Complex sentence: Chemicals dumped on the site are believed to be 1,2-Dichloroethane, arsenic, benzene, chloroform, lead, and vinyl chloride, all of which contaminated the groundwater, soil, air, and nearby creeks.
Split into simpler sentences: Chemicals dumped on the site are believed to be 1,2-Dichloroethane, arsenic, benzene, chloroform, lead, and vinyl chloride. All of which contaminated the groundwater, soil, air, and nearby creeks.