The Collaborative International Dictionary of English v.0.48:

Wellspring \Well"spring`\, n.
   A fountain; a spring; a source of continual supply.
   [1913 Webster]

         Understanding is a wellspring of life unto him that
         hath it; but the instruction of fools is folly. --Prov.
                                                  xvi. 22.
   [1913 Webster]

WordNet (r) 3.0 (2006):

wellspring
    n 1: the source of water for a well [syn: wellhead,
         wellspring]
    2: an abundant source; "she was a well of information" [syn:
       well, wellspring, fountainhead]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "wellspring":
   bonanza, cornucopia, font, fount, fountain, fountainhead,
   gold mine, headspring, headstream, headwater, inception, lode,
   mainspring, mine, origin, provenance, provenience, quarry,
   resource, riverhead, root, source, source of supply, spring,
   springhead, staple, vein, well, wellhead