The Collaborative International Dictionary of English v.0.48:

nonporous \nonporous\ adj.
   Not porous; especially, not having vessels that appear as
   pores; as, nonporous wood. Opposite of porous.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

nonporous
    adj 1: not porous; especially not having vessels that appear as
           pores; "nonporous wood" [ant: poriferous, porous]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "nonporous":
   close, close-knit, close-textured, close-woven, compact, compacted,
   compressed, concentrated, concrete, condensed, congested,
   consolidated, crammed, crammed full, crowded, dense, firm, gluey,
   hard, heavy, impenetrable, impermeable, jam-packed, jammed,
   massive, packed, serried, solid, substantial, thick, thick-growing,
   thickset, viscid, viscose, viscous