A recreational diver found about 20 concrete containers filled with medical waste in a popular Connecticut River swimming hole, officials said. Environmental and local officials in South Hadley, Mass., said the containers, with syringes and needles sticking out of their tops, were dumped intentionally. They vowed stiff prosecution of the polluters. “They sure knew what they were doing. They made forms and poured concrete around the articles,” said police Lt. William Shenker of the containers found Tuesday.