The cost of cleanup is in no way linear with the amount of coal being produced; that solution would be complicated and impractical.
A better approach is to make the mine operator come up with some sort of remediation plan as part of the approvals process and then insist that they follow it, backed with the threat of massive fines. Se a standard so that remediation has to be ongoing or commence before the economic phase of extraction is over (to prevent bankruptcy from being a rational out).
That does front-load the costs and so push out smaller operators from starting mines; but that is pretty much inevitable when tightening environmental standards. If the goal is to force a company to do something that isn't economically rational, a determined regulator is a reasonable way.
A better approach is to make the mine operator come up with some sort of remediation plan as part of the approvals process and then insist that they follow it, backed with the threat of massive fines. Se a standard so that remediation has to be ongoing or commence before the economic phase of extraction is over (to prevent bankruptcy from being a rational out).
That does front-load the costs and so push out smaller operators from starting mines; but that is pretty much inevitable when tightening environmental standards. If the goal is to force a company to do something that isn't economically rational, a determined regulator is a reasonable way.