Trace element contamination in soils, plants, and river sediments around waste dumpsites

This research is a systematic investigation of three distinct environmental components (soils, plants, river sediments) around informal electronic waste and end-of-life vehicle component recycling and dumping sites in Abuja, Nigeria’s Federal Capital Territory. The informal recycling operations involve waste dismantling, open fire burning, fabrication, soldering, painting of accident-damaged vehicles, and the disposal and dumping of waste oil/chemical fluids and other residues. Eighty-three soil samples were collected from the study sites (Apo: 31, Gudu: 28, Goza: 24), plant samples (n = 30) of okra leaves from Apo, rice leaves from Gudu, and sorghum leaves from Goza were collected from adjoining farmlands and sediment samples were taken from River Wumba (n = 10) and River Gudu (n = 13). The samples were analysed for the trace elements zinc (Zn), cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and chromium (Cr) using a microwave plasma atomic emission spectrometer (MP-AES). Pollution and contamination indices including the geoaccumulation index, enrichment factor, modified degree of contamination, pollution load index, and potential ecological risk index were used to assess the level of pollution and the associated risks posed by the trace elements. Non-carcinogenic and carcinogenic risks of trace elements in soil were also determined.

The concentration of trace elements in the study soil samples was elevated to levels ranging from 2 to 254 times compared to their corresponding background/control levels, depending upon trace element and site activities. These findings suggest high anthropogenic impacts of open burning, dismantling, and dumping of waste. The geoaccumulation index of trace elements in soil ranged from moderately to extremely contaminated, except for Cr in all the study sites/subsites, and Cu and Ni in Gudu farmland subsite. The pollution load index revealed that the soil in the study sites was highly polluted. Residents of the study sites were exposed to a high non-carcinogenic risk of trace elements and an extremely high carcinogenic risk of Cu and Cr.

High concentrations of trace elements were also found in farmland soil samples compared to the background/control sites, except for Cr at Goza. Similarly, the concentrations of trace elements in okra, rice, and sorghum were also higher than those from their respective local background farmlands. The bioaccumulation of trace elements was lower in plants in the study sites compared to the background site, except Cd, which could be due to differences in plant type, soil trace element contents, and soil properties. Cadmium was highly accumulated in sorghum, okra, and rice, with a strong positive correlation between Cd soil and okra leaves, i and Zn soil, Cd soil and rice leaves. Nickel in soil was also strongly positively correlated with sorghum leaves. Simple linear regression analysis revealed that Zn soil was strongly related to its uptake in rice while Ni soil significantly influenced its accumulation in sorghum. The main controlling factors for Zn, Cd, and Cu accumulation in okra were soil trace element content, soil pH, and soil organic matter content while soil pH was the influencing factor for Cd accumulation in sorghum.

The assessment of sediments in River Wumba and River Gudu showed that the concentration of trace elements in the midstream was higher than the upstream and downstream, suggesting that runoff from the waste areas is transported into these rivers. Sediment properties such as the organic matter content and near neutral sediment pH may be responsible for trace elements distribution and their migration from the midstream to the downstream. There was a strong positive relationship between sediment organic matter and trace elements with a severe to extremely severe enrichment of Pb, Cu, Zn, Cd, and Ni within the midstream of both rivers and ultra-high contamination of the midstream sediments by trace elements. There was a moderate to considerable risk of Ni, a high risk of Cd and Cu, and a very high ecological risk of Pb in the midstream sediments of River Wumba. In River Gudu, there was a moderate ecological risk of Cd and Ni, a considerable risk of Cu, and a high risk of Pb in the midstream sediments.

This study revealed the negative impacts of waste dumpsites on the soil, plants, and river sediments which are critical resources for human life and the environment. Therefore, there is an urgent need for adequate control measures and enforcement of waste management regulations at the study sites.