Contaminant Attenuation in a Semi-Karstic Chalk Aquifer – a New Approach Using Radiochemistry

We have investigated a potential radiochemical method for estimating the capacity of the semi-karstic Cretaceous Chalk aquifer to attenuate contaminants.

Contaminant solutes are advected by groundwater flow through fractures, but are slowed and attenuated by molecular diffusion into immobile water in the pores of the Chalk. Fracture apertures are considered to be the key factor controlling both advection and diffusion effects.

In principle apertures may be estimated by comparing dissolved radon gas in fracture water with uranium-series isotope activities in the rock matrix. Radon release and contaminant attenuation are governed by equivalent processes of molecular diffusion.

We have tested the robustness of a radon-derived transport model through a series of laboratory experiments and field observations in the Pang and Lambourn catchments, 60 km west of London, England. This has been achieved by the use of liquid-liquid extraction and luminescence spectrometry to assay Chalk core for uranium, and by energy-discriminated liquid scintillation to determine both the radium activity of Chalk core and the radon activity in Chalk groundwaters.

The data have been used to determine the distribution of radon precursors in the Chalk, sampled by reference to fractures, lithology, and stratigraphy.

We have also tested the assumption that double-porosity behaviour dominates for solute transport in the Chalk of the research area, using radial flow tracer testing, and can be characterised by an effective diffusion time despite flow heterogeneity. The use of such modelling techniques has been evaluated and compared with the results obtained by the new radiochemical model. Sensitivity of the new model to variations in radon precursor (‘source term') location and strength as well as aquifer geometry has also been assessed.

Finally, we make recommendations for the application of U-series monitoring to map spatial variations in the capacity for contaminant attenuation by double-porosity diffusive retardation in the Chalk aquifer.