Method of treating formation to remove ammonium ions without decreasing permeability

A method of treating a subterranean formation which has undergone an in situ leaching operation which utilized an ammonium carbonate and/or bicarbonate lixiviant. In such a leach operation, ammonium ions will absorb into the clay in the formation and will present a threat of contamination to any ground waters that may be present in the formation. The present method involves flushing the formation with a strong, alkaline solution, e.g., sodium or calcium hydroxide, to convert the ammonium ions to ammonia which is easily carried from the formation by the alkaline solution. The alkaline solution is modified by adding a monovalent metal salt, e.g., sodium chloride, which prevents the clay from swelling which decreases the permeability of the formation. After substantially all of the ammonium ions are removed, the formation is then flushed with water to remove any alkaline solution which may remain in the formation.