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Geochemical fluxes in sandy beach aquifers: Modulation due to major physical stressors, geologic heterogeneity, and nearshore morphology
Abstract
Coastal beach aquifers are biogeochemically active systems that mediate chemical and material fluxes across the land-sea interface. This paper provides a review of major physical stressors and geologic features that influence flow and solute fate and transport in coastal beach aquifers. We outline current understanding of the interactions between these factors and their associated impacts on water and geochemical fluxes within and across these aquifers. The physical processes that control flow, transport, and the formation and distribution of reactive zones in beach aquifers (e.g., tides, waves, density gradients, precipitation, episodic ocean events, and evaporation) operate across overlapping temporal and spatial scales, and present challenges for measuring and modeling physical flow and biogeochemical processes in coastal groundwater systems. Geologic heterogeneity introduces further complexity by modifying flowpaths, mixing patterns, and rates of biotransformation. Interactions between these physical stressors and geological controls are likely to evolve with changes in sea level, climate variability, human settlement, coastal erosion, and other natural and anthropogenic stresses, providing avenues for scientific exploration into the future role of beach aquifers as chemical mediators between the land and ocean.
... The statistical values of seawater fraction and ionic changes for PW and GW. (Geng. X et al., 2021). Comparatively, fewer samples (11.43%) from zone 3 represent saline water might be due to chloride-enriched sea spray recharging shallow groundwater (Klostowska et al., 2020) and due to mixing between freshwater and seawater due to excess removal from the aquifer resulting in seawater up coning (Vengosh and Rosenthal, 1994). The PW samp ...
... he aquifer resulting in seawater up coning (Vengosh and Rosenthal, 1994). The PW samples exhibit a signature of seawater with higher EC, TDS, and salinity, which might be due to the impact of seawater flushing driven by near-shore hydraulic and pressure gradients due to wave action, tidal pumping, and fluctuating terrestrial water table conditions (Geng. X et al., 2021). The GW samples signify heterogeneity in physicochemical parameters spatially with higher EC, TDS, and salinity along zone 3 (the southern part of the study region), suggesting the possibility of SWI. In contrast, lower EC, TDS, and salinity in zone 1 (northern part) infer freshwater discharge, and intermediate physicochemical constitue ...
Submarine groundwater discharge (SGD) and Saline water intrusion (SWI) are the two major processes that influence coastal aquifers resulting in severe water stress. It is essential to characterize groundwater dynamics and discriminate geochemical characterization to understand both approaches. The present study investigates the zones influenced by SGD and SWI along the coastal aquifers of Tamil Nadu and Pondicherry. Pore water (PW) and groundwater (GW) samples were collected during the monsoon season at low tide and analyzed for major ions adopting standard procedures. Hydrogeochemical characterization of water samples revealed by the piper, ionic ratio, ionic delta (〖∆m〗_i), and seawater fraction (f_Sea) plots suggest that the PW samples, irrespective of location and GW samples in specific areas, were attributed to SWI, while the remaining samples suggested SGD. The average chloride-attributed SGD flux calculated for PW was 23.45×10-7 L cm-2 S-1 and for GW, 0.58×10-7 L cm-S-1. Higher fluxes observed in PW suggest seawater recirculation as the primary mechanism, and GW samples were found to be influenced by fresh, recirculated, and saline intrusions. Overall, the northern parts of the study region were influenced by freshwater discharge. However, the central and southern parts of the study regions were influenced by mixed RSGD and SWI water types. The present work suggests locations influenced by fresh, recirculated, and saline water zones that can benefit the stakeholders in planning strategies to identify proper aquifer recharge and suggest ideal pumping scenarios to sustain groundwater resources.
... in SGD. In addition, geophysical techniques (e.g., measuring electrical conductivity, acoustic impedance, or temperature) could be an efficient tool to identify relatively large-scale salinity distributions and locations of fresh SGD [Taniguchi et al., 2019;Geng et al., 2021;Moosdorf et al., 2021]. ...
... Acknowledgements This work was funded by the U.S. National Science Foundation EAR1151733 to H.M. Modeling files are available from the CUAHSI HydroShare database (Geng & Michael, 2021, https://doi.org/10.4211/hs.1300844ad3c647f3ab9c6c221f8747ae). ...
Studies of coastal groundwater dynamics often assume two‐dimensional (2D) flow and transport along a shore‐perpendicular cross‐section. We show that along‐shore movement of groundwater may also be significant in heterogeneous coastal aquifers. Simulations of groundwater flow and salt transport incorporating different geologic structure show highly three‐dimensional (3D) preferential flow paths. The along‐shore movement of groundwater on average accounts for 40%–50% of the total flowpath length in both conduit‐type (e.g., volcanic) heterogeneous aquifers and statistically equivalent (e.g., deltaic) systems generated with sequential indicator simulation (SIS). Our results identify a critical role of three‐dimensionality in systems with connected high‐permeability geological features. 3D conduit features connecting land and sea cause more terrestrial groundwater flow through the inland boundary and intensify water exchange along the land‐sea interface. Therefore, conduits increase the rate of SGD compared to equivalent homogeneous, SIS and corresponding 2D models. In contrast, in SIS‐type systems, less‐connected high‐permeability features produce mixing zones and SGD nearer to shore, with comparable rates in 3D and 2D models. Onshore, 3D heterogeneous cases have longer flowpaths and travel times from recharge to discharge compared to 2D cases, but offshore travel times are much shorter, particularly for conduit‐type models in which flow is highly preferential. Flowpath lengths and travel times are also highly variable in 3D relative to 2D for all heterogeneous simulations. The results have implications for water resources management, biogeochemical reactions within coastal aquifers, and subsequent chemical fluxes to the ocean.
... Along coastlines, oceanic forces (e.g. waves, tides, and wind) create highly dynamics zones in which seawater and terrestrial groundwater strongly interact [41]. These groundwater-surface water interactions and the associated transport of chemicals and biota across the beach surface promote the formation of biochemically active zones that lead to dynamic biogeochemical conditions for oil biodegradation within beaches [4 ,42]. ...
... Taking into account coastal groundwater flow and transport processes is critical for modeling oil biodegradation within beach systems. A robust groundwater model for characterizing coastal subsurface fate and transport processes needs to incorporate multiple driving mechanisms such as buoyancy forces caused by the density difference between fresh and saline groundwater, terrestrial hydraulic gradients, tides, waves, evaporation and precipitation [41,43] (Figure 1a). For modeling oil biodegradation within beaches, a multi-species reactive transport model would also be essential for considering oil compounds, associated microbial degraders, and relevant chemical species (e.g. ...
Modeling oil biodegradation and remediation has become an increasingly important means to predict oil persistence and explore potential in-situ bioremediation strategies for oil-contaminated beaches. Beaches involve complex mixing dynamics between seawater and groundwater. Thus, numerically predicting oil biodegradation within beach systems faces major challenges in merging highly dynamic biogeochemical conditions into microbial degradation models. In this paper, we reviewed recent advances in modeling oil biodegradation from aspects of oil phases, reaction kinetics, microbial activities, environmental conditions, and beach hydrodynamics. We identified key controlling factors of oil biodegradation, highlighted the importance of fate and transport processes on nearshore oil biodegradation, and suggested some advances needed to achieve for developing a robust numerical model to predict oil biodegradation and bioremediation within beaches.
... The trend of seaward decrease in porewater exchange fluxes from the sandy beach is likely related to the saturation state of the beach at different intertidal positions. The saturation state of the sandy beach is a complex dynamic process due to the impact of tidal and wave pumping and water table (Heiss et al., 2015;Geng et al., 2021). Hence, there is the largest unsaturated zone at the high tide mark, suggesting the largest infiltration volumes of seawater. ...
The intertidal zone is an important pathway for the transport of dissolved carbon across the land-ocean interface. Quantifying carbon transformation and export from the intertidal zone remains large uncertainties due to the natural heterogeneity and various driving mechanisms. Here, we used the 224Ra/228Th disequilibrium method to quantify porewater exchange and dissolved carbon export from intertidal sandy and muddy sediments, two main intertidal wetland types in the coast. Porewater exchange fluxes from sandy sediments ranged from 1.4 ± 0.1 to 1084 ± 53 L m-2 h-1, which gradually increased in the landward direction. Porewater exchange fluxes from mangrove muddy sediments ranged from 0 to 4.4 ± 0.7 L m-2 h-1. Advective porewater exchange was recognized as the dominant solute transport process in sandy sediments, whereas bioturbation may be the key process in mangrove muddy sediments. Furthermore, marine dissolved organic carbon (DOC) was removed in sandy sediments, indicating that the sandy sediment acted as a sink of DOC, likely via aerobic remineralization. DOC consumption fluxes ranged from 3.7 ± 0.2 to 206 ± 9 mmol C m-2 d-1. The total consumption flux of DOC in the entire sandy beach was one order of magnitude higher than the globally maximum DOC removal flux in the upper ocean. Porewater-derived dissolved inorganic carbon (DIC) export fluxes from sandy sediments ranged from 5.6 ± 0.3 to 883 ± 43 mmol C m-2 d-1. Nearly 50% of DIC export originated from the consumption of DOC. The rate constants for DOC transformation ranged from 0.009 to 0.155 h-1. In comparison, porewater-derived DOC and DIC export fluxes from muddy sediments were 25 ± 4 and 206 ± 30 mmol C m-2 d-1, respectively. Porewater-derived DIC export from sandy sediments was comparable or even higher than those from muddy sediments. These results showed that sandy sediments acted as a fast lane for marine DOC remineralization. Overall, both organic-poor sandy and organic-rich mangrove muddy sediments play important roles in the delivery of dissolved carbon to the ocean.
... Drastic changes to SGD rates and associated solute fluxes can impact nearshore aquaculture and coastal biogeochemistry (e.g., Hajati et al., 2019;Ray & Fulweiler, 2020;Wang, Liu, et al., 2021;Wang, Su, et al., 2021;-especially because of the reliance of aquaculture on heavy application of artificial feeds (Cao et al., 2015). These high concentrations mean that hotspots of lateral carbon and nitrogen fluxes may be generated by SGD originating from shrimp farms, even though SGD volumes from aquaculture are generally lower than from sandy beaches (Geng et al., 2021;Taniguchi et al., 2019).The diverse feeding activities (Darodes de Tailly et al., 2021) are the primary control of nutrient loadings and carbon cycling, especially as aquaculture footprints are increasing. Because of this intensive carbon and nitrogen loading, aquaculture systems have the potential to become major anthropogenic sources of CH 4 and N 2 O emissions (Kosten et al., 2020;Yuan et al., 2019). ...
Shrimp aquaculture has expanded rapidly in coastal zones worldwide over the past few decades. Saline water stored in shrimp farm ponds can infiltrate into the underlying aquifer causing groundwater salinization and increased submarine groundwater discharge (SGD) to coastal water. However, little research has assessed salinization resulting from these shrimp ponds. To understand the impacts of shrimp farm irrigation on groundwater salinization and SGD, we numerically simulated a series of aquaculture management scenarios in a two-dimensional conceptual coastal aquifer using a coupled surface-subsurface approach. We characterized sensitivities to pond water salinity, pond water depth, and farm width. Salinization was assessed by three indicators (salinized area, infiltrated salt mass, and recovery rate), and three SGD indicators were evaluated (fresh SGD, saline SGD, and saltwater circulation rate). Our results show that pond water depth is the primary control on the mass of saltwater infiltration while farm width is the primary control for recovery rate. Pond water salinity and depth affect both fresh and saline SGD. We show that aquaculture is a previously unrecognized mechanism of salinization affecting coastal aquifer vulnerability and SGD. A regional GIS analysis shows transformation into aquacultural ponds could introduce considerable SGD variability spatially and temporally. These findings will enable coastal managers to better evaluate groundwater vulnerability in regions with expanding onshore aquaculture and demonstrates the impact of aquaculture on coastal groundwater resources and the need for further study to understand the impact of aquaculture across Asia and the globe.
... Coastal zones have been recognized to be increasingly vulnerable to contamination due to dense human populations and anthropogenic activities (Michael et al., 2017). The groundwater-seawater exchange is one of the most important hydrological processes of land/sea interactions in coastal zones, including submarine groundwater discharge (SGD) and seawater intrusion (Geng et al., 2021). SGD transports fresh groundwater from the land to the ocean, which is an important source of nutrients, heavy metals, and carbon and organic pollutants in coastal waters. ...
This note describes the development and testing of a novel, programmable reversing flow 1D (R1D) experimental column apparatus designed to investigate reaction, sorption, and transport of solutes in aquifers within dynamic reversing flow zones where waters with different chemistries mix. The motivation for constructing this apparatus was to understand the roles of mixing and reaction on arsenic discharging through a tidally fluctuating riverbank. The apparatus can simulate complex transient flux schedules similar to natural flow regimes The apparatus uses an Arduino microcontroller to control flux magnitude through two peristaltic pumps. Solenoid valves control flow direction from two separate reservoirs. In‐line probes continually measure effluent electrical conductance, pH, oxidation‐reduction potential, and temperature. To understand how sensitive physical solute transport is to deviations from the real hydrograph of the tidally fluctuating river, two experiments were performed using: 1) a simpler constant magnitude, reversing flux direction schedule (RCF); and 2) a more environmentally relevant variable magnitude, reversing flux direction schedule (RVF). Wherein, flux magnitude was ramped up and down according to a sine wave. Modeled breakthrough curves of chloride yielded nearly identical dispersivities under both flow regimes. For the RVF experiment, Peclet numbers captured the transition between diffusion and dispersion dominated transport in the intertidal interval. Therefore, the apparatus accurately simulated conservative, environmentally relevant mixing under transient, variable flux flow regimes. Accurately generating variable flux reversing flow regimes is important to simulate the interaction between flow velocity and chemical reactions where Brownian diffusion of solutes to solid‐phase reactions sites is kinetically limited.
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Submarine groundwater discharge (SGD) plays a crucial role in nutrient dynamics and eutrophication status of the typical estuarine ecosystems, which are hotspots for groundwater-borne nutrient and are sensitive to aquaculture activities. To evaluate the significant role of SGD in regulating nutrient dynamics in an aquaculture estuary, a radium mass balance model combined biological feeding experiment was carried out in the present study. The results demonstrated that SGD fluxes were estimated to be 15.9 ± 9.41 cm d-1, 18.1 ± 8.51 cm d-1, and 23.0 ± 13.7 cm d-1 during July 2019, October 2019 and April 2021, and the SGD-driven dissolved inorganic/organic nutrient fluxes were 0.6-3.1-fold, 0.2-0.9-fold and 0.4-29-fold higher than those of riverine input, respectively. Seasonal variabilities of SGD rates indicated that saline SGD is dominated and is primarily modified by the oceanic forcing stimulated by tidal and wave dynamics. The contrasting conditions between bottom-up (groundwater- and river-derived nutrient fluxes) and top-down (nutrient responses in estuarine waters), indicate the significance of seasonal differences in the biochemical mechanisms and aquaculture effects of modifying nitrogen dynamics. Dissimilatory nitrate reduction to ammonium and nitrification were responsible for the contrasting NOx (NO2- and NO3-) and NH4 conditions in July and October, respectively, and these factors jointly regulated NOx and NH4 in April. Dissolved organic nitrogen (DON) was the predominant component among the three seasons, except for DON degeneration in October, and it increased due to NH4 assimilation by the phytoplankton community. These findings indicated that biochemical transformation has potential ramifications for the dynamics of SGD-driven nutrients and the management in marine aquaculture ecosystems.
Beach recovery describes the processes by which there is a natural restoration of beach material and coastal morphology following storm events, and these processes are common across the globe. However, the effects of beach recovery on salinity distribution and solute transport in unconfined coastal aquifers are poorly understood. This study examined the changes in salinity distribution in tidally influenced aquifers in response to beach recovery, based on numerical modeling. The extent and location of the upper saline plume and saltwater wedge were found to vary with the beach recovery. The variations in salinity distribution directly changed the particle travel times in the aquifers. Compared with the erosion profile after the storm (storm profile), an increase of up to 743% of the particle travel time in the intertidal zone was observed when the beach recovered to a berm (silting) profile. The berm profile increased the residence time and peak concentration of the land-sourced solute plume in the beach aquifer compared with the storm profile. The berm profile also enhanced the aquifer–ocean mass exchange, resulting in increased intertidal saltwater infiltration and submarine groundwater discharge. On the other hand, the storm profile can generate much higher solute efflux than the berm profile. The storm profile is more favorable in diluting the land-sourced conservative solute and shortening its residence time in an aquifer.
Barrier islands in tropical regions are prone to coastal flooding and erosion during hurricane events. The Yucatán coast, characterized by karstic geology and the presence of barrier islands, was impacted by Hurricane Gamma and Hurricane Delta in October 2020. Inner shelf, coastal, and inland observations were acquired simultaneously near a coastal community (Sisal, Yucatán) located within 150 km of the hurricanes' tracks. In the study area, Gamma moved slowly and induced heavy rain, mixing in the shelf sea, and strong winds (> 20 m s −1). Similar wind and wave conditions were observed during the passage of Hurricane Delta; however, a higher storm surge was measured due to wind setup and the drop (< 1000 mbar) in atmospheric pressure. Beach morphology changes, based on GPS measurements conducted before and after the passage of the storms, show alongshore gradients ascribed to the presence of coastal structures and macrophyte wracks on the beach face. Urban flooding occurred mainly on the back barrier associated with heavy inland rain and the coastal aquifer's confinement , preventing rapid infiltration. Two different model-ing systems, aimed at providing coastal flooding early warning and coastal hazard assessment, presented difficulties in forecasting the coastal hydrodynamic response during these seaward-traveling events, regardless of the grid resolution, which might be ascribed to a lack of terrestrial processes and uncertainties in the bathymetry and boundary conditions. Compound flooding plays an important role in this region and must be incorporated in future modeling efforts.
Barrier islands in tropical regions are prone to coastal flooding and erosion during hurricane events. The Yucatan coast, characterized by karstic geology and the presence of barrier islands, was impacted by Hurricanes Gamma and Delta in October 2020. Inner shelf, coastal, and inland observations were acquired simultaneously near a coastal community (Sisal, Yucatan) located within 150 km of the hurricanes’ tracks. In the study area, Gamma moved at a slow speed and induced heavy rain, mixing in the shelf sea, and northern winds exceeding 20 m s-1. Similar wind and wave conditions were observed at this location during the passage of Hurricane Delta. However, a higher storm surge (0.5 m) was measured due to wind setup and the drop (
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.
Intrusion of saltwater into freshwater coastal aquifers poisons an essential resource. Such intrusions are occurring along coastlines worldwide due largely to the over-pumping of freshwater and sea level rise. Saltwater intrusion impacts drinking water, agriculture and industry, and causes profound changes in the biogeochemistry of the affected aquifers, the dynamic systems called subterranean estuaries. Subterranean estuaries receive freshwater from land and saltwater from the ocean and expose this fluid mixture to intense biogeochemical dynamics as it interacts with the aquifer and aquiclude solids. Increased saltwater intrusion alters the ionic strength and oxidative capacity of these systems, resulting in elevated concentrations of certain chemical species in the groundwater, which flows from subterranean estuaries into the ocean as submarine groundwater discharge (SGD). These highly altered fluids are enriched in nutrients, carbon, trace gases, sulfide, metals, and radionuclides. Seawater intrusion expands the subterranean estuary. Climate change amplifies sea level variations on short and seasonal time scales. These changes may result in higher SGD fluxes, further accelerating release of nutrients and thus promoting biological productivity in nutrient-depleted waters. But this process may also adversely affect the environment and alter the local ecology. Research on saltwater intrusion and SGD has largely been undertaken by different groups. We demonstrate that these two processes are linked in ways that neither group has articulated effectively to date.
Sandy beach aquifers are complex hydrological and biogeochemical systems where fresh groundwater and seawater mix. The extent of the intertidal mixing zone and the rates of circulating flows within beaches are a primary control on porewater chemistry and microbiology of the intertidal subsurface. Interplay between the hydrological and biogeochemical processes at these land-sea transition zones moderate fluxes of chemicals, particulates, heavy metals, and biota across the aquifer-ocean interface, affecting coastal water quality and nutrient loads to marine ecosystems. Thus, it is important to characterize hydrological and biogeochemical processes in beach aquifers when estimating material fluxes to the ocean. This can be achieved through a suite of cross-disciplinary measurements of beach groundwater flow and chemistry. In this review, we present measurement approaches that have been developed and employed to characterize the physical (geology, topography, subsurface hydrology) and biogeochemical (solute and particulate distributions, reaction rates) properties of and processes occurring within sandy intertidal aquifers. As applied to beach systems, we discuss vibracoring, sample collection, laboratory experiments, variable-density considerations, instrument construction, and sensor technologies. We discuss advantages and limitations of typical hydrologic field sampling methods when used to investigate beach aquifers and provide a measurement framework for researchers seeking to sample and collect data from these systems.
Advective flows of seawater and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary (STE). Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon (OC) from terrestrial sources. Along the groundwater flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to disentangle, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from freshwater lens groundwater, seawater, and beach porewater samples collected over different seasons. The samples were analyzed for physico-chemistry (e.g., salinity, temperature, dissolved silicate), (reduced) electron acceptors (e.g., oxygen, nitrate, and iron), and dissolved organic carbon (DOC). DOM was isolated and molecularly characterized via soft-ionization ultra-high-resolution mass spectrometry, and molecular formulae were identified in each sample. We found that the islands’ freshwater lens harbors a surprisingly high DOM molecular diversity and heterogeneity, possibly due to patchy distributions of buried peat lenses. Furthermore, a comparison of DOM composition of the endmembers indicated that the Spiekeroog high-energy beach STE conveys chemically modified, terrestrial DOM from the inland freshwater lens to the coastal ocean. In the beach intertidal zone, porewater DOC concentrations, lability of DOM and oxygen concentrations, decreased while dissolved (reduced) iron and dissolved silicate concentrations increased. This observation is consistent with the assumption of a continuous degradation of labile DOM along a cross-shore gradient, even in this dynamic environment. Accordingly, molecular properties of DOM indicated enhanced degradation, and “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial consumption. Our data indicate that the high-energy beach STE is likely a net sink of OC from the terrestrial and marine realm, and that barrier islands such as Spiekeroog may act as efficient “digestors” of organic matter.
Accurate SGD (submarine groundwater discharge) mass export calculations require detailed knowledge of the spatial and temporal variability in SGD rates. In coastal aquifers, SGD includes a terrestrial freshwater component as well as a saline component originating from circulating seawater. Representative field measurements of SGD rates are difficult to conduct, because SGD is often patchy, diffuse, and temporally variable, especially under tidal influence and high wave activity. In this study, a combination of lysimeters, seepage meters, temperature sensors, pore water radon, and numerical modeling was used to estimate the volumes of infiltrating seawater and exfiltrating groundwater in the intertidal zone of a mesotidal, high energy beach on Spiekeroog Island, northern Germany. Additionally, a 3D-laser scanner was used over short (days) and medium time scales (months) to determine changes in beach topography. The results showed net water infiltration above mean sea level (MSL) and net exfiltration below MSL. Water exchange rates fluctuated between 0.001 and 0.61 m day−1, showing similar ranges within the multiple method approaches. The beach topography was subject to strong fluctuation caused by waves, currents, wind driven erosion and sedimentation, even over short time scales. A comparison of extrapolated in- and exfiltrating water volumes along a beach transect from the mean high water to mean low water line at different times highlights the variability of total in or outflow. The results show that exchange rates depend on beach topography, which in turn changes significantly over time.
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Abstract:
Beach nourishment — the addition of sand to increase the width or sand volume of the beach — is a widespread coastal management technique to counteract coastal erosion. Globally, rising sea levels, storms and diminishing sand supplies threaten beaches and the recreational, ecosystem, groundwater and flood protection services they provide. Consequently, beach nourishment practices have evolved from focusing on maximizing the time sand stays on the beach to also encompassing human safety and water recreation, groundwater dynamics and ecosystem impacts. In this Perspective, we present a multidisciplinary overview of beach nourishment, discussing physical aspects of beach nourishment alongside ecological and socio-economic impacts. The future of beach nourishment practices will vary depending on local vulnerability, sand availability, financial resources, government regulations and efficiencies, and societal perceptions of environmental risk, recreational uses, ecological conservation and social justice. We recommend co-located, multidisciplinary research studies on the combined impacts of nourishments, and explorations of various designs to guide these globally diverse nourishment practices.
The intertidal zone of beach aquifers hosts biogeochemical transformations of terrestrially derived nutrients that are mediated by reactive organic carbon from seawater infiltration. While dissolved organic carbon is often assumed the sole reactive organic carbon component, advected and entrapped particulate organic carbon (POC) is also capable of supporting chemical reactions. Retarded advection of POC relative to groundwater flow forms pools of reactive carbon within beach sediments that support biogeochemical reactions as dissolved solutes move across them due to transient groundwater flow. In this work, we simulate the contribution of POC to beach reactions and identify parameters that control its relative contribution using a groundwater flow model (SEAWAT) and reactive transport model (PHT3D). Results show transient contributions of POC to denitrification, as the spatial extent of the saline circulation cell varies over time due to changing hydrologic factors. A decrease in POC retardation and an increase in tidal amplitude during POC deposition resulted in POC expansion, which increased the relative contributions of POC to beach reactivity. Decreased hydraulic conductivity and increased tidal amplitude post‐deposition decreased the utilization of POC for denitrification by allowing the oxic, saline water to completely encompass the pool of POC. Results highlight that POC is an intermittently utilized source of carbon that displays complex spatial relationships with groundwater flow conditions and overall beach biogeochemistry. This work demonstrates that POC may be a periodically important but overlooked contributor to biogeochemical reactions in carbon‐poor beach aquifers.
A density‐dependent, variably saturated groundwater flow and solute transport model was used to investigate the influence of swash motions on subsurface flow and moisture dynamics in beach aquifers with heterogeneous distributions of hydraulic conductivity (K) and capillarity. The numerical simulations were performed within a Monte Carlo framework using field measurements conducted in the swash zone of a sandy beach. Our results show that heterogeneous capillarity causes spatially variable capillary rise above the groundwater table. In response to swash motions, heterogeneity creates capillary barriers that result in pockets of elevated moisture content beneath the swash zone. These moisture hotspots persist within the unsaturated zone even at ebb tide when the swash motions recede seaward. Heterogeneous capillarity also results in highly tortuous preferential flow paths and alters the flow rates from the sand surface to the water table. Heterogeneous K greatly enhances the seawater infiltration into the swash zone and modulates its spatial distribution along the beach surface. Due to heterogeneous K and capillarity, complex mixing patterns emerge. Both strain‐dominated and vorticity‐dominated flow regions develop and dissipate as tides and waves move across the beach surface. Complex mixing patterns of seawater percolating from the swash zone surface to the water table, with localized areas of high and low mixing intensities, are further demonstrated by analysis of dilution index. Our findings reveal the influence of geologic heterogeneity on swash zone moisture and flow dynamics, which may have important implications for sediment transport and chemical processing in beach aquifers.
Sandy beaches have received increasing attention due to their global ubiquity and ecological services they provide. Previous investigations on their microbial diversity mostly targeted low energy beaches, lacked seasonality or geochemical information. We now have used a multidisciplinary approach to study the microbial diversity within sediments of a sandy, high energy beach that additionally exhibits submarine groundwater discharge (SGD) in the intertidal. We sampled two transects at three seasons down to a depth of one meter for 16S rRNA gene amplicon sequencing and subsequent correlation of microbial diversity to physico-chemical parameters. We found that advection driven transport of porewater constituents and constant physical reworking of the sediments prevented a distinct formation of vertically stratified redox zones. Consequently, a uniform microbial core community of generalists established independently of sampling site and depth. During spring, the community structure was disturbed by a "subsurface bloom" of Bacteroidetes and Firmicutes, probably due to deeper oxygen and nitrate penetration depths. In summer, the community returned to its equilibrium state with imprints of the subsurface bloom still visible. In our study, we did not detect a clear impact of SGD on microbial diversity, but found an unexpected homogeneous composition and resilience of the microbial community structure.
Nested groundwater flow systems (NGFS) are commonplace in various hydrogeological environments, including endorheic basins and coastal aquifers. The subsystems of the NGFS can be spatially separated by streamlines around the internal stagnation points. At the discharge zones of the topographic depressions, saline water often emerges due to high evaporation in endorheic drainage basins, or the combined effects of evaporation and intermittent seawater submersion in coastal areas. To date, there are limited studies that have considered the impact of local‐scale downward migration of saline plumes in the topographic depressions on the NGFS. In this study, the classic NGFS are revisited by considering saline water in their discharge zones. To quantify the effects of salinity in the discharge zones on the NGFS, scenarios of various salinities in the discharge zones are simulated. The displacements of the internal stagnation points are used to quantify the evolution of the NGFS in response to salinity changes in the discharge zones. The results show that, as the salinity in the discharge zones increases, the hydraulic gradient near the discharge zone can be significantly reduced, the internal stagnation points shift upward, and the local groundwater flow systems retreat upward so that their original spaces are replaced by intermediate or regional flow systems. The discharge zone is expanded and the overall groundwater flow velocity magnitude of the entire system decreases with salinity. This study may shed light on the management of saline wetlands, e.g. the control of groundwater salinization, evolution of saline groundwater basins, and seawater intrusion.
Intertidal aquifers are hotspots of biogeochemical cycling where nutrients and contaminants are processed prior to discharge to the ocean. The nature of the dynamic subsurface mixing zone is a critical control on mitigating reactions. Simulation of density‐dependent, variably saturated flow and salt transport incorporating realistic representations of aquifer heterogeneity was conducted within a Monte Carlo framework to investigate influence of nonuniform permeability on intertidal groundwater flow and salt transport dynamics. Results show that heterogeneity coupled with tides creates transient preferential flow paths within the intertidal zone, evolving multiple circulation cells and fingering‐type salinity distributions. Due to heterogeneity, strain‐dominated (intense mixing) and vorticity‐dominated (low mixing) flow regions coexist at small spatial scales, and their spatial extent reaches peaks at high tide and low tide. Such topological characteristics reveal complex tempo‐spatial mixing patterns for intertidal flow with localized areas of high and low mixing intensities, which have implications for intertidal biogeochemical processing.
Knowledge of coastal groundwater flow is critical for managing coastal groundwater resources and quantifying submarine groundwater discharge (SGD), but this flow occurs over multiple scales that can be difficult to study in an integrated way. We designed a field and modeling study to investigate groundwater flow and the distribution of salinity during sea level rise in a domain that included beaches, salt marshes and the first major confined aquifer, which reached 10–15 km offshore. Numerical models were based on the flat‐lying, passive margin coastline of North Inlet, SC, and were constrained by field studies including subsurface resistivity surveys and hydraulic head observations. Simulations that included tidal fluctuations showed that the salt marsh generated more than three times as much SGD as the beach and inner shelf, per unit length of coastline. Groundwater exchange between scales was small, suggesting that physical fluxes of groundwater can be considered independently at different scales. However, salinization of the first major confined aquifer occurred by downward transport from overlying aquifers rather than intrusion from the seaward end, suggesting that studies of aquifer salinization should consider multiscale flow. During simulated sea level rise, fresh‐to‐brackish groundwater persisted in the first confined aquifer as far as the seaward end of the overlying confining unit, 10–20 km offshore. Total fluxes of SGD decreased significantly with future sea level rise, dominated by declining SGD in the salt marsh, and portending a marked decline in the flux of nutrients and carbon to estuaries and the coastal ocean.
The mixing between fresh and saline groundwater in beach aquifers promotes biogeochemical transformations that affect nutrient fluxes to the coastal ocean. We performed variable-density groundwater flow and reactive transport simulations with geostatistical representations of sedimentary structure to understand the influence of heterogeneity on groundwater dynamics and denitrification in intertidal mixing zones. Ensemble-averaged simulation results show that heterogeneity can enhance mixing between fresh and saline groundwater and increase residence time, resulting in up to 80% higher nitrate removal relative to equivalent effective homogeneous aquifer sediment. Denitrification hotspots form in high permeability structures where DOC and nitrate are readily supplied by convergent flow. The results provide a physical explanation for the formation of denitrification hotspots observed in beach aquifers and illustrate for the first time the influence of sediment heterogeneity on rates and spatial patterns of biogeochemical processes in intertidal aquifers that are critical mediators of land-sea solute fluxes along world coastlines.
Tides in coastal rivers drive river-groundwater (hyporheic) exchange and provide opportunities for nitrate removal that may improve coastal water quality. Silt and sand layers in coastal floodplain sediments can alter the flow and transformation of nitrate. Our goal was to understand how sediment heterogeneity influences nitrogen dynamics near tidal rivers. Numerical simulations show that oxic, variably saturated sand layers and anoxic, organic-rich silt layers are sites of nitrification and denitrification, respectively. The exchange of river water and nitrate through heterogeneous sediments increases with sand fraction, as sand lenses become longer and more connected. The amount of nitrate removed from river water also increases, but represents a smaller portion of total nitrate exchange through the hyporheic zone, causing removal efficiency to decline. Our results suggest that accurate characterization of aquifer heterogeneity leads to an improved understanding of sites of nutrient transformation within floodplain sediments.
Preferential flow can result in rapid contamination of groundwater resources. This is particularly true in aquifers with connected, high permeability geologic structures and in coastal systems where the oceanic source of contamination is ubiquitous. We consider saltwater intrusion due to pumping in volcanic aquifers with lava tubes represented as connected high‐K structures and compare salinization responses to those of heterogeneous aquifers with different structure and equivalent homogeneous systems. Three‐dimensional simulations of variable‐density groundwater flow and salt transport show that conduits formed by lava flows create preferential groundwater flow in volcanic aquifers. These conduits allow fresh groundwater to extend further offshore than in other systems. However, onshore pumping causes saltwater to migrate landward quickly through the conduits relative to the other models, resulting in more severe saltwater intrusion, particularly at shallow depths. The geometry of geologic heterogeneity in volcanic aquifers leads to increased risk of salinization of fresh groundwater as well as substantial uncertainty due to significant spatial variation in saltwater intrusion. The findings illustrate the importance of considering geologic heterogeneity in assessing the vulnerability of coastal freshwater resources in volcanic and other aquifers with connected high‐permeability geologic structures.
Sandy beaches occupy more than one-third of the global coastline1 and have high socioeconomic value related to recreation, tourism and ecosystem services2. Beaches are the interface between land and ocean, providing coastal protection from marine storms and cyclones3. However the presence of sandy beaches cannot be taken for granted, as they are under constant change, driven by meteorological4,5, geological6 and anthropogenic factors1,7. A substantial proportion of the world’s sandy coastline is already eroding1,7, a situation that could be exacerbated by climate change8,9. Here, we show that ambient trends in shoreline dynamics, combined with coastal recession driven by sea level rise, could result in the near extinction of almost half of the world’s sandy beaches by the end of the century. Moderate GHG emission mitigation could prevent 40% of shoreline retreat. Projected shoreline dynamics are dominated by sea level rise for the majority of sandy beaches, but in certain regions the erosive trend is counteracted by accretive ambient shoreline changes; for example, in the Amazon, East and Southeast Asia and the north tropical Pacific. A substantial proportion of the threatened sandy shorelines are in densely populated areas, underlining the need for the design and implementation of effective adaptive measures. Erosion is a major problem facing sandy beaches that will probably worsen with climate change and sea-level rise. Half the world’s beaches, many of which are in densely populated areas, could disappear by the end of the century under current trends; mitigation could lessen retreat by 40%.
Aquifer‐ocean temperature contrasts are common worldwide. Their effects on flow and salinity distributions in unconfined coastal aquifers are, however, poorly understood. Based on laboratory experiments and numerical simulations, we examined the responses of flow processes in tidally influenced aquifers to aquifer‐ocean temperature differences. The extent of seawater intrusion and seawater circulation were found to vary with the aquifer‐ocean temperature contrast. Compared with the isothermal case, an increase of up to 40% of the tide‐induced seawater circulation rate in the intertidal zone was observed when seawater is warmer than groundwater. In contrast, saltwater circulation in the lower saltwater wedge declines notably no matter whether the seawater is warmer or colder than groundwater. As the seawater temperature rises, the contribution of tide‐induced circulation to the overall increase of submarine groundwater discharge becomes more important compared with that of density‐driven seawater circulation. Both the upper saline plume and the freshwater discharge zone expand significantly with warmer seawater whereas the lower saltwater wedge contracts.
A numerical study, based on a variably saturated groundwater flow model within a Monto Carlo framework, was conducted to investigate flow and solute transport in a heterogeneous beach aquifer subjected to tides. The numerical simulations were conducted based on our previous tracer experiments performed in a laboratory beach. Heterogeneity is assumed to be multifractal generated using the Universal Multifractal model. Our results show that heterogeneity greatly alters temporal and spatial evolution of the tracer plume migrating in the beach. The spreading coefficient of the plume shows very dynamic response to tides; it increases as the tidally driven recirculation cell overlaps with the plume and decreases as the recirculation cell moves far from the plume with tides. Descriptive statistics suggests that heterogeneity enhances spreading of the plume in the beach in an ensemble sense along with significant spatial and temporal variation. Due to heterogeneity, high spots of the pore water velocity are formed within the recirculation cell, creating transient preferential flow paths in the beach in response to tides. Contours of the Okubo‐Weiss (OW) parameter show that coupling with tides, heterogeneity creates vorticity‐dominated flow regions and also expands strain‐dominated flow regions more downward, indicating complex local‐scale mixing in the beach, compared to corresponding homogeneous case. Geologic heterogeneity also alters the spatial extent of the recirculating cell and induces highly variable transit time along the recirculating flow paths. The results provide insights into effects of geologic heterogeneity on seawater‐groundwater mixing and associated solute transport processes in tidally influenced coastal aquifers.
Plain Language Summary
The findings of this study provide insight into influences of heterogeneity on groundwater flow and solute transport processes in aquifers. Our results reveal that multifractal heterogeneity leads to focused regions of groundwater advection and creates hotspots of bending‐type vortex flow that are 3‐D and greatly deform surrounding pore water. It has strong implications for groundwater contamination and associated implementation of effective bioremediation strategies. This is because solutes/contaminants would be spun around these structures and therefore have less interaction with the soil matrix in the inner part of the vortices. We also demonstrate that the structures of the vortex flow are scale invariant in multifractal K fields and below the correlation scale of stationary K fields. It indicates that these vortex flow structures may be widely present in the field with no characteristic scale. Substantial energy dissipation along the high‐velocity zones also has strong implications for understanding activity, diversity, and the distribution of subterranean microorganisms.
Coastal marshes are an important component of the global carbon cycle, yet our understanding of how these ecosystems will respond to sea level rise (SLR) is limited. Coastal marsh hydrology varies based on elevation, distance from channel, and hydraulic properties, resulting in zones of unique water level oscillation patterns. These zones impact ecology and geochemistry and correspond to differences in carbon accumulation rates. These physical‐biogeochemical linkages enable use of a hydrological model to predict changes in marsh zonation, and in turn carbon accumulation, as well as groundwater‐surface water exchange under SLR. Here, we developed a calibrated hydrological model of a Delaware coastal marsh using HydroGeoSphere. We simulated three scenarios each of SLR, sediment accretion, and upland hydrologic response, and we quantified changes in the spatial coverage of different hydrologic zonations and groundwater‐surface water exchange. Results show that relative SLR reduces marsh area, carbon burial, and lateral water fluxes. However, the magnitudes of change are linked to the terrestrial groundwater table response as well as relative SLR. In scenarios where the upland water table does not change with SLR, the magnitude of decline in marsh area and carbon accumulation is reduced compared to scenarios where the upland water table keeps pace with SLR. In contrast, the reduction in lateral water flux is minimized in scenarios with an upland water table rise equal to SLR compared to scenarios where the upland water table is held at present‐day levels. This study highlights the importance of regional hydrologic setting in the fate of coastal marsh dynamics.
During seawater circulation in permeable intertidal sands, organic matter degradation alters the composition of percolating fluids and remineralization products discharge into surficial waters. Concurrently, coastal seawater nutrient and organic matter composition change seasonally due to variations in pelagic productivity. To assess seasonal changes in organic matter degradation in the intertidal zone of a high energy beach (Spiekeroog Island, southern North Sea, Germany), we analyzed shallow pore waters for major redox constituents (oxygen (O2), manganese (Mn), iron (Fe)) and inorganic nitrogen species (nitrite (NO2‐), nitrate (NO3‐), ammonium (NH4+)) in March, August, and October. Surface water samples from a local time series station were used to monitor seasonal changes in pelagic productivity. O2 and NO3‐ were the dominating pore water constituents in March and October. Dissolved Mn, Fe, and NH4+ were more widely distributed in August. Seasonal changes in seawater temperature as well as organic matter and nitrate supply by seawater were assumed to affect microbial rates and degradation pathways. Pore water and seawater variability led to seasonally changing constituent effluxes to surface waters. Mn, Fe, and NH4+ effluxes are minimal in March and reached their maximum in August. Furthermore, the intertidal sands switched from a net dissolved inorganic nitrogen (DIN) sink in March to a net source in August. In conclusion, seasonal effects on intertidal pore water biogeochemistry affect constituent fluxes across the sediment‐water interface. The seasonality of the beach bioreactor must be considered when fluxes are extrapolated to annual timescales.
Tidal marshes are valuable global carbon sinks, yet large uncertainties in coastal marsh carbon budgets and mediating mechanisms limit our ability to estimate fluxes and predict feedbacks with global change. To improve mechanistic understanding, we assess how net carbon storage is influenced by interactions between crab activity, water movement, and biogeochemistry. We show that crab burrows enhance carbon loss from tidal marsh sediments by physical and chemical feedback processes. Burrows increase near-creek sediment permeability in the summer by an order of magnitude compared to the winter crab dormancy period, promoting carbon-rich fluid exchange between the marsh and creek. Burrows also enhance vertical exchange by increasing the depth of the strongly carbon-oxidizing zone and reducing the capacity for carbon sequestration. Results reveal the mechanism through which crab burrows mediate the movement of carbon through tidal wetlands and highlight the importance of considering burrowing activity when making budget projections across temporal and spatial scales.
Tsunami‐induced seawater inundation causes vertical seawater infiltration into coastal aquifers and induces unexpected salinization of fresh groundwater resources. Assessing future risks of seawater intrusion in tsunami‐prone areas can provide essential information supporting disaster preparedness. In this study, we investigated seawater intrusion and aquifer recovery processes under the future Nankai earthquake and tsunami scenarios at Niijima Island, Japan. A groundwater model with a 2‐D vertical cross section was developed to solve variable‐density flow and salt transport in unsaturated‐saturated media using the numerical code, FEFLOW. Our simulation results indicate that the unsaturated zone behaves as an initial storage of the infiltrated seawater and controls the maximum amount of seawater infiltration during the anticipated tsunami inundation. The bedrock structures affect the direction of seawater movement in the saturated zone and the flushing time of the polluted aquifer. Our analysis suggests that the tsunami inundation height, the rainfall recharge rate, and hydraulic conductivity are the primary sources of uncertainties in the simulation results. These findings have implications for other tsunami‐prone zones with respect to the understanding of the relevant physical processes and model uncertainties.
Tides and seasonally varying inland freshwater input, with different fluctuation periods, are important factors affecting flow and salt transport in coastal unconfined aquifers. These processes affect submarine groundwater discharge (SGD) and associated chemical transport to the sea. While the individual effects of these forcings have previously been studied, here we conducted physical experiments and numerical simulations to evaluate the interactions between varying inland freshwater input and tidal oscillations. Varying inland freshwater input was shown to induce significant water exchange across the aquifer‐sea interface as the saltwater wedge shifted landward and seaward over the fluctuation cycle. Tidal oscillations led to seawater circulations through the intertidal zone that also enhanced the density‐driven circulation, resulting in a significant increase in the total SGD. The combination of the tide and varying inland freshwater input, however, decreased the SGD components driven by the separate forcings (e.g., tides and density). Tides restricted the landward and seaward movement of the saltwater wedge in response to the varying inland freshwater input in addition to reducing the time delay between the varying freshwater input signal and landward‐seaward movement in the saltwater wedge interface. This study revealed the nonlinear interaction between tidal fluctuations and varying inland freshwater input will help to improve our understanding of SGD, seawater intrusion, and chemical transport in coastal unconfined aquifers.
Transport processes such as the dispersion and mixing of solutes are governed by the interplay of advection and diffusion, where advection acts to organise fluid streamlines and diffusion acts to randomise solute molecules. Thus, the structure and organisation of streamlines, termed the Lagrangian kinematics of the flow, is central to the understanding and modelling of these transport processes. A key question is whether the streamlines in three-dimensional (3D) Darcy flows can wander freely through the fluid domain, or whether all streamlines of the flow are organised into a series of smooth, non-intersecting two-dimensional (2D) surfaces. The existence of such a foliation of surfaces constrains the Lagrangian kinematics in a manner similar to that of 2D flows, which in turn constrains the allowable transport processes. In a series of pioneering studies, Sposito (Water Resour. Res., 30(8):2395–2401, 1994; Adv. Water Resour., 24(7):793–801, 2001) argues that steady Darcy flow in locally isotropic media gives rise to Lamb surfaces, 2D material surfaces which are spanned by both the streamlines and vortex lines (field lines of the vorticity vector) of the flow. Hence, the existence of these surfaces renders the kinematics of such 3D steady Darcy flow as two dimensions. This topological constraint strongly affects transverse mixing and dispersion because 2D steady flow fields limit the rate of deformation of fluid elements and can only admit zero hydrodynamic transverse dispersion. In this study, however, we show that Lamb surfaces are not ubiquitous to all steady Darcy flows in locally isotropic media. We derive the conditions for when Lamb surfaces exist in such Darcy flows, and discuss the implications of these findings for the transport, mixing, and dispersion of solutes.
Biogeochemical reactions within intertidal zones of coastal aquifers have been shown to alter the concentrations of terrestrial solutes prior to their discharge to surface waters. In organic‐poor sandy aquifers, the input of marine organic matter from infiltrating seawater supports active biogeochemical reactions within the sediments. However, while the seasonality of surface water organic carbon concentrations (primary production) and groundwater mixing have been documented, there is limited understanding of the transience of various organic carbon pools (pore water particulate, dissolved, sedimentary) within the aquifer and how these relate to the location and magnitudes of biogeochemical reactions over time. To understand the relationship between changes in groundwater flow and the seasonal migration of geochemical patterns, beach pore water and sediment samples were collected and analyzed from six field sampling events spanning 2 years. While the seasonally dynamic patterns of aerobic respiration closely followed those of salinity, redox conditions and nutrient characteristics (distributions of N and P, denitrification rates) were unrelated to contemporaneous salinity patterns. This divergence was attributed to the spatial variations of reactive particulate organic carbon distributions, unrelated to salinity patterns, likely due to filtration, retardation, and immobilization dynamics during transport within the sediments. Results support a “carbon memory” effect within the beach, with the evolution and migration of reaction patterns relating to the distribution of these scattered carbon pools as more mobile solutes move over less mobile pools during changes in hydrologic conditions. This holds important implications for the prediction and quantification of biogeochemical reactions within beach systems.
Whether a coastal area is suitable for beach nourishments and can induce a growth in fresh groundwater resources depends on the appropriateness of the intended site for beach nourishments, and the attainable growth in fresh groundwater resources. In this study we presume that all eroding sandy beaches are suitable for large beach nourishments, and focus on the impact of these nourishments on fresh groundwater in various coastal settings. The growth in fresh groundwater resources – as a consequence of the construction of a beach nourishment – was quantified with 2-D variable-density groundwater models, for a global range in geological parameters and hydrological processes. Our simulation results suggest that large beach nourishments will likely lead to a (temporary) increase of fresh groundwater resources in most settings. However, for a substantial growth in fresh groundwater, the coastal site should receive sufficient groundwater recharge, consist of sediment with a low to medium hydraulic conductivity, and be subject to a limited number of land-surface inundations. Our global analysis shows that 17% of shorelines may consist of erosive sandy beaches, and of these sites 50% have a high potential suitability. This shows a considerable potential worldwide to combine coastal protection with an increase in fresh groundwater resources.
The remediation of beaches contaminated with oil includes the application of surfactants and/or the application of amendments to enhance oil biodegradation (i.e., bioremediation). This study focused on evaluating the practicability of the high pressure injection (HPI) of dissolved chemicals into the subsurface of a lentic Alaskan beach subjected to a 5 m tidal range. A conservative tracer, lithium, in a lithium bromide (LiBr) solution, was injected into the beach at 1.0 m depth near the mid-tide line. The flow rate was varied between 1.0 and 1.5 L/min, and the resulting injection pressure varied between 3 m and 6 m of water. The concentration of the injected tracer was measured from four surrounding monitoring wells at multiple depths. The HPI associated with a flow rate of 1.5 L/min resulted in a Darcy flux in the cross-shore direction at 1.15 × 10−5 m/s compared to that of 7.5 × 10−6 m/s under normal conditions. The HPI, thus, enhanced the hydraulic conveyance of the beach. The results revealed that the tracer plume dispersed an area of ~12 m2 within 24 h. These results suggest that deep injection of solutions into a gravel beach is a viable approach for remediating beaches.
Low-salinity submarine groundwater contained within continental shelves is a global phenomenon. Mechanisms for emplacing offshore groundwater include glacial processes that drove water into exposed continental shelves during sea-level low stands and active connections to onshore hydrologic systems. While low-salinity groundwater is thought to be abundant, its distribution and volume worldwide is poorly understood due to the limited number of observations. Here we image laterally continuous aquifers extending 90 km offshore New Jersey and Martha’s Vineyard, Massachusetts, on the U.S. Atlantic margin using new shallow water electromagnetic geophysical methods. Our data provide more continuous constraints on offshore groundwater than previous models and present evidence for a connection between the modern onshore hydrologic system and offshore aquifers. We identify clinoforms as a previously unknown structural control on the lateral extent of low-salinity groundwater and potentially a control on where low-salinity water rises into the seafloor. Our data suggest a continuous submarine aquifer system spans at least 350 km of the U.S. Atlantic coast and contains about 2800 km3 of low-salinity groundwater. Our findings can be used to improve models of past glacial, eustatic, tectonic, and geomorphic processes on continental shelves and provide insight into shelf geochemistry, biogeochemical cycles, and the deep biosphere.
Submarine groundwater discharge (SGD) is a ubiquitous source of meteoric fresh groundwater and recirculating seawater to the coastal ocean. Due to the hidden distribution of SGD, as well as the hydraulic-and stratigraphy-driven spatial and temporal heterogeneities, one of the biggest challenges to date is the correct assessment of SGD-driven constituent fluxes. Here, we present results from a 3-dimensional seasonal sampling campaign of a shallow subterranean estuary in a high-energy, meso-tidal beach, Spiekeroog Island, Northern Germany. We determined beach topography and analyzed physico-chemical and biogeochemical parameters such as salinity, temperature, dissolved oxygen, Fe(II) and dissolved organic matter fluorescence (FDOM). Overall, the highest gradients in pore water chemistry were found in the cross-shore direction. In particular, a strong physico-chemical differentiation between the tidal high water and low water line was found and reflected relatively stable in-and exfiltrating conditions in these areas. Contrastingly, in between, the pore water compositions in the existing foreshore ridge and runnel system were very heterogeneous on a spatial and temporal scale. The reasons for this observation may be the strong morphological changes that occur throughout the entire year, which affect the exact locations and heights of the ridge and runnel structures and associated flow paths. Further, seasonal changes in temperature and inland hydraulic head, and the associated effect on microbial mediated redox reactions likely overprint these patterns. In the long-shore direction the pore water chemistry varied less than the along the cross-shore direction. Variation in long-shore direction was probably occurring due to topography changes of the ridge-runnel structure and a physical heterogeneity of the sediment, which produced non-uniform groundwater flow conditions. We conclude that on meso-tidal high energy beaches, the rapidly changing beach morphology produces zones with different approximations to steady-state conditions. Therefore, we suggest that zone-specific endmember sampling is the optimal strategy to reduce uncertainties of SGD-driven constituent fluxes.
Although steady, isotropic Darcy flows are inherently laminar and non‐mixing in the absence of diffusion, it is well understood that transient forcing via engineered pumping schemes can induce rapid, chaotic mixing flows in groundwater. In this study we explore the propensity for such mixing to arise in natural groundwater systems subject to cyclical forcings, e.g. tidal or seasonal influences. Using a conventional linear groundwater flow model subject to tidal forcing, we show that under certain conditions these flows generate Lagrangian transport and mixing phenomena (chaotic advection) near the tidal boundary. We show that aquifer heterogeneity, storativity, and forcing magnitude cause reversals in flow direction over the forcing cycle which, in turn, generate coherent Lagrangian structures and chaos. These features significantly augment fluid mixing and transport, leading to anomalous residence time distributions, flow segregation, and the potential for profoundly altered reaction kinetics. We define the dimensionless parameter groups which govern this phenomenon and explore these groups in connection with a set of well‐characterised tidal systems. The potential for Lagrangian chaos to be present near discharge boundaries must be recognized and assessed in field studies.
Traditionally, groundwater and surface water flow models have been calibrated against two observation types: hydraulic heads and surface water discharge. It has repeatedly been demonstrated, however, that these classical observations do not contain sufficient information to calibrate flow models. To reduce the predictive uncertainty of flow models, the consideration of other observation types constitutes a promising way forward. Despite the ever-increasing availability of other observation types, however, they are still unconventional when it comes to flow model calibration. By reviewing studies that included nonclassical observations in flow model calibration, benefits and challenges associated with their integration in flow model calibration were identified, and their information content was analyzed. While explicit simulation of mass transport processes in flow models poses challenges, even simplified approaches to integrate tracer concentrations yield significantly better calibration results than using only classical observations. For a majority of calibrated flow models, observations of tracer concentrations and of exchange fluxes were beneficial. Temperature observations improved the simulation of heat transport but often worsened all other model outcomes. Only when temperature observations were made within 2 m of the surface water-groundwater interface did they have the potential to also improve flow and mass transport simulations. Surprisingly, many models were calibrated manually rather than with the widely available, mathematically robust and automated tools. There is a clear need for more systematic implementation of unconventional observations and automated flow model calibration as well as for more systematic quantification of the information content of unconventional observations.
Freshened groundwater exists offshore along many coastlines worldwide. Pumping of these offshore resources has been proposed for more efficient oil production as well as drinking and agriculture. Although pumping can occur tens to hundreds of kilometers offshore, these activities may threaten connected onshore aquifer systems. We conducted numerical simulations of variable-density groundwater flow and salt transport in coastal aquifers with different geologic structure subject to offshore pumping to assess potential impacts. Results show that offshore pumping can diminish both onshore groundwater availability and submarine groundwater discharge and can cause widespread land subsidence. Heterogeneity increases water resource vulnerability relative to equivalent homogeneous and layered systems and exacerbates the magnitude of land subsidence. More continuous geologic structure causes propagation of maximum subsidence farther landward. This work suggests that coastal aquifers may be vulnerable to offshore pumping activities and that these effects should be considered in feasibility assessments for offshore drilling.
Coastal zones constitute one of the most heavily populated and developed land zones in the world. Despite the utility and economic benefits that coasts provide, there is no reliable global-scale assessment of historical shoreline change trends. Here, via the use of freely available optical satellite images captured since 1984, in conjunction with sophisticated image interrogation and analysis methods, we present a global-scale assessment of the occurrence of sandy beaches and rates of shoreline change therein. Applying pixel-based supervised classification, we found that 31% of the world's ice-free shoreline are sandy. The application of an automated shoreline detection method to the sandy shorelines thus identified resulted in a global dataset of shoreline change rates for the 33 year period 1984-2016. Analysis of the satellite derived shoreline data indicates that 24% of the world's sandy beaches are eroding at rates exceeding 0.5 m/yr, while 28% are accreting and 48% are stable. The majority of the sandy shorelines in marine protected areas are eroding, raising cause for serious concern.
Submarine groundwater discharge (SGD) can be a significant source of chemical pollutants from land to ocean. Here, we first estimated SGD using radium isotopes and related nutrient fluxes at the local scale in Jiaozhou Bay (JZB), a typical Chinese system that is experiencing rapid urban and industrial development. We then summarized SGD studies off China to assess the large-scale implications of SGD to nutrient budgets. In JZB, the location of contaminated nearshore waters revealed by an integrative water quality index (WQI) coincided with the SGD hotspots. The total (fresh and saline) SGD flux in JZB was estimated to be (0.64–1.67) × 10⁷ m³/d or (2.12–5.59) cm/d based on ²²⁴Ra and ²²⁸Ra mass balance models. This was approximately 8 times the discharge rate of local rivers. By combining these JZB results with the literature data, we provide the first estimate of SGD and associated nutrient fluxes off China. The magnitude of SGD at the China-scale was (5.40–10.2) × 10¹² m³/yr, accounting for 5–9% of the global SGD flux. SGD-derived nutrient fluxes summarized from ∼40 previous studies were one order of magnitude higher than riverine inputs. These nutrients fluxes from SGD contributed >50% of the total dissolved inorganic nitrogen (DIN), phosphorous (DIP) and silicate (DSi) inputs into Chinese coastal waters, which can explain about 60% of the phosphorus required by primary production. The mean DIN/DIP ratio (121) in SGD was significantly higher than the Redfield ratio, with important implications for phytoplankton growth and structure. SGD can influence water quality, dominate nutrient budgets, and drive primary production not only at the local scale, but also at the regional and global scales.
Sandy beaches are places of active organic matter mineralization due to water renewal providing organic matter and electron acceptors in the porous and permeable sands. Recycled biogenic compounds are efficiently transferred to the coastal marine environment via wave and tidal-driven advective flows. The biogeochemical processes in beach aquifers were mainly studied in semi enclosed systems with low tidal amplitude, and with a connection to continental aquifers contributing to solute fluxes to the coast from terrestrial groundwater. We present here the study of a pocket beach isolated from terrestrial aquifers with a high tidal amplitude and a medium energy wave regime. In situ measurements, cross-shore profiles and vertical sampling were conducted during several tidal cycles in spring and autumn. Cross-shore transects, obtained at low tide from holes that represent a mixture of the upper 20 cm of the water saturated zone, showed concentration gradients of redox and recycled compounds. Increase in pCO2, dissolved phosphate and ammonium concentrations downslope revealed that more products from organic matter mineralization accumulated in the lower beach. The related increase in total alkalinity downslope indicated that the part of anaerobic processes in organic matter oxidation was higher in the lower beach. Concentration and δ¹³C of dissolved inorganic carbon in pore waters suggested that the carbon mineralized in pore waters came from marine plant debris that were mixed with the sand. Continuous probe records of dissolved oxygen saturation and vertical profiles revealed a tidally-driven dynamics of pore water in the first centimetres of the lower beach aquifer. Ventilation of pore waters corresponded to wave pumping and swash-induced infiltration of seawater in the upper 10–20 cm of sediment. Nutrients and reduced compounds produced through organic matter mineralization remained stored in pore water below the layer disturbed by wave. The flux of these components to seawater is possible when this interface is eroded, for example when wave energy increases after a less energetic period. The low extension of the studied aquifer, typical of pocket beaches, limits the connection with continental groundwater. Both tidally-driven and wave-driven recirculation of seawater allows pocket beaches to be efficient bioreactors for marine organic matter mineralization. As such, they provide the coastal environment with recycled nutrients, and not new nutrients.
Radium is widely used to estimate flushing time, submarine groundwater discharge (SGD), and submarine fresh groundwater discharge (SFGD), however there are important sources of uncertainty in current methods. Here an improved method is proposed, incorporating all radium quartet information to estimate flushing time, SFGD, SGD, and associated nutrient fluxes during wet and dry seasons in Laizhou Bay, China. Both SGD and SFGD in dry season are comparable to that in wet season, likely due to higher groundwater hydraulic gradients resulting from higher groundwater table and lower mean sea level in dry season. Estimated dry and wet season SFGD are of the same order of magnitude as the annually-averaged Yellow River discharge, highlighting SFGD's importance to the bay environment. Nutrient inputs into Laizhou Bay were estimated for the wet season, suggesting that SGD-derived nutrients are indeed important and significant for coastal environments compared to local river discharge estimates.
Coastal managers are increasingly faced with the challenge of disposing of stranded whale carcasses on beaches. Direct burial in the beach is often used as a cost effective method of disposal. However, whale burial management plans are often met with public resistance owing to the perceived risk of shark attraction to burial leachate that may discharge from the seabed. A reactive transport model was combined with a numerical variable-density groundwater flow model to assess buried whale leachate plume formation, transport, influence on beach aquifer reactivity, and discharge to coastal surface water for a range of burial setback distances, depths, and whale sizes. A second set of simulations was performed to evaluate aquifer nitrate removal efficiencies for a range of buried wrack scenarios and to evaluate the role of organic carbon source on beach reactivity. A sensitivity analysis was performed for both sets of models across ten physical and reaction parameters. Simulations using the best estimate parameter set show that whale burials produce DOC and ammonium leachate plumes in the beach aquifer that are transported to and discharge near the low tide line in water depths of 0.4–2.4 m. DOC and ammonium concentrations in discharging whale leachate are 1.6 and 26 times higher than typical surf zone concentrations, respectively. Of the factors tested, the burial distance inland from the high tide line is the most important factor affecting leachate fluxes to surface water. Burials placed farther inland led to smaller DOC fluxes to surface water, but increased ammonium fluxes. Burial depth also affects whale leachate to the subtidal zone, with deeper burials resulting in smaller fluxes of DOC. Leached DOC from whale decomposition and from buried wrack can fuel denitrification hotspots within beach sediments. The sensitivity analysis showed that nitrate removal supported by buried wrack and whale leachate fluxes are highly dependent on beach properties, hydrologic forcing, and reaction parameters. The wrack model results have implications for beach scraping and the whale burial models show that whale leachate can be delivered to the shallow subtidal zone via groundwater discharge pathways, with potential implications for shark attraction and whale burial management practices.
In this study, ³H-³He ages in the intertidal zone based on both field measurements and numerical simulations are presented to provide insights into the flow and transport behavior within the subterranean estuary of a meso-tidal, high-energy beach. In a systematic modelling approach, important hydrogeological parameters (dispersivity, hydraulic conductivity and fresh groundwater flux) were varied in a cross-shore transect at a beach on Spiekeroog Island in north-western Germany. Both measured and simulated salinity and age distribution within the intertidal zone suggest the existence of two– at least temporally occurring- saltwater recirculation cells accompanied by two fresh discharge location. This is in contrast to the state-of-the-art concept of a STE under tidal influence and is caused by a runnel and ridge system at the study site, which is a rather wide-spread type of beach morphology with topographical highs and interfering lows in the intertidal zone. Residence times within the two saline plumes were between weeks and several months. Freshwater from the island’s freshwater lens resided up to 18 years in the brackish beach aquifer before exfiltrating at the discharge points. Field measurements confirmed the discharge of decade-old water at the discharge points. Model results were highly dependent on the tested model parameters. A change between a one- and two-plume system, as well as overlapping conditions at the study site dependent on the change in beach topography are likely.
Geological heterogeneity is a key factor affecting rates and patterns of groundwater flow and the evolution of salinity distributions in coastal aquifers. The hydrogeologic systems of volcanic aquifers are characterized by lava flows that can form connected geologic structures in the subsurface. Surface-based geostatistical techniques were adopted to generate geologically-realistic, statistically equivalent model realizations of a hydrogeologic system based on that of the Big Island of Hawaii (conduit models). The density-dependent groundwater flow and solute transport code SEAWAT was used to perform 3D simulations to investigate subsurface flow and salt transport through these random realizations. Statistically equivalent geological systems generated with sequential indicator simulation (SIS) and equivalent homogeneous systems were also simulated for comparison. Simulation results show that conduit realizations tend to have more complex salinity distributions with larger mixing zones for which the center of mass is farther offshore compared to homogeneous and SIS realizations. The zone of groundwater discharge is also farther offshore than for homogeneous and SIS models, with higher point fluxes of both fresh and saline water and greater spatial variability. This highlights the importance of the geometry of geologic features in coastal groundwater flow and solute transport processes in highly heterogeneous aquifers and effects of preferential flow, with implications for both coastal groundwater resource management and solute fluxes to the ocean.
This review of studies that quantified fluxes with seepage meters in marine settings in the last decades shows the historical evolution of this device and the knowledge acquired during this period. Coastal environments are differentiated from freshwater settings due to water salinity and the effects of tides and waves that have important implications for the measurement approach and generated results. The framework in which seepage meters have been used in marine settings has evolved in parallel to the understanding of submarine groundwater discharge. This review of seepage meter research shows: an uneven distribution of studies in the world with some densely-studied regions and an absolute lack of data in other regions; a dominance of studies where only seepage meters were used compared to studies that combined seepage meter measurements with values determined with radioactive tracers or hydraulic calculations; and a variety of publication outlets with different focuses (hydrology, oceanography or multidisciplinary). The historical overview of the research conducted with seepage meters shows the wide range of seepage meter applications – from simply measuring fluxes at local scales to larger studies that extrapolate local results to estimate fluxes of water, nutrients, and other solutes at regional and global scales. A variety of automated seepage meters have been developed and used to better characterize short-term groundwater-seawater exchange, including the effects of waves and tides. We present recommendations and considerations to guide seepage meter deployment in marine settings, as seepage meters are still the only method that quantifies directly the interaction between groundwater and surface water.
Inadequate characterization of variability in natural tracers of submarine groundwater discharge (SGD) introduces large errors into tracer-derived SGD estimates. To address this gap, we investigated spatial variability in the natural SGD tracers ²²³Ra, ²²⁴Ra, ²²⁶Ra, ²²⁸Ra and ²²²Rn using a high-density array of piezometers and seepage meters over a nearshore area where discharging groundwater transitioned from fresh to saline. Seepage meters and piezometers were used to sample groundwater and to quantify fluxes and salinity. A series of spatial patterns was distinguished beyond the normal salinity impact on Ra activity. The discharge in the interface between saltwater and freshwater was characterized by higher activities of the longer-lived isotopes (²²⁶Ra and ²²⁸Ra), while areas dominated by benthic exchange had higher activities of the shorter-lived isotopes (²²³Ra and ²²⁴Ra). Spatial differences in Ra activities were associated with variation in salinity and residence time in the aquifer and were indicative of underlying hydrogeological processes. At the freshwater-saltwater interface, fresh discharge is driven by terrestrial hydraulic gradients and saline discharge is driven by density gradients; both result in long residence times in the aquifer. Benthic exchange of saltwater has shorter residence times, much less than required to enrich long-lived isotopes and reach secular equilibrium. The highest activities of ²²²Rn were detected in the fresh discharge zone, and the lowest activities in areas dominated by benthic exchange. Direct sampling at discharge points allowed comparison between groundwater collected from inland wells and the water discharging to the bay, and indicated significant differences in the activities despite the short distance from wells to the discharge area. Substantial spatial variability in Ra and Rn was observed on the meter scale, with trends that reflected groundwater origin and residence time. Inadequate characterization of variability, trends, and fluxes introduces large errors into tracer-derived estimates of submarine groundwater discharge. Thus, the study of submarine groundwater discharge with radioactive tracers requires adequate hydrogeological knowledge to identify processes that have strong impacts on Ra and Rn activities and associated fluxes to the ocean.
Submarine groundwater discharge (SGD) has been recognized as an important source of dissolved heavy metals to the coastal ocean. Bohai Bay, the second largest bay of Bohai Sea in China, is subjected to serious environmental problems. However, SGD and SGD-derived heavy metal fluxes in the bay are seldom reported. In this study, we
present mass balance models considering the radium losses caused by recirculated seawater to estimate water age, SGD and SGD-derived heavy metal fluxes in Bohai Bay during May 2017. The water age is estimated to be 56.7–85.0 days based on tidal prism model. By combining water and salt mass balance models, submarine fresh groundwater discharge (SFGD) is estimated to be (3.5–9.3) × 107 m3 d−1. The SGD flux estimated by the radium mass balance models is (3.2–7.7) × 108 m3 d−1, an order of magnitude larger than the discharge of the Yellow River during the sampling period. SGD-derived heavy metal fluxes were estimated to be (0.2–6.0)× 107 mol d−1 for Fe, (1.2–2.7) × 107 mol d−1 for Mn, (3.0–8.2) × 105 mol d−1 for Zn, (2.7–7.4) × 104 mol d−1 for Cr and (0.6–1.8) × 103 mol d−1 for Cd, which are significantly higher than those from local rivers. This study reveals that SGD is a significant source of heavy metals (Mn, Zn and Fe) into Bohai Bay, which may have important influences on the metal budgets and ecological environments in coastal areas。
Submarine groundwater discharge (SGD) can be an important pathway for chemical or biological pollutants from land to the ocean around the world. However, studies on the microbial communities associated with SGD in Southeast Asia, which has been hypothesized as SGD hotspot, remain scarce. In this study, we examined the microbial community composition with 16S rRNA gene sequencing along the hydrological continuum of an SGD site in a tropical urban area of Indonesia. Of the observed parameters in this study, salinity and temperature were the most determinant variables explaining patterns in microbial community composition. The bacterial taxon Burkholderiaceae was predominantly found in low salinity samples, including those from terrestrial groundwater and brackish pore water, while cyanobacteria of the genus Synechococcus sp. CC9902 were indicative of saline SGD and seawater samples. The composition of microbial taxa in each sample pointed to the influence of shallow terrestrial groundwater in the beach pore water, while seawater recirculation dominated the SGD sampling points situated further offshore. We identified taxa containing fecal indicators and potential pathogens at the SGD compartments; however, while a likely explanation, we could not conclude with certainty that SGD was a conduit for these bacteria. Overall, the results from this study show that microbial community analysis can highlight hydrological processes and water quality at the SGD site; thus, they could be useful for environmental policymakers to formulate water management strategies in coastal areas.
Recent studies have shown that, in beach aquifers, tide-induced upper saline plume (USP) may become unstable in the form of moving salt fingers under certain hydrogeological conditions. However, to what extent solute transport in beach aquifers is affected by the unstable flow associated with such salt fingers has not been quantitatively examined, despite implications inferred by these studies. This study numerically investigated this problem by considering a land-sourced conservative solute plume in a conceptual 2-D beach aquifer. The results demonstrate that unstable flow leads to salt fingers that periodically develop from the USP and then move seaward across the intertidal zone. Compared to the quasi-steady USP case, unstable flow considerably modifies the migration path of land-sourced solute plume towards the sea, moving it deeper into the aquifer until reaching the base. Also, unstable flow increases the residence time of the land-sourced solute plume in the beach aquifer and causes the solute discharge zone to vary over time and shift from the mean shoreline (in the quasi-steady USP case) to the low-tide mark. The temporal variations of total solute efflux pattern are also altered by unstable flow, changing from a unimodal to a bimodal pattern. Sensitivity analysis based on dispersivity, hydraulic conductivity, tidal amplitude and inland freshwater flux reveals that the transport path of solute plume is more sensitive to hydraulic conductivity and tidal amplitude, while the residence time and plume spreading are more sensitive to inland freshwater flux.
Subterranean estuary, as a saltwater-freshwater mixing zone in a coastal aquifer, is significantly affected by tides. While many studies have been conducted to examine the effects of tides on groundwater dynamics in subterranean estuaries in relation to submarine groundwater discharge and seawater intrusion, the focus of the investigation, particularly for those based on numerical modelling, has been on tides with few frequencies, e.g., monochromatic tide and spring-neap tides (bichromatic). In this study, we examine numerically effects of measured multi-constituent tides on a coastal unconfined aquifer. With the antecedent tidal conditions accounted for using the Gamma distribution function, we conducted regression analysis and quantified the effects of measured multi-constituent tides on coastal unconfined aquifers in a prolonged and cumulative fashion. The results showed that the response of submarine groundwater discharge to the tidal fluctuation is mostly instantaneous. The coastal aquifer behaved like a low-pass filter that smooths out high-frequency tidal signals and leaves behind low-frequency signals to affect upper saline plume and lower saltwater wedge. The upper saline plume and lower saltwater wedge were significantly affected by antecedent tidal conditions at monthly and yearly time scales, respectively. Sensitivity analysis showed that the low-pass filter capacity can be altered by soil hydraulic conductivity and beach slope. This study revealed the importance of antecedent tidal conditions for seawater intrusion and will help to improve our understanding of hydrological processes in natural coastal unconfined aquifers.
We study the mixing dynamics of solute blobs in the flow through saturated heterogeneous porous media. As the solute plume is advected through a heterogeneous porous medium it suffers a series of deformations that determine its mixing with the ambient fluid through diffusion. Key questions are the relation between the spatial disorder and the mixing dynamics and the effect of the initial solute distribution. To address these questions, we formulate the advection–diffusion problem in a coordinate system that moves and rotates along streamlines of the steady flow field. The impact of the medium heterogeneity is quantified systematically within a stochastic modelling approach. For a simple shear flow, the maximum concentration of a blob decays asymptotically as $t^{-2}$ . For heterogeneous porous media, the mixing of the solute blob is determined by the random sampling of flow and deformation heterogeneity along trajectories, a mechanism different from persistent shear. We derive explicit perturbation theory expressions for stretching-enhanced solute mixing that relate the medium structure and mixing behaviour. The solution is valid for moderate heterogeneity. The random sampling of shear along trajectories leads to a $t^{-3/2}$ decay of the maximum concentration as opposed to an equivalent homogeneous medium, for which it decays as $t^{-1}$ .