BMW: 95 BMW 325 Water pumps, long term solution, seal failures

Question
My '95 325 plastic water pump impeller disintegrated or disappeared last year at the 95K miles point and my local shop replaced it with a new plastic impeller pump. This pump lasted about 3000 miles until it started leaking out of the seal. So they replaced it with a metal impeller pump and this pump lasted less than 1000 miles until its seal started leaking also. This spring, this has happened 2 more times and in desperation they have now gone back to the metal impeller pump.  But it seems to me that replacing 5 pumps in less than 5,000 miles is missing the real problem and unless this is addressed, my car will be spending a week every month in the shop getting a new pump installed.  Any ideas what might be causing all the water pump seal failures? My theory is that particles of the original pump impeller are somehow destroying the new pumps seals but my local mechanic doesn't seem to want to consider that possibility. He just keeps replacing pumps.  What do you think and/or what do you propose for a long term solution? Thanks.

Answer
Jeff,
 Which seal is leaking?  The seal between the pump and the block?  Or is it the seal around the waterpump shaft?  If it's the seal between the pump and block, they are not cleaning the surface correctly, and it's and uneve surface.

 If it's the seal around the pulley/shaft, something is putting pressure on the shaft, like the idlerpully is stuck or something.

 I would either take it to another shop or watch them do the job.  if you stand with them, sometimes they do a better job.

 Hope this helps,
 Josh