Hydrogeology of Wales

Wales enjoys a humid westerly airstream which provides a plentiful source of water. Public supply largely depends on upland gathering and surface storage, but groundwater is also supplied. Approximately 250 Ml d-1 (91 Mm3 a-1) or about 8 per cent of the total water in public supply in Wales derives from groundwater and a further 95 Ml d-1 (34 Mm3 a-1) is abstracted for private consumption from about 21 000 boreholes, wells and springs. Private abstraction is limited and of a local scale because of the indurated and fractured nature and modest permeability of many of the aquifers. It is nevertheless of significant social and economic importance and is used for drinking water, farming, and light industry. Development of groundwater remains patchy due to a perception that it is unlikely to be present in useable quantities in areas such as the hard rock terrains typical, for example, of much of central and west Wales. Groundwater is also important as it maintains low river flows during dryer periods. The traditional perception of groundwater in Wales is as an insignificant resource that has been a hazard to the mining communities. The Water Framework Directive (European Community, 2000) has brought new impetus to the understanding of groundwater in Wales as the Directive requires that the physical and chemical status of even the smallest producing aquifers be reviewed and remedial targets set. There are also several significant innovative groundwater schemes in Wales, including the Clwyd Augmentation/Abstraction Scheme and hydrogeological investigations carried out during the development of the Cardiff Bay Barrage. Wales has a wide range of aquifers that reflect its diverse geology. These include the Triassic sandstone aquifer in the Vale of Clwyd in north Wales, the only aquifer designated as Principal A type by the Environment Agency Wales, the ‘Old Red Sandstone’ of the Brecon area and large areas of Carboniferous Limestone. Quaternary and alluvial deposits also provide some resource potential. Much of central and upland Wales comprises Lower Palaeozoic and older bedrock. There are large areas of metasedimentary strata, typified by rocks such as mudstone, siltstone and sandstone which offer shallow fracture porosity and storage with no supporting intergranular storage. The Silurian and Ordovician rocks of west Wales, for example, sustain small springs and shallow boreholes enough to supply rural demand. The alluvial deposits that floor most valleys provide additional storage if they are in hydraulic continuity with the fractured bedrock. Deeper circulation occurs in the Llandrindod Wells area of central Wales where a number of chemically mature spring sources were used as Victorians spa resorts.