ISOTOPE STUDIES OF PIPEFLOW AT PLYNLIMON, WALES, UK

Residence times and flow paths of pipe and stream flow were studied du ring low flow in the Nant Gerig and Gwy experimental catchments at Ply nlimon in mid-Wales, UK, using a two-month time series of natural deut erium and electrical conductivity data from perennial and ephemeral pi pe flow, stream flow, groundwater and rainfall. Low flow in both the p erennial pipe and the stream was maintained by 'old' groundwater disch arge. This groundwater was at least 40 days old. Flow in the ephemeral pipe was dominated by old groundwater and was only slightly affected by direct inputs of new water. Although direct rainfall inputs contrib uted minimally to runoff in the perennial pipe and the stream, rainfal l influenced the isotopic and chemical character of the groundwater, R ainfall also affected the water-table elevation, which determined the flashiness of the perennial pipe flow and whether the ephemeral pipe f lowed. The isotope and electrical conductivity data suggest that storm runoff in both the main pipe and the stream is overwhelmingly old wat er. A sensitivity analysis suggests that the old water is supplied bot h from near-stream groundwater and upslope groundwater delivered by th e ephemeral pipes.