Contaminant of the Month: Strontium

Dec. 9, 2014

Strontium is a silvery alkaline earth metal like calcium and magnesium, but it oxidizes and burns in air.

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 95th 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.

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