To help prevent their leaking into the environment, tailings ponds often have a membrane liner or are built in an area of low ground permeability. Even so, water could leak or permeate through to the groundwater and contaminate aquifers with the toxic heavy metals and radioactive elements from the tailings water. Some tailings ponds are built within dikes, and in this case, there is always the possibility that the dike fails. If some or all of the water were to be released, the toxic radioactive sediment would migrate into the environment. The exposed particles could disperse, aided by the effects of wind and erosion.
There are various examples of tailings pond breaches at mine sites. For example:
• In June 2008, 17 years after the Opemiska Copper mine near Chapais ceased production, the dike that retained the tailings gave way and a mixture of tailings and water was released into the environment, washing away the foundation of Highway 113 and the railroad. The tailings were carried downstream to the Obatagamau and Chibougamau rivers. Aside from the economic impacts incurred, local residents and land users experienced great anxiety around the contamination of fresh water resources .
• In August 2014, the tailings pond dam at the Mt. Polley mine site in British Collumbia breached and released an estimated 10 million cubic metres of water and 4.5 million cubic metres of fine sand into Polley Lake. Hazeltine Creek flows out of Polley Lake, and the flow continued into Quesnel Lake and beyond. During the initial breach of the tailings dam, the bulk of the original flow created an unstable plug at the base of Polley Lake. The balance of the tailings and water went down Hazeltine Creek and deposited at the confluence of the creek and Quesnel river. Hazeltine Creek was originally about four feet wide and is now up to 150 feet wide.
While these examples do not involve radioactive tailings, the latest breach prompted the CNSC to require operators of tailings ponds at uranium mine sites to confirm their monitoring activities and emergency response plans.