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Metal Transport in the Environment

Final Report Summary - METTRANS (Metal Transport in the Environment)

MetTrans- Metal Transport in the Environment

The MetTrans Network
Understanding the processes that control the transport of metals in the environment is essential for a wide range of fields, including environmental protection and remediation, mineral resources, and climate change. Through recent analytical developments it is possible to use natural variations in metal isotope ratios to significantly advance our understanding of metal transport in the environment. Various physical and chemical processes can alter the ratio between isotopes of an element, and so leave a signature that can be subsequently measured. The variations are very subtle but can be clearly resolved using modern high-precision techniques. Isotope ratios can be used to study different aspects of the natural cycles of metals, from release during mineral weathering, to processes controlling aqueous transport, to sequestration in solid phases. The MetTrans scientific and training network has addressed a range of important societal and industrial applications related to metal migration, including the immobilization of naturally-occurring metals and contaminants, the transport mechanisms of metals within rivers and groundwaters, the impact of submarine groundwater discharge, and the potential response of metal transport to climate change. The research has required interdisciplinary input from chemistry, geology, physics, biology, hydrology, and engineering, as well as close academic-industrial collaboration, with research scientists from both sectors providing the scientific expertise and state-of-the-art analytical techniques.

Research objectives
The network utilized recent analytical advances to understand and quantify a number of fundamental processes that control metal transport in the environment, including the release of naturally-occurring metals from geological materials, the immobilization of metals through adsorption, precipitation, and biological activity, and the influence of aqueous chemistry on metal transport and behavior. The fundamental scientific innovation within the network has been the use of ultra high precision isotopic measurements for established and new isotope methods, coupled with a combination of experimental, theoretical, and field methods and expertise to explore processes controlling metal behaviour. This was applied to a number of environmental problems, where the investigations can benefit from the synergy between parallel approaches of different metals and environments, with many cross-over techniques and insights.

MetTrans Results and Conclusions
The individual projects in MetTrans have achieved a range of scientific accomplishments.
The release of metals in the environment: Three projects determined the release of Li and Mo isotopes, which reflect various weathering processes, from terrains with different topographical, hydrological, and permafrost conditions. This established a baseline across three major Siberian rivers, the Lena, Ob, and Yenisei, that can be used to identify weathering changes as the extent of permafrost responds to climate change. Another project used Fe isotopes to determine how glacial weathering contributes to the supply of Fe to the oceans, and so established a tool for determining how this has changed during past climate changes.
The immobilization of metals through inorganic processes: Experiments were completed on how the records of Ca, Mg, and C isotopes in calcite that are used to reconstruct past conditions can depend upon kinetic factors during precipitation and subsequent interaction with surrounding waters. A study of bedrock groundwaters at the SKB hard rock laboratory determined the distribution and precipitation history of U and modeled the conditions for the release and groundwater transport of the naturally occurring radionuclides Ra and U.
The effects of microbial and biological activity on metal behaviour: A series of experiments established the isotope signature of microbial reduction of Cr, and how variations between different species and under different conditions may be further used to distinguish between different metabolic processes. The first measurements of Mo isotope changes due to biological activity were documented in an Icelandic lake, and so suggesting variations found in sediments may be linked to biological productivity in the overlying lake waters.
The aqueous chemistry of metal transport and behaviour: The Fe phase carriers of trace metals in the Lena River have been characterized using the SEM and synchrotron, more clearly defining the link between Arctic watershed processes and delivery of micronutrients to the Arctic Ocean. Elsewhere, gas-water-rock interaction times leading to the formation of metal plumes in CO2-bearing subsurface waters were determined, and the effects of carbonate species on metal solubility provides a basis for using metal concentrations to monitor carbon sequestration sites.
The transport of metals in distinct settings. The flux of metals from a mining district of metal-contaminated soils that are transported through the subsurface and discharged through submarine groundwater discharge into the Mediterranean Sea was quantified using Ra and Sr isotopes, which identified a potentially significant route for contaminants to the coast. Several studies examined processes occurring in estuarine environments, where riverine metal fluxes to marine environments can be strongly modified. Two estuaries in the northern Baltic were the focus of a study of Mo, Li, and W transport, where particle scavenging and redox cycling of Mn carriers were found to be important. Other studies found that the contrasting behaviours of particulate and colloidal Fe in boreal rivers have a significant effect on P and C cycling, and Fe affects the transport of organic matter in glacial rivers; these observations have implications for the transport of metals and nutrients from wider Arctic regions.
Method development: A series of methods for measuring the concentrations and isotope compositions of Cd, Os, and Cr in various environmental matrices were developed, along with a method for rapid determination of a wide range of metals in plant material for bio-monitoring. A method for using the concentration levels of certain metals as an early detection tool for low-intensity leakage from subsurface CO2 storage using DGTs (diffusive gradients in thin films) was also developed for sensitive monitoring of storage facilities.

MetTrans Training
The network provided an excellent platform to train young scientists in advanced scientific research that is widely applicable in academia, environmental protection, and industry. The Network focused on providing training on analytical skills, on understanding fundamental principles, and on modeling, and exposed young scientists to a wide range of opportunities for applying this training in research and industry. While training at host institutions focused on many of the skills that were specific to each individual MetTrans fellow research project, a series of group training events were completed that included workshops at mass spectrometry and radioisotope counting facilities, visits to relevant field sites in Spain and Iceland, a joint sampling trip on the frozen Baltic, tours of a synchrotron facility and nuclear waste storage facility, classroom lectures in background science and basic methods, writing seminars, student presentations, and lectures by visiting speakers.

Impact of MetTrans
MetTrans has generated a cohort of highly trained young scientists that have been attaining positions in academia and industry and, along with the network partners, form a strong intersectoral professional network. The international, intersectoral collaborations between MetTrans partners will also continue, pursuing the research advanced by the network. The network has made significant scientific contributions to environmental research, and the results have been presented to the scientific community by the research fellows through a series of presentations at major international conferences, and all original results will soon be published in the professional literature.
Specific applications are of interest to different communities. The new analytical methods that have been developed are being made available for scientific studies and industrial applications to clients by ALS, while the DGT techniques for identifying CO2 migration through trace metals are of interest for sensitive monitoring of subsurface storage facilities. A regional environmental protection agency is interested in the identification of submarine groundwater discharge of metals in coastal regions using isotope methods. The nuclear waste storage community, and in particular SKB, will utilize understanding of the migration of naturally occurring radionuclides as analogues for contaminant nuclides. Consulting companies such as Amphos find further understanding of metal transport processes essential in environmental assessment and remediation work. Isotopes have also been shown to provide innovative ways to understand bacterial reduction and electron transfer processes that have applications beyond environmental interactions. Overall, the implications for the studies in MetTrans have implications across many sectors.