Geomembranes are particularly popular as pond liners, even when the pond in question is holding mining waste rather than fresh water and fish. Ponds are the most popular option for holding large amounts of water and waste in the mining industry.
Digging into the ground to support the weight of a large storage area makes more sense than building above ground and spending extra money to reinforce the holding structures. Even if above ground tanks are preferred for holding smaller amounts of the total wastewater for processing, ponds are better for bulk storage and long-term holding.