The study area is located between 6˚81'94''N to 6˚85'00''N and 3˚90'22''E to 3˚93'33''E. It is mainly accessible by major roads, minor roads and footpaths which link to both the major and minor roads. The climate of the study area is tropical. It has two distinct seasons: wet season and dry season.

The geology of Ijebu-Ode and environs has been described by Osinowo and Olayinka (2013) to belong to the transition geological environment between the Precambrian Basement Complex rocks of southwestern Nigeria and the Cretaceous sedimentary units of the Abeokuta Group in the eastern part of the Dahomey Basin [7]. The northern (northeast, northwest and north central) is underlain by basement rocks of migmatite gneiss complex, which comprises of biotite granite gneiss, biotite-hornblende gneiss that intrude schistose host rocks such as biotite schist, quartzite schist, talc-tremolite schist, and muscovite schists (Figure 1). The southern portion is overlain by Ise Formation of the Abeokuta Group that unconformably overlies the basement rocks. The sedimentary unit comprises mainly of grits, arkosic sandstones, siltstones and clay with occasional conglomerate of predominantly arenaceous materials.

Figure 1. Geological map of the study location. Adapted from Osinowo & Olayinka (2013) [7].

The results of the geochemical analysis of the soil samples revealed varying concentrations of heavy metals (Table 3). The concentration of Vanadium (V) ranged 134 - 237 ppm with a mean value of 181.4 ppm, Chromium (Cr) ranged 151 - 236 ppm with a mean value of 197 ppm, Cobalt (Co) ranged 33 - 79 ppm with a mean value of 48 ppm, Nickel (Ni) ranged 18 - 37 ppm with a mean value of 29.6 ppm, Copper (Cu) ranged 54 - 3637 ppm with a mean value of 970.4 ppm, Zinc (Zn) ranged 131 - 1342 ppm with a mean value of 734 ppm, Arsenic (As) ranged 49 - 112 ppm with a mean value of 79 ppm, Strontium (Sr) ranged 123 - 266 ppm with a mean value of 183.6 ppm, Barium (Ba) ranged 215 - 362 ppm with a mean value of 290.2 ppm, Lead (Pb) ranged 2 - 38 ppm with a mean value of 38 ppm, and Thorium (Th) ranged 15 - 23 ppm with a mean value of 18.4 ppm (Table 3, Figure 2). The mean value of the concentration of all the metals exceeds their background concentration values of [9] except for Pb (see Table 3). This indicates that these metals, except for Pb, could be anthropogenic in origin [13]. Since the mean value of Pb concentration is lower than its crust value, this may be an indication that Pb is of natural origin [13][14]. However, at one of the dumpsites (location B1), the concentration of Pb (35 ppm) is higher than its crust value (17 ppm). Hence, the origin of Pb in the soils of the study area could be both anthropogenic and natural. The most striking enrichment pattern was observed for Cu and Zn, especially at location B1, where Cu reached 3637 ppm and Zn reached 1342 ppm. This location corresponds to the dumpsite associated with plastic waste, suggesting that plastic-rich waste may be an important contributor to metal enrichment in the study area. Plastics and plastic-associated waste materials may contain metal-bearing pigments, stabilizers, fillers, additives, coatings, wires, printed labels and other synthetic components.

During prolonged exposure to sunlight, rainfall, heat, abrasion, and microbial activity, these materials may gradually break down, releasing associated metals into the surrounding soil [15][16]. Furthermore, dumpsites often contain, in addition to plastics, other waste, including packaging materials, electrical fragments, cans, batteries, paints, rubber, textiles, and household residues, many of which may also contribute Cu and Zn to the soil system. The high Cu concentration at B1 may therefore reflect both direct input from plastic-associated materials and indirect input from mixed consumer waste disposed together with plastics. Zn enrichment may be linked to the occurrence of Zn-bearing additives, pigments, rubber residues, galvanized materials, and other domestic waste components. The simultaneous elevation of Cu and Zn at the same location supports the interpretation that waste composition, rather than natural geological background alone, influenced the observed geochemical pattern. Although this investigation is preliminary and based on five samples, the contrast between B1 and the other locations suggests that plastic-rich dumpsites may function as localized hotspots for metal accumulation in urban soils. The B1 hotspot should, however, be interpreted with caution because waste type may not be the only controlling factor. Other site-specific conditions, such as dumpsite age, burning history, drainage and leachate pathways, nearby traffic inputs, and local lithological variations, may also contribute to spatial differences in metal concentrations. Thus, B1 is best interpreted as a potential heavy-metal hotspot associated with plastic-rich waste, while further replicated sampling and source-characterization studies are required to distinguish waste-type effects from other site-specific conditions.

Table 3. Concentrations of each metal and their respective mean, and Background Concentration (BC) [9].

Figure 2. Boxplots of heavy metal concentrations in the dumpsite soil.

The calculated CF for heavy metals in the dumpsites in the study area is displayed in Table 4, with Figure 3 showing the boxplots of the contamination factor. CF for V ranged 1.97 - 3.49, CF for Cr ranged 1.70 - 2.65, CF for Ni ranged 2.25 - 4.63 and CF for Th ranged 2.00 - 3.29. These values all indicate moderate to considerable contamination. CF for Zn ranged 0.94 - 9.65, indicating low to very high contamination; CF for Co ranged 6.60 - 15.80, CF for As ranged 24.50 - 56.00 and CF for Sr ranged 7.24 - 15.65, which all indicate very high contamination. CF for Cu ranged 2.57 - 173.19 and Ba ranged 3.84 - 6.46, which both indicate considerable to high contamination, while CF for Pb ranged 0.06 - 1.09, which indicates no to low contamination (Figure 3 and Table 5). Metals with significant contamination factors in the dumpsites include As, Co, Cu, and Sr, indicating that the dumpsites are not merely passive waste storage areas but may act as active zones of soil quality deterioration. The very high CF values for Cu and As are particularly important because they indicate enrichment far above expected background levels. The extremely high CF value for Cu at the plastic-waste dumpsite (B1) further supports the interpretation that waste type may control the magnitude of contamination. Therefore, management strategies should prioritize waste segregation, plastic-waste reduction and restriction of uncontrolled dumping in areas close to residential land, agricultural soils or drainage channels.

Table 4. Contamination factor calculated using background concentration for selected heavy metals in dumpsites in Ijebu-Ode.

Figure 3. Boxplots of contamination factors of heavy metals in soils around dumpsites in Ijebu-Ode.

I_geo values for both V and Cr range 0 to 1, indicating unpolluted to moderately polluted, while I_geo value for Co ranged 2 to 3, indicating moderately polluted to highly polluted (Table 5, Figure 4).

I_geo value for Ni ranged 1 to 2, indicating moderately polluted, Cu ranged 1 to 7, indicating moderately polluted to very highly polluted, and Zn ranged −1 to 3, indicating unpolluted to very highly polluted.

Table 5. Geo-accumulation index calculated using background concentration for selected heavy metals in soils from different dumpsites in Ijebu-Ode.

Figure 4. Boxplots of the geo-accumulation index.

I_geo value for As ranged 4 to 5, indicating highly polluted to very highly polluted, Sr ranged 2 to 3, indicating moderately polluted to highly polluted, while I_geo value for Ba ranged 1 to 2, indicating moderately polluted. I_geo value for Pb ranged from −5 to 0, indicating unpolluted, while Th ranged 3 to 2, indicating moderately polluted to highly polluted.

Thus, the I_geo results reveal unpolluted for Pb, unpolluted to moderately polluted for V and Cr, moderately polluted for Ba and Ni, moderately polluted to highly polluted for Co, Sr and Th, moderately polluted to very highly polluted for Cu, unpolluted to highly polluted for Zn, and highly polluted to very highly polluted for As. The I_geo results reinforce the contamination factor pattern by showing that Pb is generally unpolluted, while Cu, As, Co, Sr, and Zn show varying degrees of pollution. The classification of Cu as moderately polluted to very highly polluted is environmentally significant because Cu is an essential micronutrient at low concentration but may become toxic to soil organisms and plants at elevated levels [17][18]. In a similar way, the highly polluted to very polluted status of As is of concern since As is toxic even at relatively low exposure levels [3][19]. These results imply that the dumpsite soils may pose ecological risks if metals are mobilized into nearby soils, crops, shallow groundwater or surface runoff pathways.

The assessed Pollution Load Index (PLI) values for the dumpsite soils ranged 2.63 - 7.42, revealing that all sampled locations exceeded the baseline value of 1. Location B1, which corresponds to the plastic dumpsite, has the highest PLI value of 7.42 (Figure 5 and Table 6), indicating the greatest deterioration in the quality of the soils. Additionally, the elevated PLI at B1 aligns with the high Cu and Zn concentrations recorded at the same location and further supports the interpretation that plastic-rich waste may contribute substantially to metal loading in the soil.

The other locations (B2, B3, B4 and B5) have PLI values above the PLI baseline level, suggesting deterioration in the quality of soils in these locations, and that contamination is not only restricted to the plastic-waste dumpsite. However, the magnitude of deterioration varies among the locations, suggesting that waste composition, degree of waste accumulation, duration of dumping, leachate generation and local soil properties may impact the level of metal enrichment [20], [21]. What this implies is that each dumpsite represents a potential source of metal release into the surrounding environment. As such, the PLI results provide evidence that the dumpsites require environmental monitoring and improved waste-management intervention.

Table 6. PLI values for dumpsites soils in Ijebu-Ode.

Figure 5. Bar chart showing PLI across all locations. The broken red line is the PLI baseline level.