Case Study of Non-Mining Prediction and Control of Acid Rock Drainage — The Vancouver Island Highway Project

Abstract

The Vancouver Island Highway Project (VIHP) was a $CAD 1.2 billion expenditure by the Province of British Columbia to improve travel safety and relieve traffic congestion between Victoria and Campbell River on Vancouver Island. In the section of highway between the Tsolum River and Oyster River, on the Inland Island Highway portion of the VIHP, over half of the rock to be cut and filled was found to be net acid generating. The Tsolum River has been impacted by metal leaching and acid rock drainage (ML/ARD) for decades from an upstream closed minesite. Geochemical testwork indicated: 1. over half of the rock along this section of highway was potentially net acid generating based on a site-specific SNPR (=NP/SAP) criterion of 1.5; 2. some core samples were already acidic; 3. pyrite occurred in disseminated framboidal and gel forms; 4. the rate of acid generation was not high; 5. measured neutralisation potential (NP) provided some lag time for the onset of net acidity; and 6. non-carbonate neutralising minerals prevented low pH (<4.0) from appearing in the humidity cells. These predictions led to ML/ARD controls involving: 1. realignment of the highway; 2. elevation control to minimise the volume of disturbed rock; 3. encapsulation of blasted-rock fills within the highway, surrounded by low-permeability till and covered with asphalt; and 4. diversion of water away from cut faces. Internal pipes drained any water reaching the fills into external ditches and ponds, where the water could be monitored and, if necessary, treated. Water from the cuts was also captured in ditches and carried to monitored ponds. Elevated aqueous sulfate concentrations in several ponds confirmed that sulfide minerals were oxidising, but the lack of both acidic pH and increasing trends in concentrations demonstrated that ML/ARD was controlled. Contingency plans for any pond that may become acidic include water treatment, which predictive testwork has shown would not require substantial amounts of limestone or lime.