MEND Project 1.17.ld
April 1990

EXECUTIVE SUMMARY

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.

Last Modified: 2003-11-26		Important Notices

Français | Contact Us | Help | Search | Canada Site
Home | What's New | CANMET-MMSL | MMS Site | NRCan Site