WordNet (r) 3.0 (2006):

well-grounded
    adj 1: logically valid; "a sound argument" [syn: reasoned,
           sound, well-grounded]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "well-grounded":
   adamantine, admissible, authoritative, binding, bulky, calculable,
   cogent, confirmed, consistent, credible, deep-dyed, deep-engraven,
   deep-fixed, deep-grounded, deep-laid, deep-rooted, deep-seated,
   deep-set, deep-settled, dense, dependable, durable,
   dyed-in-the-wool, embedded, embossed, enduring, engrafted,
   engraved, entrenched, established, etched, faithful, faithworthy,
   fast, firm, firmly established, good, graven, hard, heavy,
   implanted, impressed, imprinted, indelibly impressed, infixed,
   ingrained, ingrown, inveterate, invincible, inwrought, just,
   justifiable, lasting, lawful, legal, legitimate, logical,
   long-established, massive, old-line, on a rock, on bedrock,
   plausible, predictable, rational, reasonable, reliable, rooted,
   rugged, sane, secure, self-consistent, sensible, set, settled,
   solid, sound, stabilized, stable, staunch, steadfast, steady,
   stout, strong, sturdy, substantial, sufficient, sure, surefire,
   tough, trustworthy, trusty, unfailing, unyielding, valid, vested,
   weighty, well-argued, well-built, well-constructed, well-educated,
   well-established, well-founded, well-informed, well-made,
   well-posted, well-read, well-set, well-settled, well-versed,
   wholesome, widely read