Streamwater pH, alkalinity, p CO 2 and discharge relationships in some forested Mediterranean catchments

The streamwater chemistry of four catchments at Prades and one catchment at Montseny (NE Spain) is described and the relationships between streamwater alkalinity, pH and pCO 2 with discharge and that between pH and alkalinity are highlighted. At Montseny, results from a subsurface seepage are also presented. The Prades and Montseny catchments have similar vegetation (dense forest of holm oak. Quercus ilex L.) and lithology (phyllites). However, the catchments differ strongly in their hydrological behaviour: only 8-15% of the rainfall passes to the stream from the two gauged Prades catchments, whereas the Montseny catchment provides 50% of rainfall to the stream. Hence the mineralization of streamwaters is much higher in the Prades catchments. All the streams studied show an inverse relationship between alkalinity and discharge. Although pH also decreases with discharge in the Montseny catchment. it remains high during high flows in the Prades catchments. The Prades streamwater chemistries are determined by the solubility equilibrium of calcite: at high flows alkalinity decreases and, as a result of calcite solubility controls, this change results in an increase in pH. Consequently, pH and log(alkalinity) are either negatively correlated or uncorrelated in the Prades streams. At Montseny, the relationship between pH and alkalinity is significantly positive: calcite solubility controls do not come into play. All waters analyzed show an excess pCO 2 (epC02) with respect to atmospheric concentrations. The highest epC02 values usually correspond to low flows. However, study of the variation of epCO2 during 15 individual storm events sampled at Montseny shows that epCO 2 increases with discharge. At Prades, the degassing of epCO 2 in the stream channel produces calcite precipitation.