Abstract: A method of forming a lateral barrier to downward flow of fluid contaminants through a porous subterranean formation. A fluid medium capable of maturing into an impermeable material over a period of time, such as an aqueous composition capable of forming a gel or an aqueous slurry of portland cement, is injected into the formation at a pressure greater than the overburden pressure, lifting formations located above the level of injection, thereby creating a laterally extending void filled with the fluid medium. The lateral extent of travel of the fluid medium is monitored by utilizing tiltmeter technology. The fluid medium subsequently hardens into an impermeable barrier to downward fluid contaminant flow.