Groundwater quality assessment in the Khan- and Swakop- River catchment with respect to geogenic background concentrations of dissolved uranium Hannover, July 2010 Commissioned by: i Author: Dr. Robert Kringel, Dr. Frank Wagner & Dr. Hans Klinge Commissioned by: Federal Ministry for Economic Cooperation and Development (Bundesministerium für wirtschaftliche Zusammenarbeit und Entwicklung, BMZ) Project: Human Resources Development for the Geological Survey of Namibia, Engineering & Environmental Geology Subdivision BMZ-No.: 2008.2007.6 BGR-No.: 05-2332 BGR-Archive No.: 0129666 Date: July 2010 ii Summary Author: Dr. Robert Kringel, Dr. Frank Wagner & Dr. Hans Klinge Title: Groundwater quality assessment in the Khan- and Swakop-River catchment with respect to geogenic background concentrations of dissolved uranium Keywords: ephemeral river, geogenic background, groundwater, mine discharge, Namibia, uranium In 2009, 78 locations were sampled in the catchment areas of the ephemeral Swakop River and the tributary Khan River within the framework of a strategic environmental assessment (SEA) prior to new uranium mining activities. Samples were analysed for main components, dissolved uranium, and trace elements. 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. Nitrate concentrations are elevated yet below the WHO guideline value apart from a few exceptions. Potentially toxic trace element concentrations 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 and cobalt. Uranium is a common trace element in the groundwater of the catchment and mostly present at elevated concentrations. Only 21 % of analysed groundwater samples have uranium concentrations below the provisional WHO guideline value of 15 µg/L. 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. The chemical composition of water samples alone is insufficient and inconclusive to identify the source for the high uranium level in the respective groundwater samples. iii Table of Contents 1 Background................................................................................................................................... 5 2 Objectives ..................................................................................................................................... 5 3 Methods ........................................................................................................................................ 7 3.1 Sampling campaign................................................................................................................. 7 3.2 Analysis................................................................................................................................... 7 4 Results ........................................................................................................................................... 9 4.1 Main components and parameters.......................................................................................... 9 4.1.1 Water temperature ........................................................................................................... 9 4.1.2 Electrical Conductivity (EC)..............................................................................................9 4.1.3 Sodium adsorption ratio (SAR) .......................................................................................11 4.1.4 Cations and anions ........................................................................................................12 4.1.5 Redox conditions............................................................................................................ 18 4.2 Trace elements...................................................................................................................... 21 4.2.1 Uranium.......................................................................................................................... 21 4.2.2 Other trace elements of concern.................................................................................... 23 4.3 Classification of water types.................................................................................................. 24 4.3.1 Process water ................................................................................................................ 24 4.3.2 Drinking water samples.................................................................................................. 26 5 Conclusions ................................................................................................................................. 28 6 Extended summary ...................................................................................................................... 31 7 Acknowledgements ...................................................................................................................... 32 8 Bibliography ................................................................................................................................. 33 Appendix ......................................................................................................................................... 34 Analytical methods...................................................................................................................... 35 Correlation matrices.................................................................................................................... 36 Secondary uranium minerals ......................................................................................................39 Classification of water types........................................................................................................ 40 Water analysis sheets ................................................................................................................. 42 1 / 120 List of figures Fig. 1: Location of sampling points ................................................................................................... 8 Fig. 2: Distribution of electrical conductivity in all samples including mine process waters .............. 9 Fig. 3: Spatial distribution of electrical conductivity (EC)................................................................. 10 Fig. 4: Correlation between TDS and EC for alluvial groundwater samples (S 13 not included) .... 10 Fig. 5: E-W-projection of sampling depth for chloride as a measure of salinity (samples from Rössing and Langer Heinrich mine included) ..................................................................................11 Fig. 6: Sodium hazard versus salinity hazard of alluvial groundwater in sampled water <5000 µS/cm....................................................................................................................................11 Fig 7: Piper-diagram of all water samples, colour-table depending on percentiles of EC............... 13 Fig. 8: Br/Cl molar ratios of alluvial groundwater samples.............................................................. 14 Fig. 9: Spatial distribution of chloride .............................................................................................. 14 Fig 10: E-W-projection of the molar ratio of chloride to sodium (log10 scale)................................. 15 Fig. 11: Correlation between sulphate and electrical conductivity (EC) in alluvial groundwater ..... 16 Fig. 12: Correlation between boron and electrical conductivity (EC) in alluvial groundwater.......... 16 Fig. 13: Correlation between fluoride and electrical conductivity (EC) in alluvial groundwater....... 17 Fig. 14: Correlation between bicarbonate and electrical conductivity (EC) in alluvial groundwater 18 Fig. 15: Spatial distribution of iron (samples from Rössing and Langer Heinrich mine included) ... 19 Fig. 16: Spatial distribution of nitrate in alluvial groundwater .......................................................... 19 Fig. 18: Spatial distribution of uranium in alluvial groundwater ....................................................... 22 2 / 120 List of tables Tab. 1: Groundwater samples with dissolved uranium concentrations above the 90 % percentile. 22 Tab. 2: Linear correlation matrix of dissolved Uranium and other anion-forming elements (red (>0.75) for “strong correlation”, bold black (>0.35) for “positive correlation” and bold blue (≤0.35) “disputed correlation, but higher than background”. ....................................................................... 23 Tab. 3: List of water samples from the Langer Heinrich and Rössing uranium mines .................... 25 Tab. 4: Chemical composition of water samples from the Langer Heinrich and Rössing uranium mines .............................................................................................................................................. 26 Tab. 5: Linear correlation matrix of dissolved uranium and trace elements .................................... 36 Tab. 6: Linear correlation matrix of main components