Denitrification during Vertical Upwelling at an Alluvium-Diluvium Interface below the Upland Perimeter of a Riparian Paddy

Denitrification hotspots in riparian aquifers often develop in a relatively narrow zone at the upland-riparian interface, where nitrate-rich ground water of upland origin interacts with available soil organic carbon. In riparian paddy fields, denitrification in the aquifer has received less attention than that in the surface water and soil. This study aimed to determine the in situ activity of the denitrification hotspot formed at the vertical interface between the organic alluvial and the nitrate-rich diluvial aquifers around the depth of 2.0 to below the upland perimeter of riparian paddy, where vertical upwelling dominates the ground water recharge. The mass balances of water and solutes were approximately calculated from the one-dimensional vertical pressure head and water quality profiles with help of the stable isotopes analyses of water. The confined ground water of adjacent diluvial upland origin, with a high nitrate concentration of 1.72 +/- 0.42 mmol L(-1), mixed with the nitrate-deficient unconfined ground water at the alluvium-diluvium interface, and 63% of nitrate was removed by denitrification at a rate of 33 mg N m(-2) d(-1) and a nitrogen isotope fractionation factor of 0.988. The increase in bicarbonate concentration with the decrease in nitrate concentration suggested a heterotrophic denitrification with a stoichiometry of C:N = 5:4. These results are the first to demonstrate the quantitative importance of denitrification in the aquifer below a riparian paddy in the removal of nitrate from the ground water of upland origin and emphasize the necessity of including this process in models for predicting watershed-scale surface water mid ground water qualities.