My missus reckons that the aquifer walls would very likely contain the things that Plimer said that carbonic acid would dissolve (arsenic, cadmium, lead, all toxic).

So a cynic might say that the mammons want to dissolve the toxins and so poison everything that the aquifer waters. How much of a cynic are you Jo?

Carbonic acid (H2CO3) in this context results from carbon dioxide dissolved in water. They exist in equilibrium:
H2CO3 <--> CO2 + H20
The equilibrium lies to the right, meaning that there is more undissolved CO2 than dissolved; but when you consider the absolute not relative numbers in a system as large as the Great Artesian Basin, that is still a lot of carbonic acid.