The Bush administration tucked its tail between its legs and decided yesterday to toughen the limit for arsenic in drinking water from 50 parts per billion to the 10 ppb. The stricter standard was originally approved by the Clinton administration, but then rejected by current U.S. EPA Administrator Christie Whitman, who questioned the science behind it and said the cost would be prohibitive for small communities. Environmentalists denounced her decision, and held it up as a shining example of the Bush administration’s hostility toward the environment. Whitman changed her tune following a public comment period and a report by the National Academy of Sciences that indicated that the risks of developing cancer and other health problems from arsenic-contaminated water — even at 10 ppb — are much greater than previously thought.