Another vote here that using diesel and letting it escape into the surrounding environment is a TERRIBLE idea. Beyond terrible. Unbelievable.
Hopefully your contractor has general liability insurance. I’d be talking to an attorney about steps needed to force the contractor to
- Remove the slab
- Hire a qualified firm to certify there is no actionable contamination in the soil or groundwater
- Pay for a qualified remediation contractor to clean things up if found to be necessary
My very limited understanding of environmental law is that you could own any remaining contamination forever. Even after you sell your property and move on you might be responsible for fixing it if discovered in the future.
After highway wrecks, a crew comes out and puts absorbent material on diesel spills on the pavement, and then digs out any soil around the shoulder that got contaminated by the accidental spill. They check nearby drainage ditches to make sure the diesel didn’t get into the surface water or groundwater. All the absorbent and soil is then sealed in barrels and taken to a licensed hazmat landfill. IOW, they go to lots and lots of trouble to deal with the spilled diesel.
And now we hear about a bozo contractor who intentionally goes around creating spills on all his projects