From Colorado Public Radio (Michael Elizabeth Sakas):
Several factors are fueling the water problems in both communities. Firefighters used so much water trying to extinguish the Marshall and Middle Fork fires that pressure was lost in both water systems. Bacteria and other organisms can enter water lines that aren’t properly pressurized and contaminate water supplies.
The fires also carved a destructive path through Superior and Louisville that broke water mains and destroyed as many as 1,000 homes, damaging and exposing other pipes, leaving them open to other contaminants entering the water systems.