Environmental Impact and Geochemistry of Rare Earth Elements in Shallow Groundwater from Oban Massif and Environs, Southeastern Nigeria

Abstract

The study of hydrogeochemical processes using rare earth elements was carried out using water samples from weathered basement aquifers in Oban Massif and environs. A total of only 29 as number water samples were collected from boreholes, streams, springs and river sources. Physical parameters (pH, Eh, Ec, TDS and turbidity) were measured insitu using standard equipments. Anions (NO3-, SO42-, HCO3-, PO43-, Cl-) were measured in the laboratory using titration and chromatography, major cations (Mg2+, Ca2+, K+, Na+) were analysed using inductively coupled plasma argon emission spectrometry while the rare earth elements were assayed using inductively coupled plasma mass spectrometry. Speciation modeling for solution complexation and saturation indices was done using the software VISUAL-MINTEQ3.1. Results showed that low to medium pH(5.61-7.20) range and Fluorine mean value  20.08 ppm is common in the area which are not within  WHO (2006) recommended values of 6.5-85 and 1.5 respectively for potable water. Rare earth elements exhibited very low values with exception of Ce (0.98 ppb), Gd (0.13 ppb), La (0.56 ppb), Nd (0.66 ppb), Pr (0.15 ppb) and Sm (0.11 ppb) that recorded appreciable concentrations compared to Dy (0.09 ppb), Er (0.05 ppb), Eu (0.02 ppb), Ho (0.02 ppb), Lu (0.011 ppb), Tb (0.01 ppb), Tm( 0.01 ppb) and Yb (0.04 ppb). Trilinear plot showed that the water type is dominantly Ca+Mg, Cl+SO4. Rare earth elements concentrations are within permissible limits for potable water. Light rare earth elements had dominance over heavy rare earth elements. Rare earth elements were higher in groundwater than surface water. The dominant complexing ligand in groundwater is Carbonato complex ligand (LnCO3)3 while surface water is dominated by free ionic REE species. Oversaturated rare earth minerals at S>1 are dominantly light REE phosphates (PO43-). In surface water both light and heavy rare earth elements minerals of phosphate ligand were supersaturated. Normalized rare earth elements to granite of Honkong and PAAS showed more enrichment of HREE. Two water types classified as recharge (type1) and discharge (type 2) were identified in the area. Rare earth elements levels are not toxic and within permissible level for potable water. Minor treatment of water to reduce acidity should be expedient and a research on bioavailability of REE in humans within the area is important.