Multi-pronged approach to elucidate nitrate attenuation in shallow groundwater

It is increasingly being recognised in New Zealand that denitrification occurring in the groundwater zone can result in a substantial reduction of the nitrate load leached from agricultural land before this load can reach water supply wells or discharge into groundwater-fed surface water bodies. This natural attenuation process provides an ecosystem service with regard to the protection of the quality of our freshwater resources that to date has not been adequately accounted for. This is largely due to the major challenges involved in trying to understand and quantify the denitrification occurring in a particular water management zone. Based on the example of research conducted at the ‘Waihora’ site in the Lake Taupo catchment, we demonstrate a multi-pronged approach to elucidate the biogeochemical and hydrological controls on denitrification. The site is unique in New Zealand inasmuch as it has allowed investigating shallow groundwater underlying a pastoral hillslope in great detail using 11 multi-depth well clusters (comprising 26 wells in total). As denitrification is only active under mildly reduced conditions, a systematic approach to characterise redox conditions based on measured concentrations of dissolved oxygen, nitrate, dissolved manganese, dissolved iron, and sulphate provided fundamental initial information on the denitrification potential of the groundwater system. Determining stable isotope signatures of nitrate (δ15N, δ18O) and excess N2 dissolved in the groundwater can help differentiate between denitrification potential and denitrification that has actually occurred in a given groundwater sample. As the interpretation of these data is strongly dependent on the understanding of the temporal and spatial variation of groundwater flows at the site, hydrological understanding proved critical. Tritium, chlorofluorocarbons, and silica were determined on selected groundwater samples to gain insight into the distribution of groundwater mean residence times (‘ages’) at the field site and slug tests provided estimates of the hydraulic conductivity of the different deposits found in the shallow groundwater system. Given that most biogeochemical and hydrological parameters analysed showed substantial spatial variation, hydrological modelling of the hillslope proved the only promising way to ascertain the overall effect denitrification may have on the groundwater nitrate discharges from this site