When cities are growing, what was previously urban areas used for industries, is required for exploration and development of housing projects. Industrialized areas all of a sudden is within city limits side by side with office buildings and housing areas, giving much lower tolerances for the presence of hazardous material. As Mercury is naturally occurring in soil and rock formations, test for Mercury is getting more common at constructions sites.
Mercury is naturally occurring in the soil and ore excavated when mining. Coal deposits in particular can contain high levels of Mercury, which can be released in the excavation and handling process. In Gold mining Mercury extraction can be used as last step in purification.