Complex sentence: Until recent years, geomembranes were used mostly as canal and pond liners; however, one of the largest current applications is to the containment of hazardous or municipal wastes and their leachates.
Split into simpler sentences: Until recent years, geomembranes were used mostly as canal and pond liners. One of the largest current applications is the containment of hazardous and municipal wastes and their leachates.