『Abstract
　This study determined isotopic fractionation of nitrate-nitrogen during denitrification in riparian aquifer sediments by sequential-column experiments at two different water flow rates. The purpose was to discuss the relationships of nitrogen isotopic enrichment factor of denitrification, denitrification rate, and infiltrating condition in sediment. Sandy sediment and ground water were collected from floodplain in the middle reach of the Tama River where nitrogen isotopic enrichment factor for denitrification had been measured in a result of previous field observation. The denitrification rates and the isotopic enrichment factors of nitrate-nitrogen were estimated at 2.1 mgN dry-kg^-1 day^-1 and -32.9‰ during the low water flow condition, and at 3.5 mgN dry-kg^-1 day^-1 and -34.1‰ during the high water flow condition. The calculated water flow rates of the present column experiments were 300-400 times higher than that of the field observation. Because of the fast flow rate, nitrate was expected to continuously pass through active denitrifying sites in the small pore spaces of sediment, and nitrate supply to denitrifers by infiltration flow transport greatly exceeded the supply by diffusion. The observed denitrification rates was proportional to Peclet number as the ratio of solute nitrate transport to the diffusion, and apparent nitrogen isotopic enrichment factors during denitrification of our column experiments were lower than those obtained from the field observation. This study showed that denitrification rate and apparent nitrogen isotopic enrichment factor of denitrification in sediment depended on Peclet number.

Keywords: column experiment; denitrification; isotopic enrichment factor; nitrate; nitrogen stable isotope ratio』

1. Introduction
2. Material and methods
　2.1. Equipment for infiltration experiment
　2.2. Sample collection
　2.3. Running condition of the experiment
　2.4. Chemical analysis
3. Results and discussion
　3.1. Experimental condition
　3.2. Ion concentrations
　3.3. Denitrification rate
　3.4. Isotopic fractionation
4. Conclusion
Acknowledgment
References

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