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, ^{223,224,226,228}Ra) 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 (^{223,224,226,228}Ra) 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 × 10⁵ m³ 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 NH₄⁺ and PO₄^− 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.     

Radium mass-balance in Jamaica Bay, NY: Evidence for a substantial flux of submarine groundwater

A mass balance for the naturally-occurring radium isotopes (²²⁴Ra, ²²³Ra, ²²⁸Ra, and ²²⁶Ra) in Jamaica Bay, NY, was conducted by directly estimating the individual Ra contributions of wastewater discharge, diffusion from fine-grained subtidal sediments, water percolation through marshes, desorption from resuspended particles, and water exchange at the inlet. The mass balance revealed a major unknown source term accounting for 19–71% of the total Ra input, which could only be resolved by invoking a source from submarine groundwater. Shallow (< 2 m depth) groundwater from permeable sediments in Jamaica Bay was brackish and enriched in Ra relative to surface bay waters by over two orders of magnitude. To balance Ra fluxes, a submarine groundwater input of 0.8 × 10⁹–9.0 × 10⁹ L d^− 1 was required. This flux was similar for all four isotopes, with individual estimates varying by less than a factor of 2. Our calculated groundwater flux was 6- to 70-fold higher than the fresh groundwater discharge to the bay estimated by hydrological methods, but closely matched direct flow rates measured with seepage meters. This suggests that a substantial portion of the discharge consisted of recirculated seawater. The magnitude of submarine groundwater discharge varied seasonally, in the order: summer > autumn > spring. Chemical analyses suggest that the recirculated seawater component of submarine groundwater delivers as much dissolved nitrogen to the bay as the fresh groundwater flux.     

TIMS measurements of Ra and Ra in the Gulf of Lion, an attempt to quantify submarine groundwater discharge

Submarine groundwater discharge (SGD) is now recognized as an important pathway for water and chemical species fluxes to the coastal ocean. In order to determinate SGD to the Gulf of Lion (France), we measured the activities of ²²⁶Ra and ²²⁸Ra by thermal ionization mass spectrometry (TIMS) in coastal waters and in the deep aquifer waters of the Rhone deltaic plain after pre-concentration of radium by MnO₂. Compared to conventional counting techniques, TIMS requires lower quantities of water for the analyses, and leads to higher analytical precision. Radium isotopes were thus measured on 0.25–2 L water samples containing as little as 20 fg of ²²⁶Ra and 0.2–0.4 fg of ²²⁸Ra with precision equal to 2%. We demonstrate that coastal surface waters samples are enriched in ²²⁶Ra and ²²⁸Ra compared to the samples further offshore. The high precision radium measurements display a small but significant ²²⁶Ra and ²²⁸Ra enrichment within a strip of circa 30 km from the coast. Radium activities decrease beyond this region, entrained in the northern current along the shelf break or controlled by eddy diffusion. The radium excess in the first 30 km cannot be accounted for by the river nor by the early diagenesis. The primary source of the radium enrichment must therefore be ascribed to the discharge of submarine groundwater. Using a mass-balance model, we estimated the advective fluxes of ²²⁶Ra and ²²⁸Ra through SGD to be 5.2 × 10¹⁰ and 21 × 10¹⁰ dpm/d respectively. The ²²⁶Ra activities measured in the groundwater from the Rhone deltaic plain aquifer are comparable to those from other coastal groundwater studies throughout the world. By contrast, ²²⁸Ra activities are higher by up to one order of magnitude. Taking those groundwater radium activities as typical of the submarine groundwater end-member, a minimum volume of 0.24–4.5 × 10¹⁰ l/d is required to support the excess radium isotopes on the inner shelf. This has to be compared with the average rivers water runoff of 15.4 × 10¹⁰ l/d during the study period (1.6 to 29% of the river flow).     

Radium tracing of submarine groundwater discharge (SGD) and associated nutrient fluxes in a highly-permeable bed coastal zone, Korea

In order to estimate submarine groundwater discharge (SGD) and SGD-driven nutrient fluxes, we measured the concentrations of nutrients, ²²⁴Ra, and ²²⁶Ra in seawater, river water, and coastal groundwater of Yeongil Bay (in the southeastern coast of Korea) in August 2004 and February 2005. The bottom sediments over the shallow areas of this bay are composed mainly of coarse sands. Large excess concentrations of ²²⁴Ra, ²²⁶Ra, and Si supplied from SGD were observed in August 2004, while these excess concentrations were not apparent in February 2005. Based on the mass balance for ²²⁴Ra, ²²⁶Ra, and Si, which showed conservative mixing behavior in seawater, SGD was estimated to be approximately 6 × 10⁶ m³ day^− 1 (seepage rate = 0.2 m day^− 1) in shallow areas (< 9 m water depth) in August 2004, which is much higher than the SGD level typically found in other coastal regions worldwide. During the summer period, SGD-driven nutrients in this bay contributed approximately 98%, 12%, and 76% of the total inputs for dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicate (DSi), respectively. Our study implies that the ecosystem in this highly permeable bed coastal zone is influenced strongly by SGD during summer, while such influences are negligible in winter.     

Estimating submarine groundwater discharge around Isola La Cura, northern Venice Lagoon (Italy), by using the radium quartet

The four naturally-occurring radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) were used to estimate the submarine groundwater discharge (SGD) in the Isola La Cura marsh area in the northern Venice Lagoon (Italy). By determining the radium contributors to the study area (river, coastal ocean and sediments) the radium excess in the lagoon water was quantified through a mass balance model. This radium excess is attributed to a submarine groundwater discharge source and represents the most important input of radium. Possible endmembers were considered from analysis of groundwater samples (subtidal and marsh piezometers, marsh wells and seepage meters) that were enriched in Ra by one to two orders of magnitude relative to surface waters. In particular, a permeable layer at 80 cm depth in the surrounding marsh is considered to be representative of the most likely SGD source, although similar radium activities were measured in other subtidal porewater samples collected in the Isola La Cura area. The estimated SGD flux to the study area ranged from 1 · 10⁹ to 6 · 10⁹ L·d^− 1, the same order of magnitude as the overall riverine input to the lagoon (3 · 10⁹ L·d^− 1). A major fraction of this SGD flux is likely recirculated seawater, as evidenced by the endmember salinity. The water residence time of 2 days was estimated by both using the shortest-lived radium isotope and estimating the volume of water exchanged between the lagoon and the open sea during a tidal cycle (tidal prism approach). This SGD flux could be used to estimate the input of other chemical species (metals, nutrients, etc.) via SGD which might affect the Venice Lagoon ecosystem.     

Submarine groundwater discharge to Great South Bay, NY, estimated using Ra isotopes

There is increasing evidence that submarine groundwater discharge (SGD) in many areas represents a major source of dissolved chemical constituents to the coastal ocean. In Great South Bay, NY, previous studies have shown that the discharge of nutrients with SGD may cause harmful algal blooms. This study estimates SGD to Great South Bay during August 2006 by performing a mass balance for each of the dissolved Ra isotopes (²²⁴Ra, ²²³Ra, ²²⁸Ra, ²²⁶Ra). The budget indicates a major unknown source (between 30 and 60% of the total input) of Ra to the bay. This imbalance can be resolved by a flux of Ra-enriched groundwater on the order of 3.5–4.5 × 10⁹ L d^− 1, depending on the Ra isotope. The Ra-estimated SGD rates compare well with those previously estimated by models of flow that decreases exponentially away from shore. Compared to previous reports of fresh groundwater discharge to the bay, the Ra-estimated discharge must comprise approximately 90% recirculated seawater. The good agreement between Ra- and model-estimated flow rates indicates that the primary SGD endmember may be best sampled at shallow depths in the sediments a short distance bayward of the low tide line.     

Coincident application of a mass balance of radium and a hydrodynamic model for the seasonal quantification of groundwater flux into the Venice Lagoon, Italy

A mass balance of the naturally occurring short-lived radium isotopes (^223,224Ra) in the Venice Lagoon was conducted by an integrated approach combining the directly estimated individual Ra contributions and hydrodynamic model results. Hydrodynamic data allows for the calculation of the Ra mass balance in sub-sections of the Venice Lagoon (boxes), which are characterized by physically homogeneous properties, instead of investigating the entire lagoon. Utilizing this method, both the seasonal and the spatial variability of the submarine groundwater discharge in the Venice Lagoon have been estimated. Between 14–83 × 10⁹ L d^− 1 of water were calculated to flow across the sediment–sea interface, corresponding to 5–28 times the mean annual river input. The submarine groundwater discharge estimates were correlated with the residence time calculation to better understand spatial and seasonal variation.     

Comparison of techniques for pre-concentrating radium from seawater

In the framework of the KEOPS project (KErguelen: compared study of the Ocean and the Plateau in Surface water), we aimed to provide information on the water mass pathways and vertical mixing on the Kerguelen Plateau, Southern Ocean, based on ²²⁸Ra profiles. Because ²²⁸Ra activities are extremely low in this area (~ 0.1 dpm/100 kg or ~ 2.10^− 18 g kg^− 1), the filtration of large volumes of seawater was required in order to be able to detect it with minimal uncertainty. This challenging study was an opportunity for us to test and compare methods aimed at removing efficiently radium isotopes from seawater. We used Mn-fiber that retains radium and that allows the measurement of all four radium isotopes (²²⁶Ra, ²²⁸Ra, ²²³Ra, ²²⁴Ra). First, we used Niskin bottles or the ship's seawater intake to collect large volumes of seawater that were passed onto Mn-fiber in the laboratory. Second, we filled cartridges with Mn-fiber that we placed in tandem on in situ pumps. Finally, we fixed nylon nets filled with Mn-fiber on the frame of in situ pumps to allow the passive filtration of seawater during the pump deployment.Yields of radium fixation on the cartridges filled with Mn-fiber and placed on in situ pumps are ca. 30% when combining the two cartridges. Because large volumes of seawater can be filtered with these pumps, this yields to effective volumes of 177–280 kg (that is, higher than that recovered from fourteen 12-l Niskin bottles). Finally, the effective volume of seawater that passed through Mn-fiber placed in nylon nets and deployed during 4 h ranged between 125 and 364 kg. Consequently, the two techniques that separate Ra isotopes in situ are good alternatives for pre-concentrating radium from seawater. They can save ship-time by avoiding repeated CTD casts to obtain the large volumes of seawater. This is especially true when in situ pumps are deployed to collect suspended particles. However, both methods only provide ²²⁸Ra/²²⁶Ra ratios. The determination of the ²²⁸Ra specific activity is obtained by multiplying this ratio by the ²²⁶Ra activity measured in a discrete sample collected at the same water depth.     

Water mass ages of coastal ponds estimated using Ra and Ra as tracers

Measurements of the naturally occurring radioisotopes ²²³Ra (t_1/2 = 11.4 days) and ²²⁴Ra (t_1/2 = 3.66 days) in southern Rhode Island salt ponds were combined with a simple model to obtain independent estimates of the age of these coastal waters. Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith-Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, beginning January 2002 through August 2003. Surface water activities ranged from 1–78 dpm 100 L^− 1 and 5–885 dpm 100 L^− 1 for ²²³Ra and ²²⁴Ra, respectively. Porewater radium activities ranged from 3 to 715 dpm 100 L^− 1 for ²²³Ra, and 57–4926 dpm 100 L^− 1 for ²²⁴Ra. Results indicate seasonally varying water mass ages for Ninigret (5–12 days), Winnapaug (2–6 days) and Pt. Judith-Potter Ponds (1–9 days) and, in contrast, relatively constant ages for Green Hill (5–7 days) and Quonochontaug Ponds (3–6 days).     

The impact of groundwater discharges on mercury partitioning, speciation and bioavailability to mussels in a coastal zone

The Mussel Watch program conducted along the French coasts for the last 20 years indicates that the highest mercury concentrations in the soft tissue of the blue mussel (Mytilus edulis) occur in animals from the eastern part of Seine Bay on the south coast of the English Channel, the “Pays de Caux”. This region is characterized by the presence of intertidal and submarine groundwater discharges, and no particular mercury effluent has been reported in its vicinity. Two groundwater emergence systems in the karstic coastal zone of the Pays de Caux (Etretat and Yport with slow and fast water percolation pathways respectively) were seasonally sampled to study mercury distribution, partitioning and speciation in water. Samples were also collected in the freshwater–seawater mixing zones in order to compare mercury concentrations and speciation between these “subterranean” or “groundwater” estuaries and the adjacent macrotidal Seine estuary, characterized by a high turbidity zone (HTZ). The mercury concentrations in the soft tissue of mussels from the same areas were monitored at the same time.The means of the “dissolved” (< 0.45 μm) mercury concentrations (HgT_D) in the groundwater springs were 0.99 ± 0.15 ng l^− 1 (n = 18) and 0.44 ± 0.17 ng l^− 1 (n = 17) at Etretat and Yport respectively. High HgT_D concentrations were associated with strong runoff over short water pathways during storm periods, while low concentrations were associated with long groundwater pathways. Mean particulate mercury concentrations were 0.22 ± 0.05 ng mg^− 1 (n = 16) and 0.16 ± 0.10 ng mg^− 1 (n = 17) at Etretat and Yport respectively, and decreased with increasing particle concentration probably as a result of dilution by particles from soil erosion. Groundwater mercury speciation was characterized by high reactive-to-total mercury ratios in the dissolved phase (HgR_D/HgT_D: 44–95%), and very low total monomethylmercury concentrations (MMHg < 8 pg l^− 1). The HgT_D distributions in the Yport and Etretat mixing zones were similar (overall mean concentration of 0.73 ± 0.21 ng l^− 1, n = 43), but higher than those measured in the adjacent industrialized Seine estuary (mean: 0.31 ± 0.11 ng l^− 1, n = 67). In the coastal waters along the Pays de Caux dissolved monomethylmercury (MMHg_D) concentrations varied from 9.5 to 13.5 pg l^− 1 (2 to 8% of the HgT_D). Comparable levels were measured in the Seine estuary (range: 12.2– 21.1 pg l⁻¹; 6–12% of the HgT_D). These groundwater karstic estuaries seem to be mostly characterized by the higher HgT_D and HgR_D concentrations than in the adjacent HTZ Seine estuary. While the HTZ of the Seine estuary acts as a dissolved mercury removal system, the low turbid mixing zone of the Pays de Caux receives the dissolved mercury inputs from the groundwater seepage with an apparent Hg transfer from the particulate phase to the “dissolved” phase (< 0.45 μm). In parallel, the soft tissue of mussels collected near the groundwater discharges, at Etretat and Yport, exhibited significantly higher values than those found in the mussel from the mouth of the Seine estuary. We observe that this difference mimics the differences found in the mercury distribution in the water, and argue that the dissolved phase of the groundwater estuaries and coastal particles are significant sources of bioavailable mercury for mussels.     

Uncertainties associated with Ra and Ra measurements in water via a Delayed Coincidence Counter (RaDeCC)

The short-lived radium isotopes, ²²³Ra (T_1/2 = 11.4 days) and ²²⁴Ra (T_1/2 = 3.66 days), have been successfully used as tracers of several environmental processes, e.g., submarine groundwater discharge, coastal mixing processes, and water residence times. In this paper, the uncertainties associated with ²²³Ra and ²²⁴Ra measurements using a Radium Delayed Coincidence Counter are determined on a detailed error propagation basis with a confidence interval of 1σ. From the data analyses of several groups of coastal water samples, the calculated relative uncertainties averaged 12% for the ²²³Ra and 7% for the ²²⁴Ra. These uncertainties can decrease for radium-enriched groundwater samples although asymptotic limits have been found at 7% relative uncertainty for ²²³Ra and 4% for ²²⁴Ra. In this paper, the influence of sampling and measurement parameters on the final radium uncertainties is evaluated in order to provide guidance to optimize these factors and obtain more reliable results.     

Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge

We measured significant activities of short-lived radium isotopes, ²²³Ra (half-life = 11 days) and ²²⁴Ra (half-life = 3.7 days), around the margins of the Hawaiian Islands to water depths of 3500 m. These measurements suggest fluid inputs from the basalt to the surrounding ocean. In general ²²³Ra activities were considerably greater than ²²⁴Ra in spite of the expected higher production rate of ²²⁴Ra activity in basalt. The ²²³Ra was not supported by dissolved ²²⁷Ac. The highest enrichments of ²²³Ra were measured over the Puna Ridge (2100 m depth) east of Hawaii. Here ²²³Ra activities reached 19 dpm/m³, similar to activities measured near sites of active submarine groundwater discharge in the South Atlantic Bight. To explain the high activities of ²²³Ra unaccompanied by ²²⁴Ra, we postulate that thermally-driven circulation of seawater through the Puna Ridge deposits ²³¹Pa on basalt surfaces. With time the ²³¹Pa produces ²²⁷Ac and ²²³Ra; and ²²³Ra desorbs into the circulating fluids. These fluids then transport ²²³Ra into the overlying ocean. Based on the inventory of ²²³Ra above the Puna Ridge, we estimate the flow of fluids through the ridge to be on the order of 20–60 cm³ cm^− 2 day^− 1. In less than 1000 years the incoming seawater could provide enough ²³¹Pa to basalt surfaces to balance the inventory of ²²³Ra above the ridge if only 8% of the ²²³Ra was transported to the overlying water. These observations on the flanks of a volcanically-active ocean island have significant implications for quantifying fluid fluxes from the flanks of the mid-ocean ridge system. By mapping ²²³Ra inventories in the ocean above ridge flanks and measuring the activity of ²²³Ra in the emerging fluids, the fluid flux can be obtained.     

Submarine groundwater discharge: A large, previously unrecognized source of dissolved iron to the South Atlantic Ocean

This paper reports the initial results of a study of groundwater and coastal waters of southern Brazil adjacent to a 240 km barrier spit separating the Patos Lagoon, the largest coastal lagoon in South America, from the South Atlantic Ocean. The objective of this research is to assess the chemical alteration of freshwater and freshwater–seawater mixtures advecting through coastal permeable sands, and the influence of the submarine discharge of these fluids (SGD) on the chemistry of coastal waters. Here we focus on dissolved iron in this system and use radium isotopic tracers to quantify SGD and cross-shelf fluxes. Iron concentrations in groundwaters vary between 0.6 and 180 μM. The influence of the submarine discharge of these fluids into the surf zone produces dissolved Fe concentrations as high as several micromolar in coastal surface waters. The offshore gradient of dissolved Fe, coupled with results for Ra isotopes, is used to quantify the SGD flux of dissolved Fe from this coastline. We estimate the SGD flux to be 2 × 10⁶ mol day^− 1 and the cross-shelf flux to be 3.2 × 10⁵ mol day^− 1. This latter flux is equal to about 10% of the soluble atmospheric Fe flux to the entire South Atlantic Ocean. We speculate on the importance of this previously unrecognized iron input to regional ocean production and on the potential significance of this source to understanding variations in glacial–interglacial ocean production.     

The release of dissolved actinium to the ocean: A global comparison of different end-members

The measurement of short-lived ²²³Ra often involves a second measurement for supported activities, which represents ²²⁷Ac in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of ²²⁷Ac, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles ²²⁷Ac data from coastal regions all over the northern hemisphere, including values from ground water, from estuaries and lagoons, and from marine end-members. Deep-sea samples from a continental slope off Puerto Rico and from an active vent site near Hawaii complete the overview of ²²⁷Ac near its potential sources.The average ²²⁷Ac activities of nearshore marine end-members range from 0.4 dpm m^− 3 at the Gulf of Mexico to 3.0 dpm m^− 3 in the coastal waters of the Korean Strait. In analogy to ²²⁸Ra, we find the extension of adjacent shelf regions to play a substantial role for ²²⁷Ac activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean ²²⁷Ac inventory of 1.35 * 10¹⁸ dpm ²²⁷Ac_ex in the ocean, which corresponds to 37 moles, or 8.4 kg. This implies a flux of 127 dpm m⁻² y^− 1 from the deep-sea floor. For the shelf regions, we obtain a global inventory of ²²⁷Ac of 4.5 * 10¹⁵ dpm, which cannot be converted directly into a flux value, as the regional loss term of ²²⁷Ac to the open ocean would have to be included.Ac has so far been considered to behave similarly to Ra in the marine environment, with the exception of a strong Ac source in the deep-sea due to ²³¹Pa_ex. Here, we present evidence of geochemical differences between Ac, which is retained in a warm vent system, and Ra, which is readily released [Moore, W.S., Ussler, W. and Paull, C.K., 2008-this issue. Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in ²²⁷Ac/²²⁸Ra and daughter isotope ratios from the expected production value of lithogenic material is observed at reducing environments, where enrichment in uranium may occur. The presented data here may serve as a reference for including ²²⁷Ac in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution.     

Seasonal changes in submarine groundwater discharge to coastal salt ponds estimated using Ra and Ra as tracers

Submarine groundwater discharge (SGD) to coastal southern Rhode Island was estimated from measurements of the naturally-occurring radioisotopes ²²⁶Ra (t_1/2 = 1600 y) and ²²⁸Ra (t_1/2 = 5.75 y). Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith–Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, from January 2002 to August 2003; additional porewater samples were collected in August 2005. Surface water activities ranged from 12–83 dpm 100 L^− 1 (60 dpm = 1 Bq) and 21–256 dpm 100 L^− 1 for ²²⁶Ra and ²²⁸Ra, respectively. Porewater ²²⁶Ra activities ranged from 16–736 dpm 100 L^− 1 (2002–2003) and 95–815 dpm 100 L^− 1 (2005), while porewater ²²⁸Ra activities ranged from 23–1265 dpm 100 L^− 1. Combining these data with a simple box model provided average ²²⁶Ra-based submarine groundwater fluxes ranging from 11–159 L m^− 2 d^− 1 and average ²²⁸Ra-derived fluxes of 15–259 L m^− 2 d^− 1. Seasonal changes in Ra-derived SGD were apparent in all ponds as well as between ponds, with SGD values of 30–472 L m^− 2 d^− 1 (Winnapaug Pond), 6–20 L m^− 2 d^− 1 (Quonochontaug Pond), 36–273 L m^− 2 d^− 1 (Ninigret Pond), 29–76 L m^− 2 d^− 1 (Green Hill Pond), and 19–83 L m^− 2 d^− 1 (Pt. Judith–Potter Pond). These Ra-derived fluxes are up to two orders of magnitude higher than results predicted by a numerical model of groundwater flow, estimates of aquifer recharge for the study period, and values published in previous Ra-based SGD studies in Rhode Island. This disparity may result from differences in the type of flow (recirculated seawater versus fresh groundwater) determined using each technique, as well as variability in porewater Ra activity.     

Factors controlling excess radium in the Nakdong River estuary,Korea: submarine groundwater discharge versus desorption from riverine particles

The activities of naturally occurring radium isotopes (²²⁶Ra and ²²⁸Ra) in estuarine water were measured downstream of the dam constructed in the Nakdong River, Korea. The sampling of surface waters for radium, silicate, and suspended solid (SS) analyses was conducted at 18 stations during three periods (July 1997, April 1998, and June 1999). In general, radium activities exceeded the value expected from the mixing of two freshwater and seawater endmembers. We characterized the responses of Ra and Si according to three different conditions: (1) when the freshwater discharge and the water level of the dam relative to the sea level at low tide were lower (April 1998), the excess Ra and Si contents were lower in the estuary; (2) when the fresh water discharge was larger following heavy precipitation (July 1997), both excess Ra and Si contents were higher in the estuary with conservative mixing of Si; and (3) when the water level of the dam relative to the sea level at low tide was highest under low freshwater discharge (June 1999), high excess Ra but low Si levels were observed. The occurrence of high Ra activity in June 1999 was likely due to the large submarine brackish groundwater discharge downstream of the estuary. Since brackish groundwater in general contains high concentrations of nutrients, Ba, Ra, etc, our result suggests an important role for the submarine groundwater discharge on the biogeochemistry of estuarine/coastal waters, especially when the water level of the dam (hydraulic head) is high.     

Seasonal variation in the Ra/Ra ratio of coastal water within the Sea of Japan: Implications for the origin and circulation patterns of the Tsushima Coastal Branch Current

In the current study, low-background γ-spectrometry was employed to determine the ²²⁸Ra/²²⁶Ra activity ratio and ¹³⁷Cs activity of 84 coastal water samples collected at six sites along the main island of Japan (Honshu Island) within the Sea of Japan, including the Tsushima Strait, and two other representative sites on Honshu Island (a Pacific shore and the Tsugaru Strait) at 1-month intervals in 2006.The ²²⁸Ra/²²⁶Ra ratio of coastal waters in the Sea of Japan exhibited similar patterns of seasonal variation, with minimum values during early summer (²²⁸Ra/²²⁶Ra = 0.6–0.8), maximum values during autumn (²²⁸Ra/²²⁶Ra = 1.5–3), and a time lag in their temporal changes ( 2.5 months and over  1300 km distance). However, the 2 other sites represented no clear periodic variation.In contrast to the positive correlation between ¹³⁷Cs activity (0.6–1.7 mBq/L) and salinity (15–35), the ²²⁸Ra/²²⁶Ra ratio of coastal water samples from the Sea of Japan was not observed to correlate with salinity, and the increase in the ²²⁸Ra/²²⁶Ra ratio was not as marked (0.5–1; May–June 2004 and 2005) during the migration along Honshu Island. The input of land-derived water and/or the diffusion of radium from coastal sediments is unlikely to have affected the wide seasonal variation in the ²²⁸Ra/²²⁶Ra ratio observed in these water samples.The seasonal variation in the ²²⁸Ra/²²⁶Ra ratio recorded for the coastal waters of the Sea of Japan is considered to be mainly controlled by the remarkable changes in the mixing ratio of the ²²⁸Ra-poor Kuroshio and the ²²⁸Ra-rich continental shelf waters within the East China Sea (ECS). After passing through the Tsushima Strait, this water mass moves northeast along the coastline of the Sea of Japan as the Tsushima Coastal Branch Current (TCBC).     

Submarine groundwater discharge and nutrient addition to the coastal zone and coral reefs of leeward Hawai'i

Multiple tracers of groundwater input (salinity, Si, ²²³Ra, ²²⁴Ra, and ²²⁶Ra) were used together to determine the magnitude, character (meteoric versus seawater), and nutrient contribution associated with submarine groundwater discharge across the leeward shores of the Hawai'ian Islands Maui, Moloka'i, and Hawai'i. Tracer abundances were elevated in the unconfined coastal aquifer and the nearshore zone, decreasing to low levels offshore, indicative of groundwater discharge (near-fresh, brackish, or saline) at all locations. At several sites, we detected evidence of fresh and saline SGD occurring simultaneously. Conservative estimates of SGD fluxes ranged widely, from 0.02–0.65 m³ m^− 2 d^− 1at the various sites. Groundwater nutrient fluxes of 0.04–40 mmol N m^− 2 d^− 1 and 0.01–1.6 mmol P m^− 2 d^− 1 represent a major source of new nutrients to coastal ecosystems along these coasts. Nutrient additions were typically greatest at locations with a substantial meteoric component in groundwater, but the recirculation of seawater through the aquifer may provide a means of transferring terrestrially-derived nutrients to the coastal zone at several sites.     

Ra and Ra in the mixing zones of the Pee Dee River-Winyah Bay, Yangtze River and Delaware Bay Estuaries

²²⁶Ra and ²²⁸Ra have non-conservative excess concentrations in the mixing zones of the Pee Dee River-Winyah Bay estuary, the Yangtze River estuary, and the Delaware Bay estuary. Laboratory experiments, using Pee Dee River sediment, indicate desorption of ²²⁶Ra to increase with increasing salinities up to 20‰. In Winyah Bay desorption from river-borne sediments could contribute almost all of the increases for both isotopes. Desorption adds only a portion of the excess ²²⁸Ra measured in the Yangtse River and adjacent Shelf waters and Delaware Bay. In the Yangtze River the mixing zone extends over a considerable portion of the Continental Shelf where ²²⁸Ra is added to the water column by diffusion from bottom sediments, while ²²⁶Ra concentrations decrease from dilution. Diffusion of ²²⁸Ra from bottom sediments in Delaware Bay primarily occurs in the upper part of the bay (< 22‰ water) where fine grained sediments predominate. A diffusive flux for ²²⁸Ra of 0·33 dpm cm⁻² year was determined for Delaware Bay.     

Uncertainties in the preparation of Ra Mn fiber standards

Delayed coincidence counters (RaDeCC), used for measuring ²²³Ra and ²²⁴Ra preconcentrated from water onto MnO₂-impregnated acrylic fiber (“Mn-fiber”), require a standard Mn-fiber column that has a precisely known activity of ²²⁴Ra for calibration. This may be done by adding an aged ²²⁸Th standard solution to adsorb both ²²⁸Th and its daughter ²²⁴Ra quantitatively onto a Mn fiber. We used both seawater and deionized water (DIW) for testing the adsorption efficiency of Th and Ra onto Mn fibers. Our experimental results show that more than 50% of thorium (²³²Th and ²²⁸Th) breaks through the Mn-fiber column when DIW is used as a medium. However, near quantitative recoveries are obtained if filtered (0.45 μm) seawater is used to prepare the standard. In the case of pure DIW, the pH (initial pH  5.3) rises to > 10 after passing through the column while seawater (initial pH  7.8) changes to  7.2. Thus, the lack of thorium adsorption in DIW may be attributed to this huge increase of pH and the consequent formation of Th(OH)₄ and polyhydroxyl colloids. Based on these observations, we recommend that one should use either artificial seawater or natural seawater (which has negligible ²²⁴Ra and ²²⁸Th) as a loading solution after 0.45 μm filtration. In addition, the thorium adsorption efficiency should be confirmed either by thorium analysis of the effluent solution or long-term monitoring of the supported ²²⁴Ra on the Mn fiber using the RaDeCC. Similar cautions are likely necessary for making ²²³Ra standards by adsorption of ²²⁷Ac onto Mn fibers.