Estimating the input of submarine groundwater discharge (SGD) and SGD-derived nutrients in Geoje Bay, Korea using (222)Rn-Si mass balance model

Design of intermittent continuous measurement of radon concentration in water

The U.S. Environmental Protection Agency established the maximum contaminant level limit for radon concentration in drinking water as 11.1 Bq L^-1. A new device based on the bubbling method with a 290 mL sample bottle was designed for intermittent continuous measurement of water radon concentration. A STM32 is used to control the switch of the water pump and the valves. The Water-Radon-Measurement software written in C# is to connect RAD7 and calculate the water radon concentration automatically.

Submarine groundwater discharge: An Asian overview

Submarine groundwater discharge (SGD) is the combination of fresh and saline groundwater flux to marine system through continental boundaries regardless of its chemical composition and factors influencing the flow. We have discussed the SGD studies in the Asian context; SGD has been studied in various parts of Asia, including China, Japan, South Korea, and Southeast Asia. In China, SGD has been studied in several coastal regions, including the Yellow Sea, the East China Sea, and the South China Sea. In Japan, SGD has been studied in the Pacific coast, where it has been found to be an important source of fresh water to the coastal ocean. In South Korea, SGD has been studied in the Yellow Sea, where it has been found to be an important source of fresh water to the coastal ocean. In Southeast Asia, SGD has been studied in several countries, including Thailand, Vietnam, and Indonesia. Recently the SGD studies acquired much development India, the research on SGD in India is limited, and more studies are needed to understand the SGD process, its impact on the coastal environment, and the management strategies, Groundwater extraction for irrigation, industry, and domestic use is increasing in India, which can affect the SGD process in coastal aquifers. Overall, the studies suggest that SGD is an important process in Asian coastal regions, playing a role in the supply of fresh water and the transport of pollutants and nutrients.

Estimation of 226Ra and 228Ra Content Using Various Types of Sorbents and Their Distribution in the Surface Layer of the Black Sea

Radium isotopes have traditionally been used as tracers of surface and underground fresh waters in land-ocean interactions. The concentration of these isotopes is most effective on sorbents containing mixed oxides of manganese. During the 116 RV Professor Vodyanitsky cruise (22 April-17 May 2021), a study about the possibility and efficiency of ²²⁶Ra and ²²⁸Ra recovery from seawater using various types of sorbents was conducted. The influence of seawater flow rate on the sorption of ²²⁶Ra and ²²⁸Ra isotopes was estimated. It was indicated that the Modix, DMM, PAN-MnO₂, and CRM-Sr sorbents show the best sorption efficiency at a flow rate of 4-8 column volumes per minute. Additionally, the distribution of biogenic elements (dissolved inorganic phosphorus (DIP), silicic acid, and the sum of nitrates and nitrites), salinity, and ²²⁶Ra and ²²⁸Ra isotopes was studied in the surface layer of the Black Sea in April-May 2021. Correlation dependencies between the concentration of long-lived radium isotopes and salinity are defined for various areas of the Black Sea. Two processes control the dependence of radium isotope concentration on salinity: conservative mixing of riverine and marine end members and desorption of long-lived radium isotopes when river particulate matter meets saline seawater. Despite the high long-lived radium isotope concentration in freshwater in comparison with that in seawater, their content near the Caucasus shore is lower mainly because riverine waters meet with a great open seawater body with a low content of these radionuclides, and radium desorption processes take place in an offshore area. The ²²⁸Ra/²²⁶Ra ratio derived from our data displays freshwater inflow spreading over not only the coastal region, but also the deep-sea region. The lowered concentration of the main biogenic elements corresponds to high-temperature fields because of their intensive uptake by phytoplankton. Therefore, nutrients coupled with long-lived radium isotopes trace the hydrological and biogeochemical peculiarities of the studied region.

Tidally driven submarine groundwater discharge to a marine aquaculture embayment: Insights from radium and dissolved silicon

For understanding the significance of tidal pumping for driving submarine groundwater discharge (SGD), two time series observations were conducted over tidal cycles in typical mariculture areas of Sanmen Bay, China. Based on ²²⁴Ra and dissolved silicon (DSi), the tide-driven SGD fluxes showed significant negative correlations with tidal height, and were 16.4 ± 5.6 cm d^-1 and 21.7 ± 13.9 cm d^-1 at two coastal sites of the Sanmen Bay, highlighting the potential of DSi in SGD calculation when coastal waters are on short time scales. Furthermore, nutrient fluxes through tidally influenced SGD were estimated to be 11.5-26.5 mmol m^-2 d^-1 for dissolved inorganic nitrogen, 0.06-0.34 mmol m^-2 d^-1 for dissolved inorganic phosphorus and 11.5-32.1 mmol m^-2 d^-1 for DSi, which were higher than those from surface loadings, revealing that tide-driven SGD with large amounts of nutrient and high N:P ratios may significantly contribute to the development of marine aquaculture.

Seasonal Variability of Nutrients and Radium Isotope Fluxes from Submarine Karstic Spring at the Southwest of Crimea, Black Sea

The groundwaters of the southwestern region of Crimea are formed on the karst plateaus of the Crimean Mountains, and a significant amount of them is discharged into the Black Sea. The Crimean Peninsula is a water-deficient region; therefore, the study of its hydrogeology is an urgent task, since groundwater is a valuable freshwater resource. Through submarine groundwater discharge (SGD), the transfer of chemical compounds to the sea in the event of anthropogenic interference may also occur. In this work the fluxes of submarine groundwaters in the area of Cape Peleketo in different seasons, and also the fluxes of nutrients within them, are evaluated for the first time, as well as the factors determining their variability. During the study, hydrological (temperature, salinity (S), current velocity) and hydrochemical (concentration of biogenic elements) parameters, as well as the concentration of long-lived isotopes of 226Ra and 228Ra, were measured. The SGD fluxes were estimated through the mixing formula. As the endmember, we used groundwater concentrations of nutrients or radiotracers, defined by extrapolation of nutrients or radium concentrations to zero salinity. Significant differences in the studied region’s SGD flux values (from 4100 to 13,900 m3/day) are shown; maximum values are in winter and summer, and minimum values are in autumn and spring. The relationship between the seasonal variability of the discharge intensity and the amount of precipitation in the groundwater formation area is shown. The data obtained show that this source makes a significant contribution to the local supply of nutrients. Substantial amounts of nitrates come from the karst cavity, which can lead to eutrophication and limit the primary production of phosphorus in the local coastal sea region.

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.

Hydroclimatic variability drives submarine groundwater discharge and nutrient fluxes in an anthropogenically disturbed, semi-arid estuary

Nutrient budgets in semi-arid estuaries, with ephemeral freshwater inflows and limited nutrient sources, are likely incomplete if contributions from submarine groundwater discharge (SGD) are not included. Here, the relative importance of saline/recirculated SGD-derived nutrient fluxes spatiotemporal variability to the overall nutrient budget is quantified for Nueces Bay, Texas, U.S.A., across hydroclimatic conditions ranging from drought to normal, to flood. On average, 67% of the variance in water quality is due to temporal differences while 16% is explained by spatial differences. Principal component analysis (PCA) reveals three principal components: freshwater inflow (PC1 28.8%), saline/recirculated SGD and recycled nitrogen (PC2 15.6%), and total SGD and "new" nitrogen (PC3 11.2%). Total SGD porewater fluxes ranged from 29.9-690.3 mmol∙m^-2d^-1 for ammonium, 0.21-18.7 mmol∙m^-2d^-1 for nitrite+nitrate, 3.1-51.3 mmol∙m^-2d^-1 for phosphate, 57.1-719.7 mmol∙m^-2d^-1 for silicate, and 95.9-36,838.5 mmol∙m^-2d^-1 for dissolved organic carbon. Total and saline/recirculated SGD fluxes were on average 150-26,000 and 5.8-466 times, respectively, greater than surface runoff fluxes across all seasons. Nitrogen (N) enrichment in porewater occurs near the agricultural fields because of soil N flushing and percolation to groundwater, which facilitates N-rich groundwater fluxes. There were substantial "new" N inputs from terrestrial groundwater following precipitation while saline/recirculated SGD of recycled N accounts for only <4% of total SGD inputs. The "new" N inputs occur in the river and river mouth during flooding, and near the north shore where topography and hydraulic gradients are steeper during drought. Thus, while significant inputs of N may be associated with atmospheric deposition, or remineralization in the porewater, groundwater is the highest contributor to the nutrient budget in Nueces Bay. This result implies that nutrient management strategies should focus on land-use practices to reduce N contamination of shallow groundwater and subsequent contamination of estuaries.

222Rn, 210Pb and 210Po in coastal zone groundwater: Activities, geochemical behaviors, consideration of seawater intrusion effect, and the potential radiation human-health risk

Groundwater quality in human-influenced coastal landscapes is receiving novel attention. Radionuclides have been recognized as another important monitoring indicator in many developed countries due to the discovery of extremely high level of natural ²¹⁰Po (up to 10,000 Bq/m³) and radium and radon isotopes. This study aims to evaluate the groundwater quality in the Beibu Bulf-Guangxi coast from radiological point of view. ²¹⁰Po, ²¹⁰Pb and ²²²Rn activities in 20 wells ranged from 0.24 ± 0.05 to 6.96 ± 1.62 Bq/m³, 2.17 ± 0.12 to 13.08 ± 0.74 Bq/m³ and 1500 ± 200 to 31,800 ± 900 Bq/m³, respectively. Compared with research data of other countries, groundwaters in this area have ²¹⁰Po, ²¹⁰Pb and ²²²Rn activity within low levels. The large deficiencies of ²¹⁰Po and ²¹⁰Pb relative to ²²²Rn in groundwaters implied that ²¹⁰Po and ²¹⁰Pb are strong particle-reactive radionuclides and they might be controlled by similar scavenging processes in groundwaters due to a good positive correlation between ²¹⁰Pb and ²¹⁰Po (R² = 0.67, p < 0.01). The concentrations of ²¹⁰Po and ²¹⁰Pb decreased with increasing pH values and salinity, which indicated that geochemical behaviors of ²¹⁰Po and ²¹⁰Pb in groundwater were influenced by seawater intrusion and pH changing. Groundwater ²²²Rn activity concentrations decreased with increasing salinity in coastal zone, which may be caused by dilution due to seawater intrusion or intensified ²²²Rn escaping from well-developed pores in coastal zone. The estimated annual ingestion doses for infants, children and adults were well below the recommended reference dose level (RDL) of 0.2-0.8 mSv/a, suggesting that consumption of analyzed groundwaters is safe from radiological point of view.

Submarine groundwater discharge as sources for dissolved nutrient fluxes in Coleroon river estuary, Bay of Bengal, India

Groundwater contributed nutrients aided with increasing population threaten the global coastal ecosystems. In this study, attempt has been made using major ions and nutrients to evaluate the significance of submarine groundwater discharge (SGD) in a semi-arid estuary of south India. Surface, seepage and groundwater chemistry altered from fresh (NaK-CaMg-NO₃Cl) to mixed (NaK-NO₃Cl) to saline water (NaCl) type from upstream to outlet that connects Bay of Bengal. We predicted abundant nitrate (NO₃^-) along upstream and towards the bay due to application of fertilizers and aquaculture activities, respectively. Elevated ammonium (NH₄⁺) observed in the recirculated groundwater/sea water suggests contribution from sea water intrusion and higher phosphate (PO₄^3-) noted at the outer bay suggests sources from phosphatic nodules. Decreasing Redfield ratio towards the bay suggests anoxic aquifer condition due to salinization. The SGD driven nutrient fluxes were 40.0-47.0% for NO₃^-, 43.0-51.0% for NH₄⁺ and 9.0-32.0% for PO₄^3- from the total input fluxes. The estimated nutrient fluxes showed that NO₃^- and PO₄^3- discharges to the sea due to SGD and NH₄⁺ removed from the coast due to consumption by microorganisms that creates toxic algal blooms in the study area.

Submarine groundwater discharge and its implication for nutrient budgets in the western Bohai Bay, China

Submarine groundwater discharge (SGD) supplies substantial quantities of nutrients from land to oceans. However, SGD and associated nutrient fluxes have long been ignored in Bohai Bay, which is subjected to the serious environmental problem. Here, we investigated the concentrations of radon (²²²Rn) and nutrients in groundwater and surface water in the western Bohai Bay during May 2017. The flushing time in the bay was estimated to be 38.8-58.3 days based on tidal prism model. The SGD flux was estimated to be 7.3 ± 4.8 cm d^-1 based on ²²²Rn mass balance model. The SGD associated nutrient fluxes were estimated to be (6.3 ± 4.1) × 10⁷ mol d^-1 for dissolved inorganic nitrogen (DIN), (1.2 ± 0.8) × 10⁶ mol d^-1 for dissolved inorganic phosphorus (DIP) and (7.5 ± 4.9) × 10⁷ mol d^-1 for dissolved inorganic silicon (DSi). By establishing nutrient budgets, we found that SGD was a major source, contributing 80.8% of all source for DIN, 90.7% of all source for DIP and 78.4% of all source for DSi into the western Bohai Bay. This study shows that SGD associated nutrient fluxes may have significant impact on nutrient budgets in the western Bohai Bay.

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.

Hydrological Response of Dry Afromontane Forest to Changes in Land Use and Land Cover in Northern Ethiopia

This study analyzes the impact of land use/land cover (LULC) changes on the hydrology of the dry Afromontane forest landscape in northern Ethiopia. Landsat satellite images of thematic mapper (TM) (1986), TM (2001), and Operational Land Imager (OLI) (2018) were employed to assess LULC. All of the images were classified while using the maximum likelihood image classification technique, and the changes were assessed by post-classification comparison. Seven LULC classes were defined with an overall accuracy 83–90% and a Kappa coefficient of 0.82–0.92. The classification result for 1986 revealed dominance of shrublands (48.5%), followed by cultivated land (42%). Between 1986 and 2018, cultivated land became the dominant (39.6%) LULC type, accompanied by a decrease in shrubland to 32.2%, as well as increases in forestland (from 4.8% to 21.4%) and bare land (from 0% to 0.96%). The soil conservation systems curve number model (SCS-CN) was consequently employed to simulate forest hydrological response to climatic variations and land-cover changes during three selected years. The observed changes in direct surface runoff, the runoff coefficient, and storage capacity of the soil were partially linked to the changes in LULC that were associated with expanding bare land and built-up areas. This change in land use aggravates the runoff potential of the study area by 31.6 mm per year on average. Runoff coefficients ranged from 25.3% to 47.2% with varied storm rainfall intensities of 26.1–45.4 mm/ha. The temporal variability of climate change and potential evapotranspiration increased by 1% during 1981–2018. The observed rainfall and modelled runoff showed a strong positive correlation (R2 = 0.78; p < 0.001). Regression analysis between runoff and rainfall intensity indicates their high and significant correlation (R2 = 0.89; p < 0.0001). Changes were also common along the slope gradient and agro-ecological zones at varying proportions. The observed changes in land degradation and surface runoff are highly linked to the change in LULC. Further study is suggested on climate scenario-based modeling of hydrological processes that are related to land use changes to understand the hydrological variability of the dry Afromontane forest ecosystems.