3.1. Soil pH and heavy metal accumulation
3.1.1 Soil pH
Table 1 displays the permissible levels of metals in soil and water by Ghana EPA, WHO/FAO, whereas Tables 2-3 present pH and the four heavy metal concentrations at Sites Fand H. pH values ranged from mild acidic to high alkaline conditions (5.88- 8.03) with an average of 7.13 at for Site F (Table 2). At site H (Table 3), pH ranged from 6.07 to 7.78, with a mean of 6.94. Heavy metal adsorption and retention by soil increases generally occur within a pH range of 4-7 [23, 24], thus the pH range could partly account for the elevated levels of heavy metals in the samples. pH values recorded were within the WHO benchmark of 6.5-8.5, except for three samples (5.88 at Site F and 6.07, 6.38 at site H), where pH values were below the 6.5 minimum threshold. Relatively high pH value recorded in sample 5B (8.03) could be due to the presence of alkaline batteries, steel mill, and ashes from the incineration processes at the E-waste site.
3.1.2. Heavy metal concentration
Several factors, such as electron activity, soil texture, soil pH, ionic strength and level of organic matter, affect the metallic forms in soil matrix. Cd was found to be the least in concentration among the heavy metals in the two soil profiles at site F, from non-detection levels to a maximum of 1.57 ppm and an average concentration of 0.48 ppm (Table 2). With the exception of one sub-soil sample with a concentration of 1.57 ppm, all Cd concentrations at Site F were below the Ghana EPA permissible limit of 1 ppm and the WHO/FAO standard of 3 ppm (Tables 1-2). Cd mobility is dependent on several factors such pH and presence of organic matter that has strong affinity for the metal. The mean pH at this site was alkaline (7.13), which limits its availability, thus accounting for its low concentrations at this site [25, 26-28]. At site H, Cd concentrations were relatively higher, with a minimum of 0.29 ppm and a maximum of 13.56 ppm and an average concentration of 4.14 ppm, which exceeds both Ghana EPA and the WHO/FAO standards (Tables 1 and 3). At this site, the mean pH was 6.94. The slightly acidic conditions may have contributed to the high levels of the metal. E-waste materials with Cd at the scrapyard include printed circuit boards, batteries, accumulators, cathode ray tubes and ultraviolet lights. Site H had higher concentrations of Cd than Site F because Cd-containing E-waste materials were located more at the former site than the latter.
The minimum and maximum concentrations recorded at site F for Cr ranged from 13.97 to 162.50 ppm with an average concentration of 77.33 ppm. Most Cr concentrations exceeded the Ghana EPA threshold value of 30 ppm (Tables 1-2). Three samples also had Cr concentrations (123.07, 162.50 and 117.27 ppm) above the WHO/FAO standard of 100 ppm. However, Cr levels obtained in samples from site H were below the permissible limit of WHO/FAO and Ghana EPA with minimum and maximum concentrations of 15.95 and 30.11 ppm, respectively and an average of 21.00 ppm (Tables 1 and 3). Comparatively, Cr concentrations at site F were higher than those at site H, which could be due to the fact that the metal containers that house E-waste materials were closer to site F. They are typically composed of steel and chromium, so any wear and tear on the metal adds on Cr concentrations to the soil, pH also being a factor.
At site F (Table 2), the minimum and maximum concentrations recorded for Cu were 29.97 and 253.42 ppm, respectively, with an average concentration of 114.85 ppm. The concentrations exceeded permissible levels of Ghana EPA (20 ppm) and the WHO/FAO standards of 100 ppm (Table 1). Also, the minimum and maximum concentrations of Cu at site H ranged from 5.24 to108.76 ppm with an average concentration of 48.37 ppm, which were above the national and international limits (Tables 1 and 3). Cu finds application in most electrical and electronic appliances, such as printed circuit boards, cathode ray tubes, bare/insulated wires and in refrigeration units. Majority of the samples had high levels of Cu in the topsoil than the subsoil. These may be attributable to the strong binding between Cu and organic matter and minerals in the soil. Consequently, its mobility is supressed, and hence cannot be leached into the subsoil [29, 30]. Also, the prevailing alkaline conditions in the topsoil played a critical role (Table 3).
Ranging from 13.58 to 276.78 ppm, with an average concentration of 77.07 ppm, at site F, Pb level was found to have exceeded the WHO/FAO and Ghana EPA standard of 50 and 20 ppm, respectively (Tables 1-2). Elevated levels were detected for Pb, ranging from 17.81 to 1000.85 ppm, with an average concentration of 341.43 ppm at site H, with so obvious exceedance (Tables 1 and 3). The lowest and the highest levels were all detected on the topsoil. E-waste materials with Pb include cathode ray tubes, fluorescent bulbs, batteries and fuses. The elevated Pb levels from site F to H is reflective of low organic matter in the presence of slightly alkaline to near-neutral mean soil pH. It is contemplated that the elevated levels of Pb is a consequence of metal accumulation arising for all the operational years of the scrapyard. The extent of pollution at site F and H can be respectively expressed as follows: Cu > Cr > Pb > Cd and Pb > Cu > Cr > Cd.
Heavy metals concentrations in this study were similar to other E-waste research [4, 31, 32]. Generally, Cu and Pb were in high concentrations in most of the research studies, suggesting an extensive use of the two metals in electrical appliances, whereas Cd concentrations seem to be on the lower side in most E-waste soils in other research works (e.g., Table 4).
3.3. Statistical Studies
Tables 5a-g present the relationships between the metal levels at the following sampling sites: (i) Topsoil at site F, (ii) Subsoil at F, (iii) Topsoil at site H, (iv) Subsoil at site H, (v) Topsoil at F and H, (vi) Subsoil at F and H, and (vii) Topsoil and subsoil over the scrapyard. These were quantified using Pearson correlation coefficient metric r.
The following were the key findings at the following sampling sites: (i) In the topsoil at site F, only Cd and Cu were significant, but exhibited out-of-phase relation (r= 0.667; p<0.01; Table 5a); (ii) In the subsoil at site F (Table 5b), Cu, Cd, and Pb all exhibited in-phase relationships (Cu vs. Cd r=0.926; p<0.01; Pb vs Cd r=0.956; p<0.05; Pb vs. Cu r=0.889; p<0.05);
In the topsoil at site H (Table 5c), it is only Pb and Cu that were significant and exhibited an in-phase relationship (r=0.675; p < 0.01). In the subsoil at site H (Table 5d), Cr, Cd, and Cu showed significant in-phase relationships (Cr vs. Cd r=0.786; p<0.01; Cu vs. Cd r= 0.978; p<0.05; and Cu vs. Cr, r= 0.653; p<0.01).
In the topsoil at F and H (Table 5e), Cd, Cr and Cu at site F, and Cr and Pb at site H, showed good correlations. The in-phase relationship ranged from r=0.693 to 0.779; p<0.05 and p<0.01). These were captured for Pb vs. Cr, Cr vs. Cd, and Cd vs Cu. In contrast, Cd vs. Cr (r=-770; p<0.01) showed an out-of-phase relationship. Across the scrapyard, in the study area (Table 5g), the topsoil and subsoil revealed in-phase relationship for Cr vs. Cr (r=0.821; p<0.05), Pb vs Cd (r=0.734; p<0.01) and Pb vs. Pb (r=0.79; p<0.05).
From these results, it has been revealed that the in-phase relationships dominate the relationships and suggest synchronization of activities and chemical processes emanating from human E-waste recycling activities, whereas the out-of-phase relationships suggest otherwise. For instance, the in-phase relationships observed between pairs of heavy metals may be due to the dual complementary usage they have in certain EEE products. Cd and Pb find close applications in cathode ray tubes where Cd is used as the fluorescent powder coatings to produce color while Pb is employed to absorb the UV lights and X-rays produced. Cd is added to Cu to form alloys in Cd-Cu wire which are more resistant to softening at higher temperatures. Pb is also alloyed to Cu to act as a lubricant and also assist in chip break up, thereby increasing the machinability of the Cu metal. Since site H is used as a burning site and dumping grounds for burnt E-waste products, heavy metals may be carried from site F to site H. This could explain the positive correlation between heavy metals at different site.
A coefficient of variation (CV) analysis carried out to determine if the presence of the heavy metals was due to natural or anthropogenic source showed CV for Cd, Cr, Cu and Pb to be 137, 58, 61 and 104%, respectively at site F, and 108, 22, 125 and 93% for Cd, Cr, Cu and Pb, respectively at site H. According to Guo et al. , a CV less than 20% indicates natural sources while values greater than 50% imply anthropogenic sources. By inference the heavy metal pollutions were due to anthropogenic sources, specifically E-waste activities.
An independent t-test analyses showed statistically significant differences in mean concentrations of the heavy metals, which are as follows:
- significant difference between Cd concentrations at site F (Mean (M)=0.39, Standard deviation (SD)=0.53) and Cd concentrations at site H (M=4.14, SD=4.46), with a t-value of -2.50 and p-value of 0.02 (data are not normally distributed; skewed).
- significant difference between Cr concentrations at site F (M=77.33, SD=44.57) and Cr concentrations at site H (M=21.00, SD=4.65), with a t-value of -3.98 and p-value of 0.001 (data are normally distributed).
- significant difference between Cu concentrations at site F (M=114.85, SD=70.33) and Cu concentrations at site H (M=48.37, SD=6.58), with a t-value of 2.27 and p-value of 0.04 (data are normally distributed).
- significant difference between Pb concentrations at site F (M=77.07, SD=80.10) and Pb concentrations at site H (M=341.43, SD=317.96), with a t-value of -2.55 and p-value of 0.02 (data are not normally distributed; skewed for Pb at site F and normally distributed for same metal at site H).
3.4. Indices of pollution
3.4.1. Geoaccumulation Index
Table 6 below shows Igeo of sampled soil of the two sites. The Igeo showed site F was practically uncontaminated with Cd (average Igeo= -1.58) and Cr (average Igeo = -1.07) but unpolluted to moderately polluted with Cu (average Igeo = 0.53) and Pb (average Igeo = 0.76). At site H, index of geoaccumulation showed a moderate to strong pollution with Cd (average Igeo = 2.16) and Pb (average Igeo = 2.64), a practically unpolluted soil with Cr (average Igeo = -2.72), and Cu (average Igeo = -1.63). Site H appears more contaminated than site F, probably due to its dual role as a burning and dumping site for E-waste materials. Pb levels at site H for instance was in category 3 of the Igeo classification.
3.4.2. Contamination Factor and Pollution Load Index
Table 7 shows CF and PLI of the two sites. The CF showed site F had moderate contamination of Cd (average CF= 1.30) and Cu (average CF=2.55), low contamination of Cr (average CF= 0.86), and considerable contamination of Pb (average CF= 3.85). At site H, CF showed very high contamination of the soil with Cd (average CF= 13.80) and Pb (average CF= 17.07), no or low contamination of the soil with Cr (average CF= 0.24), moderately contaminated soil with Cu (average CF= 1.08). Again, results show more contamination of Site H than Site F, possibly because of the dumping activities in addition to the open burning. PLI of both sites were found to be moderately polluted, with averages of 1.47 and 2.16 at sites F and H, respectively.
3.5. Spatial distribution of heavy metals
Figures 2-4 show spatial distribution patterns of the heavy metals at the two sites, which were analyzed using the Inverse Distance Weighted (IDW) interpolation method. The analysis revealed elevated levels of heavy metals in subsoil (Cd, Pb at site H and Cr, Cu at site F), which indicates a possible leaching from the surface soil. The distribution pattern showed Cu retained more at the topsoil of site F and the northern section of site H, indicating high presence of organic matter. Spatial maps also revealed site H was polluted more with Pb and Cd while site F was polluted with mostly Cr and Cu.
3.6. Concentration differences at increasing distance from scrapyard
Figure 8 depicts the metal concentrations as a function of distance away from the scrapyard. Soil samples taken at 25, 50, 75 and 100 m away from the scrapyard were mostly sandy. pH values were mildly acidic, decreasing with distance away from the scrapyard, but within the WHO 6.5-8.5 thresholds. This is expected, as increasing distance from the scrapyard means decreasing heavy metals concentrations, which are mostly alkaline. This finding is comparable to a study by  where the pH at a dumpsite decreased from 5.9 to 4.7 at a distance of 18 m from the dumpsite.
The results practically revealed no Cd in these soil samples. Samples taken within the 25 m distance were found to contain respective concentrations of 20.73, 24.94 ppm for Cr and Cu and were within safe levels set by WHO/FAO but slightly above permissible levels of Ghana EPA with respect to Cu. However, Pb recorded a concentration of 155.17 ppm which exceeds the safe levels of Pb in soil as determined by both WHO/FAO and Ghana EPA. pH of the soil sample at 25 m was almost neutral at 6.97.
The levels of Cr and Cu in soil samples within the 50 m boundary were again within the safe limits set by WHO/FAO, but above permissible levels of Ghana EPA, with respective concentrations of 25.18 and 96.73 ppm. Pb levels again exceeded the 50 ppm threshold of WHO/FAO and the 20 ppm threshold of Ghana EPA, reaching levels of 74.72 ppm. pH of the soil sample at 50 m was mildly acidic at 6.58. Soil samples taken 75 m from the scrapyard had concentrations of 4.122 ppm (Cr), 4.600 ppm (Cu) and 5.965 ppm (Pb) while at a 100 m distance variation, soil samples analyzed revealed no levels of Cr, 1.260 ppm of Cu and 8.970 ppm of Pb. It can be thus inferred that the activities at the scrapyard still have effect 25 to 50 m away from it. However, since soil samples taken at 25 and 50 m were close to the Accra – Tema motorway, contamination from road dust is still possible since heavy metals can be found in tires and brake abrasion, combustion exhaust and pavements wear , which can be transported by rain, runoff, dry deposition, and atmospheric drifts. and Further research will be needed to investigate this proposition.
With a general decline in the concentrations of heavy metals from the 75 and 100 m distance, the high levels of heavy metals within the scrap yard can be attributed mainly to that of the E-waste activities. Comparably, concentrations of Cr, Cu and Pb were several times higher within the scrapyard than outside of it.
The decreasing concentrations of heavy metals with increasing distances from the scrapyard agrees with other research studies which explored the effect of increasing distance from source on concentration levels of heavy metals [36, 37].
3.7. Heavy metal concentrations in sediment and water
The metal concentrations in the drain both upstream and downstream are shown in Table 9. Soil-sediment-water samples (WSC) taken outside the scrapyard showed lower concentration of heavy metals than those obtained within the scrapyard particularly at WS 1, while the four heavy metals were absent within the water at WS 2. Water sediment outside the scrapyard (WSC) was found to contain Cd and Cr, with respective concentrations of 0.03 and 11.95 ppm, whereas the levels of Cu and Pb were 5.84 and 5.89 ppm, respectively. Water sediments within the scrapyard (WS1) were found to contain 0.49 ppm Cd, and a concentration of 217.98 ppm, for Cu. Cr had a concentration of 12.28 ppm while Pb had a concentration of 44.77 ppm. The levels of the toxic metals in the water sediments of WS1 increased significantly within the scrapyard as one moves downstream from WSC. With the wastewater drain lying in a lower plain to the two burning sites, and with movement of air current across the drain from the two burning sites, it can be fairly postulated that the E-waste activities are a possible origin for the heavy metals in the soil-sediment water samples, through the actions of wind drift and dry deposition. One other possibility is the presence of E-waste materials found near or inside the drain, causing heavy metals to dissolve into the wastewater as has been reported elsewhere .
The relatively concentrated amounts of heavy metals of soil-sediment-samples (WS1), compared to no detection in the surface wastewater samples at WS2 supports the research hypothesis, that heavy metals tend to be high in sediments and settleable particles than surface water . WSC and WS1 samples had mild alkaline pH, indicative of the presence of the heavy metals. Levels of heavy metals in the drain represented by WS1 were all above the standard permissible levels of Ghana EPA and WHO/FAO. This is of a major concern since the drain serves as irrigation source for farm crops as well as drinking water for herds of cattle near the scrapyard.