Bitter tears welled up.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means well, so the verb version is to well up.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You're fishing in a hole in the ice when suddenly the water starts to well up and to gush forth out of it. A huge walrus (わ) splashes out and jumps on you. No wonder there weren't any fish! This is a walrus hole.