Ionic Components of Rain Water in Rural Hinterland Areas Close to Urban Area

Abstract

During the period from Oct. 1 to Nov. 30, 1982, rain water samples was collected in two rural hinterland areas close to an urban area, Nara city and a hillside of Mt. Ikoma, by sequentially sampling of every 1 mm and 5 mm rainfall, respectively. And pH, electric conductivity (E. G.) and concentrations of nine dissolved ions (NH₄⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄^2-, NO₃^-, Cl^-, F^-) were measured and the property of the rain water was investigated.
Concentrations of the dissolved ions, pH and E. C. in the rain water decreased with the elapse of rainfall and became practically constant after 6 mm rainfall. Thus, the rain water was divided into two groups, the early rain water (1-5 mm rainfall amount) and the succeeding rain water (6mm-rainfall amount).
The maximum frequencies of pH in the rain water in the two sampling sites were concentrated at 4.5-5.0 for both the early rain water and the succeeding rain water. On the assumption that the acid rain water was defined by the rain water with a pH of less than 5.6, 90% of the early rain water and 100% of the succeeding rain water were found to be the acid rain water at the two sampling sites.
The dissolved ions in the rain water at the two sites were mainly consisted of the six cations and four anions determined in this study. On the equivalence basis, the cations and anions in the whole rain water were found in the order of decreasing concentration NH₄⁺, H⁺, Na⁺, Ca²⁺, K⁺, Mg²⁺, and SO₄^2-, Cl^-, NO₃^-, F^-, respectively, in Nara city, while Na⁺, H⁺, Ca²⁺, NH₄⁺, Mg²⁺, K⁺, and Cl^-, SO₄^2-, NO₃^-, F^-, respectively, at a hillside of Mt. Ikoma.
Contribution of sea salt to the dissolved ions in the rain water at a hillside of Mt. Ikoma near the seacoast relative to Nara city was speculated.