Influence of acid snowmelt on the chemistry of stream water in the forest watersheds of the cold and snowy region, Japan

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A series of observation was carried out to investigate the occurrence of acid snowmelt and its influence on the in the forest watersheds of northen Hokkaido. The results are as follows :1. The chemical composition of the snowpack of the watershed in mountain area was strongly affected by the presence of forest and had little relation with altitude. The snowpack on open area was acidic. On the other hand, those in forests showed lower acidity and higher ionic concentrations. These findings indicate that forests not only buffer the acidity of snowpack but also have an effect on trapping dry deposits.2. The pH of initial snowmelt was less than 4.0 and then rapidly increased as the progression of melting. A Continuous elution of ionic species was observed from the snowpack during the thaw period. Approximately 70% of H^+ eluted out during the thaw was lost with the first 20% of meltwater (chemical fractionation) , which indicated the strong release of acid in the beginning of the thaw.3. Although strong release of acid was evident for the meltwater, the decrease of pH was obscure in the streamwater of forest watersheds. Following reasons were considered : the meltwater penetrated into the deeper part in case of the watersheds of the rocks in tertiary period ; the streamwater had high HCO_3^- concentration in case of the watersheds of serpentine rocks. In addition, it was also possible that the forest on the watersheds decreased the acidity of the snowpack.4. As for Ca^<2+>, Mg^<2+> and HCO_3^-, their output from the watershed exceeded their input from the snowpack, which indicated they were supplied, to some extent, from the inside of the watersheds via biogeochemical processes. The respiration of microorganisms acted an important role for the generation of HCO_3^- Which was dominant proton source inside the watersheds after the thaw period.