Environmental fate and ecotoxicology of paraquat: a California perspective

The herbicide paraquat belongs to the group of the bipyridylium salts. In California, it is used primarily for control of broad-leaved grasses in fruit orchards and plantations, as a cotton defoliant, and for inter-row control in many crop and non-crop areas. In plants, paraquat causes the formation of reactive radicals leading to cell membrane damage and ultimately rapid desiccation. Soil clay minerals have a greater influence on paraquat adsorption and inactivation compared with soil organic matter following an application. Degradation mechanisms include photolysis, chemical, and microbial degradation, but these processes are generally extremely slow. In California during 2000-2014, paraquat was used primarily for the cultivation of almonds, cotton, alfalfa, and grapes: median value for an application and annual mass applied statewide were 0.53kg ion/ha and 280 Mg, respectively. Paraquat was undetected in groundwater as a non-point source pollutant. Detections in surface waters (0.42-3.6 g/L) were <1%. In earthworms and other invertebrates there is limited paraquat accumulation as toxic effects are mitigated via soil inactivation. Paraquat is among the most embryotoxic contaminants for bird eggs, but not to adult; it causes toxic and teratogenic effects in amphibians, and toxic effects in honeybees, fish, and other aquatic species.