Frequently Asked Questions
- Is my water safe to drink?
- What does that " +/ - " on my report mean?
- Gross Beta in mrems/yr?
- What does "combined radium" mean?
- What does "total radium" mean?
- What is that green stuff on the water fountain?
- How does sediment affect radiochemical results?
- Whatvdoes "gross alpha, excluding radon and uranium" mean?
First of all, this is a perfectly good question.
Secondly, no laboratory can answer it, fully and completely. There is an almost infinite number of possible contaminants that would make water unsafe to drink. The Lange's Handbook on my bookshelf contains a list of some 600 pages of organic chemicals-with a dozen or more chemicals per page- most of which should not be consumed. To say nothing of inorganic chemicals, radionuclides, or microbiological contaminants. Fortunately, for most of these contaminants, the chances of their being in the water supply are remote. But you hire a laboratory to test your water, not speculate about it's safety.
So can you test for a zillion contaminants? No; c'mon. Most analytical procedures measure a single chemical (or radionuclide, etc). There are some chromatographic procedures which can measure dozens of compounds; yet still the conclusion will be far from compete. And, as you can guess, the cost increases with every test. Got a zillion dollars?
A more practical and affordable approach is to focus on the usual suspects- the contaminants most typically found in water from your geographic location. Out west, Arsenic and Selenium are heavy metals of concern, naturally found in the water. In Florida, bacteria is the most common problem, the sub-tropics being the perfect growing environment. In many areas, where agricultural activities occurred past or present, nitrate (from fertilizer) is commonly found. We recommend contacting your local dept of health (or whatever the agency is called in your state) and ask what the primary risks are. Then, your laboratory can help.
What does that " +/- "on my report mean?
[re: Radiochemical results reported in the format 12.4 +/- 1.6 pCi/l ]
The short answer is that it is the counting error, expressed as an interval at the 95% confidence level (1.96 sigma ). The plain English answer takes a little longer. Let me use an example we're all familiar with, the opinion poll. In order to predict the outcome of an election in advance, news organizations typically survey a subset of voters, and report the results accompanied by a statement like "the survey has a margin of error of +/- 4%". What does this mean?
There are two types of error that affect a survey: sampling error and non-sampling error. Sampling error occurs when the entire population of interest is not included in the survey (you didn't think they spoke to all those millions of voters, do you?). Typically, a voter survey might only include 1000-1500 responses. This introduces sampling error, and can be quantified by well-defined statistical techniques. Non-sampling error can occur for any number of reasons: perhaps the question was confusingly worded, perhaps the response actually came from an underage non-voter, whatever. Non-sampling error cannot be mathematically quantified. It can, however, be minimized by sound methodology and experience. Since we do eventually find out the "true" values, when the election results come in, improvements in the survey methods can be made.
Back to radiochemistry. The < result > +/- < sampling error > quantifies the 95% confidence level of the measurement. Like the voter poll above, all radioactive events are not measured: measuring half the radioactive decay of Radium-226 would take 1600 years for example. Typically, measurements are made only for a few hours, leading to sampling error, which is reported along with the result. Non-sampling error is minimized like any other analytical procedure: analysis of known standards provides a basis for development of reliable methods.
Community water systems have recently been required by regulatory agencies to provide a "consumer confidence report" to users of the water they produce. Systems which monitor for gross beta have laboratory results expressed in units of picocuries per liter (pCi/l). The CCR form requests them in units of exposure, millirems per year (mrem/yr). Can the gross beta measurement be converted to mrems/yr?
The CCR form is wrong. Even if one estimates the amount of water a person drinks, and makes a few other simplifying assumptions, radioactive exposure cannot be calculated without the energy of the particle of interest. Gross beta provides no such information; it represents a mixture of nuclides with various energies: that's why it's called "gross". So the measurement cannot be converted.
Here's the correct procedure: EPA rule (40 CFR 141.26) allows for gross beta to be used as a screening procedure. If the gross beta measurement is below 50 pCi/l, the water is assumed safe for potable use. If above 50 pCi/l, the water system should be analyzed for Tritium and Strontium-90, and it is these measurements (with discrete energy values for these specific nuclides) which should be converted to units of exposure (mrem/yr.) to determine compliance.
To get a rough idea how particular measurements would compare to the exposure standard, here is an excerpt from the drinking water regulations:
Average Annual Concentration Assumed to Produce an Exposure of 4 millirem/year:
Tritium in the total body - 20,000pCi/1
Strontium-90 in the bone
marrow - 8pCi/1
What does "combined radium" mean?
In reality, it doesn't mean anything.
However, in the world of EPA compliance, it means the arithmetic sum of Radium-226 and Radium-228, results expressed in units of pCi/l. The 95% confidence level ( ±
Radium-226 is an alpha emitter, Radium-228 is a beta emitter. Both nuclides have alpha and beta-emitting daughter nuclides down the decay chain. Neither measurement includes the short-lived Radium isotopes Ra-223 and Ra-224 (both alpha's). It has never been made clear by the regulations what adding these numbers mean, and EPA has proposed changes to the radiochemical regulations to drop this issue.
What does "total radium" mean?
Probably not what you think it means. There are 4 naturally-occurring radioactive radium isotopes found in measurable quantities: Radium-223 (alpha emitter, half life = 11 days), Radium-224 (alpha emitter, half life = 3.7 days), Radium-226 (alpha emitter, half life = 1600 years), and Radium-228 (beta emitter, half life = 5.8 years). One might think that "total radium" somehow quantifies these 4 isotopes.
EPA 40 CFR 136.3. table 1E stipulates the method EPA 903.0 for "radium, total". The method itself, however, is actually entitled "Alpha-Emitting Radium Isotopes". And so it is: the method measures Ra-223, 224, and 226, quantified as a single measurement in pCi/l. Because of the relative half-lives of the isotopes, Ra-226 tends to dominate the measurement in natural waters. Ra-228 is not measured.
What is that green stuff on the water fountain?
It's copper; more specifically, copper salts from reactions with minerals in the water. The heat exchanger of the refrigeration unit of the fountain uses copper tubing- it conducts heat well and can be easily shaped and connected. The problem comes from the corrosive effect of the water on the tubing.
The EPA action level for copper is 1.3 mg/l. Most people notice a metallic taste in the water when it reaches 0.5 mg/l or so. It is considered by EPA to be more of a taste/staining nuisance than a health risk.
From our observation, this problem is quite widespread. It does not have to be this way. Bottled water companies often distribute their product in 5-gallon containers, together with a refrigeration unit for cold-water dispensing. Not a trace of green stuff there. I spoke with a vendor from Zephyrhills (a popular brand around here) and was told the metal surfaces were stainless steel. This is apparently all it takes to solve this corrosion problem.
How does sediment affect radiochemical results?
Re: the effect of sediment on measurements of radionuclides in water samples
Virtually all of the predominant naturally-occurring, alpha-emitting radionuclides (including radium) are elements commonly described as "heavy metals". As with many heavy metals, the earth's crust contains these elements in abundance; their concentration in water is typically quite low, however, due to their limited solubility at the pH of most natural waters. For this reason, a water sample which contains sediment can yield results for "total" metals or radionuclide analyses which are easily misunderstood. The analysis measures the entire concentration of the elements in the sample, yet the results are reported in terms of the water sample only. It has been our experience that for samples of this type, most of the heavy metals are actually contained in the sediment, not the water. It has been our laboratory's practice to make note of this on the analysis report to assist data interpretation. If it is desirable to measure the activity of the water only, the correct procedure would be to filter the sample prior to acidification. Laboratory results would then be reported on a “dissolved analyte” basis. If sample data is being used for compliance monitoring, consulting the regulatory agencies involved is recommended.
What does "gross alpha, excluding radon and uranium" mean?
Both federal and state regulations use this phrase in setting down the maximum contaminant level for gross alpha in drinking water (15 pCi/l). Unfortunately, there is a disconnect between the methodology and the regulation here. The method specified in the rule for gross alpha excludes radon (a gas which simply evaporates away), but includes uranium (the method makes no attempt to separate nuclides). So how does one obtain this measurement?
One option is to measure gross alpha and uranium separately, and report the difference, worded as above. The additional cost would defeat the whole purpose of using gross alpha as a screening procedure for Radium-226. What we recommend is simply measuring and reporting gross alpha, as per the method. Only if this measurement exceeded 15 pCi/l should uranium enter the picture. At that point, a separate measurement for uranium could be made, and determine whether the water system was within compliance limits.