Metribuzin (4-amino-6-tert-butyl-3-(methylthio)-1,2,4-triazin-5(4H)-one) is an herbicide used both pre- and post-emergence in crops including soy bean, potatoes, tomatoes and sugar cane.   It acts by inhibiting photosynthesis by disrupting photosystem II. It is widely used in agriculture and has been found to contaminate groundwater.