August 31, 2013
Ever since the Fukushima groundwater contamination issue emerged more than a month ago, the Press has incessantly labeled the waters “toxic”. This blog has previously covered the hazards of radioactive Cesium-137 (“How Hazardous is Cs-137?” – two parts – 11/2/12 and 11/9/12). There is so little Cesium in the stored water that it makes no sense to base the current “toxic” designation on it. The “toxic groundwater” label might due to the Strontium contained in the tank’s water. The recent deluge of Press coverage concerning a leaky storage tank at Fukushima Daiichi virtually demands a posting on the health hazards of Strontium-90. First, just how toxic is Strontium? Second, is the level of contaminated water that may have reached the sea worthy of being labeled “toxic”?
Strontium is a soft, silver-yellow, alkaline-earth metal. Its chemical properties are similar to Barium and Calcium. It is chemically reactivity, which means it readily undergoes reactions with other atoms found in nature. In its pure form it can result in an extreme reaction with water and in finely-powdered form can ignite to produce strontium oxide and strontium nitride. However, it is never in pure form when found in nature. It is contained in several mineral forms including celestite (SrSO4) and strontianite (SrCO3). Natural physical processes over the billions of years have concentrated these compounds into some sedimentary rock layers and can be mined. World production is about 140,000 tons per year. Its main use is in the special glass used in TV screens and visual display technology. Some other uses include pyrotechnics (fireworks), warning flares, and common greases. The radioactive form (Sr-90) also turns out to be valuable and has been used in some space vehicle technology, remote weather stations and navigation buoys.
Due to mining and natural weathering of Strontium-bearing sedimentary rocks, the element is found uniformly in all soils. As a result, it is in many foods – in low levels such as with corn (0.4 ppm) and oranges (0.5 ppm), and high concentrations such as with cabbage (45 ppm), onions (50 ppm), and Lettuce (74 ppm). On the average, each of us ingests 1.9 milligrams of Strontium each day, and we carry about 320 mg in our bodies. Daily intake of Strontium of less than 5 milligrams is considered harmless. The biological half-life varies with the compounds of Strontium we might take in, running from 14 days to as much as 600 days. To put it simply…once ingested, it tends to stick around. It should be noted that Strontium compounds are typically found in river waters (~ 50 ppb) and seawater (8ppm).
While Strontium-itself is not water soluble, some compounds containing Strontium are soluble. For example – SrCO3 has a solubility of 10 mg/liter and strontium chromate a solubility of 9 mg/l. Strontium is generally immobile in the environment, unless it is part of a soluble compound. Even in soluble form, it will rapidly precipitate and can be filtered out of the stream by the soil it might pass through. Strontium is not known to be carcinogenic or mutagenic. It is believed that health effects might occur with continual uptakes in the “thousands of ppm” range.
Thus Strontium itself is not worthy of being called “toxic”. However, radioactive Strontium (Sr-90) is a Beta-emitter and has a finite level of toxicity due to its radioactive emissions, and for no other reason. This also seems the case with Cesium, covered in the earlier commentaries. Is the presence of radiation itself worthy of invoking the term “toxic” with almost every Press report about water contamination at F. Daiichi?
The definition of toxic is “containing (or being) poisonous material especially when capable of causing death or serious debilitation.” The main synonyms are “poisonous or venomous”. None of the synonyms include anything about radiation itself. There is no doubt that massive radiation exposures can cause Acute Radiation Syndrome (ARS), which can be called “poisonous or venomous”. However, such huge exposures have only occurred with bomb fallout and some of the emergency workers at Chernobyl. Regardless, can the toxic moniker be reasonably applied to the wastewaters and/or contaminated groundwater at Fukushima?
Let’s look water that leaked out of the storage tank because there is actual data to utilize. We should keep in mind that the water in the tank had the Cesium filtered out of it. The health standard for water-borne Strontium is an internal exposure of 0.04 millisieverts per year. Limits on internal Sr-90 exposure are set more than 100 times below levels that have shown a few negative health effects, such as anemia and oxygen shortage. The lowest Sr-90 concentration believed to have a tiny possibility of inducing cancer or causing irreparable genetic damage is at least 10 times higher. In order to reach internal exposure levels that might result in ARS, someone would have to drink water straight from the leaky storage tank. How likely is that?
300 tons of water may have leaked out of the tank. That’s a lot of water. While the total number of Becquerels contained in the water was at least 24 trillion (all 60 contained isotopes, including Strontium), most of the escaped wastewater was absorbed into the soil where natural filtering and ion exchange removed most of the radioactive isotopes. Anything in the leaked water, other than Tritium, was filtered down to an extremely low concentration. How low? The nearby drainage ditch leading to the sea had between 200 and 500 Becquerel per liter readings in its residual water, and 21 Bq/liter at the ditch’s outlet to the sea. At those very low concentrations, drinking a liter or more of the ditch’s water would harm no-one. Again, how likely is it for that to happen? Eating the dirt holding the filtered-out Strontium is entirely different – but who eats dirt?
Is the level of radioactivity that leaked from the tank at Fukushima Daiichi, and the low level waters that flowed through the drainage ditch, worthy of being arbitrarily labeled “toxic”? And, what about the concentrations of groundwater suspected to be flowing into the Pacific? Given what you now know about Strontium…what do you think?
References:
1. Strontium – Sr; Lenntech Water Treatment Systems; Rotterdam, The Netherlands. 2013. http://www.lenntech.com/periodic/elements/sr.htm
2. Strontium (Sr) and Water; Lenntech Water Treatment Systems; Rotterdam, The Netherlands. 2013. http://www.lenntech.com/periodic/water/strontium/strontium-and-water.htm
3. Strontium-90; Washington State Department of Health, Office of Radiation Protection; July 2002. http://www.doh.wa.gov/Portals/1/Documents/Pubs/320-076_sr90_fs.pdf
4. Major Contaminants; Washington State Department of Ecology. http://www.ecy.wa.gov/programs/nwp/gwcontaminants.htm
