Abstract
The acid neutralizing capacity of Lake Okenuma, a small, acidic softwater lake in Fukushima prefecture, was estimated in order to assess the impact of acid rain on this lake. Laboratry sediment-water microcosms experiment for Lake Okenuma was conducted 23 days incubation period to simulate sediments-water interactions. Nitrate consumption and cation exchange were found to be major acid neutralizing processes in the microcosms, whereas sulfate reduction did not play an important roll. However, the nitrate consumption rate was 350μeq/m2/day in the microcosms, one eighth the rate in other lakes with high alkalinity in Japan. Cation exchange capacities of the sediments and soils of the watershed were 750μ eq/100 g and 130μeq/100 g respectively, one or two order of magnitude smaller than those of other lakes with high alkalinity in Japan and acidified lakes in Canada. The sulfate reduction rate was estimated from sulfate profile in the epilimnetic sediments pore water obtained with in situ membrane sampler. The sulfate reduction rate was only 2μeq/m2/day and this result was consistent with that from the micro cosms experiment. These results indicate that Lake Okenuma is one of the lakes in Japan most susceptible to acid rain.
