Gumpkins triggered naturally for me without having to read up on the event. As soon as I noticed I was no longer bound to 1 bucket of sacred water a day I started to make up for lost time, so to speak, and it dried up quickly. Its a nice touch. Like the other person said, I cant say if its level-based, because my character dates back awhile and was like level 23 (24 now), which I think is quite high.
After you've dealt with them, an option appears that I dont think is documented; you can go to your goblin village and order extra goblin guards posted at the well. You probably need a strong tribe to get the option. It prevents further problems there - you can also recall them (presumably to see the dried up well event again).