Radon in Chalk streams: Spatial and temporal variation of groundwater sources in the Pang and Lambourn catchments, UK

SummaryVariations in dissolved 222Rn (radon) concentrations in rivers and groundwater are observed in the Cretaceous Chalk catchments of the Pang and Lambourn. Stream radon concentrations and flow data were used to model radon inputs to rivers from groundwater, with the modelled radon input concentrations (CI) varying between 0.2 Bq l−1 and 3.8 Bq l−1, consistent with measured groundwater values. Groundwater in both catchments was found to have higher and more variable radon concentrations (2–12 Bq l−1) in the near surface, weathered horizons, compared to a consistent 1 Bq l−1 from the solid Chalk. The variations in CI can be related to flow generation pathways and hydrological events. In the Lambourn, the radon budget is controlled by diffuse groundwater inputs, supporting the hypothesis that the alluvial aquifer plays a greater role during periods of high accretion. The Pang is more complex than the Lambourn having a combination of diffuse and point source inputs, with spring inputs dominating both flow and radon signatures in the lower part of the catchment. Significant temporal and spatial variations were determined for CI in both catchments reflecting their differing geologies and flow regimes. One use of radon in hydrology is the determination of groundwater discharges to rivers, but the observed variations in CI mean this approach may not be appropriate to all situations and that changes in source need further evaluation. Nonetheless, radon is shown to be a useful tracer of flow paths and processes within these catchments.