Models test our understanding of processes and can reach beyond the spatial and temporal scales of measurements. Multi-component Reactive Transport Models (RTMs), initially developed more than three decades ago, have been used extensively to explore the interactions of geothermal, hydrologic, geochemical, and geobiological processes in subsurface systems. Driven by extensive data sets now available from intensive measurements efforts, there is a pressing need to couple RTMs with other community models to explore non-linear interactions among the atmosphere, hydrosphere, biosphere, and geosphere. Here we briefly review the history of RTM development, summarize the current state of RTM approaches, and identify new research directions, opportunities, and infrastructure needs to broaden the use of RTMs. In particular, we envision the expanded use of RTMs in advancing process understanding in the Critical Zone, the veneer of the Earth that extends from the top of vegetation to the bottom of groundwater. We argue that, although parsimonious models are essential at larger scales, process-based models are the only way to explore the highly nonlinear coupling that characterizes natural systems. We present seven testable hypotheses that emphasize the unique capabilities of process-based RTMs for (1) elucidating chemical weathering and its physical and biogeochemical drivers; (2) understanding the interactions among roots, microorganisms , carbon, water, and minerals in the rhizosphere; (3) assessing the effects of heterogeneity across spatial and temporal scales; and (4) integrating the vast quantity of novel data, including-omics‖ data (genomics, transcriptomics, proteomics, metabolomics), elemental concentration and speciation data, and isotope data into our understanding of complex earth systems. With strong support from data-driven sciences, we are now in an exciting era where integration of RTM framework into other community models will facilitate process understanding across disciplines and across scales.

Rare earth element (REE) release from weathering of accessory monazite [(REE)PO4] since the last glacial maximum at 18.1 ka was investigated in sediment recovered from an outlet lake within the glaciated Loch Vale watershed, Colorado, USA. Labile REEs in the sediments reveal monazite weathering increased during the Younger Dryas chronozone (YDC) 13.2 to 11.1 ka when alpine glaciers advanced as climate cooled and bedrock comminution increased. Monazite dissolution peaked at approximately the Pleistocene-Holocene transition ~10.5 ka. During the Holocene, REE concentrations decline, reflecting a reduction in monazite weathering. The REE cerium (Ce) may occur as trivalent or tetravalent cations, and the Ce anomaly (Cen/Ce*n) in sediment permits interpretations of paleoredox conditions. The Cen/Ce*n decreases from 18.1 ka to the Pleistocene-Holocene boundary. Initially, oxidizing conditions within the watershed increased because of enlargement of the proglacial area as the ice retreated. At the onset of the YDC, oxidation further escalated in response to enhanced chemical weathering associated with glacial advance. A more stable Holocene climate and landscape resulted in relatively small changes in Cen/Ce*n values. Slightly decreasing Cen/Ce*n values over the last several hundred years are consistent with present-day stream water values and may reflect the onset of present-day hydrobiogeochemical conditions.

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The majority of realignment schemes undertaken in the UK to date have involved breached realignment. However, banked realignment schemes can offer benefits in terms of creating habitats that have greater physical and biological connectivity with the wider estuary. These benefits are illustrated through a case study of a scheme at Welwick in the Humber Estuary, which is one of the few recent managed realignment schemes to involve the wholesale removal of the fronting flood embankment rather than the creation of breaches. In this example, the scheme design was heavily influenced by the requirement to create a compensatory mudflat habitat and, in this regard, the methods used to assess the future evolution of the scheme are described. The paper goes on to discuss the generic design issues relevant to managed realignment schemes at other locations, including the target habitat to be created. It is suggested that the benefits of banked realignment schemes, in terms of delivering more sustainable estuary morphologies, merit increasing consideration by developers in the future.

The modern sediments of Chagvan Bay, Alaska contain zones of platinum enrichment which suggest an external source of platinum-host sediments. Two mathematical sediment tracing techniques, based on vector and multiple regression analyses of geochemical data, are used to determine the provenance and dispersal patterns of the bay deposits. Criteria for the selection of natural sediment tracers are presented, and the application of tracing techniques to the exploration for high latitude marine placers is discussed.