Near-stream soil water-groundwater coupling in the headwaters of the Afon Hafren, Wales : implications for surface water quality

Summary Hard-rock acid headwater catchments typically exhibit a rapid streamflow response and concomitant rapid mobilisation of soil-derived solutes, such as aluminium, into the aquatic environment during storm events. The rapid stream responses are paradoxically associated with pre-event water dominating the storm hydrograph, however the sources and mechanisms by which ‘old’ water enters the stream channel and interacts with the soil horizons are still poorly understood. To investigate these processes a detailed and novel field study was established in the riparian zone and lower hillslopes of the Hafren catchment at Plynlimon, mid- Wales. This study showed that shallow bedrock groundwaters discharge into the stream channel. Pressure wave propagation in response to recharge further upslope caused a rapid displacement of shallow groundwaters up into the soils in the near-stream hillslope. A lateral fast flow horizon transported water down slope as interflow at the soil–bedrock interface such that the upper soil horizons remained largely unsaturated. Only where there was a discontinuity in the lateral fast flow horizon was water forced up as an ephemeral spring discharge at the soil surface. At this site, the major zone of soil water–groundwater coupling was in a narrow (20–25 m) strip next to the stream channel. The zone of soil water–groundwater interaction next to the stream channel is likely to depend on the nature of the lateral flow pathways and the hillslope characteristic. This study has shown the importance of the near-stream environment as a locus for soil waters that are bedrock groundwater derived; these groundwaters dominate processes in the deepest soil horizons from where soil components such as aluminium are sourced. Understanding these physical processes is fundamental for understanding upland catchment functioning and has important implications for solute transport modelling and for the sustainable management of surface water systems and stream ecology in these headwater regions.