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- Title
Subsurface Mixing Dynamics Across the Salt‐Freshwater Interface.
- Authors
De Vriendt, K.; Le Borgne, T.; Pool, M.; Dentz, M.
- Abstract
Mixing along the salt‐freshwater interface is critical for geochemical reactions, transport, and transformation of nutrients and contaminants in coastal ecosystems. However, the mechanisms and controls of mixing are not well understood. We develop an analytical model, based on the coupling between flow deformation and dispersion, which predicts the mixing dynamics along the interface for steady‐state flow in coastal aquifers. The analytical predictions are compared with the results of detailed numerical simulations, which show that nonuniform flow fields, inherent to seawater intrusion in coastal aquifer, result in a non‐monotonic evolution of mixing width and mixing rates along the interface. The analytical model accurately captures these dynamics over a range of freshwater flow rates and dispersivities. It predicts the evolution of the mixing width and mixing rates along the interface, offering a new framework for understanding and modeling mixing and reaction processes in coastal aquifers. Plain Language Summary: Density differences between salt and freshwater lead to the formation of a convection cell in coastal aquifers, in which seawater intrudes inland along the aquifer bottom. Fresh and mixed waters flow upward along the salt‐freshwater interface and are forced to accelerate before being discharged along the ocean seabed. The resulting nonuniform flow alters the concentration of the mixed waters along the interface, which in turn enhances mixing rates and creates local mixing hotspots. Our results show how nonuniform velocity fields result in enhanced local mixing dynamics and elucidate the mechanisms and controls of mixing processes along salt‐freshwater interfaces in coastal aquifers. Key Points: The width of the saltwater‐freshwater interface and mixing rate varies along the interfaceInterface width and mixing dynamics are related through flow deformationWe derive an analytical model that predicts the mixing width and rate based on the interface profile and transverse dispersivity
- Subjects
SALTWATER encroachment; STEADY-state flow; FRESH water; AQUIFERS; INTERFACE dynamics; NON-uniform flows (Fluid dynamics); OCEAN bottom
- Publication
Geophysical Research Letters, 2022, Vol 49, Issue 7, p1
- ISSN
0094-8276
- Publication type
Article
- DOI
10.1029/2021GL097094