HURUNUI RIVER CATCHMENT NITRATE NITROGEN DYNAMICS EXAMINED
USING CONTINUOUS CONCENTRATION MONITORING.
Jens Rekker,1 Sophie Papanek,1 Hugh Canard,1
1
Lincoln Agritech Limited
The middle reaches of the Hurunui River passes through the Culverden tectonic basin before
cutting through another basement rock gorge on its way to the Pacific Ocean. The Culverden
basin contains flat and rolling land for irrigated pasture and pine plantations. Current proposals for
former forestry land would increase the irrigation extent, grazing intensity and conversion of
plantation forestry to grazing agriculture. Ngāi Tahu Property Limited is a Māori incorporation
owning the Balmoral Forest and interests in expansion of grazed agriculture. Te Rūnanga o Ngāi
Tahu is the iwi authority representing the rūnanga and hapū covering the Hurunui River and
Culverden basin, as well as owning Ngāi Tahu Property Limited. The authors have been working
alongside both organisations to characterize the water quality trends and nutrient volumes
moving through the Hurunui River system.
Aims
The objective in approaching high frequency monitoring and data acquisition in the middle
Hurunui catchment was to obtain physio-chemical information relevant to periphyton dynamics
and build a bridge to traditional knowledge of the wider river system.
Method
A comprehensive literature and data review was undertaken. Physio-chemical sensors and dataloggers were installed and maintained in critical nodes in the mid Hurunui river system during
early 2015. A composite record was compiled across the period, including late summer, winter
and early spring. Measurement and data logging by a multi-parameter sonde in the Hurunui
gorge tracked electrical conductivity, pH, dissolved oxygen, turbidity and nitrate nitrogen
concentration. Continuous hydrographs throughout the Hurunui catchment were collected and
available from Environment Canterbury and the National Institute of Water and Atmospheric
Research (NIWA), as were rainfall recording stations.
Results
Continuous monitoring downstream of the bottom of the Culverden basin has provided high
resolution records at a frequency equal to continuous flow recording. This has allowed the
compilation of concentration chemo-graphs for the parameters measured as well as time series
mass load records when concentrations are put alongside concurrent hydrographs. The hydrochemo-graphs indicate initial dilution by fresh and flood flows. Late summer and autumn freshes
gave the appearance of flushing out nitrogen loads that had been held in various reservoirs
during the proceeding summer dry period.
Estimates of calculated nitrate nitrogen mass load, both as time series and as cumulative plots
indicated that freshes and floods in autumn and winter were correlated with spikes in the
transport of nitrogen in the river system. The Hurunui River system comprises alpine headwaters,
intermountain basins, basin & range topography and coastal ranges closer to the river mouth.
Consequently, hydrologic and land use influences have a significant bearing on recorded nitrogen
concentrations and mass loads, particularly when rainfall or pasture soil drainage patterns cause
source shifts. These effects and other features highlighted by continuous monitoring will be
examined in for the lessons that can be learned in catchment nutrient management.