Are we talking a large area? What do you have in mind for the remediated spot?
I don't know much about geothermal, but it's not something I think of as having small-scale versions. Are there likely to be heavy metals present?
This level of damage is new to me, but a lack of actual knowledge never stopped me yet!
Has the topsoil been scraped off andcarted off, or is it mixed up with the subsoil?
I'd look at bringing in organic matter to build the soil rather than adding 'topsoil', which unless you're really careful is likely to be similar to the subsoil you're alreadyy dealing with
I'm assuiming the area's extremely compacted. It might be worth looking at a one-time soil aeration, the method depending on the size of the site.
Then
I'd dump grass clippings, manure, old
hay, whatever I could lay my hands on really, mixed with a few truckloads of chipped tree prunings.
I'd let let it settle for a couple of weeks then seed tough legumes, daikon, maybe pioneer legume
trees and so on.
