Non-aqueous-phase-liquid breakthrough during evaporative drying of clay barriers.
In this study, an attempt has been made to model a real field scenario, whereby an initially almost saturated clay liner in a waste site is gradually drying, due to evaporation at its lower boundary. A detailed conceptual model that deals with the penetration and breakthrough of non-aqueous-phase-liquid (NAPL) in clay liners is introduced. Water content of clay samples was monitored during ambient evaporation through apertures at the base of sample holders. Clay drying rate served as the primary parameter for the NAPL breakthrough study. The interconnection between drying rates, structural damage formation (cracks and suction) and NAPL penetration is especially addressed. The processes taking place in the clay samples during drying appear to be associated with the capillary effects between the different fluid phases in the vicinity of either the NAPL-clay or the clay-air boundaries. A conceptual model of NAPL penetration and breakthrough of the clay layer has been considered, based on both indirect and direct observations of structural damages produced on either clay boundaries. A mutual interaction between these two boundaries is suggested and discussed. NAPL breakthrough is suggested to take place through cracks initiated on the upper soil surface.
Modeling the sensitivity of hydrogeological parameters associated with leaching of uranium transport in an unsaturated porous medium
The uranium ore residues from the legacies of past uranium mining and milling activities that resulted from the less stringent environmental standards along with the uranium residues from the existing nuclear power plants continue to be a cause of concern as the final uranium residues are not made safe from radiological and general safety point of view. The deposition of uranium in ponds increases the risk of groundwater getting contaminated as these residues essentially leach through the upper unsaturated geological formation. In this context, a numerical model has been developed in order to forecast the U and its progenies concentration in an unsaturated soil. The developed numerical model is implemented in a hypothetical uranium tailing pond consisting of sandy soil and silty soil types. The numerical results show that the U and its progenies are migrating up to the depth of 90 m and 800 m after 10 y in silty and sandy soil respectively. Essentially, silt may reduce the risk of contamination in the groundwater for longer time span and at the deeper depths. In general, a coupled effect of sorption and hydrogeological parameters (soil type, moisture context and hydraulic conductivity) decides the resultant uranium transport in subsurface environment.
On the Hygroscopic Action of Cotton
In an earlier paper by one of us an account was given of the behaviour of dry cotton-wool when immersed in air saturated with water vapour, and the relation between the hygroscopic absorption and the change of temperature which the cotton manifests was investigated. It was shown that, so long as the surrounding air is saturated, the absorption process appears to be unending, though its velocity continuously diminishes; or, in other words, true equilibrium between such an atmosphere and cotton wool, however moist, appears to be impossible, just as it would be impossible between the same atmosphere and an aqueous solution, however dilute. It was pointed out, however, that “if placed in an atmosphere containing water vapour at anything less than saturation pressure ( e. g ., in the open air) the cotton would in time reach a state of equilibrium, either by absorption or by evaporation, according to its initial condition.” As we could find no record of any complete investigation of such relationship, it seemed desirable to determine the amounts of vapour absorbed by a given quantity of cotton in atmospheres of various known degrees of humidity. This has been done; but it has been found necessary to determine the true equilibrium values by approaching them from both sides, by absorption and by evaporation, since neither process becomes really complete in any practicable time, though each leads in a few hours to what might be mistaken for true equilibrium. Reference must here be made to the interesting work published by Clayton Beadle, in 1894 and 1895, which was unfortunately entirely overlooked by Masson until his attention was called to it after publication of the paper already referred to. Beadle seems to have been the first to observe the striking elevation of temperature that dry cotton undergoes when exposed to damp air and to study the progress of absorption and the course of the temperature change. He, however, did not use atmospheres of constant and known humidity and temperature, but used the open air of the laboratory, so that his curves are affected by considerable irregularities. Apart from these, his results are in general character similar to those we have since obtained.