SHANGHAI—The seepage of chemicals into a river connecting two north China provinces is the latest accident to highlight the country's serious water-pollution problem, and to raise doubts about whether authorities acted quickly to raise the alarm.

More than 39 tons of a chemical compound called aniline used in manufacturing and agriculture leaked into a disused reservoir and a local river in Shanxi province after a pipe burst late last month, China's state-run Xinhua news agency reported Sunday. Almost a quarter of that amount...