The dataset consists of data for the UK for the Sustainable Development Goal 6.6.1: groundwater sub-indicator for the period 1990 to 2019. The dataset reports for the UK against Sub-Indicator 5 of Goal 6.6.1, following the recommended procedures in the UNEP report on monitoring methodologies for that sub-indicator. The sub-indicator is defined as the change in mean groundwater levels, averaged over a five-year period, from a mean in levels over a previous five-year reference period. Groundwater level data was obtained from BGS’ WellMaster database. 192 groundwater level monitoring stations were processed and, following quality control, 154 were used to provide estimates of regional variation in groundwater levels for 19 of the 34 HydroBASINS at Level 6 in Great Britain and Northern Ireland. As required by the guidance in the monitoring methodology, the sites chosen are representative of local and regional groundwater systems and are observation and monitoring boreholes where groundwater levels are not systematically affected by abstraction. All sites chosen have average monitoring frequencies of greater than one observation a month. The dataset is provided as a .csv file with the following headers: Column A: HYBAS_ID, HydroBASIN unique identification number. Column B: reference period (1990 to 1994). Columns C to AA: reporting periods (five-year periods starting in 1991) with data reported as percentage change (relative to reference period) in running mean five-year groundwater level by HydroBasin.