What are two precautions to reduce the risk of water pollution when mixing concrete on site?

The correct choice emphasizes the importance of maintaining a safe distance from watercourses while also ensuring that the area where concrete is mixed is impermeable. Keeping at least 10 meters away from a watercourse helps to prevent any potential runoff containing cement and other chemicals from entering the water, which could lead to significant environmental damage and harm to aquatic ecosystems. This distance provides a buffer zone that further reduces the risk of contamination.

Using an impermeable area for mixing concrete is equally critical, as it helps to contain any spills or excess materials that may occur during the mixing process. By preventing the concrete mixture from soaking into the ground, this precaution reduces the likelihood of leaching harmful substances into the soil and waterways.

In contrast, mixing near a watercourse would significantly increase the risk of pollution, and utilizing impermeable containers alone may not provide sufficient environmental protection without also maintaining an adequate distance. Mixing during rainy weather would heighten the risk of contamination due to water runoff. Finally, the suggestion of running water over the mixture and removing debris does not effectively prevent pollution and could complicate the mixing process while inadvertently causing more runoff.