地球化学

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To clarify the change in water quality caused by impoundment in a small oligotrophic to mesotrophic reservoir, we report monthly data on dissolved components at the Iwamura Dam, Gifu Prefecture, Central Japan, from April 2004 to November 2005. Concentrations of dissolved SiO₂ and NO₃^- in the dam water tended to be 10 to 20% lower than those in the inflow water. Concentrations of PO₄^3- in both the inflow and the dam waters were below the detection limit (0.1μmol/l). The degrees of decrease in SiO₂ and NO₃^- concentrations tended to be larger from spring to summer than in other seasons. This seasonal change is attributed to the blooming of phytoplankton, in particular diatom. Concentrations of dissolved Fe and Mn increased in the dam especially in November December, to ten times those in inflow water, reaching 2300 and 1500 nmol/l, respectively. This increase is attributed to the regeneration of Fe and Mn from sediment and/or suspended material in oxygen-deficient bottom water below thermocline.

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