Quarry could be used for inert waste and then become a reservoir

As I understand it one side wants to use the Ronez quarry for inert waste for 50 years after Ronez moves out, while Guernsey Water would like it for water storage – although it seems to be a ‘nice to have’ rather than it being critical to the island’s supply. Both have merit but why can’t we have both? If it represents 50 years’ worth of inert waste disposal, why not fill it with waste for 40 years or however long it takes to get say a 100 feet from the top, and then fill it with water? I’m sure someone can do the maths to see to what level it needs to be but as it’s inert waste then, as long as the disposal is carefully monitored to ensure that nothing that could affect the water quality was dumped, why not consider it?