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At times, Zhang uses his Blackberry to take pictures and occasionally throws a stone into the river to see its effect.
"If the river is polluted, there will be clearly shaped foam," Zhang said.
But in comparison, the shortage of water is a far more serious problem than pollution, Zhang continued.
"Rivers are drying up. In many years we may end up drinking the water from the dirty rivers."
Zhang said he and other hikers would trace a river to its upper stream and water source, only to find them drying out.
"Usually we find rivers dry up before reaching the urban areas and then the water volume suddenly increases with a large amount of water injected from drains and sewage," Zhang said.
"Initially I thought that through our hiking activities we could make our voices heard by the water authorities, and enable them to be better aware of the pollution problems.
"Gradually I realized that the pollutants in the river actually come from our way of life, from the cosmetics and the detergents we use in our daily lives. It has a lot to do with our lifestyles."
Zhang arranges the route for each trip in advance and answers questions from participants along the way.
So far his hiking trips have covered two-thirds of the rivers and most of the reservoirs in Beijing. He is also planning to come up with a detailed map of rivers in Beijing, including pollution, residential communities and the factories along the way.
"But it is a huge project with numerous statistics to be gathered," Zhang said, adding that he can only work on the project during his spare time.
According to Zhang, not all the participants are environmentally zealous.
"Some come to exercise, some to socialize," Zhang said.
However, that does not matter to him.
"I only want the participants to know that there is a different way to learn about nature and the environment, which is by seeing it with their own eyes and getting firsthand experience," Zhang said.
"Seeing is believing," said Cheng Mengxiang, a 60-year-old Beijing resident who admitted to having first joined the trips for exercise.
Now, as a regular participant, he tells his family to conserve water after seeing how water is drying up along the rivers.
"I also tell my co-workers to stop drinking bottled water as one bottle of water requires five times that amount to produce," Chen said.
For Zhang, that is the reason he continues the hikes.
"Although I can't make a living with this, I think it is well worth the effort."