Terrestrial Submarine Groundwater Discharge Research Articles

Neodymium Isotope Geochemistry of a Subterranean Estuary

Rare earth elements (REE) and Nd isotope compositions of surface and groundwaters from the Indian River Lagoon in Florida were measured to investigate the influence of submarine groundwater discharge (SGD) on these parameters in coastal waters. The Nd flux of the terrestrial component of SGD is around 0.7±0.03 μmol Nd/day per m of shoreline across the nearshore seepage face of the subterranean estuary. This translates to a terrestrial SGD Nd flux of 4±0.2 mmol/day for the entire 5,880 m long shoreline of the studied portion of the lagoon. The Nd flux from bioirrigation across the nearshore seepage face is 1±0.05 μmol Nd/day per m of shoreline, or 6±0.3 mmol/day for the entire shoreline. The combination of these two SGD fluxes is the same as the local, effective river water flux of Nd to the lagoon of 12.7±5.3 mmol/day. Using a similar approach, the marine-sourced SGD flux of Nd is 31.4±1.6 μmol Nd/day per m of shoreline, or 184±9.2 mmol/day for the investigated portion of the lagoon, which is 45 times higher than the terrestrial SGD Nd flux. Terrestrial-sourced SGD has an εNd(0) value of −5±0.42, which is similar to carbonate rocks (i.e., Ocala Limestone) from the Upper Floridan Aquifer (−5.6), but more radiogenic than the recirculated marine SGD, for which εNd(0) is −7±0.24. Marine SGD has a Nd isotope composition that is identical to the εNd(0) of Fe(III) oxide/oxyhydroxide coated sands of the surficial aquifer (−7.15±0.24 and −6.98±0.36). These secondary Fe(III) oxides/oxyhydroxides formed during subaerial weathering when sea level was substantially lower during the last glacial maximum. Subsequent flooding of these surficial sands by rising sea level followed by reductive dissolution of the Fe(III) oxide/oxyhydroxide coatings can explain the Nd isotope composition of the marine SGD component. Surficial waters of the Indian River Lagoon have an εNd(0) of −6.47±0.32, and are a mixture of terrestrial and marine SGD components, as well as the local rivers (−8.63 and −8.14). Nonetheless, the chief Nd source is marine SGD that has reacted with Fe(III) oxide/oxyhydroxide coatings on the surficial aquifer sands of the subterranean estuary.

Submarine Groundwater Discharge From Non-Tidal Coastal Peatlands Along the Baltic Sea

Submarine groundwater discharge (SGD) is an important pathway for water and materials within the land-ocean transition zone that can impact coastal environments and marine life. Although research from sandy shorelines has rapidly advanced in recent years, there is very little understanding of coastal areas characterized by a low hydraulic conductivity, such as carbon-rich coastal peatlands. The objective of this study was to determine the magnitude and location of terrestrial SGD to be expected from a non-tidal low-lying coastal peatland located along the Baltic Sea and to understand the controlling factors using numerical modeling. We employed the HYDRUS-2D modeling package to simulate water movement under steady-state conditions in a transect that extends from the dune dike-separated rewetted fen to the shallow sea. Soil physical properties, hydraulic gradients, geological stratifications, and topography were varied to depict the range of properties encountered in coastal peatlands. Our results show that terrestrial SGD occurs at the study site at a flux of 0.080 m2 d−1, with seepage rates of 1.05 cm d−1 (upper discharge region) and 0.16 cm d−1 (lower discharge region above submerged peat layer). These calculated seepage rates compare to observations from other wetland environments and SGD sites in the Baltic Sea. The groundwater originates mainly from the dune dike—recharged by precipitation and infiltration from ponded peatland surface water—and to a lesser extent from the sand aquifer. The scenario simulations yielded a range of potential SGD fluxes of 0.008–0.293 m2 d−1. They revealed that the location of terrestrial SGD is determined by the barrier function of the peat layer extending under the sea. However, it has little impact on volume flux as most SGD occurs near the shoreline. Magnitude of SGD is mainly driven by hydraulic gradient and the hydraulic conductivity of peat and beach/dune sands. Anisotropy in the horizontal direction, aquifer and peat thickness, and peatland elevation have little impacts on SGD. We conclude that SGD is most probable from coastal peatlands with high water levels, large Ks and/or a dune dike or belt, which could be an essential source for carbon and other materials via the SGD pathway.

Investigation of Submarine Groundwater Discharge using Thermal Satellite and Radon mapping along the East Coast of the Tamil Nadu and Pondicherry Region, India

Submarine groundwater discharge (SGD) demarcated as a significant component of hydrological cycle found to discharge greater volumes of terrestrial fresh and recirculated seawater to the ocean associated with chemical constituents (nutrients, metals, and organic compounds) aided by downward hydraulic gradient and sediment-water exchange. Delineating SGD is of primal significance due to the transport of nutrients and contaminants due to domestic, industrial, and agricultural practices that influence the coastal water quality, ecosystems, and geochemical cycles. An attempt has been made to demarcate the SGD using thermal infrared images and radon-222 (222Rn) isotope. Thermal infrared images processed from LANDSAT-8 data suggest prominent freshwater fluxes with higher temperature anomalies noted in Cuddalore and Nagapattinam districts, and lower temperature noted along northern and southern parts of the study area suggest saline/recirculated discharge. Groundwater samples were collected along the coastal regions to analyze Radon and Physico-chemical constituents. Radon in groundwater ranges between 127.39 Bq m-3 and 2643.41 Bq m-3 with an average of 767.80 Bq m-3. Calculated SGD fluxes range between -1.0 to 26.5 with an average of 10.32 m day-1. Comparison of the thermal infrared image with physio-chemical parameters and Radon suggest fresh, terrestrial SGD fluxes confined to the central parts of the study area and lower fluxes observed along with the northern and southern parts of the study area advocate impact due to seawater intrusion and recirculated seawater influence.

Submarine groundwater discharge driven nitrogen fluxes to Long Island Sound, NY: Terrestrial vs. marine sources

Bottom-waters in Smithtown Bay (Long Island Sound, NY) are subject to hypoxic conditions every summer despite limited nutrient inputs from waste-water and riverine sources, while modeling estimates of groundwater inputs are thought to be insignificant. Terrestrial and marine fluxes of submarine groundwater discharge (SGD) were quantified to Smithtown Bay using mass balances of 222Rn, 224Ra, 226Ra and 228Ra during the spring and summer of 2014/2015, in order to track this seasonal transition period. Intertidal pore waters from a coastal bluff (terrestrial SGD) and from a barrier beach (marine SGD) displayed substantial differences in N concentrations and sources, traced using a multi-isotope approach (222Rn, Ra, δ15N-NO3−, δ18O-NO3−). NO3− in terrestrial SGD did not display any seasonality and was derived from residential septic systems and fertilizer. Marine SGD N concentrations varied month-to-month because of mixing between oxic seawater and hypoxic saline pore waters; N concentrations were greatest during the summer, when NO3− was derived from the remineralization of organic matter. Short-lived 222Rn and 224Ra SGD fluxes were used to determine remineralized N loads along tidal recirculation flow paths, while long-lived 228Ra was used to trace inputs of anthropogenic N in terrestrial SGD. 228Ra-derived terrestrial N load estimates were between 20 and 55% lower than 224Ra-derived estimates (excluding spring 2014); 228Ra may be a more appropriate tracer of terrestrial SGD N loads. Terrestrial SGD NO3− (derived from 228Ra) to Smithtown Bay varied from (1.40–12.8)∗106 molN y−1, with comparable marine SGD NO3− fluxes of (1.70–6.79)∗106 molN y−1 derived from 222Rn and 224Ra. Remineralized N loads were greater during the summer compared with spring, and these may be an important driver toward the onset of seasonal hypoxic conditions in Smithtown Bay and western Long Island Sound. Seawater recirculation through the coastal aquifer can rival the N load from terrestrial SGD from a heavily polluted aquifer.

Terrestrial groundwater and nutrient discharge along the 240-km-long Aquitanian coast

We collected samples from sea water, runnel water, beach pore waters, water from the unconfined surficial aquifer discharging at the beach face, groundwater, and rainwater from the Aquitanian coast in order to determine the flux of dissolved inorganic nitrogen (DIN), phosphorus and silica from terrestrial submarine groundwater discharge (SGD). The flux of fresh groundwater was obtained from a water balance calculation based on precipitation and evapotranspiration and assessment of the coastal watershed from hydrograph separation. Waters with intermediate salinities between sea water and freshwaters are found all along the 240-km-long coast, indicating that SGD is ubiquitous. The estimated fresh water flux is 2.25m3d−1m−1 longshore. Terrestrial SGD provides a DIN flux of 9·106mol each year to the adjacent coastal zone. This flux is about four times lower than the release of DIN due to tidally driven saline SGD. The freshwater DIN flux is low because the upland land use consists almost exclusively of pine forest. Dissolved organic nitrogen represents more than 60% of the total dissolved nitrogen flux. Dissolved iron, phosphorus and silica have much higher concentrations in the anoxic forest aquifer than in the fresh-water end-member of the subterranean estuary sampled in the upper beach aquifer. This suggests that the salinity gradient of the estuary does not correspond to a redox gradient. The redox front between anoxic groundwater and fresh oxic waters occurs below the soil-depleted foredune/yellow dune. Anoxic P- and Si-rich waters seep directly on the beach face only in the north Gironde, where the foredunes are eroded. This study reveals the role of the sandy foredune aquifer in biogeochemical fluxes from SGD, which is to dilute and oxidize waters from the unconfined surficial upland aquifer.

Rare earth element cycling in a sandy subterranean estuary in Florida, USA

Rare earth element (REE) analyses were performed on 77 water samples and 5 sediment samples from the Indian River Lagoon system along the east coast of Florida. Groundwater samples were collected using nine multi-level piezometers installed along a shore-normal transect of the sandy subterranean estuary. Surface water samples were collected from Eau Gallie River and Crane Creek, the water column of the Indian River Lagoon above each piezometer, and the coastal Atlantic Ocean at Canova Beach. The REE concentrations of lagoon surface and subterranean estuary waters indicate that geochemical reactions within the subterranean estuary play a substantial role in the local REE cycle. The submarine groundwater discharge (SGD) flux of REEs calculated by using a modified form of the 1-dimensional vertical-flow equation is comprised of a nearshore heavy REE (HREE) enriched advective source chiefly composed of terrestrial SGD, and a light REE (LREE) and middle REE (MREE) enriched source that originates from reductive dissolution of Fe(III) oxides/oxyhydroxides in the subterranean estuary and subsequent transport by bioirrigation to the overlying lagoon. The total SGD flux of REEs reveals that this sandy subterranean estuary is a source for light and middle REEs and a sink for the heavy REEs to coastal waters. The SGD Nd flux is estimated at 7.69±1.02mmolday−1, which is roughly equivalent to the effective local river flux to the Indian River Lagoon. Although our re-evaluated SGD flux of Nd to the Indian River Lagoon is lower than an earlier estimate, it represents a substantial input to the coastal ocean and thus provides additional insights into the global oceanic Nd budget and the “Nd Paradox”.

A GIS typology to locate sites of submarine groundwater discharge

Although many researchers agree on the importance of submarine groundwater discharge (SGD), it remains difficult to locate and quantify this process. A groundwater typology was developed based on local digital elevation models and compared to concurrent radon mapping indicative of SGD in the Niantic River, CT USA. Areas of high radon activity were located near areas of high flow accumulation lending evidence to the utility of this approach to locate SGD. The benefits of this approach are three-fold: fresh terrestrial SGD may be quickly located through widely-available digital elevation models at little or no cost to the investigator; fresh SGD may also be quantified through the GIS approach by multiplying pixelated flow accumulation with the expected annual recharge; and, as these data necessarily quantify only fresh SGD, a comparison of these data with SGD as calculated by Rn activity may allow for the separation of the fresh and circulated fractions of SGD. This exercise was completed for the Niantic River where SGD as calculated by the GIS model is 1.2 m3/s, SGD as calculated by Rn activity is 0.73–5.5 m3/s, and SGD as calculated via a theoretical approach is 1.8–4.3 m3/s. Therefore fresh, terrestrial SGD accounts for 22–100% of total SGD in the Niantic River.

Use of 222Rn to trace submarine groundwater discharge in a tidal period along the coast of Xiangshan, Zhejiang, China

222Rn is one of the operative tracers for submarine groundwater discharge (SGD), which plays a significant role in the land–ocean interaction of the estuarine and coastal regions. By the distribution pattern of 222Rn in atmosphere, groundwater and surface seawater, in a full tidal period (25 h) in March 2012, SGD was estimated along the coast of Xiangshan, Zhejiang, China. 222Rn activity in Xiangshan coast was in range of 2.4 × 104–1.7 × 105 Bq/m3 with an average of 9.6 × 104 Bq/m3 for groundwater; 0.2 × 102–2.8 × 102 Bq/m3 with an average of 1.1 × 102 Bq/m3 for surface seawater. 222Rn activities in groundwater were much greater than those in surface water, suggesting that the major source of radon came from coastal groundwater discharge. Rn fluxes of atmospheric emissions, sediment, and of 226Ra in situ decay can be negligible in this study, but the tidal effects play a crucial role in Rn fluxes. Using a radon inventory equilibrium model, we estimated that the average SGD was 13.2 cm/day and the average terrestrial SGD flux was 1.8 × 108 m3/day. Furthermore, SGD may have a vital impact on the composition and structure of nutrients in seawater, and contribute to eutrophication events occurring in spring season along the coast of the East China Sea.

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

Porewater (i.e., groundwater) samples were collected from multi-level piezometers across the freshwater–saltwater seepage face within the Indian River Lagoon subterranean estuary along Florida’s (USA) Atlantic coast for analysis of the rare earth elements (REE). Surface water samples for REE analysis were also collected from the water column of the Indian River Lagoon as well as two local rivers (Eau Gallie River, Crane Creek) that flow into the lagoon within the study area. Concentrations of REEs in porewaters from the subterranean estuary are 10–100 times higher than typical seawater values (e.g., Nd ranges from 217 to 2409 pmol kg −1), with submarine groundwater discharge (SGD) at the freshwater–saltwater seepage face exhibiting the highest REE concentrations. The elevated REE concentrations for SGD at the seepage face are too high to be the result of simple, binary mixing between a seawater end-member and local terrestrial SGD. Instead, the high REE concentrations indicate that geochemical reactions occurring within the subterranean estuary contribute substantially to the REE cycle. A simple mass balance model is used to investigate the cycling of REEs in the Indian River Lagoon and its underlying subterranean estuary. Mass balance modeling reveals that the Indian River Lagoon is approximately at steady-state with respect to the REE fluxes into and out of the lagoon. However, the subterranean estuary is not at steady-state with respect to the REE fluxes. Specifically, the model suggests that the SGD Nd flux, for example, exported from the subterranean estuary to the overlying lagoon waters exceeds the combined input to the subterranean estuary from terrestrial SGD and recirculating marine SGD by, on average, ∼100 mmol day −1. The mass balance model also reveals that the subterranean estuary is a net source of light REEs (LREE) and middle REEs (MREE) to the overlying lagoon waters, but acts as a sink for the heavy REEs (HREE). Geochemical modeling and statistical analysis further suggests that this fractionation occurs, in part, due to the coupling between REE cycling and iron redox cycling within the Indian River Lagoon subterranean estuary. The net SGD flux of Nd to the Indian River Lagoon is ∼7-fold larger than the local effective river flux to these coastal waters. This previously unrecognized source of Nd to the coastal ocean could conceivably be important to the global oceanic Nd budget, and help to resolve the oceanic “Nd paradox” by accounting for a substantial fraction of the hypothesized missing Nd flux to the ocean.

Magnitudes of submarine groundwater discharge from marine and terrestrial sources: Indian River Lagoon, Florida

Magnitudes of terrestrial (fresh) and marine (saline) sources of submarine groundwater discharge (SGD) are estimated for a transect across Indian River Lagoon, Florida. Two independent techniques (seepage meters and pore water Cl− concentrations) show terrestrial SGD decreases linearly to around 22 m offshore, and these techniques, together with a model based on the width of the outflow face, indicate a cumulative discharge of between 0.02 and 0.9 m3/d per meter of shoreline. Seepage meters and models of the deficiencies in 222Rn activity in shallow sediments indicate marine SGD discharges of roughly 117 m3/d per meter of shoreline across the entire 1800‐m‐wide transect. Two surface streams nearest the transect have an average discharge of about 28 m3/d per meter of shoreline. Marine SGD is thus 4 times greater then surface water discharge and more than 2 orders of magnitude greater than terrestrial SGD. The magnitude of the terrestrial SGD is limited by the amount of regional precipitation, evaporation, recharge, and groundwater usage, while marine SGD is limited only by processes circulating marine water into and out of the sediments. The large magnitude of marine SGD means that it could be important for estuarine cycling of reactive components such as nutrients and metals with only slight modification from estuarine water compositions. The small magnitude of terrestrial SGD means that large differences from estuarine water composition would be required to affect chemical cycling.

Measurement of submarine groundwater discharge in Kahana Bay, O'ahu, Hawai'i

Submarine groundwater discharge (SGD) is neither well understood nor commonly investigated in HawaiÂ'i, but it is recognized as a potential pollution source to coastal environments. Between 1998 and 2000, this study located and quantified both total SGD and the terrestrial SGD fraction (ƒtgw) in Kahana Bay, OÂ'ahu. CTD casts were used to profile the water structure and identify potential areas of SGD impact in the bay. Lee‐type seepage meters were used to measure SGD rates and collect samples of SGD directly. Radon−222, Si, Cl−, and total alkalinity (Ta) were used as natural tracers to measure the terrestrial groundwater fraction within SGD. Nutrient concentrations were also measured to calculate total nutrient fluxes into the bay via SGD. Ninety percent of the SGD in Kahana Bay occurs in the inner bay within 1 km of the shoreline. The average total SGD flux measured was 90 × 106 L d−1, 16% of which was terrestrial groundwater. By comparison, the average annual surface runoff from Kahana River is 90.7 106 L d−1. Estimated fluxes of total dissolved phosphorus and nitrogen by SGD to the bay were 500 and 200% greater than fluxes via surface runoff, respectively. Thus, SGD in Kahana Bay has proved to be a significant source of both freshwater and total nutrient input comparable to that from the surface runoff of Kahana River.

Indications for Various Types of Operative Restorations for the Adult

Rare earth element (REE) analyses were performed on 77 water samples and 5 sediment samples from the Indian River Lagoon system along the east coast of Florida. Groundwater samples were collected using nine multi-level piezometers installed along a shore-normal transect of the sandy subterranean estuary. Surface water samples were collected from Eau Gallie River and Crane Creek, the water column of the Indian River Lagoon above each piezometer, and the coastal Atlantic Ocean at Canova Beach. The REE concentrations of lagoon surface and subterranean estuary waters indicate that geochemical reactions within the subterranean estuary play a substantial role in the local REE cycle. The submarine groundwater discharge (SGD) flux of REEs calculated by using a modified form of the 1-dimensional vertical-flow equation is comprised of a nearshore heavy REE (HREE) enriched advective source chiefly composed of terrestrial SGD, and a light REE (LREE) and middle REE (MREE) enriched source that originates from reductive dissolution of Fe(III) oxides/oxyhydroxides in the subterranean estuary and subsequent transport by bioirrigation to the overlying lagoon. The total SGD flux of REEs reveals that this sandy subterranean estuary is a source for light and middle REEs and a sink for the heavy REEs to coastal waters. The SGD Nd flux is estimated at 7.69 ± 1.02 mmol day− 1, which is roughly equivalent to the effective local river flux to the Indian River Lagoon. Although our re-evaluated SGD flux of Nd to the Indian River Lagoon is lower than an earlier estimate, it represents a substantial input to the coastal ocean and thus provides additional insights into the global oceanic Nd budget and the “Nd Paradox”.