Aquatic countermeasures

recommended a change of hunting season for roe deer to the spring; this change was applied voluntarily by the hunting community in the early 1990s. As a result, the radiocaesium content in roe deer meat in Gävle was reduced by approximately six times. The recommendation to shift the hunting season to the spring has remained in place until the present day [4.71].

In addition, the management of reindeer by the Sami people in northern Sweden has been altered in a variety of ways to help reduce the radio-caesium content of animals before slaughter. This includes provision of clean fodder for sufficient time to reduce the body burden below the intervention level. A similar result can be achieved by altering the time of slaughter, sometimes in combination with feeding of clean fodder [4.72].

4.5. AQUATIC COUNTERMEASURES

accident. Abstraction of drinking water for Kiev was switched to the Desna River with use of a pipeline built during the first weeks after the accident. A summary of the measures taken by the Ukrainian authorities to switch to alternative supplies from less contaminated rivers and from groundwater can be found in Refs [4.76, 4.79].

Radionuclides may be removed from drinking water supplies during the water treatment process.

Suspended particles are removed during water treatment, and filtration can remove dissolved radionuclides. In the Dnieper waterworks station, activated charcoal and zeolite were added to water filtration systems. It was found that activated charcoal was effective in removing ¹³¹I and ¹⁰⁶Ru, and zeolite was effective in removing ¹³⁷Cs, ¹³⁴Cs and ⁹⁰Sr. These sorbents were effective for the first three months, after which they became saturated and their efficiency declined [4.80, 4.81]. The average removal of these radionuclides from water (dissolved phase) was up to a factor of two.

After the accident, the upper gates of the Kiev reservoir dam were opened to release surface water.

It was believed at the time that the surface water was relatively low in radionuclide content, because suspended particles had sunk to deeper waters.

Therefore, the release of water would allow room in the reservoir to contain runoff water from the inflowing rivers, which was believed to be highly contaminated. In fact, because of direct atmospheric deposition to the reservoir surface, the surface waters in the reservoir were much more contaminated than the deep waters. As noted by Voitsekhovitch et al. [4.80], “a better approach to lowering the water level within the Kiev reservoir would have been to open the bottom dam gates and close the surface gates. This would have reduced the levels of radioactivity in downstream drinking water in the first weeks after the accident.” Although this countermeasure was not efficiently implemented after the Chernobyl accident, regulation of flow, given the correct information on contamination, could effectively reduce activity concentrations in drinking water, as it takes some time (days or more) for lakes and reservoirs to become fully mixed.

In a large river–reservoir system such as the Dnieper, control of water flows in the system can significantly reduce transfers of radioactive material downstream [4.82]. In the Dnieper River, the time it takes for water to travel from the Kiev reservoir to the Black Sea varies between three and ten months.

Over the time that the water takes to travel downstream, radioactive pollution is reduced by

decay of short lived radionuclides and transfers to reservoir bed sediments (particularly of radio-caesium) [4.82].

4.5.2. Measures to reduce direct and secondary contamination of surface waters

Standard antisoil erosion measures can be used to reduce runoff of radionuclides attached to soil particles. Note, however, that typically less than 50% of radiocaesium and less than 10% of radio-strontium and radioiodine were in the particulate phase, and this limits the potential effectiveness of this countermeasure. It should also be noted that the dissolved, rather than particulate, form of these radionuclides is important in determining activity concentrations in drinking water and freshwater biota.

Dredging of canal bed traps to intercept suspended particles in contaminated rivers was carried out in the Pripyat River [4.79]. These canal bed traps were found to be highly inefficient for two reasons: (a) the flow rates were too high to trap the small suspended particles carrying much of the radionuclide contamination; and (b) a significant proportion of the radionuclide activity (and most of the ‘available’ activity) was in dissolved form and thus would not have been intercepted by the sediment traps.

One hundred and thirty zeolite-containing dykes were constructed on smaller rivers and streams around Chernobyl in order to intercept dissolved radionuclides. These were found to be very ineffective: only 5–10% of the ⁹⁰Sr and ¹³⁷Cs in the small rivers and streams was adsorbed by these zeolite barriers [4.80]. In addition, the rivers and streams on which they were placed were later found to contribute only a few per cent to the total radio-nuclide load in the Pripyat–Dnieper system.

After the Chernobyl accident, spring flooding of the highly contaminated Pripyat floodplain resulted in increases in ⁹⁰Sr activity concentrations in the Pripyat River from annual average activity concentrations of around 1 Bq/L to a maximum of around 8 Bq/L for a flood event covering an approx-imately two week period [4.83]. In 1993 a dyke was constructed around the highly contaminated floodplain on the left bank of the Pripyat. This prevented flooding of this area and proved effective in reducing ⁹⁰Sr wash-off to the river during flood events [4.80]. A second dyke was constructed on the right bank of the Pripyat in 1999. The annual average ⁹⁰Sr activity concentration in Kiev reservoir

water, however, was below 1 Bq/L in all years from 1987 onwards. The radiological significance of the

90Sr activity concentrations in Kiev reservoir water, even during the short flood events, is therefore very low, although it has been argued that the averted collective dose to the large number of users of the river–reservoir system is significant.

It is potentially possible to increase the sedimentation of radionuclides from lakes and reservoirs by the introduction of a strongly sorbing material such as a zeolite or an (uncontaminated) mineral soil. This method has not been tested. Using a model for the removal of radiocaesium from lakes by settling of suspended particles, Smith et al. [4.78]

identified two problems with this method: (a) large, deep lakes would require extremely large amounts of sorbent; and (b) secondary contamination of the lake by remobilization of activity from the catchment and/or bottom sediments would require repeat applications in most systems.

4.5.3. Measures to reduce uptake by fish and aquatic foodstuffs

Bans on the consumption of freshwater fish have been applied in the limited zones affected by the Chernobyl accident [4.84]. In some areas, selective bans on the more contaminated predatory fish have been applied. It is believed that such bans are often ignored by fishermen. Bans on the sale of freshwater fish were applied in some areas of Norway [4.85]. Farmed fish could be used as an alternative source of freshwater fish in areas affected by fishing bans, since farmed fish fed with uncontaminated food do not accumulate radionu-clides significantly [4.86].

The addition of lime to reduce radionuclide levels in fish was tested in 18 Swedish lakes [4.87].

The results of the experiments showed that liming had no significant effect on the uptake of ¹³⁷Cs in fish in comparison with control lakes. Although the uptake of ⁹⁰Sr was not studied in these experiments, it is expected that increased calcium concentration in lakes may have an effect on the ⁹⁰Sr concen-tration in fish. Experience of lake liming, in conjunction with artificial feeding of fish in Ukraine, has been summarized by Voitsekhovitch [4.79].

It is known that the concentration factor for radiocaesium in fish is inversely related to the potassium content of the surrounding water. After

the Chernobyl accident, potassium was added to 13 lakes in Sweden, either as potash or as an additive in mixed lime [4.87]. The results of the potash treatment were somewhat inconclusive, with a small reduction in activity concentrations in perch fry observed during the two year experiment. It was found that in lakes with short water retention times it was difficult to maintain high levels of K⁺ in the lake.

In an experiment on Lake Svyatoe (a closed lake) in Belarus, Kudelsky et al. [4.88, 4.89] added potassium chloride fertilizer on to the frozen lake surface. Results showed a significant (factor of three) overall reduction in ¹³⁷Cs concentration in fish during the first years after the experiment.

However, as expected, the ¹³⁷Cs in the water increased by a factor of two to three after the countermeasure application. It is likely that potassium treatment is only feasible in lakes with very long water residence times, which allow increased potassium concentrations to be maintained. Also, the increased ¹³⁷Cs in water is unlikely to be acceptable in lakes that have water abstracted for drinking.

Manipulation of the aquatic food web by intensive fishing was carried out in four lakes in Sweden [4.87], and as a complementary measure in an additional three lakes. This resulted in a reduction of the fish population by about 5–10 kg/

ha. The species reduced were mainly pike, perch and roach. No effect of intensive fishing on ¹³⁷Cs concentrations in fish was observed. Fertilization was carried out in two Swedish lakes using Osmocoat (5% phosphorus and 15% nitrogen). The concentrations of total phosphorus generally showed no change in the long term mean value: it appears that the fertilization treatment was not carried out sufficiently effectively. No effect was observed on ¹³⁷Cs activity concentrations in fish.

Different methods of food preparation may affect the quantity of radionuclides in consumed food [4.90]. Ryabov suggested bans on the consumption of smoked and dried fish, because these processes increase concentrations of radionu-clides (per unit of weight consumed) [4.84]. Other preparation processes may reduce radionuclide levels in fish by approximately a factor of two. An effective measure to reduce the consumption of radiostrontium is to remove the bony parts of fish prior to cooking, since strontium is mainly concen-trated in the bones and skin. Various other food preparation methods are discussed in Ref. [4.91].

4.5.4. Countermeasures for groundwater

There is no evidence that measures have ever been taken to protect groundwater supplies after an atmospheric deposition of radioactivity.

Groundwater residence times are long enough that shorter lived radionuclides such as ¹³¹I will have decayed long before they affect drinking water.

Only very small amounts of radiostrontium and radiocaesium percolate from surface soils to groundwater after atmospheric deposition. A study [4.77] has shown that, after the Chernobyl accident, exposure to ⁹⁰Sr and ¹³⁷Cs via the groundwater pathway was insignificant in comparison with other pathways (food, external exposure, etc.).

Measures were taken to protect groundwater from seepage of radionuclides from the shelter and from radioactive waste sites in the CEZ. These measures focused mainly on the construction of engineering and geochemical barriers around the local hot spots to reduce groundwater fluxes to the river network. Actions to stop precipitation from entering the shelter, and drainage of rainwater collected in the bottom rooms of the shelter, have also to be considered as preventive measures to reduce groundwater contamination around the Chernobyl nuclear power plant industrial site.

4.5.5. Countermeasures for irrigation water As discussed previously, irrigation did not add significantly to the radionuclide contamination of crops that had previously been affected by the atmospheric deposition of radionuclides. Thus, in practice, no countermeasures were directly applied to irrigation waters. However, the experience described in Ref. [4.79] shows that the change from sprinkling to drainage irrigation of agricultural plants (e.g. vegetables) can reduce the transfer of radionuclides from water to crops by several times.

This, in combination with improved fertilization of irrigated lands, can effectively reduce radionuclide levels in crops irrigated with water from reservoirs affected by radioactive pollution.

4.6. CONCLUSIONS AND