Radium Isotope Distributions Research Articles

Early diagenesis of radium 226 and radium 228 in lacustrine sediments influenced by former mining sites

Radium is a naturally occurring radioactive element commonly found at low levels in natural systems such as lacustrine or marine sediments. Anthropogenic activities including former uranium mining activities can lead to the dissemination of radium isotopes having high radiological toxicities, which potentially threaten the safety of nearby environments. Although radium mobility in oxidized environments is known to be largely governed by sorption/desorption onto Fe and Mn oxyhydroxides and coprecipitation with sulfate minerals (e.g. barite), little is known regarding its behavior under reducing conditions, which are the conditions typically encountered in organic-rich systems such as wetlands and lake sediments.The present study aims at understanding the behavior of long-lived radium isotopes (226Ra and 228Ra), during early diagenesis of lake sediments contaminated by former uranium mining activities. Solid and pore water concentrations of 226Ra and 228Ra were determined using ultra low background gamma spectrometry, which allowed improvement of detection limits and measurement accuracy.This study shows that the downcore distribution of radium isotopes is closely related to the reductive dissolution of iron and manganese oxyhydroxides below the sediment-water interface. The resulting diffusive fluxes of 226Ra and 228Ra (4.1 10−25 and 4.7 10−28 mol cm−2.s−1) are however significantly lower than other radium-impacted environments, such as uranium mill tailings pond and phosphate industry-impacted sediments, and are similar to those reported for natural marine environments. Hence, in the reduced lake sediments of Saint-Clement, the major fraction of radium is trapped by the solid phase, while early diagenesis only induces a slight mobility of this radioelement.

Deltaic and Estuarine Controls on Mackenzie River Solute Fluxes to the Arctic Ocean

River discharge has an important influence on the chemistry of Arctic shelf seas and surface waters. In this study, we examined the changes in nutrient, trace metal, dissolved organic matter, dissolved inorganic carbon, alkalinity, and radium isotope distributions across the Mackenzie River delta and estuary in June 2016, and estimated trace element and isotopes fluxes from the Mackenzie River to the Western Arctic Ocean. While inorganic nutrient, dissolved inorganic carbon, alkalinity, trace metal, and radium levels remained fairly constant through the delta, dissolved organic matter concentrations were 2–3.5 times higher at the mouth of the river, reflecting inputs from the delta. In the estuarine mixing zone, radium isotope ratios indicated that both desorption from suspended sediments and benthic inputs affected solute concentrations. Similarly, most of the measured trace metals (Ba, Co, Cu, Mn, Ni) exhibited non-conservative addition in the freshwater-saltwater transition zone, while Pb was removed and U was mixed conservatively. Inorganic nutrients were affected by both addition and removal processes in the estuary. Dissolved inorganic carbon and alkalinity exhibited net removal, likely due to a combination of biological uptake, gas exchange, CaCO3 precipitation, and surface ion exchange processes. Approximately 45% and 60% of dissolved organic carbon and nitrogen, respectively, were removed rapidly at low salinities, in contrast with previous studies suggesting conservative mixing through Arctic estuaries. This study highlights the need to take deltaic and estuarine processes into account when determining the flux of riverine solutes to the coastal ocean.

Spatial Characteristics Reveal the Reactive Transport of Radium Isotopes (224Ra, 223Ra, and 228Ra) in an Intertidal Aquifer

AbstractThis study presents high‐resolution two‐dimensional distributions of radium isotopes in the shallow intertidal aquifer of Tolo Harbor, Hong Kong, illustrating the importance of salinity, groundwater residence time, and tidal flushing and mixing in controlling radium behavior in the system. The activities of radium isotopes are low in the fresh groundwater zone with activities increasing in the transition zone (1 < salinity < 25) due to desorption of radium from sediment surface coatings. In the high salinity zone (salinity > 25), the activities of radium isotopes increase with depth due to tidal flushing and mixing of the recirculating seawater in the shallow aquifer as well as increasing residence time with depth. The dissolved radium isotopes are identified to be in disequilibrium in shallow intertidal aquifer based on radium isotopic ratios and observed depletion of dissolved radium isotopes compared to the theoretical mixing line between the equilibrium activity and fresh groundwater. The influence of continuous tidal flushing of the shallow intertidal aquifer on radium is revealed, and shallow sediments are observed to have less total exchangeable radium isotopes than deeper in the aquifer. A new one‐dimensional reactive transport model that considers a depth‐dependent production rate is applied to simulate the vertical distribution of radium isotopes in the intertidal aquifer as seawater infiltrates the beach. Using least squares fitting of the model to field data, the vertical infiltration seepage velocity is estimated to be ~0.5 m/day, and the dimensionless adsorption partition coefficient of radium isotopes (K) is 350. This agrees well with K values (202–365) calculated using an adsorption/desorption model as well as measured values (266 in February and 205 in April). Based on the spatial distribution of radium isotopes and flow patterns in the intertidal aquifer, groundwater in Ra disequilibrium zone is recommended as the endmember for tide‐ or wave‐driven submarine groundwater discharge (SGD) that has shallow flow paths, and groundwater in Ra equilibrium zone is recommended as endmember for seasonally driven or density‐driven SGD that has deep flow paths.

Using Tidal Fluctuation‐Induced Dynamics of Radium Isotopes (224Ra, 223Ra, and 228Ra) to Trace the Hydrodynamics and Geochemical Reactions in a Coastal Groundwater Mixing Zone

AbstractThe reactive transport of radium isotopes (224Ra, 223Ra, and 228Ra) in coastal groundwater mixing zones (CGMZs) is sensitive to shifts of redox conditions and geochemical reactions induced by tidal fluctuation. This study presents a spatial distribution and temporal variation of radium isotopes in the CGMZ for the first time. Results show that the activity of radium isotopes in the upper saline plume (USP) is comparatively low due to a short residence time and mixing loss induced by the infiltration of low radium seawater whereas the activity of radium isotopes in the salt wedge (SW) is comparatively high due to a long residence time in the aquifer. The spatial distribution of radium isotopes is determined by the partitioning of radium isotopes, groundwater residence time, and relative ingrowth rates of radium isotopes. In addition, the variation of radium isotopes in the USP lags slightly (∼0 h) whereas the fluctuation of radium isotopes in the SW lags significantly (∼12 h) behind sea level oscillation. Tidal fluctuation affects the partitioning of radium isotopes through controlling seawater infiltration and subsequently influences the dynamics of radium isotopes in the USP. Concurrently, seawater infiltration significantly affects geochemical processes such as the production of nutrients and total alkalinity. Therefore, radium dynamics in the USP have implications for these geochemical processes. The variation of radium isotopes in the USP also has potential implications for transformation of trace metals such as iron and manganese because of the close affinity of radium isotopes to manganese and iron oxides.

Radium isotope distributions during the US GEOTRACES North Atlantic cruises

Radium isotopes are produced in sediments via the decay of thorium isotopes and are generally soluble in seawater. As such, isotopes of the radium quartet (224Ra, 223Ra, 228Ra, 226Ra) have been used as tracers of ocean boundary inputs and mixing processes. We measured radium isotopes on the US GEOTRACES North Atlantic cruises in 2010 and 2011, which crossed a number of key ocean boundary features including the Mediterranean outflow, the Mauritanian upwelling and oxygen minimum zones (OMZ) along western Africa, a hydrothermal plume over the mid-Atlantic Ridge (MAR), and the broad continental margin of the north-eastern United States. Radium isotope features at these locations are discussed in the context of their potential to quantify fluxes of trace elements and isotopes (TEIs) from common sources. Notable observations include: (1) a Mediterranean outflow spreading rate of 0.52–0.60cm/s derived from 228Ra, (2) evidence of substantial sediment–water interaction in the benthic boundary layer along the OMZ, (3) decoupling between 223Ra and the other Ra isotope sources over the MAR, and (4) significant continental inputs (e.g. submarine groundwater discharge—SGD) in the western Atlantic. Lastly, 228Ra inventories have remained constant over the past 30years, which suggests that SGD is steady-state for the North Atlantic on decadal time scales.

Determination of water ages and flushing rates using short-lived radium isotopes in large estuarine system, the Yangtze River Estuary, China

We investigated the spatial and temporal distribution of naturally-occurring short-lived radium isotopes (224Ra, t1/2 = 3.6 d and 223Ra, t1/2 = 11 d) to examine coastal water mixing dynamics of the third world largest estuary, Yangtze River Estuary (YRE) during two field trips in April 2010 and May 2011. Distributions of the 224Ra/223Ra activity ratios within the YRE area were used to calculate apparent estuarine water ages. Field-derived results were then compared to hydrodynamic assessments obtained by a Lagrangian particle tracking simulation experiment performed using the Princeton Ocean Model (POM). Water ages obtained via both geotracers and particle tracking agree very well. During both field trips an anomalously “younger” water mass (low salinity and higher radium activities) was observed at about 90–170 km offshore distance from the mouth of the river, suggesting an additional terrestrial water source influenced this area. The temporal distribution of the radium isotopes indicated a semi-diurnal tidal pattern in the YRE with relatively constant isotopic composition of less than a 20% variation during our observations. An integrated water flushing rate based on our observations (excluding the additional anomalous source area) was 8.4 km day−1.

Modeling radium distribution in coastal aquifers during sea level changes: The Dead Sea case

We present a new approach to studying the behavior of radium isotopes in a coastal aquifer. In order to simulate radium isotope distributions in the dynamic flow field of the Dead Sea aquifer, a multi-species density dependent flow model (SUTRA-MS) was used. Field data show that the activity of 226Ra decreases from 140 to 60dpm/L upon entering the aquifer from the Dead Sea, and then further decreases linearly due to mixing with Ra-poor fresh water. On the other hand, an increase is observed in the activity of the shorter-lived isotopes (up to 52dpm/L 224Ra and 31dpm/L 223Ra), which are relatively low in Dead Sea water (up to 2.5dpm/L 224Ra and 0.5dpm/L 223Ra). The activities of the short lived radium isotopes also decrease with decreasing salinity, which is due to the effect of salinity on the adsorption of radium. The relationship between 224Ra and salinity suggests that the adsorption partition coefficient (K) is linearly related to salinity. Simulations of the steady-state conditions, show that the distance where equilibrium activity is attained for each radium isotope is affected by the isotope half-life, K and the groundwater velocity, resulting in a longer distance for the long-lived radium isotopes. K affects the radium distribution in transient conditions, especially that of the long-lived radium isotopes. The transient conditions in the Dead Sea system, with a 1m/yr lake level drop, together with the radium field data, constrains K to be relatively low (<10). Thus, the sharp decrease in 226Ra cannot be explained by adsorption, and it is better explained by removal via coprecipitation, probably with barite or celestine.

Calculation of Vertical Mixing Rate Based on Radium Isotope Distributions in the Yellow Sea and East China Sea

AbstractRadium isotopes in the seawater of the Yellow Sea and the East China Sea (YSECS) were measured and their vertical distributions were described using an advection‐diffusion model. Vertical eddy diffusion coefficients and advection velocities in the YSECS were calculated by fitting the distributions of radium isotopes in the seawater using the least‐square method. The result shows that upwelling occurs in the centers of Northern and Southern Yellow Sea, Zhejiang coast sea area, and North of Taiwan. The current speeds of upwelling are 0.46×10−3 cm·s−1, 0.17×10−3~1.39×10−3 cm·s−1, 2.02×10−3~3.04×10−3 cm·s−1 and 1.06×10−3~2.51×10−3 cm·s−1 in above four sea areas, respectively. Downwelling speeds in the center of Northern Yellow Sea and Northeastern East China Sea and the current speeds are –2.30×10−3 cm·s−1 and –0.61×10−3~–2.10×10−3 cm·s−1, respectively. The vertical eddy diffusion coefficients are determined to be 5.84~48.2 cm2·s−1 with an average of 22.3 cm2·s−1 from the same calculation. Upwelling velocities of the northern Yellow Sea and Zhejiang coast area given by this study are in agreement with the results of previous work, and downwelling in Northern Yellow Sea is also supported by the literature. The agreement of the results obtained in this study with that of literatures not only confirms previous inference about upwelling distribution in the Yellow Sea and East China Sea, but also affirms the validity of our method.

Vertical mixing rate evaluation based on radium isotope distributions of Yellow Sea and East China Sea

Radium isotopes in the seawater of the Yellow Sea and the East China Sea (YSECS) were measured and their vertical distributions were described using an advection-diffusion model. Vertical eddy diffusion coefficients and advection velocities in the YSECS were calculated by fitting the distributions of radium isotopes in the seawater using least-square method.Results showed that upwellings occur in the middle of Northern and Southern Yellow Sea,coastal zone of Zhejiang,and Northeast of Taiwan.The current rates of upwellings are 0.46×10~(-3)cm·s~(-1), 0.17×10~(-3)～1.39×10~(-3)cm·s~(-1),2.02×10~(-3)～3.04×10~(-3)cm·s~(-1) and 1.06×10~(-3)～2.51× 10~(-3)cm·s~(-1) for above four sea areas respectively.Downwellings are indicated in the middle of Northern Yellow Sea and Northeastern East China Sea and the current rates are -2.30×10~(-3)cm·s~(-1) and -0.61×10~(-3)～-2.10×10~(-3)cm·s~(-1) respectively.The vertical eddy diffusion coefficients were obtained as 5.84～48.2 cm~2·s~(-1) with an average of 22.3 cm~2·s~(-1) from the same calculation.Upwelling velocity of Northern Yellow Sea and coastal zone of Zhejiang given by this study is in agreement with the results of literatures,and downwelling in Northern Yellow Sea is also supported by the literature.The agreement of the results obtained in this study with that of literatures not only confirms previous inference about upwelling distribution in the Yellow Sea and East China Sea,but also affirms the validity of our method.

Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary

We hypothesize that nutrient cycling in a Gulf of Mexico subterranean estuary (STE) is fueled by oxygen and labile organic matter supplied by tidal pumping of seawater into the coastal aquifer. We estimate nutrient production rates using the standard estuarine model and a non-steady-state box model, separate nutrient fluxes associated with fresh and saline submarine groundwater discharge (SGD), and estimate offshore fluxes from radium isotope distributions. The results indicate a large variability in nutrient concentrations over tidal and seasonal time scales. At high tide, nutrient concentrations in shallow beach groundwater were low as a result of dilution caused by seawater recirculation. During ebb tide, the concentrations increased until they reached a maximum just before the next high tide. The dominant form of nitrogen was dissolved organic nitrogen (DON) in freshwater, nitrate in brackish waters, and ammonium in saline waters. Dissolved organic carbon (DOC) production was two-fold higher in the summer than in the winter, while nitrate and DON production were one order of magnitude higher. Oxic remineralization and denitrification most likely explain these patterns. Even though fresh SGD accounted for only ∼5% of total volumetric additions, it was an important pathway of nutrients as a result of biogeochemical inputs in the mixing zone. Fresh SGD transported ∼25% of DOC and ∼50% of total dissolved nitrogen inputs into the coastal ocean, with the remainder associated with a one-dimensional vertical seawater exchange process. While SGD volumetric inputs are similar seasonally, changes in the biogeochemical conditions of this coastal plain STE led to higher summertime SGD nutrient fluxes (40% higher for DOC and 60% higher for nitrogen in the summer compared to the winter). We suggest that coastal primary production and nutrient dynamics in the STE are linked.

Seasonal distribution and flux of radium isotopes on the southeastern U.S. continental shelf

Enrichments of radium isotopes in coastal waters have served as indicators of submarine groundwater discharge (SGD). Because coastal waters exchange with the open ocean on a timescale of weeks to months, seasonal patterns of radium isotope distributions may be used to indicate changes in SGD through the year. Here I report the seasonal distributions of four naturally occurring radium isotopes measured throughout the water column of the southeastern U.S. continental shelf, commonly known as the South Atlantic Bight (SAB). The study area extended from Onslow Bay, North Carolina, to Crescent Beach, Florida. Activities of the long‐lived isotopes were highest off the coast of Georgia. In the summer, high activities extended throughout the study area, but during spring and winter, they decreased markedly off the coast of SC. The primary source of excess 226Ra and 228Ra (that is activities in excess of open ocean values) is shown to be SGD. Because the activities of these isotopes in SGD differ little with season, the lower excess activities off South Carolina imply lower rates of SGD during the spring and winter. The excess inventories and fluxes of 226Ra and 228Ra provide an estimate of the residence time of water on the shelf. These residence times range from 30 to 60 days with a mean of about 40 days.

Radium isotopes in coastal waters on the Amazon shelf

Radium isotope distributions on the continental shelf adjacent to the Amazon River are a complex function of salinity. The river and seawater endmembers for each radium isotope are low. Data from intermediate salinities plot as a highly scattered region above the conservative mixing line connecting the river and ocean endmembers. Spatial and temporal variability in sediment mixing and water residence times cause the scatter in the radium isotope data. We have used the distributions of 226Ra, 228Ra, and 224Ra collected during four cruises representing different conditions on the shelf to construct inventories of radium due to nonconservative additions from sediments. Using estimates of water residence times, we calculate fluxes of these radium isotopes from the Amazon system. The amount of fresh river sediment entering the system each year is adequate to support the nonconservative flux of 226Ra from the shelf sediments; however, the ultimate release of 226Ra from the sediments requires weeks to months. To support the nonconservative flux of 228Ra from this system, sediments of the 1–2 m thick surface mixed layer must be resuspended every two years. The resulting flux supports 15% of the 228Ra entering the surface Atlantic Ocean. Release of 224Ra from suspended sediments, primarily fluid mud, is more than adequate to support the inventory of 224Ra on the Amazon shelf. Comparing the AmasSeds low discharge cruise (November 1991) to a low discharge sampling in December 1982 revealed no significant changes in the 226Ra and 228Ra distributions.