The concept of who holds risk and how they account for it is interesting. The owner usually wants the contractor to hold all the risk (firm fixed price) and that's almost always a good thing. However, when the contractor recognizes unusual risk before he gives you his bid, he'll account for it and you'll be paying for it whether it comes to pass or not. In theory, all offerors make reasonable decisions in preparing their bids and the risks are accounted for responsibly. In practice, some contractors will low-ball their bid and argue changed conditions or unforeseens and make the owner pay for their irresponsible decision.
ALBin517 has it exactly right: many contractors want extra compensation when their risk becomes reality but don't offer any rebate when the risk doesn't come to pass. This is nothing more than shifting the risk to the owner and is counter to the concept of a FFP contract.
In my experience, whenever a risk is clearly called out in a FFP contract the owner does NOT pay if the risk becomes reality. This is why the contractor builds contingency or profit into the offer. I know nothing about your state or its laws, but I can't imagine why a state would get into how two parties can assign risk in a private contract - it's the foundation of capitalism!
My recommendation: Don't pay. It sounds like what he found was something he should have reasonably foreseen. Catacombs, not so much! Put the burden on the contractor to claim your decision. Hopefully your organization has dispute resolution procedures that could solve this before it ever goes to court. Because really, who's going to pay lawyers for a few days of extra labor and equipment?
MA_PE: You wrote
It's both unfair and unreasonable for either side to take full responsibility for localized subsurface condtions like that.
Why's that? If it's not one side or the other, how would it get resolved? In my experience, I've almost never seen the owner and contractor agree to split risk (before or after the risk becomes reality). When the risk is significant, the owner can enter into a cost plus contract and pay actual costs.