A number of elements at elevated concentrations are commonly regarded as contaminants of soil. They are also referred to as "heavy" metals, although they include metalloids and non-metals. The main elements implicated as contaminants include: arsenic, cadmium, chromium, copper, fluorine, lead, mercury, nickel and zinc. Beryllium, bismuth, selenium and vanadium may occasionally also be geochemically enriched in some soils. Contamination may result from the weathering of geological parent materials (where element concentrations exceed natural abundance values), soils contamination by industrial activities (usually by wet and dry deposition) and soil applications of waste such as sewage sludge or pig slurry, as well as fertilizer applications.