Radon gas comes from the radioactive decay of uranium which occurs naturally in all rocks and soils.

Radon is soluble in water and thus may accumulate in water as it passes over rocks and through soil. As radon is a gas it is easily lost from water when it is agitated for example when river or lake water flows over rocks or is moved by wind. This is very like what happens when a bottle of fizzy mineral water is shaken; the bubbles released represent the loss of dissolved gas. In this way surface water (lake and river water), which currently accounts for approximately 75% of drinking water supplies in Ireland, normally shows very low levels of radon.

Ground water (from wells and bore holes) on the other hand, does not tend to be agitated before reaching the domestic tap, so if there is radon present it tends to remain in the water. When the domestic tap is turned on some of the radon will be released into the air due to the agitation of the water as it is dispensed from the tap, but some will remain dissolved in the water and may be consumed.

Health implications of radon in drinking water
Ingestion of dissolved radon will result in a radiation dose to the lining of the stomach. However, a definitive link between consumption of radon-bearing water and cancer has not been established. Inhalation of radon gas that has been released from tap water will contribute to the radon content of indoor air and, if inhaled, will result in a radiation dose to the lung. Long-term exposure to high concentrations of radon in indoor air increases the risk of lung cancer.

Globally, the radiation dose from radon in drinking water is low compared with that from the inhalation of radon gas. However it is prudent to regard both of these sources of radon with similar concern and reasonable measures should be taken to minimise exposure to radon in drinking water. For this reason the Institute advises that domestic water supplies of groundwater or partial groundwater origin should be tested for radon, and if the recommended standards are exceeded the supply should be remediated.

It should be noted that while the Institute estimates that approximately 91,000 houses or 7% of the national housing stock have levels of radon in air that exceed the National Reference Level, less than 3% of the groundwater supplies tested to date contain radon levels that exceed the recommended standards.

Recommended Standards
Guidance on levels of radon and its decay products in drinking water is covered in EC Recommendation (2001/928/Euratom) on the protection of the public against exposure to radon in drinking water supplies. The Recommendation proposes 1000 becquerels per litre (Bq/l) as an action level for both public and private water supplies and can be summarised as follows:

For public water supplies, if the radon concentration exceeds 1000 Bq/l, remediation is considered justified. Where the radon level is above 100 Bq/l but below 1000 Bq/l, the local authority must consider whether this poses a risk to human health. If it is concluded that such a risk exists, then remedial action should be considered.

For private water supplies, where water is found to have levels of radon in excess of 1000 Bq/l remediation of the supply should be considered