A Water Supplies Department (WSD) official has said that carcinogenic chemicals found in five local drinking water reservoirs may not come from China’s Dongjiang River, contrary to a suggestion put forward by researchers.

Environmental NGO Greenpeace found that water samples from three out of Hong Kong’s five biggest reservoirs—which hold water from Shenzhen’s Dongjiang River—contain a higher concentration of perfluorinated chemicals (PFCs) than those without Dongjiang water.

In high concentrations, PFCs known to cause cancer and damage the human reproductive system. The group said it suspected that industrial activity near the mainland river could be one of the sources.

However, WSD Chief Chemist Kelvin Kwok Yau-ting told RTHK radio on Wednesday that PFCs do not decompose easily and can remain in the soil for long periods of time. If human activity at the sites is high, the concentration of PFCs could be higher, and may not be attributable to the Dongjiang River.

Plover Cove Reservoir in Tai Po. Photo: Greenpeace.

Kwok said the WSD has conducted its own testing at the reservoirs surveyed by Greenpeace and found about the same concentration of PFCs—the highest measuring between two and six nanograms per litre.

However, he added that the PFCs levels in the samples were still relatively low, and that there is no data suggesting that current levels could threaten human health.

Kwok said the WSD will continue to examine water samples from reservoirs and water treatment works throughout the year. If a health risk is perceived, Kwok said, the department will take measures such as using activated carbon filters to absorb the harmful chemicals.