Temporal Variation in Radon Activity of Groundwater in the UK Chalk – a Comparison with Other Karstic and Fractured Aquifers

We have monitored the temporal variation in radon activity of groundwater over a period of 20 months at a series of springs and boreholes within the Cretaceous Chalk aquifer in Berkshire, England and have investigated the correlation of radon with other environmental variables: rainfall, groundwater recharge, groundwater levels, stream flows and groundwater chemistry.

Our results demonstrate clear differences with the groundwater radon signatures previously observed by others in the karstic Carboniferous Limestone and semi-karstic Lincolnshire Chalk (both in England), a coastal carbonate aquifer in Italy and a fractured pyrite-rich orthogneiss in Switzerland.

Although a distinct seasonal pattern to activity is present, there is little direct correlation between the radon content of springs with rainfall. In addition, the timing of highest radon content in springs is found in most cases to occur during late summer, when groundwater levels are in recession and groundwater recharge is absent. This is at odds with the hypothesis of radon being flushed through the fracture system as is proposed in more classically karstic aquifers such as the Carboniferous Limestone.

From large scale pumping tests it is apparent that the Chalk matrix may contain a higher concentration of radon than is present in mobile groundwater in the adjacent fractures. We consider what the temporal variation in groundwater radon content may imply in relation to the drainage of the Chalk aquifer matrix during groundwater level recession.

Finally, we have used a double porosity diffusion model to infer the possible corresponding range in fracture aperture sizes, based on our knowledge of the distribution of the radium source term in the Chalk and the seasonal variation of radon content in associated groundwaters.