John L. Hutson

The adsorption of cyanobacterial hepatotoxins from water onto soil during batch experiments

Public health concerns associated with cyanobacteria, both chronic and acute, arise from their ability to produce toxins. Rural communities within Australia and those in developing countries require an inexpensive and low-cost method for removing toxins from drinking water. A candidate technology is bank filtration. Adsorption of cyanobacterial hepatotoxins was measured in batch studies to determine the applicability of bank filtration as an efficient removal strategy. Five soils with different physicochemical properties were collected from regions around South Australia. The soils were mixed with either nodularin or microcystin-LR in distilled water and buffered solutions (pH 4.8, 6.9 and 9.1). Additionally, nodularin was mixed in unbuffered solutions (pH 4.8 and 8.9). The three soils with the high clay and/or organic carbon contents (Paringa A, McLaren Flat and Lakes Plains) had the higher nodularin adsorption coefficients, ranging from 0.2 to 16.59 L kg-1. Soil suspensions in acetate buffer (pH 4.8) generally produced significantly higher nodularin adsorption coefficients, when compared to the other buffer systems. The background interference from the ionic strength of the buffers, however, made interpretation of the effect of pH on toxin sorption difficult. Increases in solution ionic strength, from freshwater to seawater, resulted in corresponding increases in the nodularin adsorption coefficients for all sites, except the sandy Hallett Cove site. The implications for bank filtration are that higher water pH values and lower salinities will enhance the in situ mobility of the toxins, resulting in an increased distance of filtration through the river bank before toxin free water could be abstracted for human consumption.

Laboratory Investigation of Variable-Density Flow and Solute Transport in Unsaturated–Saturated Porous Media

In many groundwater systems, fluid density and viscosity may vary in space and time as a function of changes in concentration and temperature of the fluid. When dense groundwater plumes interact with less dense ambient groundwater, these density variations can significantly affect flow and transport processes. Under certain conditions, gravitational instabilities in the form of lobe-shaped fingers can occur. This process is significant because it can lead to more rapid and spatially extensive solute transport. This paper presents new experiments carried out in a sand filled glass flow container under both fully saturated and variably-saturated conditions and focuses upon the processes that occur at the capillary fringe and below the water table, as affected by a dense contaminant plumes migration through the unsaturated zone. Source fluids stained with Rhodamine-WT were introduced at the upper boundary of the tank at a range of low and high densities. In addition to the fluid density gradients and porous medium permeability that determine the onset conditions for instabilities in fully saturated experiments, volumetric water content appears critical in the variably-saturated laboratory runs. Plume behaviour at the water table appears dependent upon the density of the fluid that accumulates there. For neutral and low density fluids, plumes accumulate at the water table and then spread laterally above it and the water table forms a barrier to further vertical flow as pore water velocities reduce with increasing water content. For medium and high density fluids, vertical movement continues as instabilities form at the capillary fringe and fingers begin to grow at the water table boundary and move downwards into the saturated zone. In these cases, lateral spreading of the plume is small. Despite these more qualitative observations, the exact nature of the relevant stability criteria for the onset and growth of instabilities in variably-saturated porous media presently remain unclear. All experimental results suggest, however, that the unsaturated zone and position of the water table must be considered in contaminant studies in order to predict the migration pathways, rates and ultimate fate of dense contaminant plumes. It is possible that the results of experiments presented in this paper could form a useful basis for the testing of variable-density (and variably-saturated) groundwater flow and solute transport numerical codes because they offer controlled physical laboratory analogs for comparison. They also provide a strong basis for the development of more rigorous mathematical formulations that are likely to be either developed or tested using numerical flow and solute transport simulators.

Variation in performance of surfactant loading and resulting nitrate removal among four selected natural zeolites

Surfactant modified zeolites (SMZs) have the capacity to target various types of water contaminants at relatively low cost and thus are being increasingly considered for use in improving water quality. It is important to know the surfactant loading performance of a zeolite before it is put into application. In this work we compare the loading capacity of a surfactant, hexadecyltrimethylammonium bromide (HDTMA-Br), onto four natural zeolites obtained from specific locations in the USA, Croatia, China, and Australia. The surfactant loading is examined using thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy. We then compare the resulting SMZs performance in removing nitrate from water. Results show that TGA is useful to determine the HDTMA loading capacity on natural zeolites. It is also useful to distinguish between a HDTMA bi-layer and a HDTMA mono-layer on the SMZ surface, which has not been previously reported in the literature. TGA results infer that HDTMA (bi-layer) loading decreases in the order of US zeolite>Croatian zeolite>Chinese zeolite>Australian zeolite. This order of loading explains variation in performance of nitrate removal between the four SMZs. The SMZs remove 8-18 times more nitrate than the raw zeolites. SMZs prepared from the selected US and Croatian zeolites were more efficient in nitrate removal than the two zeolites commercially obtained from Australia and China.

N fate and transport under variable cropping history and fertilizer rate on loamy sand and clay loam soils: I. Calibration of the LEACHMN model

The need for efficient use of agricultural chemicals and their potential adverse impact on critical water resources have increased the use of simulation models of the soil and plant system. Nevertheless, there is currently little or no agreement concerning model validity and applicability in varied soils and environments. The research version of LEACHMN (the N subroutine of LEACHM) was calibrated using field data including soil physical, hydraulic, and chemical properties, and maize (Zea mays L.) N uptake collected from a 3-yr nitrate leaching experiment. The field site consisted of plot-size lysimeters on clay loam and loamy sand soils with N fertilizer rates of 22, 100 and 134 kg N ha−1. The calibration involved adjusting nitrification, denitrification, and volatilization rate constants to optimize the fit between predicted and measured data. When calibrated for each treatment-year combination and soil type, the model simulations of soil profile NO3–N distribution were generally successful. The N transformation rate constants yielded by the calibration efforts were similar or close to those used in other model simulation studies. At both sites, the calibrated rate constants for the first year (following sod plowdown) were different from those for the subsequent two years. Denitrification rate constants were consistently higher for the clay site than for the sand site, while the nitrification rate constants were lower. N rate of application appeared not to affect the rate constants within each year-site combination, suggesting that cropping history and soil type had the greatest effect on N transformation rates.

The potential of riverbank filtration for drinking water supplies in relation to microsystin removal in brackish aquifers

In semi-arid areas, pumping town water supplies from alluvium adjacent a stream rather than the stream itself has been used to reduce turbidity and has potential to remove blue-green algal toxins, such as microsystin. However for some rivers, such as the River Murray in South Eastern Australia, the ambient groundwater of unconfined aquifers skirting some reaches of the river is saline. This paper examines the compatibility of two constraints on the quality of water recovered from bank filtration schemes; that (1) removal of cyanobacterial toxins is adequate and (2) salinity is acceptable for drinking water supplies. Adsorption and biodegradation characteristics of a cyanobacterial hepatotoxin, microsystin, in porous media were quantified and these results are summarised in the current analysis. It was found that riverbank filtration schemes could meet both criteria in a limited range of conditions, excluding locations where saline groundwater discharges to a river. However, on a river meander that had been flushed due to a hydraulic gradient induced by a lock, several feasible positions for bank filtration wells were compared and a best location meeting salinity and microsystin criteria with least-energy pumping cost was identified. The simple approach developed is intended to be used to assess feasibility of alternative designs for bank filtration schemes in semi-arid areas before commencing field studies.

Soil–water and solute movement under precision irrigation: knowledge gaps for managing sustainable root zones

Precision irrigation involves the accurate and precise application of water to meet the specific requirements of individual plants or management units and minimize adverse environmental impact. Under precision irrigation applications, water and associated solute movement will vary spatially within the root zone and excess water application will not necessarily result in deep drainage and leaching of salt below the root zone. This paper estimates that 10% of the irrigated land area (producing as much as 40% of the total annual revenue from irrigated land) could be adversely affected by root zone salinity resulting from the adoption of precision irrigation within Australia. The cost of increases in root zone salinisation due to inappropriate irrigation management in the Murray and Murrumbidgee irrigation areas was estimated at AUD 245 million (in 2000/01) or 13.5% of the revenue from these cropping systems. A review of soil–water and solute movement under precision irrigation systems highlights the gaps in current knowledge including the mismatch between the data required by complex, process-based soil–water or solute simulation models and the data that is easily available from soil survey and routine soil analyses. Other major knowledge gaps identified include: (a) effect of root distribution, surface evaporation and plant transpiration on soil wetted patterns, (b) accuracy and adequacy of using simple mean values of root zone soil salinity levels to estimate the effect of salt on the plant, (c) fate of solutes during a single irrigation and during multiple irrigation cycles, and (d) effect of soil heterogeneity on the distribution of water and solutes in relation to placement of water. Opportunities for research investment are identified across a broad range of areas including: (a) requirements for soil characterisation, (b) irrigation management effects, (c) agronomic responses to variable water and salt distributions in the root zone, (d) potential to scale or evaluate impacts at various scales, (e) requirements for simplified soil–water and solute modelling tools, and (f) the need to build skills and capacity in soil–water and solute modelling.

Floodwater infiltration through root channels on a sodic clay floodplain and the influence on a local tree species Eucalyptus largiflorens

Dieback of riparian species on floodplains has been attributed to increased soil salinisation due to raised groundwater levels, resulting from irrigation and river regulation. This is exacerbated by a reduction in flooding frequency and duration of inundation. For the Chowilla floodplain on the River Murray raised water tables have increased the amount of salts mobilised in the soil profile, causing the trees to experience salt induced water stress. For the trees to survive in the long term, salts need to be leached from the root zone.This study investigated whether floodwater infiltrates through channels created by E. largiflorens (black box) roots, flushing salts away from roots, thereby allowing the trees to increase their water uptake. Trees at different sites on the floodplain were artificially flooded, by pumping 1.5 kL of creek water into impoundments constructed around the trees. Gas exchange parameters, and pre-dawn and midday water potential were measured the day before, the day after and one week after the artificial flood and compared against trees that were not flooded. Pre-dawn and midday water potentials were also measured one month after the flood. After flooding, the trees experienced less water stress, indicated by an increase in water potential of less than 0.2 MPa, in comparison to non-flooded control trees. However, this response was not evident one month after flooding. The response to flooding did not result in increased rates of transpiration, stomatal conductance or photosynthesis, even though flooding effectively doubled the trees yearly water supply.The infiltration of floodwater in the impoundments around E. largiflorens was also compared to that of impoundments on bare ground. Floodwater infiltrated 2 – 17 times faster around trees than on adjacent bare ground, for parts of the floodplain not grazed by livestock. Tracer dye experiments indicated that bulk flow of water through pores down the profile was the reason for the enhanced infiltration. Flooding leached salts in direct vicinity of tree roots, but only leached small amounts of salts from the bulk soil.

Putative Effect of Aquifer Recharge on the Abundance and Taxonomic Composition of Endemic Microbial Communities

Drought events and the overexploitation of freshwater resources have led to the increased need to manage groundwater reserves. Aquifer storage and recovery (ASR), whereby artificial water is injected into aquifers for storage, is one of the proposed methods by which freshwater supplies can be increased. Microbial clogging following injection, however, is a major issue. Here, during laboratory simulations of ASR, we used flow cytometry and bar-coded pyrosequencing to investigate changes in microbial abundance and community dynamics. Bacterial abundance ranged from 5.0 × 104 to 1.4 × 107 cells ml-1 before the addition of synthetic wastewater. Following wastewater addition, a 25-fold decrease in abundance was observed, coinciding with a 12-fold increase in viral abundance. Taxa shifted from an overrepresentation of Sphingomonadales, Sphingobacteriales, Rhodospirillales, Caulobacterales, Legionellales, Bacillales, Fusobacteriales and Verrucomicrobiales prior to the addition of synthetic wastewater to Burkholderiales, Actinomycetales, Pseudomonadales, Xanthomonadales, Rhodobacterales, Thizobiales and Thiotrichales following the addition of synthetic wastewater. Furthermore, a significant difference in overall taxonomic composition between the groundwater samples before and after the addition of synthetic wastewater was observed, with water samples exhibiting more similarity to sediment samples after wastewater was added. Collectively, these results suggest that ASR may alter the taxonomic composition of endemic microbial communities and that complete profiles of groundwater properties, including microbial community abundance and composition need to be taken into consideration when selecting aquifers for ASR practices.

Influence of model conceptualisation on one-dimensional recharge quantification: Uley South, South Australia

Model conceptualisation is a key source of uncertainty in one-dimensional recharge modelling. The effects of different conceptualisations on transient recharge predictions for the semi-arid Uley South Basin, South Australia, were investigated. One-dimensional unsaturated zone modelling was used to quantify the effect of variations of (1) lithological complexity of the unsaturated zone, and (2) representation of preferential flow pathways. The simulations considered ranges of water-table depths, vegetation characteristics, and top soil thicknesses representative for the study area. Complex lithological profiles were more sensitive to the selected vegetation characteristics and water-table depth. Scenarios considering runoff infiltration into, and preferential flow through sinkholes resulted in higher and faster recharge rates. A comparison of modelled and field-based recharge estimates indicated that: (1) the model simulated plausible recharge rates, (2) only the models with preferential flow correctly reproduced the timing of recharge, and (3) preferential flow is probably redistributed in the unsaturated zone rather than passing to the water table directly. Because different but equally plausible conceptual models produce widely varying recharge rates, field-based recharge estimates are essential to constrain the modelling results.RésuméLa conception du modèle est la source principale d’incertitude dans la modélisation de la recharge unidimensionnelle. Les conséquences de conceptions différentes sur la prédiction de la recharge transitoire dans le semi aride Uley South Basin, South Australia, sont examinées. Un modèle unidimensionnel de la zone non saturée a été utilisé pour évaluer les effets des variations de (1) la complexité lithologique de la zone non saturée et (2) de la représentation des voies d’écoulement préférentiel. Les simulations prennent en compte des gammes de profondeurs de la surface libre de la nappe, des caractéristiques de la végétation et des épaisseurs de sol superficiel représentatives de la zone d’étude. Les profils lithologiques complexes sont plus sensibles aux caractéristiques de la végétation considérée et à la profondeur de la surface libre de la nappe. Les scénarios considérant la perte d’eau par infiltration directe et à travers des chenaux préférentiels conduisent à des taux de recharge plus élevés et plus rapides. La comparaison des évaluations de la recharge d’après le modèle et d’après le terrain indique que: (1) le modèle simule des taux de recharge plausibles, (2) seuls les modèles à écoulement préférentiel reproduisent correctement l’évolution de la recharge dans le temps (3) le flux préférentiel transite plutôt par la zone non saturée que par les zones d’écoulement direct vers la surface de la nappe. Parce que des modèles différents, mais également plausibles, conduisent à des ratios de recharge largement variables, les évaluations de la recharge sur le terrain sont essentielles pour caler le modèle utilisé.ResumenLa conceptualización de los modelos es una fuente clave de incerteza en la modelación de la recarga unidimensional. Se investigaron los efectos de diferentes conceptualizaciones en las predicciones de la recarga transitoria para la cuenca semiárida de Uley South, Australia del Sur. Se usó la modelización unidimensional de la zona no saturada para cuantificar los efectos de las variaciones de (1) la complejidad litológica de la zona no saturada, y (2) la representación de trayectorias preferenciales de flujos. Las simulaciones consideraron intervalos de profundidades del nivel freático, las características de la vegetación, y el espesor de la capa de suelo representativa para el área de estudio. Los perfiles litológicos complejos fueron más sensibles a las características de la vegetación seleccionada y a la profundidad del nivel freático. Los escenarios que consideran la infiltración y el flujo preferencial a través de los sumideros resultaron en mayores y más rápidos ritmos de recarga. Una comparación de la recarga estimada a partir del modelado y de datos de campo indicaron que: (1) el modelo simulaba ritmos de recarga razonables, (2) sólo los modelos con flujo preferencial reprodujeron correctamente el tiempo de recarga, y (3) el flujo preferencial está probablemente redistribuido en la zona no saturada en lugar de pasar directamente al nivel freático. Debido a que diferentes, pero igualmente razonables modelos conceptuales, producen una amplia variación de los ritmos de recarga, las estimaciones de la recarga provenientes de datos de campo son esenciales para restringir los resultados de la modelación.摘要模型概念化是一维补给模拟中的一个关键不确定源。调查了南澳大利亚半干旱Uley South盆地不同概念化对瞬时补给预测的影响。采用一维非饱和带模拟对非饱和带岩性复杂性的变化影响和优先流的表现变化影响进行了量化。模拟考虑了研究区有代表性的水位深度的范围、植被特征和表层土壤厚度。复杂的岩性剖面对选择的植被特征和水位深度更敏感。考虑径流渗入落水洞及优先流通过落水洞的方案导致更高和更快的补给量。模拟的和基于室外的补给估算比较显示:(1)模型模拟了似乎可信的补给量;(2)只有优先流模型正确地重现了补给时间的选择;(3)优先流可能在非饱和带被重新分配,而不是直接流到水位。由于不同的但同样似乎可信的概念模型得到变化相当大的补给量,因此,基于室外的补给估算对约束模拟结果必不可少。ResumoA concetualização do modelo é uma das principais fontes de incerteza na modelação da recarga unidimensional. Foram investigados os efeitos de diferentes concetualizações sobre as previsões da recarga variável para a Bacia semiárida de Uley South, no sul da Austrália. A modelação unidimensional da zona não saturada foi usada para quantificar o efeito das variações da (1) complexidade litológica da zona não saturada, e (2) representação de caminhos preferenciais de fluxo. As simulações consideraram os intervalos de profundidade do nível freático, as caraterísticas da vegetação e as espessuras do solo superficial representativas para a área de estudo. Perfis litológicos complexos foram mais sensíveis às caraterísticas da vegetação selecionadas e à profundidade do nível freático. Os cenários considerando infiltração do escoamento e fluxo preferencial através de sumidouros resultaram em taxas de recarga maiores e mais rápidas. A comparação entre as estimativas de recarga do modelo e as baseadas em dados de campo indica que: (1) o modelo simulou taxas de recarga plausíveis, (2) só os modelos com fluxo preferencial reproduziram corretamente os tempos de recarga, e (3) o fluxo preferencial é provavelmente redistribuído na zona não saturada, em vez de passar diretamente para a zona freática. Uma vez que modelos concetuais diferentes, mas igualmente plausíveis, produzem taxas de recarga amplamente variáveis, a estimativas da recarga com base em dados de campo são essenciais para restringir os resultados da modelação.

Drainage and Nitrate Leaching from Artificially Drained Maize Fields Simulated by the Precision Nitrogen Management Model

Environmental nitrogen (N) losses (e.g., nitrate leaching, denitrification, and ammonia volatilization) frequently occur in maize ( L.) agroecosystems. Decision support systems, designed to optimize the application of N fertilizer in these systems, have been developed using physically based models such as the Precision Nitrogen Management (PNM) model of soil and crop processes, which is an integral component of Adapt-N, a decision support tool providing N fertilizer recommendations for maize production. Such models can also be used to estimate N losses associated with particular management practices and over a range of current climates and future climate projections. The objectives of this study were to update the PNM model to include an option for simulating soil-water processes in artificially drained soils, and to calibrate the revised PNM model and test it against multiyear field studies in New York and Minnesota with different soils and management practices. Minimal calibration was required for the model. Denitrification rate constants were calibrated by minimizing the error between simulated and observed nitrate leaching for each study site. The normalized root mean squared error of cumulative daily drainage for the validation sets ranged from 10 to 23%. For cumulative daily nitrate leaching, the normalized root mean squared error ranged from 11 to 28% for the validation sets. The minimal calibration required and relatively simple data inputs make the PNM model a broadly applicable tool for simulating water and N flows in maize systems.