Assessing the influence of watershed land use patterns on the major ion chemistry of river waters in the Shimousa Upland, Japan

To assess the influence of land use pattern on the major ion chemistry of river water, seasonal changes in major ion concentrations were studied in the Shimousa Upland, Japan where urbanisation is still in progress. Water samples were collected from 24 sites from the Ohori River basin and analysed four times representing four seasons from August 2006 to April 2007 during baseflow or low flow conditions. The proportion of different land uses in the drainage area of each sampling site were estimated from a detailed digital land use map published by the Geographical Survey Institute of Japan by using Arcview/GIS. Electrical conductivity (EC) and the concentrations of Na+, K+, Cl− and NO showed significant seasonal variation (p≤0.05). The correlation analysis results showed that forested areas had negative correlations with all ions. Farmland coverage was significantly associated with elevated levels of K+, NO and SO (p≤0.05). Urban land use appeared to have the greatest influence on the major ion chemistry. Residential areas showed significant positive correlations with K+, Mg2+, Ca2+, NO and SO, commercial areas with Mg2+, Ca2+ and HCO, and urban developing areas with Ca2+ (p≤0.05). Cluster analysis (CA) on water quality parameters showed three different groups of similarity between the sampling sites and found them to be highly influenced by land use. It can be concluded that estimating the proportions of different land uses enables us to predict river water quality with respect to major ion concentrations.