Can I split a 120 circuit from a 240 circuit when there is a neutral wire available?
Assuming it's 8/4 copper wire, you could even use a 40A breaker to protect it - if aluminum wire, 30A.
You have a 4-wire feed, and you'll need to isolate the ground and neutral in the sub-panel. If your well casing is metallic, presumably it's already tied into the ground wire - if not, it should be (they make one heck of a ground rod.)