Metals such as lead, copper and zinc are found in a trace amount in all sediments. These metals are bound weakly to the hydrous clay minerals on the edges of the crystals and are held by weak bonds with hydroxyl groups. Zinc is found within carbonate minerals bound within the carbonate crystal lattice at vertices and along crystal twin planes and crystal boundaries. These metals enter the sedimentary minerals due to adsorption from the seawater which deposited them; few freshwater sediments are considered to have as much metal carrying capacity as saline waters.