Impacts of sediment compaction on iodine enrichment in deep aquifers of the North China Plain
Introduction
Iodine, an essential element for thyroid hormone synthesis, plays an important role on energy metabolism, thermoregulation and physical and mental development (Yu, 1987). Like all life-essential elements and nutrients, insufficient or excessive ingestion of iodine can result in Iodine Deficiency Disorders (IDD) and Iodine Excess Disorders (IED), respectively (May, 2011; Andersen et al., 2009). In recent years, while IDD and its prevention remain as a focus for studies (Ren et al., 2008; Zimmermann, 2009; Andersson et al., 2007), more attention has been paid to IEDs (Carvalho et al., 2012; Kwon et al., 2017; Katagirl et al., 2017). According to the results of worldwide survey of median urinary iodine, eleven countries are faced with the problem of excessive iodine intake (Andersson et al., 2012). In China, endemic goiter caused by excessive iodine intake from drinking water has been recognized as a serious public health problem (Guo et al., 2015), and the Maximum Concentration Limit (MCL) for iodine is 100 μg/L in national standard for drinking water (GB/T, 19380-2016). In areas with IEDs problems among residents, such as the North China Plain (NCP) (Zhang et al., 2013), the Datong basin (Li et al., 2013) and the Taiyuan basin (Tang et al., 2013), groundwater with iodine concentration higher than 100 μg/L has been used as the dominant source for water supply. Therefore, understanding the sources and enrichment mechanisms of iodine in aquifers is critical both for effective control of IEDs disease and for sustainable management of water resources.
Intensive human activities are associated with increasing water demand. Consequently, groundwater over-extraction has been reported in many countries such as USA, Japan, Mexico, Italy, Thailand, and China (Hu et al., 2004; Phien-wej et al., 2006; Guo et al., 2015; Kagabu et al., 2013; Erban et al., 2013; Smith et al., 2018). Groundwater, as the main source of irrigation and drinking water, has been over-exploited at NCP in the past several decades, which has caused several geo-environmental problems, such as groundwater-level decline, aquitards compaction and land subsidence (Zheng et al., 2010; Guo et al., 2015; Zhu et al., 2014). It further affects the regional aquifer recharge and solutes transport. For instance, there is a notable contribution of inter-aquifer leakage from the Quaternary compressible clay compaction to groundwater, especially in the coastal areas (Cao et al., 2013; Shi et al., 2006).
In NCP, the occurrence of IEDs (Yu, 1987) and elevated iodine concentration up to more than 1000 μg/L in deep groundwater have been reported (Zhang et al., 2013; Li et al., 2017). The previous studies mainly focused on the geochemical processes affecting iodine mobilization from the aquifer matrix into groundwater. For example, our previous studies at NCP revealed that iodine mobilization in groundwater system is related to the hydrogeochemical conditions (Li et al., 2017; Xue et al., 2018). And hydrogeochemical processes, such as the breakdown of iodine-containing organic matter and reductive dissolution of iodine-loaded metal (oxy)hydroxides, are responsible for iodine release from iodine-rich aquifer sediment into groundwater (Fuge and Johnson, 2015; Li et al., 2014a; Korobava et al., 2016; Tang et al., 2013; Zhang et al., 2013). However, little work has been done to study the impact of anthropogenic processes on iodine enrichment in groundwater.
Recent studies have shown that over-pumping of groundwater can lead to the release of naturally occurring contaminants within compressible clayey layers into aquifers (Erban et al., 2013; Smith et al., 2018). During the Quaternary Period, as a result of global climatic changes, the fluctuation of sea level triggered several events of marine transgression and regression in coastal areas of NCP (Lin and Dai, 2012; Gao and Collins, 2014; Yao et al., 2017), which caused retention of elements of marine sources in the sediments (Zhang et al., 1997). It has been well-known that ocean constitutes the major reservoir of global iodine cycle and there is a large amount of organic matter rich iodine in the ocean (Fuge and Johnson, 2015). Besides, the thermodynamically stable state of inorganic iodine in seawater is iodate, which has a greater adsorption capability than iodide (Nagata and Fukushi, 2010) and thus favors the retention of iodine during the deposition of marine sediments.
Several studies have documented that groundwater Cl and Br concentrations and Cl/Br ratio can be successfully used to trace groundwater solute transport, identify recharge sources and quantitatively evaluate groundwater systems (Cartwright et al., 2006; Xie et al., 2013; Liu et al., 2016a, Liu et al., 2016b). Groundwater samples with Cl/Br molar ratios exceeding or equal to about 650 are considered to be of marine origin or marine formation water (Davis et al., 1998). Similarly, stable isotopes (2H, 18O) and radiogenic isotope (87Sr/86Sr) are also powerful tools for understanding the hydrogeochemical processes in groundwater systems (Liu et al., 2016a, Liu et al., 2016b). Isotopes of water molecule (2H, 18O) could trace the recharge conditions over time, rainwater infiltration and mixing of different waters (Peng et al., 2012; Negrel et al., 2017). Strontium isotope has been widely used to study the origin and evolution of solutes in groundwater (Sánchez et al., 2010; Skrzypek et al., 2013).
Therefore, the objectives of this study are: (1) to identify the impact of compaction of compressible clayey layers on iodine enrichment in groundwater at NCP, and (2) to quantify the contribution of sediment compaction-released pore water to iodine enrichment in aquifers. The results help better understand the fate of iodine in the complex aquifers systems in coastal areas like NCP.
Section snippets
Study area
NCP is located in eastern China between 34°46’ - 40°25′N and 112°30’ - 119° 30′E and bordered by the Taihang Mountains to the west and the Bohai Bay to the east, by the Yanshan Mountains to the north and the Yellow River to the south. NCP has a middle-latitude continental semiarid monsoon climate with mean annual temperature of 12–13 °C, mean annual precipitation of 500–600 mm and evaporation of 1100–1800 mm (Liu et al., 1997; Chen, 1999). The plain is drained by the Haihe River, the Luanhe
Sampling
A total of 38 groundwater samples from NCP and 1 Bohai seawater sample were collected from NCP in August 2015 (Fig. 1). The groundwater samples were collected from the confined aquifers (A3 and A4) at depths ranging from 150 to 600 m, which have been used as the main source of drinking water supply in the rural areas. To avoid salinity contamination from shallow aquifers, the Holocene aquifer (A1) and late Pleistocene aquifer (A2) were sealed, and therefore, the groundwater samples collected in
Groundwater chemistry
Groundwater iodine concentration ranged from 5.8 to 1110 μg/L with a median value of 270 μg/L, with approximately 79% samples higher than 100 μg/L, the Chinese MCL value for iodine in drinking water (Table 1). As shown in Fig. 1, high iodine (>100 μg/L) groundwater is mainly distributed close to Bohai bay area where evident land subsidence occurs. The iodine concentration in compaction-released pore solution samples ranged from 18.1 to 830 μg/L, half of them being more than 100 μg/L. The
Major clues about the marine origin of iodine
According to the results of regional geological and hydrogeological studies, there were several events of marine transgression in the Quaternary Period at NCP (Zhang et al., 1997; Wang et al., 2008; Lin and Dai, 2012; Liu et al., 2016a, Liu et al., 2016b). It is worthy to note that the sediment sample (depth: 281 m) with highest iodine concentration in its compaction-released pore solution sample was collected from the reported horizon (at the depth of 270–290 m) of the Bohai marine
Conclusions
Groundwater iodine concentration at NCP ranged from 5.8 to 1110 μg/L and high iodine groundwater (>100 μg/L) is distributed mainly in the southern coastal areas, where serious land subsidence occurs. The events of marine transgression provide the provenance of iodine in deep groundwater system at NCP. The present study verifies the role that pore water compacted from deep sediments plays in the genesis of high iodine concentration in the deep aquifers. Our results show that high iodine
Acknowledgements
The research was financially supported by National Natural Science Foundation of China (No. 41521001, 41502230 and 41772255) and the 111 Program (The State Administration of Foreign Experts Affairs & the Ministry of Education of China Grant No. B18049).
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2022, Applied GeochemistryCitation Excerpt :By using GIS spatial analysis, Zhu et al. (2014) evaluated that the volume of land subsidence accounts for 57.6% of the deep groundwater yield in the Cangzhou. Our previous study estimated that the compaction-released pore water ratios range from 41.0% to 54.3%, 57.1%–70.1%, and 55.1%–79.2% to the deep groundwater based on the δ2H, 87Sr/86Sr and Cl− concentration in the NCP (Xue et al., 2019a). The simulation path 1 showed that in the non-land subsidence area, the amount of dissolved fluorite in 2021 can well explain the rising F− concentration along the flow path in 2017.