The BGS Geochemical Baseline Survey of the Environment (G-BASE) was the national strategic geochemical mapping programme in Great Britain. The project set out to establish the chemistry of the surface environment by the collection and analysis of stream sediment, stream water and soil samples. Beginning in the late 1960s in northern Scotland and moving southwards across the country, the primary focus was mineral exploration, however, the project quickly developed to address important environmental concerns. The final G-BASE samples were collected in southern England in 2014. The outputs from the G-BASE project provide an invaluable, systematic baseline of geochemical information for Great Britain, serving as a marker of the state of the environment against which to measure future change. Stream water samples were collected at each drainage site. Four different water samples were collected routinely: two filtered waters (for major and trace elements) and two unfiltered waters (for pH, conductivity and alkalinity). Samples were collected for approximately 85% of Great Britain, but a wide range of analytes were only determined from the Wales and Humber-Trent atlas areas southwards. Stream water pH, conductivity and alkalinity were determined in the field. In the later stages of the programme, stream water samples from high order streams were analysed by ICP-AES/MS for 27 elements (Al, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, La, Li, Mg, Mn, Mo, Na, Ni, P, Pb, S (as SO42-), Si, Sr, V, Y, Zn and Zr) and by quadrupole ICP-MS for 24 trace elements (Ag, Al, As, Ba, Be, Cd, Ce, Co, Cr, Cu, La, Li, Mo, Ni, Pb, Rb, Sb, Sn, Th, Tl, U, V, Y and Zr). Some samples were analysed by ion chromatography (IC). Automated colorimetric methods were used to determine Cl and NO3-, and an ion selective electrode (ISE) technique was used to determine F. Waters were also analysed for non-purgeable organic carbon (NPOC) to determine dissolved organic carbon content. All samples were routinely analysed for pH, conductivity and bicarbonate. Much of the UK coverage also includes uranium and fluoride analyses. For more information about accessing these samples and their analytical results, contact BGS Enquiries (enquries@bgs.ac.uk ).