Characterization of denitrification in the subsurface environment of the Manawatu River catchment, New Zealand

Denitrification – the microbial-mediated transformation of nitrate (NO3) to harmless dinitrogen (N2) gas – is an important NO3 reduction process in soil-water systems. This study aims (1) to identify the potential of denitrification in shallow groundwater, and (2) to quantify denitrification in the vadose and saturated zones at selected sites in the Manawatu River catchment, New Zealand. A groundwater survey was conducted sampling 56 wells in the Tararua Groundwater Management Zone during the summer of 2013-14, and laboratory denitrification incubations of vadose zone sediments and in situ push-pull denitrification tests in saturated zone (shallow groundwater) were conducted at a selected study site close to Palmerston North during 2013-2014. The groundwater survey results were analyzed to identify areas where groundwater is oxidized (not conducive to denitrification) or reduced (conducive to denitrification). A significant proportion of sampled wells in the Mangatainoka sub-catchment were found to be tapping shallow, oxidized groundwater, whereas reducing conditions were found mostly in the middle and northern parts of the TGWMZ. Preliminary results from the vadose zone denitrification assays indicate that the surface soil (0-30 cm) has a greater denitrification potential than the subsoil (100-200 cm). The single-well, push-pull tests indicate occurrence of denitrification in the shallow groundwater at the study site. Further experiments are being conducted in other study sites to help improve our understanding of variability and factors of denitrification characteristics in the subsurface environment of the catchment