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Radium isotopes are powerful proxies in oceanography and hydrology. Radium mass balance models, including assessments of submarine groundwater discharge (SGD), often overlook particle scavenging (PS) as a pathway for dissolved radium removal from the world ocean. Here, we build a global ocean 226Ra mass balance model and reevaluate the potential importance of PS. We find that PS is the major 226Ra sink for the upper ocean, removing about 96% of the total input from various sources. Aside from vertical exchange with the lower ocean, SGD is the largest 226Ra source into the upper ocean. The biological pump transfers particles to the deep ocean, resulting in a major but often overlooked impact on the global 226Ra marine budget. Our findings suggest that radium mass balance models should consider PS in systems with high siliceous algae production and export fluxes and long water residence times to prevent underestimation of large‐scale SGD fluxes. Plain Language Summary: Radium is a very powerful tracer for many oceanographic processes. A common assumption in radium investigations is that incorporation into particles plays a negligible role in mass balances. Contrary to this assumption, we found that particle scavenging related to the so‐called "biological pump" is the largest sink for 226Ra in the global ocean, and that submarine groundwater discharge is the largest terrestrial input. Particle scavenging can be even more important in highly productive coastal systems with long water residence times and high particle burial efficiencies. Particle scavenging, which has been overlooked in many mass balance calculations, may be important for 226Ra and 228Ra budgets from local to global scales. Key Points: Particle scavenging is the major 226Ra sink for the upper ocean, removing about 96% of the total input from various sourcesSubmarine groundwater discharge (SGD) is the largest terrestrial 226Ra source into the upper ocean, followed by river input and sediment diffusionParticle scavenging can be important in coastal oceans with high primary production and particle burial efficiencies [ABSTRACT FROM AUTHOR]

We explored the possibility that an underground pathway, "submarine groundwater discharge" (SGD), may contribute to the observed coastal contamination from a large industrial complex on the Gulf of Thailand. Three surveys were performed to map the area for the natural groundwater tracers radon, thoron and salinity. The results from all three surveys were internally consistent showing a point source adjacent to a large pier that serves the complex. It may be that a piling, driven into the ground to support the pier, intercepted a shallow aquifer and this resulted in an underground pathway between land and sea. Some low-density sediments are enriched in radium, we suspect from fly ash from a nearby power plant. Water quality parameters showed that total petroleum hydrocarbons (TPHs) correlated strongly to nitrite, dissolved inorganic phosphate and silica, indicating a common source. Data analysis shows that diffuse seepage accounts for more discharge than the point source. • Marine pollution present adjacent to an industrial complex on the Gulf of Thailand • Groundwater discharge is a possible pathway of contamination. • Discharge detected using the natural tracers radon, thoron, and salinity • Distinct groundwater hotspot found adjacent to a pier serving the industrial estate • Data analysis suggests that diffuse groundwater flows greater than point discharge. [ABSTRACT FROM AUTHOR]

Thoron (220Rn) in water measurements can be useful for prospecting for points of submarine groundwater discharge into surface waters. With a 56 s half-life any detection of thoron must mean that one is close to a source. To improve measurement sensitivity, the influence of airflow rates on both radon and thoron measurements have been investigated using a RAD7 detector. Based on lab experiments, 2.5 L min−1 represents an optimum airflow rate for measuring both radon and thoron. Field data from a coastal zone demonstrates that use of an external air pump produced a thoron response 65% higher than a standard setup. [ABSTRACT FROM AUTHOR]

Summary: Naturally occurring 222Rn (radon; t 1/2 =3.8days) is a good natural tracer of groundwater discharge because it is conservative and typically 2–3 orders of magnitude higher in groundwater than surface waters. In addition, new technology has allowed rapid and inexpensive field measurements of radon-in-water. Results from the C-25 Canal, a man-made canal in east-central Florida thought to be dominated by groundwater inflows, display how one can quickly assess a water body for locations of groundwater inputs. Although only the eastern portion of the canal was surveyed, use of a few assumptions together with some continuous radon measurements allowed reasonable estimates of the groundwater inflows to be made. Groundwater discharge estimates of 327,000m3/day and 331,000m3/day were measured for two stations based on determining the groundwater fraction of the total stream flow. This fraction in each case was calculated by correcting radon concentrations for decay over transit times determined from concentration differences between the apparent focal point of groundwater discharge (with a concentration of 520±80dpm/L) estimated to be ∼17.7km upstream from the downstream sample locations. During the same period, an average flow of 312,000±70,000m3/day was determined from time-series measurements of radon at a fixed downstream location. Coincident current meter readings and a measured cross-section area allowed an independent assessment of the total stream discharge of 336,000m3/day. The radon-derived estimates thus indicate that >90% of the total flow is groundwater derived, consistent with the known characteristics of this waterway. [Copyright &y& Elsevier]

Abstract: Estimates of groundwater discharge into a boat basin in Donnalucata, southeastern Sicily, based on radon measurements are reported. A continuous monitor was deployed at several locations that made repeated integrated measurements of radon concentration in the seawater once per hour. We observed large differences in the concentrations (60–2500Bqm−3) and inventories (90–3200Bqm−2) of 222Rn at various locations within the boat basin. The station closest to shore had the highest concentration by far (up to over 3000Bqm−3), reflecting inputs of radon-rich groundwater into this zone. We constructed a radon mass balance by assuming that radon may be advected via groundwater discharge into all compartments of the boat basin and the mixing outputs from an inner “box” would be an input into the adjacent box. We also made allowances for atmospheric evasion losses that were occasionally very high due to high winds and huge concentration gradients across the air–sea interface. Once the radon fluxes have been estimated, we could calculate groundwater discharge by dividing these fluxes by the radon concentration of the groundwater. Should the groundwater be entering the basin by widely disseminated seepage through bottom sediments, it would be appropriate to use values of pore water derived by sediment equilibration techniques. These values (2700±200Bqm−3) are close to those from several samples from shallow wells collected near the boat basin (1700–2700Bqm−3). However, we measured higher radon levels than this in the waters of the innermost portion of the basin. We thus elected to assume that the main radon entry was either via direct spring input with activities (15100Bqm−3) similar to those measured from a natural spring on a nearby beach or a blend of spring water with surficial aquifer water. The resulting range in our calculated groundwater discharges into the boat basin is 1200 to 7400m3 day−1. These estimates are higher than those based on seepage meters (range=300–1000m3 day−1), probably because of the wide spatial variability of the spring and seep inputs. [Copyright &y& Elsevier]

Coastal marine settings are important in terms of geochemical cycles and biological productivity. Climate change is predicted to affect coastal environment via hypoxia and Submarine Groundwater Discharge (SGD). Appropriate proxies could help to better understand oxygenation history and the role of SGD in regulating hypoxia. This would also benefit prediction of potential outcomes of future environmental changes. The sensitivity of benthic foraminiferal shell chemistry to environmental conditions opens the possibility to use them as proxies of coastal hypoxia and SGD. We report here that the average Mn/Ca ratios in tests of living benthic foraminiferal shells in the Changjiang River Estuary (CJE) is 2.3 times higher during hypoxia periods than under well-mixed conditions. In addition, Ba/Ca ratios in living benthic foraminiferal shells co-varied well with radon-inferred SGD signals. Fluctuations of Mn/Ca and Ba/Ca ratios in tests of a single foraminiferal shell along successive chambers corresponds well with seasonal-scale variations of hypoxia and SGD. We suggest that Mn/Ca and Ba/Ca ratios within intra-tests of benthic foraminifer can provide a reliable proxy for past hypoxia and SGD trends. • This work investigated proxies to record hypoxia and SGD on shells of same foraminiferal chambers. • Synchronous tendency of Mn/Ca and Ba/Ca ratios in tests of a single foraminiferal shell along successive chambers reflects seasonal-scale variations of hypoxia and SGD. • This work will provide a novel proxy for reconstructing the contribution of SGD to hypoxia over a long-term history. [ABSTRACT FROM AUTHOR]

Summary: This paper presents isotopic composition (δ18O, δ2H) of groundwater, lagoon water, and seawater collected in a coastal lagoon system in Southern Brazil as a tool to define groundwater sources within a permeable barrier that separates the lagoon from the ocean. The isotopic signature of the barrier groundwater differs both from the isotopic composition of lagoon water and seawater. The groundwater is significantly lighter, indicating that the major recharge source to the barrier is precipitation, rather than lagoon water. In the southern and central part of the barrier no interaction of groundwater with lagoon water was observed. In the northern part, however, lagoon water intrusion into the barrier was identified. This effect is apparently caused by different vegetation types on the barrier influencing subsurface hydrological processes. The central and the southern areas are characterised by natural dune vegetation (high infiltration, low evapotranspiration rates) whereas the northern part is covered by extensive pine tree plantations (low infiltration, high evapotranspiration rates). The presence of pine trees may enhance lagoon and seawater intrusion into the shallow aquifer and decrease fresh submarine groundwater discharge. [ABSTRACT FROM AUTHOR]

Abstract: The radon isotope 222Rn and salinity in coastal surface water were mapped on regional scales, to improve the understanding of coastal processes and their spatial variability. Radon was measured with a surface-towed, continuously recording multi-detector setup on a moving vessel. Numerous processes and locations of land–ocean interaction along the Central Great Barrier Reef coastline were identified and interpreted based on the data collected. These included riverine fluxes, terrestrially-derived fresh submarine groundwater discharge (SGD) and the tidal pumping of seawater through mangrove forests. Based on variations in the relationship of the tracers radon and salinity, some aspects of regional freshwater inputs to the coastal zone and to estuaries could be assessed. Concurrent mapping of radon and salinity allowed an efficient qualitative assessment of land–ocean interaction on various spatial and temporal scales, indicating that such surveys on coastal scales can be a useful tool to obtain an overview of SGD locations and processes. [Copyright &y& Elsevier]

Abstract: The natural flux of groundwater into coastal water bodies has recently been shown to contribute significant quantities of nutrients and trace metals to the coastal ocean. Groundwater discharge and hyporheic exchange to estuaries and rivers, however, is frequently overlooked though it often carries a distinctly different chemical signature than surface waters. Most studies that attempt to quantify this input to rivers use multiple geochemical tracers. However, these studies are often limited in their spatial and temporal extents because of the labor-intensive nature of integrating multiple measurement techniques. We describe here a method of using a single tracer, 222Rn, to rapidly characterize groundwater discharge into tidally-influenced rivers and streams. In less than one week of fieldwork, we determined that of six streams that empty into the Indian River Lagoon (IRL), Florida, three (Eau Gallie River, Turkey Creek, and Main Canal) did not receive substantial groundwater inputs, one canal (C-25 Canal) was dominated by groundwater exchange, and the remaining two (Sebastian River system and Crane Creek) fell somewhere in between. For more detailed discharge assessments, we focused on the Sebastian River system, a stratified tidal river estuary, during a relatively dry period (June) and a wet period (July) in 2008. Using time-series 222Rn and current velocity measurements we found that groundwater discharge into all three branches of the Sebastian River increased by 1–2 orders of magnitude during the wetter period. The estimated groundwater flow rates were higher than those reported into the adjacent IRL, suggesting that discharge into these rivers can be more important than direct discharge into the IRL. The techniques employed here should work equally well in other river/stream systems that experience significant groundwater discharge. Such assessments would allow area managers to quickly assess the distribution and magnitude of groundwater discharge nature into rivers over large spatial ranges. [Copyright &y& Elsevier]

The Changjiang (Yangtze) River Estuary (CJE) is one of the largest and most intense seasonal hypoxic zones in the world. Here we examine the possibility that submarine groundwater discharge (SGD) may contribute to the summer hypoxia. Spatial distributions of bottom water 222Rn suggest a hotspot discharge area in the northern section of the CJE. SGD fluxes were estimated based on a 222Rn mass balance model and were found to range from 0.002 ± 0.004 to 0.022 ± 0.011 m3/m2/day. Higher SGD fluxes were observed during summer hypoxia period. The well-developed overlap of the distribution patterns for SGD flux and dissolved oxygen (DO) implies that SGD could be an important contributor to summer hypoxia in the region off the CJE. We suggest that SGD contributes to the seasonal hypoxia either: (1) directly via discharge of anoxic groundwaters together with reducing substances; and/or (2) indirectly by delivering excess nutrients that stimulate primary productivity with consequent consumption of DO during organic matter decomposition. Unlabelled Image • 222Rn activities are negatively correlated with bottom water dissolved oxygen within the Changjiang (Yangtze) River Estuary. • The distribution patterns of SGD flux and dissolved oxygen in summer were overlapped. • Submarine groundwater discharge is an important contributor to summer hypoxia in the Changjiang River Estuary. [ABSTRACT FROM AUTHOR]

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 223 Ra and 224 Ra 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 m 3 /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 m 3 /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 m 3 /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. [ABSTRACT FROM AUTHOR]

Abstract: Hydrothermal fluid fluxes into deep ocean environments can be obtained from heat balance approaches. However, in shallow systems, hydrothermal heat fluxes can be masked by solar heating. In this paper, we use radon (222Rn) as a naturally occurring geochemical tracer to map the location of hydrothermal fluid inputs, as well as low-temperature groundwater discharges, and quantify fluxes into Concepcion Bay, Baja California, Mexico. This fault-bound bay contains intertidal seeps with salinities ranging from 5.3 to 25.6, temperatures reaching 64°C, and nitrate reaching 900μM. The bay is subject to natural eutrophication and frequent red tide events. A detailed 222Rn survey around the 100-km perimeter of Concepcion Bay allowed us to map the location of enhanced submarine groundwater inputs. Moorings at three contrasting coastal sites indicated that radon concentrations were higher at low tide and during the winter. Modeled hydrothermal fluid inputs ranged between 0.4cm/day in the middle of the bay and 43.9cm/day at the largest hydrothermal coastal seep site. Apparently, faults allow meteoric water to be heated and serve as conduits for its subsequent discharge through permeable marine sediments. When conservatively extrapolated to the entire bay using weighted distributions, these fluxes are estimated at 17.5 m3/s, a flow much higher than local ephemeral rivers. About 42% of the fluxes described consisted of fresh groundwater with the remaining made up of recirculated seawater. New nitrogen inputs associated with groundwater pathways are estimated to directly account for at least 15% of the local primary productivity. Our combined spatial survey/time series strategy can be very useful to quantify hydrothermal fluid inputs in particular at vent sites where a temperature signal in shallow surface waters is difficult to be observed. [Copyright &y& Elsevier]

Abstract: River discharges are usually gauged at sites farther upstream than estuarine tidal reaches. As a result, global estimates of river water and nutrient fluxes to the ocean are likely underestimated as they often neglect groundwater discharge occurring in estuaries downstream of river gauging stations. We used radon and radium isotopes as tracers of groundwater discharge into the Sebastian River Estuary, a gaining stream in Florida, USA. We developed a spatially-distributed mass balance model that accounts for radon sources and sinks in waters above and below the estuarine pycnocline. Radium isotopes (224Ra, 223Ra, and 226Ra) were not enriched in groundwater relative to surface water and thus had limited usefulness as tracers at this specific site. The detection of fresh groundwater just beneath the sediment:water interface overlain by brackish bottom water implies that fresh groundwater dominates over saline groundwater in this salt wedge estuary. Lateral groundwater inputs from sandy banks into waters above the estuarine pycnocline were about 6-fold higher than inputs into waters below the pycnocline. Groundwater discharge rates into the surface layer of the estuary estimated from a radon mass balance ranged from 5 to 18m3/s (or 18 to 62cm/day if uniformly distributed throughout the entire estuary area). The fluxes into the bottom layer ranged from 0.8 to 1.1m3/s (or 2.8 to 3.9cm/day). These groundwater inputs augmented river discharges gauged upstream of the estuary tidal reaches by about 260% during the dry period and 135% during the wet period. As nutrient and other dissolved species are often highly enriched in groundwaters, groundwater probably controls surface water quality in this and other Florida estuaries. [ABSTRACT FROM AUTHOR]