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Identifying the source of soil contaminants is vital to decision-making during an environmental cleanup. Soil in long-established cities has accumulated decades of low levels of pollutants caused by urban activity. Deposition of metals and other chemicals like polynuclear aromatic hydrocarbons (PAHs) come from historical industrial activity and manufacturing materials used to build the infrastructure of a modern society. Naturally occurring metals such as lead and arsenic may also occur in urban soil. The combination of natural and anthropogenic background levels of chemicals is known as urban background contamination.
Urban background contaminants are generally widely dispersed in low levels over large urban areas. These metals and chemicals can intermingle with higher concentrations of chemicals from spills and industrial waste. This creates challenges in understanding how an industrial site is contributing to overall soil contamination.