Internal Report 267
leGras C & Klessa D
Supervising Scientist Division
Department of the Environment, 1997
About the report
A model is proposed to enable calculation of the temporal flux of magnesium sulfate in the catchment of Ranger Retention Pond 1. The main reason for developing this model is to assess the long-term potential for solute generation within the catchment, and subsequent release to the environment. Therefore, the time frame of interest is principally the rehabilitation and postrehabilitation phases of mining. For this reason, operational releases of solutes to the catchment, for example by flood irrigation of wetland-modified Retention Pond 2 water, have not been considered in great detail.
The main medium to long term sources of magnesium sulfate within the catchment are:
- new placements of waste rock, particularly from the #3 orebody and,
- surface expression of seepage from the tailings dam (whether tailings are rehabilitated in situ or not).
To assess the rate of production of soluble erosion products from waste rock, the 'northern' waste rock dump (in the catchment of Retention Pond 4) was used as a case study. Calculations based on historical solute fluxes from this disused dump predict that soluble effluent will decline to low levels within about 20 years from last placement. Solute evolution was relative slow from this dump when operational, possibly because continual occlusion of previous rock placements impeded the initiation and propagation of weathering processes.
The volume of seepage from the tailings dam has been estimated using: the dimensions of the 'shallow' (that is, C2-horizon) aquifer; the velocity of the seepage plume, as measured by progressive water quality in observation bores; and the porosity of the aquifer material. Contaminated groundwater enters the catclunent principally by surface expression during the Wet Season. The volume reaching the surface has been estimated using discharge, water quality and isotope-composition data. Direct intersection at ground level has not been demonstrated for this aquifer. The model predicts that solute evolution from this source is likely to continue for several decades after tailings seepage is terminated, either by removal of the tails or by impermeable capping.