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

The EPA has not established an MCL for Strontium.

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.