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Posts Tagged ‘April 2019’

The Art of Reactive Transport Model Building

Much like artists use their skills of abstraction, simplification, and idealization to capture the essence of a landscape, the articles in this issue of Elements on reactive transport modeling describe how theories and assumptions about the subsurface world can be translated into constructs of a mathematical world. Such theories may encompass the actions of micro- and macroorganisms, solute and gas transport, the speciation of both solid phases and surfaces, and their myriad interactions. The resulting mathematical world, built up over decades, is then distilled and interrogated numerically using computer models made up of advanced algorithms that tenaciously step through time and space.

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Reactive Transport Modeling of Microbial Dynamics

Reactive transport modeling of microbially mediated processes has contributed significantly to an improved understanding of elemental cycling in Earth’s near-surface environments. We describe key characteristics of microbial reactive transport models, recent advances in modeling approaches, and the application of such models to terrestrial and marine environmental problems. We introduce relevant case studies and discuss ways to integrate omics data (e.g., genomics, proteomics, metabolomics) that can inform and validate microbial reactive transport models, thereby improving our ability to address some of the grand challenges in a changing world.

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Reactive Transport of Stable Isotopes

Isotopes have a rich history as tracers of biogeochemical processes, but they are commonly interpreted using distillation models that lump multiple compounding effects, including advection, diffusion, and complex chemical transformations. Today, as our ability to measure small differences in relative mass continues to improve, a new generation of process-based models are being developed that explicitly track individual isotopes across an increasingly diverse range of environments. Advances in isotopic reactive transport models are now yielding new insight into fundamental questions across the Earth sciences, including the relationships between experiments and natural systems and the conditions under which isotopes record past environments.

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Reactive Transport Models of Weathering

Continental rocks chemically weather when surficial waters and gases interact with the minerals and organisms that inhabit Earth’s critical zone. To understand and quantify this process, researchers use reactive transport models to track the kinetics and thermodynamics of weathering reactions and the transport of products and reactants. These models are powerful tools to explore how weathering sculpts the Earth’s surface from the scale of mineral grains to watersheds, and across temporal scales from seconds to millions of years. Reactive transport model simulations are now a vital tool for elucidating the complex links between climate, rock ­weathering, and biota.

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Reactive Transport Modeling: A Key Performance Assessment Tool for the Geologic Disposal of Nuclear Waste

The disposal of spent nuclear fuel and high-level radioactive waste in the subsurface represents one of the greatest challenges for the geosciences. Most disposal strategies rely on a multiple barrier system, consisting of both natural and engineered materials, to prevent or delay the contact of groundwater with the waste and radionuclide release to the environment. Reactive transport models have been central to understanding and assessing how thermal, hydrological, and geochemical processes are coupled in these containment barriers, which are expected to experience a range of temperatures and geochemical conditions, yet, must maintain their integrity for millions of years.

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The Role of Reactive Transport Modeling in Geologic Carbon Storage

The engineered storage of CO2 in Earth’s subsurface provides one of the most promising means of reducing net greenhouse gas emissions. Paramount to the success of this method is ensuring that CO2 injected into the subsurface is securely stored. Reactive transport models can be used to answer the key question regarding CO2 storage, “Will the injected CO2 be secure, and over what timescale?” Here, we explore examples of how reactive transport models have been used to simulate the range of geochemical and hydrologic processes that will take place over thousands of years and across many spatial scales to answer that key question.

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Using Reactive Transport Models to Quantify and Predict Groundwater Quality

The hydrochemical composition of most groundwater systems, whether pristine or affected by anthropogenic activities, evolves as a result of complex interactions between flow, solute transport and biogeochemical processes. An in-depth analysis of these processes and their interactions is essential for deciphering what controls groundwater quality. Reactive transport modeling has emerged as an invaluable tool for distilling complex systems into their salient components. Based on experimental data, reactive transport models have been successfully used in the rigorous, process-based quantification of coupled processes at bench and field scales. We illustrate how reactive transport modeling can aid in identifying and quantifying controls over groundwater quality.

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Book Review — Earth’s Oldest Rocks (2nd Ed)

The Earth’s oldest rocks are those which formed in the time interval 3.0–4.0 billion years ago in the mid- to early Archaean Eon. Traces of anything even earlier, which would be from the Hadean (>4.0 billion years ago), are fragmental and preserved only in detrital zircon grains and in the isotopic memory of now long-extinct isotope systems. The time interval 3.0–4.0 billion years ago is a crucial stage in Earth history, for this is when the first continents formed, when life began, and was a time during which tectonic processes were quite different from modern (Phanerozoic) plate tectonics due to the different thermal state of the young Earth.

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Book Review — Encyclopedia of Geochemistry

The field of geochemistry has grown rapidly over the past few decades, driven by significant advances in new analytical techniques, theoretical calculations, laboratory experiments, and the development of geochemical databases. This impressive growth has been further accelerated by the urgent needs of almost all the Earth sciences that use geochemistry to find resources, mitigate environmental impacts, and decipher physico-chemical processes in the Earth and the solar system. The massive two-volume Encyclopedia of Geochemistry: A Comprehensive Reference Source on the Chemistry of the Earth, edited by William M. White, is, thus, very timely and highly relevant. It represents a comprehensive update on the 1999 version, which was edited by Clare P. Marshall and Rhodes W. Fairbridge.

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Nooks and Crannies — Mountains and Clouds

I must explain the extension to my title. I wrote a Parting Shot with the title ‘Nooks and Crannies’ for Elements, 8 (2), 2012, in an issue on ‘Minerals, Microbes and Remediation’, devoted to nooks and crannies on weathered alkali feldspar surfaces. For international readers, I explained that (according to the Concise Oxford English Dictionary) a ‘nook’ is ‘a corner or recess; a secluded place’ and a ‘cranny’ is ‘a chink, a crevice, a crack’. The reactivity of complex mineral surfaces is a complicating factor in ‘reactive transport’, particularly near Earth’s surface. The present issue provides me with an opportunity to dust-off a few old micrographs, images of considerable beauty in themselves, to remind readers that minerals are not just chemical compounds. It also allows me to introduce an intriguing discovery of the last five years. Some, but not all, feldspars are extremely effective at nucleating ice in clouds, and this may be related to nooks and crannies, not just to chemistry.

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