Moving on from the agricultural sources of soil pollution, let’s explore the major soil contaminants in urban areas. Chemicals are omnipresent in cities and urban areas. From our personal care products, pesticides for urban farming to paints and solvents, a whole host of inorganic and organic compounds can be transported to the environment through transport, waste disposal, accidental leakages, or other routes, posing risks of soil pollution.
Domestic sources
Lead naturally occurs in soils at concentrations of around 10-50 mg/kg. However urban sources, including leaded paint and gasoline can increase lead concentrations above the normal background rate to around 150 mg/kg to almost 10,000 mg/kg at the base of lead-based painted homes. Lead particularly poses a large human health risk, due to its high toxicity, bioaccumulation, and persistence even in small amounts due to its high biotoxicity, bioaccumulation and persistence.
Figure 1. Diagram depicting sources and pathways of lead contamination in urban soils. Source: Duke University
Lead typically accumulates in upper layers of soil, on surfaces of fine clay and organic matter. The bioavailability of lead to plants depends on how tightly lead is held on to the soil particles. Lead is more soluble in low pH soils (higher bioavailability) and is held tightly in soils with higher organic matter (lower bioavailability). Many industrialized countries have banned lead in fuel, paint, and other products; however, lead paints are still used in low to middle-income countries. Furthermore, lead contamination from the pre-1970s still persists in urban soils, such as in Durham, North Carolina. Bioavailable lead poses human health risks if passed through airborne dust, clothing, pets, or other means.
Hazardous waste from healthcare
Hazardous waste produced from healthcare activities ranges from pharmaceutical to radioactive and infectious waste. The impact of these wastes on soil has largely been overlooked. However, when these hazardous wastes are incinerated, dioxins, POPs (persistent organic pollutants) and heavy metals can settle into soils around the incinerator bottom ash dump sites (Adama et al., 2016). Open burning or poorly managed incineration of such wastes may lead to incomplete combustion and cause the release of POPs and other contaminants such as lead and chlorine that can be transferred long distances through the atmosphere to the soil, including agricultural soils.
Electronic waste
Another major urban source of urban pollution is electronic waste (e-waste). This growing waste stream is a consequence of an increasingly high-technology world and a lack of circular economy measures such as planned obsolescence, making electronic devices increasingly affordable and replaceable. An estimated 80% of e-waste is not recycled and ends up in landfills or is incinerated. Dumping of e-waste in informal recycling facilities or urban landfills occurs mostly in developing countries such as China, Brazil, India, and West Africa which have long been on the receiving end of waste imports from industrialized countries. However, this is changing, as many countries have rolled out stricter controls and legislation on waste imports.
Organic compounds and heavy metals such as polycyclic aromatic hydrocarbons and copper used for wires are often released directly into soils or deposited via fly ash particulates. Metal release into the soil can lead to human exposure via inhalation, dermal contact, drinking water, or the food chain. In China, it has been proven that pollutants from e-waste on the surface or topsoil in rice fields are subsequently transferred to the food chain via consumption (Zhang, W.-H et al., 2012). High levels of various pollutants have also been detected in the blood, hair and placenta of residents of e-waste sites. This highlights that the urban poor, especially children and women living and working around unmanaged waste dumps are often most exposed to toxic contaminants in soil and water.
References
Adama, M., Esena, R., Fosu-Mensah, B., & Yirenya-Tawiah, D. (2016). Heavy Metal Contamination of Soils around a Hospital Waste Incinerator Bottom Ash Dumps Site. Journal of Environmental and Public Health, 2016, e8926453. https://doi.org/10.1155/2016/8926453
Ephraim P, I., Ita, A., & Eusebius I, O. (2013). Investigation of soils affected by burnt hospital wastes in Nigeria using PIXE. SpringerPlus, 2, 208. https://doi.org/10.1186/2193-1801-2-208
FAO and UNEP. (2021). Global assessment of soil pollution: Report. Chapter 3: Sources of soil pollution. https://doi.org/10.4060/cb4894en
India Today. (2016, July 19). In Pictures: The toxic cost of Kanpur’s leather industry. https://www.indiatoday.in/fyi/story/the-toxic-cost-of-kanpurs-leather-industry-329990-2016-07-19
Wang, Z., Wade, A. M., Richter, D. D., Stapleton, H. M., Kaste, J. M., & Vengosh, A. (2022).
Legacy of anthropogenic lead in urban soils: Co-occurrence with metal(loids) and fallout radionuclides, isotopic fingerprinting, and in vitro bioaccessibility. Science of The Total Environment, 806, 151276. https://doi.org/10.1016/j.scitotenv.2021.151276 Zhang, W.-H., Wu, Y.-X., & Simonnot, M. O. (2012). Soil Contamination due to E-Waste Disposal and Recycling Activities: A Review with Special Focus on China. Pedosphere, 22(4), 434–455. https://doi.org/10.1016/S1002-0160(12)60030-7