Authorities in China are investigating two Taiwan-based Apple suppliers — Foxconn and UniMicron — following accusations last week made by Chinese activist Ma Jun and five nonprofit organizations that the companies were releasing water tainted by toxic metals into the rivers, the Wall Street Journal reports.

Ding Yudong, the vice director of the local arm of China’s environmental regulator, told WSJ that local officials are carrying out investigations into the plants owned by Foxconn and UniMicron in an industrial area about 40 miles west of Shanghai, called Kunshan. The plants there are named on Apple’s 2013 supplier list.

The probe may take some time, but Ding told WSJ that if the plants are found to be “severely exceeding emissions standards,” authorities will punish the companies involved.

Foxconn, the trading name of Hon Hai Precision Industry, is the world’s largest electronics contract manufacturer by revenue, and has taken on most major electronics brands as its clients. It is most widely known as Apple’s largest supplier.

The WSJ report noted that Foxconn insisted it complies with emissions standards, while UniMicron said it inspects wastewater daily and has hired a third party to inspect its wastewater quarterly.

An Apple spokeswoman told WSJ that the company has been working closely with suppliers and one of the environmental groups to make sure that environmental standards are being met, adding that Apple regularly audits its suppliers and will not tolerate violations.

Since an initial wave of criticism in 2011, Apple started producing reports and currently maintains a greater level of transparency than most other companies, including those that use Apple suppliers like Foxconn. Last year, Apple also asked the Fair Labor Association (FLA) to “conduct special voluntary audits of Apple’s final assembly suppliers, including Foxconn factories in Shenzhen and Chengdu, China.”

However, the company has once again run into problems with its suppliers recently. Last month, a report by China Labor Watch accused Apple supplier Pegatron of “serious” labor violations in three factories, including the employment of underage workers, insufficient wages and poor working conditions.

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