The aim of the radioactivity monitoring programme in the Mooi River was to address the risk of radioactivity in water to human health, and to determine the total dose from surface and some ground water sources that are, or could be, used potentially as drinking water supplies. After a year of data collection, the results showed that of the 41 monitoring sites covered by the study, 39 sites exhibited a water quality that is ideal or acceptable for continuous lifetime use in terms of proposed interim water quality guidelines for radioactivity in drinking water. At the two remaining sites, both of which are associated with the discharge of water from gold mining activities, the elevation of radionuclides is such that the water is still radiologically acceptable for use as drinking water in the short term, but further investigation would be required to establish whether the water quality is radiologically acceptable in the longer term. It should be noted that at no site was the radiological quality such that immediate remediation was called for.
The only water in the Mooi River catchment used as a source of municipal drinking water is that supplied to Potchefstroom from the Mooi River and the ground water supplied to Welverdiend in the municipality of Carletonville. In both cases, there was no significant elevation of radioactivity above background levels, and the water falls into the ‘ideal’ classification in terms of proposed interim water quality guidelines for radioactivity in drinking water.
The search for simple indicators of radiological quality revealed the following:
Some correlation exists between annual mean gross alpha activity and annual radiation dose from ingestion, but is not particularly strong.
The correlation between gross beta activity and radiation dose is poor.
Although waters with elevated radioactivity levels generally exhibit elevated sulphate concentrations, the converse is not always true. Consequently, sulphate concentration is not a reliable indicator of radiological water quality.
An excellent linear correlation exists between annual mean uranium concentration and the annual radiation dose from ingestion. The best-fit linear relationship for the data gathered in this study is given by:
| D = 0,0012895 CU + 0,02128 (r2 =0,98) | |
| where: | D = average lifetime annual radiation dose from continuous drinking water use (mSv/a) |
| CU = uranium concentration in the water (m g/l ) |
The estimated uncertainty arising from the use of this relationship to predict annual radiation doses arising from the use of waters in the Mooi River catchment for continuous drinking purposes is less than 10%.
It remains to be seen at this stage whether different relationships will apply to different catchments or whether a single, more general relationship can be established that will be applicable across several catchments. The validity of the correlation between total dose and uranium concentration for the Mooi River catchment should be checked periodically where used for long term monitoring.
Preliminary indications were that the additional radiation dose resulting from the ingestion of the suspended solids in the water is minimal (» 2% on average) and can therefore be ignored.
Provisional guidelines for evaluating the significance of the drinking water pathway of radiation exposure were developed, based on a synthesis of the dose limits given by the World Health Organisation, the Council for Nuclear Safety, and the IAEA recommendations.
The following recommendations can be made from the results of the study:
Action
No immediate action is required to reduce radioactivity levels in surface and ground waters in the Mooi River catchment.
Management approach
The future monitoring and control of radioactivity in surface and ground waters should be integrated into the existing approach used in the management of the catchment, in terms of which all pollutants of concern are addressed.
Application of relevant International Commission on Radiation Protection (ICRP) principles
The ongoing regulation, at source, of intended radioactive discharges from mining operations affecting water quality in the catchment should be conducted in accordance with relevant ICRP principles, which are as follows:
Firstly, radiation doses should be kept as low as reasonably achievable (ALARA), economic and social factors being taken into account. This should involve a periodic review of existing practices to see that they conform to the ALARA principle.
Secondly, the exposure of individuals should not exceed 1 mSv in a year, taking into account other radiation sources subject to control. To enable regulation to be applied at source, this will require that the optimization of protection according to the ALARA principle be constrained by source-related dose constraints of less than 1 mSv in a year. Discharges of radioactivity in water should accordingly be subject to appropriate limitation on a mine-specific basis.
Monitoring strategy
The experience and knowledge gained in this study should be used as input to the formulation of a national strategy and action plan for routine and follow-up monitoring of radioactivity in public water streams, as part of an integrated approach to water quality management. The principal approach to radioactivity monitoring should be as follows:
Where no data exists, then a full nuclide analysis is advisable. For the Mooi River catchment, monitoring of uranium on a monthly basis only may be used, with use of the relationship between uranium and running average annual dose as a monitoring and evaluation tool. Within this catchment, monitoring of chemical uranium concentrations (or surveillance in the absence of water flow) should continue on a monthly basis at those sites associated with radiation doses greater than 0,1 mSv/a (sites 1, 7, 7A, 9, 11, 12 and 15) and at those sites associated with municipal water abstraction (sites 27, 35 and 36), as part of the integrated catchment management approach referred to in (ii) above. The average uranium concentrations over a year should be used to estimate the annual radiation dose at each site from sole continuous use of the water for drinking purposes, using the relationship derived in this study. Site 12 needs to be kept under surveillance if and when water flow recommences at this point.
Chemical uranium as an indicator
The use of chemical uranium as a monitoring parameter for radioactivity should be investigated for other catchments to determine its applicability outside the Mooi River catchment.
Mine closure
The potential for ongoing radiological impacts, after mine closure, on water sources in the Mooi River catchment should be taken into account in the site-specific mine decommissioning plans that are required as part of the mines’ Environmental Management Programme (EMP) obligations. Such plans address all potential sources of environmental pollution, such as acid mine drainage, in a holistic manner, so that interdependencies are taken into account. Radioactivity should be included in this holistic approach. Decommissioning plans will specify the nature and duration of any aftercare arrangements that might be required, and these will include appropriate ongoing monitoring requirements with respect to chemical and radiological pollutants.
Radioactivity in sediments
Since this study was concerned only with radioactivity in water sources, the question of radioactivity in the sediments in the Mooi River catchment remains largely unexplored. An investigation of radioactivity in sediments needs to be undertaken, with a view to understanding the role played by water chemistry, and it should be noted that this is indeed the subject of a project funded by the Water Research Commission starting in 1999 (Project No. K5/1095: Tier 1 Risk Assessment of Radionuclides in Selected Sediments of the Mooi River).
Radionuclides in fish
Because of the huge uncertainties in the uptake of radionuclides in fish, studies on the fish consumption exposure pathway should be conducted. It needs to be established whether potential radiation dose from this route in the first instance is likely to be significant or not.
Continuation of Technical Committee
The current Technical Committee should continue in order to ensure continuity of the monitoring efforts in catchments other than the Mooi River catchment as part of the national radiological monitoring programme.