Despite the decline of acid rain, its legacy still taints the rivers of the eastern US, but in an unexpected way. Following stringent air pollution controls, the acid rain that devastated forests, ponds and small streams in the eastern US has been diminishing since its peak in the 1970s.
Now the opposite problem, excessive alkalinity, has emerged in the same area. New researchhas found that 62 of 97 large rivers, from New Hampshire to Florida, have become increasinglyalkaline since the mid 20th century. "Alkalinity is typically thought of as a good thing," says Sujay Kaushal at the University of Maryland in College Park, but it can stimulate the overgrowth of algae and wreak havoc with public water supplies.
It looks like alkaline by-products of acid-neutralising processes had built up in the rocks and soil, and are now leaching into the rivers.