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.

Environmental risk of tailings pond leachate pollution: Traceable strategy for leakage channel and influence range of leachate

Identifying the leakage channel and the influencing range is essential for controlling the environmental risks of leachate from the tailings pond. The investigation of leachate pollution in tailings pond has the defect of focusing only on the scope of tailings pond in recent studies. This study innovatively built a comprehensive investigation and accurate verification system for leachate leakage of tailings pond integrated with the aeromagnetic survey, ground penetrating radar, hydrochemistry and isotope coupling methods. Geophysical exploration found that among the four fault zones, and the F1 was the channel for leachate to recharge the groundwater 2.53 km away from the tailings pond. The fissures inside the tailings pond were connected with the natural fissures outside, forming a leachate migration channel. The hydrochemistry and isotope characteristics showed that the groundwater far away from the tailings pond were polluted by arsenic containing leachate, which verified the geophysical exploration results. The significant correlation between arsenic and SO2-4 concentration indicated that arsenic in leachate originated from the oxidation release of sulfide minerals (i.e., arsenopyrite). This study sheds light on the comprehensive investigation of leachate leakage in the tailings pond. This development method also provides guidance for environmental risk identification of other contaminated sites.

Isotopic compositions (δD, δ18O) and end-member mixing for the control interface in a complex tidal region

Identifying the mixing processes of waters and currents in tidal reach is an important aspect of environmental management to protect freshwater resources and prevent water pollution. In this study, three field investigations conducted in a typical tidal reach in August, November and the following April focused on two isotopes (δD and δ¹⁸O) and salinity. A salinity-isotope conservative mixing model was established to differentiate water flows of the important control interface (CI) from freshwater, transition zone and saltwater end-members. Results suggested that the average δD and δ¹⁸O values during the ebb and flood tides depleted from August to November, then enriched significantly in the following April and were even higher than those in August. The δD and δ¹⁸O values in the saltwater zone enriched markedly compared with those in freshwater zone and transition zone due to the stronger evaporation occurring in the saltwater zone. Based on the revised model, the average contributions of freshwater end-member, transition zone end-member and saltwater end-member in three months were, respectively, 51.50 %, 36.93 % and 11.57 %. However, the contributions of freshwater and transition zones in April end-member were equivalent (47.45 % vs 44.31 %). Meanwhile the largest contribution of saltwater end-member was 20.56 % and occurred in August. The proportions of three end-members that contributed to CI changed with different evaporation scenarios and moisture sources of precipitation. Our research provides important information that furthers our understanding of the isotopes and their applications to environmental management in estuarine regions.

Redistribution effect of irrigation on shallow groundwater recharge source contributions in an arid agricultural region

Recharge sources such as precipitation, mountain front recharge, mountain block recharge and confined water are the sources usually considered in quantitative studies of groundwater recharge. Changes in recharge processes caused by irrigation practices need to be fully considered for the accurate budgeting and management of water resources. Here, we put forward a conceptual framework for evaluating the shallow groundwater recharge process in arid irrigated agricultural areas using hydrochemical and stable isotope techniques, combined with an assessment of hydrogeological conditions and quantitative models. In general, the recharge effect of atmospheric precipitation on shallow groundwater in arid areas is relatively small. The contributions made by recharge sources in the studied river irrigated area, from greater to smaller, were confined groundwater (46.98 %), river water (45.48 %) and precipitation (7.55 %). The original range in groundwater recharge levels caused by river leakage also appeared to have expanded in response to the establishment of canal irrigation networks. Lateral groundwater flow and confined groundwater were the main recharge sources of shallow groundwater in areas fed by well irrigation and well-/spring-water irrigation (not taking into account any groundwater irrigation leakage). However, had the recharge of shallow groundwater by groundwater irrigation leakage, which reached 19.8-41.1 %, not been counted as contributing to actual groundwater recharge, the recharge contributions made by lateral groundwater flow and confined groundwater to shallow groundwater would have been significantly overestimated. This is because the groundwater recharge process has been modified by the various irrigation measures employed in arid agricultural areas, leading to a redistribution effect in groundwater recharge source contributions. This study provides a new perspective and intuitive data support for the development and utilization of water resources in arid regions.

Reveal the threat of water quality risks in Yellow River Delta based on evidences from isotopic and hydrochemical analyses

This study aims to evaluate the seasonal and spatial characteristics of hydrochemistry and DO isotopes and identify the eco-environmental threats under the background of saline intrusion and human activities in Yellow River Delta (YRD). Analyses for major ions (i.e., K⁺, Na⁺, Ca²⁺, Mg²⁺, SO₄^2-, HCO₃^- and Cl^-), nitrate ion (NO₃^-) and isotopic composition are performed for precipitation, river water, wetland water and sea water. Based on the range of δ²H and δ¹⁸O as well as their relations, the mixing between multiple sources and evaporation are confirmed. Electrical conductivity (EC), concentration of NO₃^-, soluble sodium percentage (SSP) and magnesium hazard (MH) are employed as indicators to reflect the ecological risks from salinity, agricultural pollutants, sodium and magnesium. By hierarchical cluster analysis (HCA), the samples of wetland water are grouped associated with those of river water. The characteristic reflects 3 patterns of risks in wetlands, including saline intrusion, human activities and their mixed influence.

River damming and drought affect water cycle dynamics in an ephemeral river based on stable isotopes: The Dagu River of North China

The flow regime and biogeochemical cycles are greatly affected by river damming and drought, especially in ephemeral rivers. However, the combined effects have been rarely considered. This study, taking the Dagu River in Jiaodong Peninsula of North China as an example, investigated the dynamic changes in water cycle related to river damming and drought using stable water isotopes for the period 2018-2019. The results indicated that river water isotopes significantly varied temporally and spatially. The temporal variations in river water isotopes appeared to be linked with those in precipitation, but the relationship between river water and precipitation isotopes was greatly affected by river damming, river water-groundwater exchange and potential water pollution. Spatially, a single dam exhibited no significant effect on river water isotopes, but the accumulative impacts of cascade dams resulted in the enrichment of heavy isotopes in river water towards the downstream through increasing hydraulic residence time and water evaporation largely. The inter-annual variations in river water isotopes with increased evaporative fractionation were highlighted by their strong response to the drought in 2019. The combined effects of cascade dams and drought greatly changed water cycle dynamics and further exacerbated water shortage, which should thus be fully considered for water resource management, especially for regions with water-limited but heavily-regulated rivers.

Determination sources of nitrates into the Three Gorges Reservoir using nitrogen and oxygen isotopes

Identification of nitrate sources and its transformations are important for the management of large lakes and reservoirs. The Three Gorges Reservoir (TGR) in China is one of the largest reservoirs around the world. In this study, stable isotopes of nitrogen (δ¹⁵N-NO₃^-) and oxygen (δ¹⁸O-NO₃^-) of nitrate in water were used to gain insights into nitrate sources and transformations in the tail area of the TGR. Bayesian mixing model has been conducted to estimate the proportional contribute of nitrate sources. The mixing modelling results indicated that NH₄⁺ fertilizer (range 7-54%) and soil organic nitrogen (range 2-45%) were the dominant NO₃^--N sources in the tail area of the TGR during the three season study period. Nitrification contributed a part of NO₃^--N in the river water during the dry season. The nitrate from soil solution in the riparian zone with denitrified NO₃^- might be another major reason for the enrichment of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- during the normal season. Reducing the use of chemical nitrogen fertilizers, especially NH₄⁺ fertilizers, and protecting soil from erosion may be effective measures to improve water quality in the TGR.

Sequential samples reveal significant variation of mercury isotope ratios during single rainfall events

Although the investigation of mercury (Hg) isotopes in precipitation has largely improved our knowledge of the source and transformation of Hg in the atmosphere, rainwater investigated in previous studies were integrated samples collected over an event and could obscure key information about the physiochemical transformation and deposition dynamics of Hg (and its isotopes) in short precipitation events. In this study, we investigated Hg isotopic composition of filtered (Hg_F) and particulate Hg (Hg_PM) in sequential rain samples from three single rainfall events in Guiyang, China. All samples showed a decrease of total Hg concentration, as well as Hg_F and Hg_PM with time in each rainfall event, and large variation of both mass-dependent fractionation (MDF) and mass-independent fractionation of odd Hg isotopes (odd-MIF) for both phases. Isotopic data indicated variable contributions of different sources triggered by the instant change of meteorological conditions, rather than internal atmospheric processes. The rapid response of MDF and odd-MIF of precipitation samples to the incense burning on the Tomb Sweeping Day implied that Hg isotopic composition was very sensitive to the momentary anthropogenic emission, which could have at least a regional short-lived effect and should be taken into account in future studies. Hg isotopes are a powerful tool for investigating both atmospheric transformation and instant deposition dynamic of Hg, and like stable H and O isotopes, could provide useful information about local or regional meteorological changes.

Hydrochemistry of waters in snowpacks, lakes and streams of Mt. Dagu, eastern of Tibet Plateau

There is little available information on hydrochemistry of waters from glacial source to downstream of glacierized catchments. Here we examine the water chemistry of the snowpacks, lakes and streams at eight sampling sites within glacial basin in Mt. Dagu, east Tibetan Plateau. An air mass trajectory model, correlation analysis, Gibbs model, Piper diagram and hydrograph separation analysis were utilized to investigate the characteristics and solutes sources of these waters. Generally, the TDS (Total dissolved solids; 7.54, 13.95 and 18.70mg/L for snowpacks, lakes and streams respectively) and concentrations of main chemicals in all samples exhibited downstream trend from snowpacks to streams. Of the cations, Ca²⁺ appeared with the highest concentration followed by K⁺ and Na⁺. Of the anions, HCO₃^- was most abundant, followed by Cl^-, SO₄^2- and NO₃^-. For snowpack samples, the air masses arriving at the sampling sites were typically prevailing from the western Tibet Plateau, central Asia and the northern Mongolia plateau. The fine particulate matter in the Mt. Dagu snowpacks was most likely transported long range from three arid regions above-mentioned. High concentrations of SO₄^2- and NH₄⁺ in snowpacks, with twice as much NH₄⁺ as SO₄^2-, implying that the soluble part of the finer particles was transported as a form of ammonium sulfate. Rock weathering determined the ion components of the meltwater and the water could be classified as calcium and bicarbonate type based on the Piper diagram. The chemical contributions from glacier-snow meltwater were 20%-131% for lake and 5%-79% for stream, while the runoff contribution to lake varied from 65.4% to 84.9%, and 66.1% to 81.6% for stream. This study suggested that glacier-snow meltwater was the mainly runoff contributor to lake and stream water and that snowpack solutes derived from eolian additions exert a significant influence on lake and stream chemistry.