Complex sentence: Most of the dumping took place before 1960, and all dumping of radioactive wastes by the United States was terminated in 1970, after 47,500 55 gallon steel drum containers were dumped in the vicinity, with a total estimated radioactive activity of 14,500 Ci.
Split into simpler sentences: Most of the dumping took place before 1960, and all dumping of radioactive wastes by the United States was terminated in 1970. By then, 47,500 55 gallon steel drum containers were dumped in the vicinity, with a total estimated radioactive activity of 14,500 Ci.