Barium is an alkaline earth metal that occurs in nature in a combined form. It is present in rocks, minerals, soils, air, natural waters, and fossil fuels. Some barium salts (e.g., acetate, nitrate, and chloride) are quite soluble in water, whereas others (e.g., arsenate, carbonate, oxalate, chromate, fluoride, sulfate, and phosphate) are very poorly soluble. The water solubility of barium salts, except for barium sulfate, increases with decreasing pH.