In 2022 BGS was commissioned by Dŵr Cymru Welsh Water (DCWW) to undertake desk and field investigations to develop a conceptual understanding of the contribution of groundwater to streamflow during drought in the Afon Fathew, Wales. This report details the findings of these investigations. In addition to a desk study, two field visits were completed to survey water features in the catchment, take samples for groundwater residence time indicators, and undertake a passive seismic (Tromino) geophysical survey. The results of the desk study and field visits were combined with flow accretion profile data to develop a conceptual model of groundwater flow to the Afon Fathew during drought, described herein.
The Fathew is underlain by a bedrock of silty mudstones which are traditionally considered to be poor aquifers. In the Fathew catchment there is evidence from boreholes for local-scale groundwater flow in the bedrock within fractures and other discontinuities. An upper weathered layer, in combination with faulting and folding patterns, is likely to control the geometry and magnitude of bedrock groundwater flow systems and the location of springs. The residence time indicator data suggest that groundwater in the bedrock is over 40 years old. Estimated discharge from bedrock springs (< 2 l/s, 0.17 Ml/day) is very small relative to the total flow in the Fathew and tributary inflows. The Tromino has shown the superficial deposits in the catchment to be highly heterogeneous in the valley bottom. Changes in the likely permeability and areal extent of the superficial deposits going down the valley bottom correspond to changes in river flows in the Fathew based on the accretion profiles. The Fathew and its tributaries are losing over well drained alluvial gravels, and gaining over low permeability lacustrine and clay-ey alluvial deposits. The Fathew is likely to be hydraulically isolated from the Dysynni catchment. 60% of low flow inflows to the Fathew are coming directly from upland tributary inflows. where very limited superficial deposits are present. In these upland settings during dry periods it is likely that the majority of discharge is coming from baseflow from bedrock.
Baseflow support to the Fathew during drought periods can be conceptualised as a two-phase system: (1) Discharge from the superficial deposits to the river, particularly associated with the down-catchment variability in the permeability and thickness of the deposits, (2) Discharge from the weathered bedrock aquifer into the river, from both springs and tributary inflows. The contribution of these two processes is likely to vary as drought conditions develop. Moreover, flows in springs and tributaries may contribute to downstream storage within the superficial deposits, which may complicate the deconvolution of the Fathew river flow hydrograph into different flow components. This temporal sequencing requires further investigation. Further work such as groundwater and surface water monitoring during dry periods and electrical resistivity tomography may be beneficial to constrain these uncertainties
