Distribution of rare earth elements and stable isotopic constituents along the groundwater flow paths in the Quaternary deposits of Imphal valley in north‐east India

Abstract

Groundwater samples collected from three flow paths (the western, central, and eastern flows) in the shallow aquifers of the Imphal valley of north‐east India were examined to study the variation in hydrochemical and rare earth element (REE) characteristics along these flow paths. Each flow path covers a length of around 45 km towards the down gradient. In the western flow, the hydrochemical facies evolved from Na‐Cl‐Ca to Ca‐Na‐Mg‐Cl‐HCO3; in the central flow, it varies from Ca‐Mg‐Na‐Cl to Na‐Ca‐Cl‐HCO3 and in the eastern flow, it changes from Na‐Cl to Ca‐Mg‐HCO3 indicating the evolution of saline water type in the piedmont zone to fresh water type towards the discharge zones around the Loktak Lake. The REE concentration varies along the flow path as the total light REE (LREE) is more enriched than total heavy REE (HREE) in the western and central flows, while LREE is less than HREE in the eastern flow. North American Shale Composite (NASC)‐normalized REE patterns in these flow types show significant convex‐up NASC‐normalized patterns with depleted LREEs. The redox condition in the flow paths is controlled by redox‐sensitive elements such as Eh, pH, Fe, Mn, U, Er, Gd, and Nd which vary relatively along with these flows. REE fractionation depicted by (Er/Nd)SN ratios are high around the upgradient and decrease along the down gradient towards Loktak Lake. The isotopic constituents (δ18O and δD) exhibit fluctuations in their ratios along the groundwater flow paths. The residual hill and piedmont zones are characterized by depleted isotopic composition while the alluvial and flood plains show enriched isotopic composition. Thus, the present study elucidates the behavioural change in major hydrochemical parameters including REE and isotopic constituents, along with the groundwater flow, which will provide a holistic view in understanding the evolution of groundwater in terms of its quality, quantity, and origin in the study area.