We seem to be mixing and matching.

Previously we were discussing the Rio ocean waters, or at least I thought we were.

The article you cite ( which is excellent, by the way ) is discussing the source problem for shower water, specifically in Zona Sul, another issue. Contaminated shower water would certainly create a problem with beach sand anywhere in world, as the sand just sits and collects the residues. So the health warning as I read it is good advice to follow. Shower off at your Hotel, or Apto. to be safe.

E-Coli, as it is commonly known and tested for, would be a serious problem if sourced from shower water, continually at any beach in the world. Any shower water derived from " decades of " untreated effluent (which can be many things both organic and inorganic ) would be an immediate problem on sand set back from the tidal flow of the ocean. It would just accumulate and concentrate in the years.

In the sea, such organic deposits will reduce over time in ocean currents, unless pumped into a bay with no outside water current source.

It isn't unique: This became a problem at Dana Point Marina, in California, and also Boston Harbour on the US Eastern seaboard.

What is unique the volume of people that enjoy the beach in Rio. Chlorinated (Hcl) shower water is not commonly found everywhere in the world, by the way. Given the current situation in Brasil, I would count on Hcl-treated, or another form of sanitary-treated shower water being planned and implemented shortly.

This brings up a very good reminder to always swim and surf near a Lifeguard. Anyone arriving new to Rio would do well to check with the Lifeguards FIRST regarding surf, and health conditions, before entering the water. The surf and currents can be pretty wild at times, especially in Brasil's Winter months. Showers too apparently.