A column-flow cesium migration experiment has been conducted in support of the safety assessment program for the Waste Isolation Pilot Plant (WIPP) in Southeastern New Mexico. The column was a monolithic cylinder of Magenta dolomite obtained from a core sample drilled near the proposed WIPP site. The fluid was a Magenta equilibrated de-ionized water solution that had been doped with 0.001 ppM of /sup 137/Cs. The experiment ran continuously for an 18 month period at flow rates comparable to those measured for natural groundwater at the site. After flowing for 18 months, the apparatus was disassembled and the dolomite column examined for cesium distribution. Cross sectional radiographs of the sample showed that most of the cesium had penetrated only 0.4 cm into the solid Magenta column with a well defined wave front. On the other hand, trace quantities (2 x 10/sup -7/ ppM) of /sup 137/Cs were detected in effluent samples collected during the experiment. The distribution coefficient (120 ml/gm) calculated (assuming porous flow) from the average depth of cesium penetration (0.3 cm) is considerably less than the value obtained from a batch equilibration measurement made on the same system (650 ml/gm), indicating that cesium will migrate through Magenta rock …