Give an explanation (with diagram) of the natural filtration process that purifies ground water.
The water after falling on the surface of Earth through precipitation (typically rain) then permeates through the top soil layer and and filters down to the sub-surface. This movement may not always be vertical; it may be a combination of horizontal and vertical motion. It depends on a number of factors, such as soil porosity and ground slope.
The purification of water takes place through a number of processes, including filtration, metabolism of organic matter by soil microorganisms, adsorption onto soil particles, and predation of microbes by higher order microorganisms. The filtration itself occurs through a number of processes, the most common of which is physical straining. In simpler terms, there is a definite gap between adjacent soil particles, called pore space. Any particle or contaminant bigger than this inter-particle spacing will get stuck and hence removed from the infiltrating water. The attached image shows the particle straining through subsurface soil.