That efforts to dilute the cadmium content in the Longjiang River in the Guangxi Zhuang autonomous region are still being made, more than two weeks after it was first discovered, shows the seriousness of the contamination.

Despite the optimism local officials have displayed about the safety of the water supply in the city of Liuzhou and areas surrounding the lower reaches of Longjiang River, it is legitimate for local residents to question why local authorities withheld information about the pollution for days before telling the public.

Apart from the efforts to dilute the cadmium content and to supply residents with enough safe water for daily use, local authorities need to investigate thoroughly the root cause of the incident.

Seven people suspected of being involved with the contamination are being held in custody, but it is still not clear which enterprise was the main source of the pollutant. This points to lax oversight by the local environmental protection authorities over the heavy metal producers in Hechi city, where the contamination took place.

That the source of pollution is yet to be identified 15 days after such a serious contamination of a major local river sends the message that there is much to be desired when it comes to work of the local environmental protection authorities.

The authorities have argued that the karst topography of the region means there are a number of rivers and streams flowing underground, which makes it very difficult to find out whether a heavy metal plant is discharging pollutants through them.

But if that is the case, the question is whether such a location is suitable for such enterprises in the first place.

And as the industrial plants are there, local environmental protection authorities should have been responsible for making sure that such heavy metal plants treat their waste before discharging it.

So the investigation should not merely endeavor to find out the plant responsible and how it managed to flout the relevant regulations and discharge the pollutant. It also needs to find out whether the local environmental authorities are culpable and why they failed to assess the environmental impact of these enterprises before they started operations.

It is not the first time that hazardous chemicals have been leaked or dumped by enterprises into rivers. This incident should be a wake-up call to the rest of the country, those places with chemical plants in particular.

Now seven heavy metal plants in the city of Hechi have been suspended from production and 11 more are required to make amends for deficiencies in work safety or their production licenses will be revoked. Other places should learn from this lesson and tighten supervision over chemical enterprises to prevent similar incidents occurring.