A method for installing an in situ, low-permeability temporary horizontal barrier at depth for vertical containment of a soil mass including installing a low permeability vertical barrier having walls located to contain a predetermined soil volume, the vertical barrier having a horizontal surface area in a plane perpendicular to its walls; and injecting low permeability reagents capable of forming a temporary horizontal barrier in a pattern suitable for creating a horizontal subsurface barrier joined to all of the vertical barrier walls to form an in situ barrier.