GDES

Abstract: Anthropogenic carbon dioxide (CO2) emissions contribute to the greenhouse effect and global warming, which can lead to undesirable climate change and extinction of species. Besides the ongoing efforts to develop environmentally benign sources of energy and to advance technologies for the capture and sequestration of CO2, the transformation of emitted CO2 into valuable products is a pragmatic solution to curb its accumulation in the atmosphere. In this regard, electrochemical CO2 reduction (ECR) powered by renewable electricity provides an attractive approach because it not only converts CO2 to valuable fuels and chemicals but also offers a solution for the long-term storage of intermittent renewable energies. In ECR, the gas diffusion electrode (GDE) is the most critical component and has been the subject of intensive research in the last few years. This tutorial review provides an insightful guide to developing GDEs with high activity, selectivity, and stability, the three important performance metrics in ECR. First, we introduce critical fundamentals of ECR, including the chemical and physical phenomena at the electrodes as well as the electrochemical cell configurations. Next, we discuss recent advances in GDE design, focusing on their structure-performance correlation and fabrication techniques for each component of GDEs. Finally, we discuss the remaining challenges and propose promising research directions for the design of efficient GDEs. This review aims at promoting the development of industrially relevant ECR systems to bring this technology to practical applications.

Abstract: In the past, the phrase ‘environmental allocations of water’ has most often been taken to mean allocation of water to rivers. However, it is now accepted that groundwater-dependent ecosystems are an important feature of Australian landscapes and require an allocation of water to maintain their persistence in the landscape. However, moving from this theoretical realisation to the provision and implementation of a field-based management regime is extremely difficult. The following four fundamental questions are identified as being central to the effective management of groundwater-dependent ecosystems (GDEs): (1) How do we identify GDEs in the field; put another way, which species or species assemblages or habitats are reliant on a supply of groundwater for their persistence in the landscape; (2) what groundwater regime is required to ensure the persistence of a GDE; (3) how can managers of natural resources (principally water and habitats), with limited time, money and other resources, successfully manage GDEs; and (4) what measures of ecosystem function can be monitored to ensure that management is effective? This paper explicitly addresses these questions and provides a step-by-step theoretical and practical framework for providing answers. In particular, this paper provides an introduction to some of the relevant literature and from this, presents a synthesis, presented in the form of a functional methodology for managing groundwater dependent ecosystems.

Abstract: Until the early 1970s, the management of water resources in Australia was predominantly concerned with the assessment, development and harnessing of new water resources for irrigation, urban and industrial, stock and domestic water supply. The consequences of excessive and unsympathetic groundwater abstraction on groundwaterdependent (phreatophytic) vegetation, such as tree decline and mortality, have been observed throughout Australia (Arrowsmith 1996; Hatton and Evans 1998; Clifton and Evans 2001). With increasing demand for water and a changing climate regime, the need to mitigate the environmental impacts of groundwater development is increasing. Current borefield operation in Australia is largely responsive to consumption demand and often in conflict with environmental needs for groundwater, resulting in drought stress and sometimes death of phreatophytic vegetation and other impacts on GDEs. Groundwater resource managers commonly ask how much water can be taken from the aquifer while still maintaining a low level of risk to GDEs. This requires quantified information on the relationship between the health of a GDE and groundwater depth (or other parameter; see Eamus et al. 2006a). Recommendations are generally made by defining the acceptable level to which groundwater can be allowed to fall, while maintaining important environmental values (see Murray et al. 2006). The Council of Australian Governments (COAG) endorsed reforms in 1994 to achieve a sustainable water industry that included allocations for the environment

Abstract: Many rivers are classified as groundwater-dependent ecosystems (GDEs), owing to the contribution of groundwater to their base flow. However, there has been little explicit recognition of the way groundwater influences riverine biota or processes, how degrees of ecological dependency may vary, and the management implications of this dependency. The permeable beds and banks of these GDEs where surface water and groundwater exchange are termed ‘hyporheic zones’. They are often inhabited by invertebrates, with varying reliance on groundwater, although the ecological roles of these invertebrates are little known. Upwelling hyporheic water can promote surface primary productivity, influence sediment microbial activity, and affect organic matter decomposition. In many intermittent streams, variable groundwater inputs alter the duration of flow or water permanence, and the duration and timing of these largely govern the biota and rates of many ecosystem processes (e.g. leaf decomposition). Not only is the physical presence of water important, thermal and chemical conditions arising from groundwater inputs also have direct and indirect effects on riverine biota and rates or types of in-stream processes. Differing degrees of dependency of rivers on groundwater mediate all these influences, and may change over time and in response to human activities. Alteration of groundwater inputs through extraction from riparian wells or changes in local water table have an impact on these GDEs, and some current management plans aim to restrict groundwater extraction from near permeable river channels. However, these are often ‘blanket’ restrictions and the mechanisms of GDE dependency or timing of groundwater requirements are poorly understood, hampering refinement of this management approach. More effective management of these GDEs into the future can result only from a better understanding of the mechanisms of the dependency, how these vary among river types and what in-stream changes might be predicted from alteration of groundwater inputs.