This final report presents the results of studies conducted at the Waite Amulet tailings
site during the four-year period from 1985 to 1988. These studies were performed
under the national MEND (Mine Environment Neutral Drainage) program, a combined
industry-government consortium committed to the development of practical techniques
to reduce the environmental impact of acid mine drainage.

The influence of various factors (gaseous oxygen and carbon dioxide concentrations,
temperature, bacteria, sulphide content and pH) on acid generation at Waite Amulet
are presented and discussed along with saturated and saturated zone pore water
geochemistry, tailings hydrogeology, and surface hydrology. Results of simulations of
pore water flow by a two-dimensional steady state finite element model are also given.
The evolution of acidic pore waters at the tailings site is discussed.

The study verified that the most important control on the pyrite oxidation process is the
availability of oxygen. In addition to contributing directly to oxidation, oxygen influences
the production of ferric iron and the density of bacterial populations. Oxygen
concentration distribution with depth in the tailings defines the depth of active
oxidation, and show that oxygen movement within the tailings is controlled by diffusion.

The oxidation of sulphides in the deep unsaturated zone occurs at a rate dependent on
pH, and will be very low. Acidic porewater is partly neutralized by mineral dissolution
and precipitation reactions in the deeper unsaturated zone. As sulphides in the shallow
zone are depleted, the oxidation front will move downwards. The remaining acidity and
dissolved metals will flow downwards with the pore water in the unsaturated zone until
it reaches the water table. Acid conditions in the shallow saturated zone become more
neutral with depth due to more buffering.

Pore water in the saturated zone flows in a direction which varies across the area of
the tailings. Flow in the central part of the tailings is vertical and downwards while flow
along the perimeter of the tailings is horizontal. Anisotropy in the hydraulic properties of
the tailings is a major control on the flow of porewater in the tailings. The anisotropy is
produced by the presence of fine-grained, horizontal (“slime”) layers. It has the effect of
promoting horizontal flow over vertical downward flow.

Along the west side of the tailings, a surface drainage ditch collects most of the water
infiltrating the tailings. Analysis of porewater quality and calculation of groundwater
velocity suggested that tailings porewater does not penetrate deep into the clayey
horizon underlying the south section of the tailings. In the northwest section, a small
portion of the porewater (approximately 10%) flows through the bottom of the tailings
into the underlying geological units. Sulphate levels above background values,
observed in the deeper layer of the clay, may be attributed to migration by diffusion.