Reveal the threat of water quality risks in Yellow River Delta based on evidences from isotopic and hydrochemical analyses
Introduction
Yellow River is the second longest river in China. The river is faced with many types of environmental problems in its upper, middle and lower parts because of the natural background as well as human activities. In Yellow River Basin (YRB), the shortage of water is very severe (Li, 2020). The deterioration of water quality is one of the most important contributors to this shortage, which is mainly led by pollutants from heavy metal discharge (Song et al., 2013), concentrated fluoride and nitrate in groundwater (Li et al., 2019a; J. Wu et al., 2015a) and the organic contaminants (Li et al., 2015a). From upper part to the estuary of Yellow River, the level of pollution grows heavier due to the cumulation of pollutants.
In the Yellow River Delta (YRD), the representative landscape is the coastal wetland, which is characterized by rapid exchange of materials between land and sea and the vulnerability of ecosystem (Barbier, 2013; Zhang et al., 2016). The wetlands serve as the habitat for rare wildlife such as red-crowned crane, hooded crane and golden eagle, and thus have great value in the maintenance of biodiversity (H. Wang et al., 2014a). Therefore, it is a very important study area for ecologists and ecohydrologists and has drawn considerable attentions (Zhang et al., 2016). However, the wetlands in YRD are confronted with increasing risk of degradation caused by the overlapped influences of human activities and environmental changes (Cui et al., 2009; Kong et al., 2015). On the one hand, the rapid changes in various geological and physical variables in the delta have exacerbated coastal erosion, and the globally climate warming has led to the rising of sea level (Qi and Liu, 2017). On the other hand, the exploitation of oil and natural gas by industrial corporations as well as the development of agriculture has caused dramatic changes in coastal and marine environment, leading to disturbance of water-salt balance and modification of groundwater dynamics (Kuenzer et al., 2014; Liu et al., 2016a). Saline intrusion is a severe and typical environmental problem in YRD area and pronouncedly influences the suitability of water for ecological, domestic and agricultural utilizations (Xia et al., 2021). In recent years, the water quality shows marked deterioration in YRD area, whose potential causes are complex, including water sources, human disturbance, saline intrusion and their interaction (An et al., 2011; Chen et al., 2012; Gao et al., 2014). In previous studies, multiple techniques such as GIS, geochemical exploration and remote sensing were employed to investigation the influence of salinization, heavy metal and organic contaminants on water environment in YRD (Guan et al., 2001; Li et al., 2015b; Song et al., 2013). Whereas, the mechanisms of water interaction and degree of ecological risks need to be further understood.
Hydrogen and oxygen isotopes (i.e., 2H and 18O) can be used as tracers to identify the water sources and interaction between water pools due to their stable chemical properties and consistent behavior with water flow (Barbieri, 2019; Krishan et al., 2021a; Li et al., 2016a; Qian et al., 2013a; Xia et al., 2020b). Hydrochemical characteristics of water is the result of long-term interaction between waters and environments. Major ions determine the basic features of hydrochemistry, which are useful in identifying the controlling factors of hydrochemical evolution, reflecting the water-rock interaction, deciphering the sources of water pollutants and revealing the formation, recharge and flow path of groundwater as well as surface water (Barbieri et al., 2017; Karunanidhi et al., 2021a; Krishan et al., 2021b; J. Wu et al., 2015b). They are direct indicators for water quality which can be applied to determine the extent of influence by pollutants or salinity on water (Barbieri et al., 2019; Karunanidhi et al., 2021c; Krishan et al., 2021c, Krishan et al., 2020; Li et al., 2019b, Li et al., 2016b). In previous studies focusing on water environment in wetland area, isotopic and hydrochemical indicators were used to identify the sources of water and ions in surface water and groundwater, and determine the influence by natural factors and anthropogenic activities (Prasanna et al., 2011; Yang et al., 2021; Yuyu et al., 2021; Zhao et al., 2018). The combined application of isotopic and hydrochemical tracers will provide a clearer insight into the regimes of water recharge and migration of salt and pollutants, and help better delineate the risk of water quality in areas with high complexity.
In this study, samples for precipitation, river water, sea water and wetland water were collected in different seasons. Based on the isotopic and hydrochemical analyses, the specific objectives of this paper are as follows: (1) to provide an overview of hydrogen and oxygen stable isotope composition and hydrochemical background in different waters in YRD area; (2) to determine the distribution and sources of contaminants in different water pools and (3) to assess the ecological risks in wetland ecosystems brought by salinity and anthropic contaminants. This study will help understand the mechanisms of ecological deterioration and environmental pollution caused by overlapped seawater intrusion and human activities, and provide important information for the water resources management in wetland ecosystems of YRD area.
Section snippets
Geographic and geological background
The Yellow River Delta is located in the northeast of Shandong Province, the south coast of Bohai Bay and the west coast of Laizhou Bay (shown in Fig. 1). It is a fan-shaped alluvial plain where the Yellow River enters the sea. The terrain in YRD is low and flat, with the elevation of most area lower than 50 m. The main hydrogeological type is loose pore water, with the aquifer mainly composed of porous Quaternary sediments. Sediments show a variation of thickness between 8 and 20 m. From the
Sample collection
The site for precipitation collection is located in an open land in Gudao Town, Dongying City. The ground where the equipment was established is flat and there's no tall buildings or trees around the site. During the observation period, the precipitation samples were collected after each event. For precipitation sampling, a specific tool was made following the “Option 3: Totalizer, table tennis ball” in the guidebook published by IAEA/WMO (//www-naweb.iaea.org/napc/ih/IHS_resources_gnip.html
Characterization of hydrogen and oxygen stable isotopes in waters
During the observation period, the δ2H in precipitation varies from −97.13 to 2.86, and the range of δ18O is −14.16–0.02. Information from the δ2H-δ18O relation (local meteoric water line, LMWL) in precipitation can be used to indicate the source of moisture and the degree of local evaporation (H. Wu et al., 2015c; Xia et al., 2020a). The LMWL (shown in Fig. 3a) for YRD is calculated by least square regression. The equation is δ2H = (7.24 ± 0.25) δ18O + (3.95 ± 2.13), whose slope and
Conclusions
The protection of coastal wetland ecosystems is of great importance due to its extreme vulnerability and complex environmental background. Based on stable isotopes, hydrochemistry and water quality parameters, this study evaluates the potential ecological risks in wetlands of YRD and provides a new insight into the spatio-temporal heterogeneity of water interaction and water quality. The main findings are obtained as follows:
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Relationship between δ2H and δ18O isotopes in precipitation shows
Funding
This study was supported by China Scholarship Council (No. 202006240192).
Availability of data and material
Readers can get the original data by contacting the corresponding author.
CRediT authorship contribution statement
Conceptualization, methodology, writing—review and editing and funding acquisition, G.L.; data curation and writing—original draft preparation, C.X.; formal analysis, F.J. and Y.M.; experiment, F.J. and C.X.; investigation, F.J.
Ethics approval
Not applicable.
Consent to participate
No human participant is involved in this study.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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