Evidence of springwater acidification in the Vosges mountains (North-East of France): Influence of bedrock buffering capacity

Investigations on springwater acidity were carried out in the Vosges mounta ins (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 equa lly spread between 5.0 and 6.8 whereas on sandstone a majority of springs i s in the range 5.6 to 6.2. As a whole, but mainly on sandstone, from the 19 60's to 1990's, the shape of the pH distributions shifts toward greater aci dity. In the sandstone area, trends in pH, alkalinity, total hardness (corr esponding to divalent cations), sulfate and nitrate were considered over th e 30 yr period (1963-1996) in relation to the bedrock chemical composition. Kendall seasonal tau coefficients indicate that decreasing trends were sig nificant 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 drai ning clay enriched sandstone. This can be interpreted as a titration proces s by acid atmospheric inputs of the buffering capacity of weathering and ex change processes in the soils and the catchment bedrock. The nitrate presen ts an increasing step in the early seventies but possibly as a result of ch ange in analytical technics and/or increase in atmospheric inputs mainly re sulting from increase in fertiliser inputs in agricultural areas or in car traffic. Surprisingly no change in sulfate was noticed in any groups of spr ings probably as a result of the adsorption/mobilisation in the soils. Thes e long-term trends in spring waters (1963-1996) confirmed the soil and stre amwater acidification trends already mentioned in this region, in relation to acid atmospheric inputs since no climate nor forestry practice changes h ave been detected over the period. Moreover, in spite of acid atmospheric i nput reductions, no recovery can presently be detected.