The main reason why River Yamuna is polluted near Delhi is due to the addition of untreated sewage.

- Untreated sewage: One of the major sources of pollution in River Yamuna is the untreated sewage that is discharged into the river. The sewage contains harmful chemicals, bacteria, and other pollutants that contaminate the water, making it unsafe for human use and harmful to the aquatic life in the river.

- Population density: The high population density in Delhi and the surrounding areas results in a large volume of sewage being generated daily. The existing sewage treatment plants are not able to handle this huge volume, leading to a significant amount of untreated sewage being directly discharged into the river.

- Industrial waste: Industries located along the banks of River Yamuna also contribute to its pollution by releasing untreated industrial waste into the river. This waste contains various chemicals and toxins that further degrade the water quality.

- Impact on ecosystem: The pollution in River Yamuna has a detrimental impact on the ecosystem of the river. It leads to the death of aquatic life, loss of biodiversity, and contamination of groundwater sources. The polluted water is also a health hazard for the communities living along the riverbanks.

In conclusion, the addition of untreated sewage is the primary reason for the pollution of River Yamuna near Delhi. Addressing this issue requires proper waste management practices, the establishment of more sewage treatment plants, and strict enforcement of environmental regulations to prevent further degradation of the river.