• Denitrification and indirect N2O emissions in groundwater: Hydrologic and biogeochemical influences

      Jahangir, Mohammad M. R.; Johnston, Paul; Barrett, Maria; Khalil, Mohammed I.; Groffman, P.M.; Boeckx, P.; Fenton, Owen; Murphy, John; Richards, Karl G.; Department of Agriculture, Food and the Marine, Ireland; et al. (Elsevier, 08/07/2013)
      Identification of specific landscape areas with high and low groundwater denitrification potential is critical for improved management of agricultural nitrogen (N) export to ground and surface waters and indirect nitrous oxide (N2O) emissions. Denitrification products together with concurrent hydrogeochemical properties were analysed over two years at three depths at two low (L) and two high (H) permeability agricultural sites in Ireland. Mean N2O–N at H sites were significantly higher than L sites, and decreased with depth. Conversely, excess N2–N were significantly higher at L sites than H sites and did not vary with depth. Denitrification was a significant pathway of nitrate (NO3−–N) reduction at L sites but not at H sites, reducing 46–77% and 4–8% of delivered N with resulting mean NO3−–N concentrations of 1–4 and 12–15 mg N L− 1 at L and H sites, respectively. Mean N2O–N emission factors (EF5g) were higher than the most recent Intergovernmental Panel on Climate Change (IPCC, 2006) default value and more similar to the older IPCC (1997) values. Recharge during winter increased N2O but decreased excess dinitrogen (excess N2–N) at both sites, probably due to increased dissolved oxygen (DO) coupled with low groundwater temperatures. Denitrifier functional genes were similar at all sites and depths. Data showed that highly favourable conditions prevailed for denitrification to occur — multiple electron donors, low redox potential (Eh < 100 mV), low DO (< 2 mg L− 1), low permeability (ks < 0.005 m·d− 1) and a shallow unsaturated zone (< 2 m). Quantification of excess N2–N in groundwater helps to close N balances at the local, regional and global scales.
    • Linking hydrogeochemistry to nitrate abundance in groundwater in agricultural settings in Ireland

      Jahangir, Mohammad M. R.; Johnston, Paul; Khalil, Mohammed I.; Richards, Karl G.; Department of Agriculture, Food and the Marine, Ireland; Department of Civil, Structural and Environmental Engineering, Trinity College Dublin; RSF 06383 (Elsevier, 11/05/2012)
      Nitrate (NO3-–N) contamination of groundwater and associated surface waters is an increasingly important global issue with multiple impacts on terrestrial, aquatic and atmospheric environments. Investigation of the distribution of hydrogeochemical variables and their connection with the occurrence of NO3-–N provides better insights into the prediction of the environmental risk associated with nitrogen use within agricultural systems. The research objective was to evaluate the effect of hydrogeological setting on agriculturally derived groundwater NO3-–N occurrence. Piezometers (n = 36) were installed at three depths across four contrasting agricultural research sites. Groundwater was sampled monthly for chemistry and dissolved gases, between February 2009 and January 2011. Mean groundwater NO3-–N ranged 0.7–14.6 mg L−1, with site and groundwater depth being statistically significant (p < 0.001). Unsaturated zone thickness and saturated hydraulic conductivity (Ksat) were significantly correlated with dissolved oxygen (DO) and redox potential (Eh) across sites. Groundwater NO3-–N occurrence was significantly negatively related to DOC and methane and positively related with Eh and Ksat. Reduction of NO3-–N started at Eh potentials <150 mV while significant nitrate reduction occurred <100 mV. Indications of heterotrophic and autotrophic denitrification were observed through elevated dissolved organic carbon (DOC) and oxidation of metal bound sulphur, as indicated by sulphate (SO42-). Land application of waste water created denitrification hot spots due to high DOC losses. Hydrogeological settings significantly influenced groundwater nitrate occurrence and suggested denitrification as the main control.