The Collaborative International Dictionary of English v.0.48:

Turbidness \Tur"bid*ness\, n.
   The quality or state of being turbid; muddiness; foulness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

turbidness
    n 1: muddiness created by stirring up sediment or having foreign
         particles suspended [syn: turbidity, turbidness]