We have used the Scheutjens-Fleer theory for polymers at interfaces to study depletion zone effects in polyelectrolyte systems. The segment density profiles of depletion regions are independent of the chain length, $N$, and the depletion interaction is often fully repulsive. In polyelectrolyte systems, a depletion zone develops both at adsorbing and at non-adsorbing interfaces, especially at low ionic strengths. Interactions which are repulsive at some surface separations and attractive at others can also be found, e. g. for intermediate ionic strength conditions or for very low charge density in the chain. At very high ionic strength or very low charge density, the classical neutral polymer depletion layers develop, causing attraction between particles in solution. Effects of polydispersity of the polyelectrolytes on the segment density profiles are small. There is a minor effect on the depletion region at low ionic strengths, provided that there is not a large fraction of very short chainspresent. Only depletion zones near adsorbing surfaces tend to be preferentially populated by low molecular weight polymer. Under these conditions, long chains are still depleted from this filled-up depletion region, which has immediate implications for the equilibration of the adsorbed layer: The diffusion of long chains through this region to the surface is slow. At an adsorbing surface, preferential adsorption of the longer polyelectrolytes over the shorter ones results in a fractionation of the chains at the adsorbing interface, if equilibrium can be reached.