The fibers come mainly from clothing. The fashion industry is the second largest polluter on the planet, right behind the oil industry. Thus, these fibers are generated during the fabric manufacture, during the disposal of the clothes manufacture remains (remains of seams), and in the act of washing the clothes, since several washing machines do not have filters.
This study proved that contamination by plastics is everywhere. In oceans, estuaries, rivers, and even river springs.