Pathways from land to stream: lessons from Pukemanga

Oral presentation at the New Zealand Hydrological Society Annual Conference, 20-23 November 2007, Rotorua, New Zealand.In the Pukemanga catchment, nitrate is leaching from soil under agricultural land use, and is being transported by subsurface water flow to surface waters. Groundwater is the suspected dominant transport pathway. This study proposes to determine the proportion of groundwater discharge to streamflow by partitioning of daily and hourly streamflow on the basis of groundwater dynamics

Fate of urine nitrogen through a volcanic vadose zone

© CSIRO 2014. To investigate the fate of nitrogen (N) from urine, dairy cow urine was amended with bromide (Br) and chloride (Cl), and applied onto a loamy sand soil with an underlying vadose zone of gritty coarse sands and pumice fragments with groundwater at ∼5.5m depth. Textural changes and hydrophobicity resulted in heterogeneous flow and high variability in the Cl, Br and N masses captured. Three forms of N derived from the urine, organic-N (org-N), ammonium-N (NH4-N) and nitrate-N (NO3-N), were measured at 0.4m depth. At 1.0m depth, effectively all measured N was NO3-N. At 4.2m, the mass of recovered N (average 33% of applied N, s.d. 21%), although solely speciated as NO3-N, was not significantly different from that at 0.4m (average 24.5% of applied N, s.d. 0.1%), suggesting that no substantial assimilation of NO3-N had occurred in this vadose zone. Below the interface of the Taupo Ignimbrite and the Palaeosol at 4.2m depth, recoveries of the Cl and Br tracers were negligible. In addition, the isotopic signatures (δ18O and δ15N) of the nitrate were different and the NO3-N concentrations were higher than in the upper vadose zone. These results all suggest that the Palaeosol was acting as a hydraulically limiting layer resulting in lateral unsaturated flow occurring at this interface. The fact that no nitrate assimilation was observed in this field study, despite previous laboratory studies showing substantial assimilative capacity, underlines that that the nitrate assimilative capacity in the vadose zone is a function of both hydrological and biogeochemical factors

Variability of unsaturated bromide fluxes as measured through a layered volcanic vadose zone in New Zealand

The measured drainage fluxes through a layered volcanic vadose zone exhibited high spatial variability as a consequence of heterogeneous flow conditions. The drainage flux variability was quantified using automated equilibrium tension lysimeters, installed in close-proximity and resulted in high variability in the Br masses recovered from a conservative tracer experiment. The primary cause of the heterogeneous flow was attributed to textural changes occurring at the interface between volcanic layers, resulting in development of funnel-flow patterns, and further enhanced by the existence of hydrophobic conditions. The Br recoveries in individual automated equilibrium tension lysimeters were used to determine the corresponding variable sizes of the surface areas contributing drainage to the lysimeters. The tracer experiment confirmed the existence of unsaturated lateral transport occurring at the sloping interface of the coarse Taupo Ignimbrite material with the silty Palaeosol layer at approximately 4.2 m depth. This study demonstrates that measurements of both flux and solute concentrations at multiple locations are essential when heterogeneous flow is suspected to be present, to be able to determine reliable estimates of contaminant leaching through the vadose zone at the plot scale