Evidence of springwater acidification in the Vosges mountains (North-East of France): influence of bedrock buffering capacity
Résumé
Investigations on springwater acidity were carried out in the Vosges mountains (north-
eastern France). Acid or poorly buffered spring and streamwaters were detected in the same area.
The proportion of acid springwaters (pH < 5.6) is about 20% among 220 springs. The springwater
pH on granite are equally spread between 5.0 and 6.8 whereas on sandstone a majority of springs is
in the range 5.6 to 6.2. As a whole, but mainly on sandstone, from the 1960’s to 1990’s, the shape
of the pH distributions shifts toward greater acidity. In the sandstone area, trends in pH, alkalinity,
total hardness (corresponding to divalent cations), sulfate and nitrate were considered over the 30
yr period (1963-1996) in relation to the bedrock chemical composition. Kendall seasonal tau coeffi-
cients indicate that decreasing trends were significant for the first three parameters. Linear regression
on the smoothed mean value revealed 18 and 90% decrease for pH and alkalinity respectively, for
springwaters draining poor-base cation sandstone whereas only 8 and 30% decrease respectively, was
observed on clay-enriched sandstone. On silica-enriched sandstone, alkalinity began to decrease in
the early 70’s as well as pH. Loss of alkalinity only occurred in the early 80’s for springs draining
clay enriched sandstone. This can be interpreted as a titration process by acid atmospheric inputs of
the buffering capacity of weathering and exchange processes in the soils and the catchment bedrock.
The nitrate presents an increasing step in the early seventies but possibly as a result of change in
analytical technics and/or increase in atmospheric inputs mainly resulting from increase in fertiliser
inputs in agricultural areas or in car traffic. Surprisingly no change in sulfate was noticed in any
groups of springs probably as a result of the adsorption/mobilisation in the soils. These long-term
trends in spring waters (1963–1996) confirmed the soil and streamwater acidification trends already
mentioned in this region, in relation to acid atmospheric inputs since no climate nor forestry practice
changes have been detected over the period. Moreover, in spite of acid atmospheric input reductions,
no recovery can presently be detected.
Domaines
Hydrologie
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