Insights into mercury in glacier snow and its incorporation into meltwater runoff based on observations in the southern Tibetan Plateau.

The Tibetan Plateau (TP) is recognized as "Water Tower of Asia". Yet our understanding of mechanisms influencing incorporation of mercury (Hg) into freshwater in mountain glaciers on the TP remains quite limited. Extensive sampling of environmental matrices (e.g., snow/ice) were conducted on the East Rongbuk glacier on Mt. Everest and Zhadang glacier on Mt. Nyainqentanglha for Hg speciation analysis. Speciated Hg behaved quite different during snowmelt: a preferential early release of DHg (dissolved Hg) was observed at the onset of snowmelt, whereas PHg (particulate-bound Hg) and THg (total Hg) become relatively enriched in snow and released later. Small fraction of Hg in snow was lost during a snowmelt day (18.9%-34.7%) with a large proportion (58.1%-87.3%) contributed by PHg decrease, indicating that the deposited Hg is most likely retained in glacier snow/ice. Furthermore, THg were positively correlated with PHg and crustal major ions (e.g., Ca, Mg) during snowmelt, indicating that Hg is mainly migrated with particulates. The main pathway of Hg loss during snowmelt was most probably associated with release of PHg with meltwater, which was greatly influenced by ablation intensity of snow/ice. This should be paid particular concern as Hg preserved in mountain glaciers will mostly enter aquatic ecosystem as climate warms, impacting on downstream ecosystems adversely. Obvious decrease of THg during the downstream transport from glacier was observed with a large proportion contributed by PHg decrease. The main removal mechanism of Hg was associated with sedimentation of PHg during the transport process.