CHAPTER FIVE

CHAPTER FIVE.

SUMMARY AND CONCLUSION.

SUMMARY.

The study performed on street soil and road dusts samples from Benin City, Nigeria revealed a clear accumulation of Lead, Copper, Zinc and Cobalt beyond the ‘heavily contaminated’ level while both Strontium and Barium reach the ‘moderately to heavily contaminated’ point, Table 4.7. The increases in the anthropogenic metals such as Lead, Zinc, Copper and Cobalt in the soil samples in Benin City can most likely be attributed to rapid development, increased traffic emissions to the atmosphere, industrial activities, metal smelting activities and lack of efficient management of wastes and effluents from plants and industries. Metals in aerosols can be inhaled by people and animals but their greatest environmental impact in the long term is through deposition under gravity or wash-out on to vegetation, soils, rivers, lakes, and the sea.

As evident from this study, no significant level of contamination risk is expected from metals like Chromium, Thorium and Lanthanum as all the samples are uncontaminated with any of these metals. Metals like Cadmium, Gold, Uranium, Arsenic, Bismuth, Tungsten, and Antimony were below analytical detection limit.

The highest metal values were observed in the soil/dust samples around heavy traffic sites and metal dumpsites, while the lowest metal concentrations were found around less built-up areas of the City believed to be protected from all the emissions and pollutions of the urban areas. The mean metal concentration values in soil samples are in the order Zn (1447ppm) > Mn (587.5ppm) > Cu (189ppm) > Pb (127.93ppm) > Ba (88.214ppm) > V (53.929ppm) > Sr (45.571ppm) > Cr (41.25ppm) > Co (29.429ppm) > Ni (13.929ppm) > La (7.786ppm) > Th (6.07ppm). Results showed that soil samples contained higher significant concentration values of the heavy metals than the dust samples.

Principal Components analysis coupled with correlation analysis was used to gain additional insight into the origin of these metals in the soil/dust samples of Benin City. The examined metals were observed to have been anthropogenically contributed by metal smelting activities, combustion of fossil fuels, industrial effluents, and improper disposal of metal/e-wastes and from sewage sludge. The trend of increasing industrial and traffic activities in the city indicates the need for pollution control in the city environment

Highly significant correlation values (>0.65 from Table 4.3) were observed between Copper-Lead (0.905), Nickel-Copper (0.743), Strontium-Copper (0.893), Barium-Copper (0.696), Nickel-Lead (0.841), Cobalt-Zinc (0.953), Barium-Lead (0.746), Strontium-Lead (0.964), Manganese-Lead (0.671), Barium-Zinc (0.616), Nickel-Manganese (0.848), Nickel-Strontium (0.886), Manganese-Strontium (0.765), Vanadium-Chromium (0.710), Thorium-Vanadium (0.825). These high correlation values show that the origin of the metals in the investigated area is highly related and may have resulted from heavy traffic, Industrial activities, Structure of the soil, Street dusts emissions, oil spills in mechanic workshops and metal smelting activities of the natives.

As earlier mentioned, the study area has anomalous elevated concentrations of Lead, Copper, Zinc and Cobalt and moderate contamination with metals like Strontium, Barium, Nickel, Vanadium and Manganese. These metals could infiltrate through the dominant permeable sand which characterizes Benin to contaminate the groundwater resource of the study area. The predominance of the sand and ‘likely’ absence of clay minerals explains for the lesser concentrations of radioactive metals like Thorium and Uranium.

Furthermore, there is a gradual anthropogenic increase in the level of metals in the soils around the waste/metal dumpsites. It is advised that the practice of dumping wastes within the residential areas be stopped and appropriate disposal systems should be constructed according to specification and located at the outskirts of the City to avoid further contamination from these metals. It is evident from findings (colour and taste, no chemical analysis) that the surface water and wells within the vicinity of waste dumpsites within the study area have been influenced negatively by leachates from the waste dumps. Also, adequate monitoring effort should be put in place to ensure that untreated wastes and effluents are not introduced into rivers and streams as seen within the study area.

Future studies should include the collection of the health data of the people and animals living within the study area (if possible blood and tissue samples should be analysed), water samples and also the collection and analysis of plants to ascertain the level of the impact of contamination caused by the heavy metals.