Rare earth elements in surface waters and sediments of the Mgoua watershed, south western Cameroon

Abstract

The concentrations and distribution spectra of 14 stable rare earth elements (REE) were studied for surface waters (n = 30) and sediments (n = 30) of the tropical zone Mgoua watershed in southwest of Cameroon. Samples were analyzed using the inductively coupled plasma mass spectrometer (ICP-MS) method. The ∑REE concentrations of the surface waters vary from 0.11 to 6.60 µg/L with an average value of 1.18 µg/L. This average is 150 times lower than the average values of the Post-Archean Australian Average Shale (PAAS). In the sediments, the concentrations range from 282.12 to 727.67 ppm with an average value of 550.05 ppm, making it 2.5 times higher than the PAAS. Rare earth spectra normalized to PAAS are spread out and show heavy rare earth enrichment; positive Eu anomalies; negative, low, or no Ce anomalies; and a gradual decrease in heavy earths in river and well waters and sediments, but are sawtooth with strong negative Ce anomalies in effluent waters and sediments. A low rare earth fractionation is noted in river and well waters [(La/Yb)N ~ 0.35–1.19], while it is high in effluent waters [(La/Yb)N ~ 0.42–56.99]; a slight depletion of rare earths in all environments [(La/Sm)N ~ 0.69–2.35], as well as a depletion of heavy rare earths [(Gd/Yb)N ~ 0.58–1.94]. In the sediments, the same characteristics are found with elemental ratio values differing slightly [(La/Yb)N ~ 1.15–1.99]; [(La/Sm)N ~ 0.75–1.36]; and [(Gd/Yb)N ~ 1.15–2.01]. This study shows that the Mgoua watershed does not have a high concentration of REE, although in the sediments they are well above the average PASS value while they are insignificant in the waters.