VARIABILITY IN HYDROGEOCHEMICAL CONDITIONS IN SHALLOW
GROUNDWATER IN THE MANAWATU RIVER CATCHMENT AND
IMPLICATIONS FOR DENITRIFICATION POTENTIAL
Rivas, A.,1 Singh, R.,1 Horne, D.,1 Roygard, J.,2 Matthews, A.2, Hedley, M.1
1
Massey University
2
Horizons Regional Council
Aims
Favourable hydrogeochemical conditions in soil-water systems can attenuate nitrate (NO3-)
leached from agricultural lands before it has impact on receiving surface waters. Denitrification has
been identified as an important NO3- attenuation process in groundwater systems. However, the
denitrification characteristics of groundwater systems vary in space and time depending on the
properties of the surface and subsurface environments. The magnitude of, and variability in,
attenuation capacity has implications for both the policy related to, and management of, NO3- losses
from farm systems. Therefore it is important to investigate denitrification. However, in-situ
measurements of denitrification are costly and time consuming. On the other hand, some water
quality parameters that are commonly measured in groundwater monitoring programmes, such as
redox conditions, may be used as indicators of denitrification capacity. The aims of this study were
(a) to monitor hydrological and redox related water quality parameters at four selected sites in the
Manawatu River catchment, (b) to determine the spatial and temporal variability in redox processes
in the shallow groundwater, and (c) to identify the implications of hydrological and redox processes
for denitrification at selected sites.
Method
Detailed monitoring of shallow groundwater was conducted at four study sites in the Manawatu
River catchment. These sites were at: Palmerston North, Pahiatua, Woodville and Dannevirke. At
each site, two to three piezometers were installed to allow sampling of shallow groundwater at
depths ranging from 4.4 m to 7.7 m below ground level. Shallow groundwater samples have been
collected monthly since October 2014 and analysed for a range of water quality parameters. In
addition to water quality parameters measured on site (pH, dissolved oxygen, electrical
conductivity, temperature, and oxidation-reduction potential or ORP), samples were taken for
laboratory determinations of nitrate and ammonium-nitrogen (analysed by flow injection analysis),
sulphate (by hydriodic acid reagent reduction), dissolved organic carbon (by potassium dichromate
wet oxidation and titration), bicarbonate (based on alkalinity measurements and on site pH
measurements), and the major cations namely, iron, manganese, calcium, magnesium, potassium,
and sodium (by microwave plasma - atomic emission spectrometry). The ambient redox conditions
in groundwater were determined in accordance with the biochemical succession of electronaccepting processes and sequential production of final products (McMahon and Chapelle, 2008).
Redox processes were inferred by comparing concentrations of relevant parameters with threshold
values as follows: dissolved oxygen (1.0 mg L-1), nitrate-nitrogen (0.5 mg L-1), manganese (0.05
mg L-1), iron (0.1 mg L-1) and sulphate (0.5 mg L-1).
Results
As expected, water tables decreased (by 23 cm to 156 cm) from October 2014 to March/April 2015,
the drier period of the year, at all sites. Thereafter, water tables rose and reached the highest levels
at all sites in July 2015. Dissolved oxygen (DO) levels varied significantly across the sites with two
sites (Palmerston North and Woodville) showing reduced conditions (DO < 0.70 mg L-1) (Figure
1a). Significantly higher DO levels were observed at the Pahiatua site and in the shallower
piezometers at the Dannevirke site. In general, the DO concentrations seemed to correspond with
changes in groundwater level, with DO levels decreasing as groundwater levels fell (from October
to March) and increasing as the groundwater levels rose in the wet months (from May to August).
This indicates that conditions for denitrification are more favorable during the summer period. Other
studies have made similar observations (e.g., Anderson et al., 2014). This trend appears to be
reflected in the decreasing nitrate-N concentrations at the Dannevirke site (Figure 1b). However,
this was not observed at the Pahiatua site where the nitrate-N concentrations were found to
increase during the drier months. This may indicate low denitrification potential at this site and the
observed increase in nitrate-N concentrations could be attributed to the reduced dilution during the
drier months. In reduced conditions such as in Palmerston North and Woodville sites as well as in
the deepest piezometer at the Dannevirke site, very low nitrate-N concentrations were found. The
significant amount of iron, which could act as electron donor in the denitrification process, in these
groundwaters (0.5-4.2 mg L-1) is a key indicator of the denitrification potential at these locations.
Initial results from measurements of redox processes support these findings with oxygen reduction
identified as the dominant process at the Pahiatua site, whereas anoxic conditions and iron
reduction are the dominant processes at the Palmerston North, Woodville and Dannevirke (deepest
piezometer) sites.
The initial results presented here clearly indicate the variability of denitrification characteristics
among sites as well as within sites (particularly at the Dannevirke site), and the temporal dynamics
of hydrogeochemical properties of shallow groundwater. Given that the groundwater quality
parameters used in this study are included in the monitoring programmes of regional councils,
conducting such analysis with existing databases would help the assessment of spatial and
temporal dynamics of denitrification characteristics in groundwater systems at a larger scale.
a
b
Figure 1: The variation in (a) dissolved oxygen and (b) nitrate-nitrogen concentrations at four sites in the
Manawatu River catchment. Notes: PNR – Palmerston North site; PAH – Pahiatua site; WDV –
Woodville site; and DAN – Dannevirke site.
Acknowledgment
This research is part of a wider collaborative research programme by Massey University's Institute
of Agriculture and Environment and Horizons Regional Council to investigate the transport and fate
of nitrogen as it moves from farms to waterways in the catchment.
References
Anderson, T., Groffman, P., Kaushal, S., & Todd Walter, M. (2014) Shallow groundwater denitrification in
riparian zones of a headwater agricultural landscape. Journal of Environmental Quality, 43, 732-744.
McMahon, P. & Chapelle, F. (2008) Redox processes and water quality of selected principal aquifer systems.
Ground Water, 46, 259-271.