Field-scale modelling reveals dynamic groundwater flow and transport patterns in a high-energy subterranean estuary

Terrestrial groundwater travels through subterranean estuaries before reaching the sea. Groundwater‐derived nutrients drive coastal water quality, primary production, and eutrophication. We determined how dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved organic nitrogen (DON) are transformed within subterranean estuaries and estimated submarine groundwater discharge (SGD) nutrient loads compiling > 10,000 groundwater samples from 216 sites worldwide. Nutrients exhibited complex, nonconservative behavior in subterranean estuaries. Fresh groundwater DIN and DIP are usually produced, and DON is consumed during transport. Median total SGD (saline and fresh) fluxes globally were 5.4, 2.6, and 0.18 Tmol yr−1 for DIN, DON, and DIP, respectively. Despite large natural variability, total SGD fluxes likely exceed global riverine nutrient export. Fresh SGD is a small source of new nutrients, but saline SGD is an important source of mostly recycled nutrients. Nutrients exported via SGD via subterranean estuaries are critical to coastal biogeochemistry and a significant nutrient source to the oceans.

Groundwater Quality Restoration and Coastal Ecosystem Productivity

Groundwater dynamics in an aquifer adjacent to a coastal lagoon

Non-conservative behavior of fluorescent dissolved organic matter (FDOM) within a subterranean estuary

Groundwater-surface water interactions are an important process in coastal and estuarine environments that influence water budgets and bring bioactive solutes, such as nutrients, gases, and trace metals, yet are often overlooked. Submarine groundwater discharge (SGD), or the exchange of water between coastal sediments and surface waters, is a critical component of the global hydrologic and biogeochemical cycles that link terrestrial waters to marine environments. Fresh SGD, which may account for up to 10% of total freshwater inflows to the ocean globally, is a source of new nutrients to a system, whereas saline SGD is considered a source of recycled nutrients from sediments. Like surface water estuaries, within the coastal aquifer terrestrial and marine waters meet in a subterranean estuary where groundwater from land drainage is measurably diluted by seawater recirculating through the aquifer and altered to become biogeochemically distinct. SGD has been quantified in three Texas estuaries with Nueces Estuary consistently having the highest fresh SGD rates and Mission-Aransas and Upper Laguna Madre Estuaries being dominated by saline SGD. SGD-derived nutrient fluxes have been shown to be substantial in Texas estuaries but the influence of these nutrients on coastal ecosystem functions and services still requires further investigation.

Dissolved strontium in the subterranean estuary – Implications for the marine strontium isotope budget

Carbon dynamics driven by seawater recirculation and groundwater discharge along a forest-dune-beach continuum of a high-energy meso-macro-tidal sandy coast

Previous studies have revealed hysteretic behavior in subterranean estuaries in response to intensified wave conditions caused by offshore storm events, showing dependence of submarine groundwater discharge (SGD) and subsurface salt distribution on historic wave conditions. Although most shorelines worldwide are also exposed to tidal fluctuations, it is unclear how tides moderate wave‐induced SGD and salinity distribution in subterranean estuaries. This study presents numerical simulations that explore the combined influence of intensified wave conditions and tides on groundwater flow and salt transport in a subterranean estuary. The results show that tides weaken the hysteretic wave effect on SGD, suggesting that a tidally influenced subterranean estuary is less sensitive to intensified wave conditions with respect to the water fluxes across the aquifer‐sea interface. However, due to enhancement of salt‐freshwater mixing, tides strengthen the hysteretic wave effect on the salt fluxes across the aquifer‐sea interface, prolonging the recovery of salt distribution in the subterranean estuary to the prestorm state. These findings reveal the nonlinear, coupling nature of processes driven by oceanic oscillations at different time scales in subterranean estuaries.

Submarine groundwater discharge (SGD) to coastal zones contributes terrestrial freshwater and nutrients that may support harmful algal blooms (HABs). The magnitude of nutrient exports via SGD depends on volumes of fresh groundwater discharge, its chemical composition, and modifications by biogeochemical processing within subterranean estuaries. Thus, the ability to upscale SGD exports requires knowing the range of chemical composition of inland groundwater and how those compositions may be transformed as fresh and saltwater mix within subterranean estuaries. These processes may create heterogeneous magnitudes of solute exports, even at small spatial scales, and such heterogeneities have rarely been assessed for regional or global SGD nutrient export estimates. To evaluate heterogeneity in subterranean estuary processes and nutrient export, we collected seasonal pore water samples in 2015–2016 at three proximal (<20 km) subterranean estuary sites in Indian River Lagoon, FL. Sites have homogenous hydrogeological settings, but differ in land use and coastal features, and include a mangrove site, an urban site, and a site offshore of a natural wetland. All sites exhibit little variation through time in nutrient concentrations and modeled SGD rates. In contrast, each site exhibits significantly different nutrient concentrations of potential fresh groundwater sources, fresh groundwater discharge volumes, and nutrient transformations within subterranean estuaries. Groundwater specific discharge correlates with nutrient concentrations, suggesting that higher residence times in the subterranean estuary increase biogeochemical transformations that reduce anthropogenic nutrient loads but increase in situ nutrient sources derived from organic matter remineralization. The differences in transformations lead to SGD nutrient contributions that differ by orders of magnitude between sites and have N:P ratios that are greater than the Redfield ratio (15) for the mangrove (29) and urban sites (28), but less than the Redfield ratio for the wetland site (8). These results indicate that heterogeneity of both absolute and relative nutrient export via SGD complicates integration of nutrient fluxes across regional coastal zones and evaluations of its impacts to coastal ecosystems. A better understanding of the drivers of heterogeneity, including subterranean estuary processes, land use, coastal topography, and vegetation dynamics could improve assessments of regional nutrient loading and upscaling for estimates of global solute cycles.

Terrestrial and marine environments merge at the land-sea transition zone. This zone is important as ~38% of the world’s population live by and depend on the coastal regions, and oceans are considerably affected by it. Furthermore, terrestrial and marine groundwater and seawater mix in the subterranean estuary (STE), where submarine groundwater discharge (SGD), i.e., discharging fresh groundwater and recirculated seawater, results in significant solute fluxes to the sea. With this article, we focus on advances of geochemical, microbiological, and technological aspects related to fresh groundwater, SGD, and STE in sandy coastal areas, using the barrier island Spiekeroog as a case study area. Previous studies showed that the fresh groundwater composition in sandy coastal aquifers is governed by calcareous shell dissolution, cation exchange, and organic matter degradation. Biogeochemical reactions in the STE further modify the water composition of SGD, with a dependence on residence time. Microbial communities, which are present in coastal sediments and usually follow salinity and redox gradients, are the driver for the degradation of organic matter. Regarding organic matter sources in the STE, it is evident that dissolved organic matter is primarily of marine origin and that SGD delivers degraded dissolved organic matter back into the ocean. Furthermore, recent studies used radiotracers, such as radium and radon, and seepage meters as reliable tools to quantify rates and fluxes associated with SGD. We conclude that, despite the advances being made, the complexity and interactions of the different processes at land-sea transition zones require multidisciplinary scientific approaches.

Submarine groundwater discharge is an important net source of light and middle REEs to coastal waters of the Indian River Lagoon, Florida, USA

Submarine groundwater discharge (SGD) links terrestrial and marine systems, but has often been overlooked in coastal nutrient budgets because it is difficult to quantify. In this Review, we examine SGD nutrient fluxes in over 200 locations globally, explain their impact on biogeochemistry and discuss broader management implications. SGD nutrient fluxes exceed river inputs in ~60% of study sites, with median total SGD fluxes of 6.0 mmol m−2 per day for dissolved inorganic nitrogen, 0.1 mmol m−2 per day for dissolved inorganic phosphorus and 6.5 mmol m−2 per day for dissolved silicate. SGD nitrogen input (mostly in the form of ammonium and dissolved organic nitrogen) often mitigates nitrogen limitation in coastal waters, since SGD tends to have high nitrogen concentrations relative to phosphorus (76% of studies showed N:P values above the Redfield ratio). It is notable that most investigations do not distinguish saline and fresh SGD, although they have different properties. Saline SGD is a ubiquitous, diffuse pathway releasing mostly recycled nutrients to global coastal waters, whereas fresh SGD is occasionally a local, point source of new nutrients. SGD-derived nutrient fluxes must be considered in water quality management plans, as these inputs can promote eutrophication if not properly managed. Submarine groundwater discharge transports nutrients from terrestrial to marine systems, but is often ignored in coastal biogeochemistry. In this Review, the fluxes, impacts and management implications of this discharge are examined and compared with riverine fluxes globally.

More than 60% of the global population lives in coastal areas, especially within 100 km from the coastlines, relying mostly on shallow groundwater resources. Seawater intrusion and submarine groundwater discharge (SGD) occur in the coastal aquifer systems, threatening these critical freshwater resources. Salinity seawater and fresh groundwater complexly interact with each other via SGD and SI. The SGD drives the discharge of not only a large volume of freshwater, but also terrestrial geochemical substances into the ocean through a mixing zone between discharging freshwater and recirculating seawater. The flux of SGD may be even greater than that of surface water through rivers and estuaries. For example, the SGD was estimated to be ~40 % and 80 %~160 % of the river water discharging flux into the South Atlantic Bight and Atlantic Ocean, respectively, and as a major source of dissolved organic matter and nutrients to Arctic coastal waters and the Mediterranean Sea.A few hydrological models, including MARUN, SEAWAT, SUTRA, and PHT3D, are commonly used for SGD studies. The recently developed TOUGHREACT is robust in simulating coupled hydrodynamic, thermodynamic, and geochemical processes. From TOUGH2 (Transport Of Unsaturated Groundwater and Heat, version 2), a multi-dimensional numerical model for simulating coupled transport of water, vapor, non- condensable gas, and heat in porous and fractured media. However, TOUGHREACT is rarely used for SGD analysis, despite it being a well-rounded model with wide applications. Additionally, relevant studies on the iron (Fe) precipitation during SGD have focused predominantly on its spatial distribution and the adsorption of dissolved species, and studies on the genesis and geochemical evolution are scarce.Therefore, we developed a systematic method using TOUGHREACT to simulate the hydrological processes in STEs and benchmarked the estimations; and then we numerically explored the groundwater flow and salt transport dur SGD by considering the influencing factors of tidal amplitude, freshwater head, seawater diffusion coefficient, and beach slope ratio. Consequently, by employing TOUGHREACT simulation, we analyzed the formation and spatiotemporal distribution of the Fe precipitation in the shallow beach aquifer due to the mixing of freshwater and seawater, and identified the key influencing factors during SGD.The results show that, freshwater-derived Fe2+ is oxidized by O2(aq) in seawater during SGD, then precipitates as Fe (hydr)oxides (Fe(OH)3) to form an Fe precipitation zone. Fe(OH)3 tends to accumulate in the freshwater side of the mixing zone, whereas Fe(OH)3 precipitation in the seaward side of the mixing zone is inhibited by locally high H+ concentrations. The Fe(OH)3 first precipitates in the shallow aquifer, then extends to deeper layers over time, which is attributed to the increase in the residence time with the depth of both freshwater and seawater. The spatial distribution, and particularly, the extent of the iron curtain are influenced by the water flux and the concentration ratio of O2(aq) to Fe2+. These results are beneficial for better understanding the formation and distribution of iron curtains, and shed light on enhancing the understanding of the hydrogeochemical processes in subterranean estuaries.

Subterranean estuaries (STEs) play an important role in linking nutrient cycling between marine and terrestrial systems. As being the primary drivers of nutrient cycling, the composition of microbial communities and their adaptation toward both, terrestrial and marine conditions are of special interest. While bacterial communities of STEs have received increasing scientific attention, archaeal and meiofaunal diversity was mostly neglected. Previous studies at the investigated sampling site, the STE of a mesotidal beach at the German North Sea island of Spiekeroog, focused on spatial and seasonal patterns of geochemical and bacterial diversity. By additionally investigating the archaeal and meiofaunal diversity and distribution, we now aimed to fill this gap of knowledge to understand the microbial response to submarine groundwater discharge (SGD). The topography of Spiekeroog beach and associated geochemical gradients in porewater displayed a distinct cross-shore zonation, with seawater infiltration on the upper beach at the high water line (HWL), and saline and brackish porewater exfiltration (SGD) at the ridge-runnel structure and the low water line (LWL) on the lower beach. This led to a higher evenness of prokaryotic communities in lower beach areas impacted by SGD compared to unimpacted areas. Archaea contributed 1–4% to the 16S rRNA gene sequence dataset. Those were dominated by Nitrosopumilaceae, corresponding well to higher concentrations of NH4+ in the discharge area of the STE. The unimpacted sites had elevated abundances of Wosearchaeia, which were also detected previously in impacted areas of an STE at Mobile Bay (Gulf of Mexico). While a large proportion of prokaryotes were present in the entire intertidal area, meiofaunal community compositions were site specific and dominated by nematodes. Nematode communities of the high-water line differed distinctively from the other sites. Overall, our data indicates that the three domains of life display distinctly different adaptations when facing the same conditions within the STE. Therefore, distribution patterns of any domain can only be understood if all of them, together with basic environmental information are investigated in an integrated context.

Subterranean estuaries below high-energy beaches are understudied, despite being potential powerful biogeochemical reactors at the land/sea transition zone affecting the quality of coastal waters. Highly transient hydro(geo)logical boundary conditions and density-effects lead to dynamic subsurface flow and transport patterns which are difficult to understand and hard to replicate by models. A comprehensive and unique 1-year dataset of hydraulic heads, salinity and temperature data in combination with apparent 3H/He ages was obtained at a beach research site on Spiekeroog Island in North Germany. The site includes 3 multilevel groundwater monitoring wells and a vertical electrode chain with 10 temperature sensors, all positioned on a transect aligned along the principal cross-shore flow direction and all reaching down to 24 m depth below ground surface. The data-set was used to set up and calibrate a site-specific groundwater flow and transport model, aiming to approximate the highly dynamic groundwater flow patterns on that transect. The simulation time needed to be 20 years because of the long model spin-up. Due to the complex and nonlinear nature of the system, model calibration was carried out via particle swarm optimization, which is superior to gradient-based optimization techniques with respect to finding a global minimum of the objective function. The calibration results were reasonable. The dynamics of hydraulic head data were well captured, however, simulated values were constantly higher than those observed. The observed salinities were best captured for the multilevel wells near the mean high water and low water line. At the highest multilevel well located at the upper beach right at the dune base, simulations matched observations less well. Similarly, groundwater temperatures and ages were best replicated at the location in the infiltration zone near the high-water line. Groundwater ages and their temporal dynamics at the dune base and mean low water line could only be replicated down to 12 m depth. Deviations between simulations and observations are likely due to 3D flow effects in longshore direction, which could not be captured with the 2D vertical cross-sectional model approach. However, long model run times hindered calibration of a full-blown 3D density-dependent, 20-year long-term groundwater flow and transport model. The next step is to estimating the importance of longshore hydraulic gradients. Finally, the model will be extended for hydrobiogeochemical reactions to assist in the analysis and understanding of the observed hydrochemical data at this site.