The results of a two year laboratory test program demon strated that the hydraulic conductivity of a marginal quality sodium bentonite could be significantly enhanced with the addition of a small quantity of specially formulated commercial polymer. The first step in the laboratory program involved tests to establish the rheological prop erties of slurries prepared using 8 % marginal quality (MQ) and 8 % high quality (HQ) bentonite. The marginal bentonite was then modified with various percentages of polymer. A comparison of shear stress ver sus shear rate characteristics, as a function of polymer addition rate, was used to establish a range of polymer addition rates for hydraulic conductivity tests. Ten low gradient triaxial permeability/volume change tests were conducted on compacted Ottawa sand-8 % bentonite tests samples. The concentration of polymer used in these tests varied from 0 to 0.5 % based on the weight of bentonite. Approximately half of the tests were conducted on newly prepared polymer and the other half on polymer aged for one year prior to being used.