Tracking Dissolved Trace and Heavy Metals in the Ganga River From Source to Sink: A Baseline to Judge Future Changes

Understanding how dissolved trace elements chemically evolve in the Ganga River from source to sink is important to understand subcatchment contributions and chemical variability across space and time but remains poorly constrained. What exists is site-specific data sets that are focused on capturing contamination "hotspots." Here, we present riverine trace element concentrations of 38 targeted locations in the Ganga Basin. Samples in the headwater and the upstream segments of the river were collected during the premonsoon, monsoon, and postmonsoon seasons of 2014, 2015, and 2016, and the downstream samples were collected in 2016. In addition, monthly time-series samples were collected at a downstream site to capture the geochemical variability at a higher temporal-resolution. To evaluate the geogenic contributions, groundwater, rainwater, snow, glacier-ice, and sediment samples were also analyzed. We find that the river chemistry displays a wide spatio-temporal variability. Headwater samples are characterized by high concentrations of trace elements that are primarily controlled by ice meltwater, intense weathering, and interactions with glacial flour and are therefore geogenic in nature. Moreover, high concentrations of trace metals were also observed in a few localized downstream sites. However, such enriched signals are not persistent further downstream as they get diluted by the joining of large tributaries. We show that the dissolved trace element concentrations in the Ganga River are low compared to existing datasets and are comparable to the global average river water composition. We additionally quantified the present-day "baseline" concentration ranges to facilitate future water quality assessment in the Ganga Basin.