TC Namibia: Geogenic Background Concentration of Dissolved Uranium in Groundwaters of the Ephemeral Khan- and Swakop River
Report of the project:
- Namibia - Human Resources Development for the Geological Survey of Namibia, Engineering & Environmental Geology Subdivision
- Namibia – Beratung des Geologischen Dienstes (Phase II)
Sampling alluvial groundwater
Contributing to issues "Water" and "Radiation" within the framework of the "Strategic Environmental Assessment (SEA) for the central Namib Uranium Rush", 78 locations in the catchment areas of the ephemeral Swakop River and the tributary ephemeral Khan River were sampled in 2009. Sampling was organized by Bittner Water Consult (BIWAC) and carried out jointly by BIWAC, Federal Institute for Geosciences and Natural Resources (BGR) and Institute of Hydrology, University of Freiburg (IHF). The SEA was issued by the Ministry of Mines and Energy (MME) of the Republic of Namibia supported by BGR within the technical cooperation project "Human Resources Development for the Geological Survey of Namibia, Engineering & Environmental Geology Subdivision". It was carried out prior to potential new uranium mining activities in the Central Namib. Water samples were analysed at the BGR laboratory for main cations and anions, dissolved uranium and other trace elements.
The groundwater resources of the two rivers are limited. The alluvial aquifers are not homogenous, but separated into compartments created by outcropping bedrock or narrowing of the river gorge. These compartments are replenished by surface water infiltration from occasional floods. Currently the groundwater resources are not used for domestic supply. The alluvial aquifer of the Swakop River currently supply a small proportion of the total mining demand at Langer Heinrich uranium mine, as well as irrigation water for farmers in the lower Swakop (Ministry of Mines and Energy 2010).
Chloride in groundwater
Alluvial groundwater in the upper Khan and Swakop River catchments is freshwater of drinking water quality, whereas groundwater in the lower river catchment is saline. Salinity is due to sodium chloride whose main source can be attributed. The molar ratio of bromide to chloride plotted versus electrical conductivity equally points to a non-seawater sodium chloride-source. Apart from evapotranspiration and groundwater evaporation, characteristic bromide/chloride molar ratios as well as high boron concentrations indicate that dissolution of halite beds in a regional sedimentary formation (Gevers & Van der Westhuizen 1931) is most likely a major source of the sodium chloride in saline water.
Nitrate concentrations are elevated yet below the World Health Organization (WHO) guideline value apart from a few exceptions.
Potentially toxic trace element concentrations (arsenic, thorium, lead) are without critical implications for drinking water quality. Drinking water samples from the municipalities of Swakopmund and Walvis Bay meet the requirements of the Namibian and the WHO drinking water standards. Process and seepage water samples from uranium mines have elevated concentrations of uranium, arsenic and fluoride, manganese and a number of other trace elements like lithium, nickel, vanadine and cobalt.
Uranium in groundwater
Uranium is a common trace element in the groundwater of the catchment and mostly present at elevated concentrations as in the frequency distribution of all geogenic waters, process waters are excluded. Only 21 % of analysed groundwater samples have uranium concentrations below the provisional WHO guideline value of 15 µg/L. These include the majority of the freshwater samples in the upper river catchments. The 90 % percentile value (p90) is 230 µg/L and the median concentration (p50) is 39 µg/L. Concentrations of uranium in the upper and middle parts of the Swakop and Khan ephemeral rivers tend to be lower than in their lower parts, with some exceptions, and uranium concentrations tend to increase with increasing salinity towards the lowest parts of the Swakop and Khan rivers, again with some exceptions (lower values). Six groundwater samples in the vicinity of the Rössing and Langer Heinrich Uranium Mines and the lower Swakop River Valley have uranium concentrations above the regional background level. In an attempt to 'fingerprint' potential mine seepage, Schubert et al. (2010) investigated radon concentrations in the groundwater. The authors concluded that the distribution pattern in groundwater appear to correspond with the (elevated) radium background concentration of the sediments in the river beds and thus do not indicate seepage of tailings water into the alluvial aquifers.
Literature:
- BIWAC (2009): Overview of the sampling campaign conducted in June/July 2009 – SEA of the uranium Province: Water Balance Study of the Swakop and Khan Rivers. Unpublished report by Bittner Water Consult CC issued by the Ministry of Mines and Energy / Republic of Namibia, august 2009, appendices, 14 pages.
- GEVERS, T.W. & VAN DER WESTHUIZEN, J.P. (1931): The occurrence of salt in the Swakopmund area, South West Africa. Trans. Geol. Soc. S. Afr., 34, pp 61-80.
- KRINGEL, R., WAGNER, F. & KLINGE, H. (2010): Groundwater quality assessment in the Khan- and Swakop-River catchment with respect to geogenic background concentrations of dissolved uranium. Unpublished report ordered by GSN / BGR Namibia (Technical cooperation project BMZ-No.: 2008.2007.6) within the framework of the Namibian "Strategic Environmental Assessment for the central Namib Uranium Rush", by Ministry of Mines and Energy (MME) and Federal Institute for Geosciences and Natural Resources (BGR) (archive-no. 0129666), 18 figures, 9 tables, 5 appendices, 33 pages. Hannover. (PDF, 3 MB)
- MILLER, R. McG. (2008): The Geology of Namibia. Chapter: 25 Namib Group, Vol 3, Geological Survey of Namibia, Windhoek.
- MINISTRY OF MINES AND ENERGY (2010): Strategic Environmental Assessment for the central Namib Uranium Rush. Ministry of Mines and Energy, Windhoek. Pdf-document (multi-part) hosted by Southern African Institute for Environmental Assessment (SAIEA), Windhoek under www.saiea.com/uranium/index.html, 07.10.2011, (Citations in chapter 7 Cumulative effects: 7.4 - Water and 7.12 - Radiation).
- SCHUBERT, M. & KNÖLLER, K. (2010): Application of naturally occurring radionuclides and stable isotopes as environmental tracers in line with the “SEA Uranium Rush”. Helmholtz Centre for Environmental Research (UFZ), Leipzig/Halle.
- VAN WYK, A.E., STRUB, H. & STRUCKMEIER, W.F. (Eds.) (2001): Hydrogeological Map of Namibia 1 : 1,000,000. Published by DWA, GSN, Namwater and BGR, Windhoek.
- WORLD HEALTH ORGANIZATION (2004): Guidelines for Drinking Water Quality; Vol 1., 3.ed., Geneva.
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