3.1. Concentration Distribution of PTEs
The coefficient of variation (CV) and concentration of PTEs in river water and sediments along the Danshui River are showed in
Table 2 and
Figure 2. The CV can intuitively show the spatial distribution characteristics of PTEs. The high variability (100% < CV) suggests that a significantly uneven spatial distribution, the medium variability (10% ≤ CV ≤ 100%) show a relatively homogeneous spatial distribution, and the low variability (CV < 10%) indicate a relatively even spatial distribution [
42,
43,
44]. After the analysis, it was found that the concentration of Mn was 0.03~92.8 mg·L
−1 and 3010~14150 mg·kg
−1 in river water samples and sediments, respectively. The CV for the river water samples and sediments were 103 ± 2% and 206 ± 12%, respectively. These results indicate that the distribution of Mn in the water and sediments of the Danshui River was uneven [
42,
43,
44]. The difference in the Mn of the river water and sediments was obvious, and was concentrated upstream and downstream relative to the mining operation with the average concentrations of 28.4 mg·L
−1 and 5.9 mg·L
−1 in waters. These values were 284 and 59 times higher than the GBIII levels, and 4733 and 983 times higher than the C
BG levels. The average concentration of Mn in the water of the midstream was lower than the GBIII, but it was 8.3 times higher than the C
BG. Mn in the sediments from upstream to downstream were 10,915 mg·kg
−1, 4367 mg·kg
−1 and 6380 mg·kg
−1, which were 22 times, 9 times and 13 times higher than the B
i, respectively. According to literature research and field investigations, the over-standard phenomenon and spatial distribution of Mn in the study area were dominated by the combination of natural processes and anthropogenic activities. The Mn in the electrolytic manganese residue mainly existed in the form of oxides and the water environment at the sampling cross-sections with high Mn concentration in the study area (K6–K8 in upstream, K13–K15 in downstream) was mainly acidic and oxidizing conditions (295 < Eh, pH < 6.3,
Table 3,
Figure 3). Mn in river water existed in the valence state of Mn
2+ and easily migrated in this environment, but the water environment at the other cross-sections with relatively low Mn concentration in river water (K1–K5 in the control section, K9 in lower upstream, K10–K12 in midstream, K16 in lower downstream) was mainly at moderately alkaline and oxidizing conditions (288 < Eh, 6.9 < pH,
Table 3,
Figure 3) where Mn existed in the form of Mn
2O
3, which is relatively stable [
45,
46,
47]. In addition, based on the field investigation, PET pollutants were released mainly from upstream sources. Although a dam that sits between upstream and midstream ideally blocks the pollutants, the acid sewage from upstream was discharged to a downstream location after being utilized by the hydropower station. Thus, it is possible that the Mn from the river water samples and sediments mainly originated from the manganese mining activities. The excessive degree of Mn in the upstream and downstream of the study area was relatively high, while the midstream was lower affected and the control section formed a good contrast.
The concentrations of Pb and As in the river water were 0.002~0.1 mg·L
−1 and 0.001~0.07 mg·L
−1, and their CV were 35 ± 2% and 36 ± 3%, respectively. Similarly, the concentrations of Pb and As in the river sediments were 7.6~18.1 mg·kg
−1 and 11.8~19.6 mg·kg
−1, respectively, and their CV were 37 ± 2% and 35 ± 2%, respectively. It suggested that the distributions of Pb and As in river water and sediments were relatively uneven [
42,
43,
44]. The concentration of Pb in the river water at the K16 location downstream (0.1 mg·L
−1) was higher than the GBIII and C
BG levels. As distributed similarly to Pb, but it did not exceed the GBIII in the river. This metal was also high at the K16 sampling location with a concentration of 0.07 mg·L
−1, which was 14 times higher than the C
BG. In addition, Pb and As in the sediments were both high at the K16 sampling location with 18.1 mg·Kg
−1 and 19.6 mg·Kg
−1, which was 1.2 times and 1.03 times higher than the B
i. These results suggest that As and Pb in the river water and sediments at K16 was highly affected by human activities. In addition, the water environment at the sampling cross-sections with high As and Pb concentrations in the study area (K6 in downstream) was mainly at alkaline and oxidizing conditions (288 < Eh, 8 < pH,
Table 3,
Figure 3). As and Pb in the river water existed in the valence state of As(V) (H
2AsO
4− and HAsO
42−) and PbSO
4 and were stable in the river water in this environment [
45,
46,
47]. As and Pb concentrations in the other cross-sections in the river water were relatively uniform and irregular and their speciation had little influence on their spatial distribution. According to the field investigations, the K16 sampling location was near a wasteyard and it was inferred that the high concentrations of As and Pb in the river water and sediments at K16 were related to this facility. As and Pb at other sites along the river were all lower than the GBIII levels and their distributions were relatively uniform and irregular, thus they could be a result of natural origins [
43].
Cu in the river water were 0.001~0.1 mg·L
−1 and lower than the GBIII levels. Cu in the river sediments were 7.3 to 19.3 mg·Kg
−1 and also lower than the B
i. However, the CV of Cu in river water and sediments was 36 ± 3% and 55 ± 4%, respectively, which were uneven [
42,
43,
44]. The average concentration of Cu in the water at the K7 sampling location which was upstream was higher than the C
BG levels, which was 0.1 mg·L
−1. Although Cu in sediments at K7 with 19.1 mg·Kg
−1 was lower than the B
i, it was higher than other sites. However, the other sampling cross-sections did not exceed the standard, and the K7 was located in the main pollution discharge area of manganese mining activities. In addition, the Cu mainly exists in the form of oxides in the karst strata, and Cu existed in the state of CuSO
4 in the river water of the study area and always easily migrated in acid and oxidizing environment (295 < Eh, pH < 6.3
Table 3,
Figure 3) [
12,
46,
47]. Therefore, Cu in river water and surface sediments at K7 may be related to manganese mining activities. However, Cu in river water and sediments at the other cross-sections were relatively uniform and irregular, so it was determined that Cu at the other cross-sections also originated from nature [
43].
Cd, Cr and Zn in the river water were 0.001~0.002 mg·L
−1, 0.001~0.002 mg·L
−1 and 0.001~0.005 mg·L
−1, respectively, which were all lower than the GBIII levels. Their CV were 9 ± 1%, 10 ± 1% and 13 ± 1%, respectively. While, Cd, Cr, and Zn in the river sediments were 0.08~0.12 mg·kg
−1, 17~23 mg·kg
−1 and 23~25 mg·kg
−1, respectively, and all were lower than the B
i. Their CV were 11 ± 1%, 13 ± 2%, and 15 ± 2%, respectively. These results indicated that Cd, Cr and Zn in the river water samples and sediments all were distributed uniformly and irregularly. In addition, since the water environment in the study area is mainly dominated by acidic and oxidizing conditions (
Table 3) and the rock strata in the study area are dominated by carbonate, these conditions will actually accelerate the dissolution of carbonate and promote the release of PTEs [
47,
48], which is the main reason why the water environment in the study area is rich in Cd, Cr and Zn. Therefore, it infers that Cd, Cr and Zn were mainly from the natural environment [
44].
3.2. Identification of Sources of PTEs
The manganese mining and wasteyard are the main reasons for the serious PET pollution of river water and sediments in the research area. To further prove these observations, correlation matrix and multivariate statistical analyses were carried out. The Pearson’s correlation matrix and principal component analysis of PTEs in the water and sediments in the study area are shown in
Table 4 and
Table 5 and
Figure 4.
In river water, the correlations among Cd, Cr and Zn were both significant (0.99 ≤ r ≤ 1,
p < 0.01), indicating that these metals might originate from common sources [
49]. Likewise, As and Pb showed significantly positive correlations (r = 0.99,
p < 0.01) and weaker correlation with the elements of the first group (0.56 ≤ r ≤ 0.69,
p < 0.05). At 0.05 significance level, the correlation coefficients of Cu with As, Cd, Cr, Pb, and Zn were 0.59, 0.64, 0.67, 0.61 and 0.69, respectively, and similar correlations also existed between Cu and Mn. These trends suggest that the Cu had many potential sources. In addition, three principal components (PCs) were obtained that explain 78.79% of the variation in the river water. PC1 was dominated by Cd, Cr, and Zn, accounting for 46.30% of the total variance. Cd was widely scattered in the lithosphere in the Karst area, and these three PTEs were at relatively low concentrations and low CV values. This trend indicates these elements mainly originated from nature [
49]. Cu had a lower factor loading (0.57) than those of the other PTEs in PC1, whereas it had a higher factor loading of 0.75 in PC2. Similarly, As and Pb had relatively lower factor loadings of 0.47 and 0.48 in PC1, but they also had higher factor loadings in PC3. These showed that the three PTEs all had many sources [
50]. PC2 explained 16.78% of the total variance and was dominated by Mn (0.98) and Cu (0.75). Generally, Mn is a common element in the lithosphere with a relatively high abundance and its concentration is not easily affected by artificial factors [
9,
50]. Despite being a common element in the lithosphere, the concentration and CV value of Mn is abnormally high in the study area, suggesting it mainly originated from the mining of manganese. Cu had a high concentration in K7 and a high CV value in the study area, but its concentrations were homogeneous in other sampling cross-sections. Cu was also always considered to be associated with Mn [
22]. Therefore, Cu originated from both nature and manganese mining [
51]. PC3 explained 15.71% of the total variance and was characterized with higher factor loadings of As (0.79) and Pb (0.83). These two PTEs both had a higher concentration in K16 and homogeneous concentration in other sampling cross-sections and they were typical Anthropogenic pollutants, suggesting they originated from both nature and the wasteyard [
48,
51].
In the case of river sediments, similar results were obtained. Cd, Cr and Zn showed strong positive correlation factors (0.98 ≤ r ≤ 0.99,
p < 0.01), reinforcing the idea of a similar origin [
51,
52]. As and Pb showed similar significant positive correlations (r = 0.99,
p < 0.01) and weaker correlation with the elements of Cd, Cr and Zn (0.63 ≤ r ≤ 0.67,
p < 0.05). Cu also had some correlations with As, Cd, Cr, Pb, Zn and Mn (0.58 ≤ r ≤ 0.68,
p < 0.05), indicating it had various sources in line with the river water. Similarly, three PCs were also obtained, explaining 83.23% of the variation in the river sediments. PC1 accounted for 48.89% of the variance and exhibited the higher loadings of Cd (0.97), Cr (0.99), Zn (0.95), as well as a moderate loading of As (0.48), Cu (0.53), Pb (0.50). PC2 and PC3 explained 17.67% and 16.67% of the total variance, respectively, and were dominated by Mn (0.945), Cu (0.71) and As (0.71), Pb (0.79). The results of the principal component analysis in the river sediments were consistent with those in the water samples in the study area, indicating that the sources of the river sediments were consistent with each other.
In order to validate the above analysis, the Pearson correlation coefficients between PET contents in the river water samples and sediments for those sampling cross-sections were showed in
Figure 5. All of the sampling cross-sections exhibited a high relationship (r ≥ 0.99) between the PET concentration in both environmental compartments, indicating that they had common sources [
12]. These are consistent with the above analysis, which further validates the accuracy of source analysis of PTEs in the river water and sediments.
3.3. Pollution Assessment of PTEs
According to the contamination factor (CF) for the PTEs (As, Cd, Cr, Cu, Mn, Pb and Zn) in river water (
Figure 6), Cu represented an extreme level at K7 in upstream, and As and Pb represented extreme level at K16 in downstream. Cu, As and Pb were low at other sites, while Mn showed extreme contamination at all the sites in the river. All sampling sections in the control area were at the critical contamination (CF = 1), which belonged to the low moderate contamination and they had a good contrast with the downstream.
Additionally, to further reflect the integrated influence of the PTEs in the river water of the study area, the I
PL of these seven PTEs are discussed (
Figure 6). Results indicated that upstream and downstream were the main polluted sections of the Danshui River, which represented progressive contamination (I
PL > 1). Especially at K7 of upstream and K16 of downstream, it showed high I
PL values with 4 and 3. All sampling sites from midstream presented a level of non-pollution (I
PL < 1). The same as the CF, all sampling sections in the control section were also at the critical contamination (I
PL = 1), which belonged to the baseline pollution level, and they also had a good contrast with the downstream.
According to the Geo-accumulation Index (I
geo) for the river sediments (
Figure 7), only the Mn in the river sediments represented contamination (2 ≤ I
geo < 5), and the upstream and downstream are the main contaminated sections, which are heavily contaminated (3 ≤ I
geo < 4) and heavily to extremely contaminated (3 ≤ I
geo < 5), respectively. However, Mn in the river sediments in the midstream represented a moderate to heavy contamination (2 ≤ I
geo < 3), which suggested lower contamination
. This was mainly due to the block by the dam at down part of upstream. Meanwhile, at other sites, there was lower contamination (I
geo < 0).
In order to determine the comprehensive influence from all PTEs, the I
PER was applied to evaluate the ecological risk from PTEs in river sediments (
Table 6). Similar to the results of I
geo, Mn upstream and downstream showed a level of moderate ecological risk and Mn was mainly related to the high concentration caused by manganese mining activities. The average ecological risk factors for the other PTEs were generally at low levels, which means the ecological risk in river sediments at acceptable levels.
According to these various indices (CF, IPL, Igeo and IPER), the main polluted sections by PTEs were upstream and downstream. PET pollutants were released from upstream and the sewage from upstream is discharged in the upper part of downstream after being utilized for hydropower. Ideally, these pollutants could be blocked by the dam at the lower part upstream. Mn was the main polluted PET. As and Pb in river water were at a relatively low pollution level downstream, and Cu was low upstream. The pollution levels of PTEs from river sediments were lower than in river water due to the PET pollutants are discharged mainly in the form of sewage, and the PET pollutants in river water are likely to precipitate into river sediments when the water environment changes. Therefore, the emission of sewage is the main pollution source of the study area and water conservancy facilities were the key impacts to the pollution distribution of PTEs. Therefore, the control of sewage emission should be strengthened and the water conservancy facilities should be utilized later on for additional pollution control. These are of great significance to the continued pollution control of the river in the mining areas.