Assessment of soil and leachate heavy metal contamination at the Weltevreden Landfill, Polokwane

Abstract

In recent times, there has been increasing environmental concern that municipal solid waste (MSW) landfills are potentially a sink of persistent heavy metal contaminants. The contamination of landfill soil by heavy metals seems to be widespread in MSW landfills. Overtime, the accumulation of heavy metals in landfill soils resulting from the disposal of municipal waste represents a threat to the environment. Despite this, the provenance of heavy metals in landfill soils, leachates and groundwater quality is rarely known with certainty and whether concentrations of the heavy metals exceed the statutory recommended thresholds for safety. Therefore, the present study was conducted to (1) assess heavy metal contamination of soil and leachates from the Weltevreden landfill site and (2) examine the spatial variability and distribution patterns of heavy metals across the landfill site using geostatistical techniques. Soil samples were collected from the landfill site and an area located 100 m away from the landfill site (reference site). Soil samples were collected at a depth of 0-30 cm using a 50 m x 50 m grid. At the nodes of the grid, landfill characteristics, including the landscape position, elevation, latitude and longitude of each sample location were recorded to analyse and map the spatial distribution of the heavy metal concentrations across the site. Collected soil samples were analysed for total concentration of arsenic (As), cadmium (Cd), lead (Pb), nickel (Ni), manganese (Mn), zinc (Zn), cobalt (Co), copper (Cu) and iron (Fe) and physicochemical properties such as soil pH, soil organic carbon (SOC) and soil texture were analysed. Leachable heavy metals in the collected soil samples at the landfill site were determined using the toxicity characteristics leaching procedure (TCLP). The extent of pollution in the landfill soil was determined using the contamination factor (CF), enrichment factor (EF), pollution load index (PLI), ecological risk factor (Er) and ecological risk index (RI). The study also characterised the spatial structure of the soil heavy metals in the landfill site by computing semivariograms and determining their spatial distribution using the ordinary kriging (OK) geostatistical technique. The results obtained in this study revealed that the concentration of all heavy metals in the soil was higher in the landfill site compared to the reference site. The only heavy metal which was an exception to this pattern was Fe. The results also showed that the mean concentration of As and Cd in the landfill soil was above the acceptable limits of the South African National Norms and Standards for Remediation of Contaminated xiii Land and Soil Quality. The correlation analysis revealed that most of the heavy metals were strongly positively correlated with each other (As, Cd, Pb, Ni, Zn and Co). The results of the different indices show moderate contamination of the landfill soil by As, Cd, Cr, Pb, Ni, Mn, Zn, Co and Cu. The PLI index shows that the landfill site is highly polluted with heavy metals (PLI>1). The calculated enrichment factors suggest an anthropogenic origin for Cd and Pb, and natural origins for As, Cr, Mn, Co and Cu. Among the analysed heavy metals, a moderate ecological risk was observed only in the case of Cd concentration in the landfill soil. Meanwhile, the other investigated heavy metals had a low ecological risk. The overall ecological risk status of the heavy metals in the landfill soils was found to be low. Geostatistical analysis of the data revealed that Cu was characterised by a strong spatial dependence while Cd, Pb and Fe showed moderate spatial dependence and As, Cr, Ni Mn, Zn and Co were characterised by a weak spatial dependence. Copper accumulation in the soil was linked to natural factors such as soil mineralogy while As, Cr, Co, Ni, Mn and Zn accrual were linked to anthropogenic activities such as waste disposal. Iron, Pb and Cd were driven by both natural and anthropogenic factors. Based on the interpolated maps, similar spatial patterns were observed for Cd and Pb kriged maps, whereby hotspots were in the northwestern parts of the landfill site. Meanwhile, Mn, Zn and Fe were also found to exhibit similar concentration hotspots in the southwest part of the landfill. The distribution of Cr displayed high concentrations in the central part of the landfill. Similarly, As, Ni and Co showed similar distribution with the southwest parts also having high concentrations. Results from TCLP confirmed concentrations of Cd, Ni and Mn that were found to be above the South African National Norms and Standards for Remediation of Contaminated Land and Soil Quality while Zn, Co and Cu were within acceptable levels. Overall, the results show that the disposal of waste at the Weltevreden landfill site led to differences in the concentration of heavy metals. The current study has offered a better understanding of the current state and potential pollution risks as well as the spatial distribution of heavy metals in the landfill soil. It is envisaged that these findings will help landfill managers to properly manage the municipal solid waste at the Weltevreden landfill site to minimise heavy metal contamination.