Contaminant of the Month: Strontium

What it is:

Strontium is a silvery alkaline earth metal like calcium and magnesium, but it oxidizes and burns in air.
It forms water soluble Sr⁺⁺ salts, such as strontium chloride (SrCl₂) and strontium carbonate (SrCO₃), by losing two electrons.
Its atomic number is 38 and it has four stable natural isotopes and 12 mostly short-lived (days) radioactive isotopes.
The half-life of Sr-90, produced by nuclear plants and detonations, is the longest — 28.9 years; it is present at trace levels in some surface waters from old fallout and catastrophic nuclear plant releases (e.g. Chernobyl).
Strontium salts burn at very high temperatures with a crimson flame, so they are used in fireworks.
It is a minor contributor to hard water.

Occurrence:

Strontium makes up about 0.02 percent of the Earth’s crust, but its salts are detectable in essentially all drinking waters.
It was found in more than 99 percent of 989 drinking water groundwaters in the 1988 National Inorganics Reconnaissance Survey (NIRS).
Concentrations ranged from 1 ppb to > 10 ppm with a mean of 0.6 ppm (600 ppb).
Preliminary results from the current Unregulated Contaminants Monitoring Rule (UCMR 3) had 100 percent detections, including surface and groundwaters, with the 95^th percentile at 1.7 ppm (1,700 ppb).
In the UCMR 3, 1.1 percent of reporting public water systems exceeded the Health Reference Level (HRL 4,000 ppb).
Dietary exposure estimates are approximately 0.5 to 1.5 mg/day.

Health effects:

Strontium chemistry is similar to calcium’s chemistry, so it accumulates in bone and can affect bone formation at sufficient doses.
The EPA Lifetime Drinking Water Health Advisory and HRL were both 4,000 ppb (4 ppm).
It is not highly toxic, however, there are concerns about excess exposure in the early bone forming years.
Strontium-based drugs (ranelate) are used in osteoporosis treatments and are also available as over-the-counter supplements.

Water treatment:

Water softening is effective treatment for strontium.
Precipitative lime softening with calcium hydroxide and lime-soda ash softening are used by municipal water plants.
Point-of-entry (POE) softening with a cation exchange resin is common practice.
Point-of-use (POU) reverse osmosis (RO) softening is another option in the home.

Regulation:

There are no current health-based regulations for natural strontium.
Sr-90 is dealt with in radioactivity regulations and is not a general concern because of declining concentrations in water from old nuclear tests.
On October 20, EPA announced its preliminary intention to regulate strontium, and it changed the HRL of 4,000 ppb to 1,500 ppb by choosing a different rat study and changing the uncertainty (safety) factor to 1,000 from 300.
It based the decision to regulate on the 1988 NIRS occurrence data rather than the current UCMR3 data.
Seven percent of NIRS groundwaters exceeded 1,500 ppb and 14.3 percent exceeded half of the HRL. If borne out, the regulatory impact would be significant.
Monitoring requirements would affect all public water systems, and any health benefits would most likely be small.
Water softening would provide concurrent benefits.
A final regulatory determination would probably appear in 2015, proposal in 2017 and final regulation in 2019.
Some well-designed epidemiology studies would appear to be prudent prior to a regulation. Stay tuned.

Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.