This preliminary study highlights the vulnerability of water resources to mercury (Hg) contamination in the northern Indian plains, the global hotspot of atmospheric Hg emission. The total dissolved Hg in the river, lake and groundwater samples ranged from 5 to 1414 ng/L (n = 13). The dissolved Hg concentration increased by two orders of magnitude in the Gomati River due to the untreated effluent inputs from Lucknow urban center and by three fold in the Ganga River due to economic development during the last two decades. The dissolved Hg concentration in water resources was reported several folds high during the monsoon season and above the prescribed drinking water standards of World Health Organization. Atmospheric deposition was predominantly identified as source of Hg in the Karela Lake water (364 ng/L). The present study underscores the significance of monsoon-controlled anthroposphere-to-aquatic Hg transfer in the alluvial plain that acts as the caterer of 7% of the world's population.

To evaluate the mercury sources and its transportation mechanism from a global hotspot region (the Ganga Alluvial Plain, northern India), two sets of time-series samples of shallow groundwater (n = 31) and river water (n = 31) were collected in alternate days, from the Behta River Basin, in July and August 2019. The estimation of total dissolved mercury was performed by using the Flow Injection Atomic Absorption Spectroscopy-Mercury Hydride System. The mercury concentrations ranged high (> 1400 ng/L), in the three orders of magnitude, emphasizing the importance of mercury transportation, through the unconfined alluvial aquifer system, linked with the monsoon precipitation. In July, the accumulative intensity of monsoon precipitation favored the increasing mercury concentrations in the river water, as well as, in the groundwater. The infiltration of mercury contaminated water is the prominent hydrological process accountable for the peaked mercury concentration (above the World Health Organization's drinking water standard of 1000 ng/L) in the river basin. In August, about 67% of the collected samples reported base-level mercury concentrations. Temporal variations of mercury concentration in the groundwater and the river water showed significantly higher values in July, than in August. These results illustrate the importance of the interaction between the monsoon precipitation, and anthropogenic mercury, in the river basin, for the riverine and groundwater mercury transportation as the fundamental hydraulic control. The present study underscores the significance of monsoon precipitation on the groundwater transportation of global mercury cycling by the Ganga River.