The delivery system is old and can cause a myriad of problems for water. This helps explain why folks have to boil water for 10 days after a water main break.
Also, the pipes inside the home can cause problems, affected by any number of factors, including the corrosiveness of water. Water softeners dont help.
The EPA has even modified some sampling techniques for detecting lead in water, whether with a flush or first draw sample. Now, the recommedation is to leave the aerator IN, when sampling for lead.
Water filters can increase the heterotrophic plate count, associated with water, providing a fertile environment for all sorts of bacteria.
We cover all of this, and more, in the CWS course.
BTW… the EPA is making a move to actually get states to cover their reservoirs…