Disturbance and resilience of a granitic critical zone submitted to acid atmospheric influence (the Ringelbach catchment, Vosges Mountains, France): Lessons from a hydrogeochemical survey in the nineties

Abstract

The chemistry of precipitations and stream waters in the critical zone of a small granitic catchment mainly covered by grassland has been investigated backward (period 1990–1997). Major elements concentrations, fluxes and budgets at annual and seasonal scales allowed evaluating the catchment response to variation trends in atmospheric deposition and hydrological patterns. Acid precipitation was efficiently buffered by soil cations exchange and mineral weathering processes, as attested by the dominance of Ca and HCO3 in stream waters. A decrease of sulfate acidity in precipitation following clean air measures was accompanied by an increase of alkalinity and a decrease of sulfate in stream waters. Waters of short residence time from water-saturated areas in the valley bottom and rapid shallow circulations within slopes were a very effective diluted weathering end-member contributing to stream outlet in high flow conditions, whereas evapotranspiration from saturated areas and/or deep waters with long residence time influenced the stream water concentration pattern in low flow conditions.Water discharge controlled the variations of the annual and seasonal budgets of major elements, except for nitrate and sulfate, mainly stored during summer. Soil legacy sulfate was mainly released during the first autumn stormflows, with high peak concentrations decreasing rapidly from 1990 to 1992 and disappearing afterwards. The output water flux was the main driver of the weathering rate in the acidification recovery period 1993–97, contrary to the first period 1990–92 when acidification was still under way, as attested by the weathering plateau (constant Si/ΣBC ratio). At that time, the intense weathering testifies the disturbance caused by acidification process. However, this critical zone was resilient enough to allow rapid and significant recovery over a few years following sulfur atmospheric abatement. For the future, the atmospheric nitrogen deposition pressure remains still challenging in a global change context, which argues for the necessity of long-term observatories.