A radioactivity monitoring study was conducted by the Institute for Water Quality Studies (IWQS) of the Department of Water and Sanitation (DWAF) in collaboration with a wide group of interested parties, in the Mooi River Catchment during 1997. The study served to establish the drinking water health risk, as well as the radiological status of the water resources, in the catchment from the viewpoint of drinking water. The intensive monitoring, both in time as well as in number of radionuclides measured served to clear up many areas of doubt, and has established with reasonable certainty the representative radiological status of the water resources in the catchment. The study covered surface streams and groundwater sources in the catchment. The evaluation of health risk was based on the levels of radioactivity in raw water samples that had been filtered prior to analysis, and on the use of such water for drinking purposes on a continuous basis. The relative contributions to the health risk from ingestion of the suspended solids in the water and from radiation exposure scenarios other than drinking water use were, with the possible exception of fish consumption, shown to be insignificant. The study did not consider radioactivity in sediments.
The radiological variables measured were all from the natural radioactive decay chains of uranium-238, uranium-235, and thorium-232. In addition to radiological variables, a full set of chemical variables was also monitored.
The radiation doses calculated in the study were based on the conservative assumption that the water at every sampling point was used continuously as the sole source of drinking water.
In view of the controversy surrounding the radiological status of water sources in the catchment, extensive efforts were made to validate the accuracy of the radiological measurements, as well as to cross check the validity of the total yearly doses calculated for each monitoring point. The total yearly dose was independently calculated by two different methods, which gave very similar radiation doses.
The set of dose criteria, used to evaluate the dose values found for drinking water, ranged from the ideal level of the World Health Organisation of 0,1 mSv/year, through the 0,25 mSv/year single facility dose limit used by the Council for Nuclear Safety, to the 1,0 mSv/year dose limit of the International Atomic Energy Agency for public exposure from anthropogenic sources. These dose criteria have been incorporated into proposed interim radioactivity water quality guidelines, with associated actions and interpretation.
The natural background radiation dose in drinking water in the catchment was estimated at 0,020 mSv/year. The great majority of sampling sites in the catchment showed a total drinking water radiation dose below 0,1 mSv/year, implying that no radiological problem exists from the viewpoint of drinking water. The general conclusion was that of the 41 sites monitored, 39 showed a water quality which is either ideal or acceptable for continuous lifetime use in terms of the proposed interim water quality guidelines for radioactivity in drinking water. Five sites had a dose between 0,1 and 0,25 mSv/year, showing a slightly larger increase above local natural background, but still fully acceptable for lifetime use with no significant detrimental effects to the user. Only two sites had significant elevation of the radiation dose which showed the need for planning to reduce the exposure over the course of time. Both these sites involved the discharge of mine water that had been pumped to the surface.
A highly relevant and comforting finding of the study was that the total radiation dose for both Potchefstroom untreated raw drinking water supply points was very low, and in fact not significantly different from the natural background dose value estimated for the study.
A valuable finding of the study was the good linear correlation between total radiation dose from all radionuclides and the uranium concentration. This will, in the future, make it possible to use the uranium concentration for screening and routine monitoring purposes within the catchment.
As regards the two classical screening parameters for radiation, viz., alpha and beta activity, the former showed a reasonably good correlation with total radiation dose, when compared on an annual average basis. The gross beta activity measurements were considered to be unreliable because of measurement difficulties at the low levels encountered. As regards chemical variables, while it was found that elevated radiation dose is usually associated with elevated sulphate concentrations, the converse was not true, consequently sulphate concentration cannot be used as an indicator of radioactivity in the water.
The water analysis technique involves filtering of the raw water samples prior to radiometric analysis, and the primary intention of the study was to measure only the radioactivity in the water passing through the filter. In the final month of the study, however, the radioactivity in the suspended solids trapped by the filter was also measured, as a preliminary indication of whether the suspended solids were of any significance as regard the possible radiation dose from ingestion of untreated water.
While an important aim of the study was to measure the concentrations of a large range of radionuclides in the natural uranium and thorium decay chains, it was not the intention to look at radon gas dissolved in the water. Dissolved radon, even at relatively high concentrations, does not contribute significantly to the drinking water health risk, and is generally considered to be of possible concern only where significantly elevated radon concentrations are associated with conditions that promote the dissolution and release of the gas into poorly ventilated enclosures. Such scenarios, which might include indoor spa baths and underground water treatment plants, are not known to exist in the Mooi River catchment.