Salinity concerns from coal mine spoil

Last modified by Vanessa Glenn on 2017/01/31 14:50

Title/TopicSalinity Concerns from Coal Mine Spoil by Melinda Hilton, The University of QLD, SMI - Environment Centres - PhD Student
Summary

The rate of salt generation from coal mine spoil is strongly dependent on the solubility of the available minerals. Soluble salts, such as NaCl and KCl, dissolve rapidly and create a heavy initial salt load. Continued leaching removes these ions, and salt generation becomes dependent on the slower rates of mineral oxidation and weathering. This releases ions such as Ca2+, Mg2+, and SO42-, which may be taken up by plants, or react further to produce secondary minerals. Salt accumulation due to evaporation is more of a problem in dry, arid areas than in tropical zones which have a heavy rainfall. Fine and weathered sediment types generate more salinity than fresh, coarse sediments, which are relatively stable. A geological assessment of waste material early in the mine cycle may help to predict and prevent future salinity issues. A knowledge of the salt potential of various geological units could be better utilised as a guide to choosing suitable cover materials.

 

Background

Coal mine spoil, which is comprised of overburden and waste rock interburden, is often used as backfill or dumped in above-ground piles. Spoil contains a wide variety of particle sizes, is usually fresh (unweathered), and contains very little organic matter.  The rehabilitation of a closed mined site is severely impacted if revegetation efforts are ineffective, which is commonly the case if the spoil is salt-affected.

Elevated salinity levels in natural soils have long been known to adversely affect agricultural crops and increase erosion. In spoil, the effects of salinity are more pronounced, as the lack of organic matter removes the pH buffer, generating greater extremes of acidity and alkalinity. Salt-affected spoil is usually alkaline (pH 8-9), and high in specific salt ions, namely Na+, Ca2+, Mg2+, K+, Cl-, and SO42-.

Salt-affected soil may be categorised as saline, sodic, or saline-sodic. Approximately 26% of Australia‚Äôs land is classified as sodic, and a further 5% is saline (Rengasamy and Olsson 1991). Recently, concern has grown that mining activities may be bringing fossilised salt reserves to the surface, and that leaching due to rainfall may be raising salinity levels in ground and surface waters.  

Objectives

The combined objectives of the relevant literature examined were:

-          To assess the source the salts in spoil, and the associated geochemistry;

-          To model the kinetics of salt release i.e. the rate of leaching from the spoil.

Method/ techniquesColumn leaching is commonly used as an experimental method to determine the species of salt in the mine spoil leachate, and the rate at which the salts are released. In the laboratory, researchers try to accelerate the weathering process through a series of leaching cycles or events. Real-world mine spoil may require 10-20 years to weather properly, but in the laboratory, this process may be simulated over only a few months.

Figure 1.jpg

Figure 1: Schematic of a column leach experiment (Washington State 2003).

Alternatively, a large-scale field trial may be undertaken, accompanied by the use of lysimeters, to study available plant nutrients. Field studies will usually rely on natural precipitation rates, with increased monitoring around storm events. Weathering rates vary according to the amount of rainfall, but in general, large-scale experiments are carried out over a period of 1-2 years.  

Results and observations

The processes involved in the salt dynamics of mine spoil have been summarised in Figure 2. The saline ion pool is made up of cations and anions dissolved in aqueous solution, which are held in the pore spaces between particles. If the grains are large and porous, then water escapes quickly and easily, travelling down through the spoil profile into groundwater. If the spoil contains fine particles such as clay, then water becomes trapped, and leaching is slow.

Figure 2.jpg

Figure 2: Chemical reactions in a salt system (Li et al. 2014).

Soluble salts, such as NaCl and KCl, dissolve readily into solution, and may become trapped in the root zone of growing plants. Cations, such as Na+, are readily adsorbed onto the surface of clay particles, which increases dispersion and friability of the spoil. This makes it very difficult for plants to acquire aeration. Reactions of dissolution/precipitation and adsorption/desorption take place relatively quickly, and are reversible until the solution finds a stable equilibrium.

The second phase of salt release is much slower, and takes place through the oxidation of minerals (Li et al. 2014). For example,

-          Pyrite (FeS) weathers to sulphate (SO42-);

-          Muscovite (KAl2(AlSi3O10)(F,OH)2) and albite (NaAlSi3O8) weather to kaolinite (Al2Si2O5(OH)4);

-          Calcite (CaCO3), magnesite (MgCO3), and dolomite (CaMg(CO3)2) weather to release soluble Ca2+ and Mg2+ ions.  

The release of salts from minerals is an irreversible process, and results in the general weathering of the spoil over time.

 

The kinetics of the release of salt from spoil can be represented by two phases (Figure 3, Fityus et al. 2007). The initial rate of leaching is rapid due to the dissolution of readily soluble minerals such halite, NaCl. As soon as these minerals are leached or flushed out, the concentration of salt in the leachate decreases rapidly. The second, slower oxidation phase then begins. Fe, Al, and Si minerals are much less soluble, and release salt when they become exposed. Over time, this second phase becomes slow and steady, but does not completely diminish.

Figure 3.jpg

Figure 3: Relationship between salt concentration and leaching intensity for various mine spoil fractions (Fityus et al. 2007).

Attempts have been made to model the predicted release of salts from mine spoil based on the geology of the material. Park et al. (2013) provided a description of mapping categories (A to H) based on particle size, pH, the amount of weathering, and the type of parent rock (Figure 4). By using statistical modelling it was found that each category could be associated with a high, or low median salinity group (Figure 5).

Figure 4.jpg

Figure 4: Mapping categories for geological units of coal mine spoil (Park et al. 2013).

Figure 5.jpg

Figure 5: Dendrogram showing salinity clustering of mapping category groups (Li et al. 2014).

Based on the clustering on the dendrogram, mapping categories A, B, D, and F were predicted to produce a relatively high amount of salinity, while groups E, G, and H were low. The fine and weathered sediments tended to produce the most salt, while the fresh, coarse, and relatively stable units produced less. Acid-generating rocks containing pyrite may have reduced salinity by the creation of Na2SO4, a soluble mineral that is easily leached out.

Outcomes

What was learned?

It is unreasonable to assume that all mine spoil creates salinity problems. The amount of salt generated depends upon the geology of the spoil material, as well as the rate of precipitation. Other factors that may affect leaching include temperature, the dimensions of the pile (which affects bulk density and porosity), the creation of preferential flow paths along erosional channels, the mining method, and the particle size and shear strength (Edraki et al. 2015).

What were the benefits delivered?

With adequate modelling, the potential salinity issues of coal mine spoil can be predicted by studying the geology of the waste material, even before it has started to weather. Any additional costs can then be factored into the rehabilitation strategy in the planning stage.

Existing and abandoned mines with salt-affected spoil may be able to be reclaimed by choosing salt-tolerant vegetation species and/or an adequate application (at least 30cm) of suitable topsoil (Grigg et al. 2003). Low salt-producing spoil could be co-disposed, or used as a cover over top of high salinity spoil, thereby reducing the cost of topsoil and avoiding the accumulation of salt in the rootzone.

References

Edraki, M., et al. (2016). Prediction of Long-Term Salt Generation from Coal Spoils. CMLR, University of Queensland, ACARP.

Fityus, S., et al. (2007). The environmental geotechnics of coal mine spoil. Common Ground (07) Proceedings - 10th Australia New Zealand Conference on Geomechanics Brisbane, Australia.

Grigg, A. and T. Baumgartl (2005). A Model of Long-Term Salt Movement in Reconstructed Soil Profiles Following Open-Cut Coal Mining in Central Queensland. ACARP, Centre of Mined Land Rehabilitation, University of Queensland.

Grigg, A., et al. (2003). The effect of organic mulches on crusting, infiltration and salinity in the revegetation of a salin-sodic coal mine spoil from central Queensland, Australia. American Society of Mining and Reclamation, 2003 National meeting of the American Society of Mining and Reclamation.

Li, X., et al. (2014). "Understanding the salinity issue of coal mine spoils in the context of salt cycle." Environmental geochemistry and health 36(3): 453-465.

Park, J. H., et al. (2013). "Geochemical assessments and classification of coal mine spoils for better understanding of potential salinity issues at closure." Environmental Science: Processes & Impacts 15(6): 1235-1244.

Rengasamy, P. and K. Olsson (1991). "Sodicity and soil structure." Soil Research 29(6): 935-952.

Washington State (2003). An Assessment of Laboratory Leaching Tests for Predicting the Impacts of Fill Material on Ground Water and Surface Water Quality. D. o. Ecology.

Date 

Soil reconstruction and the testing of organic mulches on Central QLD mine spoil was carried out by Grigg et al. from 2003-2005.  Collection and analysis of the Hunter Valley samples by Fityus et al. took place in a series of experiments from 2004-2007. Column leaching experiments and statistical analysis by Park et al. on QLD spoil were conducted from 2012-2013.
 

Location(s)

Bowen Basin - QLD, Hunter Valley - NSW

Keywords

Salinity; mine spoil; column leaching; weathering; solubility

    
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