Geology and hydrogeology ... …
Report No. SRL/FP/004.1 (10/05/2006)
The biggest uncertainty is estimating the evaporative losses. Thus the experiment would be best attempted in weather conditions (winter) when such losses are likely to be negligible. Another problem would be the existence of springs feeding water into the lake from the underlying (Corallian) aquifer. This possibility is considered in Appendix 1. Since such springs would be driven by a hydraulic potential greater than that of the groundwater, the above equation would no longer be valid. It would therefore be necessary to establish
first that springwater was not entering the lake. However, were a negative value of τ to be yielded by the above, this would be evidence of springs and the issue of containment would then have to be resolved differently.
OTHER MECHANISMS FOR ESCAPE OF POLLUTANTS
Npower have assumed that the only mechanism for escape of pollutants is by advection seepage, ie the pollutants are carried in the leachate, which seeps through the clay, and quote numbers such as the 10-10 m/s hydraulic permeability19 of the Kimmeridge Clay, to support the contention that pollutants would be adequately contained. In an ideal world, this would imply containment times of the order of several hundred years and slow leach rates (up to ~60 cu m/year, thereafter20 ). As noted above, things are far from ideal in the real world.
However, we are forgetting that the purpose in sealing the lakes is not to contain the water, but rather the pollutants dissolved in the water. In this case, the main pollutants of concern are heavy metals, which, when in solution, take the form of positive ions (cations). Since these ions are positively charged single atoms, they are much smaller than water molecules, and can move more readily through the clay matrix. The mechanisms for the migration of metal ions through clay are affected by many physical and electrochemical factors: Concentration gradients, pH gradients, electric fields generated by differential migration of different ion species, and adsorption due to affinity between the ions and the clay itself. In general, the mechanisms driving the diffusion of metal ions through wet clay are complicated and difficult to model. Nevertheless studies have shown21,22 that some chemical species, including metal ions, can diffuse much more rapidly through clay than by advection seepage (eg 1.5 metres through clay in 10 years, in circumstances when advection seepage would have penetrated only a few centimetres). The reason for mentioning this is not because we believe it to be an important mechanism for leakage of metal ions through Kimmeridge Clay, but because it is a possibility and Npower have failed to consider it.
As with leaching, diffusion rates are significantly increased by imperfections in the clay: voids, inclusions and compaction fractures all occur and warn that, in the real world, the clay linings may not perform, or be performing, quite as intended.
An observation is that placing toxic landfill into sealed pits possibly places underlying deep aquifers in greater danger, since the intervening clay barrier, whose thickness is, we recall, not known with any certainty, is subject to a concentration gradient for far longer, than if the pits were unsealed and pollutants allowed to disperse relatively rapidly within the surface groundwater.
19 ES, page 132.
20 This calculation has been performed for Lake E, whose contents would be raised significantly (~3.7 m) above the surrounding water
t a b l e , r e s u l t i n g i n t h e b u n d s b e i n g s u b j e c t t o a h y d r a u l i c p r e s s u r e g r a d i e n t ~ 3 m / m a c r o s s t h e s e a l i n g l a y e r . 2 1 Quigley, R M and Rowe, R K, Leachate Migration Through Clay below a Domestic Waste Landfill, Sarnia, Ontario, Canada: Chemical Interpretation and Modelling Philosophies, Hazardous and Industrial Solid Waste Testing and Disposal: Volume 6. American Society for Testing and Materials, Philadelphia PA. (1986) p. 93-103.
22 Quigley, R M; Yanful, E K; Fernandez, F, Ion Transfer by Diffusion Through Clayey Barriers in: Geotechnical Special Publication No.
13. Geotechnical Practice for Waste Disposal 1987, American Society of Civil Engineers, New York. (1987) p. 137-158.
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