『Abstract
　This work was held at the Corumbatai（iの頭は´） River basin that is inserted within the giant Parana（最後のaの頭に´） sedimentary basin (Paleozoic-Cenozoic) in South America. The Corumbatai（iの頭は´） River is the major river draining the area and its water is extensively used by water supply systems in the basin. Its surface waters were collected at two sampling points, upstream and downstream from Rio Claro city, the principal municipality within the basin. We report chemical and radionuclides (²²²Rn and ²¹⁰Po) analyses for rainwater and river water samples in order to estimate chemical weathering fluxes. All major chemical data indicated poorer conditions of the water quality in Corumbatai（iの頭は´） River after reaching Rio Claro city. However, one very important finding was that the weighted mean of the ²¹⁰Po activity concentration is the same (0.21 dpm/L) upstream and downstream from Rio Claro city, indicating that ²¹⁰Po is a conservative nuclide. The net output flux in Corumbatai（iの頭は´） River basin estimated from the difference between the total discharge flux and the input flux based on wet precipitation yielded a negative value for polonium as it is a very particle-reactive radionuclide, tending to accumulate into fluvial sediments. The chemical weathering rate (removed material quantity) corresponded to 76.5 t/km² yr when Po data in sediments and rocks were utilized in the calculations. This rate is compatible with others determined elsewhere, indicating the usefulness of Po in studies of weathering processes, even in areas characterized by anthropogenic inputs.

Keywords: Hydrochemistry; Rainwater; Surface water; Weathering rate; Corumbatai（iの頭は´） River basin』

Introduction
Area of investigation
Experimental
Results and discussion
　Major ions in rainwater
　　²¹⁰Po and ²²²Rn in rainwater
　River water chemical composition
　The annual flux of ions and ²¹⁰Po in the basin
　Negative dissolved fluxes
　Polonium mobility
　Chemical weathering rates in the basin
Conclusion
Acknowledgements
References