Stream sediment geochemistry for the UK

The stream sediment geochemistry map layers show interpolated concentrations of 31 chemical elements analysed in approximately 111 000 stream sediment samples collected across the UK. Sampling was carried out annually from 1968 to 2013; less than 150 µm size-fraction sediment samples were collected from first and second order (that is, small) streams across the country at an approximate sample density of one sample per 2.7 km². These map layers are designed to visually highlight geochemical anomalies — areas where significantly high values of each element were measured in stream-sediment samples.

Stream sediment geochemical mapping is a key survey method for mineral reconnaissance and accordingly it can be used to identify potential targets for critical raw material resources. It is, however, important to note that the visualisation scheme used here is relative: it shows the location of samples that have high concentrations in comparison with most others in the dataset. This means that, in isolation, the layers should not be taken as verifying the presence of any prospective mineral deposit(s). Likewise, high values of any potentially harmful element should not be taken to indicate the presence of contaminated land or watercourses.

While element concentrations are primarily influenced by natural factors, including bedrock geology, weathering processes and mineralisation, human effects such as urbanisation, industry, mining and agriculture are also important influences. For example, in the case of lead (Pb), copper (Cu) and antimony (Sb), high values are associated with some of the UK's most heavily urbanised and industrialised areas such as Leeds, Bristol, Glasgow and Birmingham.

The geochemistry map layers are derived from research conducted by BGS's Geochemical Baseline Survey of the Environment (G-BASE) and GSNI's TellusNI Geochemical Survey. The G-BASE survey was funded by the UK's Natural Environment Research Council (NERC). The TellusNI survey was funded by the Northern Ireland Department of Enterprise, Trade and Investment (DETI).

Element concentrations were determined by several different analytical methods, which changed during the data collection. These methods included:

• inductively coupled plasma mass spectrometry (ICP-MS)
• atomic absorption spectroscopy (AAS)
• optical emission spectroscopy (OES)
• delayed neutron activation (DNA)
• X-ray fluorescence spectrometry (XRFS)

Data quality was assured by inclusion of certified reference materials, internal standards and field and analytical duplicates. A programme of cross-reference samples and data levelling ensured data consistency between analytical methods and sample analysis batches (Everett et al., 2019).

Each analytical method used offers the ability to determine a different range of elements; as a result, the spatial coverage of each element map varies and, for some elements, the available data covers only a limited area of the UK. For many elements, some areas in the UK have no coverage because the analyses of samples collected in these areas did not measure concentrations of that element. However, baseline stream-sediment data is not presented for the Isle of Man and the area within the boundary of the Greater London Authority as no surface-drainage sampling was carried out in these areas. The limitations of the analytical methods also means that data is not available for certain critical metals, for example tellurium (Te) and palladium (Pd).

Further details of the analytical methods used and the spatial distribution of samples analysed by each analytical method are documented in Everett et al. (2019).

Concentrations of the major elements are expressed as oxide weight per cent and for all other elements as mg/kg. A tabulated summary of key statistics (number of samples; lower limit of detection.) for each element is provided in Table 1 and Table 2.

To produce the map layers, interpolation to convert the original stream-sediment geochemistry point sample data into a continuous surface was performed by ordinary kriging of the log10 transformed concentration values (except for potassium [K₂O] and scandium [Sc]) using the nine nearest neighbour samples at a 500 m grid resolution. The log transformation was applied because element concentrations show a highly skewed distribution, except in the cases of K₂O and Sc, which show near-normal distribution. The log10 transformed values assigned to each grid cell were subsequently back-transformed to concentration values in the final map layers. Because the interpolation requires each measurement to be assigned a numerical value, concentrations below the lower limit of detection (LLD) were assigned a value of half the LLD prior to interpolation.

The colour scheme applied to the map layers is intended to highlight the highest concentrations and it uses 11 class divisions based on a percentile scale. The concentration ranges for each class division are indicated on the legend provided for each layer. The values that each of these ranges correspond to are shown in Table 1.

Additional information

Everett et al. (2019) provides more information about the history of the G-BASE and TellusNI projects, and further detail about the sampling, sample preparation, analysis and data quality control methods.

Map layers for 18 selected elements were previously published by BGS as individually downloadable element raster files in ASCII format, along with a supporting stream sediment geochemistry user guide.

References

EVERETT, P A, LISTER, T R, FORDYCE, F M, FERREIRA, A M P J, DONALD, A W, GOWING, C J, B, and LAWLEY, R S. 2019. Stream sediment geochemical atlas of the United Kingdom. British Geological Survey Open Report OR/18/048. (Nottingham, UK: British Geological Survey.)