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

摘要

There are currently few reliable data available for the concentrations of trace elements inudScottish groundwaters. A new project Baseline Scotland, jointly funded by the BritishudGeological Survey (BGS) and the Scottish Environment Protection Agency (SEPA), seeks toudimprove the data availability and general understanding of the chemistry of Scotland’sudgroundwater. However, this is a major undertaking and these new data will take several yearsudto collect and interpret across the whole of Scotland.udIn the interim, SEPA have asked BGS to use their existing knowledge and data to give audrough estimate of where certain elements are more likely to be elevated in groundwater. Thisudinformation will be used to help focus future monitoring and give background for BaselineudScotland. Predicting trace element concentrations is difficult, in part due to lack ofudknowledge on the distribution of mineral phases, the reactivity of different minerals and theudgeochemical environment, particularly the redox status.udThis report scopes the potential scale of naturally elevated trace elements in Scottishudgroundwater, in particular those elements that are potentially harmful to health: e.g.udaluminium, arsenic, barium, cadmium, chromium, lead, manganese, nickel, uranium and zinc.udThe problems and limitations of prediction are discussed in the report and this work does notudreplace a proper assessment based on actual chemical analyses of groundwater.udThe method uses information on the geochemistry of the Scottish environment derived fromudthe most comprehensive geochemical data set for Scotland, the BGS Geochemical BaselineudSurvey of the Environment (G-BASE), combined with the limited data available on theudchemistry of Scottish groundwaters. The conditions under which each of the elements canudbecome elevated in groundwater are discussed and the geological and geochemicaludinformation interpreted to produce a series of maps highlighting areas where each traceudelement may be elevated in groundwater relative to the Scottish average.udThe maps are based primarily on the 1:625 000 scale bedrock geology map of Scotland. Inudorder to make the scheme and the maps simple and manageable, we have used the sameudnumbers to describe the individual rock units (1 to 114) that are usedd on the Geological mapudof the UK (Solid Geology): North sheet. Some rock units have been subdivided, and otherudsmall areas highlighted where additional information is known, either from G-BASE orudprevious studies.udAfter assessing the results of the exercise the following conclusions can be drawn:ud1. The study has provided a useful summary of geochemical information for traceudelements in Scotland, and detail the conditions in which these elements may becomeudelevated in groundwater. This provides essential background to the Baseline Scotlandudproject, which aims to improve the availability of groundwater chemistry data and theudgeneral understanding of the chemistry of Scotland’s groundwater.ud2. The predictions can be used as a first pass to help focus and prioritise additionaludmonitoring and for helping to interpret groundwater chemistry data from differentudareas. The predictions are only preliminary and will be modified in the future byuddetailed groundwater sampling and interpretation.udThere are several caveats:ud• For all of the trace elements considered, the lack of available groundwater chemistryuddata with detailed analysis of trace elements, and their restricted spatial distribution, means that it is not possible to rigorously test whether the groundwater qualityudpredictions are accurate or not.ud• More groundwater chemistry data are available for three elements, barium, manganeseudand zinc, allowing a rudimentary test of the predictive maps. For barium theudprediction appears to work well, but there is poor correlation for zinc. For manganese,udsome correlation is evident, but the complexity and variability of local conditions areudsuch that much variation is observed.ud• This approach, using broad, national scale geological and environmental data, cannotudaccount for the complexity of the controls on groundwater chemistry: i.e. theudheterogeneous nature of the Scottish environment, not least the aquifer mineralogy andudglacial history, and the complex behaviour of trace elements in groundwater,uddetermined by aspects such as flow pathways, residence times, and the geochemicaludenvironment (for example, oxidising/reducing or acidic/alkaline conditions).udIn summary, this approach appears to be a useful first step in trying to estimate the likelyuddistribution of trace elements in Scottish groundwater, in the absence of much reliableudgroundwater quality data. However, only by systematically collecting reliable groundwaterudchemistry data, across different aquifers and regions and from different depths, can theudvariation in trace elements in groundwater across Scotland be understood. Careful modellingudand interpretation of these new data in the context of the geology and environmentaludconditions will help make future predictions of groundwater quality more reliable and provideudreference information for the Water Framework Directive.