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Baseline Scotland : an overview of available groundwater chemistry data for Scotland

By Alan M. MacDonald and Brighid E. O Dochartaigh


This report forms a national overview of existing groundwater chemistry data for Scotland, carried\ud out to provide a foundation for the new project Baseline Scotland. This project is jointly funded by\ud the British Geological Survey (BGS) and the Scottish Environment Protection Agency (SEPA), and\ud aims to improve data availability and understanding of the chemistry of Scotland’s groundwater.\ud Understanding baseline groundwater chemistry is fundamental to helping to implement the Water\ud Framework Directive: it is difficult to determine whether groundwater is polluted unless the\ud baseline is known. Also, the presence or absence of various elements in water can occasionally\ud give rise to health concerns or make the water unacceptable for human use.\ud Chemistry data from Scottish groundwater samples collected prior to the start of the Baseline\ud Scotland project in June 2004 have been collated and described. Many of the data came from\ud previous BGS studies, but data were also available from Scottish Water, councils and SEPA. In\ud total, more than 800 groundwater chemistry analyses were identified.\ud The data are of variable quality and were subject to a thorough quality-assurance assessment. This\ud involved gathering additional information on sources, checking the units for consistency, removing\ud data from highly contaminated sites and calculating ionic balances. After this process the final\ud dataset was as follows:\ud • The dataset comprised 428 sites with full major ion chemistry with an error in ionic balance\ud of less than 10%;\ud • Of these 428 sites, 31 sites had a wellhead measurement of dissolved oxygen, and 103 had a\ud wellhead measurement of the redox potential;\ud • Information on trace elements was generally limited. Only iron and manganese had more\ud than 200 measurements across Scotland. For arsenic and fluoride (two trace elements\ud generally considered the greatest risk for health in groundwater) data are particularly\ud limited: 65 meaningful measurements for arsenic, and 149 for fluoride. The quality of the\ud data is also variable – samples from Scottish Water and SEPA are generally not filtered at\ud the sample site.\ud The dataset is highly skewed, with most good quality data being available for the Devonian aquifer\ud in Fife and parts of Morayshire, and the Permian aquifer in Dumfries. There are few data available\ud for Precambrian or Silurian/Ordovician rocks.\ud The major ion data were used to characterise Scottish groundwaters. Summary statistics have been\ud calculated and the data presented on box plots, cumulative frequency plots and tri-linear diagrams.\ud The data illustrate the variability of Scottish groundwater, reflecting the diverse geology and local\ud environmental conditions.\ud • The most highly mineralised groundwater is found within the Carboniferous rocks. There is\ud a marked bimodal distribution for chloride and sodium which may indicate mixing with\ud formation water. Sulphate concentrations are considerably higher than for other aquifers\ud which may reflect the oxidisation of sulphide minerals within mudstones and legacy of coal\ud mining.\ud • Groundwaters from the Permian sedimentary aquifers in Scotland have well-defined\ud chemistry and the least variation in concentrations of major ions of all Scottish\ud groundwaters. Groundwater is of Ca-Mg-HCO3 type with a narrow range in Mg/Ca ratio,\ud and near-neutral pH. Most data are available for the Devonian aquifers in Scotland. Groundwaters tend to be of\ud Ca-HCO3 to Ca-Mg-HCO3\ud type and show a wider range in Mg/Ca ratio than for the Permian\ud rocks, but similar bicarbonate concentrations (median Devonian HCO3 178 mg/l, median\ud Permian HCO3 157 mg/l). Median pH from the Devonian groundwaters is 7.5.\ud • The available data for the superficial deposits, Precambrian rocks and Silurian/Ordovician\ud rocks indicate similar variability of major ion chemistry across the three hydrogeological\ud environments. This may reflect the short residence time of much of the water, and the high\ud proportion of samples from springs and shallow wells. The waters have typically low\ud mineralization and pH less than 7.\ud • Groundwaters in volcanic aquifers are generally of Ca-HCO3 type, and similar to the\ud sandstone aquifers, with the exception of potassium which varies considerably across the\ud volcanic aquifers. A small proportion of samples have chloride, sodium, sulphate and\ud calcium concentrations.\ud • Most of the small island sources have high chloride concentrations, probably as a result of\ud increased chloride concentrations in recharge. Other notable elevated chloride\ud concentrations are observed in the coastal areas of East Lothian, Arbroath, Morayshire and\ud northwest Fife The majority of inland sources have chloride concentrations less than\ud 25 mg/l, and in upland areas concentrations are less than 10 mg/l.\ud • Nitrate data indicate a clear pattern across Scotland, with concentrations in excess of\ud 25 mg/l as NO3 across the east and southwest. Concentrations over much of the rest of\ud Scotland are less than 10 mg/l as NO3. The elevated concentrations of nitrate largely\ud coincide with the agricultural areas of Scotland and are attributable to fertiliser applications\ud and the presence of intensive farming; this is consistent with other recent studies.\ud There are insufficient reliable data to identify clear trends in the concentration of trace elements in\ud Scottish groundwater. However, the following have been highlighted from the existing data.\ud • Iron and manganese concentrations greater than the EC maximum permissible value are\ud observed in all hydrogeological units in Scotland. Iron and manganese are ubiquitous in\ud rocks and the dominant controls on their presence in groundwater are the pH and redox\ud status.\ud • The limited data for aluminium indicate that concentrations approach or exceed the EC\ud maximum permissible value for drinking water in Precambrian and volcanic rocks and\ud granite. This is linked to the acidic conditions of groundwater in these environments.\ud • The available data indicate that natural chromium concentrations in Scottish groundwater\ud are typically well below the EC maximum permissible value, and usually below detection\ud limit. There are no data from the Glasgow area, where elevated chromium concentrations\ud may be expected due to widespread contamination from the chrome and steel industry.\ud • Fluoride concentrations greater than 0.5 mg/l have been measured in samples from the\ud Permo-Triassic sandstone in Morayshire and the Permo-Triassic sandstone in Mauchline,\ud southwest Scotland. There are several isolated samples scattered throughout Scotland with\ud fluoride concentrations greater than the EC maximum permissible value of 1.5 mg/l.\ud In conclusion, there is a lack of systematically collected groundwater quality data for Scotland.\ud Particularly lacking are data on trace elements and pH and redox status. The following\ud recommendations should be implemented to give a reliable national dataset.\ud 1. New data are required that are collected in a consistent, reliable and systematic manner, and\ud analysed to a consistent degree of accuracy. There is not widespread coverage of groundwater chemistry data in Scotland. Certain\ud hydrogeological units have few reliable data and should be targeted for further sampling.\ud 3. Wellhead measurements of redox potential and dissolved oxygen are required in order to\ud understand the groundwater geochemical processes and element distributions.\ud 4. There is evidence that the sampled chemistry is affected by the type of water source (e.g.\ud well, borehole or spring). It is essential that comprehensive data are collected on each\ud sampled source.\ud 5. Routine sampling for age dating would be helpful for understanding residence times of\ud groundwater and interpreting chemical variations – particularly nitrate data.\ud 6. There are insufficient data on many trace elements in Scotland to characterise their\ud occurrence or understand their origin. Comprehensive analysis of groundwater chemistry,\ud including trace elements, should be carried out wherever possible

Topics: Earth Sciences
Publisher: British Geological Survey
Year: 2005
OAI identifier: oai:nora.nerc.ac.uk:11335

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