In response to the increase in world demand for uranium, mines in the Elliot Lake area of Northern Ontario are being reopened. As part of the rehabilitation program, James F. MacLaren Ltd. was commissioned to undertake studies related to the safe disposal of the radioactive tailings which result from the mining–milling process.Tailings disposal is usually achieved by discharging the tailings slurry and waste rock to an isolated impoundment area. Process water from the slurry and natural runoff from the tailings basin and its drainage area collect at the lowest end of the tailings area. This water must be treated to meet water quality standards set by regulatory agencies. However, no firm criteria (at least for Ontario) for the frequency with which spillage of untreated water might be permitted were discovered. Equally, no criteria regarding the design of an emergency spillway for this type of impoundment were found. In view of this, criteria were developed during these studies using engineering judgement.The evolution of these criteria is discussed in this paper. The hydrologic methods used in the analysis of both long term water level variations and the response to severe storm events are outlined. An example of the application of the criteria for the Elliot Lake area is presented. A method of reducing the cost of meeting these criteria is also discussed. Finally, the need for further independent research on the problem is pointed out.