Quinn, S; (2012) Evaluation of a radiochemical method to estimate the rate of solute diffusion within a dual porosity Chalk aquifer. Doctoral thesis , UCL (University College London).

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Abstract

This thesis investigates a potential radiochemical method for estimating the capacity of the dual porosity Chalk aquifer to attenuate solutes. Solutes are advected by groundwater flow through fractures, but are slowed and attenuated by molecular diffusion into immobile water in the Chalk matrix. Fracture apertures are a key factor controlling rates of both advection and diffusion. The radiochemical model suggests that apertures may be estimated by comparing radon activity in groundwater with uranium-series isotope activities in the matrix. This estimate would be of great value if both radon release and solute attenuation are dominated by molecular diffusion. The thesis tests the assumptions made in the radiochemical method through a series of laboratory experiments and field observations. This has been achieved by use of • liquid-liquid extraction and luminescence spectrometry to assay Chalk core for uranium; and, • energy-discriminated liquid scintillation to determine both the radium activity of Chalk core and the radon activity in springs and pumped groundwater. The data demonstrate that • the Chalk does not possess a homogeneous distribution of radon precursors, which are dependent on both lithology and disequilibrium within the decay chain; • radon activity of pumped groundwater is highly variable and dependent on both the rate and duration of pumping; and, • spring sources demonstrate variation in radon activity which are not readily explained by the prevailing hydrogeological conditions. At a research site in Berkshire, double-porosity behaviour is shown to dominate solute transport, which can be characterised by an effective diffusion time. However, there is a clear disparity between diffusion times calculated from artificial tracer testing and estimated by the radiochemical method. This suggests that the radiochemical diffusion model is not appropriate in its current form to estimate rates of solute diffusion between fractures and the surrounding Chalk matrix.

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