Role Of Submarine Groundwater Discharge Research Articles

Assessing hydrologic and biogeochemical controls on pore-water dissolved inorganic carbon cycling in a subterranean estuary: A 14C and 13C mass balance approach

Submarine groundwater discharge (SGD) studies have focused on nutrient fluxes to coastal zones in many regions of the world. However the role of SGD in carbon cycling and fluxes is less well studied. We traced SGD-derived dissolved inorganic carbon (DIC) through a subterranean estuary using Δ 14C and δ 13C to evaluate how water residence times and carbon sources affected cycling and potential DIC fluxes to coastal waters. Samples were collected from the Indian River Lagoon subterranean and surface estuaries for analysis of DIC concentrations and natural ∆ 14C-DIC and δ 13C-DIC abundances. Pore-water samples were collected from multiple depths at three piezometer stations along a 250-m long transect perpendicular to shore and extending across the seepage face (about 20–22 m from shore). This approach captured the gradient from rapid, fresh groundwater discharge at the shoreline, across the groundwater–seawater subterranean mixing zone, and into purely marine offshore sediments beyond the seepage face. Freshwater and marine end-member DIC samples were also collected from a beach well and overlying lagoon surface waters beyond the seepage face, respectively. Older ∆ 14C-DIC and more enriched δ 13C-DIC values were observed at the shoreline than at the beach well (∆ 14C: − 431‰ ± 31, δ 13C: − 9.1‰ ± 1.0 versus ∆ 14C: − 108‰, δ 13C: − 16.9‰, respectively), indicating the influence of carbonate dissolution along the groundwater flow path. Pore-water DIC isotopic values indicate the groundwater plume extends from the base of the shoreline profile (115cmbsf) to the distal end of the freshwater seepage face 20–22 m offshore. The application of dual isotope, two- and three-end-member mixing models shows DIC sources change with increasing distance from shore, transitioning from a groundwater dominated system nearshore, to a mixing zone, and then into the seawater recirculation system beyond the mixing zone seepage face. Contributions from organic carbon remineralization increased with distance from shore and had varying effects on carbonate dissolution, depending on flow paths, residence times, and redox conditions. Our study demonstrates the connectivity of unconfined coastal aquifers to surface water estuaries and illustrates how hydrologic and biogeochemical complexities within the subterranean estuary can significantly contribute to the variations in sources of DIC. Furthermore, these findings highlight the importance of qualitative tracing of DIC sources together with an in-depth consideration of in situ biogeochemistry and quantitative SGD estimates. Understanding how spatial changes in hydrology and biogeochemical conditions contribute to the composition and flux of SGD-derived older DIC to coastal waters elucidates the role subterranean estuaries may ultimately play in the global carbon budget.

Large fluxes of rare earth elements through submarine groundwater discharge (SGD) from a volcanic island, Jeju, Korea

To evaluate the role of submarine groundwater discharge (SGD) as a source of rare earth elements (REEs) in the coastal ocean, we estimated the SGD associated discharge of REEs into two semi-enclosed coastal bays off a volcanic island, Jeju, Korea. The coastal brackish groundwater showed pronounced enrichments of middle REEs (MREE) relative to light REEs (LREE) and heavy REEs (HREE) when normalized against the upper continental crust (UCC), whereas seawater samples outside the bays showed a HREE enrichment pattern. The enrichment of both MREE and HREE was clearly identified in bay waters, resulting from mixing between groundwater and offshore seawater. The mass balances of REEs demonstrated that the REE fluxes through SGD were two to three orders of magnitude higher than those that occurred through the other sources, such as diffusion from bottom sediments and atmospheric dust fallout. The SGD-driven Nd flux from the entire Jeju Island during this summer was approximately 120 ± 60 mol d − 1 , which is comparable to the Nd fluxes from major rivers (i.e., Mississippi River). Our results imply that highly permeable oceanic islands are particularly important for REE fluxes to the ocean.

The role of submarine groundwater discharge (SGD) in nutrient budgets of Gamak Bay, a shellfish farming bay, in Korea

In order to evaluate the main source of nutrients fueling primary production in a shellfish farming bay (Gamak Bay) in the southern sea of Korea, we measured the concentrations of nutrients and radium isotopes in stream water, surface seawater, and coastal groundwater in May 2006, August 2006, and November 2007. Using a 226Ra mass balance model, submarine groundwater discharge (SGD) into the bay was estimated to be 1.2–1.8 × 10 7 m 3 d − 1 (8–12 cm d − 1 ) during the three sampling periods. SGD-driven nutrient fluxes were approximately 85–90% and 10–30% of the total input fluxes for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP), respectively. The estimated nutrient budgets show that almost all the groundwater-borne nutrients were utilized inside the bay. Thus, our study suggests that the biological production of this shellfish farming bay is controlled mainly by SGD-driven nutrients.

Biogeochemical transport in the Loxahatchee River estuary, Florida: The role of submarine groundwater discharge

The distributions of dissolved organic carbon (DOC), Ba, U, and a suite of naturally occurring radionuclides in the U/Th decay series ( 222Rn, 223,224,226,228Ra) were studied during high- and low-discharge conditions in the Loxahatchee River estuary, Florida to examine the role of submarine groundwater discharge in estuarine transport. The fresh water endmember of this still relatively pristine estuary may reflect not only river-borne constituents, but also those advected during active groundwater/surface water (hyporheic) exchange. During both discharge conditions, Ba concentrations indicated slight non-conservative mixing. Such Ba excesses could be attributed either to submarine groundwater discharge or particle desorption processes. Estuarine dissolved organic carbon concentrations were highest at salinities closest to zero. Uranium distributions were lowest in the fresh water sites and mixed mostly conservatively with an increase in salinity. Suspended particulate matter (SPM) concentrations were generally lowest (< 5 mg L − 1 ) close to zero salinity and increased several-fold (∼ 18 mg L − 1 ; low discharge) toward the seaward endmember, which may be attributed to dynamic resuspension of bottom sediments within Jupiter Inlet. Surface water-column 222Rn activities were most elevated (> 28 dpm L − 1 ) at the freshwater endmember of the estuary and appear to identify regions of the river most influenced by the discharge of fresh groundwater. Activities of four naturally occurring isotopes of Ra ( 223,224,226,228Ra) in this estuary and select adjacent shallow groundwater wells yield mean estuarine water-mass transit times of less than 1 day; these values are in close agreement to those calculated by tidal prism and tidal frequency. Submarine groundwater discharge rates to the Loxahatchee River estuary were calculated using a tidal prism approach, an excess 226Ra mass balance, and an electromagnetic seepage meter. Average SGD rates ranged from 1.0 to 3.8 × 10 5 m 3 d − 1 (20–74 L m − 2 d − 1 ), depending on river-discharge stage. Such calculated SGD estimates, which must include both a recirculated as well as fresh water component, are in close agreement with results obtained from a first-order watershed mass balance. Average submarine groundwater discharge rates yield NH 4 + and PO 4 − 3 flux estimates to the Loxahatchee River estuary that range from 62.7 to 1063.1 and 69.2 to 378.5 μmol m − 2 d − 1 , respectively, depending on river stage. SGD-derived nutrient flux rates are compared to yearly computed riverine total N and total P load estimates.

Submarine groundwater discharge: An important source of new inorganic nitrogen to coral reef ecosystems

Submarine groundwater discharge: An important source of new inorganic nitrogen to coral reef ecosystems

The role of submarine groundwater discharge in coastal biogeochemistry

The advection of fluids from sediments into the coastal ocean provides a source of nutrients, metals, carbon, and chemical tracers. Of these tracers radium isotopes have proved to be useful in quantifying fluxes of the other components and evaluating sources. Models that use radium must evaluate assumptions that are inherent in the models. The results of these studies demonstrate that submarine groundwater discharge must be considered as a significant source of nutrients, carbon, and metals to the coastal ocean.

The problem of direct groundwater discharge to the seas

The problem of groundwater discharge to the seas and oceans covers investigations of: (1) the role of submarine groundwater discharge in the world water balance; (2) the effect of groundwater on forming the water and salt balances of the seas; (3) the interrelationship between sea and groundwaters in coastal areas; (4) the effect of groundwater discharge on forming mineral deposits at sea and ocean floors; and (5) points of fresh groundwater discharge on the sea floor for water-supply purposes. The methods of investigating groundwater outflow to the seas and oceans are divided into two large groups: those based on studying coastal drainage areas and those based on studying the sea. The coastal techniques are subdivided into the hydrodynamic method, the average long-term water-balance method, the combined hydrological and hydrogeological method, and the analog-simulation method. The sea techniques include those based on studying seawater and those based on studying the sea bottom. The seawater methods incorporate investigations of the electric conductivity, the distribution of temperature, the anomalies of salt composition of seawater and the groundwater discharge studies with tracers. The sea-bottom methods include locating groundwater discharge from data on physical or chemical anomalies in bottom sediments (geothermal and geochemical techniques) and investigating structural features of the sea bottom (geophysical method, geomorphological method and observations by divers and submersibles). In conclusion, some experience in studying groundwater outflow to the seas, submarine springs investigations and results of studying groundwater discharge to the Baltic and Caspian Seas are presented.