Reading it in whole it looks that the concern is the large...

Reading it in whole it looks that the concern is the large amount of waste which would be 80% of whats dug up and finding sterile ground to but it on ,so drilling has to find barren ground for waste and major infrastructure ..

Throwing the waste back in the hole as you go makes what ever is underneath sterile /lost,
so drilling has to confirm whats at depth..

As we know the area is very flat , so a dam for the slurry would be large and shallow , the commentary talks of juggling "competent material " to build the dam , with that in mind that the natural ground out there is porous maybe a membrane can be used to line the Dam.

So looks like its up to the drilling to see whats underneath , and make decisions from there .