Hydrological processes: insights from a three-year study in Alpine basins based on a multichemical approach

In mountainous regions, snow, glaciers, and rock glaciers play a significant role as valuable hydrological assets by supplying substantial quantities of meltwater for both surface and subsurface flows. The current climate changes are resulting in a global phenomenon of fast glacier retreat and an earlier onset of the snowmelt runoff season. Consequently, this trend is leading to a reduction in mountain water resources, which has repercussions on aquatic ecosystems and human societies.

But how can these processes be estimated and modeled?
Is it possible to quantify the relative importance of the ice melt, snowmelt and rainwater to stream runoff?
How is the stream water chemistry and quality affected by the contribution from different runoff components?

A recent study, conducted by the esteemed collaboration of the Free University of Bozen-Bolzano, the Office of Hydrology and Dams of the Autonomous Province of Bozen-Bolzano, and Eco Research, and subsequently published in the Journal of Hydrology, has successfully tackled these inquiries through an extensive three-year research project in two alpine catchments. The study employed a multichemical approach to attain a comprehensive understanding of the hydrological mechanisms associated with snow, glaciers, and rock glaciers. The primary aim of the study was to ascertain the relative significance of glaciers and rock glaciers in influencing the quality of freshwater resources on a catchment scale. The outcomes of this study furnish invaluable insights into the intricate hydrological processes involved, elucidating the impact of diverse runoff components on the chemistry and quality of stream water.

Glaciers and rock glaciers are key elements of mountain hydrological systems, but their relative influence on the chemical and isotopic conditions of streams within the river continuum is still overlooked. During three consecutive years (2019–2021), we studied 24 stream sections in two catchments (Plima and Schnals, Eastern Italian Alps) with varying cover of glaciers and rock glaciers. End-member mixing models based on δ2H and d-excess revealed a large spatial and temporal variability in the contribution of different water sources to stream runoff. Overall, snowmelt (77 ± 17 %) and rainwater (5 ± 5 %) were the largest and the smallest runoff components, respectively. The ice melt contribution was high in streams fed by glaciers (23 ± 15 %) and rock glaciers (16 ± 16 %). In the highly-glacierised Plima basin, the tracer-based estimation of annual ice melt fraction matched reasonably well (90–167%) the mean annual glacial ice loss estimated by geodetic mass balance. In contrast, we found a large overestimation of the ice melt component derived from mixing models in the poorly glacierised (but rock glacier-rich) Schnals catchment. In streams influenced by rock glaciers, at both catchments the particular temporal patterns of electrical conductivity resulted in unreliable estimates of the meltwater/ groundwater fractions of runoff. Depending on the local lithology, concentrations of trace elements (Sr, Ni, Ba, Mn, Zn, Al) were high in streams fed by rock glaciers and glaciers, and close/below the limits of quantification in non-glacial streams. In alpine areas, the abundance of rock glaciers can confound the isotopic and chemical signature imparted by glaciers, thus hindering the use of tracer-based methods for hydrograph separation. Under the combined influence from glaciers and rock glaciers, concentrations of trace elements can surpass the limits for drinking water quality even in downstream areas, as we observed at the Schnals catchment for nickel.

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