The Collaborative International Dictionary of English v.0.48:

Impoverish \Im*pov"er*ish\, v. t. [imp. & p. p. Impoverished;
   p. pr. & vb. n. Impoverishing.] [OF. empovrir; pref. em-
   (L. in) + povre poor, F. pauvre; cf. OF. apovrir, F.
   appauvrir, where the prefix is a-, L. ad. Cf. Empoverish,
   and see Poor, and -ish.]
   1. To make poor; to reduce to poverty or indigence; as,
      misfortune and disease impoverish families.
      [1913 Webster]

   2. To exhaust the strength, richness, or fertility of; to
      make sterile; as, to impoverish land.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impoverish
    v 1: make poor [ant: enrich]
    2: take away [syn: deprive, impoverish] [ant: enrich]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "impoverish":
   ablate, absorb, assimilate, bankrupt, beggar, bleed, bleed white,
   break, burn up, bust, consume, denude, deplete, deplume, despoil,
   digest, displume, divest, drain, drain of resources, draw,
   draw down, dry, eat, eat up, erode, exhaust, expend, finish,
   finish off, flay, fleece, fold up, gobble, gobble up, ingest, milk,
   pauper, pauperize, pick clean, pluck, reduce, ruin, scuttle, shear,
   sink, skin, spend, squander, strip, strip bare, suck dry, swallow,
   swallow up, use up, waste away, wear away