Sources of dissolved ions revealed by chemical and isotopic tracers in the Geum River, South Korea

Abstract

Water samples were collected in the main channel of the Geum River, South Korea, and measured dissolved elemental concentrations and isotopic compositions of nitrate in order to identify the factors controlling water chemistry. Elemental concentrations significantly increased location-wise after the confluence from urban areas, indicating the changes in solute sources from chemical weathering to anthropogenic inputs such as manure, fertilizers, and sewage. In particular, the effect of sewage input is manifested in the concentrations of Cl−, SO4 2−, and Na+, while the NO3 − concentration is influenced mainly by soil inputs with minor contributions from manure and fertilizer because both δ15N–NO3 and δ18O–NO3 indicate NO3 − mostly consists of soil-derived nitrates in the upper reaches but manure/sewage nitrates in the lower reaches. The relative proportion of three factors, Cl−, Ca2++Mg2+, and NO3 −, indicates that water chemistry in the upstream is controlled by the soil weathering but that in the downstream by the sewage. Seasonally, water chemistry during summer is dominated by the soil weathering due to the flushing effect but that during winter by the sewage. This study suggests that the relative proportion of three factors can be used for tracing natural and anthropogenic sources in water chemistry.