Preliminary study on evaluation of effects of acid deposition on forested ecosystem in East Tanzawa Mountains estimated from chemical characteristics of stream waters

It has been reported that the nitrate concentration in stream water in the mountainside on Mt. Oyama (1252 m a.s.l), which is located at the western part of Kanagawa Prefecture, is higher than that in stream water in other regions, indicating that nitrogen saturation is occurring in the forested ecosystem. However, there has been no report on nitrogen saturation across the Tanzawa mountains, except Mt. Oyama. We investigated the current status of the stream water chemistry and the effect of acid deposition on the stream water chemistry across the East Tanzawa mountains. Thirty-eight samples were collected in nine mountain streams in this region. The mean pH and alkalinity in stream water in this region were 7.315 and 650 μeq/L, respectively. There was no symptom of stream-water acidification by acid deposition in East Tanzawa, probably because of the high acid buffering capacity by the weathering of carbonate and silicate minerals. Nitrate concentrations in the stream water ranged from 27.6 to 87.8 μeq/L and NH ₄ ⁺ was detected. High nitrate concentration in stream water suggests that nitrogen saturation is occurring in the forested ecosystem in East Tanzawa mountains. We divided our sampling sites into southeast (SE), southwest (SW), and north (N) regions by the ridge line of Mt. Tanzawa-Mt. Tonodake-Shindainichi-Mt. Sannoto-Mt. Takenodai-Mt. Oyama-Mt. Mitsumine. The stream water pH was lower and the concentrations of Na ⁺ and NO ₃ ^- were higher in south regions. There were high correlations between the concentrations of Na ₃ ⁺ (r = -0.854), Cl ^- (r = -0.815) and NO ₃ ^- (r = -0.922) and the distance from the coast of Odawara City, Oisomachi, and Ninomiyamachi. In addition, a high correlation between the NO ₃ ^- and Cl ^- concentrations (r = 0.741), which is an indicator of atmospheric deposition, in stream water was observed. Our preliminary investigation indicates that the runoff of nitrate, originated from atmospheric deposition, might occur in the forested ecosystem in the southern regions in East Tanzawa.