Simplified method to predict final void water levels

Abstract

At the completion of open cut coal mining, the open cut pit may remain, becoming a final void. In the majority of cases, the BHP final voids will collect water until they reach an equilibrium level where net inflows equal net outflows. BHP currently has over 100 open cut coal mine pits across Queensland and New South Wales coal operations, some of which may be final voids at closure. A decision framework is being developed by BHP to facilitate more efficient and consistent decisions about rehabilitation of final voids. Within this framework, an understanding of the pit lake equilibrium water level is needed to assess the interaction with groundwater aquifers (whether the void will be a groundwater sink or a source of groundwater recharge) and the likelihood of discharges (overflowing) to surface waters. At present, the most common method for predicting the final void pit lake level is to use a daily time step water balance model (WBM), often derived from complex WBMs developed for site operations. Considering the number of pits BHP needs to assess at various stages of mine planning, with frequent mine plan changes in the decades prior to closure, developing and continuously updating complex WBMs for each pit would result in inefficient closure planning. Consequently, BHP recognised the benefit in developing a simple and efficient screening method to predict the long-term pit lake water level. To facilitate the development, a suite of daily time step WBMs were developed based on the current methodology BHP adopts to assess final voids. The aim of the WBMs was to evaluate the typical behaviour of the pit lake using a range of representative final void configurations from a representative sample of seven BHP operations. Recognising the uncertainty in the key parameters adopted in the BHP operational WBMs, a sensitivity analyses of four key parameters associated with runoff, evaporation, groundwater and spoil storage were undertaken. The results were used to guide the development of the screening method by identifying which parameters have the most influence and should therefore be accounted for in predicting the behaviour of pit lakes. A ‘pit lake prediction tool’ has been developed and benchmarked to the WBM results to show confidence in the screening method. A risk assessment was included to allow effective communication of the pit lakes’ risk of interacting with groundwater or spilling to the receiving environment. This pit lake prediction tool forms part of the BHP final void decision framework, which facilitates more efficient and consistent decisions about rehabilitation of final voids.