Evidence for the redissolution of soil spodic horizons under the influence of acid deposition in the Vosges mountains (north-eastern France)

The speciation of soluble Al present in leaching waters was studied in an acid watershed with podzolic soils from the upper soil horizons to two spring waters. The main pattern of soil water circulation during high rainfall events was found to be vertical leaching through the upper soil eluvial horizons, then lateral flowing within the upper part of the deep spodic horizons. Between the bottom of the eluvial horizons and the spring waters, organic Al disappeared, whereas new inorganic monomeric Al species appeared. The latter were ascribed to the redissolution of the amorphous Al previously accumulated within the soil spodic horizons rather than to the dissociation of pre-existing F-bound Al or organic Al species. Atmospheric pollution-derived acidity was involved in this process, rather than soil-derived acidity. As such horizons contain high amounts of potentially toxic Al and trace metals in readily mobilizable forms, such a redissolution constitutes a true chemical time bomb for the future.