Genesis of uranium containing groundwater in the Brahmaputra floodplain of Assam: Hydrogeochemistry and its associated health risk

Abstract

Groundwater uranium (U) content and the major hydrogeochemical processes were evaluated for six districts of Assam, lying mostly on the central and northern region of the Brahmaputra floodplain. A total of 504 groundwater samples were collected using the grid sampling technique. Uranium concentrations analysed using LED fluorimeter model LF-2a, ranged from below detection limit (BDL) to 13.5 μgL−1. All the U concentrations were found to be well below the prescribed limits of AERB, WHO and USEPA. The highest groundwater U was recorded in a sample from Nagaon, a central Brahmaputra floodplain district. The SEM-EDX spectra of the sediment samples revealed geogenic presence of U in the floodplain. A comparative assessment between the relationships of U with other groundwater parameters revealed that shallow aquifers provided more feasibility for elemental interactions and hence more mobility of U species. The correlation statistics in shallow aquifers depicted the importance of alkalinity on U release and mobilization. Uranium release during weathering of the geological strata was signified by the relationship of U with EC, TDS and salinity. The role played by pH and ORP in dissolution of U in the floodplain was not strong. The dominant water type in the study was Mg–HCO3, which was associated with higher U concentration. This revealed the positive role of carbonate dissolution towards U release in the floodplain. Hydrogeochemical investigation further implied for greater silicate weathering and ion exchange compared to that of carbonate weathering and reverse ion exchange processes. The calculated undersaturation of calcite, dolomite and gypsum explained their role as source minerals in the floodplain. Uranium dissolution and mobilization was found to have positive association with carbonate weathering, as depicted by its significant relationship with calcite, dolomite and gypsum. Health risk, both radiological and chemical, relating to U ingestion through drinking water was absent within the floodplain.