Answer to Question #6477 Submitted to "Ask the Experts"

Q

We are organic farmers in Maine and need to find a new source of irrigation for our fields. There is a drilled well on our property that we have never used since the water in it has high levels of radon (we plan eventually to buy a radon filter so that we can use the water for the house, but have not found a filter that seems reliable and thorough enough for us to be comfortable with). Our question is: Is this water safe to use on crops? We would mostly be irrigating with drip irrigation (i.e., not spraying the water through the air, but running it through tubes with holes in them directly on the soil). Could the radon damage, at some level, the plants, the vegetables, the soil, or the people who eat the vegetables or work with the plants? We are having a very dry spring and are eager to find a new source for irrigation but want to do the right and safe thing!!

A

Granitic rocks, as found in Maine and other locations in the United States, release radon from their surface. Water in rocky areas has reached the well by moving through a network of rock fractures. The small amount of water in the fracture is in contact with large surface areas of rock, which leads to high radon concentrations in the water. Radon is a noble gas and, as such, is not chemically reactive so it is unaffected by the water chemistry that usually precipitates the other radioactive elements that are present in the rock. This means that high radon concentrations are not a sign that there are other radioactive materials present in the water.

The mineral component of soil also releases radon, so soil water naturally contains some radon. If the well water is used for drip irrigation, you are right to expect that the radon concentration in the soil water will increase, but the resulting increase in radiation dose to the plant roots will be very small and far too low to affect their growth or the bio-flora in the soil. The concentration of radon in the plants will decrease rapidly once the plant is harvested, so the additional amount of radon that someone might take in from food will be negligible.

As the water will be dripped onto the soil, most of the radon will remain dissolved in the water, and the radon concentrations in the air over the field will not be increased much over the current level, which is determined by the mineral component of the soil. Again, the effect on plant workers will be negligible.

The high radon concentrations do not make the water unsafe to use. However, conventional chemical composition tests (pH, dissolved elements, etc.) would verify that the water is suitable for irrigation.

Arthur Scott, PhD