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A Model of Long-Term Salt Movement in Reconstructed Soil Profiles Following Open Cut Coal Mining in Central Queensland

Open Cut » Environment

Published: May 05Project Number: C12043

Get ReportAuthor: Andrew Grigg, T Baumgartl | Centre for Mined Land Rehabilition, University of Qld

Waste rock materials exposed at the surface following open-cut coal mining in central Queensland are commonly saline, impacting on the success of revegetation efforts. The aim of this study was to develop a simple model that predicts the removal of salts from reconstructed soil profiles by leaching, thereby demonstrating an improving growth medium over time.

The study was able to draw upon salinity profile data from 49 plots established as part of routine rehabilitation monitoring programs across four mines. Plots sampled rehabilitation ranging in age from <1 to 27 years since pasture establishment and had been monitored for periods of up to eight years. The dataset was extended to include plots from a rehabilitation trial at a fifth mine where a range of spoil amendments had been applied and monitoring had been conducted for 12 years.

Results from the monitoring plot dataset revealed a general decline in average salinity in the rootzone with time, with evidence of a distinct upper boundary. Plots with initially high salinity commonly decreased rapidly, while plots with either lower initial salinity or older plots presumably subjected to leaching losses prior to measurement decreased more slowly. Using data from plots with at least four assessments over time, a simple model was able to be developed that predicts average salinity in the rootzone (EC p ) from the initial average salinity (EC 0 ), cumulative rainfall since rehabilitation establishment (P), and two terms describing the amount of rainfall required to initiate leaching (a) and the rate of leaching (). The model takes the form

The rate-of-leaching term was found to have a relatively consistent value of 2 across a number of mines and a range of situations. Variation around this figure was shown to be related to factors affecting spoil hydraulic conductivity, and with additional data, the model may be further improved by replacing the single value with a suitable function that describes this.

The model predicts downward salt migration over longer time periods, and is not intended to predict periods of capillary rise. This was shown to occur over the shorter term during years of below-average rainfall even in profiles with net long-term leaching. Net removal of salts over time requires adequate infiltration in combination with a sufficient depth of material at the surface with at least moderate permeability to combat capillary rise. The model is not applicable to saline spoils with very low permeability, which are prone to fluctuating or increasing salinity. A more complex modelling approach is needed in these cases to resolve the appropriate depth of replaced topsoil to instigate net leaching. However, from data examined in this study, a minimum depth of 30cm of replaced topsoil appears to be required for spoils with very poor permeability.

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