Biogeochemical Influences on the Determination of Water Chemistry in a Temperate Forest Basin: Factors Determining the pH value

Abstract

In order to clarify the mechanism of pH determination in a temperate forest watershed in Japan, intensive hydrochemical observations that included in situ measurement of dissolved pCO2 were carried out in 1991 and 1992. From the variations of observed pCO2 and pH and estimated alkalinity associated with the hydrological process, the factors determining pH were described. There were two hydrological processes which have different determining hydrochemical processes: (1) rainfall and throughfall to infiltration in the soil layer to stable groundwater and (2) stable groundwater to spring water to stream water. In the first process, pH is influenced by infiltration from the low pCO2 layer to the high CO2 layer and by an increase of alkalinity, which is mainly caused by an exchange reaction and chemical weathering. In the shallow soil layer the protons for alkalinity generation are supplied by acid deposits from rainfall and throughfall, microbial acid production, and CO2 dissolution reaction. In the deeper layer an increase of alkalinity caused by Na+ generation becomes remarkable as depth increases. This process is strongly controlled by chemical weathering. In the second process, pH increases with CO2 degassing around the spring point. The alkalinity is kept at the same level as that of the stable groundwater. These results suggests that the biochemically supplied CO2 in soil not only directly controls the pH determination, but also has influences on the alkalinity generation as another determining factor of pH.