Rare earth elements geochemistry of groundwater from Shanmuganadhi, Tamilnadu, India: Chemical weathering implications using geochemical mass-balance calculations

Abstract

Groundwater samples were collected from Shanmughanadhi in Dindigul district of Tamilnadu, India, to determine the concentration and fractionation of REEs during chemical weathering. Groundwater samples collected from aquifers hosted in weathered and fractured crystalline formations were analyzed by ICP-MS for Rare earth elements (REEs) after filtering by 0.45 μm membrane filters. ΣREEs in Charnockite accounts 8.49 × 10−4mgL−1 and 1.08 × 10−3mgL−1 in hornblende biotite gneiss signifying higher REEs in Gneiss than Charnockite, due to higher flushing rates in gneiss and relatively shorter water residence time in Charnockite aquifer. Higher LREEs were observed upstream in the study area, followed by enrichment of MREEs and HREEs in midstream and discharge areas, respectively. Irrespective of lithology, the groundwater samples mostly exhibited a positive europium anomaly, which might be due to higher dissolution and mobilization of europium from plagioclase group minerals through rock-water interaction. The plagioclase feldspar, orthopyroxene, and alkali feldspars mineral weathering rates were 124, 17.5, and 20.0 mol/hectare/year. The rare earth minerals enriched in the host rocks provided the referenced material for the REE enrichment in the groundwater. Variations in pH and redox conditions in groundwater environments have influenced the enrichment of REE aided by continuous aquifer residence time, chemical weathering, and groundwater flow path.