Approval of an operating license for the McClean Lake uranium mill and tailings disposal site in northern Saskatchewan, required that Cogema Resources Inc. be able to limit and predict maximum As concentrations that could be released from their tailings management facility (TMF) to ground and surface waters for up to 10,000 years. The approach to such prediction involved applying the concepts of thermodynamic equilibrium and chemical kinetics to arsenic-controlling reactions. These reactions were studied in the laboratory as a function of the Fe/As ratio, pH and aging time of processed acid mill raffinates and neutralized tailings discharged from the mill. Arsenic concentrations of <1-2 mg/L in limed tailings (pH 7-8) were found controlled chiefly by the precipitation of ferric and nickel arsenate solids (scorodite and annabergite), and crystallization of the scorodite with time. The experimental results and associated geochemical modeling persuaded the regulators to grant an operating license for the mill and TMF in 1999. The license required that As concentrations in the TMF be monitored for a number of years to validate the laboratory and modeling results, and that As released from TMF pore waters to local ground waters not exceed 5 mg/L. Sampling and analysis of pore waters and solids after two years of TMF operation has confirmed the laboratory results and predictive modeling. The conditional licensing approach taken by the Canadian regulators is the only sensible way to license waste facilities in geological environments that have long-term performance requirements.