The influence of unsaturated zone drainage status on denitrification and the redox succession in shallow groundwater

Abstract

Since nitrate is a major agricultural freshwater contaminant, denitrification is the environmentally most important step in the ecological succession of redox processes that can occur in groundwater. Understanding where and to what extent denitrification occurs would enable spatially differentiated land management and regulation. We investigated in a dairy farming catchment in the North Island of New Zealand the influence of the unsaturated zone's drainage status on the redox succession in shallow groundwater along a well transect spanning drainage conditions from well drained to very poorly drained. Groundwater samples were analysed for a variety of parameters including nitrate, tritium, dinitrogen, argon, methane and nitrous oxide.The redox classification based on measured redox-sensitive parameters broadly matched the a priori assessed drainage status of the overlying unsaturated zone. Only the groundwater underlying the well-drained soil was oxic and reflected the N losses from the intensive pastoral land use, with nitrate nitrogen concentrations up to 9.6 mg L−1. All other sites had mildly to strongly reduced groundwater and concomitantly decreasing or low nitrate concentrations, even at the water table.The tritium-derived mean residence time (MRT) estimates for the oxic groundwater (12 and 14 y) were within the range found in mildly reduced groundwater from the imperfectly drained sites (6–24 y), with the exception of one sample from below an aquitard (105 y). In contrast, the strongly reduced groundwater observed at the poorly and very poorly drained sites was relatively immobile (55 to >110 y).Denitrification was confirmed by the nitrate dual isotope signatures, and by the occurrence of excess dinitrogen, and likely occurred in both the unsaturated and saturated zones. A coherent sampling scheme throughout the unsaturated zone – saturated zone continuum should be used in future studies to allow ascertaining the exact location of denitrification activity in vertical profiles.