Radionuclides of cosmic (3H, 14C…) and tellurian origins (members of the uranium and thorium groups), together with 40K, are present naturally in the environment and constitute a source of exposure for the population. However, the proportion involved in drinking water contamination is usually low and mainly due to 40K.
Groundwater radioactivity is mainly caused by the radium that is present in all rocks. Radium is an element that is not very soluble. However, its descendant, radon 222 (a chemically inert rare gas that is odourless and colourless) is highly soluble in cold water under pressure and, therefore, is easily conveyed up to the water tap.
The presence of other radionuclides such as uranium, thorium, lead and polonium comes from granite rocks, uranium, lignite and phosphate deposits. The main forms are uranium 238 which can be found at levels of more than 99% in deposits and its descendant, uranium 234.
As an illustration, table 6 provides ranges recorded in French water; even higher levels have been recorded in water in Spanish (60 Bq·L–1) or Finnish (100 Bq·L–1) granite terrains.
Radionuclides present in the atmosphere attach themselves to aerosols and are carried away by rainwater: 3H and 222Rn mainly, with argon, beryllium, phosphorus.
The solubilisation of radiolements present in the soil will also show up as radon and uranium … Surface water radioactivity is usually quite low and any radioactivity found in this water is caused by human activities.
Downstream from the sites, radioactivity levels will be high, regardless of any operations. In the regions concerned, water drawn for consumption from individual wells is a common practice whereas it should be compulsory for it to be processed through a treatment plant. Surface water contamination can be seen as increased mineralisation and radioactivity that is primarily caused by:
- 230Th, 226 Ra (α emitters);
- 228Ra,210Pb (β emitters).
It should be noted that gold mine discharge often creates identical problems because of the frequent presence of uranium (see mining industries).
Most b emitters (excluding potassium 40 and radium 228) are linked to Man’s military “nuclear” (atmospheric nuclear testing), industrial (energy generation), medical (radiodiagnostic and treatment) and research activities. The following are the main radionuclides discharged into the environment:
- 58Co, 60Co, 54Mn, 3H (liquid effluent produced by nuclear power stations);
- 134Cs, 137Cs, 90Sr, 3H, 106Ru, 131I, 239Pu (gas effluent produced by power stations and atmospheric atomic tests);
- 131I (Hospitals).
The normally low contamination levels occurring in surface water can have a number of causes: atmospheric fallout, deposits leached into the soil through runoff water, liquid effluent discharges, accidental emissions; for instance, the products discharged into the atmosphere by the Chernobyl power station disaster were evaluated as 5·1017 Bq iodine 131 and 7·1016 Bq caesium 137.
Tritium3H (T) is produced naturally in the atmosphere. However, this radionuclide is also discharged in large amounts into the environment (power stations, nuclear explosions).
In the environment, 99% of tritium is found as tritiated water ("THO"): it diffuses through the soil, is easily combined with biological fluids but, unlike other radionuclides, does not attach itself to sediments or suspended solids.
In general, water tritium levels will be as follows:
- rain water ~ 2 Bq·L–1
- groundwater < 10 Bq·L–1
- surface water < 20 Bq·L–1
As a reminder, the European standard on drinking water has set 100 Bq·L–1 as the maximum tritium content.