The levels of contamination at which foodstuffs or water supplies should be banned is based upon the potential dose to the consumer. This topic is discussed in Chapter 10 of this report.
9.5 Water
The Radiological Hazard
Hong Kong has no natural lakes or rivers or substantial underground water sources. The main water sources are therefore natural catchment and supply from the People's Republic of China. The supply from China is taken from the Shenzhen reservoir which is part of the East River --Shenzhen Scheme.
In the event of a major release of radioactivity from Daya Bay the wind could carry the plume over one or more reservoirs and radioactive material could be deposited on the surface of the reservoir and its associated catchment area. There is hence the possibility of contamination of water supplies.
In practice it is highly unlikely that any credible release of radioactivity would have any significant effect upon water supplies.
The main reservoirs close to Daya Bay which store Hong Kong's water are High Island and Plover Cove in Hong Kong and Shenzhen in the People's Republic of China. Taking the High Island reservoir as an example; this has a capacity of 280 million cubic metres of water, and a surface area of approximately 10 square kilometres. The reservoir is located about 30 km from the Daya Bay site. If the wind were such that the plume travelled towards High Island reservoir, after travelling 30 km the plume would be fairly dispersed and also any deposition on the reservoir would be subject to massive dilution by the volume of water in the reservoir.
In average dry weather conditions it would require a release of depositable activity (ie excluding noble gases) from Daya Bay of the order of 1017 Bq before the activity in the reservoir water could theoretically approach the lower derived emergency reference level (DERL) given in reference 38. Such a release would be comparable with the release from Chernobyl. Calculation of deposition rates during rainfall is difficult (ref 24), however in the UK during the passage of the Chernobyl cloud it was found that deposition in areas of heavy rainfall could be a factor of 100 or more greater than those areas with little or no rainfall, (ref 25). This would imply that for the highly pessimistic scenario of the plume travelling towards the High Island reservoir and meeting heavy rainfall only as it passes over the reservoir, a release of 1015 Bq of depositable radioactivity could just give rise to a concentration of radioactivity in the water equivalent to the lower DERL for drinking water. The level of activity in the water would, however, be greatly reduced by the standard water treatment processes and so the level of radioactivity in the customer's supply would be much less than the lower DERL.
The hazard associated with contaminated water is principally due to its consumption. Using such water for washing, etc., is unlikely to result in any significant hazard.
Behaviour of the Deposited Activity
Radioactivity may be deposited on the surface of the reservoir and on its catchment area. Only a small fraction of the radioactivity deposited on the catchment area is likely to be transferred to the reservoir (ref 26). For example less than 2% of the Cs-137 is likely to find its way into the reservoir (ref 27). The catchment area for the High Island reservoir is about 70 km2 compared with a surface area of the reservoir of about 10 km2. It is clear therefore that the activity in the reservoir will initially be dominated by the activity deposited directly on the surface of the reservoir. Any activity which does enter the reservoir from the catchment area will take time to do so. In Sweden after the Chernobyl accident it was found that I-131 and Cs-137 entering lakes from run-off from the catchment area
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