The permeability of barrier layers formed from sodium bentonite clay may be significantly and unpredictably impacted upon contacting various ionic substances. Modifying agents may be admixed with sodium bentonite clay to lessen the impacts resulting from its contact with an ionic substance. Suitable modifying agents may comprise an anionic substance selected from the group consisting of a diutan, a polymer containing a 2-acrylamido-2-methylpropanesulfonic acid monomer, any salt thereof, and any combination thereof. Methods for forming a barrier layer may comprise: providing a barrier material comprising sodium bentonite clay and a modifying agent admixed with the sodium bentonite clay, the modifying agent comprising an anionic substance selected from the group consisting of a diutan, a polymer containing a 2-acrylamido-2-methylpropanesulfonic acid monomer, any salt thereof, and any combination thereof; and forming a barrier layer comprising the barrier material.