Answer to Question #11022 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I have a private well that was drilled into highly fractured granite. I have had the raw water tested and it has high levels of radon (440–850 Bq L-1), uranium (700–1,200 µg L-1), 226Ra (0.48 Bq L-1), gross alpha (35 Bq L-1), and gross beta (9.3 Bq L-1). No 228Ra is present at detectable levels.
I have a whole-house reverse osmosis system, prefiltered through a sediment filter and a cation softener. The reverse osmosis system is followed by an aerator to remove radon from the water, and we also have remediation for airborne radon.
The reverse osmosis system flushes at a 1:1 ratio; the cation system flushes with about 110 L of salt for every 1,900 L of water. The effluent “brine” exits into the perimeter drain, passes through underground perforated piping, and daylights into a dry well away from the house.
I have measured gamma rays, x rays, and beta particles passing through the 1.3-cm plastic of the sediment filter (upstream from the softener and reverse osmosis system) using a Geiger counter. When the water is running, Geiger counter readings are up to 1,000 counts per minute or 5 µSv per hour. When the water is not running, Geiger counter readings are about two times background.
Given this data, can I estimate the radioactivity of the effluent? I am concerned about the level of radiation from both the raw water before it goes into the sediment filter and the concentrated effluent accumulating in the ground outside the house over time.
It was good that you had the forethought to have your water tested! This is clearly not water that you want to be drinking on a regular basis without treatment. You indicate that you also have remediation for airborne radon in place. I take that to mean that you have a subslab depressurization system or something similar in your home. The primary hazard from radon is breathing it, not drinking it. Based on what you are saying, I am assuming that the level of radon in the air in your house is less than the Environmental Protection Agency’s recommended limit of 0.15 Bq L-1.
So you have two questions. First, can you estimate the effluent concentration? In a word, I would say “no,” at least not with your Geiger counter. As you indicated, the Geiger counter is only effective for gamma rays, x rays, and beta particles. Depending on the counter that you have, it may not be very effective at detecting beta particles.
No matter what type of Geiger counter you have, however, it cannot detect alpha particles in your water under any condition. It is the alpha-emitting radionuclides that are the issue here. If your water treatment system is effectively removing the alpha-emitting radionuclides then it will be removing the beta-emitting radionuclides as well. No convenient means to measure an alpha emitter in water is available to the nonprofessional.
You had the raw water characterized, so I recommend that you have the treated water characterized. You need to have only gross alpha/gross beta and radon quantified in the treated water, and then you can use that result to calculate the removal efficiency of your water treatment system. At the same time that you are drawing this treated water sample for analysis, you can measure your treatment system with your Geiger counter. Henceforth you can test the continued efficiency of your treatment system by scaling your Geiger counter measurements to your removal efficiency. The Geiger counter reading that you indicated of the water treatment system seems normal to me and an indication that the system is running properly. If the external count rate drops when you measure your treatment system then the removal efficiency is not as good as it used to be and should be checked by a professional.
Unfortunately the Geiger counter does not provide a sensitive measurement; your best assurance that the water is safe to drink is to have the treated water tested annually. There is little hazard to an individual standing by the water treatment system when the water is running. I can only assume that the system was placed in a location that was not conducive to spending a lot of time around anyway.
Your second question is what to do with the brine solution from the water treatment process. This brine solution will have a higher concentration of radionuclides than the raw water. Assuming that the dry well and the perforated pipe are more than 30 cm below the surface of the soil, you will have all the shielding that is necessary to mitigate any external radiation dose. To say that in a different way, if the pipe and tank are at sufficient depth, then you will not be able to locate them with your Geiger counter. The soil will likely remove the radionuclides from the brine and concentrate them in the soil. So you are effectively just putting the radioactive material back where it came from.
The concentrations of radioactive materials in your water are high but not so high that they cannot be effectively treated. So assuming that the water treatment system is functioning properly, the water should be safe for consumption.
Timothy A. DeVol, PhD, CHP