Moruroa Atoll is part of the Tuamotu Archipelago, in French Polynesia. It is located at a longitude of 138o 54' W, and latitude 21o 50' S (Figure 3). The quantity of radioactive material contained within the atoll is not precisely known, but has been estimated to be equivalent to approximately 3.2x1017 Becquerels (Bq) of Cesium-137 (137Cs) (RT90). It should be noted that there remains considerable uncertainty as to this figure. Therefore, it was decided in this study to use an arbitrary unit concentration (1 Bqm-3). To obtain a prediction of radionuclide concentration in any given scenario therefore only requires scaling of our model results by the net quantity released.
The variables incorporated into the tracer release function are (i) the depth of the water column to which the tracer is released, (ii) the duration of the release of the tracer, and (iii) the climatic conditions under which the tracer is released. In all tracer experiments, the model is run over a simulated period of ten years.
Releases of tracer are simulated at two different depth layers within the water column. The first is to the uppermost depth level of the model, corresponding to the top 25m of the water column. Such a release could be associated with venting or upwelling of contaminated material through the atoll structure into the lagoon (Rougerie and Wauthy, 1992), or a geological event such as a rockslide affecting the uppermost level of the atoll structure (IAEA, 1998). The second scenario involves a release over the depth range of 360-510m, corresponding to model depth level nine. This depth range is the location of the karst layer; the interface between the basalt foundation of the atoll and the carbonate upper layer. This layer has been suggested as a potential conducting medium for the escape of radionuclides coming from the volcanic formations in which weapons tests were conducted (IAEA, 1998).
Tracer experiments are conducted using two different release periods. The first is an instantaneous (single time-step) release of the entire quantity of radioactive material. Such a release may result from a geologically catastrophic incident such as a landslide or earthquake (IAEA, 1998). The second is the release of the same quantity of material over the duration of ten years. Gradual hydrological processes such as venting or endo-geothermal upwelling (Rougerie and Wauthy, 1992) may be the cause of such a scenario. The duration of ten years is, however, somewhat arbitrary, as determination of an accurate time-scale for such processes is not currently feasible (Mittelstaedt et al, 1999).
In this study an attempt is made to investigate the possible influence that climate variability can have on the transport of a tracer. Examples of seasonal and interannual climate variability have been incorporated into the model experiments by releasing the tracer into the corresponding seasonal or interannual velocity fields diagnosed from the POCM. To allow a comparison across experiments and with previous studies, each of the above release scenarios is run using the annual-mean and three examples of time-varying velocity fields.
The annual-mean case uses the average of all of the monthly velocity fields of the twenty-years of the POCM to advect the tracer. Two seasonal climate variability cases are conducted. These use a twelve-monthly cycle of velocity fields averaged from the POCM. Release times are simulated for January and July, corresponding to the summer and winter velocity fields. One simple interannual climate variability case is also conducted. In this experiment the tracer is released into a velocity field corresonding to the mean 1982/1983 circulation (a strong ENSO event). After two years of integration, the model uses the annual-mean velocity field to advect the tracer for the remainder of the experiment. The 1982/1983 velocity field is averaged from May 1982 to April 1983, corresponding to the height of the strong ENSO event (Kiladis et al., 1989).
http://www.maths.unsw.edu.au/~doughaze Page created by Douglas R. Hazell 7/6/2001 Last update of this page: 1/8/2001