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

In order to clarify the mechanism of p H determination in a temperate forest watershed in Japan, intensive hydrochemical observations that included in situ measurement of dissolved p CO 2 were carried out in 1991 and 1992. From the variations of observed p CO 2 and p H and estimated alkalinity associated with the hydrological process, the factors determining p H 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, p H is influenced by infiltration from the low p CO 2 layer to the high CO 2 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 CO 2 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, p H increases with CO 2 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 CO 2 in soil not only directly controls the p H determination, but also has influences on the alkalinity generation as another determining factor of p H.