Distribution, pollution levels, and health risk assessment of heavy metals in groundwater in the main pepper production area of China

Mamattursun Eziz; Nazupar Sidikjan; Qing Zhong; Atiyagul Rixit; Xinguo Li

doi:10.1515/geo-2022-0491

Article Open Access

Distribution, pollution levels, and health risk assessment of heavy metals in groundwater in the main pepper production area of China

Mamattursun Eziz , Nazupar Sidikjan , Qing Zhong , Atiyagul Rixit and Xinguo Li

Published/Copyright: June 15, 2023

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From the journal Open Geosciences Volume 15 Issue 1

Abstract

Heavy metal pollution of groundwater can have severe potential impacts on human health and the environment. The aim of this study was to evaluate the spatial distribution, pollution, and health risk of heavy metals in groundwater in the main pepper production area of China. A total of 67 groundwater samples were collected, and the concentrations of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), manganese (Mn), and zinc (Zn) elements were analyzed. The results indicated that the average concentrations of Cd, Cr, Cu, Mn, Ni, and Zn in groundwater are lower than the limits of the Standard for Groundwater Quality of China (Class III). However, the maximum concentrations of Mn, Cd, and Ni surpass the Standard for Groundwater Quality of China. The spatial distribution patterns of the concentrations and pollution levels of the six heavy metals in groundwater to be substantially heterogeneous. Furthermore, the pollution level of heavy metals in groundwater decrease in the following order: Mn > Cd > Ni > Cr > Zn > Cu, and the collected groundwater samples are found to be slightly polluted by heavy metals. Overall, the non-carcinogenic risk of the investigated metals, instigated primarily by oral ingestion of groundwater, are found to be higher than the acceptable range for children, and Cd poses the most significant health risk among the investigated metals.

Keywords: non-carcinogenic risk; water quality; toxicity; Nemerow comprehensive index

1 Introduction

Groundwater is an important and indispensable freshwater resource, with nearly one-third of all humans drinking it [1]. As a result of rapid development of society, pollution of groundwater with heavy metals from natural and anthropogenic sources has become one of the main environmental problems in many countries [2,3,4]. Heavy metals are highly toxic and harmful to the human health and entire ecosystems, if they go beyond certain limits [5]. Specifically, heavy metals that accumulate in groundwater can result in the deterioration of groundwater quality which affect human health [6], and also can threaten the natural ecosystems [7]. Some heavy metals, such as copper (Cu), manganese (Mn), nickel (Ni), zinc (Zn), and chromium (Cr) are essential in limited quantities for metabolism, but they become toxic and harmful to the human health at higher levels [8,9,10]. Meanwhile, some other elements, such as cadmium (Cd) and mercury are hazardous even at lower concentrations [11,12,13]. To help control and prevent these harmful consequences, regular assessment of the distribution, levels, and the potential health risk of heavy metals in groundwater has become necessary.

Groundwater quality assessment is a crucial approach towards realizing sustainable groundwater quality management [14]. Hierarchical clustering [15], artificial neural network modeling [16], and multiple linear regression have been commonly used for the simulation and prediction of groundwater quality parameters. Multiple graphical, stoichiometric, indexical, and multivariate statistical models were also integrated for the monitoring and assessment of the quality and hydrogeochemical signatures of groundwater [17]. These indices have proven to be effective in analyzing the pollution levels of water resources in different regions of the world.

Due to extensive agricultural irrigation and fertilization, China now faces a severe problem of non-point source groundwater pollution [18]. Over the past several decades, numerous studies have been carried out to investigate the human exposure and potential health risks of heavy metals in groundwater in China [19,20,21]. Many of these literature show that drinking of groundwater is the main pathway of human exposure to dissolved toxic heavy metals, and the residents faced a significant potential health risk by consumption of polluted groundwater, which result in cancer risk. For example, Wen et al. [22] reported the groundwater investigation progress in China, and provided the main information about the aggravated heavy metal pollution of groundwater. Zhang et al. [18] established a GIS-based groundwater contamination risk assessment model for a typical agricultural area with high fertilizer application, and indicated that groundwater pollution risk decreases most significantly under the scenario of irrigation amount reduced by 25%. Du et al. [23] investigated the potential health risks of heavy metals in groundwater in an irrigation area in northeastern China, and found harmful health risks of heavy metals in groundwater in the studied area. Qiao et al. [21] evaluated the quality and related health risk of groundwater in the Guanzhong Plain region of China, and suggested that local residents and government departments should be made aware of Cr and Arsenic pollution in shallow groundwater in the studied area. More recently, Shi et al. [24] reported the sources and health risks of heavy metals in groundwater in Leizhou Peninsula, China, and find a pollution of As, Mn, and Cd elements. Long et al. [25] investigated the spatial distribution and health risk of heavy metals in Dongting Lake Plain in China, and found both the carcinogenic and non-carcinogenic health risks caused by heavy metals in some areas. Similarly, Gao et al. [26] assessed the health risks of heavy metals of groundwater in the coastal areas of Tianjin, and found a higher potential health risk caused by carcinogenic heavy metals in groundwater in the study area.

However, the aforementioned research works are mainly focused on the pollution risk assessment of heavy metals in groundwater in the eastern developed regions, or eastern coastal areas of China. Knowing that the northwestern arid regions of China have been rapidly developed with the promotion of the Silk Road Economic Belt [27], and environmental problems, especially heavy metal pollution, in this area have garnered more attention [28,29]. The Baghrash Lake Basin (BLB) is one of the most important pepper production areas in China. The natural conditions in the BLB provide advantages for the development of pepper planting, allowing the area to become the main pepper production region in NW China. One recent study [27] reports that heavy metal pollution is a serious problem in the BLB due to the intensive agricultural activities. Groundwater plays an important role in drinking and irrigation water supply in the BLB that makes its quality as very important to be assessed in order to know its potential for health risks. So far, however, pollution risks of heavy metals in groundwater in the BLB have attracted less attention. Thus, some effective measures must be carried out for the protection and sustainable development of groundwater resources.

The main objectives of this research are to (1) identify the concentrations and spatial distribution patterns of six heavy metal elements, such as Cd, Cr, Cu, Ni, Mn, and Zn, in groundwater in the BLB based on statistical and geostatistical analysis, (2) perform groundwater quality assessment using the Nemerow Comprehensive Index, and (3) quantify potential human health risks through oral intake and dermal contact of groundwater based on the health risk assessment model introduced by the United States Environmental Protection Agency (US EPA). The results will guide authorities in taking the necessary measures to ensure the safety of groundwater supply in this region.

2 Study area

Field research is conducted in the BLB, which is one of the main pepper production areas in China. The BLB is located at 85°50′–87°15′ E and 41°52′–42°22′ N, with a flat agricultural land of about 213,000 ha, and with an altitude ranging from 1,050 to 1,250 m (Figure 1). It is an active area of agricultural activity, and it is also the main production area of pepper in the northwestern arid zones of China. The climate of the BLB is a continental dry type with an average annual temperature of 10.88°C, an average annual precipitation of 70 mm, and an average annual evaporation of about 1,900 mm [27]. Groundwater in the BLB is primarily in the aquifers of the diluvial and alluvial layers in front of the Tianshan Mountains. The phreatic aquifer is the top layer in the BLB, and the thickness of the phreatic aquifer is about 10–20 m. Groundwater in the BLB flows from the piedmont areas to the Baghrash Lake. Groundwater recharge is mainly from river infiltration before irrigation seasons. During the irrigation seasons, part of groundwater is from the infiltration of the rivers and irrigation canals. Approximately 40% of the irrigation water recharge to the aquifer. The discharge of groundwater is mainly phreatic evaporation and drainage for irrigation and daily use [30].

Figure 1

Location of the Baghrash Lake Basin and sample sites.

3 Materials and methods

3.1 Sample collection, chemical analysis, and quality control

A total of 67 groundwater samples were collected from the BLB in August 2016, and the sample sites are illustrated on the map in Figure 1. Groundwater samples were collected from pumping wells, which are used by local residents for irrigation, drinking, and other daily uses. The pumping wells were pumped for about 5 min to eliminate the impacts of stagnant water.

At each sample site, 500 mL of groundwater was collected in a high-grade plastic bottle, and filtered through 0.45 μm Millipore nitrocellulose filters, preserved with ultrapure HNO₃ to pH < 2, and then transferred to the laboratory, where they were stored at a temperature of 4°C. Finally, the concentrations of Cd, Cr, Cu, Mn, Ni, and Zn were determined using a Flame atomic absorption spectrometer (Agilent 200AA, USA). The groundwater samples were brought to room temperature before testing, and distilled water was used for blank analyses in order to ensure the reliability of the results. Calibration curves were produced, using quality control standards, for assessing data from each set of groundwater samples. Reagent blanks and duplicates were tested five times in parallel and the average value of the last three values was used to avoid contamination caused by the last groundwater sample. Then, the average of the last three values was used as the result. The precision of the measurement was ensured through RSD not exceeding 3%.

3.2 Pollution assessment

The Nemerow Comprehensive Index, including I _i and NI, introduced by Nemerow [31] was used to evaluate the pollution level of heavy metals in groundwater. I _i and NI are calculated as follows:

(1) I i = P i S i ,

(2) NI = I max 2 + I ave 2 2 ,

where I _i represents the pollution index for a single element i, P _i is the concentration of element i (mg/L), and S _i is the evaluation standard of element i (mg/L). NI represents the pollution index caused by the overall pollution by all investigated elements, and I _max and I _ave are the maximum and the average value of I _i, respectively. The Standard for Groundwater Quality of China (GB/T 14848-2017) introduced by AQSIQ [32] is selected as the evaluation standard.

Pollution levels of I _i are classified as follows: no pollution (I _i ≤ 0.7), slight pollution (0.7 < I _i ≤ 1), light pollution (1 < I _i ≤ 2), moderate pollution (2 < I _i ≤ 3), and high pollution (I _i > 3). The pollution levels of NI are classified as follows: no pollution (NI ≤ 0.7), slight pollution (0.7 < NI ≤ 1), light pollution (1 < NI ≤ 2), moderate pollution (2 < NI ≤ 3), and high pollution (NI > 3) [31].

3.3 Health risk assessment

In general, oral ingestion and dermal contact of water are two main pathways of exposure of heavy metals in groundwater [33]. Considering the age of residents, the residents were classified into two groups: adults and children. The exposure of heavy metals in groundwater was characterized by the chronic daily intake (CDI, mg/kg/day). The CDI in the two exposure pathways can be computed using the following equations [34]:

(3) CDI oral = ( C i × IR × EF × ED ) / ( BW × AT ) ,

(4) CDI dermal = ( C i × SA × K p × ET × EF × ED × CF ) / ( BW × AT ) ,

(5) CDI total = CDI oral + CDI dermal ,

where C _i indicates the concentration of heavy metal i (mg/L), IR is the ingestion dose of groundwater (1.8 L/day for adults, and 0.7 L/day for children), ED is the exposure period (24 years each for adults and children) [35]. BW is the average body weight (62.4 kg for adults, and 21.2 kg for children) [27]. EF is the exposure frequency (350 days/year), CF is the conversion factor (0.001 L/cm³), AT is the averaging time of non-carcinogenic effect (8,760 days for adults, and 2,190 days for children), K _p is the absorption coefficient of the human body (2 × 10⁻³ cm/h for Cr, 2 × 10⁻⁴ cm/h for Ni, 6 × 10⁻⁴ cm/h for Zn, and 1 × 10⁻³ cm/h for Cd, Cu, and Mn [36]. SA indicates the skin surface area (16,600 cm² for adults, and 12,000 cm² for children) [37], and ET is the contact duration (0.33 h/day) [20].

The potential non-carcinogenic health risk of heavy metals in groundwater is calculated as the hazard quotient (HQ) as in equation (6):

(6) HQ = CDI/RfD,

where RfD indicates the reference dose (mg/kg/day). The RfDs for oral ingestion pathway for Cu, Mn, Cd, Cr, Ni, and Zn elements are 0.04, 0.024, 0.0005, 0.003, 0.02, and 0.30 mg/kg/day, respectively. The RfDs for dermal contact pathway for Cu, Mn, Cd, Cr, Ni, and Zn elements are 0.012, 0.0008, 0.000005, 0.00006, 0.0054, and 0.06 mg/kg/day, respectively [38,39]. Considering the total potential health risks posed by investigated heavy metal elements in groundwater, the hazard index (HI) is computed using the following formula [34]:

(7) HI = ∑ HQ = HQ oral + HQ dermal ,

When HI < 1, the exposed individual will not have the non-carcinogenic health risks from heavy metals in groundwater. On the contrary, HI > 1 indicates a potential non-carcinogenic health risks [40,41,42].

3.4 Statistical analysis

The basic statistical parameters including minimum, maximum, average, median, and standard deviation were computed. The normality of the groundwater data was checked using the normal QQ plot, and logarithmic transformation or square root transformation of non-normal distribution for the heavy metal concentrations were used to obtain the Gaussian distributions. Ordinary Kriging interpolation method was employed for mapping the spatial distribution patterns of concentration, pollution levels, and health risks of heavy metals. Statistical analysis was performed using the SPSS 25.0 software, while the Geostatistical Analysis based on GIS technology was performed using the ArcGIS 10.2 software and the GS+10 software.

4 Results and discussions

4.1 Concentrations of heavy metals in groundwater

The basic statistical values of concentrations of the investigated heavy metals in groundwater and the third class of the Standard for Groundwater Quality of China (GB/T 14848-2017) are given in Table 1. As shown in Table 1, on average, the concentrations of Cu, Mn, Cd, Cr, Ni, and Zn in the collected groundwater samples are 0.027, 0.069, 0.007, 0.012, 0.022, and 0.065 mg/L, respectively. The average concentrations of all elements are less than the limits of the Standard for Groundwater Quality of China. In addition, the maximum concentrations of Mn, Cd, and Ni surpass the Standard for Groundwater Quality of China by factors of 2.79, 1.10, and 2.18, respectively. The exceedance ratios for Mn, Cd, and Ni elements in groundwater are 10.45, 1.49, and 10.45%, respectively. Meanwhile, the pH value of groundwater varied from 7.13 to 8.05, with an average value of 7.45, with weak alkalinity. Depending on the above analysis, it appears that the groundwater in the BLB was affected by heavy metals in varying degrees, and Mn, Cd, and Ni are particularly more abundant in groundwater in the BLB.

Table 1

Descriptive statistics of heavy metal concentrations in groundwater (n = 67)

Items	Cu	Mn	Cd	Cr	Ni	Zn
Minimum/(mg/L)	0.009	0.005	0.002	0.005	0.005	0.005
Maximum/(mg/L)	0.108	0.153	0.011	0.042	0.109	0.454
Average/(mg/L)	0.027	0.069	0.007	0.012	0.022	0.065
Median/(mg/L)	0.020	0.065	0.007	0.012	0.020	0.064
SD/(mg/L)	0.020	0.061	0.002	0.007	0.024	0.086
CV	0.741	0.884	0.286	0.583	1.091	1.323
*National standard/(mg/L)	1.000	0.100	0.010	0.050	0.050	1.000
Exceedance ratios/(%)	0	10.45	1.49	0	10.45	0

Note: *The Standard for Groundwater Quality of China (Class III).

The standard deviations (SDs) for concentrations of heavy metals in groundwater were relatively high, as shown in Table 1. This indicates that the concentration of each heavy metal varied significantly from one sample site to the other. Based on this criterion and the calculated coefficient of variations (CVs) of the analyzed groundwater samples, the CVs of Cu, Mn, Cr, Ni, and Zn are all higher than 0.50, indicating high variability for these elements. Cadmium shows a moderate variability concerning spatial distribution. The large CVs of these elements indicated a higher possibility of being affected by extrinsic factors. The heavy metals falling into the moderate or high variability classes are likely to be influenced by anthropogenic factors. Therefore, it can be concluded that the heavy metals in groundwater in the study area are influenced by activities to a certain extent.

4.2 Spatial distribution of heavy metals in groundwater

The heavy metal concentrations in groundwater are influenced by both natural and anthropogenic factors, which lead to the diverse spatial distributions of the heavy metal concentrations. The geostatistical analysis method is an effective way for identifying the spatial heterogeneity and spatial distribution patterns of heavy metal concentrations in groundwater [4]. A GIS-based ordinary Kriging interpolation method was applied in order to map the spatial distribution of the concentrations of analyzed heavy metals in groundwater in the BLB (Figure 2).

Figure 2

Spatial distributions of the concentrations of heavy metals in groundwater. (a) Cu, (b) Mn, (c) Cd, (d) Cr, (e) Ni and (f) Zn.

The spatial distribution patterns of the concentrations of the six heavy metals in groundwater illustrated in Figure 2 are substantially heterogeneous. In the case of Cu, the most accumulation in the western areas and least accumulation in the eastern areas are investigated in this study, and the concentrations of Cu decreased from the western to the eastern areas of the BLB. Copper enters into the aquatic medium through the anthropogenic factors of metal production, fertilizers production, and other industrial activities. Copper also originates from agriculture activities [43]. The industrial park of Hejing county is located in the western parts of the BLB, and industrial activities in this industrial park may lead to an increase in Cu concentration in surface soil, and may enter into the groundwater system through the leaching effects of irrigation and rainfall.

Higher concentrations of Mn are observed mainly in the western and central areas, whereas lower concentrations are observed in eastern areas in the BLB. Anthropogenic factors in these areas, such as relatively higher population density and agricultural practices, may affect the accumulation of Mn in groundwater. Higher concentrations of Cd are observed mainly around the southern areas, whereas the lower concentrations are detected in the eastern areas in the BLB. Cadmium occurs naturally in the earth’s crust at a low level, and anthropogenic factors have led to its increase beyond permissible limits [44]. Cadmium mainly originates from the utilization of chemical fertilizers and pesticides [45]. Previous report [28] in this area shows that the average concentration of Cd, which mainly originated from agricultural activities, in soil in this study area exceeded the corresponding background values of soils by a factor of 54.0. Cadmium attached with soil particles and sediments may dissolve in irrigation water and move toward groundwater, and lead to an increase in the concentration of Cd in groundwater.

Higher concentrations of Cr are observed mainly in the southern areas, whereas the lower concentrations were detected mainly around the eastern and northern areas in the BLB. Chromium is originated from geogenic and anthropogenic processes [43]. The concentrations of heavy metal in groundwater are affected by the leaching of metal elements from the surface, and the geochemistry of aquifers also influences the availability of heavy metals in groundwater [21]. The average concentration of Cr, which also originated from agricultural activities, in soil in this study area exceeded the corresponding background values of soils by a factor of 1.34 [28]. Relatively intensive agricultural activities in the southern parts in the study area may lead to the increase in Cr concentration in southern areas in the BLB.

Higher concentrations of Ni is observed mainly around the eastern and central areas, whereas lower concentrations are detected in the western and southern areas in the BLB. Based on field investigation, the higher concentrated areas of Ni may be affected by the agricultural and domestic runoff, which reaches the groundwater system, from the catchment area. A zonal distribution pattern is found for Zn element, with the most accumulation in the northern areas and least accumulation in the southern areas investigated in this study, and the concentrations of Zn decreased from the northern to the southern areas in the BLB. The concentration of Zn in groundwater in the study area is very low. Zinc is one of the most abundant elements on earth. Therefore, the spatial distribution of Zn in groundwater in the BLB may be controlled by natural factors.

4.3 Pollution of heavy metals in groundwater

The pollution of groundwater by heavy metals in the BLB was categorized based on the values of the I _i and the NI (Table 2). Depending on the average value of I _i, the pollution grade of I _i for all metals decrease in the following order: Mn > Cd > Ni > Cr > Zn > Cu, with greater I _i values showing higher pollution levels. Based on the classification standard of I _i and the calculated average values of I _i of heavy metals in groundwater, the groundwater in the BLB is slightly polluted by Mn (I _i = 0.71), and not polluted by Cd (I _i = 0.68), Ni (I _i = 0.46), Cr (I _i = 0.25), Zn (I _i = 0.07), and Cu (I _i = 0.03).

Table 2

Statistics of I _i and NI values of heavy metals in groundwater

Statistics	I _i						NI
Statistics	Cu	Mn	Cd	Cr	Ni	Zn	NI
Minimum	0.01	0.10	0.20	0.20	0.01	0.01	0.23
Maximum	0.11	3.00	1.10	0.80	2.20	0.45	2.25
Average	0.03	0.71	0.68	0.25	0.46	0.07	0.73
SD	0.05	0.61	0.23	0.12	0.49	0.09	0.41
CV	1.67	0.86	0.34	0.48	1.07	1.29	0.56

It should be noted that the maximum values of I _i for Mn and Ni elements in groundwater in the BLB fall into the moderate pollution level, while the maximum value of I _i for Cd in groundwater falls into the light pollution level, and the maximum value of I _i for Cr in groundwater falls into the slight pollution level. More specifically, the average values of I _i for Cu and Zn elements in all groundwater samples fall into the unpolluted level, while Mn in 25.37% of the samples, Cd in 41.79% of the samples, Cr in 1.49% of the samples, and Ni in 7.46% of the samples fall into the slight pollution level, respectively. At the same time, Mn in 5.97% of the samples, Cd in 1.49% of the samples, and Ni in 7.46% of the samples fall into the light pollution level, while Mn in 4.48% of the samples and Ni in 2.98% of the samples fall into the moderate pollution level. The results suggest that Mn, Ni, Cd, and Cr in groundwater are the main pollutants of concern in the BLB, and further analysis should focus on them.

Table 2 also indicates that groundwater in the BLB are polluted by heavy metals that may have originated from anthropogenic factors. The calculated CVs of I _i for Cu, Mn, Cr, and Zn in groundwater show high variability, while Cd and Cr show moderate variability concerning spatial distribution. Clearly, the pollution levels of these six heavy metals in groundwater vary significantly from one sample site to the other. It can be concluded that the pollution level of these heavy metals in groundwater are likely to be correlated with anthropogenic activity.

The NI values of groundwater in the BLB range from 0.23 to 2.25 with an average value of 0.73, which is at a slight pollution level compared to the classification standard of NI. Among the 67 groundwater samples, 59.70% of the samples show an unpolluted level, while 28.36% of the samples show a slight pollution level, 7.46% of the samples show a light pollution level, and 4.48% of the groundwater samples show a moderate pollution level.

The spatial distribution patterns of the I _i values for Cd, Mn, and Ni elements in groundwater are shown in Figure 3a–c, and the NI value of the six heavy metals in groundwater is illustrated in Figure 3d. Here higher I _i values for Cd and Mn are observed mainly in the western and southern areas, while the lower I _i values are observed in the eastern areas in the BLB. Higher I _i values for Ni is observed around the eastern and central areas, and lower concentrations are observed in the southern areas in the BLB. Moreover, the spatial distribution patterns of the NI and the spatial distribution patterns of I _i values of Cd, Mn, and Ni in groundwater overlap substantially, further indicating that Cd, Mn, and Ni are the main pollution factors in groundwater system in the BLB. Overall, the pollution level of heavy metals in groundwater in the BLB depends on the intensity of anthropogenic activities, as discussed earlier.

Figure 3

Spatial distributions of the I _i of (a) cadmium, (b), manganese, (c) nickel, and (d) NI values of heavy metals in groundwater.

4.4 Health risks of heavy metals in groundwater

The calculated average non-carcinogenic health risk, both children and adults, of heavy metals in groundwater in the BLB is given in Table 3. As shown in the table, the total chronic daily intakes (CDI_total) of Mn element for children and adults are 1.93 × 10⁻³ and 2.21 × 10⁻³ mg/kg/day, respectively, at the highest total chronic daily intakes. The CDI_total of Cd element for children and adults are 1.88 × 10⁻⁴ and 2.15 × 10⁻⁴ mg/kg/day, respectively, at the lowest CDI_total. The CDI_total of six heavy metal elements in groundwater in the BLB decrease in the order of Mn > Zn > Cu > Ni > Cr > Cd. The CDI_total of six heavy metal elements for children are relatively higher than that for adults. Moreover, the CDI_total from the oral ingestion pathway is higher than dermal contact pathway, for both children and adults. Therefore, the oral ingestion of the groundwater is the major pathway for the non-carcinogenic health risk of analyzed heavy metals in the BLB.

Table 3

Estimated daily exposure for non-carcinogenic risk from heavy metals

Metals	CDI_oral (mg/L/day)		CDI_dermal (mg/L/day)		CDI_total (mg/L/day)
Metals	Adults	Children	Adults	Children	Adults	Children
Cu	7.60 × 10⁻⁴	8.70 × 10⁻⁴	2.31 × 10⁻⁶	4.92 × 10⁻⁶	7.62 × 10⁻⁴	8.74 × 10⁻⁴
Mn	1.92 × 10⁻³	2.20 × 10⁻³	5.85 × 10⁻⁶	1.25 × 10⁻⁵	1.93 × 10⁻³	2.21 × 10⁻³
Cd	1.87 × 10⁻⁴	2.14 × 10⁻⁴	5.69 × 10⁻⁷	1.21 × 10⁻⁶	1.88 × 10⁻⁴	2.15 × 10⁻⁴
Cr	3.47 × 10⁻⁴	3.97 × 10⁻⁴	2.11 × 10⁻⁶	4.49 × 10⁻⁶	3.49 × 10⁻⁴	4.01 × 10⁻⁴
Ni	6.32 × 10⁻⁴	7.23 × 10⁻⁴	3.84 × 10⁻⁷	8.18 × 10⁻⁷	6.32 × 10⁻⁴	7.24 × 10⁻⁴
Zn	1.80 × 10⁻³	2.07 × 10⁻³	3.29 × 10⁻⁶	7.01 × 10⁻⁶	1.81 × 10⁻³	2.07 × 10⁻³

The calculated HQ values of heavy metals in groundwater for two exposure pathways and the HI for children and adults are given in Table 4. As shown in the table, the average values of HQ of heavy metals in groundwater decrease in the order of HQ_Cd > HQ_Cr > HQ_Mn > HQ_Ni > HQ_Cu > HQ_Zn. As for adults, the total HQ values of Cu, Mn, Cd, Cr, Ni, and Zn elements in groundwater account for 2.45, 11.17, 62.32, 19.28, 4.05, and 0.78% of the entire HI value, respectively. As for children, the total HQ values of Cu, Mn, Cd, Cr, Ni, and Zn elements account for 2.10, 10.19, 63.81, 19.71, 3.46, and 0.67% of the entire HI value, respectively. It indicates that Cd contributes the most to the potential non-carcinogenic human health risks of heavy metals in groundwater in the BLB. Therefore, Cd is the main non-carcinogenic factor in groundwater in the BLB.

Table 4

The non-carcinogenic risk index of heavy metals in groundwater

Metals	HQ_oral		HQ_dermal		HQ_total		HI
Metals	Adults	Children	Adults	Children	Adults	Children	Adults	Children
Cu	1.90 × 10⁻²	2.17 × 10⁻²	1.93 × 10⁻⁴	4.10 × 10⁻⁴	1.92 × 10⁻²	2.21 × 10⁻²	0.78	1.05
Mn	8.02 × 10⁻²	9.18 × 10⁻²	7.32 × 10⁻³	1.56 × 10⁻²	8.75 × 10⁻²	1.07 × 10⁻¹
Cd	3.74 × 10⁻¹	4.28 × 10⁻¹	1.14 × 10⁻¹	2.42 × 10⁻¹	4.88 × 10⁻¹	6.70 × 10⁻¹
Cr	1.16 × 10⁻¹	1.32 × 10⁻¹	3.52 × 10⁻²	7.49 × 10⁻²	1.51 × 10⁻¹	2.07 × 10⁻¹
Ni	3.16 × 10⁻²	3.62 × 10⁻²	7.12 × 10⁻⁵	1.51 × 10⁻⁴	3.17 × 10⁻²	3.63 × 10⁻²
Zn	6.01 × 10⁻³	6.88 × 10⁻³	5.49 × 10⁻⁵	1.17 × 10⁻⁴	6.07 × 10⁻³	7.00 × 10⁻³

The HI values for adults and children are 0.78 and 1.05, respectively. It indicates that the heavy metals in groundwater in the BLB had no non-carcinogenic health risks to adults, but may present a low non-carcinogenic health risk to children. The spatial distribution patterns of the HI value of the heavy metals in groundwater are shown in Figure 4. Here higher HI values of heavy metals are observed mainly in groundwater of the western and southern areas in the BLB. This is because higher concentrations of heavy metals are observed mainly in groundwater of the western and southern areas in the BLB. Cadmium is a highly toxic element, and the RfDs of Cd for oral ingestion and dermal contact are 0.0005 and 0.000005 mg/kg/day, respectively, which are significantly smaller than other elements analyzed in this study. It resulted in the greatest impact of Cd on HI values of groundwater. Moreover, the spatial distribution patterns of the HI value and spatial distribution patterns of Cd concentration in groundwater overlap substantially, further indicating that Cd is the prominent health risk factor in groundwater. Due to this, Cd can be selected as the priority control heavy metal in groundwater in the BLB.

Figure 4

Spatial distributions of the HI values of heavy metals in groundwater for (a) children and (b) adults.

Generally, heavy metals in groundwater originate from either anthropogenic sources or natural sources [46]. Based on field investigation in the BLB, the heavy metal concentrations in groundwater near industrial parks and the areas with intensive agricultural activities are relatively higher, which seems to be an indication that pollution emissions from industry and agriculture may influence heavy metal pollution of groundwater, especially in irrigation areas. It indicates that there should be concern about the potential health risk from ingesting heavy metals in groundwater system in the BLB, and the safety of rural domestic water in this area should be emphasized in future to protect the health of rural residents and to ensure the safety of groundwater.

The results obtained in this study indicated that Cd can be selected as the priority control heavy metal in groundwater in the BLB. It is worth highlighting that Cd is harmful to human health even at lower concentrations, owing to their high toxicity [9]. Cadmium toxicity affects the respiratory system, skeleton, and kidney. Cadmium is termed a carcinogenic heavy metal for humans [44]. Mamattursun et al. [27] reported that the fertilizer application rates in agricultural lands in the BLB have increased in past decades, and local farmers have used relatively larger amounts of fertilizers and pesticides, and they have overirrigated the agricultural lands to achieve relatively more yields. Application of fertilizer and pesticides and the large amount of irrigation water may cause increased levels of Cd in both soil and groundwater system in the BLB. These findings explained that Cd in groundwater in the BLB may be mainly derived from agricultural activities. However, special attention by the local government should be paid to effectively control Cd pollution of groundwater system in order to target the lowest threats to human health, and ensure the safety of groundwater supply in the BLB. Further, measurements, such as advanced treatment of agricultural wastewater, and reducing chemical fertilizers and pesticides, can be helpful to lower heavy metal pollution spread to groundwater system in the BLB.

The results of the present study can provide some implications for groundwater management efforts. First, great differences in the concentrations, pollution levels, and potential health risk of heavy metals exist among different parts in the BLB. Specific policies should be made for different parts in the BLB, and anthropogenic activities in the BLB should be cautious. The most effective way to mitigate accumulation of heavy metals in groundwater is the control of origins of heavy metals and strict enforcement of environmental regulations. Second, accumulation of heavy metals in groundwater is a dynamic process. A monitoring network for groundwater system should be established to ensure long-term monitoring on the dynamic change process of groundwater quality, which could provide more affective, continuous, and updated information of groundwater system for decision-makers. Finally, the exposure parameters (such as the ingestion dose, the exposure period, the average body weight, and the skin surface area) for assessing the CDI used in the present study were obtained from the US EPA Exposure Handbook or other related research works, which might not be very appropriate for assessing the potential health risk of heavy metals in groundwater in the BLB. Some of the exposure parameters are different in different seasons (e.g., heavy metal concentration and daily water intake), and it may probably overvalue or undervalue the actual potential human health risks of heavy metals in groundwater. Further studies should focus on the more suitable exposure parameters to obtain a more accurate estimation of the potential human health risks of heavy metals in groundwater in the BLB. Moreover, the toxicity from simultaneous exposure to multiple heavy metal elements in groundwater system is needed to gain systematic estimation of the potential human health risk [27]. It is recommended that the exposure parameters should be modified for defining the more precise health risk assessment.

5 Conclusion

In this study, groundwater samples were collected from the BLB in northwestern arid regions of China, and investigated the samples for Cd, Cr, Cu, Ni, Mn, and Zn concentrations. Further, the spatial distribution patterns, pollution levels, and health risks of these heavy metals were analyzed. Specifically, results of this study show that the average concentrations of Cd, Cr, Cu, Mn, Ni, and Zn elements in groundwater in the BLB are lower than the limits of the Standard for Groundwater Quality of China, but the maximum concentrations of Mn, Cd, and Ni in groundwater surpass this Standard. Moreover, the pollution level of heavy metals in groundwater decrease in the following order: Mn > Cd > Ni > Cr > Zn > Cu. The NI values of groundwater show a slight pollution level. In addition, spatial distribution patterns of heavy metals in groundwater are found to be substantially heterogeneous. Results of health risk assessment indicates that the investigated heavy metals in groundwater in the BLB may present a low non-carcinogenic health risk to children. Similarly, the oral ingestion of groundwater is the main pathway for the health risk of heavy metals. However, Cd is the main non-carcinogenic factor in groundwater in the BLB, and regular monitoring of heavy metals in groundwater is recommended.

Funding information: This work was supported jointly by the National Natural Science Foundation of China (No. U2003301, No. 41661047 and the Tianshan Talent Plan Project).
Conflict of interest: Authors state no conflict of interest.

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Received: 2022-02-07

Revised: 2023-04-26

Accepted: 2023-05-08

Published Online: 2023-06-15

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/geo-2022-0491

Keywords for this article

non-carcinogenic risk; water quality; toxicity; Nemerow comprehensive index

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