Complex sentence: Pipeline workers on December 3 had been repairing sections of the 16-inch pipe in a rural area, left on the afternoon of December 3, and the spill occurred during the evening hours of December 3--4.
Split into simpler sentences: Pipeline workers on December 3 had been repairing sections of the 16-inch pipe in a rural area. They left on the afternoon of December 3, and the spill occurred during the evening hours of December 3--4.