19. If both the gross alpha and gross beta screening levels are not exceeded, the individual dose criterion (IDC) of 0.1 mSv in a year shall also not be exceeded, except under exceptional circum-stances (see below). Monitoring of drinking water shall continue at the locations and frequency that has been agreed with the water quality regulator for radiological parameters.
(a) If it is suspected that the water may contain a radionuclide that will not be detected by the screening methods. Table 2 provides information on the main radionuclides that will not be detected by the gross alpha and beta screening methods. Different sources of information may indicate that radionuclides in drinking-water could be present but would not be detected e.g. other environmental monitoring data in the area and the catchment that the water is drawn from, knowledge of sites that could have led to discharges of radionuclides to the catchment and local geology. In this case, radionuclide-specific measurements should be made and compared with the relevant guidance values.
(b) There are a few naturally occurring radionuclides (notably Ra-228 and Po-210) where the IDC of 0.1 mSv per year could be exceeded, even if the screening levels are not exceeded, in the exceptional situation where these radionuclides are the only significant contributors to the to-tal activity concentration.