BackPacker, I thik it is clear to everybody, this is not about fuel contamination of ground water anymore, it has something to do with cash flow. Follow the money...
Yeah, I think the contractor would've loved to dig a hole of about 2400m3 (8 meters deep) for free. Especially considering the health hazard - they've got to work with airtight/overpressurized cabins and so forth. Not to mention the 100 or so truckloads of soil that need to be taken off-site, cleaned somehow and then brought back onsite. And that's just the primary spill site.
From that spill site the lighter fuel components washed into the surrounding soil over the decades. That area is too vast to excavate, so they are going to use air injection somehow for the next few years, so that the benzenes remaining in the ground can finally oxidize while they're being washed out by the ground water.
Now I agree that this is a government project and it could possibly have been done cheaper, but I don't think it can be done *significantly* cheaper. (But you're welcome to submit a counter-offer. Just PM it to me, and I'll be happy to forward it to my friend for consideration.)