『Abstract
　Iron and Mn concentrations in fresh groundwaters of alluvial aquifers are generally high in reducing conditions reflecting low SO4 concentrations. The mass balance and isotopic approaches of this study demonstrate that reduction of SO4, supplied from agricultural activities such as fertilization and irrigation, is important in lowering Fe and Mn levels in alluvial groundwaters underneath a paddy field. This study was performed to investigate the processes regulating Fe and Mn levels in groundwaters of a point bar area, which has been intensively used for flood cultivation. Four multilevel-groundwater samples were installed to examine the relationship between geology and the vertical changes in water chemistry. The results show that Fe and Mn levels are regulated by the presence of NO3 at shallow depths and by SO4 reduction at the greater depths. Isotopic and mass balance analyses revealed that NO3 and SO4 in groundwater are mostly supplied from the paddy field, suggesting that the Fe- and Mn-rich zone of the study area is confined by the agricultural activities. For this reason, the geologic conditions controlling the infiltration of agrochemicals are also important for the occurrence of Fe/Mn-rich groundwaters in the paddy field area.』

1. Introduction
2. study area
3. Methods
4. Results and discussion
　4.1. Well logging results
　4.2. Vertical variations in groundwater chemistry
　　4.2.1. B3, B4 and B5 sites
　　4.2.2. B6 site
　4.3. Groundwater classification
　4.4. Water origin
　　4.4.1. Evaporation in the flooded paddy field
　　4.4.2. Two-way mixing of the gravel-sand groundwater
　4.5. Evaluation of SO4-reduction processes for Fe and Mn removal in group 3 based on mass balance approach
　4.6. Importance of agricultural activities and the geology for the occurrence of Fe/Mn-rich groundwaters
　　4.6.1. SO4 and NO3 sources group 1 and group 2
　　4.6.2. Group 3
　　4.6.3. B6 site
5. Conclusions and implications
Acknowledgements
References