2.2. Samples Collection and Analysis
Various media samples, including 145 surface soil samples (0–20 cm), 24 surface water samples, 6 sediment samples, and 12 air deposits were collected to obtain the mean and ranges of trace metal concentrations in the study area.
Each soil sample was the mixture of nine subsamples around the designated location. The soil samples were air-dried, picked for residues/stones, and sieved twice using a 2- and 0.149-mm mesh. Of the sieved soil sample, 0.25 g was digested in a mixture of HCl-HNO3-HClO4-HF, until the solution was translucent or reduced to about 2–3 mL. The final solution was diluted to 50 mL, using double-distilled water (ddH2O).
Atmospheric depositions were collected from three sampling sites in the study area. A total 12 samples of air samples were collected monthly with membrane filter, by using an air sampler with a larger volume (20 cm × 38.5 cm). Samples were digested with an HCl-HNO3-HF-HClO4 solution for the metal concentrations.
A total of 24 irrigation water samples were collected in 1 L acid-washed polyethylene bottles from eight sites at random to represent the entire area. Microwave digestion of water samples for metal analysis was done by measuring 50 mL into 5 mL HNO3.
Six different types of plant species were harvested from the study area. The edible portions of the plants were harvested for trace metal analysis. The six species plants were cabbage (n = 12), oil wheat (n = 6), tomato (n = 10), radish (n = 15), corn (n = 14), and pepper (n = 3). Plant samples were washed by using ddH2O and were dried in a hot air oven at 75 °C for 24 h. The dried residues were mashed into fine particles before microwave digestion, 0.2 g of mashed samples were digested with 8 mL of HNO3 and H2O2 with 3:1 ratio.
Hair samples were collected from 76 farmers in Tielong town who settled in the town, with a residence time of at least 10 years (except children). Farmers with a history of trace metal exposure from other sources rather than the study area were not included in the study, the hair was selected after the natural growth of the occipital without heat dyed. Hair samples from human subjects were collected after informing farmers of the purpose of the research and subsequently soliciting consent from farmers. Hair samples were washed with neutral detergent and three times with ddH2O and put in a drying furnace at 70 °C over night. After cooling, the samples were caught in small pieces of 2 mm length and 0.1 g of hair samples was weighted, mixed with 6 mL HNO3 and 1 mL HClO4.
The concentrations of As, Cd, Cr, Cu, Ni, Pb, Zn in the samples were analyzed by inductive coupled plasma mass spectrometer (ICP-MS) (7500a, Agilent Technologies Inc., Santa Clara, CA, USA). All of the experiments were conducted within strict experiment conditions to eliminate/minimize possible contamination and interference. Standard reference samples (including GSS-5, GSB-26, and GSH-1) and blank samples were used for quality control and recovery analysis. Recovery rates for metal contents were controlled between 80% and 120%.
2.4. Risk Assessment
Currently, there is no agreed limit for acceptable maximum carcinogenic and non-carcinogenic risk levels in China, we therefore employed the U.S. EPA model and their threshold values in order to assess the potential human health risk that is posed by trace metal pollution in the study [
34]. Six exposure routes are considered, including: (1) direct ingestion of soil particles; (2) dermal contact with soil particles; (3) diet through the food chain; (4) inhalation of soil particles from the air; (5) oral intake from groundwater; and (6) dermal intake from groundwater [
17].
The calculation for the average daily dose of contaminants via various exposure pathways are as follows [
35,
36]:
where
is the metal concentration in the samples,
is the ingestion rate,
is the inhalation rate,
is the particle emission factor,
is the surface area of skin exposed to pollutants,
is the skin adherence factor,
is the dermal absorption factor,
is the exposure frequency,
is the exposure duration,
is the body weight, and AT is the average time for non-carcinogens or carcinogens,
is the units conversion factor.
Both the carcinogenic and non-carcinogenic risk for all of the metals through ingestion, inhalation, dermal, and diet exposure pathways were calculated. The non-carcinogenic risk from individual trace metals can be expressed as the hazard quotient (HQ).
where the non-cancer HQ is the ratio of exposure to hazardous substances, and
is the estimated maximum permissible risk that is imposed on humans through daily exposure. Experiencing adverse health effects is unlikely when HQ ≤ 1, whereas potential non-carcinogenic effects can occur when HQ > 1.
Moreover, the HQ calculated for each metal is summarized to assess the overall potential non-carcinogenic effects posed by more than one trace metal. If multiple pathways are available, a total exposure hazard index (HI) can be utilized to communicate non-cancer risks through different pathways:
HI values > 1 shows that there is a chance that non-carcinogenic risk may occur; and when HI ≤ 1 the reverse applies.
Cancer risk (CR) and total cancer risk (TCR) can be evaluated from:
where cancer risk represents the probability of an individual lifetime health risk from carcinogens; SF is the slope factor of hazardous substances. The acceptable of tolerable risk for regulatory purposes is within the range of 10
−6–10
−4. In accordance with Agency for Research on Cancer, As and Cd are treated as having a potential carcinogen effect.
Human health risk from trace metals in multi-media are related to total metal concentrations, ingestion rate, and the bioavailability. In reality, trace metals in multi-media cannot be absorbed by humans to much of an extent. So in our study, total metal concentrations in multi-media will overestimate the residents’ potential risks.
2.5. Statistical Analysis
Descriptive statistics were calculated by using the SPSS 20.0 software (IBM corporation, Armonk, NY, USA) and Excel 2013 (Microsoft corporation, Redmond, WA, USA). The data were displayed using the parameters of the minimum value, maximum value, mean value, the median, and standard deviation. Statistical analysis including Pearson correlation analysis, two significant levels at 5% and 1% were used in the statistics.