A means of expressing small concentrations, usually of an element, compound, contaminant, or particle in water or a mixed medium such as soil and normally on a mass basis. Units that are equivalent to ppm include micrograms per gram, milligrams per kilogram, and milligrams per liter. One ppm times 10,000 equals one part per hundred or one percent. The nutrient phosphorus and the contaminant lead found in soil are commonly measured in ppm.