A Cesium Products Facility (CPF) at Bernic Lake, Manitoba, manufactures a Cs-formate drilling mud from pollucite (CsAlSi2O6•H2O) ore. The waste residue is dry-stacked over pegmatite tailings. This thesis studied the mineralogy of the residue and tailings, and the mobility of residue related elements: Ca, Cs, Rb, S and Sr. Major minerals in the residue include gypsum, quartz, barite, spodumene, feldspar, pollucite, and Cs-sulphate, and in the tailings, quartz, feldspar, spodumene, pollucite and micas. The residue contains an order of magnitude more Cs (<2 wt. %) than the tailings (<0.3 wt. %), with 47 % being mobile from Cs-sulphate and Cs-carbonate. Cs in the residue is immobile in microcline, pollucite, Cs-formate, and Cs in Fe-oxy-hydroxides. Elements from the residue are mobilized by water from several sources: meteoric water and surface runoff from the residue pile, precipitation leaching through the residue pile into the tailings, and direct contact of residue with groundwater.