Numerous empty bottles, plastic food items and other rubbish with simplified Chinese characters were found washed up on Hong Kong's beaches.
The volunteers' report suggests that the five most polluted beaches were Sha Chau and Lung Kwu Chau Marine Park, Discovery Bay, Pak Kok Tsuen and Yung Shue Wan on Lamma island, Chi Ma Wan on Lantau island south, and Fan Lau on the southwest side of Lantau island.
ERC manager Carol Liu Shui-kam says the simplified Chinese characters on rubbish suggests that it is from the mainland.
"Most of the more polluted beaches are located in the southwest of Hong Kong," she says.
"Given that the wind blows from north to south during Hong Kong's winter, the rubbish could come from the east coast of mainland China."
The campaign was started last July, when ERC teamed up with two other city-based environmental groups - Lamma Corner and Green Power - to help clean up millions of plastic pellets, which fell from a transport ship during last July's typhoon and washed up on the city's beaches.
Although most of the sites were cleared of the pellets by September, the groups continued to send voluntary squads to observe and help collect rubbish from beaches.
Liu says ERC has not yet notified the Hong Kong and central governments about its findings. But it plans to make a full report after it has carried out further scientific research and gathered more evidence of pollution on beaches in the coming months.