Export of dissolved iron from river catchments in northeast Japan

This study investigated the export of dissolved iron (DFe) from river catchments with emphasis on land use and land cover (LULC). The DFe concentrations were measured at 61 and 63 sites during summer and winter, respectively, in the Kitakami Mountain in northeast Japan. LULC was classified as broadleaf forest, conifer, grassland, paddy field, cropland, and urban area using GRASS GIS software. In the downstream areas, the riverine DFe concentrations ranged from 0.84 to 115 μg/L and from 1.5 to 103 μg/L in winter and summer, respectively, whereas the DFe in the upstream areas ranged from 0.16 to 7.6 μg/L and from 1.1 to 20.0 μg/L. Regression analysis was applied to the DFe and LULC data, and the unit load for DFe was estimated for each LULC type, the unit load values being 1.6, 139, and 439 g/km2/day for broadleaf forest, paddy fields, and urban area, respectively, in winter and 61.9, 564, and 727 g/km2/day for grassland, paddy fields, and urban area, respectively, in summer, reflecting the impact of distinct seasonality. A significant positive correlation was found between urban area and riverine DFe concentration in each season, emphasizing the influence of human activity on DFe export to river catchments. Overall, our investigation provides a series of quantitative evidence showing the importance of LULC to understand DFe export from river catchments.