Strontium (Sr) is similar in some ways to calcium and magnesium and it appears as a constituent of hardness. It's concentration in water is low, and it does not appear in all locales. Strontium sulfate is a reverse osmosis membrane foulant, especially when in significant concentration.

The most commonly found radioactive isotope in the environment, strontium-90, was a legacy left behind by above-ground testing of the atomic bomb, according to the EPA.

The EPA has not established an MCL for Strontium. 4 PPM was the initial proposed limit, but it is expected that the number will be lowered.

"The human body absorbs strontium as if it were calcium. Due to the chemical similarity of the elements, the stable forms of strontium might not pose a significant health threat — in fact, the levels found naturally may actually be beneficial (see below) – but the radioactive 90Sr can lead to various bone disorders and diseases, including bone cancer."

Treatment: Strontium is easily removed by strong acid cation exchange resin, as in a water softener. Reverse osmosis also reduces strontium, but as noted strontium sulfate is a membrane foulant.