The influence of atmospheric deposition and climate change driven catchment internal processes on the recovery from acidification of high-altitude Alpine lakes.

The impact of atmospheric deposition and environmental factors on catchment processes and water chemistry of 20 high-altitude Alpine lakes in Southern Switzerland was investigated over four decades. Through the analysis of input-output budgets of sulphur (S), nitrogen (N), base cations and alkalinity significant trends emerged. Notably, S and N input concentrations significantly declined since the 1980s, by approximately 78 % and 22 %, respectively, with N primarily declining after 2000. Recovery from acidification was slightly delayed, likely due to the increased release of S, possibly originating from legacy S pools, alongside the simultaneous reduction in leaching of base cations from exchange sites. Catchments heavily impacted by thawing cryospheric features increasingly released S and base cations due to enhanced weathering processes, with hardly any impact on the recovery process, as evidenced by the balanced releases of S and base cations. N output concentrations followed the decrease of N input concentrations, while the relative N retention in the catchments remained relatively stable. Recently, both input concentrations of S and N have stabilised, while output concentrations of base cations began to increase across all catchments. The trend likely arises from the stabilisation of S and N input concentrations and/or the ongoing increase in weathering rates induced by climate change. Consequently, there was a consistent rise in alkalinity output concentrations even after the stabilisation of the S and N input concentrations. Ion ratio analysis suggests that carbonation primarily drives weathering processes in catchment areas unaffected by thawing cryosphere, while in areas impacted by thawing cryosphere, sulphide oxidation (or sulphate dissolution) is the dominant process. Further recovery depends on future N deposition and the effects of climate change.

Factors controlling the water quality of rock glacier springs in European and American mountain ranges.

Rock glaciers (RGs) provide significant water resources in mountain areas under climate change. Recent research has highlighted high concentrations of solutes including trace elements in RG-fed waters, with negative implications on water quality. Yet, sparse studies from a few locations hinder conclusions about the main drivers of solute export from RGs. Here, in an unprecedented effort, we collected published and unpublished data on rock glacier hydrochemistry around the globe. We considered 201 RG springs from mountain ranges across Europe, North and South America, using a combination of machine learning, multivariate and univariate analyses, and geochemical modeling. We found that 35 % of springs issuing from intact RGs (containing internal ice) have water quality below drinking water standards, compared to 5 % of springs connected to relict RGs (without internal ice). The interaction of ice and bedrock lithology is responsible for solute concentrations in RG springs. Indeed, we found higher concentrations of sulfate and trace elements in springs sourcing from intact RGs compared to water originating from relict RGs, mostly in specific lithological settings. Enhanced sulfide oxidation in intact RGs is responsible for the elevated trace element concentrations. Challenges for water management may arise in mountain catchments rich in intact RGs, and where the predisposing geology would make these areas geochemical RG hotspots. Our work represents a first comprehensive attempt to identify the main drivers of solute concentrations in RG waters.

Response of atmospheric deposition and surface water chemistry to the COVID-19 lockdown in an alpine area.

The effects of the COVID-19 lockdown on deposition and surface water chemistry were investigated in an area south of the Alps. Long-term data provided by the monitoring networks revealed that the deposition of sulfur and nitrogen compounds in this area has stabilized since around 2010; in 2020, however, both concentrations and deposition were significantly below the average values of the previous decade for SO4 and NO3. Less evident changes were observed for NH4 and base cation. The estimated decrease of deposition in 2020 with respect to the previous decade was on average - 54% and - 46% for SO4 and NO3, respectively. The lower deposition of SO4 and NO3 recorded in 2020 was caused by the sharp decrease of SO2 and particularly of NOx air concentrations mainly due to the mobility restrictions consequent to the COVID-19 lockdown. The limited effects on NH4 deposition can be explained by the fact that NH3 emission was not affected by the lockdown, being mainly related to agricultural activities. A widespread response to the decreased deposition of S and N compounds was observed in a group of pristine freshwater sites, with NO3 concentrations in 2020 clearly below the long-term average. The rapid chemical recovery observed at freshwater sites in response to the sharp decrease of deposition put in evidence the high resilience potential of freshwater ecosystems in pristine regions and demonstrated the great potential of emission reduction policy in producing further substantial ameliorations of the water quality at sensitive sites.