Hydropower plants are not a source of pollution in soil.

Explanation:

1. Transportation Sector:
- The transportation sector includes vehicles such as cars, trucks, buses, motorcycles, trains, ships, airplanes, etc.
- These vehicles emit various pollutants such as carbon dioxide (CO2), nitrogen oxides (NOx), sulfur oxides (SOx), particulate matter (PM), and volatile organic compounds (VOCs).
- These pollutants can contribute to air pollution, but they can also have indirect effects on soil pollution.
- For example, air pollutants can deposit onto the soil through dry or wet deposition, which can lead to the contamination of soil with heavy metals and other harmful substances.

2. Agriculture Sector:
- The agriculture sector includes activities such as farming, livestock rearing, pesticide and fertilizer application, etc.
- Agricultural practices can contribute to soil pollution in several ways.
- The use of chemical fertilizers and pesticides can contaminate the soil with harmful substances, including heavy metals and toxic chemicals.
- Improper disposal of agricultural waste, such as crop residues and animal manure, can also lead to soil pollution.
- Soil erosion caused by unsustainable farming practices can result in the loss of topsoil, which is rich in nutrients and essential for plant growth.

3. Thermal Power Plants:
- Thermal power plants generate electricity by burning fossil fuels such as coal, oil, and natural gas.
- The combustion of these fossil fuels releases pollutants into the atmosphere, including carbon dioxide, sulfur dioxide, nitrogen oxides, and particulate matter.
- These pollutants can contribute to air pollution, which can have indirect effects on soil pollution through deposition.
- Acid rain, which is caused by the emission of sulfur dioxide and nitrogen oxides, can also contribute to soil acidification and contamination.

4. Hydropower Plants:
- Unlike other sources mentioned above, hydropower plants do not directly contribute to soil pollution.
- Hydropower plants generate electricity by harnessing the energy of flowing water, such as rivers or waterfalls.
- This process does not involve the combustion of fossil fuels or the release of pollutants into the atmosphere.
- However, it is worth noting that the construction of hydropower plants can have environmental impacts, including the alteration of river ecosystems, displacement of communities, and changes in sediment flow, which can indirectly affect soil quality in the surrounding areas.