This dataset includes rainfall, river and stream hydro-chemistry data from the River Hafren (Severn). The dataset represents high-frequency (7 hourly) monitoring of stream hydrochemistry at both the Lower and Upper Hafren site from 2007-2009, as well as rainfall hydrochemistry near the Carreg Wen meteorological site. Data for over 50 chemical determinands are presented alongside data for some in-situ measurements such as water temperature. Full descriptions of the analytical methods used for each determinand is included. The Plynlimon research catchments lie within the headwaters of the River Severn in the uplands of mid-Wales. Intensive and long-term monitoring within the catchments underpins a wealth of hydrological and hydro-chemical research; other linked datasets include river flow, meteorology and a variety of detailed spatial datasets representing the topography, soils and rivers of the catchments. Monitoring is funded by the Centre for Ecology & Hydrology, and is ongoing since 1968. Full details about this dataset can be found at https://doi.org/10.5285/551a10ae-b8ed-4ebd-ab38-033dd597a374
