Final answer:
The soil at the bottom of a retention basin contains organic matter, native plants, and soil organisms that filter nutrients and pollutants from the water, thereby reducing environmental runoff effects and enhancing water quality.
Step-by-step explanation:
The soil at the bottom of a retention basin is integral in reducing pollutants in rainwater. It does this by utilizing organic matter, native plants, and soil organisms, such as bacteria and zooplankton, which filter nutrients and small particles from the water. These natural filters work together to break down and absorb excess minerals and pollutants such as carbon, nitrogen, phosphorus, and sulfur, which are cycled from land to water through rain and surface runoff. Healthy soil rich in organic matter not only helps in pollutant reduction but also in water retention, which supports a diverse ecosystem and assists in preventing soil erosion.
Retention basins themselves are designed to store excess floodwater, slow down runoff, help prevent erosion, remove excess nutrients before water enters rivers or lakes, and they can also be habitats for diverse plant and animal life. The soil's ability to hold water thus complements the basin's function and enhances the quality of water that eventually becomes part of surface or groundwater systems.