At many locations in Australia the decommissioning of mining operations
will require the disposal of excess water. If the water is contaminated its
disposal may be restricted by government guidelines. The timing of
water disposal may be critical to the schedule of close-out. The
uncertainty of the annual rainfall input to the water storage and disposal
system may determine the success or failure of close-out. The uncertainty
associated with any outcome can be used to determine the reliability of
the system, or inversely its risk. The technique of Monte Carlo simulation, using the uncertainty in
annual rainfall, is applied to quantify the risks associated with water
disposal as part of the requirements for close-out in two case studies. In
both case studies it is assumed that water disposal is via evaporation only.
The first case study is the Nabarlek operation, located about 270 km east
of Darwin. This project is required to complete the engineering work
associated with its close-out by December 1995. The second study
investigates the situation of close-out for the Ranger Uranium Mine,
about 250 km east of Darwin. It should be noted that the Ranger mine
was examined as part of the annual update of its contingency
rehabilitation plan and the probability of implementing the plan is
extremely small. The probability of success for the Nabarlek case varied from less than
ten per cent to greater than 95 per cent depending on the management
strategy. Four possible strategies were investigated for the Ranger mine
with the chance of success varying between 20 and 95 per cent. The
estimation of the evaporation rate and the area allocated to evaporation
was critical in both cases. By applying seasonally based evaporation pan
coefficients determined on site, rather than an average monthly value of
0.75, the chance of success was improved by up to 35 per cent for Ranger. Simulation modelling is able to provide information on the likely
response of mine water management systems and the level of risk. This
information is required to efficiently plan and manage the disposal of
water for close-out.