Denitrification in aquifer soil and nearshore marine sediments influenced by groundwater nitrate.

We estimated rates of denitrification at various depths in sediments known to be affected by submarine discharge of groundwater, and also in the parent aquifer. Surface denitrification was only measured in the autumn; at 40-cm depth, where groundwater-imported nitrate has been measured, denitrification occurred consistently throughout the year, at rates from 0.14 to 2.8 ng-atom of N g day. Denitrification consistently occurred below the zone of sulfate reduction and was sometimes comparable to it in magnitude. Denitrification occurred deep (14 to 40 cm) in the sediments along 30 km of shoreline, with highest rates occurring where groundwater input was greatest. Denitrification rates decreased with distance offshore, as does groundwater influx. Added glucose greatly stimulated denitrification at depth, but added nitrate did not. High rates of denitrification were measured in the aquifer (17 ng-atom of N g day), and added nitrate did stimulate denitrification there. The denitrification measured was enough to remove 46% of the nitrate decrease observed between 40- and 14-cm depth in the sediment.