Uranium concentrations in the groundwaters of the Derbyshire Dome
Abesser, C.; Smedley, P.. 2006 Uranium concentrations in the groundwaters of the Derbyshire Dome. Nottingham, UK, British Geological Survey, 43pp. (IR/06/072) (Unpublished)Before downloading, please read NORA policies.
This report describes the concentrations and distribution of uranium (U) in the groundwater of the Derbyshire Dome and discusses potential sources and controls on U mobility within the aquifer. The Derbyshire Dome is an anticlinal structure which consists of marine Carboniferous Limestone (Dinantian), flanked by shales and sandstones of the Millstone Grit (Namurian). The limestone is extensively mineralised in some parts, in particular in the east of the study area and different zones of mineralisation are distinguished, reflecting the west-east progression from calcite to barite to fluorite as the dominant gangue minerals. The hydrogeology of the study area is dominated by natural karstic features but even more by the ‘anthropogenic karst’ which has developed as a result of the extensive mining activities that have created a network of mine passages and drainage adits (“soughs”). Water movement through the aquifer is generally very rapid, but varies temporally as well as spatially. Thermal springs issue from a number of locations, some of which are known or believed to have bulk ages of up to several thousand years. Uranium is present in the bedrock as well as in the groundwater of the study area. The mineralogy and precise nature of U in the bedrock(s) is not known but a number of potential sources within the Dinantian and Namurian have been identified. These include organic matter fragments and hydrocarbon globules within the matrix of the Carboniferous Limestone, the organic-rich shales of the Namurian (Millstone Grit Series) and bedrock mineralization. A total of 26 groundwater samples were collected in July 2005 and analysed for a comprehensive suite of major, minor and trace elements, including U. Samples were collected from various sources including natural springs, decommissioned mines and soughs as well as from industrial and farm boreholes. The results show that aquifer lithology is the dominant control on the groundwater chemistry in the study area, hence Ca-(Mg)-bicarbonate waters are the most common groundwater type. Some groundwaters of Ca-(SO)-Cl type and Na-HCO43 type are also present in the study area and they are due to different thermal sources, mixing with deep (connate) waters as well as ion exchange and mineral dissolution processes. -1Uranium concentrations in the groundwaters range between < 0.2 and 4.23 μg l, and do not exceed the WHO provisional guideline value for U in drinking water of 15 μg l-1. The concentrations of U in the groundwaters are strongly controlled by pH, redox conditions and the presence of complexing agents such as carbonate or phosphate. Bedrock mineralization appears to be a major control on U concentrations in the groundwater as indicated by the spatial distribution of dissolved U in the study area as well as by the relatively high concentration associated with drainage from soughs and mines. The observed U concentrations are low compared with U-mineralised areas worldwide. This could be due to the limited solubility of uranyl minerals at pH > 7 and increased adsorption of U onto hydrous ferrous oxide (Hfo) surfaces. Alternatively, uranyl carbonate solubility controls or the ‘exhaustion’ of easily leachable U source may have contributed to the low U concentrations. While U concentrations are well below the WHO provisional guideline value for U concentrations in drinking water, it is possible that U activities (234238U and U) in some of the sampled groundwaters exceed the WHO screening value for total alpha emissions in drinking water, in particular where 234U dominates over 238U. This study has shown that the highest observed U concentrations in the groundwater of the Derbyshire Dome are largely associated with bedrock mineralisation, probably due to (1) the abundance of U-bearing minerals in mineral veins and secondary iron deposits and (2) the high surface area provided by cavities and rock debris resulting in enhanced bedrock weathering. High U concentrations are also present in the geochemically more evolved thermal waters and U enrichment is likely to result from prolonged water-rock interactions and increased mineral solubility at higher temperatures.
|Item Type:||Report (UNSPECIFIED)|
|Programmes:||BGS Programmes > Groundwater Management|
|Additional Information:||This item has been internally reviewed but not externally peer-reviewed|
|Additional Keywords:||Derbyshire, Groundwater, Uranium, GroundwaterBGS, Groundwater quality|
|NORA Subject Terms:||Earth Sciences
|Date made live:||28 May 2009 09:26|
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