Answer to Question #9291 Submitted to "Ask the Experts"
Category: Radiation Basics
The following question was answered by an expert in the appropriate field:
We have a well drilled in bedrock and recently had a water test that showed a uranium level of 30 ppb, unfiltered. The level goes down to about 20 ppb after a standard water filter, but I borrowed a Geiger counter from a friend and on holding it up to the filter the instrument reads around 22 cps. How concerned should I be with this level of radiation in the home? The level is below the Environmental Protection Agency limit for drinking, though I am wondering about the filter being a source of radiation. Should I be looking for another water source?
The unfiltered level of uranium that you cite is at the Environmental Protection Agency (EPA) Maximum Contaminant Level as specified in the EPA Safe Drinking Water Act, which applies to public water supplies. Since the EPA does not regulate private water supplies, the recommended limits do not put any specific requirements on you, although you are correct in wanting to ensure that your water is safe to drink. You do not specify what type of filter you have installed, but it does appear to have reduced the uranium concentration somewhat. You may want to keep a reasonable check on this to see if the filter remains effective. I do not view the present level of uranium in the water as a cause for rejecting use of this well.
The interpretation of the reading of 22 cps that you obtained with the Geiger Mueller (GM) detector would require more information, particularly the manufacturer and model of the GM detector to translate the reading to a more meaningful biological impact quantity, such as tissue dose rate. Assuming that the GM is one of the more common varieties, I would judge that the reading does not represent a level of radioactivity that presents any significant health concern. I must point out, however, that the reading is not coming from the uranium, since the uranium does not emit significant radiations measurable with a GM under the likely conditions that prevail here. It is much more likely that the reading is associated with gamma radiation being emitted from radioactive progeny that result from the decay of uranium, and this prompts additional consideration.
The fact that you are seeing significant uranium in the water means, naturally, that there is uranium in the bedrock. You can also be fairly certain that there is radium (226Ra) in the bedrock since aged uranium produces the radium through a series of radioactive decay steps. The radium then decays to radon gas, 222Rn, which can permeate the rock—e.g., through small cracks and fractures—and enter the water much more readily than either the uranium or the radium. The radon decays to other species that produce significant gamma rays, and it is likely these radiations that are producing the elevated reading on the detector when you hold it near the filter. You might want to have the water tested for radon to make sure that it is not present at excessive levels.
While specific requirements do not apply, a rule of thumb for assessment purposes is that a water concentration of 0.370 Bq cm-3 of water may result in an air concentration of 37 Bq m-3. It is generally the air concentration that is of most concern because of the potential inhalation hazard (lung cancer) associated with the deposition and decay of radon’s radioactive decay products in the lungs. If you haven’t done any screening measurements to estimate radon air concentrations in your home, you should. The airborne level at which the EPA recommends taking more informative measurements and possible corrective action is 148 Bq m-3. If you find radon water levels approaching 0.370 Bq cm-3 you may want to consider mitigation methods to reduce the radon concentration in the water.
You should also attempt to make occasional measurements of the filter to determine whether there is any significant buildup of radioactive species on the filter. There have been instances of individuals who have had high radon concentrations in their well water that resulted in the buildup of significant radioactivity on the filter system. If the high radon levels are accompanied by high radium levels in the water, the radioactivity on the filter may build up on the filter and remain at undesirably high levels even after the filter has been removed from the system. If radium is not present in significant amounts, the radioactivity on the filter may be high when the filter is in operation but will decline fairly rapidly and continuously, likely decreasing by a factor of two every 30 to 60 minutes, after the filter has been removed.
I would encourage you to do some additional investigation to evaluate the possible significance of radon in the water (and air) and possibly radium in the water. Based only on the uranium measurements, I would judge your well to be acceptable, but the additional measurements are necessary to rule out other possible problems. Good luck.
George Chabot, PhD, CHP