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Item Measurement and modelling of salt and nutrient dynamics under Salicornia irrigated with saline groundwater, desalination reject-brine, and aquabrine : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Soil and Environmental Sciences, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand(Massey University, 2024-07-27) Al Tamimi, MansoorEnvironment Agency-Abu Dhabi (EAD) has implemented several scientific research projects with Plant and Food Research (PFR) New Zealand and OnlyFromNZ Limited (OFNZ) to determine the irrigation requirements for date palms, arid forestry, and field crops, for Law 5 which states that groundwater in the Emirate is the property of the Government which has the responsibility for management, organization and licensing the activities related to groundwater. The Government entity is EAD-Environment Agency of Abu Dhabi). can be implemented using sound scientific bases to protect the interests of all. This is a cooperation with external partners, plus both governmental and non-governmental agencies. These projects aimed to determine the irrigation requirements for date palms, forests, and field crops. While these projects have determined irrigation requirements, they have not, to date, addressed the environmental impacts of farming on groundwater quantity and quality. This Ph.D. research explored the trade-offs between improved technologies for the use of alternative water supplies, such as desalination brines, and the environmental consequences of brine re-use. The results will form part of the future solutions at the nexus among food, energy, and water in Abu Dhabi. This doctoral research builds on nearly a decade of scientific knowledge developed in collaboration with the New Zealand teams. It identifies the opportunities for reject brine from desalination units in aquaculture and halophytic agriculture, as well as the environmental consequences of the fate of this salt and nutrients. Measurements are critical to understand groundwater impacts and the benefits of using saline groundwater and brines from desalination plants to irrigate halophytes in hyper-arid environments. In 2020, a pilot trial was set up using irrigation with highly saline waters. However, two difficulties were encountered, and the pilot trial was a failure. The first part of the thesis describes how the practical challenges of the measurement technologies used in this saline environment were overcome. Further, experiments were then carried out to quantify the efficiency and impact of salt leaching in removing salt from the rootzone. Two years of field experimentation were undertaken to determine the economic productivity and environmental impact on groundwater of irrigating the halophyte Salicornia bigelovii with three types of saline waters in the hyper-arid United Arab Emirates. In the first year, the irrigation waters employed were groundwater (GW) at 25 dS m⁻¹, reverse-osmosis brine (RO) from a desalination unit at 40 dS m⁻¹, and Aquabrine (AQ) effluent from land-based aquaculture in tanks filled with RO brine, also at 40 dS m⁻¹. Bubblers (BUB), pressure-compensated drippers (PCD), or subsurface irrigation tape (SUB) were used to apply the three waters Salicornia fresh tip, harvest forage, and seed yields were highest for AQ applied through BUB, reaching 650 g m⁻². The dry forage yield with AQ through BUB was found to be 2-2.6 kg m⁻², compared to 1-2.3 kg m⁻² for the other irrigation waters and emitter devices. The highest water productivity WPI (kg m⁻³) across all three crop outputs resulted from Aquabrine applied by pressure-compensated drippers. Gross economic water productivity (GEWP₁, $ m⁻³) was assessed based solely on gross revenue. The highest GEWP₁, at US$5.8-6.2, was achieved with AQ applied through PCD and SUB, primarily due to revenue from fresh tips. Notably, the GEWP₁ significantly exceeded the cost of desalination at $1.5 m⁻³. Drainage and leaching were measured using fluxmeters. The greatest salt load into the groundwater, at 135-195 kg m⁻², was observed with BUB irrigation. For PCD and SUB, the salt load ranged between 14-36 kg m⁻². Simple mass-balance calculations of these salt loadings were then employed to predict the impact on the saline quality of aquifers. An exemplar loading of 75 kg m⁻² was used, resulting in a projected annual salinity rise of 2.6 dS m⁻¹ y⁻¹ for an aquifer with a saturated depth of 100 m. This significant increase in groundwater salinity would represent a continual decline in the resource's utility. This simple mass-balance arithmetic highlighted the need for modelling. New data from the following year’s experiments highlighted the economic value of using nitrogen-rich saline waters, either from groundwater or reject brines from desalination units, to irrigate the halophytic crop Salicornia bigelovii for food, fodder, and fuel in a hyper-arid environment. The greatest benefit was, again, achieved using pressure compensated drippers. Field measurements of drainage and leaching under the crop showed that, in sum, all of the salt, as well as the nitrogen drawn up from the groundwater were returned back to the aquifer as leachate. The only loss of water to the system was through crop evapotranspiration ET𝒸. A simple heuristic model of groundwater quantity and quality was developed to infer the environmental impacts of irrigating crops with saline and high-nitrate groundwater in a hyper-arid environment. The time-rise in solute concentration in groundwater is found to be a hyperbola. The parameters needed for this simple model are the fraction of the land above the aquifer that is irrigated, the initial depth of the saturated thickness of the aquifer, the saturated water content of the aquifer, and the annual rate of crop evapotranspiration. An indicator of the rate-of-rise in solute concentration, akin to a half-life, is the numbers of years to double the solute concentration in groundwater. This can be found as Ө hₒ /2 ET𝒸, where Ө is the saturated water content, hₒ is the original thickness of the saturated layer, and ET𝒸 is the annual rate of crop evapotranspiration. The general model is simple and straightforward to parameterise. It is easily understood and useful for assessing the impacts and trade-offs of policy and regulatory options. The knowledge gained from these experiments and the predictions resulting from the heuristic modelling have been used to highlight future needs to be addressed for the critical issues at the food-water-energy nexus in hyper-arid regions.Item Water and solutes in soil : hydraulic characterisation, sustainable production, and environmental protection : application for the degree of Doctor of Science from Massey University, Palmerston North, New Zealand(Massey University, 2002) Clothier, Brent EThe soil of the rootzone, the fragile and fertile interface between the atmosphere and the subterranean realm, is characterised by massive transfers of water and solutes. Our understanding of the biophysical transport processes into, and through, soil has been enhanced by the research endeavours of the applicant, Brent Euan Clothier. Dr Clothier, a 1977 Ph.D. graduate of Massey University, has developed tools and techniques that increased the acuity of our vision of transport processes of water and solutes in soil, as well it has sharpened our ability to hydraulically characterise those mechanisms for the purpose of modelling and risk assessment. His research has also enhanced our understanding of how these biophysical processes affect sustainable agriculture, environmental protection, and the bioremediation of contamination. These endeavours are grouped, in this thesis, into four overlapping areas of research: • Processes and properties of water movement into and through soil • Processes and properties of solute movement through soil • Root uptake processes and sustainable irrigation • Plants, groundwater protection and bioremediation of contaminated soil. The key elements of these four themes, and their contribution to knowledge, form Chapters 2-5 of this thesis. Dr Clothier's awards, honours, and impact are discussed in Chapter 6.Item Assessment of biogeochemical transformation of nitrate in shallow groundwater in the agricultural landscape : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science at Massey University, Manawatu, New Zealand(Massey University, 2019) Gonzalez Moreno, Marcela AngelicaIn groundwaters, denitrification or subsurface denitrification (SD) has been identified as a key attenuation process. Where leached nitrate (NO₃⁻) can be reduced to dinitrogen (N₂ — a harmless gas), offering an ecosystem service in terms of reducing water pollution. However, partial denitrification (PD) can release nitrous oxide (N₂ — a greenhouse gas), resulting in a pollution swap from liquid to gaseous pollution and adding to greenhouse gas emission. There is very limited information available about occurrence, characteristics and dynamics of subsurface denitrification in shallow groundwaters across New Zealand agricultural catchments. We studied 6 pastoral farms (DF 1, 2, 3; SC 1, 2, 3; ARM 1, 2, 3; CAM 1, 3; SR 1, 2, 3; BUR 1, 2 and 3) located in various hydrogeological settings in the Manawatu and Rangitikei Rivers catchments, in the lower North Island of New Zealand. We collected 7 sets of monthly groundwater observations at 17 piezometers from March to September 2018 to characterize the groundwater monthly chemical variations. The collected groundwater samples were analyzed for groundwater redox status, including dissolved oxygen (DO), oxidation-reduction potential, pH, NO₃⁻-N, iron (Fe²⁺), manganese (Mn²⁺) and sulphate (SO₄²⁻). We also conducted a set of push-and-pull tests to gain insights into dynamics of subsurface denitrification occurring in the groundwater samples at the study sites. We quantified changes in concentration of NO₃⁻-N, Br⁻ (tracer), dissolved N₂O-N and excess N₂ during the push-and-pull tests. Our results suggested a spatially variable groundwater redox conditions and SD occurring across the study sites. The piezometers DF 2, 3; SC 1, 2; CAM 3; ARM 1, 2 and 3 showed anoxic redox status. Only the piezometers SC 3 and CAM 1 presented mixed redox condition. While the piezometers DF 1; SR 1, 2, 3; BUR 1, 2 and 3 indicated oxic conditions with some variability over the study. Nitrate is being reduced in the anoxic piezometers DF 2, 3; SC 1, 3; ARM 1, 2, 3 and CAM 3, showing no NO₃⁻-N accumulation (< 0.5 mg L⁻¹). One of the piezometers with mixed redox condition (CAM 1) showed NO₃⁻-N accumulation (> 6 mg L⁻¹) while the piezometer SC 3 showed variability in NO₃⁻-N accumulation ranging from 0.02 mg L⁻¹ to 22.56 mg L⁻¹. The oxic piezometers SR 1, 2, 3; BUR 1, 2 and 3 showed NO₃⁻-N accumulation (> 3 mg L⁻¹) except for piezometer DF 1 that showed variability in NO₃⁻-N concentrations ranging from 0.01 mg L⁻¹ to 3.75 mg L⁻¹ over the study. The concentrations of the electron donors Fe²⁺ and Mn²⁺ were found to be suitable for SD on anoxic piezometers DF 2, 3; SC 1, 2; CAM 3 and ARM 1, 2, 3 (> 1 mg L⁻¹ and > 0.05 mg L⁻¹ respectively). The piezometers with mixed redox status SC 3 and CAM 1 ranged just over the redox threshold for identifying redox processes (0.1 – 1.0 mg L⁻¹ and > 0.05 mg L⁻¹ respectively). In general, the piezometer with oxic redox status (DF 1, SR 1, 2, 3 and BUR 1, 2, 3) showed [Fe²⁺] and [Mn²⁺] below the threshold for identifying redox processes (< 0.1 mg L⁻¹ and < 0.05 mg L⁻¹ respectively) and not suitable to support SD. The dominant terminal product of SD, whether was complete denitrification (N₂ — as end product) or partial SD (N₂O — as end product) spatially varied according to the redox status of the groundwater. Push-pull test results showed an increase in excess N₂ and N₂O-N concentrations at DF 3, ARM 3, CAM 3, BUR 3. The push-pull test conducted at SR 3 and SC 3 showed inconclusive results. Piezometers CAM 3 and ARM 3 showed the highest suitable conditions for SD followed by DF 3. Piezometer BUR 3 showed the highest partial SD rate. Therefore BUR 3 is considered in general, the less suitable piezometer for SD. Our observations highlight the influence of different hydrogeological settings on spatial variability of partial (pollution swamp) or complete (ecosystem service) SD in shallow groundwaters. A better understanding and quantification of spatial and temporal variability of SD process will support information, design and formulation of targeted and effective management measures for sustainable agricultural production while protecting soil, water and air quality.Item Coupled effects of irrigation management and water salinity on date palm cultivars in the hyper-arid environment of the United Arab Emirates : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Soil and Environment Sciences, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand(Massey University, 2019) Al Muaini, Ahmed HassanDates, and the farming of date palms (Phoenix dactylifera L.), are culturally, aesthetically and economically important in the United Arab Emirates. In this hyper-arid region, dates require irrigation, as rainfall is virtually non-existent. Groundwater is relied upon as the source of this irrigation water. Yet, the groundwater reserves in the Emirates are expected to run-out in about 55 years. Furthermore they are becoming more saline. In the Emirate of Abu Dhabi, Law 5 has been passed and that will limit the amount of water that can be withdrawn for agriculture, or any other purposes. Thus there are imperatives to minimise the amount of water being used for the irrigation of date palms, and to limit the amount of salt leaching from the rootzone of the date palms. These critical issues provide the underpinning reasons for the research described in this thesis. Environment Agency – Abu Dhabi (EAD) has invested in two research projects to determine the minimum amount of irrigation water, as a function of salinity that needs to be applied to date palms to ensure economic returns from date production. These two projects underpin my doctoral research. Using the Compensation Heat Pulse Method (CHPM) of monitoring sapflow has enabled quantification of palm-tree water use, ETc. This was carried out on three cultivars of differing salt tolerances: the salt-tolerant ‘Lulu, the moderately tolerant ‘Khalas’, and the salt-intolerant ‘Shahlah’. Two salinities of groundwater were considered: 5 dS m⁻¹ and 15 dS m⁻¹. The sustainable daily rate of irrigation was considered to be 1.5 ETc, which accounts for a 25% factor-of-safety, and a 25% salt-leaching fraction. This represents considerable savings over current practices. As well, both proximal and remote sensing were used to extrapolate these findings onto commercial date farms. Finally, an assessment of the green, blue and grey water footprints of date production was made. The grey-water footprint from salt leaching was found to be the largest. A benefit-cost assessment was made of the option of using desalinated water to augment and dilute the brackish groundwater used for irrigation. To dilute 15 dS m⁻¹ groundwater to 5 dS m⁻¹ irrigation water was shown to have a benefit-cost ratio of 1.4. However, the environmental impact of the reject brine will need to be considered.Item Maintaining the hyper-arid forests of Abu Dhabi by sustainable irrigation using treated sewage effluent in conjunction with groundwater : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Soil and Environmental Sciences, School of Agriculture and Environment, Massey University, Palmerston North, New Zealand(2019) Al Yamani, WafaThe late H.H Sheikh Zayed bin Sultan Al Nahyan, the founding President of the United Arab Emirates sought to ‘green the desert’ through planting of trees. These forests in the hyperarid desert of Abu Dhabi depend on irrigation with groundwater (GW). A wide range of valuable ecosystem services are delivered by the forests. In the 2017 State of the Environment Report, Environment Agency – Abu Dhabi (EAD) noted that “… considerable water resources are required to maintain these forests”. Over-consumption of GW, and the increasing salinity of the aquifers means that the GW of Abu Dhabi is under threat. To manage sustainably the GW resources, in 2016 the government of Abu Dhabi passed Law 5 on GW management and identified the requirement for limits to be placed on GW allocation for vegetation. The means to realise reductions in GW use are: minimised irrigation schedules for GW; and the replacement of GW with treated sewage effluent (TSE). To achieve this, a Government-to-Government partnership was established between EAD and the New Zealand Government. The NZ partners are Maven International and Plant & Food Research. This doctoral research was carried out under this larger partnership. The actual water uses, ETc, of the 3 major forest species of Al Ghaf, Al Sidr and Al Samr were measured by heat-pulse sapflow methods in trees irrigated with either GW or TSE. The impacts on ETc and tree health of the lower salinity TSE are detailed. The complex links between tree water-use, the reference ETo, and trees’ phenology are described. Relationships between the crop factor, Kc (=ETc/ETo), and tree canopy characteristics were inferred using a light-stick to measure the percentage light intercepted by the trees’ canopy. From this research, guidelines have been proposed for Law 5 for the water-allocation limits for these 3 species. These guideline values for GW are based on 1.5 ETc to account for a 25% factor-of-safety, and a 25% salt-leaching fraction. For TSE, there is no need for salt leaching. These recommendations will lead to GW savings of 35-70% compared to current practice. Eventually TSE should replace GW to sustain the forests.Item Accounting of nitrogen attenuation in agricultural catchments : 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(Massey University, 2018) Elwan, AhmedThe transport and fate of the nitrate that leaches from the root zone of farms, via groundwaters, to receiving surface waters is poorly understood, particularly for New Zealand’s agricultural catchments. Monitoring nitrate concentrations in rivers clearly demonstrates that not all of the nitrate leached across the catchment enters the river. As nitrate moves from land to receiving waters there is potential for subsurface denitrification and hence the attenuation of the nitrate flux to receiving surface waters. A good understanding of the influence of catchment characteristics on the spatial variations of nitrate attenuation is essential for targeted and effective water quality outcomes across agricultural landscapes. This thesis analysed large datasets of geographical information (land use, soils and geology) and water quality records at 20 sites in two large agricultural catchments, the Tararua and Rangitikei, which are located in the lower parts of the North Island New Zealand. The results demonstrated that the influence of land use on river soluble inorganic nitrogen (SIN) concentrations in the Tararua catchment was outweighed by other catchment characteristics such as soil type and hydrological indices. A simple approach, that is not data-intensive, was developed and applied to quantify the capacity of a catchment to attenuate nitrogen. The nitrogen attenuation factor (AFN) is a key component of this approach. AFN is defined as the average annual land use nitrogen leaching losses minus the average annual river SIN river loads, divided by the average annual land use nitrogen leaching losses. AFN was determined for 5 and 15 sub-catchments in the Rangitikei and Tararua catchments, respectively, and was found to be highly spatially variable with values ranging from 0.14 to 0.94. To assess the uncertainty associated with AFN, the uncertainty in the average annual river SIN loads was evaluated. Five load calculation methods (global mean GM, rating curve RC, ratio estimator RE, flow-stratified FS, and flow-weighted FW) and four sampling frequencies (2 days, weekly, fortnightly, and monthly) were investigated to calculate average annual river loads at one of the long-term, representative water quality monitoring sites in the study catchment. The FS method using a monthly sampling frequency resulted in the lowest bias (0.9%) for average annual river SIN loads and therefore was used in the quantification of AFN across the study catchments. A robust uncertainty analysis of AFN showed two distinct groups of sub-catchments; sub-catchments with higher (>0.7) and less uncertain nitrogen attenuation factors, and sub-catchments with lower (<0.4) and more uncertain nitrogen attenuation factors. This supports the use and applicability of AFN as a sub-catchment descriptor of the capacity of a sub-catchment to attenuate nitrogen. AFN was positively related to poorly drained soils and mudstones, and negatively related to well-drained soils and gravels in the study catchments. A novel but simple hydrogeologic-based model was developed to evaluate the potential to use soil and rock indices to predict average annual river SIN loads from different land uses in a catchment. Four different versions of the model (uniform nitrogen attenuation, variable nitrogen attenuation based on soil indices only; variable nitrogen attenuation based on rock indices only; and variable nitrogen attenuation based on both soil and rock indices) were developed. Accounting for the spatial distribution of the nitrogen attenuation capacities of both soils and rocks resulted in markedly better predictions of river SIN loads in the Tararua and Rangitikei sub-catchments. The novel findings of this thesis clearly suggest that effective and targeted measures to improve water quality at a catchment scale should account not only for land use but also for other catchment characteristics, such as the subsurface nitrogen attenuation capacity. This new knowledge will be instrumental in the future development of the models and planning tools required to reduce the detrimental impacts of agriculture, by aligning spatially intensive land use practices with high nitrogen attenuation pathways in sensitive agricultural catchments.Item Redox characteristics of shallow groundwater in the Tararua Ground Water Management Zone : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Manawatū, New Zealand(Massey University, 2018) McGowan, Peter GrantGroundwater redox conditions have a major influence on transport and transformation of nutrients such as nitrate from farms to rivers and lakes. This study focused on measurement and analysis of chemical and physical characteristics of groundwater to determine the spatial distribution of redox characteristics across the Tararua Ground Water Management Zone in the Manawatu River catchment. The influence of catchment characteristics such as soil texture and drainage, and rock type have on groundwater chemistry and its redox characteristics across the Tararua GWMZ is investigated using multivariate statistical analysis. Existing geographical information was collated and analysed to map spatial distributions of land use, soil characteristics and lithologies across the study area. This information was utilised to identify potential site locations for sampling and analysis of shallow groundwater in the Tararua GWMZ. A direct-push system capable of penetrating a range of substrates including deep, imbricated, and coarse gravels was developed. Using this system, shallow groundwater samples were recovered from contrasting hydrogeological settings, areas where water wells are rarely installed; such as along the margins of the axial ranges, and from areas considered not to have groundwater; e.g. the mudstone country on the east of the Tararua District. Data collected with the direct-push method was combined with similar data collected from existing wells by Rivas et al. (2017) and classified according to redox status. The data was subjected to multivariate statistical assessment using Hierarchical Cluster Analysis to determine the water type, and Principal Component Analysis to determine the influence of discrete catchment characteristics on redox reactions occurring in shallow groundwater of the Tararua GWMZ. The in-field and chemical analysis revealed significant variation of groundwater quality parameters and redox characteristics across the Tararua GWMZ. The regional trend was for reducing conditions in gravel aquifers in the north western areas of the Tararua GWMZ and oxidising in gravel aquifers of the south western; although statistically significant variations of redox characteristics is also recognised within these areas. Groundwater samples were collected from mudstone where little, if any, groundwater research has been conducted previously. Groundwater characteristics from mudstone are generally classified as anoxic and strongly reducing, with very high specific conductivity and analyte levels such as bromide, chlorine, sodium, fluorine, dissolved inorganic carbon and magnesium. Identifying the influence of discrete catchment characteristics on groundwater chemistry and redox characteristics was complex and difficult to quantify. Extrapolation of the principal component inferred to be associated with redox characteristics provides a useful means to evaluate the influence of discrete catchment characteristics on redox conditions in shallow groundwater of the Tararua GWMZ. The direct-push method provided an opportunity to compare groundwater chemistry between samples collected proximal and distal to production wells. Statistically significant differences in redox related parameters such as DOC, Eh, Fe2+, Mn2+, NH4+-N, and N02--N were detected in groundwater samples collected from existing wells compared to groundwater samples collected with the direct-push method. Factors contributing to this effect were explored but found to be difficult to isolate.Item Investigating the transport and fate of nitrogen from farms to river in the Lower Rangitikei catchment : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Manawatu, New Zealand(Massey University, 2015) Collins, Stephen BrianA sound understanding of the transport and fate of leached nitrate-nitrogen (NO3--N) in shallow groundwater is key to understanding the impacts of land use intensification on the quality of groundwater and surface water bodies. However, these are not well understood in the Lower Rangitikei catchment. This study was undertaken to assess the groundwater flow pattern and its interactions with the Rangitikei River; the redox conditions of the groundwater; and the extent of NO3--N attenuation in shallow groundwater in the Lower Rangitikei catchment. Groundwater depths were collected from more than 100 wells to map the piezometric surface to inform the groundwater flow pattern within the study area. Groundwater interactions with the Rangitikei River were estimated qualitatively from two longitudinal river flow and water quality surveys (on 6th and 20th January 2015) under low-flow conditions. Fifteen wells were sampled and analysed in the study area during December 2014 to characterise the groundwater redox condition. A total of nine piezometers were installed at a range of depths (3 m and 6 m) on two dairy farms (sand country and river terrace) and one cropping farm (sand country). In these piezometers, NO3--N, dissolved oxygen (DO) and other parameters were monitored over March, April and May 2015. Single-well push-pull tests were used to measure NO3--N attenuation in shallow groundwater during May 2015. Groundwater flow was largely influenced by the regional topography, particularly shallow groundwater (<30 m), where it flows from elevated areas such as Marton in a southerly direction towards the Rangitikei River. The longitudinal river flow and water quality surveys revealed a dynamic relationship between the river and the underlying aquifer. The surveys suggested groundwater discharges into the river both upstream and downstream of Bulls. The groundwater redox characterisation showed generally anoxic/reduced groundwater across the lower Rangitikei catchment area. Groundwater typically has a low DO concentration (<1 mg/L) with elevated levels of available electron donors, particularly dissolved organic carbon and Fe2+. These groundwater characteristics provide for generally favourable conditions for NO3--N reduction. Monitoring at the installed piezometers showed a generally low NO3--N concentration at these sites. The push-pull tests revealed NO3--N reduction occurring at all three sites, with the rate of reduction varying between 0.04 mg N L-1 hr-1 to 1.57 mg N L-1 hr-1. These results suggest that groundwater is likely to be connected with the Lower Rangitikei River. However, NO3--N concentrations in the river and groundwater were generally low, especially for the river at low flows. This suggests NO3--N may be undergoing reduction within shallow groundwater before it has a chance to seep into the river. Further evidence for appreciable levels of NO3--N reduction in the shallow groundwater is provided by the redox characterisation of reduced groundwater and the push-pull tests. However, more spatial and temporal surveys and in-situ measurements of denitrification occurrence in the shallow groundwater of the study area are required.Item The convection dispersion equation -- not the question, the answer! : anion and cation transport through undisturbed soil columns during unsaturated flow : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University(Massey University, 1997) Vogeler, Iris; Vogeler, IrisPrediction of solute movement through the unsaturated zone is important in determining the risk of groundwater contamination from both "natural" and surface applied chemicals. In order to understand better the mechanisms controlling this water-borne transport, unsaturated leaching experiments were carried out on undisturbed soil columns, about 3 litres in volume, for two contrasting soils. One was the weakly-structured Manawatu fine sandy loam, and the other the well-aggregated Ramiha silt loam. Anion transport was satisfactorily described using the convection dispersion equation (CDE), provided that anion exclusion for the Manawatu soil, and adsorption for the Ramiha soil were taken into account. At water flux densities of about 3 mm h-1, a dispersivity of about 40 mm was obtained for the Manawatu soil, and a dispersivity of about 15 mm for the Ramiha soil. The difference was probably due to the contrasting structures of the two soils. Increasing the water flux density in the Manawatu soil to about 13 mm h-1 resulted in a slightly higher dispersivity of about 60 mm. Flow interruption resulted in a subsequent drop in the effluent concentration for the Manawatu soil but not in the Ramiha soil. This suggests that the lag time for transverse molecular diffusion from "mobile" to "immobile" water domains was important in the Manawatu soil, but not in the Ramiha soil. In both soils cation transport was described satisfactorily with the CDE in conjunction with cation exchange theory, providing that only 80% of the cations replaced by 1 M ammonium acetate were assumed to be involved in exchange reactions. Column leaching experiments were also carried out using a rainfall simulator and larger columns of about 22 litres of the Manawatu soil with a short pasture on top. Solid chemical was applied to both a dry and a wet soil surface. Neither the pasture nor the initial water content had a significant effect on solute movement. Slightly higher dispersivities of about 70 mm were found. Time Domain Reflectometry (TDR) was found to be valuable for monitoring solute transport in a repacked soil under transient water flow conditions. But in undisturbed soils TDR only proved to be accurate under steady-state water flow when absolute values of solute concentration were not sought. The CDE was thus found to satisfactorily answer the question of how to describe transport of non-reactive and reactive solutes under bare soil and under short pasture. This applied during both steady-flow and transient wetting.