Open pit mines that have ceased production are increasingly being considered for the permanent and environmentally acceptable disposal of mine waste rock and tailings that are, or have the demonstrated potential to become, sources of acidic drainage. This report addresses key aspects that need to be taken into consideration when evaluating the in-pit disposal of wastes and presents 12 case studies of actual and planned in-pit disposal of mine wastes.

There are four basic concepts for the placement of wastes in pits:

Option 1 – Underwater disposal
Option 2 – Elevated water tables
Option 3 – Dry disposal
Option 4 – Perched water tables

The four options are described in terms of the theoretical concepts and practical aspects; selected examples of research and field applications are provided from the published literature.

Not all pits are suitable for the in-pit disposal of wastes. The success of an application would depend on many technical factors, including:

* the acid generation potential of the wastes and pit walls;
* the geotechnical characteristics and properties of the wastes and the pit walls;
* predicted pore water, pit water, and groundwater quality;
* hydrogeology of the open pit; and
* the hydrology of the open pit.

Mine related constraints must also be taken into consideration. These include: limiting access to remaining mineralization below the pit; wall stability and related safety concerns; available access; and the proximity of underground workings to the open pit.

Consideration must be given to both the short term and long term implications of the in-pit disposal concept; these include ecological and human health protection and closure planning perspectives. Each site is unique and may have special constraints with respect to the quality and use of surface water and groundwater, land use, and sensitive ecological communities.

The potential costs for future in-pit disposal of wastes should be considered in the preliminary economic evaluations that are used to establish final pit limits. Greater than expected waste disposal costs could have an unfavourable impact on the future profitability of some open pit mines.

Volume 2 of this report presents twelve case studies: seven case studies describe in-pit disposal programs that have been implemented; two case studies describe proposed in-pit disposal programs; and three case studies provide technical information from other pits which will likely be of interest to persons evaluating in-pit disposal programs.

The following case studies describe pits that have been used for in-pit disposal of mine wastes:

The Owl Creek pit – To prevent discharge of acidic drainage from waste rock piles, the waste rock was relocated to the pit and flooded.

The Rabbit Lake pit – This pit is likely the first application of an engineered pit disposal concept, which involved the use of a bottom rock drain, an engineered pervious envelope, placement of tailings as a dry filter cake, and closure with a surface lake and a soil/sand diffusion barrier.

The Collins Bay B-Zone pit – Following extensive decommissioning studies, the special waste (which contains elevated arsenic, nickel, sulphur, or uranium content) was placed in the bottom of the pit and the pit was flooded, creating a pit lake.

The Solbec pit – A crown pillar pit was filled with reactive waste as a means of inhibiting further acidic drainage.

The Udden pit – The pit was backfilled with reactive waste rock and allowed to flood.

The Stratabound CNE pit – The in-pit disposal of reactive waste rock was planned at the design stage, and the waste rock has been relocated to the pit and clay capped.

The Cluff Lake “D” Zone pit: The pit flooded after the end of mining operations, and an extensive monitoring program has been carried out to evaluate the physical and chemical changes occurring in the pit lake water column.

In-pit disposal programs have been proposed at the following case study sites:

The Island Copper pit – As part of planned closure, it is proposed that the pit be flooded to create a meromictic lake.

The Deilmann pit – Insufficient capacity exists within an existing surface tailings management facility to accommodate future tailings production therefore tailings disposal options are being reviewed; one option involves the conversion of the Deilmann pit to a full side drain and under-drain tailings management facility.

Additional case studies which may provide useful information to persons evaluating in-pit disposal programs are:

Robinson Mining District – Extensive numerical modelling was carried out to evaluate the hydrogeochemistry and to support the prediction of environmental impacts from several pit lakes.

The Gunnar pit – This pit lake is interesting because of unique and well-documented limnological characteristics.

The Berkeley pit – This mine site is part of a U.S. Superfund site and is a well-known example of a serious acidic drainage problem in a flooding open pit.