What causes this brown discoloration in residential water supply?
Almost certain to be iron rust; a complex oxide/hydrate/hydroxide. Almost certain it came from a component in the water system such as a valve. I have a few steel valves in my sprinkler, they cause red/brown water for a few seconds when turned on. Rust is ( practically) insoluble in tap water. Iron can be soluble in water when it is ferrous ( Fe +2). I once had a home with deep ( 100' ) well. I filled a large above ground swimming pool all afternoon , great looking clear water. The next morning there was a a few inches of reddish loose gel-like stuff covering the bottom . Ferrous iron was dissolved in the well water, overnight the air oxidized it to ferric ( Fe +3) and it settled as "rust". Point being ,the rust is not coming from your well.