A fingerprinting method for the identification of uranium sources in alluvial aquifers: An example from the Khan and Swakop Rivers, Namibia

Abstract

A fingerprinting method for identifying sources of uranium in shallow groundwater alluvial aquifers of the Khan and Swakop Rivers was established using 234U/238U ratios and 235U/238U ratios in the areas that drain the Rossing Uranium mine and the Langer Heinrich Uranium mine, in Namibia. In most groundwater aquifers that drain basement granitoids enriched in uranium the contribution of the total uranium in the shallow alluvial aquifers may be significant. Another source of uranium in shallow alluvial aquifers maybe from anthropogenic sources associated with mining activities as is the case in our study area. The distribution of radionuclides in water depend on various factors that influence their solubility and mobility and control their concentration in water such as pH, Eh, O2 and availability of ligands. The study identified a methodology that can fingerprint the two sources i.e., a natural source where 234U/238U ratios are above unity and a second one where this ratio is below unity implying that the source is anthropogenic. In the study area, 234U/238U activity ratio is above unity (1.3–1.7) and 235U/238U is 0.045±0.015 that both identify a natural source for all elevated uranium and other radionuclides in groundwater of the study area. The uranium values in groundwater exceed the WHO guideline value of 15μg/l and it increases in the lowest part of Swakop River; but there is no gradual or systematic change in uranium concentration thus indicating that concentration is related to local factors such as the geology and lithology of the aquifer material, Eh and pH for each borehole. The 238U decay series exhibits disequilibrium due to different fractionation processes that include decaying of radioactive elements and alpha recoiling.