Submarine Groundwater Discharge Estimates Research Articles

Modeling time series radon inventory and constraints on the submarine groundwater discharge mass balance of a well-mixed, highly dynamic estuary

Although often diffuse and over-shadowed by surface-water inputs, submarine groundwater discharge (SGD) represents a significant source of freshwater and solutes to many sensitive marine environments. Radon (222Rn) is an effective naturally occurring tracer of SGD due to its non-reactive nature and dominant terrestrial source. However, limited continuous measurements and poor constraints on temporal 222Rn variability in shallow-turbulent estuaries introduce large uncertainties to SGD rate estimation. We hypothesize that nonlinear regression analysis (e.g., generalized additive model [GAM]) and machine learning (ML) algorithms could explain radon trends and resolve limitations of the traditionally used mass balance for estimating SGD in shallow and vertically mixed estuaries, thus enabling preliminary insights related to wind-driven mixing losses using sporadic groundwater tracer measurements and publicly available hydroclimatic parameters. Cross-correlation functions and a generalized additive model [GAM] were employed to quantify atmospheric and hydrologic controls and predict, respectively, 222Rn inventory variability modulated by windspeed and direction. Two ML models, neural network (deep-learning neural network [DNN]) and decision tree (random forest [RF]) based models, were trained on an intermittent 24-month 222Rn (n = 10,660, 30 min interval) dataset (2019–2021) collected from a well-mixed northwestern Gulf of Mexico estuary (Corpus Christi Bay, Texas) with publicly available inputs (e.g., windspeed/direction, tide levels, water temperature, creek discharge and barometric pressure, n = 35,088). Both ML models can predict observed radon concentrations with r2 > 0.90, however, only the RF model is able to predict within the observed range of the 222Rn magnitude dataset and finds the relevant hydroclimatic parameters (e.g., streamflow and wind direction) highly responsible for the seasonal and daily variability in the inventory, respectively. The DNN model can more closely predict extremely high inventories, but it overpredicts lows, resulting in numerous negative values, which suggests that large data gaps in the groundwater tracer cause overfitting and underperformance of this model in a wind-mixed estuary. Using the GAM predicted 222Rn dependency on wind direction and speed, the adjusted mass balance reduces overestimation of wind-mixing losses and results in overall lower fluxes. This research provides new opportunities to: 1) predict radon inventories in regions with large temporal data gaps using only publicly available hydroclimatic parameters and 2) better constrain SGD estimates in windy coastal areas, allowing for insights that can be used to plan field excursions and management strategies for coastal water and solute budgets.

Seasonality in groundwater recharge in Coastal Southwestern India and its hydrological implications based on stable isotopes (δ18O, δD)

This study observes and interprets the spatio-temporal variation in the oxygen and hydrogen isotopic composition (δ18O, δD) of shallow groundwater in the Southwestern Coastal state of India, Kerala, in terms of seasonally varying rainfall, physiography, and hydrogeological settings of this region. This study identifies effective groundwater recharge sources and their relative contributions and provides first-order estimates of submarine groundwater discharge by interpreting the spatio-temporal variations in the stable isotopes of oxygen and hydrogen in groundwater, collected from 225 locations for pre-monsoon and post-monsoon seasons in conjunction with the isotopic composition of rainfall. Spatio-temporal variations in the δ18O and d-excess, with other hydrogeological data, reveals that: (1) ∼4.9 Billion Cubic Meter (BCM) of fresh water from rainfall during May to October mixes with ∼5.9 BCM of residual pre-monsoon groundwater across Kerala. (2) ∼ 11% area of Kerala is additionally recharged by the NE monsoon rainfall. (3) The annual groundwater recharge varies physiographically from ∼2.2 BCM in the lowland to ∼1.5 BCM in the midland and ∼1.2 BCM in highland regions. (4) On an annual basis, 41% of replenishable groundwater is drained as SGD into the Arabian Sea.

A New Simulation‐Optimization Framework for Estimation of Submarine Groundwater Discharge Based on Hydrodynamic Modeling and Isotopic Data

AbstractEstimation of submarine groundwater discharge (SGD) has traditionally relied on isotopic mass balance calculation, which is a form of regional box modeling. Here we propose an entirely new approach for SGD estimation by coupling a general marine hydrodynamic simulation model with a global optimization algorithm. The newly developed approach has the distinct advantage of being able to map the spatial distribution of SGD. The results of this study show that the estimated SGD exhibits a strong spatial heterogeneity. Furthermore, this study underscores the important effect of isotope sampling time on SGD estimation. Thus, the fluctuation of seawater level and a lag in isotope activity to water level changes should be considered to assure the representativeness of measured isotope activities. Overall, the simulation‐optimization framework adds a versatile and powerful new tool for more detailed SGD estimation.

Extreme precipitation events induce high fluxes of groundwater and associated nutrients to coastal ocean

Abstract. Current studies of submarine groundwater discharge (SGD) are commonly conducted under aquifer baseflow conditions, neglecting the influence of episodic events that can significantly increase the supply of nutrients and water. This limits our understanding of the social, biogeochemical, and ecological impacts of SGD. In this study, we evaluated the influence of an extreme precipitation event (EPE) on the magnitude of SGD. To do so, three seawater sampling campaigns were performed at a Mediterranean ephemeral stream-dominated basin after an EPE (∼ 90 mm in few hours) and in baseflow conditions. Results indicate that the groundwater flows after the EPE were 1 order of magnitude higher than those in baseflow conditions. The SGD induced by EPEs, which only take place a few days per year, represented up to one third of the annual discharge of groundwater and associated nutrients at the study site. This work accentuates the need to account for episodic increases in the supply of water and nutrients when aiming to provide reliable annual SGD estimates, particularly in the current context of climate change, since the occurrence of such events is expected to increase worldwide.

Application of radon (222Rn) as an environmental tracer in hydrogeological and geological investigations: An overview.

Radon (222Rn) is a colourless, odourless, inert, and radioactive noble gas (t1/2=3.8 days) that emanates from rocks and soils as a result of the alpha decay of its parent, radium (226Ra) in the decay series of uranium-238, is the focus of this study. Radon is produced in the crystal lattice of the minerals and emanates out through alpha recoil. It dissolves in water, and is also found in soil and air. Its distribution in water is more pertinent for scientific investigations. It can be measured by various methods. Certain properties of radon enable it to serve as an ideal tracer, viz., short-half life, inertness, high abundance in groundwater than surface water, preferential partitioning, sensitivity to sudden changes in subsurface conditions, non-invasiveness etc. This paper reviews the state-of-the-art techniques on the measurement of dissolved radon in water and its potential applications as a tracer and precursor in several hydrogeological and geological applications like understanding the surface water - groundwater interactions, hydrograph separation of streams, estimation of Submarine Groundwater Discharge (SGD), study of hydrodynamics and water balance of lakes, earthquake predictions, locating geological structures (faults/lineaments), geochemical explorations, NAPL contamination studies etc. Among the various applications presented, radon based approach is found to be more reliable in water resources domain than seismic precursory studies. The interpretations based on radon study in the above applications will pave the way for the improved understanding of the hydrological processes, and thus, help the planners and water managers for the sustainable development and management of water resources.

Submarine groundwater discharge enhances primary productivity in the Yellow Sea, China: Insight from the separation of fresh and recirculated components

Submarine groundwater discharge (SGD) is being increasingly recognized as a significant source of nutrient into coastal waters, and generally comprises two components: submarine fresh groundwater discharge (SFGD) and recirculated saline groundwater discharge (RSGD). The separate evaluation of SFGD and RSGD is extremely limited as compared to the conventional estimation of total SGD and associated nutrient fluxes, especially in marginal-scale regions. In this study, new high-resolution radium isotopes data in seawater and coastal groundwater enabled an estimation of SGD flux in a typical marginal sea of the Yellow Sea. By establishing 226Ra and 228Ra mass balance models, we obtained the SGD-derived radium fluxes, and then estimated the SFGD and RSGD fluxes through a two end-member model. The results showed that the total SGD flux into the Yellow Sea was equivalent to approximately 6.6 times the total freshwater discharge of surrounding rivers, and the SFGD flux accounted for only 5.2%–8.8% of the total SGD. Considering the nutrient concentrations in coastal fresh and saline groundwater, we obtained the dissolved inorganic nutrient fluxes (mmol m−2 yr−1) to be 52–353 for nitrogen (DIN), 0.21–1.4 for phosphorus (DIP), 34–226 for silicon (DSi) via SFGD, and 69–262 for DIN, 1.0–3.9 for DIP, 70–368 for DSi via RSGD, with the sum of nutrient fluxes equaling to (1.8–9.3)-fold, (1.3–5.6)-fold and (2.0–9.5)-fold of the riverine inputs. Compared to the conventional estimation of the total SGD flux, the nutrient fluxes derived from the separation of SFGD and RSGD were (1.6–2.1), (1.6–1.8) and (4.0–4.9) times lower for DIN, DIP and DSi, respectively, indicating that the estimates by separating SFGD and RSGD could be conservative and representative results of the Yellow Sea. Furthermore, we suggested that SGD played an important role in nutrient sources among all the traditional nutrient inputs sources, providing 15%–48%, 33%–68% and 14%–43% of the total DIN, DIP and DSi input fluxes into the Yellow Sea, and the high nutrient stoichiometric ratios (i.e., DIN/DIP) in SGD probably contributed to the increasing ratios in the Yellow Sea. In addition delivering large amounts of nutrient into the Yellow Sea, SGD would create primary productivity of 10–49, 1.6–6.8 and 8.8–42 g C m−2 yr−1 based on N, P and Si, which were equivalent to 5.2%–27%, 0.9%–3.7% and 4.7%–23% of the total primary productivity, respectively. In particular, the SFGD-derived DIN flux can be converted to primary productivity of 4.2–28 g C m−2 yr−1 thus demonstrating the disproportionately large role of SFGD in ecological environment of the Yellow Sea relative to its flux. Therefore, we conclude that SGD, particularly SFGD, plays an important role as a nutrient source for the Yellow Sea, and not only affects nutrient budgets and structures but also enhances the primary productivity.

Rethinking tracer-based (Ra, Rn, salinity) approaches to estimate point-source submarine groundwater discharge (SGD) into coastal systems

Submarine groundwater discharge (SGD) has been shown to potentially be a more important source of solutes to the global ocean than river flow. As a result, the accuracy of predictions of the magnitude and seasonal variability of SGD to the sea is critical for the management of coastal marine ecosystems. For a model site (Kinvarra Bay, Ireland) dominated by point source SGD inputs, we show that the difference between estimates provided by different marine tracer-based methods can be large, depending on the way water residence time within the system and water exchange across its boundaries are described. For example, results from tidal-prism and freshwater fraction models, the most widely used approaches to quantify freshwater fluxes into marine areas of restricted exchange, vary by one order of magnitude depending on the way the water exchange across the outer system boundaries is considered and whether Lagrangian or Eulerian data is used. We show that SGD estimates based on single point time series (Eulerian) outside of a retention area can lead to consistently lower estimates than when using transect data (Langrangian). Moreover, the outcome of radon (222Rn) based SGD estimates is dependent on the way the effect of water age variability on radon decay and degassing is accounted for. We provide a solution to this conceptual problem by developing a correction to 222Rn budgetary approaches to estimate SGD, based on the spatial variation of water ages estimated from radium isotopes (223Ra, 224Ra) in solution. We use new data to compare the different sets of approaches while examining the validity of their underlying assumptions to verify internal consistency and use the substantial range of published data on Kinvarra bay to assess the quality of results produced with the new method.

Submarine groundwater discharge drives coastal water quality and nutrient budgets at small and large scales

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.

Radioactive and stable isotope measurements reveal saline submarine groundwater discharge in a semiarid estuary

There is little information on submarine groundwater discharge (SGD) in hypersaline estuaries in semi-arid climates where SGD may be the dominant nutrient source. Here, we assess the spatial and temporal variability of SGD in the hypersaline Baffin Bay (Texas) using radon [222Rn], radium [226Ra, 224Ra, and 223Ra], and water isotopes [δ18O and δD]. Continuous electrical resistivity surveys revealed potential SGD at nearshore serpulid reefs. High spatial and temporal resolution radon measurements revealed slightly higher SGD inputs at the shoreline near course-grained sediments and relic serpulid reefs. Mass balance models of 222Rn and 226Ra produced equal SGD estimates within the range of uncertainty, while 223Ra yielded substantially higher SGD rates. Baywide SGD rates from 226Ra, 223Ra, and 222Rn ranged from 1.6 ± 0.2 to 41.3 ± 4.1. Larger SGD rates obtained from short-lived isotopes imply higher recirculated/saline SGD rates over short time scales. Radiogenic and stable isotopes as well as resistivity indicate that saline rather than freshwater accounts for most SGD in this hypersaline estuary. Because saline porewater exchange can play a significant role in coastal biogeochemical budgets, SGD inputs should be considered in management strategies in semi-arid areas where surface inflows are almost absent.

Quantification of submarine groundwater discharge (SGD) using radon, radium tracers and nutrient inputs in Punnakayal, south coast of India

The present study focused on the estimation of submarine groundwater discharge (SGD) and the effects of nutrient fluxes due to the SGD process. The parameters of SGD such as magnitude, character, and nutrient flux in Punnakayal region of South East coast of India were evaluated using multiple tracers of groundwater inputs in 2019. It was found that the elevated values for the tracers in the study area, displayed a gradational change in the values as move from estuarine part to the offshore. Simultaneous occurrence of fresh and saline SGD is observed on the study sites. Also, indicated that the SGD fluxes ranged from 0.04 to 0.12 ​m3 ​m−2 ​d−1 ​at the estuary and 0.03–0.15 ​m3 ​m−2 ​d−1 ​at the groundwater site. A substantially increased value for 222Rn activities is distinguished in the estuary to values over 312 dpm L−1. Nutrient embellishments were generally greatest at locations with substantial meteoric elements in groundwater; however, the recirculation of saltwater through the geological formation could provide a way of transferring terrestrially-derived nutrients to the coastal zone at many places.

Radium Mass Balance Sensitivity Analysis for Submarine Groundwater Discharge Estimation in Semi-Enclosed Basins: The Case Study of Long Island Sound

Estimation of submarine groundwater discharge (SGD) to semi-enclosed basins by Ra isotope mass balance is herein assessed. We evaluate 224Ra, 226Ra and 228Ra distributions in surface and bottom waters of Long Island Sound (CT-NY, USA) collected during spring 2009 and summer 2010. Surface water and bottom water Ra activities display an apparent seasonality, with greater activities during the summer. Long-lived Ra isotope mass balances are highly sensitive to boundary fluxes (water flux and Ra activity). Variation (50%) in the 224Ra, 226Ra and 228Ra offshore seawater activity results in a 63 – 74% change in the basin-wide 226Ra SGD flux and a 58 – 60% change in the 228Ra SGD flux, but only a 4 – 9% change in the 224Ra SGD flux. This highlights the need to accurately constrain long-lived Ra activities in the inflowing and outflowing water, as well as water fluxes across boundaries. Short-lived Ra isotope mass balances are sensitive to internal Ra fluxes, including desorption from resuspended particles and inputs from sediment diffusion and bioturbation. A 50% increase in the sediment diffusive flux of 224Ra, 226Ra and 228Ra results in a ~30% decrease in the 224Ra SGD flux, but only a ~6 – 10% decrease in the 226Ra and 228Ra SGD flux. When boundary mixing is uncertain, 224Ra is the preferred tracer of SGD if sediment contributions are adequately constrained. When boundary mixing is well-constrained, 226Ra and 228Ra are the preferred tracers of SGD, as sediment contributions become less important. A three-dimensional numerical model is used to constrain boundary mixing in Long Island Sound, with mean SGD fluxes of 1.2 ± 0.9 *1013 L y-1 during spring 2009 and 3.3 ± 0.7 *1013 L y-1 during summer 2010. The SGD flux to Long Island Sound during summer 2010 was one order of magnitude greater than the freshwater inflow from the Connecticut River. The maximum marine SGD-driven N flux is 14 ± 11 *108 mol N y-1 and rivals the N load of the Connecticut River.

Global Research Trends and Hotspots on Submarine Groundwater Discharge (SGD): A Bibliometric Analysis.

Submarine groundwater discharge (SGD), a major component of the hydrological cycle, has significant impacts on the sustainable development of the marine environment. This study aimed to examine the literature characteristics and research hotspots of SGD based on Web of Science’s citation database from 1998–2019. With systematic bibliometric analysis, insights were made into multiple aspects including research output, subject categories, journals, countries/territories, institutions, authors, and hotspots and research trends. Results showed that the current amount of publications on SGD has increased exponentially. The characteristics of multi-subject, active international and inter-institutional collaborations were identified. There were 11 core journals publishing the research on SGD, and the number of covered journals increased linearly from 1998. USA had distinct advantages in publication outputs and took the core position in international collaborations. At present, the research hotspots of SGD mainly include the following: dynamics process and estimation of SGD with hydrogeological methods, tracer techniques, geochemical process in subterranean estuary, and dissolved material inputs to coastal waters via SGD. Citation analysis implied much development space in carbon flux transported by SGD and the implement of head as groundwater tracer. These results provided an instructive perspective of the present situation and future research direction on SGD.

Hydrogeological processes and near shore spatial variability of radium and radon isotopes for the characterization of submarine groundwater discharge

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.

Quantitative estimation of submarine groundwater discharge using airborne thermal infrared data acquired at two different tidal heights

AbstractSubmarine groundwater discharge (SGD) plays an important role in coastal biogeochemical processes and hydrological cycles, particularly off volcanic islands in oligotrophic oceans. However, the spatial and temporal variations of SGD are still poorly understood owing to difficulty in taking rapid SGD measurements over a large scale. In this study, we used four airborne thermal infrared surveys (twice each during high and low tides) to quantify the spatiotemporal variations of SGD over the entire coast of Jeju Island, Korea. On the basis of an analytical model, we found a linear positive correlation between the thermal anomaly and squares of the groundwater discharge velocity and a negative exponential correlation between the anomaly and water depth (including tide height and bathymetry). We then derived a new equation for quantitatively estimating the SGD flow rates from thermal anomalies acquired at two different tide heights. The proposed method was validated with the measured SGD flow rates using a current meter at Gongcheonpo Beach. We believe that the method can be effectively applied for rapid estimation of SGD over coastal areas, where fresh groundwater discharge is significant, using airborne thermal infrared surveys.

Estimation of submarine groundwater discharge and its impact on the nutrient environment at Kamaiso beach, Yamagata, Japan

Estimation of submarine groundwater discharge and its impact on the nutrient environment at Kamaiso beach, Yamagata, Japan

Cross‐Shelf Transport of Submarine Groundwater Discharge Tracers: A Sensitivity Analysis

AbstractThe coastal mass balance for environmental tracers can be used to evaluate submarine groundwater discharge (SGD) and is often estimated using advective‐dispersive cross‐shelf transport models. Here we evaluate the sensitivity of different types of SGD tracers to key variables in this transport model under a wide range of environmental conditions. Salinity and long‐lived radionuclides (here represented by 226Ra; half‐life = 1,600 years) were transported by a combination of advection and dispersion under the scenarios considered, but short‐lived radionuclides (here represented by 224Ra; half‐life = 3.66 days) were mostly dispersed unless the offshore coefficient of solute dispersivity (Kx) was very low (<1 m2/s). Sensitivity to variations in key transport variables was further evaluated by reinterpreting the cross‐shelf trends in Ra activity in the semiarid Gulf St Vincent of South Australia (GSV). In GSV, the trends in most Ra isotopes (223Ra, 228Ra, and 226Ra) in seawater could be explained similarly by a large number of parameter combinations, implying these tracers would provide highly uncertain SGD estimates in this environment. However, the range of possible parameter combinations was relatively smaller for 224Ra, suggesting that SGD estimates derived from this tracer would be most reliable for GSV. In both the sensitivity analysis and the reevaluation of the GSV data, Kx was the most sensitive parameter in the transport equation. Because Kx is an empirical parameter potentially encompassing different dispersion processes in time and space (tidal currents, storms, etc.), it is likely to be tracer, time, and site specific.

The influence of submarine groundwater discharge on nearshore marine dissolved organic carbon reactivity, concentration dynamics, and offshore export

Dissolved organic carbon (DOC) is the largest pool of reduced carbon in the oceans, with a reservoir equivalent to atmospheric CO2. In nearshore marine regions, DOC sources include primary production, terrestrial DOC delivered by river discharge, and/or terrestrial and marine DOC delivered via submarine groundwater discharge (SGD). While the importance of SGD to coastal carbon cycling has been implicated, the actual influence of this process on nearshore carbon dynamics and offshore export has not been explicitly identified. This study, conducted at a predominantly marine-influenced intertidal beach-nearshore ocean system along the Santa Barbara, California coastline, aimed to address this knowledge gap. Dark, temperature-controlled laboratory incubations, radioisotopic (radon-222) SGD estimates, and an SGD-DOC mixing-reaction box model were coupled to identify the influence of pore water mixing with seawater on nearshore DOC reactivity, concentration dynamics, and offshore export. Even with a relatively low volumetric contribution, SGD pore water mixing altered nearshore DOC reactivity, and elevated the nearshore DOC concentration by 0.4–6.6 µmol L−1 over nearshore seawater residence times spanning 1–6 days. These elevated DOC concentrations were equivalent to 0.5% to 9% of the mean offshore DOC concentration for the summer months in the Santa Barbara Channel, when the coastal water column is highly thermally stratified. Despite the challenge of assessing carbon dynamics in physically and biogeochemically complex nearshore marine regions, this study demonstrates the need for future investigations to assess and account for SGD as a non-trivial component of coastal marine carbon cycles.

Lingering radioactivity at the Bikini and Enewetak Atolls

We made an assessment of the levels of radionuclides in the ocean waters, seafloor and groundwater at Bikini and Enewetak Atolls where the US conducted nuclear weapons tests in the 1940's and 50's. This included the first estimates of submarine groundwater discharge (SGD) derived from radium isotopes that can be used here to calculate radionuclide fluxes in to the lagoon waters. While there is significant variability between sites and sample types, levels of plutonium (239,240Pu) remain several orders of magnitude higher in lagoon seawater and sediments than what is found in rest of the world's oceans. In contrast, levels of cesium-137 (137Cs) while relatively elevated in brackish groundwater are only slightly higher in the lagoon water relative to North Pacific surface waters. Of special interest was the Runit dome, a nuclear waste repository created in the 1970's within the Enewetak Atoll. Low seawater ratios of 240Pu/239Pu suggest that this area is the source of about half of the Pu in the Enewetak lagoon water column, yet radium isotopes suggest that SGD from below the dome is not a significant Pu source. SGD fluxes of Pu and Cs at Bikini were also relatively low. Thus radioactivity associated with seafloor sediments remains the largest source and long term repository for radioactive contamination. Overall, Bikini and Enewetak Atolls are an ongoing source of Pu and Cs to the North Pacific, but at annual rates that are orders of magnitude smaller than delivered via close-in fallout to the same area.

Combining airborne thermal infrared images and radium isotopes to study submarine groundwater discharge along the French Mediterranean coastline

Study regionThe French Mediterranean coastline, which includes karstic springs discharging into coastal seas and coastal lagoons. Study focusWe investigated submarine groundwater discharge (SGD), an important vector for many chemical elements that may impact the quality of the coastal environment. First, we acquired airborne thermal infrared (TIR) images to detect terrestrial groundwater inputs. Then we report in situ data (salinity; temperature; radium isotopes). We use these data i) to confirm the presence of groundwater discharge and to characterize the different systems, and ii) to quantify SGD fluxes and estimate the residence time of the water bodies. New hydrological insights for the regionFew studies have been conducted on SGD along the French Mediterranean coastline. The terrestrial groundwater spring inputs in La Palme and Salses-Leucate coastal lagoons are in the range (0.04–0.11)m3s−1,≤2% of the local river inputs. In comparison, total SGD estimates to La Palme lagoon (0.56–1.7m3s−1) suggest that the recirculation of lagoon water through the sediment is two orders of magnitude greater than the terrestrial groundwater inputs. At the Calanque of Port-Miou, the terrestrial groundwater flux to the coastal seas was between 0.6 and 1.2m3s−1 in July 2009. This study demonstrates the application of airborne TIR remote sensing for detecting surficial groundwater springs, and the inability of the method to detect deeper, submerged springs.

Applications of radon and radium isotopes to determine submarine groundwater discharge and flushing times in Todos os Santos Bay, Brazil

Todos os Santos Bay (BTS) is the 2nd largest bay in Brazil and an important resource for the people of the State of Bahia. We made measurements of radon and radium in selected areas of the bay to evaluate if these tracers could provide estimates of submarine groundwater discharge (SGD) and flushing times of the Paraguaçu Estuary and BTS. We found that there were a few areas along the eastern and northeastern shorelines that displayed relatively high radon and low salinities, indicating possible sites of enhanced SGD. A time-series mooring over a tidal cycle at Marina do Bonfim showed a systematic enrichment of the short-lived radium isotopes 223Ra and 224Ra during the falling tide. Assuming that the elevated radium isotopes were related to SGD and using measured radium activities from a shallow well at the site, we estimated groundwater seepage at about 70 m3/day per unit width of shoreline. Extrapolating to an estimated total shoreline length provided a first approximation of total (fresh + saline) SGD into BTS of 300 m3/s, about 3 times the average river discharge into the bay. Just applying the shoreline lengths from areas identified with high radon and reduced salinity results in a lower SGD estimate of 20 m3/s. Flushing times of the Paraguaçu Estuary were estimated at about 3–4 days based on changing radium isotope ratios from low to high salinities. The flushing time for the entire BTS was also attempted using the same approach and resulted in a surprisingly low value of only 6–8 days. Although physical oceanographic models have proposed flushing times on the order of months, a simple tidal prism calculation provided results in the range of 4–7 days, consistent with the radium approach. Based on these initial results, we recommend a strategy for refining both SGD and flushing time estimates.