Developing and validating a decision support tool for media selection to mitigate drainage waters

The nitrate nitrogen (NO3-N) and ammonium (NH4-N) and/or dissolved reactive phosphorus (DRP) load in drainage water from farms can be managed by reactive or biological media filters. The nutrient content of the drainage water can be obtained directly from water analysis, which immediately focuses attention on filter media selection. There are many factors that may be important before choosing a medium or media e.g. nutrient removal capacity, lifetime, hydraulic conductivity, the potential for “pollution swapping”, attenuation of non-target contaminants (e.g. pesticides, organic carbon, etc.), and local availability and transportation cost of media to site. In this study, a novel decision support tool (DST) was developed, which brought all these factors together in one place for five nutrient scenarios. A systematic literature review was conducted to create a database containing 75 media with an associated static scoring system across seven criteria (% of nutrient concentration reduction, removal of other pollutants, lifetime, hydraulic conductivity, negative externalities) and a dynamic scoring system across two criteria (delivery cost and availability). The DST was tested using case studies from Ireland, Belgium and USA with different agricultural practices and nutrient scenarios. It was then validated by SWOT (strength, weakness, opportunities and threats) analysis. The DST provided a rapid, easily modifiable screening of many media-based treatments for specific dual or single nutrient-based water drainage problems. This provides stakeholders (farmers/regulators/advisors) with a versatile, flexible and robust yet easy-to-understand framework to make informed choices on appropriate media-based mitigation measures according to users’ relevant technical, economic and logistical factors

Impact of P inputs on source-sink P dynamics of sediment along an agricultural ditch network

peer-reviewedPhosphorus (P) loss from intensive dairy farms is a pressure on water quality in agricultural catchments. At farm scale, P sources can enter in-field drains and open ditches, resulting in transfer along ditch networks and delivery into nearby streams. Open ditches could be a potential location for P mitigation if the right location was identified, depending on P sources entering the ditch and the source-sink dynamics at the sediment-water interface. The objective of this study was to identify the right location along a ditch to mitigate P losses on an intensive dairy farm. High spatial resolution grab samples for water quality, along with sediment and bankside samples, were collected along an open ditch network to characterise the P dynamics within the ditch. Phosphorus inputs to the ditch adversely affected water quality, and a step change in P concentrations (increase in mean dissolved reactive phosphorus (DRP) from 0.054 to 0.228 mg L−1) midway along the section of the ditch sampled, signalled the influence of a point source entering the ditch. Phosphorus inputs altered sediment P sorption properties as P accumulated along the length of the ditch. Accumulation of bankside and sediment labile extractable P, Mehlich 3 P (M3P) (from 13 to 97 mg kg−1) resulted in a decrease in P binding energies (k) to < 1 L mg−1 at downstream points and raised the equilibrium P concentrations (EPC0) from 0.07 to 4.61 mg L−1 along the ditch. The increase in EPC0 was in line with increasing dissolved and total P in water, demonstrating the role of sediment downstream in this ditch as a secondary source of P to water. Implementation of intervention measures are needed to both mitigate P loss and remediate sediment to restore the sink properties. In-ditch measures need to account for a physicochemical lag time before improvements in water quality will be observed

Modelling oxide thin films

Three simulation methodologies have been employed to investigate the growth, nucleation, and structure of oxides supported on oxide substrates, these are atom-by-atom deposition, layer-by-layer deposition and finally amorphisation of a structure followed by recrystallisation. The materials which have been investigated include the rocksalt-structured oxides; MgO, CaO, SrO, and BaO, the perovskite structured SrTiO3 and also fluorite structured CeO2 and ZrO2. The work has shown that the substrate influences critically the structure of the supported thin film by determining the nature and interactions of defects, dislocations and grain-boundaries, as well as influencing the interfacial ion densities and various epitaxial relationships. In addition, graphical techniques have been employed to show the three-dimensional atomistic structure of each structural and epitaxial feature. Moreover, by considering large simulation cell sizes (approaching the mesoscale, 18 nm square), it has been possible to accommodate the synergistic interactions between neighbouring structural features, which can lead to changes in their basic structure. We also show that the particular surface termination of the substrate can influence the structure (and tentatively, the critical thickness) of the supported film through the example of SrO and TiO2 terminated faces of a SrTiO3(001) substrate. © 2002 Taylor &amp; Francis Ltd

Atomistic simulation methodologies for modelling the nucleation, growth and structure of interfaces

There have been many studies applying atomistic simulation techniques to investigate the structure and energetics of surfaces and interfaces. Almost all start by defining the basic structure of the interface, which is then simulated by static or dynamical methods. A different approach is adopted here, where we allow interfacial structures to evolve during the course of the simulation. In particular, three atomistic simulation methodologies for constructing models for thin film interfaces have been developed, including 'atom deposition', where the thin film is 'grown' by sequentially depositing atoms onto a support material to obtain information on nucleation and growth mechanisms; 'layer-by-layer' growth, where monatomic layers of a material are successively deposited on top of a substrate surface; and finally, 'cube-on- cube' whereby the whole of the thin film is placed directly on top of the substrate, before dynamical simulation and energy minimisation. The methodologies developed in this study provide a basis for simulating the nucleation, growth and structure of interface systems ranging from small supported clusters to monolayer and multilayer thin film interfaces. In addition, the layer-by-layer methodology is ideally suited to explore the critical thickness of thin films. We illustrate these techniques with studies on systems with large negative misfits. The calculations suggest that the thin films (initially constrained under tension due to the misfit) relax back to their natural lattice parameter resulting in the formation of surface cracks and island formation. The cube-on-cube methodology was then applied to the SrO/MgO system, which has a large (+ 20%) positive misfit. For this system, the SrO thin film underwent an amorphous transition which, under prolonged dynamical simulation, recrystallised revealing misfit-induced structural modifications, including screw-edge dislocations and low angle lattice rotations