Can't seem to find the water bridge o-ring part #. After replacing it during my intake refresh, it appears that I've popped it again. Coolant dripping down from that area, coolant in the valley behind the water bridge..... blah.

[edit] ...on a 88' S4

 

looks to be "999 701 627 40" O Ring, they shouldn't pop, most just dry out and crack w/ age.
How did you install it and was the metal surface clean and smooth, did you use and lube?

 

It was not installed with any lube. The metal surface was clean and smooth. That definitely appears to be the part. I'll likely install a new oil filler neck gasket as well. I believe its also leaking [was also replaced as a part of the intake refresh].

 

on second thought, it might be the thermostat housing gasket. probably more likely. I'll let it get nice and hot and see if I can't track down the leak.

 

Make sure your hoses are nice and snug. They can be sneaky.