While grading the ground where the old silos and barn were, the previously plugged underground water line started to leak. I noticed a wet spot and started digging the next day, ultimately renting a small excavator to dig six feet down to expose the water line. One the line was exposed it was a relatively simple job to cut the old line and put a compression fitting on the end of the line to seal it up. I'll make note of the location so if I need to run a water line out that direction I can find it again.
The concrete box is a cache basin for the drain that came from the old dairy barn. It's no longer in use and so will be capped and buried.