Enhanced mitigation of nitrate in groundwater

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As a result of funding for Freshwater research announced by the government in the budget in 2016, ESR has a new project aimed at developing new methods for inducing denitrification in groundwater systems.

New Zealand’s alluvial aquifers in Canterbury, Hawkes Bay, Wairarapa, Southland, Waimea and Marlborough support highly productive agricultural systems through the supply of irrigation water. However, the enhanced production increases fertiliser use and production of animal waste and the nitrogen from these products washes straight through the alluvial soils, into the aquifers, which have little capacity to reduce the nitrates. These aquifers feed lakes and rivers, putting ever increasing quantities of nitrates into the environments loved by New Zealanders. The tension between primary sector productivity goals and environmental limits is increasing with the likelihood of production being significantly constrained in many catchments (e.g. Canterbury, Hawkes' Bay).

Our over-arching research question is “Can we sustainably reduce N fluxes in fast-flowing alluvial aquifers, in which natural N attenuation is insignificant, by inducing conditions conducive to denitrification?”

Alluvial aquifers are characterised by fast and heterogeneous groundwater flow patterns and these present significant challenges for the development, design and implementation of remediation options. Permeable channels exist which have much higher flow within them meaning that residence times within a barrier may be reduced and hence sufficient denitrification may not occur. We are going to develop and use shallow geophysical techniques such as ground penetrating radar and electrical resistivity to identify where these permeable channels are located and adjust the design of the barrier in these areas.

We are developing innovative approaches such as Biogas Induced Denitrification (BID) in groundwater and denitrifying Permeable Reactive Barriers (PRB). These are being coupled with aquifer characterisation using advanced shallow depth geophysics, DNA tracers and groundwater microbial community analysis to enable the effective design, delivery and implementation of these mitigation tools. A third technology being trialed is the use of denitrifying bioreactors to treat nitrate from artificial drains. These new methods for on-farm denitrification will enable more sustainable farming systems. It is estimated that our technology can reduce nitrogen by around 13 kgN/ha/yr, which is a significant proportion of required reductions (typically 10 – 30 kgN/ha/yr.

Our research team includes people from ESR, Lincoln Agritech(external link) (external link) Aqualinc Research(external link) (external link) Southern Geophysical(external link) (external link)  and University of Canterbury(external link) (external link) . They encompass the necessary areas of expertise in subsurface nitrate and denitrification research, groundwater redox conditions, shallow geophysics, subsurface microbial ecology, vadose zone processes, and modelling of key groundwater processes. Particularly interesting aspects of the research includes using geophysics to identify where water is moving rapidly underground and using genomic techniques to monitor which microbes remove nitrates from water. Ngāi Tahu Farming, Environment Canterbury, and Hawkes Bay Regional Council have all been involved in proposal development and will assist with field site selection, actively monitor progress and results, be involved in testing and look for application opportunities for our new denitrification technology.

Schematics of how PRB and BID systems work are shown below. A related project is being carried out in the Barkers Creek catchment, South Canterbury where an instream bioreactor using woodchips (external link)  is being installed near the end of a drainage system that collects shallow groundwater. The bioreactor stimulates denitrification and reduces the nitrate before it enters Barkers Creek. Denitrification walls are a tried and tested concept in slow moving sandy aquifer systems where they have proven effective at treating nitrate from point pollution sources. There are no examples, however, of these remediation systems having been installed in gravel aquifers such as those found in Canterbury. In this regard, our PRB pilot study represents a world-first. 

Figure 1: Schematic of a permeable reactive barrier

Figure 2: Schematic of biogas induced denitrification

Contacts:

Murray Close, ESR Christchurch, murray.close@esr.cri.nz, DDI: 03 351 0014

Lee Burbery, ESR Christchurch, lee.burbery@esr.cri.nz, DDI: 03 351 0087

Other water research projects

The Groundwater Assimilative Capacity project was a six year research (2010-16) project led by ESR that determined how different contaminants were transported and attenuated in NZ groundwater systems. The project focused on nitrate, phosphorus and microbial contaminants such as bacteria and viruses and examined the significance of denitrification processes that occur naturally in some groundwater systems and the role that the redox status (whether there was oxygen present or not) played in determining whether denitrification took place. A method was developed to predict the redox status of groundwater at a regional scale. The project also used a novel smoke tracer to infer connectivity of open framework (highly permeable) channels within an alluvial gravel system, and determined the transport and fate of pathogenic microbes relative to indicator microbes in a range of aquifer media.

The current Transfer Pathways project (TPP), which is led by Lincoln Agritech Ltd (LAL), is focussed on finding out where water in a particular spring or stream comes from and how it has travelled along shallow or deep groundwater flow paths. This is important as the different flow paths have different travel times and different attenuation (or contaminant reduction) processes. One of the case studies for the project is the Ashley-Waimakariri catchment and we are focussing on the Silverstream system. 

Hazel Clemens, a summer student with ESR, is currently working on her masters under the guidance of Dr Louise Weaver, Senior Scientist, ESR and Dr Leanne Morgan, Senior Lecturer in Groundwater at the Waterways Centre. Hazel's project is to investigate how viruses are transported from on-site wastewater treatment systems [PDF, 280 KB] (external link)  (OWTS) through the soil below.