There are many ways in which nature affects the water purification process. The first is through the way that rainwater isTorrentised. Rainwater falls from the sky and comes into contact with the Earth's surface. Fluids such as salt, oil, and bacteria burst out of the ground and attach themselves to raindrops as they fall through the atmosphere. This mixture of droplets and chemicals makes its way to the ground, where it quickly seeps through underground rocks and soil. The mix of soil, groundwater, and pollutants helps to create what are know as urban runoff basins. These basins can be found throughout large cities, and contain high concentrations of pollutants including bacteria, oil, and metals.
Urban runoff does not just affect major cities; it can also contaminate small bodies of water near urban areas. For example, treated effluent from a municipal wastewater plant may end up in a local creek or lakeside if it is not properly disposed of. This untreated sewage water