Denitrification in a seashore sandy deposit influenced by groundwater discharge

The chemical compositions of ground water and organic matter in sediments were investigated at a sandy shore of Tokyo Bay, Japan to determine the fate of ground water NO₃^–. On the basis of Cl^– distribution in ground water, the beach was classified into freshwater (FR)-, transition (TR)-, and seawater (SW)-zones from the land toward the shoreline. The NO₃^– and N₂O did not behave conservatively with respect to Cl^– during subsurface mixing of freshwater and seawater, suggesting NO₃^– consumption and N₂O production in the TR-zone. Absence of beach vegetation indicated that NO₃^– assimilation by higher plants was not as important as NO₃^– sink. Low NH₄⁺ concentrations in ground water revealed little reduction of NO₃^– to NH₄⁺. These facts implied that microbial denitrification and assimilation were the likely sinks for ground water NO₃^–. The potential activity and number of denitrifiers in water-saturated sediment were highest in the low-chlorinity part of the TR-zone. The location of the highest potential denitrification activity (DN-zone) overlapped with that of the highest NO₃^– concentration. The C/N ratio and carbon isotope ratio (¹³C) of organic matter in sediment (< 100 -m) varied from 12.0 to 22.5 and from –22.5 to –25.5, respectively. The ¹³C value was inversely related to the C/N ratio (r² = 0.968, n = 11), which was explained by the mixing of organic matters of terrestrial and marine origins. In the DN-zone, the fine sediments were rich in organic matters with high C/N ratios and low ¹³C values, implying that dissolved organic matters of terrestrial origin might have been immobilized under slightly saline conditions. A concurrent supply of NO₃^– and organic matter to the TR-zone by ground water discharge probably generates favorable conditions for denitrifiers. Ground water NO₃^– discharged to the beach is thus partially denitrified and fixed as microbial biomass before it enters the sea. Further studies are necessary to determine the relative contribution of these processes for NO₃^– removal.