Relationship between river water quality and land use in a small river basin running through the urbanizing area of Central Japan

Abstract

In this study, the relationship between water quality (as represented by major inorganic ion concentra- tions) and land use characteristics is examined for a small river basin which runs through the urbanizing area of central Japan. Water samples were taken from 24 sites at base flow and analyzed, and the proportions of the various land uses associated with the respective drainage basins were calcu- lated using a digital land-use map (scale: 1:25000). The electrical conductivity (EC) of the water ranged from 84.5 to 600 l Sc m -1 .C a 2+ and Na + were the major cations, accounting for 77% of all cations. Among the anions, HCO3 - was dominant (56%), followed by Cl - (24%), SO4 2- (13%) and NO3 - (7%). Applying principal component analysis to land use in the drainage basin yielded three principal com- ponents. The first principal component expressed the degree of occupation by residential areas, the second indicated the degree of urban developing area (i.e., fast-developing and industrial areas), and the third showed the degree of coverage with farmland and green space. The residential area showed significant positive correlations with K + ,M g 2+ ,C a 2+ ,N O3 - , HCO3 - , EC and TMI (total major ions). Urban developing area showed significant positive correlations with Ca 2+ ,C l - , HCO3 - , EC and TMI as well as weak negative correlations with NO3 - and SO4 2- . Industrial area showed weak positive correlations with Na + and Cl - and a moderate negative correlation with NO3 - . Farmland showed significant posi- tive correlations with NO3 - and SO4 2- ; these ions are present due to fertilizers and the biological activity of plants. Forest area is inversely related to almost all ions, indicating the need for this form of land use in order to maintain river water quality.