Terrestrial Groundwater Discharge Research Articles

Groundwater nutrient loading into the northern Indian River lagoon: measurements and modeling

The Indian River Lagoon System (IRLS) has been impacted by the surrounding development, leading to excessive nutrient loads that have resulted in frequent and prolonged phytoplankton blooms in the northern reaches. Our study focused on estimating terrestrial groundwater discharge (TGD) and associated nutrient loads by combining field measurements and hydrogeologic modeling at four transects: Eau Gallie (EGT), River Walk (RWT), Banana River (BRT), and Mosquito Lagoon (MLT) across the IRLS. Multiple monitoring stations were installed to collect groundwater and surface water levels, salinity, and nutrient concentrations during 2014-2015. Samples were analyzed for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). Numerical modeling was accomplished using SEAWAT to simulate TGD rates, whereas nutrient loads were calculated by multiplying simulated TGD by measured concentrations. TGD rates and nutrient loads were also estimated specifically for the “near-shore zone” along each transect. The effect of recharge from underlying Hawthorn Formation was also evaluated by incorporating estimated recharge rates into the models. Porewater and lagoon water samples showed that ammonium predominated over (NO2+NO3) and PO4 at all sites, resulting in DIN/DIP ratio surpassing the Redfield ratio. Low nitrite/nitrate, coupled with elevated ammonium concentrations at RWT, BRT, and MLT, may be attributed to biogeochemical transformations catalyzed by mangroves and wetlands. Simulated TGD showed mild temporal but significant spatial variation, especially between EGT and RWT compared to BRT and MLT. The highest average TGD of 0.73 and 0.77 m3/d.m occurred at RWT and EGT, respectively, whereas the lowest rates were predicted at BRT and MLT. The highest estimated average DIN loads of 507 and 428 g/yr.m were received at EGT and RWT, respectively, whereas MLT and BRT exhibited lower loads. The DIP loads were remarkably lower than the DIN loads and were significantly different in space and time between sites. Elevated DIN combined with reduced DIP resulted in DIN/DIP exceeding the Redfield ratio, thereby encouraging the blooming of harmful algae. Although the majority of seepage occurs through the near-shore zone, small amounts are received along the entire transect at all sites. The Hawthorn Formation does not contribute significant recharge to the aquifer at the transect locations.

Tracing terrestrial groundwater discharge and porewater exchange derived dissolved carbon export in a tropical estuary using multiple isotopes

Coastal terrestrial groundwater discharge and porewater exchange are two different submarine groundwater discharge (SGD) pathways. The contribution of both pathways in dissolved carbon export to the coastal water remains almost unknown. Here, we investigated terrestrial groundwater discharge, porewater exchange, and dissolved carbon export in a tropical estuary (Moyangjiang, China) using stable water (δ2H and δ18O) and radioactive (222Rn, 224Ra, and 228Th) isotopes. The average terrestrial groundwater discharge was estimated to be (3.2±0.2) × 106 m3 d−1 by a three end-member mixing model of δ18O and salinity. The porewater exchange was estimated to be 11.3±0.14 cm d−1 by the 224Ra/228Th disequilibrium method. Accordingly, dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) fluxes from terrestrial groundwater discharge were estimated to be (1.19±0.10) × 107 and (3.23±0.25) × 105 mol d−1, respectively. In comparison, when upscaling to the whole estuary, DIC and DOC export fluxes from porewater exchange were estimated to be (1.56±0.21) × 105 and (2.12±0.29) × 105 mol d−1, respectively. Terrestrial groundwater discharge derived DIC flux was two orders of magnitude greater than that from porewater exchange, and was three times greater than that from river discharge. Terrestrial groundwater discharge was recognized as a significant contributor to DIC export. Porewater exchange played a disproportionately important role in the delivery of DOC to the ocean, and its contribution to DOC needed to be emphasized. Overall, both pathways contributed a large amount of dissolved carbon export to the coastal water, and play an important role in delivering terrestrial carbon across land-ocean interface. Evaluating the contributions of different pathways of SGD to terrestrial solutes will help to improve the accuracy of SGD associated terrestrial solute fluxes.

Arsenic enriched groundwater discharge to a tropical ocean: Understanding controls and processes

The role of submarine groundwater discharge (SGD) in transporting terrestrial-sourced arsenic (As) to the global oceans is not well documented. In the present study, executed on a coast adjoining the extensive groundwater As-contaminated Ganges river delta, we hypothesize that As-enriched groundwater discharges to the adjoining Bay of Bengal (BoB) through SGD flow paths. We conducted high-resolution, field-based investigations and thermodynamic modeling to understand the SGD-sourced As discharge and geochemical cycling of As and other redox-sensitive solutes along the discharge path under varying redox conditions and water sediment interactions. The As distribution and other solutes were measured in a series of multi-depth observation wells and sediment cores, extending from the high tide line (HTL) to 100 m toward the sea, for pre- and post-monsoon seasons. Results reveal the presence of a plume carrying up to 30 μg/L dissolved load of As toward the sea. Arsenic is associated with a plume of Fe and exhibits similar shore-perpendicular variability. Arsenic distribution and transport is controlled by the Fe-Mn redox cycle and influenced by terrestrial groundwater discharge. Field-observations and geochemical modeling demonstrate that Fe-hydroxide precipitates in the subterranean estuary and acts as an interim sink for As , which is eventually mobilized on alteration of geochemical conditions with the season. Fluctuating plume size can be attributed to seasonal variation in fresh groundwater input to the site. Estimates indicate up to 55mg/m2/d As is released to BoB from the site. Based on physicochemical observations this study demonstrates the yet to be studied SGD derived As cycles and the role of SGD dynamics in controlling the fate of redox-sensitive contaminants and their discharge into global oceans.

Effect of Variable-Density and Constant-Density Representations of Flow on Simulating Terrestrial Groundwater Discharge into a Coastal Lagoon

Effect of Variable-Density and Constant-Density Representations of Flow on Simulating Terrestrial Groundwater Discharge into a Coastal Lagoon

Submarine groundwater discharge as a significant export of dissolved inorganic carbon from a mangrove tidal creek to Qinglan Bay (Hainan Island, China)

Submarine groundwater discharge (SGD) is an important pathway for the transport of dissolved chemical substances to the ocean. In this study, we performed a time-series measurement of 222Rn, the concentrations and isotopic compositions of the different carbon species isotopes (particulate organic carbon, POC, dissolved organic carbon, DOC, dissolved inorganic carbon, DIC), physico-chemical parameters during a tidal cycle in a mangrove tidal creek without upstream freshwater inputs in northeastern Hainan Island, China. Our results show mangrove detritus provide the major sources of POC and DOC for the mangrove tidal creek water. Organic matter degradation and terrestrial groundwater discharge are two main processes of providing DIC for the pore water of the mangrove tidal flat sediments. A quantitative evaluation indicates that the amount of DIC production from the organic matter degradation process via sulfate reduction accounts for 59.8% of the total pore water DIC addition, while the proportion of DIC derived from terrestrial groundwater discharge is 40.2%. Using 222Rn mass balance model, we quantify the rates of SGD and its associated dissolved carbon: the average SGD rate in the mangrove tidal creek is approximately 165 (σ=68.73) cm d−1, and the SGD-derived DIC and DOC are approximately 107 and 2.18 g C m−2 d−1, respectively. DIC is the dominant form of dissolved carbon export (~98%) from mangroves tidal creek to the adjacent Qinglan Bay. The high DIC loading via SGD potentially represents the mangrove missing carbon, and has significant implications for the coastal ocean carbon budget.

Nutrient Fluxes Associated With Submarine Groundwater Discharge From Karstic Coastal Aquifers (Côte Bleue, French Mediterranean Coastline)

Determination of submarine groundwater discharge (SGD) from karstic coastal aquifers is important to constrain hydrological and biogeochemical cycles. However, SGD quantification using commonly employed geochemical methods can be difficult to constrain under the presence of large riverine inputs, and is further complicated by the determination of the karstic groundwater endmember. Here, we investigated a coastal region where groundwater discharges from a karstic aquifer system using airborne thermal infrared mapping and geochemical sampling conducted along offshore transects. We report radium data (223Ra, 224Ra, 228Ra) that we used to derive fluxes (water, nutrients) associated with terrestrial groundwater discharge and/or seawater circulation through the nearshore permeable sediments and coastal aquifer. Field work was conducted at different periods of the year to study the temporal variability of the chemical fluxes. Offshore transects of 223Ra and 224Ra were used to derive horizontal eddy diffusivity coefficients that were subsequently combined with surface water nutrient gradients (NO2- + NO3-, DSi) to determine the net nutrient flux from SGD. The estimated DSi and (NO2- + NO3-) fluxes are 6.2 ± 5.0 *103 and 4.0 ± 2.0 *103 mol d-1 per kilometer of coastline, respectively. We attempted to further constrain these SGD fluxes by combining horizontal eddy diffusivity and 228Ra gradients. SGD endmember selection in this area (terrestrial groundwater discharge versus porewater) adds further uncertainty to the flux calculation and thus prevented us from calculating a reliable flux. Additionally, the relatively long half-life of 228Ra (5.75 y) makes it sensitive to specific circulation patterns in this coastal region, including sporadic intrusions of Rhone river waters that impact both the 228Ra and nutrient surface water distributions. We show that SGD nutrient fluxes locally reach up to 20 times the nutrient fluxes from a small river (Huveaune River). On a regional scale, DSi fluxes driven by SGD vary between 0.1 and 1.4 % of the DSi inputs of the Rhone River, while the (NO2- + NO3-) fluxes driven by SGD on this 22 km long coastline are between 0.1 – 0.3 % of the Rhone River inputs, the largest river that discharges into the Mediterranean Sea.

DSi as a Tracer for Submarine Groundwater Discharge

Submarine groundwater discharge (SGD) is an important source of nutrients and metals to the coastal ocean, affects coastal ecosystems, and is gaining recognition as a relevant water resource. SGD is usually quantified using geochemical tracers such as radon or radium. However, a few studies have also used dissolved silicon (DSi) as a tracer for SGD, as DSi is usually enriched in groundwater when compared to surface waters. In this study, we discuss the potential of DSi as a tracer in SGD studies based on a literature review and two case studies from contrasting environments. In the first case study, DSi is used to calculate SGD fluxes in a tropical volcanic-carbonate karstic region (southern Java, Indonesia), where SGD is dominated by terrestrial groundwater discharge. The second case study discusses DSi as a tracer for marine SGD (i.e. recirculated seawater) in the tidal flat area of Spiekeroog (southern North Sea), where SGD is dominantly driven by tidal pumping through beach sands. Our results indicate that DSi is a useful tracer for SGD in various lithologies (e.g. karstic, volcanic, complex) to quantify terrestrial and marine SGD fluxes. DSi can also be used to trace groundwater transport processes in the sediment and the coastal aquifer. Care has to be taken that all sources and sinks of DSi are known and can be quantified or neglected. One major limitation is that DSi is used by siliceous phytoplankton and therefore limits its applicability to times of the year when primary production of siliceous phytoplankton is low. In general, DSi is a powerful tracer for SGD in many environments. We recommend that DSi should be used to complement other conventionally used tracers, such as radon or radium, to help account for their own shortcomings.

Significant chemical fluxes from natural terrestrial groundwater rival anthropogenic and fluvial input in a large-river deltaic estuary

The shores of the Pearl River estuary are home to 35 million people. Their wastes are discharged into the large river delta-front estuary (LDE), one of the most highly polluted systems in the world. Here we construct a radium reactive transport model to estimate the terrestrial groundwater discharge (TGD) into the highly urbanized Pearl River LDE. We find the TGD comprises only approximately 0.9% in term of water discharge compared to the river discharge. The TGD in the Pearl River LDE delivers significant chemical fluxes to the coast, which are comparable to the fluvial loadings from Pearl River and other world major rivers. Of particular importance is the flux of ammonium because of its considerable role in Pearl River estuary eutrophication and hypoxia. Unlike the ammonium in many other aquifers, the ammonium in the Pearl River aquifer system is natural and originated from organic matter remineralization by sulfate reduction in the extremely reducing environment. The TGD derived NH4+ is as much as 5% of the upstream Pearl River fluvial loading and 42% of the anthropogenic inputs. This high groundwater NH4+ flux may greatly intensify the eutrophication, shift the trophic states, and lead to alarming hypoxia within the affected ecosystems in the Pearl River LDE. The large TGD derived chemical fluxes will lead to deterioration of water and will potentially affect human health.

Groundwater-driven nutrient inputs to coastal lagoons: The relevance of lagoon water recirculation as a conveyor of dissolved nutrients

Evaluating the sources of nutrient inputs to coastal lagoons is required to understand the functioning of these ecosystems and their vulnerability to eutrophication. Whereas terrestrial groundwater processes are increasingly recognized as relevant sources of nutrients to coastal lagoons, there are still limited studies evaluating separately nutrient fluxes driven by terrestrial groundwater discharge and lagoon water recirculation through sediments. In this study, we assess the relative significance of these sources in conveying dissolved inorganic nutrients (NO3−, NH4+ and PO43−) to a coastal lagoon (La Palme lagoon; France, Mediterranean Sea) using concurrent water and radon mass balances. The recirculation of lagoon water through sediments represents a source of NH4+ (1900–5500 mol d−1) and PO43− (22–71 mol d−1), but acts as a sink of NO3−. Estimated karstic groundwater-driven inputs of NO3−, NH4+ and PO43− to the lagoon are on the order of 200–1200, 1–12 and 1.5–8.7 mol d−1, respectively. A comparison between the main nutrient sources to the lagoon (karstic groundwater, recirculation, diffusion from sediments, inputs from a sewage treatment plant and atmospheric deposition) reveals that the recirculation of lagoon water through sediments is the main source of both dissolved inorganic nitrogen (DIN) and phosphorous (DIP) to La Palme lagoon. These results are in contrast with several studies conducted in systems influenced by terrestrial groundwater inputs, where groundwater is often assumed to be the main pathway for dissolved inorganic nutrient loads. This work highlights the important role of lagoon water recirculation through permeable sediments as a major conveyor of dissolved nutrients to coastal lagoons and, thus, the need for a sound understanding of the recirculation-driven nutrient fluxes and their ecological implications to sustainably manage lagoonal ecosystems.

Effect of Low Energy Waves on the Accumulation and Transport of Fecal Indicator Bacteria in Sand and Pore Water at Freshwater Beaches

Elevated fecal indicator bacteria (FIB) in beach sand and pore water represent an important nonpoint source of contamination to surface waters. This study examines the physical processes governing the accumulation and distribution of FIB in a beach aquifer. Field data indicate E. coli and enterococci can be transported 1 and 2 m, respectively, below the water table. Data were used to calibrate a numerical model whereby FIB are delivered to a beach aquifer by wave-induced infiltration across the beach face. Simulations indicate FIB rapidly accumulate in a beach aquifer with FIB primarily associated with sand rather than freely residing in the pore water. Simulated transport of E. coli in a beach aquifer is complex and does not correlate with conservative tracer transport. Beaches with higher wave-induced infiltration rate and vertical infiltration velocity (i.e., beaches with higher beach slope and wave height, and lower terrestrial groundwater discharge) had greater E. coli accumulation and E. coli was transported deeper below the beach face. For certain beach conditions, the amount of FIB accumulated in sand over 5-6 days was found to be sufficient to trigger a beach advisory if eroded to surface water.

Application of 224Ra and 222Rn for evaluating seawater residence times in a tidal subterranean estuary

The length of time in which seawater circulates through the beach face and mixes with terrestrial groundwater will have a profound influence on the geochemical cycling of nutrient deliveries to surface waters through submarine groundwater discharge. The goal of this work is to utilize pore water measurements of 222Rn and Ra isotopes to evaluate the time-scale of tidal seawater circulation in a coastal bluff and a barrier beach subterranean estuary (STE) on Long Island, NY. Intertidal pore water profiles sampled monthly between 2014 and 2015 revealed distinct geochemical differences between the two sites that were driven by the primary hydrogeologic forcing mechanisms. The coastal bluff site was dominated by a large hydraulic gradient that drove terrestrial groundwater discharge, while the barrier beach site had saline pore waters and was physically controlled by tidal forcing mechanisms. These mechanisms resulted in notably different pore water radionuclide (222Rn, 223,224,226,228Ra) and trace metal (Fe, Mn, Ba) distributions in the STE. A one-dimensional advective transport model was applied to estimate the residence time of circulated seawater within the STE using measured radionuclide activities. There are inherent uncertainties within the model, set by the activity of deep groundwater that has mixed with the circulated seawater, the initial activity of seawater that infiltrated the coastal aquifer and the radionuclide production rates. Residence times generally increased with depth from 0.1 to ≥15d and were greater in the low tide well of the barrier beach (4.9±3.4d) compared to the coastal bluff (1.1±0.4d). Residence times were fairly constant over time within the coastal bluff STE but varied seasonally in the barrier beach STE, suggesting that the barrier beach may have not been in “steady-state.” Variable pore water residence times within the STE may have a profound effect on the cycling of carbon, nutrient and trace metal fluxes to the coastal ocean.

Submarine groundwater discharge from the South Australian Limestone Coast region estimated using radium and salinity

The Tertiary Limestone Aquifer (TLA) is one of the major regional hydrogeological systems of southern Australia. Submarine groundwater discharge (SGD) of freshwater from the TLA occurs through spring creeks, beach springs and diffusively through beach sands, but the magnitude of the total flux is not known. Here, a range of potential environmental tracers (including temperature, salinity, 222Rn, 223Ra, 224Ra, 226Ra, 228Ra, and 4He) were measured in potential sources of SGD and in seawater along a 45 km transect off the coastline to evaluate SGD from the TLA. Whilst most tracers had a distinct signature in the sources of water to the coastline, salinity and the radium quartet had the most distinct SGD signal in seawater. A one-dimensional advection-dispersion model was used to estimate the terrestrial freshwater component of SGD (Qfw) using salinity and the recirculated seawater component (Qrsw) using radium activity in seawater. Qfw was estimated at 1.2–4.6 m3 s−1, similar in magnitude to previously measured spring creek discharge (∼3 m3 s−1) for the area. This suggests that other terrestrial groundwater discharge processes (beach springs and diffuse discharge through beach sands) were no more than 50% of spring creek discharge. The largest component of total SGD was Qrsw, estimated at 500–1000 m3 s−1 and possibly greater. The potential for wave, storm, or buoyancy-driven porewater displacement from the seafloor could explain the large recirculation flux for this section of the Southern Ocean Continental Shelf.

Identification and characterization of the encrusting materials in a coastal liquefied petroleum gas storage cavern

An examination was carried out of the encrusting materials on the seepage removal pumps in a bottom sump of an undersea liquefied petroleum gas (LPG) cavern. The studied cavern, constructed at 8 km off the western coast of Korea, facing the Yellow Sea, is at 130–150 m below the seabed. Since the first filling of the LPG into the cavern, it has suffered from the unexpected problem of a thick encrustation of unknown materials on the seepage removal pumps of the cavern. The XRD and XRF analyses revealed that the encrustation materials were mainly iron (hydro)oxides and carbonate mineral (aragonite). Based on the geological setting below the cavern site, it was inferred that iron oxides found in the Pleistocene sandy sediment and the Precambrian gneiss might be supplying ferrous iron to the cavern waters under an anaerobic condition, evidenced by low dissolved oxygen and negative redox potential in the cavern water. A significant change in the hydraulic condition near the pump intake and mixing the cavern water with oxic waters supplied through re-circulated seawater and terrestrial groundwater discharge, would precipitate the dissolved iron. Precipitation of the carbonate mineral is thought to have occurred due to over-saturation of calcium and bicarbonate, which may have resulted from the dissolution of cement grouting materials used during the undersea cavern construction and submarine groundwater discharge (SGD). This study reports the iron and carbonate precipitation in the man-made undersea cavern, which is affected by the surrounding hydrogeological condition and the SGD.

Tidally regulated chemical fluxes across the sediment—water interface in Elkhorn Slough, California: Evidence from a coupled geochemical and hydrodynamic approach

Measurements of four Ra isotopes, Ba, Mn, U, and dissolved nutrients were used in concert with hydrodynamic modeling to reveal a large, tidally regulated sediment‐surface‐water exchange within Elkhorn Slough, an estuary in Monterey Bay, California. Hydrologic and oceanographic data from in situ sensors and geophysical surveys of estuarine bottom sediments were integrated with the chemical measurements to develop Ra flux boundary conditions for a three‐dimensional hydrodynamic transport model of the estuary. Sediment and surface‐water exchange is the dominant source of Ra isotopes within this system. This process occurs without significant fresh terrestrial groundwater discharge. A number of known solute transport and pore‐water flushing mechanisms can explain the 224Ra input, including diffusion, evaporative transport, bioirrigation, and inundation and drainage of borrows and macropores. The 226Ra results suggest that some of these mechanisms, or others, are capable of mobilizing Ra from deeper sediments. The sediments supplying this flux may cover an area as large as the entire intertidal zone, primarily salt marsh, to a depth of 1–2 cm. In addition, the Ra delivery to the estuary appears to be temporally regulated by the tidal inundation of the salt marsh and subsequent drainage through tidal creek channels. These results illustrate the importance of the intertidal zone as a flowpath in the estuarine water cycle and the tidal phase lag that results when solute from this zone is transported into the estuary. This process is essential to understanding estuarine biogeochemical budgets of elements with significant sedimentary sources.