Source, mobilization and distribution of uranium in a complex aquifer system: a spatial and temporal evaluation using geochemical, statistics and GIS approach

The presence of uranium (U) in potable groundwater must be treated with extreme care because of its health concerns. The study area consists of complex lithology and various land use activities including natural and anthropogenic processes. The present study aims to assess the seasonal distribution, probable sources and factors responsible for U mobilization in groundwater and provides a holistic picture of geochemical processes in the study area. To achieve these objectives, a total of 680 groundwater samples were collected during four different seasons (pre-monsoon, southwest monsoon, monsoon and post-monsoon). The samples were collected with lithological differences to determine the effects of natural geochemical processes in addition to the anthropogenic impacts. The trend of anions reflected the following order during PRM and NEM: Cl > HCO3- > NO3 > SO4 > PO4, and cations Na > Ca > Mg > K during PRM and SWM. The U concentration in the samples ranged from 10 to 70 ppb, being higher during the NEM season. The samples were classified into three groups based on U concentration (< 10 ppb, 10-15 ppb and > 15 ppb) and studied using various geochemical diagrams, statistical application, thermodynamic study and saturation states for different minerals. The maximum concentration of U in groundwater during different seasons reflects that NEM > POM > SWM > PRM. The higher U values were observed in the hornblende-biotite-gneiss irrespective of the season. U in groundwater migrates away from the source due to its high solubility, where it tends to form complexes, especially in the presence of phosphates or carbonates. Analysis of the data shows that the majority of the samples in the monsoon season reflect dilution and recharge processes. In contrast, during SWM and PRM, ion exchange and anthropogenic influences predominate the processes. The PCA study shows that secondary salt leaching, weathering, anthropogenic influences and ion exchange play a significant role in determining the groundwater chemistry in this region.