Multiple pollutants in groundwater near an abandoned Chinese fluorine chemical park: concentrations, correlations and health risk assessments

Characteristics of various pollutants in water

NO⁻
₂-N, NO⁻
₃-N, NH⁺
₄-N and F^–

Pollutant concentrations in groundwater from the FCP of Fuxin City are shown in Table 1. The NO⁻₂-N and NO⁻₃-N concentrations ranged from no detection (nd) to 0.063 mg·L⁻¹ and nd to 10.77 mg·L⁻¹, which were below the standard values of 1.00 mg·L⁻¹ and 20.00 mg·L⁻¹, respectively (Table 1). The mean NH⁺₄-N concentration (1.02 mg·L⁻¹) was two times that of the standard value (0.5 mg·L⁻¹) in GB/T14848-2017²⁰. Three major sources of NH ⁺₄-N were identified: leather industry production, introduction of nitrogen chemicals, and human activities²⁶.

A high F⁻ background always exists in the local groundwater. Abundant fluorite (CaF₂) resources can dissolve and disperse along the water flow direction, which was identified as the main source of F⁻ in groundwater. The highest F concentration appeared in G3 (4.42 mg·L⁻¹), followed by G8 (3.43 mg·L⁻¹) and G6 (1.50 mg·L⁻¹) (Table 1 and Fig. 1). These concentrations far exceeded the limit (1.0 mg·L⁻¹) for Class III Chinese drinking water, although F concentration standards from different organizations and institutes are always controversial²⁰. The U.S. Environmental Protection Agency (EPA) recommends fluoridated community water systems adjust fluoride to approximately 0.7 mg·L⁻¹²⁷. Additionally, other studies have indicated that only F⁻ concentrations below 2 mg·L⁻¹ preclude adverse effects on the human body from drinking water²⁸. Dissanayake et al. suggested that if the F concentration in drinking water exceeds 1.5 mg·L⁻¹, the probability of dental fluorosis increases for children²⁹. Regardless of which standard values are used, all F values in our study exceeded the screening or risk values, which indicated that F contamination in groundwater may pose a threat to the human body.

Heavy metals

As, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were all detected, whereas Cd and Hg were assigned nd status. The predominant heavy metals were Mn (0.0005–4.91 mg·L⁻¹), Fe (1.45–5.61 mg·L⁻¹) and As (0.0025–0.01 mg·L⁻¹) (Table 1 and Fig. 1). In our previous studies, the levels of Ni (63.01 mg·kg⁻¹), Cr (48.26 mg·kg⁻¹), Cu (23.29 mg·kg⁻¹), Pb (12.54 mg·kg⁻¹) and As (9.80 mg·kg⁻¹) in soil were higher than those in groundwater¹⁹. Surprisingly, Cd and Hg were detected in soil but not in groundwater. Heavy metals entering the soil have weak transport capacity and are easily adsorbed by soil colloids; moreover, the physical and chemical properties of soil affect the form adopted, especially for some metals that have difficulty migrating from soil to water^2,12. The concentrations of the two heavy metals Fe and Mn were much higher than 0.3 mg·L⁻¹ and 0.1 mg·L⁻¹, especially for Fe, which exceeded the standard value at all sampling points. Rich ore resources are the main sources of excess Fe and Mn in groundwater, and metal powder can enter groundwater during the extraction process.

PFASs

Eighteen PFASs groundwater concentrations from the FCP of Fuxin City are shown in Table 2. The ∑₁₈PFAS concentrations ranged from 72.49 ng·L⁻¹ to 68,142.14 ng·L⁻¹, and the mean value was 11,108.42 ng·L⁻¹. Short chain PFBA (4.14–2501.42 ng·L⁻¹), PFBS (17.07–51,818.61 ng·L⁻¹) and PFHxA (0.47–936.32 ng·L⁻¹) were the predominant substances (Table 2 and Fig. 1). The highest ∑₁₈PFAS concentrations were found at G5 (68,142.16 ng·L⁻¹), G4 (26,464.56 ng·L⁻¹) and G6 (3334.00 ng·L⁻¹), which were very near the center of the FCP (Fig. 1 and Table 2). Short chain PFASs, as alternatives to long chain substrates, have been widely used in the FCP of Fuxin City or in other countries²¹. The PFASs contamination in our study area was much higher than those in other regions, including Taiwan³⁰, the Maozhou River basin²⁸, a Tianjin suburb³¹ and Hubei Province³². Short chain PFASs (PFBA, PFBS, PFHpA, PFHxA and PFHxS) and PFOA are commonly detected currently due to the heavy use of long chain substitutes and different industry types^17,32. There is no current environmental standard for PFASs in China, whereas newly emerging pollutants have been of concern in Canada³³, Swedish¹³, American³⁴ and so on. The U.S. Environmental Protection Agency (USEPA) limit for PFOA in drinking water is 400 ng·L⁻¹, and the PFOA concentration (35.34–11,305.65 ng·L⁻¹) exceeds the standard value by several orders of magnitude³⁵. The low concentration detected for long-chain PFASs (C ≥ 8) may result mainly from historical industry production and emissions. Short chain PFASs applied to improve hydrophilicity and fluidity tend to migrate easily in water bodies, whereas long chain PFASs are readily adsorbed by large particles or sediments³⁶. The Xi River is the main river flowing through the FCP and receives many industrial pollutants throughout the year. Surface water recharge and infiltration moves PFASs into the groundwater system. Based on contamination in the Maozhou River basin, a potential link between the surface and groundwater was identified, and the PFASs concentration in surface water had a highly positive relationship with that in groundwater³⁷. A PFOS substitute of 6:2 Cl-PFSEA was detected at low concentrations (0.01–0.17 ng·L⁻¹), which indicates that the emerging PFASs substitute has been used in local industrial production^38,39 (Table 2). In addition, PFSAs concentrations were much higher than PFCAs concentrations. Different functional groups cause PFASs to exhibit varied environmental behaviour, especially short chain PFASs, which migrate easily in water bodies⁴⁰.

Correlation relationship analysis

Correlations among various pollutants and physicochemical properties (see Supplementary Table S6 online) are shown in Fig. 2. The relationship between F⁻ concentration and physicochemical properties was weak in groundwater except for DO, and the result was the same as that of Loganathan et al. for soil⁴¹. The F⁻ concentration had a strongly negative relationship (R² = − 0.717, p = 0.02) with DO, which indicates that DO content may influence F⁻ migration or behaviour, but this also needs to be studied further. In the present study, the weak correlation between F⁻ and heavy metals was due to frequent rain, which dilutes the pollutants in water. The F⁻ in groundwater has many sources, including atmospheric deposition, agricultural activities, geographical factors, fertilizers and pesticides⁴². Although the F⁻ concentration had no significant relationship with TH (R² = − 0.337, p = 0.3) in groundwater, F⁻ concentrations are continuously enriched, even after the groundwater reaches equilibrium with respect to CaF₂ levels, due to removal of Ca by precipitation of calcite (CaCO₃)^42,43. In addition, plant cover type and amount, rain and soil properties also affect the migration of F⁻ from the surface to groundwater⁴⁰. The F⁻ concentration showed the only significant relationship with heavy metal levels (p < 0.1) in soils from Shifang County of Sichuan Province; similarly, this relationship and the solubility of F⁻ in groundwater needs to be further studied⁴⁴.

Correlations between physicochemical properties and multiple pollutants in groundwater from the FCP of Fuxin City, Liaoning Province.

The environmental behaviour of PFASs in water bodies is influenced by various physicochemical properties, but the correlations between PFASs and physicochemical properties were limited in this study. pH, DO, RP, SC, TH, TDS and TC showed no significant positive or negative relationships with individual PFAS concentrations, whereas the water level had a positive correlation with short-chain PFASs and a negative correlation with long-chain PFASs. PFBA (R² = 0.703, p = 0.03), PFBS (R² = 0.803, p = 0.009), PFPeA (R² = 0.619, p = 0.052), PFHxA (R² = 0.710, p = 0.03), PFHxS (R² = 0.802, p = 0.001), and PFHpA (R² = 0.620, p = 0.054) had positive relationships with water level. PFASs tend to migrate to deep groundwater and soil along the path for movement and penetration of water. Short-chain PFASs were more inclined to migrate in water environments than long-chain PFASs because of their hydrophilicities. In addition, frequent rainfall may also accelerate flow migration and affect the PFAS concentration distribution in the groundwater system. Nevertheless, long chain PFDoA (R² = − 0.607, p = 0.07), PFTrDA (R² = 0.786, p = 0.01), PFTeDA(R² = − 0.807, p = 0.002) and PFHxDA(R² = − 0.670, p = 0.04) showed negative relationships with the water level. The strong hydrophobicities of long chain PFAS facilitate adsorption by particles in water and limit migration and mobility capabilities. The correlations indicate the fates of short- and long-chain PFASs in groundwater. The positive relationships between short chain PFASs or long chain PFASs indicates that carbon chain length plays a key role in migration. The solubility in water in the water body is inversely proportional to the carbon chain length³². A previous study from New Jersey, USA, also showed that long chain PFDA, PFUdA and PFDoA have higher affinities for organic material and preferentially partition into sediments⁴⁵. Importantly, there were no relationships between PFASs and F⁻ or heavy metal concentrations in our study. The reason may be that PFASs and heavy metals come from different sources or their environmental concentrations are low, but this requires more study.

Health risk assessments

Health risk assessments are widely used to evaluate harmful effects on the human body. The assessment method with the quotient is a simple model that effectively calculates the risk values for different pollutants in the groundwater system. However, the model only considers the risk of a single pollutant and does not apply complex mechanisms for different pollutants. The main detected pollutants, including heavy metals, individual PFASs, and F⁻, were assessed, and the results are shown in Fig. 3.

Health risk assessment for heavy metals, F⁻ and PFASs in groundwater from the FCP of Fuxin City, Liaoning Province; (a) HQ value for different age groups; (b) HQ_mix value for different age groups and (c) HQ contributions from different heavy metals and individual PFASs for subjects aged 21–65 year old.

Figure 3a shows that children aged 6–12 months were more vulnerable to toxic pollutants than residents in the other age groups. The mean HQ values for heavy metals and F⁻ reached 4.82 and 5.13, respectively, which poses potential risk for the 6–12 month age group. The HQ value reflected a decreasing trend with increasing age. The mean HQ or HQ_mix value for F or heavy metals exceeded 1 for all age groups, which indicates that residents may suffer the negative effects of drinking contaminated groundwater. In Fig. 3b, the potential sources of risk for adults aged 21–65 years came from the heavy metals As (45.0%), Mn (27.1%), and Fe (24.9%). Although the quotients for As, Mn, and Fe were not exceeded 1 for the 21–65 year group, the HQ values were 2.34, 1.41, and 1.29 in the 6–12 month group (Fig. 3c). In studies of the risk due to heavy metals, groups of infants were exposed to toxic pollutants^46,47,48,49. Abundant CaF₂ resources were the main reason for the high F⁻ concentrations in groundwater, and earlier industrial production was also identified as another source. The HQ value for F⁻ exceeded 1 for all age groups (4.82 for 6–12 months, 1.75 for 6–11 years, 1.32 for 11–16 years, 1.38 for 16–18 years, 1.53 for 18–21 years, 1.47 for 21–65 years and 1.45 for > 65 years) (Fig. 3c). A previous study by Ozsvath et al. indicated that if a child ingests excess F⁻, a variety of adverse effects can occur in a healthy body, including dental fluorosis, skeletal fluorosis, increased rates of bone fractures and decreased birth rates²⁸. Importantly, high F⁻ levels in groundwater are also introduced to farmland or residential areas by irrigation, although the risk from soil is lower than that from groundwater¹⁹. A study of a phosphate industrial area from Sichuan Province found that the main exposure pathway for pollutants was ingestion or particulate inhalation by groups of children and adults, respectively, while the highest risk value was less than 3–4 times that in our study²². F⁻ and PFASs were the predominant substances in the FCP, especially PFOA and PFOS, which were the common PFASs in the groundwater system. The toxicities of long-chain PFASs are much higher than those of short-chain PFASs, and they accumulate more easily in the human body^50,51. In Fig. 3b, PFOA accounted for the largest proportion of risk among PFASs (89.4%), followed by PFOS (7.0%) and PFHxS (1.80%). PFASs in teenagers may influence the reproductive system and character development of residents living near a Chinese fluorine chemical plant, as shown previously⁵². Drinking water provides the major ingestion pathway for PFASs, and they have a long half-life in the human body.