Mudd, Gavin Mark (2001) Solute Transport Modelling of Latrobe Valley Ash Disposal Sites. PhD thesis, Victoria University.

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.

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