The nitrate load to come: a tale of two porosities
Helen Rutter 1
1. Aqualinc Research Ltd, Christchurch, Canterbury, New Zealand
In 1975, Foster described the “Tritium anomaly” in the British Chalk, where very low levels of post-bomb tritium were observed in groundwater, as compared with
concentrations in the vadose zone above it. This was hypothesised to occur due to
99 | P a g e the concentration gradient between rapidly recharging water through fissure, and relatively immobile pore water. Since then, research in the UK has focused on this process, and more recent work has highlighted the critical role that dual
porosity/permeability mechanisms play in controlling the “nitrate load to come”.
Simplistically, when recharge events occur, water moves through high permeability pathways (fissures), and diffusion occurs from this high nitrate recharge into the low concentration matric water. As the concentration in the matric water increases over time, the concentration gradient can be reversed, and concentrations in the
recharging water may then be enhanced (load to come). This has been observed in the UK Chalk, with the matric water becoming a ‘source’ of nitrate as land use
changes have resulted in lower nitrate concentrations in recharge. Similar processes occur in the saturated zone and have been observed through failed aquifer storage and recovery trials, and tracer experiments.
In New Zealand alluvial gravel systems, as in the Chalk, there are two contrasting components of flow, with the same opportunity for movement of solutes between the two phases. The fine-grained matrix is almost always close to saturation, but has very low intrinsic permeability, whereas the open framework gravels (OFGs) within the gravel sequence, as with the fissures in the Chalk, transport most of the flow but are usually unsaturated in the vadose zone.
This paper explains the process of nitrate transport in dual porosity/permeability systems, providing evidence from the UK and New Zealand. Whilst the work in the UK has been carried out over several decades, quantification of the nitrate load to come has not yet started in New Zealand. There is a crucial need to fully quantify the extent to which nitrate has been retained in immobile pore water. Until we measure this, we cannot have confidence in modelling the impacts of land use change on groundwater and surface water quality.
Nutrient transfer through the vadose zone under sugarcane in the Wet Tropics
Rezaul Karim 1 2 , Lucy Reading 3 , Leslie Dawes 4
1. School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD, Australia
2. Environmental Science and Technology, Jashore University of Science and Technology, Jashore, Khulna, Bangladesh
3. School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD, Australia
4. School of Civil Engineering and Built Environment, Queensland University of Technology, Brisbane, QLD, Australia
Agriculture has a substantial impact on the quality and quantity of groundwater. The excessive usage of nitrogen fertilizers and manure on agricultural lands can increase the nitrate concentrations in surface water and groundwater. Over the past 150 years, the catchments adjacent to the Great Barrier Reef (GBR), Australia, the world’s largest coral reef ecosystem, have been drastically changed with the development of the catchments for farming activities. Therefore, there is potential for nutrients (carbon, nitrogen and phosphorus) to be discharged beneath the plant root zone to the vadose zone, then to groundwater or be transported in runoff from sugarcane fields and eventually into creeks and rivers that feed into the GBR. This paper has investigated the fate of nutrients that leach through the vadose zone under a sugarcane field in the South Johnstone catchment, in the Wet Tropics. Using a real time monitoring technique, the vadose zone monitoring system (VMS), the
100 | P a g e temporal and spatial distribution of nutrients in the vadose zone has been studied.
NOx-N concentrations in leachate under sugarcane varied over time at different depths. Generally, the concentration of Nitrate decreased over time but increased with depths. Ammonia was found to range from 0.01 to 0.49 mg/L, showing a difference between samples collected before and after fertilization in 2016 -2018.
Total Organic Carbon (TOC) losses in the vadose zone also varied with time.
Phosphate in the vadose zone was found below the detection limit, demonstrating plant uptake and hereafter the phosphate attenuation in the soil. The processes affecting the transport of nutrients through the vadose zone and time lags that occur before these contaminants reach ground water have also been determined.
Denitrification walls as a tool to reduce nitrate load to the Greats Barrier Reef whilst reducing nitrous oxide emissions: results from the southeast Queensland trials
Fabio Manca 1 , Daniele De Rosa 1 , Lucy Reading 1 , David Rowlings 1 , Clemens Scheer 1 , Peter Grace 1
1. Queensland University of Technology, Brisbane, QLD, Australia
Nitrogen (N) used in excess in agricultural systems can leach to shallow groundwater and reach the ocean via submarine groundwater discharge. Excess N inputs can also be lost into the environment as greenhouse gas (GHG) emissions, contributing to anthropic climate change via nitrous oxide (N2O) production. The Great Barrier Reef (GBR) receives increasing harmful nutrient loads including nitrate (NO3-), linked to the dramatic growth of corallivores. The GBR is also subject to coral bleaching as a response to elevated sea surface temperatures as a direct consequence of GHG emissions induced by global warming.
Denitrification walls are a low-cost technology for NO3- remediation and consist of organic carbon (OC) media-filled permeable trenches able to intercept NO3- polluted groundwater and catalyse denitrification. Denitrification progressively reduces NO3- to dinitrogen with N2O as an intermediary. The process is performed by microbes that use OC as an electron donor to perform their respiration under anaerobic conditions.
Two denitrification walls (30 m3 volume) filled with different OC sources were
installed on a sandy aquifer in Southeast Queensland. The water table height in the bioreactors and aquifer was monitored using a dipmeter and pressure transducers.
Water samples were collected and analysed photometrically to evaluate NO3- concentrations. Chemo-physical parameters were collected with portable instruments. Dissolved N2O in groundwater was determined using gas
chromatographic techniques. The hydraulic characteristics of the bioreactors were tested using saline tracing tests.
Both the walls supported full removal of NO3- and N2O and resulted suitable to reduce NO3- load to the GBR whilst reducing N2O emissions. The results of this study will provide the Queensland Government with a technical tool to improve the use of this cost-limited technology.
The study was funded by the Queensland Government Office of the Great Barrier Reef in collaboration with the Queensland Department of Agriculture and Fisheries.
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Removing nitrate from artificial subsurface drainage under pastoral agriculture using woodchip bioreactors
Aldrin Rivas 1 , Greg Barkle 2 , Roland Stenger 1 , Juliet Clague 1 , Brian Moorhead 1 1. Lincoln Agritech Ltd, Hamilton, Waikato, New Zealand
2. Land and Water Research Ltd, Hamilton, Waikato, New Zealand
Objectives: Artificial subsurface drainage controlling high seasonal groundwater has been found to be a substantial pathway for nutrients from agricultural lands into surface waters. Thus, mitigating the impacts of agriculture on surface water quality needs to address nutrient transport via subsurface drainage. Woodchip bioreactors are a promising mitigation option as demonstrated in arable agriculture in the US.
However, research is needed to understand their efficiency in removing nutrients from very flashy drainage flows from NZ pastoral agriculture and any possible pollution swapping.
Methods: A lined 60-m3 woodchip bioreactor was constructed on a dairy farm in the Hauraki Plains (Waikato, NZ). Rainfall, flow, hydrochemistry and dissolved gases in the inflow and outflow were monitored for two drainage seasons (part of 2017, 2018).
Results: The mean nitrate-N concentrations in the inflow and outflow respectively, were 5.59 and 0.01 mg/L in 2017, and 13.72 and 7.45 mg/L in 2018. Based on the nitrate-N fluxes, the estimated nitrate removal efficiency of the bioreactor was 99 and 48% in 2017 and 2018, respectively. The higher removal efficiency in 2017 could be attributed to; the much longer residence time of the water in the bioreactor (mean=22 days vs 5 days in 2018) allowing more opportunity for microorganisms to reduce the nitrate in the water; and the availability of electron donor (DOC) to support denitrification. In 2017, greater DOC available within the bioreactor was indicated by the higher DOC flux from the bioreactor (17.9 kg vs 9.3 kg in 2018).
Very long residence times in 2017 promoted strongly reduced conditions, resulting in the production of hydrogen sulphide and methane. However, short residence times constrained complete reduction of nitrate resulting in higher nitrous oxide
concentrations in the outflow vs inflow in 2018. Elevated discharges of DOC and DRP were evident during the start-up phase of the bioreactor in 2017. Significant removal (89%) of DRP was observed in 2018.
Conclusion: Woodchip bioreactors are a useful tool in removing nitrate, and possibly DRP, from subsurface drainage water. Enhancing their efficiency may require a combination of; adding another electron donor (e.g. methanol) to promote complete denitrification during flow and N load peaks and preventing very long residence times to minimise the production of odorous or greenhouse gases.
Acknowledgement: This work is part of the SSIF-funded ‘Enhanced Mitigation of Nitrate in Groundwater’ programme led by ESR. We gratefully acknowledge the co- funding by Waikato Regional Council and the co-operation of the Mourits family.
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Reducing N-discharges from agriculture: modelling the potential benefits of spatially targeted regulation
Theo S. Sarris 1 , David M. Scott 1 , Murray E. Close 1 , Lee F. Burbery 1 , Bronwyn Humphries 1
1. Institute of Environmental Science and Research, Christchurch, New Zealand
Diffuse nitrate leaching from agricultural areas is a major environmental problem in many parts of the world. Understanding where in a catchment nitrate is removed is key for designing effective land use management strategies that protect water quality, while minimizing the impact on economic development.
The aim of this study is to assess the effects of spatially targeted nitrate leaching regulation in a basin with limited knowledge of the complexity of chemical
heterogeneity. We incorporate three alternative nitrate reactivity spatial
parameterizations in a catchment-scale flow and transport model. These are used to evaluate the effectiveness of four possible spatially targeted regulation options.
Our findings confirm that there are potential benefits of implementing spatially targeted regulation compared to spatially uniform regulation. Focusing regulation in areas where nitrate residence time is short, such as riparian zones or areas with low natural N-reduction, results in greater reduction of N-discharges through
groundwater. Management efficiencies are significantly improved when delineation of management zones considers the chemical heterogeneity and groundwater flow paths. These improved efficiencies are achieved by adopting management rules that regulate land use in discharge sensitive areas, where leaching changes contribute the most to the catchment nitrate discharges. In our case study, regulation in discharge sensitive zones was twice as efficient compared to other management options.
A participatory approach to better understand well water quality in Canterbury, New Zealand
Abie Horrocks 1
1. FAR (Foundation for Arable Research), Christchurch, CANTERBURY, New Zealand
Groundwater nitrate nitrogen levels in Canterbury, New Zealand have been gradually increasing. A project was carried out from 2016 to 2019 to seasonally monitor 51 randomly selected wells of varying depths between the Ashburton and Rakaia rivers.
The objective was to identify the variability in nitrate nitrogen levels in groundwater and to see how concentrations vary seasonally, sub-regionally, by depth and over time. This area is primarily farmland built on gravelly alluvial plains overlaying
tertiary sediments of braided river deposits and greywacke. The project was initiated by farmers wanting to engage with the process of better understanding trends in water quality in their region. The sampling was carried out by the Foundation for Arable Research working closely with hydrologists to ensure protocols and
procedures were consistent and valid. Recharge of aquifers further from the rivers and coast is achieved through a combination of rainfall, irrigation, snow melt and infiltration from rivers. Piezometric contours running between the two rivers illustrate that water movement is from the foothills to the coast, with groundwater levels becoming shallower approaching the coast. The sampling found there to be no differences in nitrate levels from north to south, but concentrations increased as
103 | P a g e groundwater moved down the catchment accumulating as it moved towards the coast. Four wells were consistently over the maximum acceptable level for safe drinking water of 11.3mg NO3-N/L but the majority of wells had acceptable levels of nitrate nitrogen over the duration of the project. The information has enabled
farmers to develop a better understanding of the high and low risk zones in the area.
Importantly the participatory approach involving researchers and farmers has enabled the farmers to engage with regulatory bodies in constructive dialogue to develop outcomes that meet the environmental, economic and social needs of farmers and the community.