If it's not leaking right now, and doesn't start back up, wait until spring and dig it up. You should be able to see how deep it is from inside.
I'm not sure about your physical abilities (chemo?), But regardless of the issue, that will have to be dug. If it were a waterline problem, it'd still have to be dug for repair/replacement, so it'll only save you money.
Digging sucks but it doesn't even have to be done in the same day! I'm not sure why everyone is always so quick to say "call the professionals and spend the money. If you want to save and don't mind the work to do so, then save that money.
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