Survey and Risk Assessment of Contaminants in Soil from a Nitrogenous Fertilizer Plant Located in North China

China is the world’s largest consumer of fertilizer, with fertilizer plants widely distributed throughout the country. With the removal and closing of fertilizer factories in recent years, pollutant surveys and risk assessments (human health risks) for these sites have become increasingly necessary. However, there has been little research on contaminated fertilizer factory sites. This study aimed to characterize the distribution of pollutants, assess the health risk of the site, and calculate the remediation area and volume in a typical fertilizer plant site in North China. A total of 443 samples were collected in 2019; they indicated that the study site had high concentrations of copper (Cu), ammonia-nitrogen (NH3-N), total petroleum hydrocarbons (TPH), and fluoride at maximum ratios (the ratio of the highest value of all test data for a particular pollutant to the standard value of the pollutant) of 3.30, 2.55, 19.69, and 1.10, respectively. The health risk assessment results suggested that some hazard quotients exceeded the threshold safe level (>1 established by environmental regulations). The risk control values of soil were 2000 mg/g (Cu), 826 mg/g (TPH), and 1549 mg/g (NH3-N), and the total remediation soil volume was 72860.71 m3. The results provided basic information on soil pollution control and environmental management in a contaminated fertilizer plant site.


Introduction
Nitrogen is the first element in soil for plant growth. e nitrogenous fertilizer has been intensively used in agricultural systems to achieve higher yields in China. China is the largest consumer of fertilizer and has the biggest nitrogen production in the world [1][2][3]. Fertilizer plants are widely distributed throughout China. A large amount of chemicals have been released from these plants into soil and groundwater during their production. In recent years, with the change of fertilizer demand from chemical fertilizer to organic fertilizer, lots of nitrogen fertilizer plants have been closed. e land of these plants had been planned to be used for residential or recreational purposes [4]. Investigation and assessment of contaminants left in these sites is necessary for their safe redevelopment. A large number of studies have focused on the investigation and assessment of contaminants on petroleum [5], coking [6], and chemical-contaminated sites [7]. However, there has been little concern about the fertilizer-contaminated sites, but in fact, this kind of site could have serious environmental impacts and human health risks.
Ammonia-nitrogen (NH 3 -N) is the most typical pollutant found at fertilizer-contaminated sites [8]. As a major element determining plant growth and productivity, inorganic and organic forms of NH 3 -N in the soil can be absorbed by plants through their roots [9,10]. However, high levels of NH 3 -N have become an increasingly significant environmental problem [11], along with the acidification and eutrophication of ecosystems and climate change [12]. Although NH 3 -N is not considered a carcinogenic pollutant when assessing human health risks, the exposure to NH 3 -N at high concentrations could be harmful to the respiratory tract, eyes, and skin [13,14]. Compared with NH 3 -N, other typical pollutants, such as copper (Cu), total petroleum hydrocarbons (TPH), and fluoride, are at relatively low concentrations in fertilizer-contaminated sites. Even so, they cannot be neglected because of potential bioaccumulation and carcinogenicity.
Understanding the fate and transport of the contaminants present on fertilizer plant sites and identifying their environmental exposure risks are not only the preconditions for soil pollution prevention and control but also to provide important information for making decisions on polluted site remediation. e specific objectives of this study were to (1) characterize the distribution of pollutants in the fertilizercontaminated site, (2) assess the health risk of pollutants in the study site and identify health risk exposure pathways, and (3) calculate the remediation soil area and volume.

Contaminated Site Characterization.
e fertilizer plant in this study is located in Linzhang County in the southern part of Hebei Province, belonging to Taihang Mountain's foreland and flood plain. Construction waste and miscellaneous fill (0.1-2.8 m) are distributed on the surface of the study area, and below them are silt (8.8-11.4 m), silty clay (0.6-2.3 m), and silt (0.4-7.0 m) formed in alluvial and diluvial sediments of quaternary. e site began operating in 1975 and shut down completely in 1999; it covers an area of 53 × 10 3 m 2 ( Figure 1). e plant received anthracite, water, electrolytic copper, and catalyst as the raw material. e main product of the plant was ammonia (30 × 10 3 t/a), and the byproduct was ammonium bicarbonate (100 × 10 3 t/a). However, poor pollution controls and production technology during the operational period caused serious environmental pollution. Many hazardous substances were released into the soil, resulting in potential damage to the surrounding environment and the plant site. A detailed survey of the contaminants present along with a risk assessment is essential if the site is to be redeveloped, in particular as residential land.

Soil Sampling.
A total of 55 soil sampling sites were planned in the plant (Figure 2). e depths of soil sampling from a soil-drilling core were decided by soil lithology and the transport character of contaminants. e soil cores were collected by a drilling rig (SH-30, drilling rig manufacturer). e sampling standard is that one sample is taken from 0.5 m on the surface, and the vertical sampling interval is less than 2 m between 0.5 m and 6 m; if the sampling distance is less than 6 m, one sample is taken every 3 m until no obvious pollution odor is found on the spot. e specific locations of the sampling points this time are 0-0.5 m, 1-1.5 m, 3-3.5 m, 5-5.5 m, 8-8.5 m, 11-11.5 m, 14-14.5 m, 17-17.5 m, and 20-20.5 m. e vertical sampling interval was less than 2 m in 0.5-6 m, and the sampling interval was 3 m at most when the depth exceeded 6 m. To avoid cross-contamination, the sampling personnel replaced their disposable gloves after retrieving each sample. e samples to be analyzed for TPH and NH 3 -N were transferred to 250 mL brown glass bottles as soon as possible, and the samples to be analyzed for copper and fluoride were transferred to valve bags. All the samples were stored at 4°C and transported to the laboratory for analysis immediately. In total, 443 samples were collected in April and August of 2019.

Sample
Analyses. All analyses were completed by the Pony Testing International Group. Table 1 provides the selected analytical methods employed to measure copper, TPH, fluoride, and NH 3 -N. To corroborate the results and determine the accuracy of each method, 82 duplicate samples were analyzed by Hebei Shipu Testing Technology Service Co., Ltd. e relative deviation and relative standard deviation can meet the national standard (technical code for soil environmental monitoring HJ/T 166-2004).

Assessment
Standard. TPH and copper concentrations were converted using the risk screening and risk intervention values in accordance with the soil environmental quality risk control standard for soil contamination of development land (GB36600-2018) [18]. However, fluoride and NH 3 -N, which could not be found in GB36600-2018, were calculated using the technical guidelines for the assessment of contaminated sites (HJ25.3-2014) [19]. e risk assessment of soil contaminants was carried out according to HJ 25.3-2014 [19]. To simplify calculations, the soil was divided into 6 layers as given in Table 2. It is worth mentioning that because the pollutants did not exceed the standard in the 20-25 m depth layer, the layer was ignored.
According to the plan for land utilization of Linzhang County, the future land use pattern of the fertilizer site was residential. erefore, both adults and children were considered the sensitive human receptors under the residential scenario. e soil exposure of the local population was estimated by considering six different routes: oral ingestion, dermal contact, inhalation of soil particles by mouth, inhalation of gaseous pollutants from the surface soil in the outdoor air, and inhalation of gaseous pollutants from the underlying soil in outdoor and indoor air. e hazard quotient (HQ) of exposure pathways was estimated using the equations given in Table 3.

Contaminated Soil Remediation.
According to the health risk assessment guidelines, human health might be harmed when the hazard quotient exceeds 1, and soil remediation is necessary. e risk control values of soil (RCVS) were calculated using the equations given in Tables 4  and 5. Furthermore, to avoid excessive remediation, RCVS was compared with the screening value of GB36600-2018 [18].

Contaminant Characteristics and Sources.
e soil sample results are given in Table 6. Cu concentrations ranged from 5.94 mg/kg to 6590 mg/kg, with an average of 107.82 mg/kg and a maximum ratio of 3.3. As shown in Dermal contact 6 ways receptors are exposed to pollutants Inhalation of soil particles by mouth HQ pis � PISER nc × C sur /RfD i × SAF (3) Inhalation of gaseous pollutants from the surface soil in outdoor air Inhalation of gaseous pollutants from the underlying soil in outdoor air Inhalation of gaseous pollutants from the underlying soil in indoor air Dermal contact Formula for calculating the amount of contaminants ingested by skin contact Inhalation of soil particles Inhaled soil particle pollutant quantity calculation formula RCVS pis � ACR/PISER ca × SF i (9)  us, it can be concluded that the main reason for high Cu concentrations was the corrosion and aging of the machine that accelerated the release of wastewater [20,21].
TPH ranged from <3 mg/kg to 16263 mg/kg with an average concentration of 129.19 mg/g. e maximum ratio was 19.69, and the distribution of TPH was relatively concentrated (Figure 4). However, TPH was not a raw material used in the fertilizer plant. It could be concluded that TPH was from the lubricating oil leakage of machines in the process of operation or migration (Figure 3(b)). e pollution level decreased with increasing depth due to  interception by the soil (Table 7). e low volatility, low solubility, and high hydrophobicity and sorption capacity of TPH made remediation difficult [22,23].
As the main product of the fertilizer plant, NH 3 -N concentrations ranged from 0.14 mg/kg to 3951 mg/kg, with a mean concentration of 445.1 mg/kg and a maximum ratio of 2.55. e main layers of the plant site were silt and silty clay with low water permeability. With the migration of rainfall and wastewater, NH 3 -N could permeate down to a high depth. e deepest depth that the sample drill could reach was 27 m, and NH 3 -N was present in those samples. As shown in Figures 5(c) and 3, NH 3 -N was distributed near the copper washing and carbonization transformation workshops. e pollution level was not typically consistent with the depth, which suggests that the area had been polluted with NH 3 -N for many years (Table 7). is could be attributed to the leakage of wastewater and the leaching of solid waste. Furthermore, poor pollution control and environmental protection awareness during the operational period could have been key factors affecting the contamination of the site.
Fluoride ranged from 127 mg/kg to 1150 mg/kg with a mean concentration of 424.94 mg/kg and a maximum ratio of 1.10. e overstandard point was at the ammonia pool of the carbonation transformation workshop. e fluoridepolluted area was relatively small compared with the other contaminants, and fluoride was only present in the surface soil (first layer) ( Table 7). e presence of fluoride may be attributed to the atmospheric deposition of coal burn or the high background value [24,25]. e polluted areas were 2199.6 m 2 , 3048.7 m 2 , 14564.81 m 2 , and 7 m 2 for Cu, TPH, NH 3 -N, and fluoride, respectively. ese areas could be potentially harmful for human health. erefore, it was necessary to conduct a health risk assessment as per HJ25.3-2014 [26]. It should be mentioned that the total polluted area of the last 4 layers was not consistent with the increasing depth. e reason for this requires further investigation and analysis.

Health Risk Assessment.
e hazard quotients in soils of the study area were calculated from Table 3 and are given in Table 8. Based on the values obtained, the main potential exposure pathway of Cu, fluoride, and TPH could be from oral ingestion. Inhalation of soil particles and dermal contact were the second most likely pathways for fluoride and TPH, respectively. However, the values with different exposure pathways of NH 3 -N varied significantly in the following order: inhalation of gaseous pollutants from the surface soil in outdoor air > inhalation of soil particles. e contribution of inhalation of gaseous pollutants from the underlying soil in the outdoor and indoor air for NH 3 -N was insignificant and may be ignored.
In general, the contribution rates of oral ingestion, dermal contact, and inhalation of gaseous pollutants from the surface soil in the outdoor air were 43.3%, 30.6%, and 25.9%, respectively. ese results suggested that wearing safety masks would be an effective measure to reduce the negative health effects for workers and residents in the study site.
As given in Table 8, as the soil depth increased, the concentration of most of the pollutants tested decreased gradually. In the first two layers (0-6.5 m), the hazard quotients of Cu, NH 3 -N, and TPH were greater than 1. In the third layer (6.5 m-10 m), Cu and NH 3 -N were higher than the standard HQ. Until the fourth (10 m-15 m) and fifth

e Target Value of Contaminated Soil Remediation and the Amount of Pollution.
e RSVSs of Cu, TPH, and NH 3 -N were calculated and are presented in Table 9, except for fluoride, which had an HQ < 1. e calculated RSVS of Cu and TPH were lower than GB36600-2018 [22]; thus, the final RSVS was based on GB36600-2018 .   T51   T20   T13   T24   T50   T25   T19  T32  T34  T4  T5   T55  T35  T33   T37   T41   T42   T46  T47  T10   Old town avenu e   T43  T9  T44   T8   T41   T39   T7  T23   T36   T11   T16   T31   2 T45   T15  0 10 20m   T17   T6  T38   T30   T26   T29   T28   T12  T27   T14   T3  T22   T15   T49   T53  T21  Journal of Chemistry e remediation area and volume for the targeted pollutants for each different layer were determined by interpolation and manual correction; results are given in Table 10. e spatial distribution of the remediation area for Cu, TPH, and NH 3 -N is shown in Figure 6. Cu and TPH need to be remediated only in the first three layers. e Cu and TPH remediation areas overlapped almost completely in the second and third layers. In contrast, the area that requires remediation of NH 3 -N comprises almost five layers. e total remediation volume was 72860.71 m 3 after superposition.
ese results show that a risk assessment is necessary where fertilization plants have been operational, and they can be used to compare, develop, and select remediation options.

Conclusion
In this study, a pollutant survey and risk assessment of soils were carried out in a fertilizer plant in Linzhang County, North China. e results indicated that the site was polluted by four main contaminants: fluoride, copper, TPH, and NH 3 -N. ese contaminants were mainly attributed to coal burning, the leakage of wastewater, and lubricating oil. eir presence also reflected the poor pollution controls and environmental protection awareness during the operation period. e total areas polluted by fluoride, Cu, TPH, and NH 3 -N were 7 m 2 , 2199.6 m 2 , 3048.7 m 2 , and 14564.81 m 2 , respectively. e hazard quotients found were relatively high and indicate that oral ingestion, inhalation of soil particles, and inhalation of gaseous pollutants from the surface soil in the outdoor air could be the main exposure pathways of this site without any remediation steps. RSVS was calculated and compared with GB36600-2018, and the final total remediation soil volume was 72860.71 m 3 after superposition. To solve the problem of soil pollution and protect public health in the study area, the enterprise and local government should make more efforts to remediate the soil pollution. is study provides useful information and reference for environmental management in fertilizer plant sites.

Data Availability
e data used to support the findings of this study are included within the article, and any data in the full-text can be quoted and does not involve the situation of leakage.

Conflicts of Interest
e authors declare that they have no conflicts of interest.