A method destroys organic liquid contaminants contained in a plurality of below-ground volumes by smoldering combustion. The method applies heat to at least a portion of a first one of the volumes of organic liquid and forces oxidant into the first volume of organic liquid so as to initiate self-sustaining smoldering combustion of the first volume of organic liquid. The method may terminate the heat applied to the first volume of organic liquid. Next, the method modulates the flow of the oxidant into the first volume of organic liquid so as to cause at least a portion of the first volume of organic liquid to migrate and come into contact with another one of the volumes of organic liquid, so as to propagate the smoldering combustion. In an alternative embodiment, the flow of the oxidant may be modulated to establish a substantially stationary combustion front.