HYDROGEOLOGICAL ASSESSMENT AND DEVELOPMENT OF AMD CONTROL TECHNOLOGY FOR MYRA FALLS WASTE ROCK

Abstract

The Myra Falls Minesite on Vancouver Island, British Columbia contains a relatively large waste rock pile built against a valley wall and having a lateral extent of 800 meters by 300 meters with a height of up to 40 meters. The waste rock dump has been generating acid for at least a decade. Acid-base accounting of 230 borehole samples has shown that the most active areas of oxidation occur within a 10 meter depth of exposed surfaces of the waste rock dump and in deeper zones where relatively high contents of sulfide minerals are located. Water moving through the waste rock dump originates from infiltration of precipitation and lateral groundwater discharge from the valley wall. During periods of significant rainfall, the shallow acid-generating zones are flushed with water and acidic water appears beneath the water table. During periods of negligible rainfall, neutral-pH water is present beneath the water table. However, general calculations suggest that a significant portion of the annual production of acidity is retained in the dump and therefore remains available for flushing. As a result, remediation and decommissioning planning must address the neutralization of this acidity or the control of infiltration and water-table variation. Various approaches to preventing acid mine drainage were evaluated. The most promising approach was the development of a cementitious solidification mixture incorporating mine waste materials for use as a waste rock dump surface sealant and grouting matrix. Mine waste water sludge and mine tailings were used as principal components of the solidification mixtures. A field assessment program was also carried out to evaluate the application and durability of the test mixtures on waste rock test piles. The solidified materials prepared have achieved similar properties to construction concrete in terms of compression strengths, setting times, workability and durability as measured by freeze/thaw testing. In addition, the solidification mixtures can be prepared at costs less than costs associated with other surface sealants such as high density polyethylene.