This paper describes a case study on the design, construction and performance of an underground barrier intended to contain copper mine leachates in central Arizona. The barrier was a composite slurry wall composed of high density polyethylene (HDPE) and soil-bentonite (SB). The purpose of the barrier was to retain low pH (<2) and high copper sulfate leachate seeps and groundwater from an existing tailings mine. Challenges to barrier performance included resisting the high hydraulic gradients from the 100 feet + high dam, chemical compatibility of the barrier materials and the leachate, and a maximum allowable hydraulic conductivity of 1×10^-7 cm/sec.