Authors: Oldham, C; Salmon, U; Hipsey, M; Ivey, G; Wake, G

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Oldham, C, Salmon, U, Hipsey, M, Ivey, G & Wake, G 2006, 'Prediction of Long Term Water Quality in Acidic Pit Lakes', in AB Fourie & M Tibbett (eds), Mine Closure 2006: Proceedings of the First International Seminar on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 679-687,

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Stewardship of the post-mining environment relies, amongst other things, on our ability to predict the long- term fate and transport of contaminants. Ideally, if remediation is deemed necessary, scenarios should be tested numerically prior to scale-up. Pit lakes exemplify post-mining environments that must be appropriately managed for optimal benefits to local communities. Yet the potential benefits are dependent on our ability to predict, and possibly manipulate, long term water quality in the lakes. However this predictive capability requires a detailed understanding of the biogeochemical cycling of contaminants in the post- mining environment. Our understanding of metal and nutrient cycling in pH neutral conditions is reasonably advanced, based on the past 100 years of limnological, hydrological and catchment research. However many post-mining environments are highly acidic, and the impact of acidity on biogeochemical cycling is poorly understood. In particular our understanding of the interactions between groundwater and surface inflows, meteorological forcing, lake stratification cycles and local geochemical conditions is limited, in part due to the lack of appropriate numerical models that bring all of these controls together. In acidic pit lakes, high levels of dissolved species consist predominantly of Fe and SO4, coming from iron sulphide oxidation, and Al, coming from accelerated mineral weathering processes at low pH (Eary, 1999). Addition of alkalinity leads to precipitation of Fe(III) and Al out of solution as (oxy)hydroxides or hydroxysulphates; these reactions are however associated with proton release. Precipitation of Fe and Al thus constitute “buffering systems” which must be overcome before the pH will increase. Compilations of data from multiple mine lakes indicates grouping into the “Fe” and “Al” buffering systems (e.g. Klapper and Schultz, 1995), which maintain pH at approximately <3.8 and 3.2 - 4.2 respectively, with the more normal carbonate buffering system coming into play at pH > 6. The buffering capacity is critical for the long term remediation potential of pit lakes, and must be described in any lake water quality model. Our understanding of how the three major buffering systems interact, and how transitions occur from one buffer system to the next, is limited. Yet most of the key contaminant cycles are highly pH sensitive, and thus it is critical that we quantify the controls on pH, and the subsequent impact on alkalinity generating processes, such as Fe and SO4 diagenesis in the lake sediments. Over the last three years, a consortium including the Australian Coal Association, Wesfarmers Premier Coal Ltd, Griffin Coal Mining Company Pty Ltd and the Western Australian Department of Premier and Cabinet, has funded us to develop a coupled lake stratification - biogeochemical model for long-term prediction of water quality in legacy pit lakes. In 2006, we have completed the inclusion of an aqueous and mineral geochemical module and a dynamic diagenesis module into the previously existing stratification-ecological model (DYRESM-CAEDYM). All of these components are required to allow investigation of the interaction between mineral dissolution processes under low pH conditions (geochemical module), buffering capabilities (geochemical module), nitrate, iron, manganese and sulfate reduction processes (diagenesis module), the cycling of allocthonous and autocthonous carbon sources (CAEDYM) and how all of these processes are affected by seasonal stratification cycles (DYRESM). In this paper we present the initial results of this modelling effort, with a focus on water column processes, and finally with some validation against collected field data. Mine Closure 2006 ― Andy Fourie and Mark Tibbett (eds) © 2006 Australian Centre for Geomechanics, Perth, ISBN 0-9756756-6-4 Mine Closure 2006, Perth, Australia 679

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