Which of the following does not cause water pollution? 

The correct answer is  Gravels 

Important Points 

Water pollution

• Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater.
• It is the contamination of water bodies, usually as a result of human activities.
• Water pollution results when contaminants mix with these water bodies

Gravels in water:

• The gravel or stone are vital to the role of the trench.
• Using gravel or small stone enables the water to flow through them slower, allowing the soil to absorb more water over time and limiting runoff and eventually reducing water pollution.

Additional Information

Water Pollution from paints:

• All paints, solvents and adhesives contain chemicals that are harmful to aquatic life in creeks, lakes and the ocean
• Leaching from paint products happens as water washes down a painted surface or as paint deteriorates, allowing chemicals to bind to runoff waters and find their way into soils, thus leading to soil and plant contamination and potential groundwater pollution.

Water Pollution from Silt:

• Siltation is water pollution caused by particulate terrestrial clastic material, with a particle size dominated by silt or clay.

Water Pollution from detergents:

• Regular laundry detergents are bad for the environment 
• Detergents don't completely biodegrade and they contaminate our water supplies, rivers, and oceans with toxic heavy metals like cadmium and arsenic.