Characterisation of denitrification in the subsurface environment of the Manawatū catchment, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand

Figures 2.1 & 2.2 have been removed for copyright reasons but may be accessed via their source listed in the References (Rivett et al., 2008, Fig. 2 & Saggar et al., 2013, Fig. 3).A sound understanding of the quantity of nitrate lost from agricultural soils, as well as their transport and transformation in soil-water systems is essential for targeted and effective management and/or mitigation of their impacts on the quality of receiving waters. However, there is currently little known about the occurrence, variability, or factors affecting, nitrate attenuation by subsurface (below the root zone) denitrification in New Zealand, particularly in the Manawatū River catchment. This thesis developed and applied a combination of regional- and local-scale hydrogeochemical surveys and experiments, to gain an insight into the occurrence, variability, and hydrogeological features of subsurface denitrification in the Manawatū River catchment, particularly in the Tararua Groundwater Management Zone (GWMZ). A regional survey and analysis of samples from 56 groundwater wells conducted in the Tararua GWMZ revealed mainly oxic groundwater with low denitrification potential in the southern part of the catchment (Mangatainoka sub-catchment), whereas mainly anoxic/reduced groundwaters with high potential to denitrify in the middle and northern parts (Upper Manawatū sub-catchments). Oxic groundwaters with enriched nitrate concentrations were generally correlated with coarse textured soil types and aquifer materials (e.g., well-drained soil, gravel rock type), allowing faster movement of percolating water and oxygen diffusion from surface to subsurface environments. Local-scale laboratory incubations and in-field, push-pull test techniques were evaluated and optimised to measure and quantify denitrification in unsaturated (vadose) and saturated (shallow groundwater) parts of the subsurface environment. A novel incubation technique using vacuum pouches was found to be more reliable than traditional Erlenmeyer flasks in determining denitrifying enzyme activity (DEA) in subsurface soils (>0.3 m depth) with low denitrification activity. A combination of 75 μg N g-1 dry soil and 400 μg C g-1 dry soil was also found to provide the optimum DEA in subsurface soils. In the evaluation of the push-pull test, denitrification rates estimated using the measurements of denitrification reactant (nitrate) were found to be significantly higher (6 to 60 times) as compared to the rates estimated using the measurements of denitrification product (nitrous oxide). The estimates of denitrification rates also differed depending on whether a zero-order or first-order kinetic model was assumed. However, either a zero-order or a first-order model appears to be valid to estimate the denitrification rate from push-pull test data. The optimised laboratory incubation technique and in-field, push-pull test were applied at four sites with contrasting redox properties; Palmerston North, Pahiatua, Woodville, and Dannevirke. The incubation technique revealed that denitrification potential in terms of DEA is highest in the surface soil and generally decreased with soil depth. The push-pull test measured large denitrification rates of 0.04 to 1.07 mg N L-1 h-1 in the reduced groundwaters at depths of 4.5-7.5 m below ground level at two of the sites (Woodville and Palmerston North), whereas there were no clear indications of denitrification in the oxidised shallow groundwaters at the other two sites (Pahiatua and Dannevirke). This new knowledge, information and techniques advance our scientific capability to assess and map subsurface denitrification potential for targeted and effective land use planning and water quality measures in the Manawatū catchment and other catchments across New Zealand’s agricultural landscapes and worldwide

Bangkok, Thailand

This chapter demonstrates that substantial progress has been made during 1998–2008 in terms of expanding water supply and wastewater treatment services and alleviating flood problems in Bangkok. However, there exists a huge potential for further improvements in water demand management through suitable water pricing, NRW reduction, level of service in terms of water pressure in the distribution lines, wastewater treatment and tariff, and flood-retention ponds, among others. Sustained efforts, appropriate policies and institutions including improved coordination among the entities responsible are indispensable to achieve integrated and efficient urban water management, especially in the light of global changes including changing climate conditions

Nitrate removal efficiency and secondary effects of a woodchip bioreactor for the treatment of agricultural drainage

Artificial drainage has been instrumental in the viable use of poorly drained soils for agriculture. However, artificial drains can also provide a pathway for fast and unattenuated nutrient transfers to streams and rivers. To remove nitrate from drainage water, bioreactors have recently been widely adopted as an edge-of-field mitigation measure, particularly in the USA. Bioreactors are fundamentally a lined pit filled with woodchips as a source of carbon, which microorganisms use to transform nitrate through the process of denitrification into gaseous forms of nitrogen, mostly N₂. However, there is a lack of information on the performance of these bioreactors under the very flashy agricultural drainage flow conditions typical for New Zealand. Moreover, to avoid pollution-swapping, any possibly occurring negative side effects need to be investigated. A pilot-scale woodchip bioreactor was constructed on a dairy farm on the Hauraki Plains in Waikato and was monitored for one and half drainage seasons (part of 2017, 2018). The nitrate removal efficiency of the bioreactor, calculated from the difference in nitrate load between the bioreactor inflow and the outflow, was 99% and 48% in 2017 and 2018, respectively. The difference in removal efficiencies can be attributed to the much longer residence times and greater organic carbon (OC) availability in the bioreactor in 2017. While the long residence times in 2017 resulted in nearly complete denitrification with reduced concentrations of the greenhouse gas nitrous oxide in the bioreactor outflow, it also led to very strongly reduced conditions with production of methane (another greenhouse gas) and hydrogen sulphide (“rotten egg smell”). The shorter residence times occurring in 2018 following the modification of the bioreactor inlet manifold rectified this strongly reduced condition; however the nitrate removal efficiency concomitantly decreased. Elevated discharges of OC and dissolved reactive phosphorus (DRP) were evident during the first start-up phase of the bioreactor in 2017. In 2018 significant removal (89%) of DRP was measured over the drainage season, with no initial elevated DRP discharge. Ongoing investigations aim to optimise installation costs and treatment efficiency, while minimising any potential side effects. Specifically, options to improve the poor treatment during high flows will be investigated in the 2019 drainage season (e.g. by adding readily available OC source such as methanol)

Denitrifying bioreactor technology to reduce nitrate discharges from artificial drainage - a novel tool to enable viable farming within limits?

Aims - Artificial drainage is essential for viable use of poorly drained soils, which account for approximately 40% of dairying land in New Zealand. However, subsurface and surface drains can also provide a pathway for fast and unattenuated nutrient transfers to our streams and rivers. A denitrifying bioreactor, fundamentally a pit filled with carbon source such as woodchips, is a recently developed technology for treating drainage water at the edge of the field (Schipper et al. 2010). Naturally occurring microorganisms utilise carbon in woodchips to transform nitrate in the drainage water into gaseous forms of nitrogen (largely N2) through the denitrification process. The technology has been widely adopted in cropped lands in the USA (Christianson et al. 2012). However, a different bioreactor design is necessary in New Zealand due to the shallower subsurface drainage systems in our flat pastoral lowland areas. While bioreactors have been found to effectively remove nitrate in the drainage water, possible pollution swapping (particularly N2O emissions) and other unwanted side effects (including high concentration of dissolved organic matter in the outflow) also need careful consideration (Schipper et al. 2010; Weigelhofer and Hein 2015). Thus the main objective of this research is to assess the applicability and performance of denitrifying bioreactor technology in reducing nitrate loads from subsurface drains in New Zealand pastoral lands. We aim to identify the factors affecting the performance as well as potentially occurring detrimental side effects of denitrifying bioreactor technology to optimise the cost and efficiency of future installations in New Zealand. Method - We designed and constructed a pilot-scale denitrifying bioreactor at a farm in the Hauraki Plains where high nitrate concentrations (>10 mg nitrate-N L-1) were found in the drainage water (Figure 1). The bioreactor has an effective volume of approximately 60 m3 filled with locally sourced untreated pine (Pinus radiata) woodchips. We route the drainage water from a lateral subsurface drain into the bioreactor through an inlet control structure and the flow rate through the bioreactor is controlled by the difference between the heights of weirs in the inlet and outlet control structures (Figure 2). The inlet control structure allows excess drainage water during high flow events to by-pass the bioreactor. We continuously monitor flow through the bioreactor and any by-pass flow, electrical conductivity at the inlet and outlet, temperature at the inlet, outlet and within the bioreactor, and rainfall at the site. Inlet and outlet waters are proportionally sampled for nitrogen and carbon species to assess the effectiveness of the bioreactor in attenuating nitrate and for a range of other analytes to investigate the possible occurrence of negative side effects. Results - We will present our approach to the design of the bioreactor for typical New Zealand subsurface drainage systems in comparison with the approach applied in other countries, such as the USA. Monitoring data from the first season of the bioreactor’s operation will also be presented to show the performance of the bioreactor in reducing nitrate in the subsurface drainage water and to assess any potentially occurring negative side effects

Measuring denitrification in the subsurface environment of Manawatu River Catchment

Denitrification is an important nitrate (NO₃⁻) attenuation process in soil water systems. A sound understanding of this process will aid in the management and mitigation of the impacts of NO₃⁻ on groundwater and surface water quality. Denitrification in surface soils has been widely studied, but there are relatively few studies of its occurrence and distribution in the subsurface environment, particularly in the Manawatu River catchment, New Zealand. Challenges around the measurement of denitrification in the subsurface environment is one of the reasons that there has been limited research in this important area. Acetylene inhibition (AI) is a commonly employed method to measure denitrification in soil-water systems. However, subsurface denitrification studies using the AI method vary in methodological details, and this variation has implications for the reliability and comparability of results

Evaluation of the acetylene inhibition method to measure denitrification in unsaturated zone

A sound understanding of the transport and fate of farm nutrients (such as nitrogen and phosphorus) is a key component in our understanding and management of the likely impacts of these nutrients on fresh water quality and ecosystem health. Denitrification is an important nitrate attenuation process in soil-water systems that significantly influences the quality of groundwater and receiving surface waters. While denitrification in surface soils is widely studied, there is limited information available about its occurrence and distribution in the subsurface environment, especially below the root zone. One of the most common methods used to quantify denitrification is the acetylene inhibition (AI) method. On the other hand, subsurface denitrification studies using the AI method vary in methodological details, particularly in the determination of the denitrifying enzyme activity (DEA), which gives a snapshot of the potential of the soil to denitrify at the time of sampling. Luo et al. (1996) conducted a study to standardise DEA measurements for New Zealand soils, but it was carried out with samples from the surface soil layer (0-10 cm). Given the much lower biological activity in subsurface soils, this study aims to evaluate and determine the appropriate procedures for AI method to measure DEA in soil samples from the unsaturated zone

An assessment of the denitrification potential in shallow groundwaters of the Manawatu River catchment

Denitrification in shallow groundwaters is an important nitrate attenuation process which is dependent on the characteristics of the contributing surface and subsurface environment. Little is known about the spatial variability and factors contributing to denitrification potential in shallow groundwater systems in the Manawatu River catchment. The objectives of the study, therefore, are (1) to determine the spatial variability of the denitrification potential in shallow groundwater, (2) to identify the factors affecting this denitrification potential, and (3) to quantify denitrification in selected sites in the catchment

Variability in hydrogeochemical conditions in shallow groundwater in the Manawatu River catchment and implications for denitrification potential

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

Assessment of nitrogen flow pathways and its potential attenuation in shallow groundwaters in the Lower Rangitikei catchment

We assessed surface water and groundwater interactions, and nitrogen flow and its potential attenuation in shallow groundwater in the lower Rangitikei catchment. 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. A piezometric map, developed from measured depths to groundwater in about 100 wells in October 2014, suggests that groundwater flow, in particular the shallow groundwater (30 m) showed relatively less topographic influence. Groundwater discharges to the river upstream and downstream of Bulls, while groundwater recharge or no interaction with the river is more likely to occur near the coast. The groundwater redox characterisation, based on sampling and analysis of 15 groundwater wells, suggests that in general the groundwater across the lower Rangitikei catchment is under anoxic/reduced conditions. The groundwater typically has low dissolved oxygen concentrations (<1 mg/L) and elevated levels of electron donors (particularly DOC and Fe²⁺) that are suitable for denitrification. We further measured NO₃⁻-N attenuation in shallow groundwater piezometers (3-6 m bgl) using the single-well push-pull tests. The push-pull tests showed N0₃⁻-N reduction at four of the five piezometers, with the rates of reduction varying from 0.04 mg N L⁻¹ hr⁻¹ to 1.57 mg N L⁻¹ hr⁻¹

Redox characteristics of shallow groundwater in the Tararua Groundwater Management Zone

We aimed to (i) develop a direct-push method to effectively sample groundwater from a wide range of geological settings; (ii) analyse chemical and physical characteristics of groundwater and determine their redox potential,; and (iii) identify the influence different catchment characteristics such as soil texture and drainage and rock types on groundwater chemistry and its redox characteristics across the Tararua GWMZ