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• Using radioactive and stable water isotopes to quantify the water age and SGD. • SFGD was estimated by combining water and salt mass balance models. • The SFGD accounted for 4.5% of the total SGD. • SGD associated nutrient fluxes were significantly higher than riverine input. • The estimated water age of Laizhou Bay ranged from 23.5 to 50.0 days. Nutrient inputs from submarine groundwater discharge (SGD) play a significant role in sustaining primary productivity and nutrient cycling in the coastal area. Currently, various geochemical isotopes are used to trace the SGD processes. However, mass balance models of stable water isotopes (δ2H and δ18O) are seldom used in SGD estimating. In this study, mass balance models of 18O and radium (228Ra) isotopes were used to quantify the water age and SGD in Laizhou Bay, China based on the isotope data sampled in August 2017. The estimated age of water masses in Laizhou Bay ranged from 23.5 to 50.0 days with an average of 32.1 ± 16.3 days. The average SGD flux was (2.07 ± 1.04) × 108 m3 d−1, which was an order of magnitude larger than the Yellow River discharge during the sampling period. The sensitivity analysis revealed that estimated results of the water age and SGD are sensitive to the δ18O value in evaporation, as well as δ18O and 228Ra values in groundwater end-members. Based on the isotope method, the proportion of the Yellow River discharging into Laizhou Bay was estimated to be less than 27% of the total discharge. Furthermore, based on water and salt mass balance models, the estimated submarine fresh groundwater discharge (SFGD) flux ranged from (0.54–1.31) × 107 m3 d−1 with an average of (0.93 ± 0.46) × 107 m3 d−1. The SFGD accounted for 4.5% of the total SGD. Nutrient fluxes from SGD were significantly greater than those from the Yellow River, and SGD may have important effects on the marine ecosystem environments. This study reveals that stable and radium isotopes can be effectively combined to calculate the water age and SGD, which may be applied to the coastal area elsewhere. [ABSTRACT FROM AUTHOR]

• Location of polluted nearshore waters coincided with SGD hotspots on a local scale. • SGD-driven nutrients were one order of magnitude higher than riverine inputs in China. • Nutrients via SGD contributed >50% of the total sources to Chinese coastal waters. • SGD influences coastal water quality and nutrient budgets at small and large scales. Submarine groundwater discharge (SGD) can be a significant source of chemical pollutants from land to ocean. Here, we first estimated SGD using radium isotopes and related nutrient fluxes at the local scale in Jiaozhou Bay (JZB), a typical Chinese system that is experiencing rapid urban and industrial development. We then summarized SGD studies off China to assess the large-scale implications of SGD to nutrient budgets. In JZB, the location of contaminated nearshore waters revealed by an integrative water quality index (WQI) coincided with the SGD hotspots. The total (fresh and saline) SGD flux in JZB was estimated to be (0.64–1.67) × 107 m3/d or (2.12–5.59) cm/d based on 224Ra and 228Ra mass balance models. This was approximately 8 times the discharge rate of local rivers. By combining these JZB results with the literature data, we provide the first estimate of SGD and associated nutrient fluxes off China. The magnitude of SGD at the China-scale was (5.40–10.2) × 1012 m3/yr, accounting for 5–9% of the global SGD flux. SGD-derived nutrient fluxes summarized from ∼40 previous studies were one order of magnitude higher than riverine inputs. These nutrients fluxes from SGD contributed >50% of the total dissolved inorganic nitrogen (DIN), phosphorous (DIP) and silicate (DSi) inputs into Chinese coastal waters, which can explain about 60% of the phosphorus required by primary production. The mean DIN/DIP ratio (121) in SGD was significantly higher than the Redfield ratio, with important implications for phytoplankton growth and structure. SGD can influence water quality, dominate nutrient budgets, and drive primary production not only at the local scale, but also at the regional and global scales. [ABSTRACT FROM AUTHOR]

Many coal mining subsidence lakes have formed in eastern China due to underground coal mining. Lacustrine groundwater discharge (LGD) is of great importance to the hydrological cycle and the eco-environment of lakes. However, LGD in coal mining area is rarely reported. In the study, we quantified groundwater discharge into a Huainan coal mining subsidence lake (China) using water stable (D and 18O) and radioactive (222Rn) isotopes. 222Rn and 18O mass balance models were used to independently estimate LGD. The lake water was sampled based on the horizontal and vertical profile sampling. The isotope depth profiles were weighted according to the lake bathymetry to obtain a representative isotope inventory by geographic information system (GIS) analysis. The LGD rates estimated from two models were comparable, and the values were 18 mm d−1 for the 18O and 14 ± 8 mm d−1 for the 222Rn. Compared with the LGD of Ammelshainer See lake (Germany), a subsidence lake formed by coal mining, the LGD in the study was larger, which may be related to the different hydrological conditions in different areas. By integrating the LGD rates from these two models, this study quantitatively analyzed the role of groundwater in maintaining the water balance in subsidence lakes, which is of great significance to water resources assessment and sustainable utilization of coal mining subsidence lakes in eastern China. • Using radioactive and stable isotopes to estimate LGD rate of the coal mining subsidence lake. • Combining horizontal and vertical sampling improves the accuracy of radon decay loss assessment. • Isotope depth profile is weighted to obtain representative isotope inventory. • Groundwater discharge contributes a large amount of water to coal mining subsidence lakes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Seasonal sampling of heavy metals in the groundwater was conducted in a subtropical mangrove wetland. • The enrichment trend of heavy metals exhibited seasonal variations along the intertidal transect. • COVID-19-induced attenuation of anthropogenic activity reduced terrestrial input of heavy metals to the sea. • The intertidal aquifer served as a buffer zone for the transport of heavy metals to the sea. Mangrove wetlands connecting the land and sea are critical and dynamic ecosystems which act as biogeochemical reactors that influence the concentrations and distributions of heavy metals in the groundwater and adjacent seawater. In this paper, a subtropical mangrove wetland in China was selected to investigate the spatiotemporal patterns of groundwater heavy metals and the driving mechanism along an intertidal transect with distinct undulated topography and vegetation zonation. Along the transect, five groundwater observation wells (W1-5) were set to collect groundwater samples at three different sediment depths (0.5, 1.2, and 2.0 m) and the advective flux between groundwater and surface water were calculated during four seasonal sampling campaigns from 2019 to 2020. The results showed that the seaward W1-3 acted as the groundwater discharge area while the landward W4-5 served as the surface water recharge area. The results of single-factor contamination index showed that a potential pollution risk of Hg existed at the study sites. The distribution of other heavy metals in the mangrove groundwater exhibited a seasonal variability along the transect during the sampling campaigns. For example, the relative high concentrations of Fe, Pb, Cu, Cr, and Zn focused in the landward zones in the dry seasons (autumn and winter 2019), whereas they changed to the seaward zones in the wet seasons (summer and autumn 2020). Moreover, the intertidal aquifer can act as a sink to enrich heavy metals in winter 2019 and autumn 2020 but become a source to discharge heavy metals to the sea in summer 2020. The above migration patterns of heavy metals were ascribed to the multiple complex factors including the seawater-groundwater interactions, vegetation zonation, biogeochemical reactions, and anthropogenic activities. This study contributed to the better understanding of heavy metal behaviors in mangrove groundwater so as to improve the sustainable management of mangrove wetlands under the pressure of anthropogenic activities and climate change. [ABSTRACT FROM AUTHOR]

[Display omitted] • Seasonal sampling of heavy metals in the groundwater was conducted in a subtropical mangrove wetland. • The enrichment trend of heavy metals exhibited seasonal variations along the intertidal transect. • COVID-19-induced attenuation of anthropogenic activity reduced terrestrial input of heavy metals to the sea. • The intertidal aquifer served as a buffer zone for the transport of heavy metals to the sea. Mangrove wetlands connecting the land and sea are critical and dynamic ecosystems which act as biogeochemical reactors that influence the concentrations and distributions of heavy metals in the groundwater and adjacent seawater. In this paper, a subtropical mangrove wetland in China was selected to investigate the spatiotemporal patterns of groundwater heavy metals and the driving mechanism along an intertidal transect with distinct undulated topography and vegetation zonation. Along the transect, five groundwater observation wells (W1-5) were set to collect groundwater samples at three different sediment depths (0.5, 1.2, and 2.0 m) and the advective flux between groundwater and surface water were calculated during four seasonal sampling campaigns from 2019 to 2020. The results showed that the seaward W1-3 acted as the groundwater discharge area while the landward W4-5 served as the surface water recharge area. The results of single-factor contamination index showed that a potential pollution risk of Hg existed at the study sites. The distribution of other heavy metals in the mangrove groundwater exhibited a seasonal variability along the transect during the sampling campaigns. For example, the relative high concentrations of Fe, Pb, Cu, Cr, and Zn focused in the landward zones in the dry seasons (autumn and winter 2019), whereas they changed to the seaward zones in the wet seasons (summer and autumn 2020). Moreover, the intertidal aquifer can act as a sink to enrich heavy metals in winter 2019 and autumn 2020 but become a source to discharge heavy metals to the sea in summer 2020. The above migration patterns of heavy metals were ascribed to the multiple complex factors including the seawater-groundwater interactions, vegetation zonation, biogeochemical reactions, and anthropogenic activities. This study contributed to the better understanding of heavy metal behaviors in mangrove groundwater so as to improve the sustainable management of mangrove wetlands under the pressure of anthropogenic activities and climate change. [ABSTRACT FROM AUTHOR]