injecting spent fluids into underground layers is not a new...

injecting spent fluids into underground layers is not a new thing mate - its been used in many forms of mineral and oil extraction for many many years. engineers who work on this kind of thing don't just forget to allow for pumps, it is a common practice.

you can inject fluid into ground without pressure. it all depends on the porosity of the ground you inject it into. it also depends on how deep you try to inject it- if they go into the pressurised layer were the fluid originally came from - then yeah, you would need to pump it against the existing pressure.
but if you inject it into a shallower, porous layer, often gravity can do the work for you.