Small water treatment plant with typical open top tank designs. Image credit:Flickr, Shnib Snuff
According to a report in the English version of Kyodo News, the Japanese government has asked ...treatment plants not take in rainwater, due to radiation risk. This is nationwide guidance. Underscore that national guidance part. The government has suggested covering up open water treatment process equipment with tarps, "or to use powdered activated carbon that can help get rid of radioactive materials." I explain why this is likely being done and comment on what it means, below.
This is over-simplified to satisfy the average blog post attention span: around 4 minutes. So please, no going after me if you are a health physicist. Comments of explanation always appreciated, though.
For the average American, exposure to dangerous levels of Japanese-emitted radio-nuclides appears unlikely right now, although radio-iodine is being detected in rain over America, per this story in the Boston Herald. Not that Americans should ignore the possiblility of worsening risk, should reactor emissions increase.
Here are some of the key subjects of concern relative to drinking water exposures in either place: Japan or America.
Why radio-iodine danger is high, if exposed via water or food.
Prevalent radio-iodine forms have a half life of a little over a week. Half is decayed away in a week and a few days, then the remainder is again halved in another week, and so on until after a few months little remains as radio-iodine. While radio-iodine is decaying however, it unleashes potentially cell destroying energy, leading to smaller but less hazardous radio-nuclides.
Important: Measuring the energy produced by radio-iodine decay in the open air or water with a Geiger Counter is fairly meaningless.
What low levels of radio-iodine are ingested or breathed, over time, they bio-concentrate in the thyroid gland. Low levels become high levels in that target organ. Decay occurs right up close to individual cells of that organ, so there is little or no dissipation of the rays before 'impact.'
Take home point:
For most people, rradiation intensity measurements, like with a Geiger counter, have at best an indirect relationship to human health risk posed by radio-iodine in the environment. To reduce the danger, (in this case) you have to either keep radio-iodine out of food and water as much as possible and/or take potassium iodide tablets to prevent bio-magnification in the thyroid gland. (By taking the pills, the radio-iodine is increasingly excreted instead of bio-magnified.)
Surface water versus groundwater versus direct rain.
I was tempted to substitute the term 'surface water' for "rain" in the headline of this post because most surface water - defined as rain-fed reservoirs, lakes, rivers, streams, and ponds - comes from rain or melting snow and many water plants draw on surface water.
The only reason to focus just on direct rain as a water source to a treatment plant is that rain landing in a treatment tank could have higher concentrations of radio-nuclides than in resulting runoff or receiving waters - due to dilution and settling. On the other hand, if Japanese water plants tended mostly to withdraw from groundwater sources, about the only route of entry to water supply of radio-iodine would be through direct precipitation, into uncovered treatment equipment.
Because we don't have any data on Japanese public water sources - dependency on groundwater versus surface water - it's impossible to fully understand sufficiency of the government recommendations.
It might be a stretch to assume that throwing a layer of powdered activated carbon on a water treatment plant's sand filters will prevent all radio-iodine from entry into the distribution system. It can't hurt to try, though, if paired with water treatment flocculants. That's probably why they also recommend covering up the treatment tanks.
Do not assume that the statements made above about radio-iodine equally pertain to radio-cesium or radio-strontium emissions (two other dangerous types of radio-nuclides that have been or may be emitted from the failing Japanese reactors). These have longer half-lives, different toxic endpoints, different solubility and different densities than radio-iodine.
What's in a particle?
I refer in this post to certain radio-nuclides as 'particulate' in form, assuming they are in some portion either agglomerations or amalgams or radio-isotopes , gaseous or solid, variously bound to soil material, or aerosols, or smoke particles from a variety of potential sources (more was on fire than just the power plant). Have not seen any data on this yet.
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