  The accumulation of positive ions, produced by ionizing particles crossing
Liquid Argon Time Projection Chambers (LAr-TPCs), may generate distortions of
the electric drift field affecting the track reconstruction of the ionizing
events. These effects could become relevant for large LAr-TPCs operating at
surface or at shallow depth, where the detectors are exposed to a copious flux
of cosmic rays. A detailed study of such possible field distortions in the
ICARUS T600 LAr-TPC has been performed analyzing a sample of cosmic muon tracks
recorded with one T600 module operated at surface in 2001. The maximum track
distortion turns out to be of few mm in good agreement with the prediction by a
numerical calculation. As a cross-check, the same analysis has been performed
on a cosmic muon sample recorded during the ICARUS T600 run at the LNGS
underground laboratory, where the cosmic ray flux was suppressed by a factor
$\sim 10^6$ by 3400 m water equivalent shielding. No appreciable distortion has
been observed, confirming that the effects measured on surface are actually due
to ion space charge.
