This data provides the results of a survey of the water quality of small streams draining forested and felled catchments across Wales. The water quality measurements are extensive, including analysis of major, minor, trace and ultra-trace elements together with nutrient and standard water quality measures such as pH and Gran alkalinity. Opportunistic sampling was undertaken with the aid for Forest Enterprise staff to sample sites at periods of both dry and very wet weather in order to assess the water quality under baseflow and stormflow conditions, respectively, to assess groundwater and soil endmember chemistries. The work was undertaken as part of a joint NERC, Environment Agency and Forestry Commission funded study to examine the impacts of conifer harvesting and replanting on upland water quality (Neal et al., 1998). Small catchment sites (2 to 5 ha) were chosen single tree and soil type at each location. Across the sites, the number of samplings varied between 1 and 10 depending upon feasibility of sampling. The monitoring period was from the 7th September 1995 up to the 18th November 1997.The scope and range of the Welsh survey work together with the findings are provided by Neal et al., 1998. Full details about this dataset can be found at https://doi.org/10.5285/6361c484-42bd-4e0c-874f-ef22dc55129f