According to Mr. Brown, founder of the veterans' organization Gulf Watch Intelligent Networking System, the US Military threw a pentration nuclear bomb (that explodes once underground) in that area east of Basra, probably with the intention of sending a strong message to the Iraqi President that they were determined to win the war by all means necessary.
It is the third nuclear weapon used ever in warfare apparently, after those thrown against Hiroshima and Nagasaki in WWII. The Basra bomb was smaller than those used against Japan, which had 16 and 22 kilotons respectively, and certainly much smaller than any intercontinental weapon whose destruction power is measured in megatons.
But still it was the use of a nuclear weapon, with painful consequences for the locals. Consequences that, as Brown denounces, are largely hidden by he widespread use of other radioactive weapons: depleted uranium ammunition. The problem of radioactivity effects in the Basra area has a long a story but it was so far believed it had been caused by depleted uranium ammo, not a full fledged nuclear bomb.