Deciphering dissolved load geochemistry of the Upper Ganga Basin: Natural weathering vs human impact

Abstract

Traversing through one of Earth's most densely populated regions, the Ganga River carries the undeniable imprints of human activities. The present study aims to elucidate the relative contributions of natural weathering and anthropogenic processes in driving the dissolved load in the Upper Ganga Basin, investigating how these factors vary seasonally in mountainous regions while considering the diverse range of human influences. The study analyzed daily variations in major ions and trace elements in water samples collected from the Ganga River at Rishikesh during the year 2020, along with effluent, rainwater, faecal coliform, air quality, mobility data, and previously published data on Himalayan Rivers. A mass balance based forward model was applied to quantify contributions from atmospheric deposition, erosion, weathering, wastewater, and urban runoff. Co-occurrence network and principal component analysis were applied to decipher the interplay between natural weathering and human activities. Results indicate a seasonal shift in cation dominance, with anthropogenic activities peaking in the post-monsoon season. Carbonate weathering (73.6 % ± 7.9 %) followed by silicate weathering (15.2 % ± 5.1 %) dominated the cation load. Although urban runoff contributes to the dissolved load, geogenic sources (particularly carbonate rock weathering) are the primary contributors to heavy metals. Faecal bacteria levels of sewage samples were found to mirror chloride concentrations in the river. While natural weathering contributes significantly, residential pollution is a notable factor in altering the Upper Ganga River's dissolved load composition. To effectively restore the Ganga River, our findings emphasize understanding diverse pollution pathways and incorporating natural weathering processes into management plans.