For now, read the contract and pay the contract rates to show good faith in your word and level of intelligence when you signed.
I see responsibility split equally between owner and engineer, provided the contract did not pile it all on the contractor. They need to answer a couple of questions, How could the owner reasonably expect the engineer to know about the spring beforehand? Should he not have taken responsibility to adequately investigate subsurface conditions himself, or was that the engineer's (expert's) job to recommend that approach to his client. Why didn't the engineer request the owner to do a geotech investigation beforehand? The engineer is paid to make those recommendations to a client whenever he feels it is necessary to avoid risk to the client. If not, then the engineer effectively accepts all risks associated with not warning his client. If none of that was contemplated, then they both blindly accepted the risk that underground conditions could vary. Since the engineer is the professional here, paid for his expert opinions and recommendations, IMO most of that risk should fall on him.
Unless it was specifically stated in the contract, in writing, that the contractor would accept the risk of any discovery of unknown underground conditions, it would be difficult to find him at fault, unless those conditions were prevalent everywhere in the area and the owner & engineer can demonstrate beyond a doubt that the contractor had prior knowledge of same in the area and it was likely to encounter the same on the specific site. Otherwise they have the responsiblity to deliver to the contractor a site that is fit for purpose to do the work as contracted. The contractor is only liable for the KNOWN conditions described to him before or at the time of contract signing. That's what he bid on.
My best advice; settle any disagreements man to man in the most friendly manner possible. Getting lawyers involved will cost all parties many times more than 2 weeks of standby charges in money and 1000 x more in aggravation.
BTW dik, I liked the "Call a lawyer and you'll need a lawyer" line. Great one. Nothing more true than that.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek