DEFICIENCIES IN MODELS USED TO DEMONSTRATE HYDRAULIC CONTAINMENT AT THE PROPOSED ADAMS MINE LANDFILL, ONTARIO, CANADA

Large excavated pits such as the South Pit at the Adams Mine in northern Ontario, Canada, have always been attractive options for waste disposal. Today’s advanced engineering technologies (liners, drainage blankets and pumping controls etc.) allow such pits to be developed as landfills with a measure of environmental protection, particularly during the early decades of landfill development when the landfill leachate is often most potent. Ultimately, however, pumping must cease, engineered components fail, and the impacts of contaminants on the environment will depend on the hydrogeological properties of the host rock in its natural state. Technical studies for the Environmental Assessment for the Adams Mine Landfill began in 1995 and the potential “billion dollar” project was conditionally approved in June 1998. A key condition was the installation and testing of two additional deep boreholes (later known as DH 98-1 and DH 98-2) to supplement the one existing deep borehole (DH 95-12). In April, 1999, the Director of the Ministry of the Environment issued a Certificate of Approval (CofA) for the site, evidently deciding that data from the two new deep boreholes were sufficient to demonstrate that hydraulic containment of leachate would be sustained throughout the landfill’s 1000-year contaminating lifespan. It now transpires that the sophisticated models used to demonstrate long-term containment at the site were seriously flawed. Problems included the use of a two-dimensional vertical slice model that failed to acknowledge potential flow of groundwater into and out of the model domain via cross-cutting faults, inadequate calibration with head data deep beneath the pit, and unverified boundary conditions. In fact, the outcome of the model simulations used to demonstrate that relatively low, deep groundwater heads observed in DH 98-1 and DH 98-2 would recover sufficiently to maintain long-term hydraulic containment, was entirely predetermined by the head conditions selected for the model boundaries. While the Ministry of the Environment has never acknowledged any deficiencies in the model work, the CofA was eventually revoked in the summer of 2004.