I would suggest you dip a finger or cloth down into that reservoir and get a sample of that contaminant. If it is just sooty brown stuff, that is a mild case of rust. You haven't been keeping up with your coolant changes. :nono: If there is solid matter in there, that is hard core rust. You may need to have your radiator rodded. Have a radiator shop check it out and tell you how bad it is. The contaminants in the block can be handled two ways 1) get an over the counter flush at your favorite store. Follow the instructions, then refill with clean 50/50 coolant. These are generally mild and wont clean out too much, so they are only usable for mild cases. Make certain it says it is safe for aluminum blocks and radiators. 2) if you have a severe case of rust, take it to a radiator shop, auto shop or dealer and have them do a thorough rust flushing that they can warranty. 3)The ******* way would be to change your coolant once every 6 months and do a flush each time. This will clean everything out over the course of 3 years maybe. The best medicine is to never let it get this bad.
:bigthumbsup