Potentially toxic elements are naturally occurring, ubiquitous substances in the human environment, which typically originate from the weathering of parent materials. Nevertheless, due to a variety of human activities, including mineral resources development, metal processing and smelting, industrial emissions, application of fertilizers and pesticides, sewage irrigation, and atmospheric transportation [1
], potentially toxic elements have substantially accumulated in the global environment in recent years, particularly in soil and sediment environments. According to the national communique of soil pollution survey by the Ministry of Environmental Protection of China and the Ministry of Land and Resources in 2014 [3
], soil has been contaminated heavily in some regions, and the quality of cultivated land soil is particularly concerning. Potentially toxic elements are transported in various forms through the exchange of substances among ecosystems, as they are highly mobile in air and water.
The transformation and migration patterns of potentially toxic elements in the aquatic environment are complex processes. Potentially toxic elements in the sediments are mainly from two sources, i.e., natural sources such as rock weathering and anthropogenic activities such as mining and agriculture. For example, Cd is considered to derive mainly from phosphorous fertilizers [4
]. Zinc and Cd found in water are typically from mining activities or industrial effluents, as well as combustion of fossil fuels [5
]. In addition, some studies have also indicated that fine sediments are carriers of potentially toxic elements in the surface land, flowing into the aquatic environment by surface runoff [5
]. In the aquatic environment, potentially toxic elements can migrate with a carrier, and are eventually absorbed by fine mineral particles in sediments and soils, leading to potentially toxic element pollution in aquatic environments, which has the potential to cause serious aquatic ecosystem and human health impairments [6
]. Elements such as metals dissolved in natural waters are easily absorbed by aquatic organisms and can rapidly bioaccumulate/biomagnify within the aquatic food web. Chronic potentially toxic element exposure in aquatic ecosystems may adversely affect the activity, growth, metabolism, and reproduction of aquatic organisms [7
]. Major Chinese rivers including the Chang River, Huang River and Zhu River are contaminated with potentially toxic elements to various degrees; Cd and Hg are the most heavily polluting elements, and are categorized as having high and extremely high ecological risks, respectively [8
]. In China, approximately 10 million ha of arable land has been polluted, and approximately 12 million tons of grain is contaminated every year due to potentially toxic elements in soils [4
In recent years, review and research articles have provided assessments of various kinds of soil heavy metal pollution all over the world, especially in developing countries, including urban soil, agricultural soil and mining area soil, such as the mining areas in Shanxi [9
] and other provinces in China [11
], as well as agricultural areas in southeast and northwest China [12
]. Such studies have helped to raise public awareness of soil contamination and to facilitate research for pollution control. On the national scale, Niu et al. [14
] reviewed metal accumulation across Chinese main farmland soil, Cd was found to have the highest pollution index (PI) of 5.28; Chen et al. [2
] identified the contamination characteristics of heavy metals in Chinese soils, the results revealed that cadimium and mercury were identified as the priority control metals, and indicated moderate contamination level. Yang et al. [15
] assessed potentially toxic element pollution and associated risks of industrial and agricultural soils in China, cadmium, lead, and arsenic showed more serious effects, and those in southeast China were severer than those in northwest China. In addition to China, potentially toxic element pollution studies were also conducted in other countries, i.e., Vietnam, Brazil, Congo, Thailand, where potentially toxic elements showed various contamination levels, and coal combustion, industrial and domestic waste discharges, and agricultural activity are the main sources [16
Potentially toxic elements, especially heavy metals, can enter food chains by enrichment, posing risks to the ecosystem and human health through long-term exposure, even at very low concentrations. For example, Pb is a non-essential element for humans, and excessive intake can damage the nervous, skeletal, circulatory, enzymatic, endocrine, and immune systems [20
]. Chronic exposure to Cd can have adverse effects such as lung cancer, pulmonary adenocarcinomas, prostatic proliferative lesions, bone fractures, kidney dysfunction, and hypertension. Meanwhile, the chronic effects of As exposure include dermal lesions, peripheral neuropathy, skin cancer, and peripheral vascular disease [21
]. Nowadays, A “war” to conquer soil pollution is occurring in China to improve soil quality, ensure the quality of agricultural products and protect the health of humans as well as animals. For example, a national action plan called “Soil Ten Chapter” has been implemented for the sake of the prevention and control of soil heavy metal pollution [3
In recent years, large numbers of studies have been conducted on soil and sediment potentially toxic element pollution and risk assessment, however, studies focused on metal pollution and risk in drinking water source areas are relatively few. The Miyun Reservior is the most important drinking water source for Beijing in China, and thus its soil and aquatic environmental safety are essential to the health of its residents. To our knowledge, some studies have been reported on potentially toxic element pollution levels, remobilization characteristics and source identification [22
]. However, no work has been reported the health risks induced by potentially toxic elements of soil and sediment in the Miyun Reservoir. Herein we present our study assessing the potentially toxic element pollution level, ecological risk, and human health risk of soils and sediments in the Chao River and Baimaguan River, which are located upstream of the Miyun Reservoir. The results of this study will provide useful reference information for potentially toxic element pollution control and risk management in this region.
Compared with the potentially toxic elements concentrations in the soil and sediments of other nationally important water sources, the potentially toxic element concentrations in the upstream of Miyun Reservoir seems lower. For example, the soil Cd, As and Cu in Danjiangkou Reservoir located in central China, were measured in the range of 0.02–1.17, 14.8–48.7, 17.03–52.4 mg kg−1
, and the average values were 0.27, 24.28, 27.76 mg kg−1
, respectively [45
]. The mean contents of Cu, As and Cd in the Danjiangkou Reservoir were the main pollution elements in arable land and were strongly affected by anthropogenic sources (such as agricultural fertilizers and pesticides) and were higher than other land-use types, showing the same trend to the Miyun Reservoir. According to the study of Han River from Liu et al. [11
], the average concentrations of Cd, Cr, Cu, Pb, Ni and Zn in the soils along the Han River were 0.54, 84.6, 37.7, 25.4, 111, and 95.5 mg/kg. Land uses had the most significant effect on the metal concentrations. Another study focused on Poyang Lake in Jiangxi Province, the contents range of Cu, Zn, Pb, Cd, Cr, and As were 37.92–709.28, 70.23–362.64, 54.26–160.63, 0.49–8.79, 51.32–153.63, and 9.72–47.66 mg/kg. Compared to rivers in the other developing countries, the metal contents in soils of the upstream of Miyun Reservoir also presented lower levels. For example, the concentration of Cd, Cr, Cu, Ni, and Pb in soils along Curu River watershed in Brazil were 0.38, 130, 34.6, 88, and 19.8 mg/kg, respectively; As for another research of the Khorat Basin in the northeast Thailand, the average concentration of As, Cd, Cr, Cu, Pb, and Zn were 1.7, 0.34, 64, 20.7, 23.3, and 61mg/kg, respectively. Application of fertilizers is the main reason for Cd, Cu and Zn pollution in the soil. According to research by Zhang [46
], Cd reached significant enrichment levels (5 < EF < 20), and approximately 97.14% of soil samples were heavily polluted with Cd in the Yellow River Delta of China. EF values for Cd in all soil samples were greater than 1.5, which means that Cd pollution in the YRD might be seriously affected by anthropogenic inputs. Similar results were found in the study by Zhang [47
], wherein average EF values for both Cd and Pb in the Bortala River in Xinjiang indicated extreme pollution, and the average ecological risk indices for Cd and Pb were 73 and 95, respectively, which indicated moderate pollution. High concentrations of Cd and Pb were primarily distributed in sediments from certain land use types, generally farmland and built-up urban areas with high populations and economic activity.
As for the sediment, there are also quantities of studies focused on potentially toxic elements. For example, a study conducted in five reservoirs in Liaoning and Jilin Province, which located in northern China, were detected for the sediment potentially toxic elements [48
]. The concentration ranges for Cu, Cd, Pb, Zn and Cr were 26.20–42.74 mg/kg, 1.33–2.43 mg/kg, 38.38–82.40 mg/kg, 101.11–148.06 mg/kg and 71.97–113.59 mg/kg, respectively. The contamination of potentially toxic elements in the sediments is higher than those in Miyun Reservoir, showing a closely relation to the mining industries located in the upstream basins. As for the western region in China, for example, Zhang et al. studied the sediment potentially toxic elements in the Bortala River in Xinjiang Province, the concentration of Cu, Pb, Cd, Zn, As and Cr were 30.09, 31.98, 0.17, 99.19, 9.67, 51.55 mg/kg, Cd, Hg, and Pb showed the similar concentration with those in Miyun Reservoir, and presented the main potential ecological risk factors with contributions of 25.43, 22.23%, and 21.26%. There are also other studies focused on potentially toxic elements pollution in the Miyun Reservoir, for instance, Qiao et al. [49
] showed that the average concentrations of sediment Cu, Cd, Pb, Zn, Cr, and As in Chao River, the largest tributaries of the Miyun Reservior, were 69.80, 3.32, 20.49, 94.79, 54.48, 4.62 mg/kg, respectively. Their values are significantly higher than our results, which was attributed to that the samples sites of Qiao et al. [49
] were located in the inflow and outflow tributaries of the Miyun Reservoir. However, in another study conducted for sediment core in the Miyun Reservior by Wu et al. [13
], the average concentration of Cd, Cr, and Zn are 0.028, 21.74, and 19.28 mg/kg, respectively. While in this research, the sampling sites are in the reservoir area, the water volume of which rises significantly after South-to-North Water Transfer Project, and are less affected by the potentially toxic elements concentration of tributaries. Another study conducted in the Pearl River Estuary of China by Zhang et al. [50
] showed that the Igeo values for Cd in all sediments of urban, rural and reclamation-affected rivers were categorized as “heavily to extremely polluted” (4 < Igeo < 5). Dusts and aerosols stemming from human activities, such as industrial and energy production, construction, vehicle exhaust and waste disposal are easily deposited into surrounding river sediments where they cause serious metallic pollution, and were considered the main source of potentially toxic element pollution in Pearl River sediments. Generally speaking, Cd was considered the primary contaminating metal in both soil and sediments samples from the Baimaguan and Chao Rivers. Its concentration was most likely influenced by extrinsic factors, such as human activities, automobile exhaust, and deposition of aerosols [31
]. The results described above are in accordance with the survey conducted by Chen et al. [2
], who reported that Cd and Hg are the main elements contributing to soil potentially toxic element contamination in China. In particular, increased anthropogenic inputs caused by rapid economic development since the last 1970s, establishment of industrial operations, and rapid urban expansion in China have drastically increased industrial and municipal wastewater discharges. In addition, similar trends in potential ecological risks were also found in other studies. For instance, among the potentially toxic elements assessed, namely As, Cd, Cr, Cu, Hg, Pb, Sb, and Zn, in soils from Wen’an County, Hebei Province in China, the potential ecological risk caused by Cd was greatest, and Cd posed considerable potential eco-risk (E(Cd) 125; [51
]). In the soils of other megacities, such as Beijing and Shenzhen, cadmium was also observed at much higher levels than other potentially toxic elements investigated with respect to potential ecological risk.
Overall, the comparison above reflects the variation in potentially toxic element concentrations among different regions, as well as the impact of anthropogenic activities on potentially toxic element enrichment in sediments.
The above results support treating Cd with concern in terms of risk to the ecosystem of Miyun Reservoir. The area of highest eco-risk was at the entrance of Miyun Reservoir, which may be influenced by dust, solid waste, accumulated mill tailings and acid drainage generated by mining, smelting and other activities. As is evident from the survey results, there are several iron ore mines in upstream of Miyun Reservoir, mainly distribute in the Chao River and Baimaguan River catchments, and mining history has been going on for more than 30 years. Since 2005, these iron ore mines has been closed one after another, despite this, due to the chronic deposition of potentially toxic elements in soil, there is still residual contamination in the soil surrounding these iron ore mines. Combined with the effect of soil erosion under rainfall on agricultural land in Miyun Reservior, which preferentially carries away potentially toxic elements and other organic matter. Ultimately, the carried potentially toxic elements import into aquatic environment, which furthermore induced potentially toxic elements accumulation in sediments.
In general, enrichment of Cd is considered to be caused by agricultural activities, including livestock and poultry breeding, as well as pesticide and fertilizer application. It was found that there are some fishponds and large numbers of orchards, and especially for orchards, large amounts of fertilizer are used, which could induce soil potentially toxic element contamination. According to another study in the Chao River in the upstream of Miyun Reservoir, the sources of Cd in the soil were identified by the PCA and cluster analysis methods and the results showed that the factor loading of Zn was very close to that of Cd, and distribution map also display a similar uniformity of these two elements’ distributions. Weathering erosion of the parent rock, mining activity and fertilizer application were the main origins of Cd and Zn [23
]. For the sediment Cd, it also has been identified by Wu et al. [13
], in the study of which found that due to agricultural land is the major land use type in Miyun Reservoir catchment. Thus, Cd and Zn in sediment were mainly from the phosphate fertilizer-related non-point source pollution. In addition, there are very few resident homes and tourist activities around, suggesting little domestic sewage discharge would be another source. Overall, phosphorous fertilizer and soil erosion are the two main sources for Cd pollution in the soils and sediments of the Miyun Reservoir.
In recent years, the Beijing Government has enacted a series of policies and strategies to control potentially toxic element pollution, such as industrial restructuring, clean energy development and coal consumption controls. Particularly in Miyun District, the most important drinking water source in Beijing, the assessment and enforcement of potentially toxic element pollution is strict. Therefore, the contribution of potentially toxic element pollution from other adjacent places should be paid attention, particularly Cd. For example, Hebei Provinces and Tianjin City, which are areas with more serious potentially toxic element pollution. Atmospheric deposition is an important pathway of soil and sediment contamination [52
]. Key pollutants such as potentially toxic elements, which can be related to industrial or transportation emissions, will either accumulate directly on ground surfaces or initially accumulate in the atmosphere. Over time, these atmospheric pollutants will also be deposited on ground surfaces for eventual transport by rivers and groundwater. Eventually, potentially toxic elements in the water are absorbed by organic matter in the sediment.
Potentially toxic element pollution control is arduous, and large-scale control must be carried out by governments. China has promoted coordinated management in Beijing-Tianjin-Hebei, namely establishing a collaborative governance mechanism and system. However, achieving successful treatment may take several decades. In addition, it should be noted that a number of other emerging metal contaminants (e.g., platinum, rhodium, and palladium) should also be considered in future environmental studies, as they are now poorly understood in terms of potential ecological and human health risks. Most previous studies on these pollutants have focused on detection methods and on their chemical forms in the environment.
It should be noticed that although the risk assessment model used in this study are useful and powerful tools to distinguish the toxic chemicals and various exposure pathways in the environment, however, the risk assessment of heavy metals in this study remained some uncertainties, which are inherent in quantitative risk assessment. First, the bioavailable or bioaccessible concentration of heavy metals is considered more reliable and accurate in ecological and human risk assessments [53
] which suggests that the human risk are assessed based on the total concentration of heavy metals; In addition, organic matter can absorb Cu, Pb, and Cd through its groups such as -RCOOH and -ROH, which made these elements no more be available, leading to the assessed risks in this study may be overestimated [54
]. In future work some models also can be considered to be used to avoid these uncertainties, such as study form Di Bonito [55
]. However, despite a lack of a completely accurate risk assessment, this study scored the health effects based on a well-defined investigation on oral ingestion and dermal contact exposure pathways and various heavy metals leading to potential ecological and human health risks in a typical county.