Groundwater chemistry varies widely as a function of complex and interacting geological, geochemical, hydrogeological and climatic factors. These give rise to large variations in chemical quality — in both space and time — at a range of scales. The ‘baseline’ for a given chemical will vary significantly both between and within aquifers. It is scale-dependent and should be considered as a range rather than a single value.
Sampling for microplastics in groundwater, Oxfordshire. BGS © UKRI.
What do we mean by baseline?
Opinions differ on the precise meaning of the concept of ‘baseline’ in the context of groundwater chemistry. Our aim has been to establish the chemistry of water under natural conditions.
For some parameters, the definition is clear-cut: the presence of purely artificial substances such as pesticides or CFCs represents a departure from natural conditions. However, many solutes can be derived either from pollution or from natural sources. Examples include nitrate, phosphorus and arsenic. To add further complication, a given groundwater body may contain some specific pollutants alongside other chemicals that are entirely naturally derived.
Our working definition of ‘baseline’ is:
The range in concentration (within a specified system) of an element, species or chemical substance present in solution that is derived by natural processes from natural geological, biological or atmospheric sources.
How do we investigate the baseline?
We have been characterising the groundwater chemistry and interpreting the main geochemical processes in each of our study areas. This has involved:
- collating existing, reliable data for groundwater chemistry (spatial and time-series data), rainfall, land-use and aquifer mineralogy and geochemistry
- new sampling of groundwater sources for a comprehensive suite of inorganic constituents (typically 25 to 35 samples per study area)
Data has been interpreted and mapped to characterise local groundwater conditions. We have compared these with similar aquifers elsewhere and investigated changes down the groundwater flow gradient and, where possible, with depth and time. Statistical methods, including summary data (medians and percentiles), box plots and cumulative probability diagrams, also provide valuable analytical tools for assessment of the chemical data.
Find out more about UK baseline sampling and analysis.
Sampling groundwater in Andover. BGS © UKRI.
Baseline reports
BGS has an established programme of research on the baseline chemistry of groundwater from aquifers in England, Wales and Scotland. Investigations have helped to:
- interpret the baseline groundwater conditions, against which impacts of pollution can be assessed
- characterise groundwater quality
- establish the dominant controlling processes
Measuring wellhead chemistry. BGS © UKRI.
The reports provide a summary of the inorganic chemical status of groundwaters in a given study area or aquifer, and the key pressures on regional water quality. Results of the investigations are useful to groundwater users and potential users, water suppliers, and regulators with a remit to manage available resources, establish the current status of groundwater quality and ensure national compliance with European legislation such as the Water Framework Directive or the Drinking Water Directive.
Contact
Please contact Pauline Smedley for further information.