The Collaborative International Dictionary of English v.0.48:

groundage \ground"age\ (ground"[asl]j), n.
   A local tax paid by a ship for the ground or space it
   occupies while in port. --Bouvier.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

GROUNDAGE, mar. law. The consideration paid for standing a ship in a port.
Jacobs, Dict. h. t., Vide Demurrage.