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. Sampling Stream sediments were the primary sample medium for G-BASE. The sediment was collected from the centre of the stream and sieved through two sieves (2 mm and 150 µm) to obtain a fine grain-size fraction of <150 µm. Excess material from the <2 mm fraction was panned to collect a heavy mineral concentrate. Stream water samples were collected also 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). The routine collection of soil samples was introduced in 1986 in areas of poor drainage density, and urban soil sampling commenced in 1991. Further details of the sampling methodology can be found in the G-BASE field procedures manual (https://nora.nerc.ac.uk/id/eprint/5190/). Sample analysis Stream water pH, conductivity and alkalinity were determined in the field. Water samples were returned to our laboratories and analysed by inductively coupled atomic emission and mass spectrometry (ICP-AES/MS), ion chromatography (IC) / ion selective electrode (ISE) for up to 50 chemical parameters. Waters were also analysed for non-purgeable organic carbon (NPOC) to determine dissolved organic carbon content. Most water samples were analysed for alkalinity, pH, conductivity, F and U and some for multi-element analyses including Al, Cl, Na, Si, SO4, NO4, and TOC. The <150 µm fraction of the stream sediment and the <2 mm fraction of topsoil were analysed by techniques including X-ray fluorescence (XRF) / direct reading optical emission spectrometry (DR-OES) to determine the concentration of up to 53 major and trace elements. Loss on ignition (LOI) and pH were determined in topsoil samples. Analytical data for the 150 micron fraction of soil and stream sediment samples are available for some or all of: Ag, As, B, Ba, Bi, Be, Ca, Ce, Cd, Co, Cr, Cs, Cu, Fe, Ga, K, La, Li, Mg, Mn, Mo, Nb, Ni, P, Pb, Rb, Sb, Se, Sn, Sr, Th, Ti, U, V, Y, Zn, Zr. In later stages, the project also routinely determined the elements listed in the <2 mm fraction of surface soils. Sample storage and archive After preparation and analyses all soil, stream sediment and panned concentrate samples and excess material were stored and archived at the National Geoscience Data Centre (NGDC). The archive holds more than 40 years’ worth of material collected across the UK. Archived samples can be made available for further tests and analyses on request by contacting BGS Enquiries (mailto:enquiries@bgs.ac.uk ).