TRANSPORT AND FATE OF NITROGEN IN THE LOWER RANGITIKEI CATCHMENT
Collins, S1; Singh, R1; Rivas, A1; Palmer, A1; Horne, D1, Roygard, J2 & Matthews, A2
1
Institute of Agriculture and Environment, Massey University
2
Horizons Regional Council
Aims
A sound understanding of the transport and fate of leached nitrate-nitrogen in shallow groundwater is key
to understanding the impacts of land use intensification. However these are not well understood in the
Rangitikei sand country. This study was undertaken to assess the groundwater flow pattern and its
interactions with the Rangitikei River; redox conditions of the groundwater; and the extent of nitrate-nitrogen
attenuation in shallow groundwater in the lower Rangitikei catchment area.
Methods and Materials
The study area covers about 850 km2 between the townships of Bulls and Sanson in the east, Tangimoana
in the south and Santoft in the north (Figure 1). Groundwater flow patterns were interpreted from a
groundwater-level survey conducted during October 2014 from approximately 100 existing wells installed
at a range of depths in the catchment area. A piezometric map was created from the measured
groundwater-level data indicating the general groundwater flow direction of shallow and deep groundwaters
(wells screened at <30 m and >30 m, respectively). Groundwater interactions with the Rangitikei River were
estimated qualitatively using two longitudinal surveys (6th and 20th January 2015) of river-flow gauging and
water quality analyses under low-flow conditions. The river flow was measured at six locations along the
lower reach of the river and grab water-quality samples were taken at the same sites for analysis of nitrate,
chloride and sulphate. We also sampled and analysed 15 wells in the study area during December 2014.
The collected groundwater quality parameters were used to characterise the groundwater conditions as
oxic, anoxic or mixed, according to the concentration of redox parameters (McMahon & Chapelle, 2007).
We further measured nitrate-nitrogen attenuation in shallow groundwater using the single-well push-pull
tests (Istok, 2013). A total of nine piezometers were installed at a range of depths (3 m and 6 m) on two
dairy farms (sand country and river terrace) and one cropping farm (sand country). Nitrate-nitrogen, DO
and other parameters were monitored over several months.
Results and Discussion
The piezometric levels of the study area showed groundwater flow was heavily influenced by the regional
topography, particularly shallow groundwater (<30 m), and so it flows mostly from elevated areas such as
Marton in a south-westerly direction towards the Rangitikei River. Groundwater-river water interaction was
also inferred from the groundwater contour lines where they show some refraction as they cross the
Rangitikei River. This suggested groundwater discharge into the river, particularly around Bulls. River-flow
gauging and water-quality analysis also suggested groundwater discharge at locations upstream and
downstream of Bulls. In comparison groundwater recharge or no interaction between the river and
groundwater is more likely to occur near the coast. The groundwater redox characterisation showed
generally anoxic/reduced groundwater across the lower Rangitikei catchment area. Groundwater typically
has a low dissolved oxygen concentration (<1 mg/L) with elevated levels of available electron donors,
particularly DOC and Fe2+. These groundwater characteristics provide for generally favourable conditions
for nitrate reduction (Figure 1; Rivett et al., 2008), which may account for the generally low nitrate
concentrations in groundwater in the area. In the installed shallow groundwater piezometers, nitratenitrogen levels were low except at one location (cropping), where the nitrate-nitrogen varied between 2-5
mg/L. DO was also low, generally <0.3 mg/L. The push-pull tests showed nitrate reduction at four of five
piezometers, with the rate of reduction varying between 0.036 mg N L-1 hr-1 to 1.158 mg N L-1 hr-1.
Figure 1: Lower Rangitikei catchment with groundwater redox conditions of December 2014 and locations of installed
piezometers.
Conclusions
Our results suggest that groundwater is likely to be connected with the lower Rangitikei River. However,
nitrate-nitrogen concentrations in the river and groundwater were generally low, especially for the river at
low flows. This suggests nitrate-nitrogen may be undergoing reduction within shallow groundwater before
it has a chance to seep into the river. Further evidence for appreciable levels of nitrate-nitrogen reduction
in the shallow groundwater is provided by the redox characterisation of reduced groundwater and the pushpull tests. However, more spatial and temporal surveys and in-situ measurements of denitrification
occurrence in the shallow groundwater of the study area are required.
Acknowledgements: This study is part of a collaborative project between Massey University’s Institute of Agriculture
and Environment (IAE) and Fertilizer and Lime Research Centre (FLRC), and Horizons Regional Council (HRC). This
project is partially funded by HRC. The authors thank HRC for the financial support, in-kind support for field
measurements and survey, and provision of the river flow and water quality datasets used in this study.
References
Istok, J. D. (2013). Push-Pull Tests for Site Characterisation. Berlin: Springer-Verlag.
McMahon, P. B., & Chapelle, F. H. (2007). Redox processes and water quality of selected principal aquifer systems.
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Rivett, M. O., Buss, S. R., Morgan, P., Smith, J. W. N., & Bemment, C. D. (2008). Nitrate attenuation in groundwater:
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