Integrating blue-green and grey infrastructure through an adaptation pathways approach to surface water flooding.

A range of solutions to future flood risk are available ranging from blue-green infrastructure (BGI) as commonly incorporated in sustainable drainage systems (SuDS) to traditional grey infrastructure (e.g. pipe networks, storage tanks, flood walls). Each offers a different profile with respect to costs, flexibility of implementation and the ability to deliver a range of wider benefits beyond their flood protection function. An important question that must be addressed when considering these approaches is what is the most suitable mix of grey and blue-green solutions to urban flooding at any location and at any future time? This paper uses an adaptation pathways approach to compare a range of alternative options to deal with current and expected future flood risk in part of a London borough. Solutions considered separately and in combination include grey pipe expansion, bioretention cells, permeable pavements and storage ponds. A methodological framework combines a range of existing tools to develop, assess and characterize each pathway, including a storm water management model (SWMM), a SuDs opportunity selection tool, an adaptation pathway generator and the CIRIA B£ST tool for monetizing multiple benefits. Climate change is represented by the UK Water Industry Research method for establishing future rainfall intensities for sewer and BGI design. The results showed that by extending the way in which adaptation pathways are compared and evaluated through the wider consideration of multiple benefits there is a trade-off between deferring interventions until they are needed for flood risk mitigation and delivering the multiple benefits associated with interventions so that performance thresholds do not need to be met before introducing new options. The relative contribution of each option's capital and operation and maintenance costs has implications on when the option is implemented as well as the rate of implementation. The monetization of the multiple benefits associated with each pathway shows that their economic co-evaluation alongside infrastructure costs can change the preference for one pathway over another. This article is part of the theme issue 'Urban flood resilience'.

Water allocation and governance in multi-stakeholder environments: Insight from Axios Delta, Greece.

This article deals with stakeholders' interactions and institutional capacity influencing water resource management where competitive demands co-exist. For the case study area of Axios Delta, Northern Greece, a water deficit in the agricultural sector, an unmet environmental flow and a reduced capacity for urban supply during drought conditions are observed. An egocentric network analysis based on desk-study and a series of semi-structured stakeholder interviews reveals how weak stakeholder ties lead to ineffective multilevel governance and, as a result, low water efficiency practices. There is a lack of understanding of other users' priorities as well as of the risks related to climate change and/or seasonal variability. This is reflected in the flat rate abstraction licence for agricultural purposes which reduces environmental flow to below acceptable standards. There is no transboundary cooperation between Greece and the Republic of North Macedonia which hinders an integrated management approach. A limited exchange of information to support an evidence-based allocation plan is observed. Suitable interventions identified through a DPSIR approach are evaluated in a multi-criteria analysis considering cost effectiveness, delivered benefits as well as ease of implementation. Suitable technical practices include the development of a local and catchment-scale monitoring network for surface water and groundwater, climate-adaptive agriculture and treated-water reclamation. Updated management policies involve the institutional prioritisation of environmental flow through an adaptive allocation plan as well as the strengthening of transboundary cooperation. This research shows how the coordination of aggregated diverging interests in multilevel multi-stakeholder environments appears to be key in supporting positive water budgets in an uncertain climate future.

Transport and viability of Escherichia coli cells in clean and iron oxide coated sand following coating with silver nanoparticles.

A mechanistic understanding of processes controlling the transport and viability of bacteria in porous media is critical for designing in situ bioremediation and microbiological water decontamination programs. We investigated the combined influence of coating sand with iron oxide and silver nanoparticles on the transport and viability of Escherichia coli cells under saturated conditions. Results showed that iron oxide coatings increase cell deposition which was generally reversed by silver nanoparticle coatings in the early stages of injection. These observations are consistent with short-term, particle surface charge controls on bacteria transport, where a negatively charged surface induced by silver nanoparticles reverses the positive charge due to iron oxide coatings, but columns eventually recovered irreversible cell deposition. Silver nanoparticle coatings significantly increased cell inactivation during transit through the columns. However, when viability data is normalised to volume throughput, only a small improvement in cell inactivation is observed for silver nanoparticle coated sands relative to iron oxide coating alone. This counterintuitive result underscores the importance of net surface charge in controlling cell transport and inactivation and implies that the extra cost for implementing silver nanoparticle coatings on porous beds coated with iron oxides may not be justified in designing point of use water filters in low income countries.

Evidence of preferential path formation and path memory effect during successive infiltration and drainage cycles in uniform sand columns.

The formation of preferential flow paths in the partially saturated zone, and in naturally structured media, is well known. This study examines non-uniform flow in uniform sand columns under different pressure and infiltration/drainage conditions. Experiments were carried out in a vacuum box, with applied suction set to three different heads, and with infiltration fixed at two different flow rates. Tailing observed in some conservative tracer breakthrough curves suggests the formation of immobile resident water pockets which slowly exchange mass with the flowing water fraction. The applied suction controlled the degree of water immobilization whereas flow rate had minimal effect on the dynamic behavior. Trapping and exchange of water occurred repeatedly during successive infiltration and drainage cycles, implying a (hysteretic) memory effect of the previously formed preferential flow paths. Flow and solute transport modeling suggests that these dynamics can be described by a mobile-immobile model that corroborates measurements suggesting preferential flow path formation. These findings have implications for the natural attenuation of contaminants in the partially saturated zone, but also for the persistence of a contamination source exposed to repeated conditions of infiltration and drainage.

Thermodynamic and kinetic controls on cotransport of Pantoea agglomerans cells and Zn through clean and iron oxide coated sand columns.

Recent observations that subsurface bacteria quickly adsorb metal contaminants raise concerns that they may enhance metal transport, given the high mobility of bacteria themselves. However, metal adsorption to bacteria is also reversible, suggesting that mobility within porous medium will depend on the interplay between adsorption-desorption kinetics and thermodynamic driving forces for adsorption. Till now there has been no systematic investigation of these important interactions. This study investigates the thermodynamic and kinetic controls of cotransport of Pantoea agglomerans cells and Zn in quartz and iron-oxide coated sand (IOCS) packed columns. Batch kinetic studies show that significant Zn sorption on IOCS takes place within two hours. Adsorption onto P. agglomerans surfaces reaches equilibrium within 30 min. Experiments in flow through quartz sand systems demonstrate that bacteria have negligible effect on zinc mobility, regardless of ionic strength and pH conditions. Zinc transport exhibits significant retardation in IOCS columns at high pH in the absence of cells. Yet, when mobile bacteria (non attached) are passed through simultaneously with zinc, no facilitated transport is observed. Adsorption onto cells becomes significant and plays a role in mobile metal speciation only once the IOCS is saturated with zinc. This suggests that IOCS exhibits stronger affinity for Zn than cell surfaces. However, when bacteria and Zn are preassociated on entering the column, zinc transport is initially facilitated. Subsequently, zinc partly desorbs from the cells and redistributes onto the IOCS as a result of the higher thermodynamic affinity for IOCS.

Kinetics of bacterial potentiometric titrations: the effect of equilibration time on buffering capacity of Pantoea agglomerans suspensions.

Several recent studies have made use of continuous acid-base titration data to describe the surface chemistry of bacterial cells as a basis for accurately modelling metal adsorption to bacteria and other biomaterials of potential industrial importance. These studies do not share a common protocol; rather they titrate in different pH ranges and they use different stability criteria to define equilibration time during titration. In the present study we investigate the kinetics of bacterial titrations and test the effect they have on the derivation of functional group concentrations and acidity constants. We titrated suspensions of Pantoea agglomerans by varying the equilibration time between successive titrant additions until stability of 0.1 or 0.001 mV s(-1) was attained. We show that under longer equilibration times, titration results are less reproducible and suspensions exhibit marginally higher buffering. Fluorescence images suggest that cell lysis is not responsible for these effects. Rather, high DOC values and titration reversibility hysterisis after long equilibration times suggest that variability in buffering is due to the presence of bacterial exudates, as demonstrated by titrating supernatants separated from suspensions of different equilibration times. It is recommended that an optimal equilibration time is always determined with variable stability control and preliminary reversibility titration experiments.