Abstract
Subterranean estuaries the, subsurface mixing zones of terrestrial groundwater and seawater, substantially influence solute fluxes to the oceans. Solutes brought by groundwater from land and solutes brought from the sea can undergo biogeochemical reactions. These are often mediated by microbes and controlled by reactions with coastal sediments, and determine the composition of fluids discharging from STEs (i.e., submarine groundwater discharge), which may have consequences showing in coastal ecosystems. While at the local scale (meters), processes have been intensively studied, the impact of subterranean estuary processes on solute fluxes to the coastal ocean remains poorly constrained at the regional scale (kilometers). In the present communication, we review the processes that occur in STEs, focusing mainly on fluid flow and biogeochemical transformations of nitrogen, phosphorus, carbon, sulfur and trace metals. We highlight the spatio-temporal dynamics and measurable manifestations of those processes. The objective of this contribution is to provide a perspective on how tracer studies, geophysical methods, remote sensing and hydrogeological modeling could exploit such manifestations to estimate the regional-scale impact of processes in STEs on solute fluxes to the coastal ocean.
Highlights
Along global coastlines, meteoric groundwater discharges into the ocean (Church, 1996; Taniguchi et al, 2002; Zhou et al, 2019; Luijendijk et al, 2020)
A drawback of remote sensing methods is that it is challenging to link discrete, in-situ measurements to remote sensing data that integrate a multitude of effects other than those caused by an subterranean estuary (STE) presence
This review of biogeochemical processes in subterranean estuaries highlighted the spatial and temporal variability of these systems. Their reactions and corresponding rates depend on the hydrology of the STEs and the chemical composition of sediments and source waters
Summary
Meteoric groundwater discharges into the ocean (Church, 1996; Taniguchi et al, 2002; Zhou et al, 2019; Luijendijk et al, 2020). The N transformation processes are controlled mainly by groundwater residence time (Santos et al, 2008; Gonneea and Charette, 2014), redox condition (Slomp and Van Cappellen, 2004), availability of electron donors (Santoro, 2010), and mixing rate of freshwater and saltwater (Kroeger and Charette, 2008) They have been detected and measured by geochemistry or molecular biology approaches. While at the local scale geochemistry or molecular biology assessment methods exist, regional scale tracers for these processes have yet to be identified They could be indirectly inferred from STE physical properties (e.g., organic matter content and quality in host sediment, residence time, redox conditions) that control reaction rates and types. Research in this direction is emerging (Lecher and Mackey, 2018), but additional knowledge is required before ecology can be used as a tracer for STE processes
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