Spatial variability of heavy metal concentrations in soils adjacent to Oshakati solid waste disposal site, Namibia

Abstract

Human existence generates massive amount of waste. These wastes are usually dumped at disposal sites especially in developing countries due to lack of landfills. As a result of continuous waste dumping, soils in disposal sites have become sinks of toxic heavy metals. Oshakati disposal site is open and all waste from the town irrespective of the source get dumped at the site. The scoping assessment which was done at the site in 2018 found high levels of heavy metals way above the standard limits. This study was then undertaken to investigate the spatial variability of cadmium (Cd), lead (Pb), arsenic (As), chromium (Cr), manganese (Mn), nickel (Ni), copper (Cu) and Zinc (Zn) concentrations in soils adjacent to Oshakati solid waste disposal site in Namibia, to understand the impacts of the disposal site on soils nearby. This was done by assessing heavy metal occurrence and levels in soil samples taken from an area adjacent to the Oshakati disposal site. A total of 95 topsoil samples at a depth of 30 cm were collected from Oshakati disposal site vicinity (56 samples), control site 1 (28 soil samples) and control site 2(11 soil samples) using a soil auger. Soil samples were collected from four transects that were laid according to cardinal direction from 10 metres to 150 metres away from the disposal site. A similar method was employed at control sites. The disposal site vicinity was an area comprised mainly of farm fields and homesteads, while control site 1 was an open area, located about 700m away from the disposal site where human activities were limited while control site 2 was a homestead with a farm field situated approximately 2 km away from the disposal site. Soil samples were analysed for their physicochemical properties such as pH, particle size analysis, texture and electrical conductivity (EC) using a pH meter, pipette method, USDA classification system and conductivity meter respectively. Soil samples were further analysed for the concentrations of the following heavy metals: cadmium, lead, arsenic, chromium, manganese, nickel, copper and Zinc using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) as a standard analytic technique. Results of physicochemical parameters showed that soil at disposal site vicinity was slightly acidic with high EC compared to control sites and all study sites recorded a sand texture. Heavy metals were found in various concentrations at all study sites except for cadmium which was not detected in the disposal site vicinity. Manganese recorded the highest mean values and arsenic the least in all study sites. The mean metal concentrations recorded at disposal site vicinity where : Mn > Zn > Cr > Pb > Cu > Ni > As while at control site 1 mean values where : Mn > Zn > Cr > Cu >Ni > Pb > Cd > As and at control site 2: Mn > Zn > Cr > Ni > Cu > Pb > Cd > As. The result showed that the disposal site contributed to the presence of heavy metals present in the disposal site vicinity. A MANOVA test indicated a significant difference (P < 0.05) in mean metal values between disposal site vicinity and control sites. These differences stated that beside zinc and lead, heavy metal concentrations were higher at control sites compared to disposal site vicinity. This was attributed to contamination from another anthropogenic source other than the disposal site. However, all heavy metals analysed in this study from all study sites were found below the standard limits for heavy metals in soil established by SA and WHO. Furthermore, distance did not influence the spatial variability of heavy metal concentrations, as no correlation was detected between them. A Pearson’s correlation matrix test indicated that only at 100m distance were significant linear relationship existed in the interaction between mean metal concentrations and distance. Manganese positively correlated strongly with nickel, copper and arsenic (r = 0.9046, r = 0.8283, r = 0.6563, p > 0.05) while nickel values correlated strongly with those of manganese, copper, zinc and arsenic (r = 0.9046, r = 0.8686, r = 0.6015, r = 0.5704, p = 0.05). Nevertheless, chromium, cadmium and lead did not show any interactions with other metals. The interactions were attributed to soil physicochemical parameters such as pH, EC and water holding capacity. The occurrence of heavy metals, although in low concentrations in soils at the disposal site vicinity requires periodic monitoring, as heavy metals do not degrade. With continuous dumping of waste in the disposal site, heavy metals will continue to accumulate in soil and cause potential threat to soil properties and living organisms inhabiting these soils. The researcher, therefore, recommends that local authorities develop and implement a soil quality monitoring system that will regularly asses heavy metal levels in soils adjacent to disposal sites. This is necessary to ensure healthy environments.