This dataset contains the physicochemical parameters, ion concentrations and isotopic compositions (Si, Mg and Sr isotopes) of groundwater, river water, rainwater, tap water, thermal spring, rocks and estuarine water samples collected from Cardiff and Severn Estuary in the UK. Groundwater samples were collected from boreholes in Cardiff using a 1 L groundwater bailer, estuarine water samples were collected from surface (<2m) and at depth (~10m) at four sites along the Severn Estuary using a 2.5 L Niskin bottle. Two rainwater samples were collected at Bristol (51°27'24.0"N, 2°36'09.6"W) and Cardiff(51°27'45.4"N, 003°10'46.9"W) , one thermal spring sample was collected from the Taffs Well in Cardiff (51°32'41.3"N, 003°16'16.8"W) and one tap water sample was obtained within the main building, Cardiff University, Cardiff (51°29'20.3"N, 3°10'47.3"W) . Temperature, pH, salinity, dissolved oxygen and electrical conductivity of the water samples were measured in the field using a Hach meter (Hach HQ40d portable meter), the salinities of the groundwater samples were examined using a hand-held salinity meter. Apart from the tap wate sample, all water samples were immediately filtered upon collection, using pre-cleaned 20 mm syringes and 0.22 μm filters (Sartorius Minisart® Syringe Filter, Polyethersulfone), and subsequently stored in acid-cleaned LDEP bottles. Subsamples for cation concentration and isotope analyses were acidified with in-house double-distilled hydrochloric acid (HCl) to pH around 2. Three fresh bedrock samples were collected from the outcrop at Penarth Head, Wales, UK (51°26'35.4"N, 003°09'55.9"W), including one red mudstone, one green mudstone and one evaporite (alabaster). One glacial gravel sample was obtained from 10 - 11m depth (top of aquifer) from the Observation OBS2 Borehole in the Marl Park, Cardiff, provided by the British Geological Survey. All the measurements of ion concentrations and isotope compositions of the samples were carried out at laboratories in the University of Bristol.