Shallow groundwater in drift and Lower Palaeozoic bedrock: the Afon Teifi valley in west Wales

Abstract

Abstract Detailed geological surveying in the Afon Teifi valley of west Wales has identified the distribution and geometry of a complex array of drift deposits over shales and greywackes of Silurian and Ordovician age. A well and borehole field inventory and groundwater sampling programme provides the basis for evaluating the groundwater flow patterns within these strata. Sustainable yields are mostly small with exceptional yields up to 345 m 3 d −1 . Potential infiltration derived from an estimate of baseflow is 535 mm a −1 ; much of this discharges locally and only 9 Mla −1 is estimated to flow longitudinally down the valley as groundwater throughflow. Total groundwater abstraction amounts to 760 Mla −1 . Groundwater generally satisfies the EC maximum admissible concentration guideline levels and is Ca-HCO 3 to Ca-Cl type. Differences in bedrock groundwater chemistry between the western and the eastern parts of the study area are largely controlled by the drift type. By analogy, the groundwater potential demonstrated in the Afon Teifi valley is likely to be indicative of the potential in geologically similar valleys elsewhere in west and mid Wales.