Using geological and geochemical information to estimate the potential distribution of trace elements in Scottish groundwater

There are currently few reliable data available for the concentrations of trace elements in Scottish groundwaters. A new project Baseline Scotland, jointly funded by the British Geological Survey (BGS) and the Scottish Environment Protection Agency (SEPA), seeks to improve the data availability and general understanding of the chemistry of Scotland’s groundwater. However, this is a major undertaking and these new data will take several years to collect and interpret across the whole of Scotland. In the interim, SEPA have asked BGS to use their existing knowledge and data to give a rough estimate of where certain elements are more likely to be elevated in groundwater. This information will be used to help focus future monitoring and give background for Baseline Scotland. Predicting trace element concentrations is difficult, in part due to lack of knowledge on the distribution of mineral phases, the reactivity of different minerals and the geochemical environment, particularly the redox status. This report scopes the potential scale of naturally elevated trace elements in Scottish groundwater, in particular those elements that are potentially harmful to health: e.g. aluminium, arsenic, barium, cadmium, chromium, lead, manganese, nickel, uranium and zinc. The problems and limitations of prediction are discussed in the report and this work does not replace a proper assessment based on actual chemical analyses of groundwater. The method uses information on the geochemistry of the Scottish environment derived from the most comprehensive geochemical data set for Scotland, the BGS Geochemical Baseline Survey of the Environment (G-BASE), combined with the limited data available on the chemistry of Scottish groundwaters. The conditions under which each of the elements can become elevated in groundwater are discussed and the geological and geochemical information interpreted to produce a series of maps highlighting areas where each trace element may be elevated in groundwater relative to the Scottish average. The maps are based primarily on the 1:625 000 scale bedrock geology map of Scotland. In order to make the scheme and the maps simple and manageable, we have used the same numbers to describe the individual rock units (1 to 114) that are usedd on the Geological map of the UK (Solid Geology): North sheet. Some rock units have been subdivided, and other small areas highlighted where additional information is known, either from G-BASE or previous studies. After assessing the results of the exercise the following conclusions can be drawn: 1. The study has provided a useful summary of geochemical information for trace elements in Scotland, and detail the conditions in which these elements may become elevated in groundwater. This provides essential background to the Baseline Scotland project, which aims to improve the availability of groundwater chemistry data and the general understanding of the chemistry of Scotland’s groundwater. 2. The predictions can be used as a first pass to help focus and prioritise additional monitoring and for helping to interpret groundwater chemistry data from different areas. The predictions are only preliminary and will be modified in the future by detailed groundwater sampling and interpretation. There are several caveats: • For all of the trace elements considered, the lack of available groundwater chemistry data with detailed analysis of trace elements, and their restricted spatial distribution, means that it is not possible to rigorously test whether the groundwater quality predictions are accurate or not. • More groundwater chemistry data are available for three elements, barium, manganese and zinc, allowing a rudimentary test of the predictive maps. For barium the prediction appears to work well, but there is poor correlation for zinc. For manganese, some correlation is evident, but the complexity and variability of local conditions are such that much variation is observed. • This approach, using broad, national scale geological and environmental data, cannot account for the complexity of the controls on groundwater chemistry: i.e. the heterogeneous nature of the Scottish environment, not least the aquifer mineralogy and glacial history, and the complex behaviour of trace elements in groundwater, determined by aspects such as flow pathways, residence times, and the geochemical environment (for example, oxidising/reducing or acidic/alkaline conditions). In summary, this approach appears to be a useful first step in trying to estimate the likely distribution of trace elements in Scottish groundwater, in the absence of much reliable groundwater quality data. However, only by systematically collecting reliable groundwater chemistry data, across different aquifers and regions and from different depths, can the variation in trace elements in groundwater across Scotland be understood. Careful modelling and interpretation of these new data in the context of the geology and environmental conditions will help make future predictions of groundwater quality more reliable and provide reference information for the Water Framework Directive