
disposal pond after a period of some 6 to 12 months.
The principal environmental concerns relating to the disposal of ash are the potential for
groundwater contamination from salt fluxes and the transport of trace elements. Thus
long term disposal requires a thorough understanding of both the solute fluxes from the
ash as well as the controls on the transport of these solutes through groundwater.
Predicting the behaviour of ash and the leached solutes under field conditions is difficult
and common laboratory tests have been found to be inadequate.
The transport of sulfate in seepage was investigated through back analysis of monitoring
data, field monitoring, bacterial analysis and modelling. Sulfate was shown to be
undergoing strong biogeochemical reactions which attenuate its rate of migration in
shallow groundwaters at the Loy Yang power station. The application of a kinetic solute
transport model was able to model the monitoring data obtained to date.
The chemical quality of the ash, and its source from the power station, is a critical
aspect of disposal since this primarily determines the leachability and potential fluxes.
After slurrying and disposal in a saturated pond, the amount of soluble minerals is lower
and therefore the ash presents a lower potential for groundwater impacts.
For ash excavated from a disposal pond and placed within a low moisture environment,
such as an Overburden Dump, the potential for leaching and solute transport must be
considered differently to that in a saturated disposal pond. Two field trial cells were
operated for about 14 months to investigate such behaviour, one artificially irrigated
(Wet) and a second open to rainfall only (Dry). Both cells showed the importance of
unsaturated flow mechanisms in controlling the water balance and leachate generation,
due mainly to the potential of ash to retain moisture in its pores. The irrigated cell
showed a marked reduction in leachate salinity as irrigation continued, although some
trace elements demonstrated complex leaching patterns.
To further quantify ash leaching rates, a series of laboratory leaching columns were
constructed and operated, with electronic logging of soil moisture using Time Domain
Reflectometry (TDR). The use of TDR, although able to detect relative changes in soil
moisture, was less than successful. The leachate results from the columns were
encouraging and provided additional confirmation of leaching curves for particular
solutes and trace elements. The soil water characteristic curve (SWCC) was established
for the ash through a Tempe Cell test. This quantified more accurately the water
retention properties highlighted through the field and laboratory research. Importantly,
analysis of the SWCC for the ash shows that it appears to maintain high hydraulic
conductivity over typical ranges of matric suctions.
The geochemical controls on solutes in the various ash leachates generated in the field
and laboratory were investigated through geochemical speciation modelling and
plotting. The major solutes in leachate appeared to be controlled by dissolution from
more soluble minerals, such as gypsum and halite, while for other species the controls
were more complex. Most trace elements appeared to be controlled by a mix of mineral
dissolution, co-precipitation and adsorption mechanisms.
A solute transport and leaching model was developed and applied to the various data
sets obtained for this thesis. The model, describing the leaching and transport of solute
in one-dimensional steady state flow, gave reasonable calibration to the different
column experiments. Extension of this approach to unsaturated flow and solute transport
is discussed in light of the experience from the field trials. The conversion of this model
to non-dimensional form was then examined and provided a useful approach for
assessing the scale effects from different sized column leaching experiments and field
trials. The use of batch leaching tests, although not generally representative of field
conditions, can be incorporated into this approach and used to estimate the initial
concentration of a solute in leachate. The use of these models provides the methodology
to quantify leaching over time and at various scales, important in the engineering design
of ash disposal sites.
In summary, this thesis presents a detailed qualitative study of ash leaching and solute
transport mechanisms, and develops a quantitative methodology for the design and
assessment of ash disposal sites. |
