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From soil to sea: the role of groundwater in coastal critical zone processes

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Near coasts, surface water–groundwater interactions control many biogeochemical processes associated with the critical zone, which extends from shallow aquifer to vegetative canopy. For example, submarine groundwater discharge delivers a significant fraction of weathering products such as silica and calcium to the world's oceans. Owing to changing fertilizer and land use practices, submarine groundwater discharge is also responsible for high nitrogen loads that drive eutrophication in marine waters. Submarine groundwater discharge is generally unmonitored due to its heterogeneous and diffuse spatial patterns and complex temporal dynamics. Here, we review the physical processes that drive submarine groundwater discharge at various spatial and temporal scales and highlight examples of interdependent critical zone processes. Like the inland critical zone, the coastal critical zone is undergoing rapid change in the Anthropocene. Disturbances include warming air and sea temperatures, sea‐level rise, increasing storm severity, increasing nutrient and contaminant inputs, and ocean acidification. In a changing world, it is more important than ever to understand complex feedbacks between dynamic surface water‐groundwater interaction, rocks, and life through long‐term monitoring efforts that extend beyond inland rivers to coastal groundwater. WIREs Water 2016, 3:706–726. doi: 10.1002/wat2.1157

Mechanisms of fresh and saline groundwater flow in the intertidal beach zone. Solid black lines represent flow paths (1: saltwater exchange driven by tides and wave setup, 2: density‐driven saltwater exchange). Gray shading represents salinity. Colors indicate key geochemical zones (A: high dissolved oxygen in shallow swash zone, B: high pH in saline pore waters, C: transient zone where high pH, oxygenated saltwater mixes with anoxic, Fe(II) rich freshwater to form Fe(oxy)hydroxides within the Fe curtain.
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Idealized isotherms (dashed lines) and convection patterns (solid lines) in a carbonate platform. Gray zone illustrates approximate region of maximum dolomite precipitation, after Whitaker and Xiao.
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In young permeable volcanic islands, most precipitation (P) infiltrates (I) and transports subsurface weathering products to the coast through submarine groundwater discharge (Q). River flow (R) represents a small component of water and solute fluxes. Weathering along groundwater flow paths produces base cations and clay minerals in conjunction with alkalinity and consumes CO2 and acidity. Example silicate weathering reactions include potassium feldspar (felsic silicate) weathering: (KAlSi3O8(s) + CO2(g) + 1.5H2O → 0.5Al2Si2O5(OH)4(s) + K++ HCO3‐ + 2H4SiO4°), and olivine (mafic silicate) weathering: (Mg2SiO4(s) + 4H2O + 4CO2(g) → 2 Mg2++ H4SiO4° + 4HCO3 ).
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Physical processes that drive coastal surface water‐groundwater exchange and solute transport. 1: Fresh submarine groundwater discharge. 2: Deep, density‐driven circulation of saline groundwater. 3: Shallow exchange of saline groundwater due to tides and swash. 4: Subtidal exchange of saline groundwater due to hydraulic head gradients associated with interactions of currents and waves with the sediment‐water interface. 5: Onshore infiltration of saline water due to storm surge.
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Effect of ocean surges on the critical zone. (a) Photograph of coastal Thailand 3 weeks after the 2004 Indian Ocean tsunami. Saline floodwater is still visible in surface depressions. (b) Schematic of infiltration of saline floodwater into a fresh aquifer and associated changes in chemistry and mobilization of trace metals.
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(a) Salt marsh environments. (b) Salt marshes fix atmospheric carbon. A portion is sequestered through burial, while another portion is decomposed or mineralized and exported as dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) to surrounding estuary and coastal waters via surface water and groundwater flow.
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(a) Exchange of salt water through shallow, microbially active subtidal sediments enhances delivery of oxygen and organic matter (OM), which increases OM mineralization. (b) I. Under calm conditions, the aerobic zone is thin and the carbon mineralization rate per unit area of seafloor is low. II. An increase in wave energy mobilizes sediments, which leads to bedform organization, deepening of the aerobic zone, exposure of labile OM to oxidants, and increased carbon mineralization. III. When calm conditions resume, bedform‐current interaction ceases, the aerobic zone contracts, and carbon mineralization declines.
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