Groundwater quality of crystalline bedrock aquifers in Norway
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In cooperation with the Geological Survey of Norway, the Norwegian Radiation Protection Authority, and local health authorities, 1604 groundwater samples collected from Precambrian and Palaeozoic crystalline bedrock aquifers and 72 groundwater samples from Quaternary aquifers were analysed for radon, pH, and major and minor elements. A representative subset of 476 samples was further analysed for trace elements at the Bundesanstalt für Geowissenschaften und Rohstoffe, Germany. There appears to be a degree of lithological control on radon, fluoride, uranium and several other trace elements, shown as elevated contents in granitic groundwaters and depletions in anorthositic groundwaters. However, the concentration of a particular element in groundwater from a distinct lithology may span over 4 orders of magnitude, suggesting that other (kinetic and hydrodynamic) factors are equally important. A significant percentage of the analysed hard rock groundwaters do not fulfil the Norwegian standards for drinking water quality (the following as percentages of the n=476 subset), e.g. pH: 7%, Rn: 16%, F: 16%, Na: 1.5%, Fe: 14%, Mn: 26%, Al: 3.8%, Zn: 2.3%, Ba: 8%, As: 1.5 %, and U: 12 % (using US norm). pH values range from 5.6 to 9.8 in crystalline bedrock groundwaters with a predominance between 8.0 and 8.3, irrespective of rock type, suggesting buffering by the calcium-carbonate system. Geochemical modelling indicates that pH values above 8.3 may possibly be due to exhaustion of this buffering system by calcite precipitation, as well as by cation exchange against sodium. pH has also been shown to influence the concentrations of many elements in groundwater. Groundwaters from Quaternary sedimentary aquifers in Norway are generally less hydrochemically mature than their bedrock counterparts, with lower pH values and concentrations of Rn and F well below drinking water norms. Regular sampling of five boreholes during one year indicates that crystalline bedrock hydrogeochemistry is relatively stable over time, but intrusion of waters of short residence time and "surficial nature" (possibly due to poor well construction) can affect the water quality significantly. The groundwater chemistry of Norwegian hard rock aquifers resembles their Canadian and Fennoscandian counterparts, but differs systematically from lithologically comparable British aquifers. This suggests a distinct geochemical evolution for groundwaters from areas strongly affected by glacial erosion during Quaternary time. This may be related to the fact that aquifer host-rock is relatively chemically and physically "unweathered" in glacially scoured terrains, compared with deeper weathering profiles present in landscapes not recently glaciated.