"hole dug for water, spring of water," Old English wielle (West Saxon), welle (Anglian) "spring of water, fountain," from wiellan (see well (v.)). "As soon as a spring begins to be utilized as a source of water-supply it is more or less thoroughly transformed into a well" [Century Dictionary]. Figurative sense of "source from which anything is drawn" was in Old English.
"source of a stream or river, flow of water rising to the surface of the earth from below," Old English spring "spring, source, sprinkling," from spring (v.) on the notion of the water "bursting forth" from the ground. Rarely used alone in Old English, appearing more often in compounds, such as wyllspring "wellspring," espryng "water spring." Figurative sense of "source or origin of something" is attested from early 13c. Cognate with Old High German sprung "source of water," Middle High German sprinc "leap, jump; source of water."
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Harper, D. (n.d.). Etymology of wellspring. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/wellspring