Last blog post I focused on the some of the methodology associated with monitoring radionuclide dispersion. This time, I will seek to highlight some one of the specific problems we face regarding muddy estuaries!
Routine Discharges to Coastal Waters
The following is taken from part of an abstract on a paper written for critical group radiation exposure calculation:
Most models currently employed to calculate critical group doses arising from discharges of activity to estuaries or seas employ a single well mixed compartment to represent local dispersion. This paper presents more general methods for calculating radionuclide dispersion, enabling many important factors to be considered ... like turbulence generated by the discharge, the effects of a sloping sea bed and the variation with time of the lateral dispersion coefficient. The importance of the timing of discharges can be assessed, as can the variation of mean radionuclide concentrations along the coastline. The methods that are used in conjunction with the dispersion model to estimate radiation exposure are described, with particular attention being paid to external exposure over muddy estuarine sediments which is frequently found to be the most important pathway. In each stage of the exposure calculations the simplest modelling approach possible is employed consistent with the quality of input data available.
Estuarine Sediments
What is it about the soil composition that makes it a viable pathway for radionuclide release? In a study conducted by the US Geological Survey, 9 separate Florida estuary sites were tested for texture and composition of bottom sediments (including hydrologic and geologic factors that influence them). I had no idea until skimming through this that there was so much interest in the coastal estuaries content of carbon, nitrogen, minerals, and other ingredients. Surveying is just as tough to do correctly as engineering, in many respects.
Some estuaries, however, are sited in tropical regions and are heavily impacted by exposure to solar radiation. When sediment rinses in with influxes of fresh water, turbidity becomes an issue for photon attenuation and so some flora are affected. The only paper I could find on tropical estuaries and radiation stated that no relationsihp was found between the attenuation coefficient and amount of suspended matter in the water.
French estuaries have been under scrutiny since 14 of their many power producing nuclear sites are adjacent to the Loire river estuary. This paper talks about sources, transport, and fate of the radionuclides that have been released over time into the environment. Fallout from nuclear testing and accidents were also considered. It was found that sediment turnover rate has the biggest impact on how these harmful emitters travel around. There has been a decreasing trend since 1980 of how much activity is ongoing due to radionuclides (attributed to the die off of testing and accident emissions).
References
http://rpd.oxfordjournals.org/content/15/3/153.short
http://pubs.usgs.gov/pp/0742/report.pdf
http://www.sciencedirect.com/science/article/pii/0022098168900154
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