The Waters of the Spas, Mineral Springs and Holy Wells of Wales. C.SAXON and F.M. SLATER. Department of Applied Biology, UWIST, Llysdinam Field Centre, Newbridge-on-Wye, Powys. (Received February 1982: in revised form January 1983) Abstract This investigation was initiated because of the recent renewed interest in the main Welsh spas and the absence of any modern chemical analyses of them. As many mineral waters as possible within the Principality were located and analysed and their distrib- ution and chemistry were related to the geology of the area. Large differences in the quantity of certain ions were found between samples of the same water, taken at different times. A number of Holy Wells were also analysed, to see if they could be considered "mineral in character" and some of these had ion concentrations similar to those of the sulphur rich mineral waters. Introduction Mineral waters have been found throughout Wales but there is an abundance of them in mid-Powys. Many have been used for medic- inal purposes and cures for which they were reputed were similar to those claimed for water from the Holy Wells which are abundant in Wales (Table 1). The Holy Wells have a much longer history than the mineral waters, many dating back to Celtic deities (F.Jones, 1954). Development of Welsh Spas The Romans discovered the Trefriw mineral waters in Clwyd early in the third century and are supposed to have used Builth and Llan- drindod mineral waters. The earliest written record of the medicinal use of mineral waters since Roman times was in 1696 when the Vaughans of Herefordshire visited Llandrindod to take the waters. The use of these springs increased in the mid eighteenth century by which time Trefriw Wells, Caergwrle Wells, Llanwrtyd Wells and Llandegley Wells were operating. Most spas declined in popularity in the early nineteenth century due, among other things, to the in-