Radon and radioactivity

Chapter 12 Radon and radioactivity

12.1 Introduction The main source of radiation is from naturally occurring radionuclides. In fact,

the combined effect of all these natural sources accounts for, on average, 87% of the total radiation exposure to a person over their lifetime. So man-made sources only account for 13% of radiation exposure, of which 11.5% is from medical sources, 0.5% from fallout, 0.4% due to occupational exposure and 0.1% from nuclear discharges, with various other miscellaneous sources accounting for the remainder. Of the natural sources, radon accounts for 32% of this total exposure. The majority of water supplies have very low levels of radionuclides that represent no health risk. Those that are a cause for concern are due to naturally occurring radioactivity. Many types of rock contain mildly radioactive elements, which are known as parent radionuclides, that decay producing other radioactive contaminants known as daughter radionuclides. Depending on their chemical properties both can accumulate in water sources, often at dangerous concen- trations. In the environment the parent radionuclides may behave differently from their daughter radionuclides resulting in very different patterns of occurrence in water resources. For example, groundwater with high radium levels tends to have low uranium levels and vice versa, even though uranium- 238 is the parent of radium-226.

The intensity or activity of a radionuclide was originally measured by the curie (Ci), which is still in use in the USA. Elsewhere this has largely been replaced by the becquerel (Bq). In practice the Ci is too large for use with normal environmental

monitoring so the picocurie (pCi) is used, so 1 pCi ¼ 10 12 Ci. Conversion of the two units is 1 Bq ¼ 2.7 · 10 11 Ci or 1 Ci ¼ 3.7 · 10 10 Bq.

Uranium-238 series radionuclides are the major contributors to natural radiation associated with drinking water, the most important being radium-222 (radon), which is also released from water into the air. Other long-lived radionuclides include uranium-234, radium-226, lead-210 and polonium-210, which are mostly alpha-emitters, as well as their shorter-lived progeny that also emit beta and gamma radiation. Uranium-235 is a minor fraction of natural uranium also found occasionally in water.

Radon and radioactivity

12.2 Radon Radon is a natural radioactive gas that has no taste, smell or colour; in fact, special

equipment is required to detect its presence. Analysis of drinking water for radionuclides is normally by an ultra-low liquid scintillation counter equipped with an alpha–beta discrimination device (Forte et al., 2007 ). Radon is formed in the ground by the decay (breakdown) of uranium, which is found in all soils and rocks to some extent. The highest levels of uranium-238 are found in areas of granite with maximum concentrations in the UK, for example, just below 2 ppm (parts per million) in granite from Devon. The decay of uranium-238 results in the formation of radium-226, which subsequently decays to radon-222(Rn). The radon gas slowly migrates through the soil to the surface and is quickly dispersed into the atmosphere, where it is diluted to safe concentrations. Concern has been expressed, however, over modernized houses that are well draught-proofed. Radon percolates up through the foundations of the building, or occasionally from the granite stone used to build the house, and because there is so little air exchange in the building, the radon can accumulate to dangerous concentrations in the enclosed space of the room. Even where newly constructed buildings have adequate radon-proof membranes laid in the foundations to stop the gas from percolating through, radon gas can still accumulate in the buildings from the use of well water in the bathroom and kitchen. Radon is known to be carcinogenic, and as it is primarily inhaled as a gas it causes lung cancer. The US Public Health Service considers radon to be a major environmental health problem and evidence shows it to be the second major cause of lung cancer after smoking. The US Environmental Protection Agency (USEPA) suggest that when ingested with water radon can also increase the risk of stomach cancer (Cross et al., 1985 ).

Radon is very soluble in water, so when the gas comes into contact with groundwater it dissolves. It is occasionally seen at elevated levels in bottled mineral waters (Section 29.2 ). For example, in a study of 28 commercially available mineral waters in Hungary, six had activity levels >0.1 Bq l 1 with one with a mean activity level of 2.9 Bq l 1 (Somlai et al., 2002 ). In a national survey of groundwaters in the USA the average radon level was 900 pCi l 1 (median 300 pCi l 1 ), although in some states the mean value was considerably higher (e.g. New Hampshire 1716 pCi l 1 ). Some groundwaters have been found to contain in excess of l 00 000 pCi l 1 , with levels in excess of 10 000 pCi l 1 common in the west and north-east of the USA (Dupuy et al., 1992 ; Helms and Rydell, 1992 ). Levels in European groundwaters are largely unknown and it is only relatively recently that radon in drinking water has been taken seriously in Europe.

Radon can be consumed by drinking contaminated water or through the inhalation of radon released from water. Radon is primarily released from water when agitated, with up to 40 and 30 times higher concentrations recorded in the bathroom and kitchen respectively, than the living room, due to the radon released from flushing the toilet and running large volumes of water.

12.3 Non-radon radionuclides

Some groundwaters may contain high levels of radon, but those that are affected will mainly be those on private supplies, especially sealed boreholes. This water will have lost very little of its natural radiation before reaching the house. Having a water storage tank in a vented attic significantly reduces radon levels, and if radon is a problem, then consideration should be given to supplying all the water from the attic storage tank. Installing a large aquarium-type aerator in the tank will help to release the radon, whereas installing a small activated carbon filter will remove the radon from the supply to the kitchen tap, which is used for drinking. Installing even a small aquarium-style aerator in the water storage tank will cause some degree of noise, especially at night, so a time switch is required to turn it off at night. Alternatively the tank and motor can be insulated to reduce the vibration that causes the noise. The implications of doing this should be discussed with, and approved by, the local authority or water inspector. Of course, as the aerator will cause considerable mixing of the water, all sediment and debris must

be removed from the tank before installation or two tanks should be used in series, the first for radon removal and the second for settlement. Alternatively, as iron and manganese will be constantly precipitated out in the tank the installation of a point-of-use physical filter will be most effective in most cases.

12.3 Non-radon radionuclides Apart from radon-222, other radionuclides are occasionally found in water; for

example, the alpha-emitting radionuclides radium-226, which is associated with an increased risk of bone and head carcinomas, and uranium-234 and -238 associated with an increased risk of bone cancer (Longtin, 1988 ). With the exception of radium-228, most beta-emitters are associated with human activity, whereas the alpha-emitters are of natural origin and so are far more likely to be

detected in groundwaters (Table 12.1 ).

Generally the concentration of radionuclides in drinking water is very low compared to the maximum contaminant levels (MCLs) in the USA although some parts of the mid-West have significantly higher average combined radium- 226 and -228 levels than the rest of the country. This is also true for some Western states that have elevated uranium levels compared to the national average. In a national survey of groundwater supplies in the USA mean uranium

1 with about 3% of water supplies with

concentrations

1 (Longtin, 1988 ). Elevated concentrations are found

in groundwater in the Colorado Plateau, Western Central Plateau, Rocky Mountain System, Basin and Range, and the Pacific Mountain System. Pockets

of elevated uranium are found scattered in South Carolina, Connecticut and other eastern states, although in general uranium concentrations are low in groundwaters in eastern USA. For example, in one study of the groundwater used by residents in Greenville County in South Carolina, the mean uranium

1 (Orloff et al., 2004 ). Other

Radon and radioactivity

Table 12.1 Radionuclides found in drinking water that have a significant health risk

Alpha-emitting radionuclides 210 Po

Beta-emitting radionuclides

a Also have alpha-emitting daughters.

radionuclides have been reported in a small number of drinking water supplies, although compared to radium-226, radium-228 and uranium their occurrence is thought to be rare.

It is inevitable that some water supplies are located in areas that have potential sources of man-made radioactive contamination from facilities that use, manufacture or dispose of radioactive substances. While water supplies can readily become contaminated through accidental releases of radioactivity or through improper disposal practices, there is also some evidence that water supplies can become contaminated from the normal operation of nuclear power plants and reprocessing facilities. There is understandable concern over the proposal to store radioactive wastes underground at Sellafield in the UK, where there are extensive aquifers in the area important for water supply. Water utilities using supplies vulnerable to radioactive contamination are required to perform extensive monitoring for beta particle and photon radioactivity to ensure the safety of drinking water.

Ionizing radiation is perhaps the most established human carcinogen (IARC, 2001 ). Alpha and beta radiation have a short range compared to gamma radiation, and so need to enter the body in order to damage internal organs. Radionuclides are largely excreted in the urine and so the kidneys and bladder are particularly at risk (Kurttio et al., 2006 ). Between 0.8% and 7.8% of uranium ingested via drinking water is absorbed, concentrating in the bones and to a lesser extent in the kidneys, liver and other soft tissue, the rest is rapidly lost from the body via the urine although Orloff et al. ( 2004 ) found that uranium can go on being excreted for up to ten months after the contaminated water had been replaced. Prolonged exposure to drinking water containing alpha-emitters in excess of the USEPA MCL increases the risk of getting cancer, while uranium, which is also carcinogenic, also has toxic effects on the kidney. Beta- and photon-emitters in excess of the MCL are also carcinogenic.

12.4 Standards and treatment The EC Drinking Water Directive (98/83/EEC) includes radioactivity under

Indicator Parameters in Part C (Appendix 1). This sets maximum values for

12.4 Standards and treatment

tritium at 100 Bq l 1 and the total indicative dose at 0.1 mSv yr 1 . Total

indicative dose is measured excluding tritium, potassium-40, carbon-14, radon and its decay products, but including all other natural decay series radionuclides. While radon and its decay products in drinking water are not covered by the Directive, an EC Recommendation (2001/928/Euratom) on the protection of the public against exposure to radon in drinking water supplies proposes an action level for both public and private water supplies of

1000 Bq l 1 (EU, 2001 ). The action level is considered similar to the risk that

would arise from breathing air containing radon at 200 Bq m 3 . Where radon levels in water are above 100 Bq l 1 but below 1000 Bq l 1 , then 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. The EC Recommendation makes it clear that the action level does not mark a boundary between safe and unsafe, but rather a level at which action will usually be justified. In Ireland, if the radon level in public water supplies

exceeds 500 Bq l 1 , then remediation is considered justified for the protection of human health. The World Health Organization (WHO) specifies guideline values for gross

alpha and beta activity of 0.5 and 1.0 Bq l 1 respectively. If samples exceed

these levels of activity then further radiological examination is recommended, although higher values do not necessarily imply that the water is unfit for human

1 has been set for uranium

(WHO, 2004 ). The USEPA has proposed a combined MCL for radium-226 and radium-228 of 5 pCi l 1 and has set a separate MCL for uranium as well as alpha and beta activity (Table 12.2 ). Although the National Primary Drinking Water Regulations do not apply to private water supplies, they do apply to small water schemes serving more than 25 people (or with 15 or more service connections) using groundwater.

Individual supplies will require a point-of-entry treatment system where radon is present. In a study carried out by the USEPA, granular activated carbon (GAC) was compared with various bubble aeration systems to treat a groundwater supply

containing 35 620 pCi l 1 radon. It was found that removal by GAC decreased

over time from 99.7% to 79%. This was improved if the activated carbon was preceded by ion-exchange, although the performance still decreased from 99.7% to 85% over time. Ion-exchange was necessary to remove iron, which was impeding radon adsorption by fouling the surface of the GAC. In contrast, the bubble aeration systems were highly efficient (>99%) at removing radon from water without any loss of efficiency over time (Kinner et al., 1990 ).

There is no risk with mains supply, even if the supply is from an aquifer contaminated by high levels of radon. Water treatment of groundwaters normally includes aeration so that 99% of the radon is removed at this point. The removal efficiency varies with the aeration technique, with packed tower

Radon and radioactivity

Table 12.2 Current maximum contaminant levels (MCLs) for radionuclides in US Drinking Water. Reproduced by permission of the US Environmental Protection Agency

Combined radium-226 and

5 pCi l 1 Naturally occurs in

radium-228

some drinking water sources

Gross alpha activity

15 pCi l 1 Naturally occurs in

(not including radon

some drinking water

or uranium)

sources

Beta particle and photon

4 mrem yr 1 May occur due to

radioactivity

contamination from facilities using or producing radioactive materials

Uranium

1 Naturally occurs in some drinking water

sources

aeration particularly effective ( >99% removal), compared with only 60–70% removal when using spray aeration. Water treatment plants also use GAC filters

to remove radon and other radioactive isotopes. However, there is a concern that filters could become radioactive and then be a major disposal problem.