3.1. Concentration and Ecological Assessment of PTEs
The grain size analysis of the studied coastline sediments showed a dominance of fine to very coarse sands (87.92%), along with mud (9.05%) and gravels (3.03%). The average concentrations of the investigated potentially toxic elements (PTEs) in the 25 surface sediment samples (dry weight) were as follows: Fe (14259 μg/g), V (28.30 μg/g), Zn (22.74 μg/g), Cr (16.81 μg/g), Cu (12.41 μg/g), Ni (10.63 μg/g), As (2.66 μg/g), and Pb (2.46 μg/g).
Table S.5 presents the PTE concentrations across the study area, indicating that the highest levels were found in sample S21 (for Fe, As, Ni, V, Zn, Cr, Pb, and Cu), while the lowest levels were in sample S8 (for the same elements). Overall, PTE concentrations varied throughout the study area without a clear pattern. However,
Figure 2 shows a significant increase in PTE levels in samples 16-21, collected at the mouth of Wadi Al-Lith. This suggests that the coastal sediments and associated PTEs might have originated from the igneous and metamorphic rocks of the Arabian Shield.
The average Fe value was higher than those listed in
Table 1, except for the background references [
40,
41]. The average Cr value exceeded those reported for Ras Abu Ali, Aqeer, and Al-Jubail - Al-Khafji coastlines along the Arabian Gulf [
5,
42,
43]. Furthermore, the average As value was greater than those from Al-Khobar, Saudi Arabia [
44], and the background continental crust [
41]. On the other hand, the average levels of Zn and Cu were generally lower than the reported values in the table, except for those found along the Red Sea coast in Egypt [
45], Ras Abu Ali, Arabian Gulf [
42], Aqeer coastline, Arabian Gulf [
43], and Al-Jubail - Al-Khafji, Arabian Gulf [
5].
The enrichment factor (EF) is a valuable tool for determining the origin of heavy metals [
49]. The average EF values for potentially toxic elements (PTEs) in descending order are: As (1.12) > Zn (0.75) > V (0.70) > Cr (0.69) > Cu (0.69) > Pb (0.67) > Ni (0.46). This suggests that the Al-Lith coastal sediments exhibit deficiency to minimal enrichment with PTEs (
Table 2). However, moderate enrichment of As was observed in samples S1 and S9 (8% of the studied samples) [
34]. The contamination factor (CF) results indicated low contamination for all PTEs, with average CF values less than 1. The pollution load index (PLI), used to assess contamination at specific sediment sites [
50], ranged from 0.06 to 0.68, with an average of 0.21, indicating unpolluted sediment [
5]. The risk index (RI), which helps understand and control heavy metal pollution at a site [
43], ranged from 1.61 to 25.80, with an average of 6.84, suggesting low risk from heavy metals in the present sediments (
Table S.5). All PTE levels were below the effects range-low (ERL), indicating that the Al-Lith coastal sediments do not pose a risk to benthic communities due to these PTEs [
48].
A significant positive correlation was found between Zn and As, Zn and Cr, Zn and Cu, Zn and Fe, Zn and Ni, Zn and Pb, and Zn and V (
Table 3), indicating a common source for these elements [
11]. Iron also showed strong positive correlations with other elements, suggesting natural sources, primarily from the chemical weathering of the basement rocks in the nearby Arabian Shield mountains located to the east of the study area [
51,
52]. In contrast, there was a weak correlation between As and Pb, indicating a different source for Pb in the investigated sediments. Principal Component Analysis (PCA) identified two principal components (PCs) that largely supported the correlation matrix (
Table 4). PC1 had high loadings for As, Cr, Cu, Fe, Ni, Pb, V, and Zn (0.666, 0.981, 0.975, 0.988, 0.710, 0.935, and 0.988), while PC2 had high loading for Pb (0.597). Samples S16 to S21, located at the mouth of Wadi Al-Lith, showed high concentrations of Cr, Ni, As, V, Cu, Pb, Zn, and Fe. This suggests that the coastal sediments and associated PTEs originated from the igneous and metamorphic rocks of the Arabian Shield, which were weathered by rainwater and subsequently eroded and transported through Wadi Al-Lith. The lower EF values for these PTEs confirm their geogenic origin [
53].
The basement rocks of the Arabian Shield in Saudi Arabia are rich in various metallic minerals, including PTEs. These include sphalerite (ZnS) found in hydrothermal vein systems, vanadiferous magnetite (Fe,V)₃O₄ associated with mafic to ultramafic rocks, chromite (FeCr₂O₄) within peridotite units, and chalcopyrite (CuFeS₂) linked to volcanic rocks and intrusive bodies. Additionally, pentlandite ((Fe,Ni)₉S₈) occurs in mafic-ultramafic complexes, arsenopyrite (FeAsS) is found with sulfide mineralization in hydrothermal systems, galena (PbS) is associated with hydrothermal veins, and both magnetite (Fe₃O₄) and hematite (Fe₂O₃) are found in volcanic and sedimentary rocks [
54,
55].
3.2. Health Risk Assessment
Various essential potentially toxic elements (PTEs), such as Co, Cr, Fe, Zn, Ni, and Mn, play crucial roles in nutrition at trace levels. However, excessive exposure to these PTEs can lead to severe health issues in humans [
15,
45,
56]. In the investigated area, the average chronic daily intake (CDI) values (mg/kg/day) for non-carcinogenic risk in adults ranged from 3.37392E-06 (Pb) to 0.01953 (Fe) through ingestion and from 1.34619E-08 (Pb) to 1.54661E-07 (V) through dermal pathways (
Table 5). In children, the average CDI values varied from 3.14899E-05 (Pb) to 0.182310164 (Fe) through ingestion and from 6.28224E-08 (Pb) to 3.16474E-07 (Cu) through dermal pathways. These findings indicate that children are at a heightened risk of non-carcinogenic exposure compared to adults.
The average hazard index (HI) values for potentially toxic elements (PTEs) in both adults and children, in descending order, were Fe, As, Cr, V, Pb, Cu, Ni, and Zn (
Table 5). The distribution of HI values across sample locations revealed that the highest HI values for Cr, Pb, V, Zn, and Fe were found in sample S21, while the highest value for As was in sample S16 (
Table S.6). This trend is likely due to elevated levels of PTEs in these specific samples. In adults, HI values ranged from 0.00010 (Zn) to 0.0280 (Fe), while in children, they ranged from 0.00097 (Zn) to 0.260 (Fe), indicating that children have a higher hazard index compared to adults for non-carcinogenic risk. Despite this, all HI values for PTEs were below 1.0, suggesting no significant non-carcinogenic risk for residents along the Al-Lith coastline [
57,
58]. However, it is important to note that the HI value for iron exceeded 0.2 in children, highlighting the need to protect their health.
The accumulation of potentially toxic elements (PTEs) such as As, Cr, and Pb in the human body can result in serious health complications, including an increased risk of lung, stomach, and skin cancers, as well as potential effects on the nervous system [
59,
60]. Carcinogenic risks (CRs) associated with Cr, Pb, and As were evaluated in the examined samples (
Table 6,
Table S.7). In adults, the average CR values ranged from 2.87E-08 (Pb) to 1.15E-05 (Cr) through ingestion and from 2.18E-08 (As) to 4.60E-08 (Cr) through dermal exposure. In children, the average CR values ranged from 2.68E-07 (Pb) to 0.000107 (Cr) through ingestion and from 5.11E-05 (As) to 2.14E-07 (Cr) through dermal exposure. Lifetime cancer risk (LCR) values for adults varied from 2.87E-08 (Pb) to 1.16E-05 (Cr), while for children, they ranged from 2.68E-07 (Pb) to 1.08E-04 (Cr).
The distribution of lifetime cancer risk (LCR) values across sample locations revealed hot spots in S1, S10, S16, and S21 for As; S2, S7, and S16-S21 for Cr; and S21 for Pb (
Table S.7 and
Figure 4). All LCR values for Pb, As, and Cr in both adults and children were within acceptable or tolerable carcinogenic risk levels, posing no significant health threats (ranging from 1 × 10⁻⁵ to less than 1 × 10⁻⁶). However, six samples (S2 and S16-S21) showed LCR values exceeding 1 × 10⁻⁴ for Cr in children, indicating potential carcinogenic risks [
19,
38,
61]. These elevated values were mainly found in samples collected at the mouth of Wadi Al-Lith and were associated with higher levels of PTEs, suggesting an origin from the basement rocks of the Arabian Shield.