I have a 1 inch line buried from my house pressure system to the barn, and it has a very small leak somewhere. Is there some additive that could be circulated through this line to stop the leak? Rather than digging it all up.
Start digging. And next time, use 160 or 200 PSI black polyethylene (NSF-PW rated - ie, suitable for potable water, per Fiasco's comment) rather than PVC. (You may not "need" the pressure rating, but it gets you a thicker wall which makes the pipe even more durable .vs. the normal 100 PSI stuff. And digging is something you don't want to have to do over.)
In any case PE pipe is much more durable than PVC when faced with rocks, movement, etc.
Probably the leak is at one of the fittings. Dig out both ends and wipe the fittings dry and clean. After an hour or so run your fingers under the fittings and check for water drops. If the leak proves to be somewhere in the pipe it may be easier to rent a ditch witch (trench digger) and just replace the whole line with proper hose.