The Environment Agency were alerted to a pollution of oil on the River Anton close to Andover College on 4 April.
Since then, Environment Officers have been working on site to determine the source and reduce the impact on the environment, and booms have been deployed across the river to contain the pollution.
Adam Fulton, Area Duty Manager for the Environment Agency, said:
Our Environment Officers are working hard at the site to manage the environmental impact, and at the moment we have had no reports of dead fish.
We will continue to investigate the source of pollution, and are asking anyone who might be able to assist our enquiries to call our Incident Hotline on 0800 807060.