Last month, Chinese President Xi Jinping directed his government to build a new city for the "millennium to come." It would rise on rural land about 95 km south of Beijing, guided by the principles of "ecological protection and green development." And it would become a model for a new kind of urban expansion.

It was an attractive vision. Over the next few weeks, however, reports emerged of vast pollution in and around Xiongan, the area Xi hopes to develop. That's no surprise: China's four-decade economic boom has exacted a punishing price on the environment. But it does present an enormous challenge. Xiongan, intended as the green city of the future, will have to serve as a model for how China can clean up its past.

Although China's urban smog may get the headlines, water and soil pollution are just as bad in the countryside. Nearly 20 percent of farmland is dangerously polluted, and 80 percent of groundwater is undrinkable. City dwellers have often worsened these problems by pushing their most polluting activities — power generation, manufacturing, waste management — to the rural fringes where they can't be seen, heard or smelled. As China's cities expand, many of those once-hidden problems are now being exposed, and becoming nationwide scandals.