Carbon export from submarine groundwater discharge in a semi-enclosed bay: Impact for the buffering capacity against coastal ocean acidification.

Submarine groundwater discharge (SGD) serves as an important pathway for the transport of dissolved carbon from land to ocean, significantly affecting the coastal biogeochemical cycles. However, the impact of SGD-derived dissolved carbon on the coastal carbon budget remains poorly understood. This study first quantified SGD and associated dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and total alkalinity (TA) fluxes in Daya Bay using mass balance models based on radium isotopes (223Ra, 224Ra, 226Ra and 228Ra). We then constructed carbon mass balance models to evaluate the impact of SGD-derived carbon on the buffering capacity against coastal ocean acidification. The estimated SGD fluxes ranged from 0.80 × 107 to 2.64 × 107 m3d-1. The DIC, DOC and TA fluxes from SGD were 17.90-36.44 mmol m-2d-1, 0.93-2.13 mmol m-2d-1, and 21.19-28.47 mmol m-2d-1, respectively. Based on carbon mass balances, the DIC flux from SGD was 19-39 times the riverine input, accounting for 27.16 % ∼ 37.64 % of the total carbon source. These results suggest that SGD is a major contributor to DIC, significantly affecting the coastal carbon budget. Furthermore, the average TA:DIC ratio of groundwater discharging into Daya Bay was approximately 1.13. High TA exports enhance the buffering capacity of the coastal ocean and contribute bicarbonate to the ocean, playing a significant role in the ocean carbon sequestration process. This study demonstrates the importance of SGD-derived dissolved carbon in the assessment of coastal carbon budgets.

Research methods for seawater intrusion in China and recommendations for novel radium-radon technologies.

Seawater intrusion has been a globally significant environmental issue. This paper comprehensively reviews and highlights the research methods of seawater intrusion in China, recommending the potential application of novel radioactive radium-radon isotopes. Geochemical and geophysical techniques have been extensively utilized in studying seawater intrusion in China, including methods such as hydrochemical analysis, groundwater level observations, geophysical survey techniques, and isotope tracing. The former three methodologies boast a lengthier historical application in seawater intrusion field, while the radium-radon tools in isotope tracing, as newcomers, can specifically indicate crucial scientific questions such as seawater intrusion rates, salt groundwater age, water-rock reactions, and preferential flow dynamics. However, it is imperative to acknowledge the limitations inherent in the utilization of radium-radon tools within the realm of seawater intrusion research, as with any other methodologies. Strategic integration of radium-radon tools with other methodologies will propel advancements in the investigation of seawater intrusion in China. While the primary focus is on research methods in China, insights gained from novel radium-radon tools could have broader value for seawater intrusion research and coastal management globally.

The Radium triplet 226Ra, 228Ra, 224Ra in saline deep water - a valuable information source.

The occurrence of enhanced concentration of the radium triplet 226Ra, 228Ra and 224Ra is a frequently observed property of highly saline anoxic deep water as used e.g. in geothermal plants. In the present study we develop a model to explain the observed activity levels in the brines. The model considers processes at the rock-fluid interface of the aquifer like alpha recoil, sorption and surface precipitation and is implemented by means of a Monte Carlo simulation. The outcomes of the simulations indicate the dominating role of fine-grained constituents of the reservoir rock, e.g. claystone with enhanced specific activities of the natural decay chains. Mass fractions of such material in the order of a few percent are sufficient to result in radium fluid concentrations >1 Bq l-1. Also a generally valid relation between the Th/U ratio in the aquifer rock and the 228Ra/226Ra activity ratio in the fluid was found. This link improves the agreement between radium fluid data and the mean Th/U ratio of the Earth's crust. The 224Ra/228Ra fluid ratios reflect the transport time from the location of last radium release to the sampling point. The model findings were applied to a well investigated aquifer used in a geothermal plant in the North German Basin. An eight component system of the aquifer rock was established as the basis for the simulation of the radium concentrations in the deep fluid. The comparison between simulation and fluid analyses revealed a degree of radium sorption of about 50 %, which is necessary to match the model's results with the measurements. On the other hand, the 228Ra/226Ra fluid ratio of the brine was well reproduced by the simulation, showing the suitability of the model even in complex heterogeneous reservoirs. From the 224Ra/228Ra fluid ratios a transition from pore-to fracture-guided transport < 10 m distance from the production well is suggested. Precipitates from such deep fluids occurring after changes of the thermodynamic conditions are able to accumulate radium isotopes in Ba/Sr-sulphate phases. The time dependence of the radioactive disequilibrium between 226Ra, 228Ra and its child 228Th in such scales is described by a mathematical model and is applied to two different uptake models. Based on this approach, age determinations on precipitates found in different components of a geothermal plant are conducted. They reveal the triggering of scale formation due to modifications in the plant. The results are suitable for drawing conclusions about the operation of the system, which result in a reduction in the amount of scale and a reduction in downtimes.

Unique transport paths of 137Cs from the Indian to Southern Oceans.

To assess ocean-scale transport systems, we examined the latitudinal cross-sectional distribution of 137Cs activity concentrations in the Indian and Southern Oceans between December 2019 and January 2020 using low-background γ-spectrometry. At 0°-20°S, 137Cs concentrations exhibited a gradual decrease below the mixing layer (1-0.1 mBq/L). However, the concentrations steeply decreased toward the Southern Ocean along a transect of 30°-60°S (from 0.8 to 0.02 mBq/L) with minor vertical variation at each site. For the 137Cs inventories (0-800 m depth) from 15 to 600 Bq/m2, a maximum value was recorded at 30°S, indicating the downwelling of 137Cs as a reservoir for the Subantarctic Mode Water. The significantly low concentrations (0.02 mBq/L) at 60°S suggest minimal transport of 137Cs to the Southern Ocean. These findings assist in understanding 137Cs circulation patterns and provide valuable insights into the transport pathways of soluble contaminants.

Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef.

Rivers are often assumed to be the main source of nutrients triggering eutrophication in the Great Barrier Reef (GBR). However, existing nutrient budgets suggest a major missing source of nitrogen and phosphorus sustaining primary production. Here, we used radium isotopes to resolve submarine groundwater discharge (SGD)-derived, shelf-scale nutrient inputs to the GBR. The total SGD was ∼10-15 times greater than average river inputs, with nearshore groundwater discharge accounting for ∼30% of this. Total SGD accounted for >30% of all known dissolved inorganic N and >60% of inorganic P inputs and exceeded regional river inputs. However, SGD was only a small proportion of the nutrients necessary to sustain primary productivity, suggesting that internal recycling processes still dominate the nutrient budget. With millions of dollars spent managing surface water nutrient inputs to reef systems globally, we argue for a shift in the focus of management to safeguard reefs from the impacts of excess nutrients.

Submarine groundwater discharge and associated metal elements into an urbanized bay.

In this study, geochemical tracers (radium isotopes) and heavy metals (Pb, Zn, Cd, Cr and As) were analyzed to derive the submarine groundwater discharge (SGD) and associated metal fluxes during four seasons in an urbanized bay (Daya Bay, China). Results showed that Pb and Zn were the main pollutants in bay water. SGD was found to exhibit an obvious seasonal trend (autumn > summer > spring > winter). Such seasonal patterns may be related to the hydraulic gradient between groundwater level and sea level, storm surges and tidal range. SGD was a dominant source of marine metal elements, contributing 19 %-51 % of the total inputs of metals into Daya Bay. The bay water was classified as slight pollution to heavy pollution, which could be linked to SGD-derived metal fluxes. This study provides a better understanding of the important role that SGD plays in metal budgets and ecological environments of coastal waters.

Study of Ra desorption processes in an estuary system with high-turbidity at the Southeast China.

Radium (Ra) isotopes are extensively used as geochemical tracers for studying water mass mixing and submarine groundwater discharge in marginal and coastal seas. However, river-borne particles and seafloor sediments are an important source of Ra in marine systems due to Ra desorption. Therefore, it is necessary to study the desorption behaviors of Ra isotopes in river sediment or suspended particles. Here, the desorption behaviors of four Ra isotopes (223Ra, 224Ra, 226Ra, and 228Ra) in the Zhangjiang River sediments were investigated by a series of designed variable-controlling experiments in the laboratory. Within the designed salinity range, desorption amounts of Ra isotopes increased with increasing salinity, and when the salinity was greater than 15 ppt, Ra desorption reached an equilibrium state. Overall, desorption of Ra isotopes increased with the decrease of particle grain size, however, the desorption fractions of 224Ra and 228Ra decreased with decreasing particle size due to the increase of original Ra activities in smaller sediment particles. In the experiments, we found that two sediment samples with similar mean grain size (3.8 µm and 3.3 µm) and similar grain size distributions had significantly different Ra desorption under the same conditions. The results of mineral composition analysis based on X-ray diffraction showed that these two samples had different percentages of kaolinite, quartz, and plagioclase, which indicated that the mineral composition of particles had an important effect on Ra isotope desorption. In conclusion, salinity, particle grain size, and mineral composition all had significant effects on Ra desorption behaviors of sediment particles. Based on the above desorption experiments, the desorbed fluxes of four Ra isotopes from river-borne sediments to the Dongshan Bay were estimated to be (5.95 ± 1.47) × 107 Bq yr-1 for 223Ra, (1.95 ± 0.27) × 109 Bq yr-1 for 224Ra, (2.73 ± 0.47) × 108 Bq yr-1 for 226Ra, and (1.26 ± 0.20) × 109 Bq yr-1 for 228Ra, respectively.

Quantifying the groundwater seepage along a glacier originated river by integrated use of radium isotopes and hydrochemistry.

Glaciers, as the core part of the cryosphere, are very sensitive to climate change. As an indicator of glacier changes, the characteristics of glacier-originated rivers have profound significance to global climate change and local water resources. In this paper, the Mingyong River, a glacier-originated river replenished by groundwater, was selected to study the river hydrological cycle characteristics by integrating natural tracer radium isotopes with hydrochemical parameters. The results showed that there were significant differences in radium isotope activities and hydrochemical parameters between groundwater and river water, and the radium activities increased along the river, which reflected the fact that the river was supplied by groundwater seepage. We also found that the activity ratios of 224Ra/228Ra in river and groundwater were less than one unit, which indicated that the groundwater and river water circulated rapidly and that the radioactive equilibrium of short-time radium isotopes had not yet been reached. According to the geochemical behavior of radium in river water body, the mass balance equation of radium was established. 228Ra and 224Ra were used to estimate the groundwater seepage of different segments of the Mingyong River. The results demonstrated that the groundwater seepage fluxes calculated by 224Ra and 228Ra were similar and increased along the river from 123.12 to 657.68 m3 m-1 d-1. Our results have certain significance in revealing the characteristics of the local hydrological cycle and demonstrate that radium isotopes can be used as a tool to estimate the groundwater discharge of rivers in glacial environments.

Effects of salinity and particle size on radium desorption from river sediments in the Qinghai-Tibet Plateau.

Natural radium isotopes have been widely used to study groundwater discharge in different systems. Therefore, it is of great significance to understand the desorption behavior of radium isotopes on sediments to trace water-land exchange processes. However, there is very limited studies observing the desorption Ra isotopes to lake water of the brine lake. 224Ra desorption experiments with different salinities and particle sizes were carried out by collecting samples of brackish water from Qinghai Lake, brine from Dabuxun Lake and river sediments entering the lakes. The results show that the desorption activity of 224Ra from the river sediments to lake water of Qinghai Lake is 0.2 dpm/g when the salinity is 10.07‰. The maximum desorption activity of 224Ra from river sediments to lake water of Dabuxun Lake is 0.195 dpm/g at a salinity of 40.81‰. A salinity of 41.81‰ and particle size of 16.28 µm are the threshold points affecting the desorption behavior of Ra. When the salinity is less than 40.81‰, the desorption activity of Ra increases linearly with increasing salinity. When the salinity is greater than 40.81‰, the desorption activity of Ra decreases nonlinearly with increasing salinity and tends toward a stable low value. When the particle size is larger than 16.28 µm, the small particle size promotes desorption. The smaller the particle size is, the greater the desorption activity is. When the particle size is less than 16.28 µm, the small particle size inhibits desorption. The smaller the particle size is, the smaller the desorption activity. The co-precipitation of Ra2+ with supersaturated Ca2+, SO42- and other ions may be the main reason for the threshold point of salinity and particle size in Ra desorption process in salt lake system.

Radiological monitoring in some coastal regions of the Saudi Arabian Gulf close to the Iranian Bushehr nuclear plant.

We studied the concentrations of terrestrial and anthropogenic radionuclides in seawater and shore sediment/sand of three selected regions; Khafji, Safaniyah and Menifah along the Saudi Arabian Gulf coast. The mean activity concentrations of the 228Ra, 226Ra, and 40K in the analyzed sand samples are 5.9, 3.5 and 113.5 Bq/kg, and the respective values in seawater samples are 1.6, 0.8 and 10.4 Bq/L. All data show lower than the corresponding UNSCEAR (2000) reported world average values of 35, 30 and 400 Bq/kg for soil matrix. A few relevant radiological hazards were quantified by the estimation of the absorbed dose rate, and the results are compared with the prescribed limits set by international regulatory bodies. Measured data indicates that the studied coastal regions pose a negligible radiological hazards to the public, and show an insignificant radioactive loading to this coastal region by the Busher nuclear power plant.

Radium desorption behavior of riverine suspended sediment: Theoretical and experimental.

Radium desorption from riverine or suspended sediment is an important source term of Ra isotopes in estuarine water, which is one of the significant factors affecting the source/sink material balance for Ra applications, such as estimating submarine groundwater discharge flux in coastal zones. In this paper, a theoretical model is proposed to study Ra desorption considering several influencing factors, including salinity, particle size and size distribution, alpha recoil range of atoms in the sediment grains, and grain surface roughness. The results of the model parameter sensitivity analysis show that the alpha-decay recoil can improve the number of total exchangeable Ra on grain surfaces, but it is more significant for sediment of small grain size. Sediment with large mean grain size may contain a considerable number of smaller particles, which may facilitate the Ra desorption quantity. Due to the relatively low concentrations of Ra in natural mineral, the Ra desorption quantity is not sensitive to the roughness of the sediment grain surfaces. The model fits well with the indoor experimental data, by fitting the experimental data or quoting the literature values, the desorption parameters (A, B, α, ß) in the model can be determined to estimate the total number of exchangeable Ra of a sediment and also predict the Ra desorption of sediments in different conditions.

Effects of nutrient-rich submarine groundwater discharge on marine aquaculture: A case in Lianjiang, East China Sea.

Submarine groundwater discharge (SGD) and associated nutrient have long been received insufficient attention in the aquaculture areas of Lianjiang, East China Sea. In this study, we used 224Ra mass balance model to evaluate the importance of SGD in the aquaculture areas of Lianjiang in different seasons. The results showed that SGD fluxes were 0.14 ± 0.070 m3 m-2 d-1 in July and 0.077 ± 0.040 m3 m-2 d-1 in October 2019, which corresponded to approximately 22% and 32% of the river discharge, respectively. Nevertheless, the dissolved inorganic nitrogen (DIN) and phosphorus (DIP) fluxes contributed by SGD over total nutrient inputs were approximately 58% and 73%, respectively in July, 11% and 33%, respectively in October 2019. The ratio of DIN to DIP in seawater was slightly higher than that derived from SGD in July and significantly lower than that derived from the river and SGD in October, which was prone to profound modulated by shellfish (e.g., clam Ruditapes philippinarum and Sinonovacula constricta) and nori (e.g., Pyropia haitanensis) that selectively acquire nitrogen and phosphorous, suggesting that SGD could affect the nutrient structure and aquaculture activities in Lianjiang. Moreover, the new primary production via SGD yielded (2.47 ± 1.32) × 103 mg C m-2 d-1 in July and (3.52 ± 1.76) × 102 mg C m-2 d-1 in October 2019, which were approximately 8-25% and 10-31% of the production in the entire study area. These meaningful discoveries indicated that SGD and associated nutrient are the predominant regulator of nutrient and aquaculture structure under significant seasonal differences.

Role of lithology in the presence of natural radioactivity in drinking water samples from Tarragona province.

One hundred and ninety-six drinking water samples from the different regions of Tarragona province (Catalonia, Spain) were analysed to determine the gross alpha and beta activity. Individual alpha emitting isotope activities were also determined to evaluate a possible relationship between their radiological content and the lithological and hydrogeological formations present in the studied area. The results obtained showed that approximately 23% of the analysed samples, mainly from five of the evaluated regions, had a gross alpha index exceeding the parametric value of 0.1 Bq/L for waters intended for human consumption according to the current legislation. This could be related to the presence of natural radionuclides in these water samples. The differences between the radiological content in these samples could be related to the different lithological conditions of the areas included in this study. High activity levels of 234U, 238U, 224Ra, 226Ra and 228Ra were detected in specific samples, mainly from granitic and carbonate areas. This research also focuses on evaluating the radiological risk associated with water ingestion. In this regard, consuming 95.5% of the drinking water samples analysed would not imply a health risk to the population as the annual effective doses calculated were below 0.1 mSv/year. There was only one sample that exceeded this level with a value of 0.33 mSv/year. 226Ra activity concentration was the radionuclide that mainly contributed to this dose.

Utility of radium quartet for evaluating porewater-derived carbon to a saltmarsh nearshore water: Implications for blue carbon export.

Saltmarshes are global hotspots of carbon sequestration and storage and are known as effective blue carbon ecosystems. However, the role of porewater exchange in saltmarshes as a source of carbon to the nearshore waters is still poorly constrained. Herein, we examined the radium quartet, dissolved inorganic (DIC) and organic (DOC) carbon in the porewater and nearshore surface water of Chongming Dongtan saltmarsh, China. Multiple methods based on the radium quartet were applied to estimate the porewater exchange, including the three-endmember model, mass balance model and time series observation. All methods revealed that the porewater exchange rate in Chongming Dongtan saltmarsh equaled 3.37 ± 1.23 cm d-1. The porewater-derived DIC and DOC fluxes were then estimated to be (1.51 ± 0.64) × 107 and (9.97 ± 6.96) × 105 mol d-1, respectively, which correspondingly made up 64.6% and 35.6%, of the total inputs into the Chongming Dongtan saltmarsh nearshore water. Considering the intertidal area covered by saltmarsh vegetation, carbon export through the porewater exchange was 3.87 ± 1.55 g C m-2 d-1, and was 1.2-fold greater than the carbon burial rate, accounting for approximately 29% of carbon outwelling in Chongming Dongtan saltmarsh. This study highlights the significance of porewater exchange for evaluating carbon sequestration capacity, and suggests that porewater exchange should not be overlooked in blue carbon assessments of saltmarshes.

Radium and nitrogen isotopes tracing fluxes and sources of submarine groundwater discharge driven nitrate in an urbanized coastal area.

The quantitative evaluations of nutrients delivered by submarine groundwater discharge (SGD) have been widely conducted worldwide, but sources of nutrients in the discharged submarine groundwater remain unclear. Identifying these sources of nutrients is essential to the protection and management of marine ecological environments. This study aims to evaluate the magnitudes of SGD and the associated nitrate in the Guangdong-Hongkong-Macao Greater Bay Area (GHM Greater Bay Area), China, and identify the sources of SGD-driven nitrate in this region using radioactive radium (Ra) isotopes (223Ra, 224Ra, and 228Ra) and stable nitrogen (N) and oxygen (O) isotope composition of nitrate (δ15N-NO3- and δ18O-NO3-). The results of the Ra mixing model show that the estimated SGD and the associated nitrate fluxes into the Greater Bay Area are (9.15 ± 1.26) × 108 m3/d and (3.77 ± 0.52) × 107 mol/d, respectively, both of which are comparable to the contributions from the Pearl River. Combing NO3- dual isotopic signatures of sampled coastal groundwater and five kinds of potential nitrate sources, we found that ammonium (NH4+) fertilizer and natural soil N are the two main sources of nitrate in discharged submarine groundwater and rivers. No anthropogenic inputs from manure or sewage waste were identified. This study provides significant insights into the establishment of effective management strategies for controlling SGD-nutrients into the bay and protecting the marine ecological environment.

Identifying locations and sources of groundwater discharge into Poyang Lake (eastern China) using radium and stable isotopes (deuterium and oxygen-18).

Knowledge of groundwater discharge (location and sources) into Poyang Lake is needed for water resources management and ecological security. In this study, hydrochemical and stable (δD and δ18O) and radium (223Ra, 224Ra, 226Ra, and 228Ra) isotopic approaches were employed to study the hydrochemical and isotopic characteristics of groundwater and surface water (river water and lake water) to identify the places where groundwater discharged into Poyang Lake and the groundwater discharge sources. The results showed that the groundwater discharge area was extensive during the dry season. The locations of predominant groundwater discharge were indicated by the evolution of radium and stable isotopes in lake water along two water flow profiles. At the confluence of Ganjiang and Xiushui rivers, groundwater with more negative δ18O value than that of the lake water discharged into this area, and the estimated groundwater discharge proportion in this area was close to that of the river water input. The main sources of groundwater input for Poyang Lake were inferred to originate from clastic rock pore-fissure aquifer and bedrock fissured aquifer around this lake. This study also found that groundwater affected by the anthropogenic activities may have discharged into Poyang Lake. Future studies are required to focus on groundwater discharge into Poyang Lake for its management and protection.

Submarine karstic springs as a source of nutrients and bioactive trace metals for the oligotrophic Northwest Mediterranean Sea.

Groundwater springs in karstified carbonate aquifers are known to transport carbon, nutrients and trace elements to the coastal ocean. The biogeochemical significance of submarine karstic springs and their impact on coastal primary production are often difficult to quantify. We investigated several karstic springs, including the first-order Port-Miou spring, in an urbanized watershed that is also severely impacted by sewage effluent (Calanques of Marseille-Cassis, France). Karstic springs were elevated in major nutrients and bioactive trace metals over Mediterranean seawater, with relatively low concentration ranges. Groundwater NO3- was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO2- and NH4+ was autochthonously produced during mixing between karst groundwater and seawater. Submarine groundwater discharge (SGD) during March 2018 (wet season, baseflow conditions) was 6.7 ± 2.0 m3 s-1 for the entire investigated coastline, determined from simultaneous 224Ra and 226Ra mass balances. The contribution of groundwater PO43-, the major limiting nutrient of the Mediterranean Sea, sustained only 1% of primary production adjacent to sewage outfall, but between 7 and 100% of the local primary production in areas that were not impacted by sewage. Groundwater and seawater Fe:DIN and Fe:DIP ratios suggest that Fe was not a limiting micro-nutrient during the period of study, where bioactive trace metal fluxes were dominated by sewage and atmospheric deposition, although excess Fe from groundwater may locally enhance N fixation. Groundwater solute fluxes may easily vary by a factor of two or more over time because karst aquifers are sensitive to precipitation, as is the case of the regional carbonate karstified aquifer of Port-Miou, highlighting the critical importance of properly characterizing nutrient and trace metal inputs in these coastal environments.

Evaluations of submarine groundwater discharge and associated heavy metal fluxes in Bohai Bay, China.

Submarine groundwater discharge (SGD) has been recognized as an important source of dissolved heavy metals to the coastal ocean. Bohai Bay, the second largest bay of Bohai Sea in China, is subjected to serious environmental problems. However, SGD and SGD-derived heavy metal fluxes in the bay are seldom reported. In this study, we present mass balance models considering the radium losses caused by recirculated seawater to estimate water age, SGD and SGD-derived heavy metal fluxes in Bohai Bay during May 2017. The water age is estimated to be 56.7-85.0 days based on tidal prism model. By combining water and salt mass balance models, submarine fresh groundwater discharge (SFGD) is estimated to be (3.5-9.3) × 107 m3 d-1. The SGD flux estimated by the radium mass balance models is (3.2-7.7) × 108 m3 d-1, an order of magnitude larger than the discharge of the Yellow River during the sampling period. SGD-derived heavy metal fluxes were estimated to be (0.2-6.0) × 107 mol d-1 for Fe, (1.2-2.7) × 107 mol d-1 for Mn, (3.0-8.2) × 105 mol d-1 for Zn, (2.7-7.4) × 104 mol d-1 for Cr and (0.6-1.8) × 103 mol d-1 for Cd, which are significantly higher than those from local rivers. This study reveals that SGD is a significant source of heavy metals (Mn, Zn and Fe) into Bohai Bay, which may have important influences on the metal budgets and ecological environments in coastal areas.

Combining hydrological investigations and radium isotopes to understand the environmental effect of groundwater discharge to a typical urbanized estuary in China.

Pollution of urbanized rivers with excess nutrients due to groundwater discharge is an increasing environmental concern worldwide. Dan'ao river, a typical urbanized river in the Guangdong-Hong Kong-Macao Greater Bay Area, is experiencing heavy water pollution. However, the groundwater-derived nutrient loads had not yet been thoroughly quantified. In order to quantify the contribution of groundwater-derived nutrient inputs, we combined the methods of hydrological investigations and radium isotopes. Groundwater and river water samples were collected from the river upstream to the estuary for the analyses of radium quartets and nutrients including DIN, DIP and DSi. The results showed that the radium activities in both surface water and groundwater decreased from the estuary to the upstream. The groundwater discharge rate was estimated by the radium mass balance model using short-lived radium isotopes (223Ra and 224Ra). The estimated groundwater discharge rate ranged from 1.99 × 105 to 6.67 × 105 m3 d-1, comparable to the upstream river discharge rate of 4.23 × 105 m3 d-1. The groundwater-derived nutrient fluxes were 165.66-554.98 mmol m-2 d-1 for DIN, 2.47-8.26 mmol m-2 d-1 for DIP and 63.73-213.49 mmol m-2 d-1 for DSi, respectively. They contributed 19%~44% DIN, 16%~39% DIP, and 31%~60% DSi of all the nutrient inputs into the Dan'ao River, respectively. In addition, the nutrient inputs by groundwater discharge has an average DIN:DIP ratio of as high as 190, which is able to potentially affect the riverine and marine nutrient structures. These findings may provide useful information for designing control strategies for reducing massive nutrient inputs to Dan'ao River in the future.

Radium isotopes-suspended sediment relationships in a muddy river.

Radium isotopes are known to be excellent geochemical tracers for study of oceanographic processes. We show here that radium isotopes can also be used to assess adsorption/desorption and transport processes in rivers. The Yellow River (Huanghe), one of the longest, most turbid and heavily regulated rivers in the world, is used as an example. We first investigated the temporal and spatial behavior of radium isotopes (224Ra and 226Ra) in the lower reaches of the river, and found that this zone displayed some of the highest known riverine radium concentrations and fluxes in the world. Suspended particulate matter (SPM) is shown to be the dominant factor controlling radium activities. Laboratory simulation experiments showed that radium desorption from SPM obeys an exponential relationship in fresh water (S = 0). When salinities are >10, the increase in radium concentration follows a linear increase with respect to the amounts of SPM added. Significantly higher radium concentrations (3-5 times), especially for short-lived 224Ra, were observed during the "Water-Sediment Regulation Scheme" (WSRS), an annual management event when ∼15%-55% of the annual water discharge and ∼30%-75% of the annual sediment load are released from a reservoir to control sedimentation in the Yellow River. The radium fluxes during WSRS periods (∼2 weeks long) accounted for more than half of the entire annual load during the periods studied. Sediment erosion and pore water release are also thought to be important processes supplying radium to the river. After a WSRS, Ra desorption from SPM increases and becomes the prevailing process.