KUNSHAN, China—Chinese regulators are scrutinizing two suppliers to Apple Inc. and other gadget makers following allegations by environmental groups that plants run by the two companies are pumping large amounts of toxic heavy metals into nearby rivers.

The allegations cast a spotlight on this industrial area roughly 40 miles west of Shanghai that has long supplied the global electronics industry. They come as China's leaders and public are growing increasingly concerned about pollutants in China's soil, air and water.

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