The article mentions streams as being a safe source of drinking water for rural communities — as most of the population were during the medieval period.

My mother from rural north-west Ireland drank water from a stream for the first 25 years of her life. There was no running water in her home-place until the late 70s (after she had moved out). Rain water was collected from the gutters in barrels for washing and other uses but water for drinking had to be carried by bucket from a stream 1km away across fields on the side of a hill that were muddy when it rained. I’m guessing drinking water from a stream would have been even more common in the preceding decades and centuries.