Complex sentence: It was owned and operated by the Omega Processing Co. On April 20,1989, a massive chemical spill occurred that released around 200 gallons of cadmium cyanide into the Chicopee Brook, Monson's main water supply.
Split into simpler sentences: It was owned by the Omega Processing Co. until, on April 20,1989, when a massive chemical spill occurred. About 200 gallons of cadmium cyanide spilled into the Chicopee Brook, Monson's main water supply.