ReviewThe driving forces of porewater and groundwater flow in permeable coastal sediments: A review
Introduction
Coastal and shelf areas are the main sites of organic matter processing in the global ocean. Studies about shelf sediment biogeochemistry have traditionally focused on muddy sediments, where molecular diffusion is the dominant solute transport process (Viollier et al., 2003, Eyre and Ferguson, 2005). In the last decade or so, however, organic-poor sandy sediments have emerged as sites where biogeochemical reaction rates can be as high as or even higher than organic-rich muddy sediments (Boudreau et al., 2001, Sansone et al., 2008). Sandy sediments cover >40% of coastal and shelf areas worldwide (Riedl et al., 1972) and are characterized by a relatively high-permeability. Depending on compaction, grain size and sorting, shelf sands typically have permeabilities ranging from 10−12 to 10−9 m2 (Wilson et al., 2008). Porewater flows through these high-permeability beds facilitate sediment-water exchange of solutes and particles. Porewater advection transports oxygenated seawater, dissolved substances, fine particles, bacteria, viruses and phytoplankton into sediments, as well as releasing degradation products, refractory particles, and organisms from sediments (Huettel et al., 1996a, Huettel et al., 2007, Patten et al., 2008). As a result, seawater circulation through permeable sands can exert a major control on the biogeochemistry of sediments and overlying waters (Eyre et al., 2008, Anschutz et al., 2009, Santos et al., 2011a).
What causes porewater (or groundwater) flow in permeable sand sediments? The physical forces driving fluid advection affect porewater composition and residence times, and may control the extent and rates of biogeochemical reactions in sediments. Advection in sands may have been an understudied process for decades in part because of a limited and fragmented understanding about its driving mechanisms. The following drivers may occur: (1) terrestrial hydraulic gradients, (2) seasonal changes in the aquifer level on land moving the location of the subterranean estuary, (3) wave setup and tidal pumping, (4) water level differences across permeable barriers, (5) flow- and topography-induced pressure gradients, (6) wave pumping; (7) ripple and other bed form migration, (8) fluid shear, (9) convection induced by density inversions, (10) bioirrigation, (11) gas bubble upwelling, and (12) sediment compaction (Fig. 1). All these mechanisms force flow across the sediment-water interface and thus influence the flux of new and recycled nutrients to seawater, fuelling and maintaining primary production. They can also influence the release and uptake of toxic substances from the seabed.
These different processes can be difficult to quantify in situ and to simulate experimentally. As a result, several publications provide information on the overall rate of porewater transport without discriminating between those individual physical processes. Alternatively, studies tend to focus on a single driving mechanism, neglecting its interactions with other drivers. The complex interplay that occurs among those drivers can result in porewater exchange rates that are highly variable over time scales ranging from seconds to years and spatial scales ranging from mm to km. Hence, new insights into the global biogeochemical significance of sandy sediments depend on a better understanding of the drivers of porewater flow. These drivers strongly affect the transformation, removal, transport and release of biogeochemically-important species.
In this paper, we review the known drivers of porewater and groundwater advective flow, emphasizing processes associated with seawater recirculation in permeable coastal sediments. We define each of the drivers listed above and discuss their controls. We later address how the current approaches used to estimate porewater advection rates capture these drivers. Finally, we make an attempt to generalize the spatial and temporal magnitude of the different drivers across a typical coastal transect. This review does not cover the biogeochemical implications of porewater flow and other solute transport mechanisms such as molecular diffusion and hydrodynamic dispersion.
The 12 mechanisms identified may overlap in time and space, yet are rarely studied in an integrated fashion. Small (cm) and short (minutes) scale advective flow drivers have been studied primarily by biologists with the goal of assessing how porewater exchange affects coastal marine communities and how sands act as biocatalytical filters recycling organic matter. Large (meters) and long (days) scale advective flow drivers (including fresh groundwater flow) have been studied usually from a hydrological perspective with the goal of quantifying how much groundwater enters the ocean. While the small scale processes are often referred to as “porewater exchange”, “seawater recirculation in sediments” or similar, the large-scale processes are often referred to as “submarine groundwater discharge” (SGD). The term SGD has been defined as “any and all flow of water on continental margins from the sea bed to the coastal ocean, with scale lengths of meters to kilometres, regardless of fluid composition or driving force” (Moore, 2010). This definition thus includes both fresh groundwater flow and seawater recirculation through the seabed. The “meters to kilometres” scale in the SGD definition excludes small scale processes that are not necessarily a source of natural tracers commonly used to quantify SGD. We feel that the “SGD” and “porewater advection” scientific communities have evolved independently in the last few decades. Hence, this review aims at bridging gaps between different groups studying related processes from different perspectives.
Section snippets
Terrestrial hydraulic gradients
Fresh groundwater discharge into the coastal ocean is driven by hydraulic gradients in coastal aquifers (Fig. 1.1). It occurs as long as the water table remains above sea level, the coastal aquifer is hydraulically connected to the ocean, and evaporation does not exceed groundwater recharge (Burnett et al., 2003). As sandy beaches are typically unconfined, highly permeable aquifers, seepage of fresh groundwater can occur either as submarine springs or seepage at the beach face (Holliday et al.,
What mechanisms are captured by the approaches used to quantify porewater flow?
Porewater advection in marine sands remains understudied largely because of technical difficulties. While molecular diffusion in muddy sediments has traditionally been modelled by applying Fick’s Laws to porewater profiles or measured using benthic chambers and cores, advection in sands is more complicated to evaluate. There are at least seven approaches that have been used to quantify porewater (or groundwater) flow in permeable sediments: (1) numerical models, (2) seepage meters, (3) natural
Toward a global estimate
How much seawater is filtered by permeable sediments if all the driving mechanisms discussed here are taken into account? We make an attempt to provide a first order, conservative estimate of global porewater advective exchange rates. We first discuss the spatial scales where the different porewater (or groundwater) driving mechanisms are important in order to confine the actual shelf areas that are likely to be affected by each of the drivers (Fig. 2).
Most known advective transport mechanisms
Summary and conclusions
Porewater (or groundwater) flow in permeable coastal sediments can be driven by at least 12 independent processes that occur under different temporal and spatial scales. While there are multiple, overlapping driving forces at work in most locations, previous research tended to focus on a single process with little reference to its relative contribution to the total porewater exchange rates. This has caused confusion (i.e., different units, different terms used for the same physical process)
Acknowledgements
Bill Burnett introduced the topic to IRS. IRS and BDE acknowledge the support of ARC grants (DP110103638 and LP100200732) during the preparation of this manuscript. MH acknowledges the support of NSF grants (OCE-424967 and OCE-726754).
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