Submarine groundwater discharge and chemical behavior of tracers in Laizhou Bay, China

A comprehensive analysis of submarine groundwater discharge and nutrient fluxes in the Bohai Sea, China

Groundwater discharge and associated nutrient fluxes in the Bohai Sea, China has attracted great attention, but most studies lacked high spatial resolution for the whole sea. As the largest semi-enclosed sea in China, the Bohai Sea is confronted with strong environmental pollution problems such as eutrophication induced by terrestrial nutrient inputs. However, the role of SGD has not been evaluated well for the whole Bohai Sea. In this study, stable isotopes (hydrogen and oxygen), radioactive isotope (228Ra), salinity, and temperature were combined to trace the diluted seawater. Mass balances of 228Ra, oxygen isotope, and salinity were used to quantify SGD and nutrient fluxes to the Bohai Sea. The estimated submarine fresh groundwater discharge (SFGD) and SGD to the Bohai Sea were (6.0 ± 0.5) × 109 and (2.7 ± 1.6) × 1011 m3 a-1, respectively. SFGD represents 10 % to 11 % of the total river discharge and SGD is about 2 to 8 folds of the total river discharge to the sea. Moreover, SGD derived dissolved nutrients to the Bohai Sea were (4.8 ± 4.0) × 1010 mol a-1 for dissolved inorganic nitrogen, (1.9 ± 1.7) × 1010 mol a-1 for dissolved inorganic phosphorus, and (6.7 ± 5.5) × 1010 mol a-1 for silicon. These nutrient inputs were about 10 to 20 folds of the total riverine inputs. Overall, this study underscores the importance of evaluating SGD to better understand the terrestrial imported nutrients in regional scale.

Review of drivers and threats to coastal groundwater quality in China

With rapid socio-economic development, China's coastal areas are among the fastest growing and most economically dynamic regions in the world. Under the influence of climate change and human activities, protecting the quality of coastal groundwater has emerged as one of the key environmental and resource management issues for these areas. This paper reviews (for the first time) groundwater quality data for the coastal basins of China, where over 600 million people live, focussing on key inorganic indicators/pollutants; groundwater salinity, nitrate, fluoride, and arsenic. These pollutants present major water quality issues and are also valuable as indicators of wider processes and influences impacting coastal groundwater quality - e.g. saltwater intrusion, agricultural pollution and release of geo-genic contaminants. We discuss the major drivers causing water quality problems in different regions and assess future trajectories and challenges for controlling changes in coastal groundwater quality in China. Multiple processes, including modern and palaeo seawater/brine migration, groundwater pumping for agricultural irrigation, pollution from agrochemical application, rapid development of aquaculture, urban growth, and water transfer projects, may all be responsible (to different degrees) for changes observed in coastal groundwater quality, and associated long-term health and ecological effects. We discuss implications for sustainable coastal aquifer management in China, arguing that groundwater monitoring and contamination control measures require urgent improvement. The evolution and treatment of coastal groundwater quality problems in China will serve as an important warning and example for other countries facing similar pressures, due to climate change, coastal development, and intensification of anthropogenic activity in coming decades.

Radon traced seasonal variations of water mixing and accompanying nutrient and carbon transport in the Yellow-Bohai Sea

Radon (²²²Rn) is a natural radioactive tracer widely utilized to evaluate water exchange and mixing processes; however, minimal studies have investigated the ²²²Rn distribution in the Yellow-Bohai Sea (YBS) and its behavior is poorly understood. In this study, the seasonal distribution of ²²²Rn in the YBS was investigated. The results found that the ²²²Rn distribution in surface waters is significantly affected by rivers, while ²²²Rn activity in bottom waters is highly affected by submarine groundwater discharge. The eddy diffusivity and advection velocities of the YBS were obtained utilizing an improved 1D steady-state ²²²Rn diffusion-advection model. The average horizontal eddy diffusivities in the wet (August 2015) and dry (November 2014) seasons were 4.54 × 10⁸ and 2.28 × 10⁸ cm² s^-1 in dry season, respectively and the average vertical eddy diffusivity was 4.99 cm² s^-1. The dissolved inorganic nutrient (N, P, and Si) and dissolved inorganic carbon flux outputs from vertical eddy diffusion were determined to be 4.85, 0.29, 3.59, and 61.6 mmol m^-2 d^-1, respectively. These results demonstrate that eddy diffusion tracing in coastal ocean is conducive to interpreting water mixing processes and can be utilized to understand offshore nutrient and carbon transport better.

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.

Tidal groundwater flow and its potential effect on the hydrochemical characteristics in a mud-sand-layered aquifer in Daya Bay, China

Tidal groundwater dynamics and hydrochemistry can play important roles in influencing nearshore ecological and environmental systems. However, the potential relationship between the groundwater dynamics and the hydrochemical characteristics was not well understood. In this study, we conducted an integrated investigation by field work and numerical simulations to explore the potential effect of tidal groundwater dynamics on hydrochemistry in an intertidal mudflat in Daya Bay, China. The time series of groundwater level were monitored over a spring-neap tidal cycle along a 200-m-long intertidal transect, which had a mud-sand-layered aquifer. The shallow groundwater samples were collected to analyze the spatial distributions of hydrochemical characteristics, including major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄^2-, HCO₃^-, and Cl^-), heavy metals (As and Cu), and short-lived radium isotopes (²²³Ra and ²²⁴Ra). The groundwater transport process along the transect was simulated for understanding the groundwater flow field and quantifying the groundwater-seawater exchange rates across the water-sediment interface. The integrated results indicated that the seawater-groundwater interactions play a great influence on the groundwater hydrochemistry. For example, the major ions showed various degrees of enrichment and loss, such as losses of SO₄^2- due to microbial sulfate reduction, and enrichment of HCO₃^-, Ca²⁺, and Mg²⁺ under the water-rock interactions. Heavy metals were transported by groundwater and accumulated in the intertidal sediments. In addition, there was a negative correlation between short-lived radium isotope activities and oxidation-reduction potential. However, the relationship between seawater-groundwater exchange rates and the short-lived radium isotope activities was not significant.

Enhanced nitrogen imbalances in agroecosystems driven by changing cropping systems in a coastal area of eastern China: from field to watershed scale

Agricultural activities exacerbate nitrogen (N) imbalances in the agroecosystem by disturbing the N inputs and outputs, yet the influence of changes in cropping systems on the N balance of agroecosystems remains unclear. In this study, at the field scale, we calculated the N balance of four cropping systems, (1) traditional crops with traditional crop rotation (G-G), (2) vegetables with traditional crop rotation (V-G), (3) vegetables with vegetable rotation (V-V), and (4) greenhouse vegetables (GHV); then analyzed the influence of changes in cropping systems from 1995 to 2015 on the N balances in the agroecosystems in sub-watersheds of the Dagu River. The results indicate that N balances were higher in GHV, V-V, and V-G than G-G, due to significantly higher inputs of N fertilizers and lower N use efficiency (NUE) in vegetable cultivation compared to traditional crops. Driven by economic benefits between 1995 and 2015, V-G, V-V, and GHV replaced G-G in a considerable number of cultivation areas in the sub-watersheds. These changes resulted in an increase of 109.9-170.1% in the N balance in the agroecosystem in the sub-watersheds between 1995 and 2015. In the entire watershed, the total N surplus contribution by V-V, V-G, and GHV increased from 39.3% to 79.1% between 1995 and 2015. These findings suggest that increased vegetable cultivation contributed to the increased risk of N pollution in agricultural production. Thus, there should be a focus on the management of cropping systems to control N loss from agricultural lands.