The true scope of the problem from urban sources is still not very well understood, but it is clearly a major contributor. We do not discuss it in detail here because we have already covered it in a previous section. Nevertheless, it is a classic example of the multi-media role of wastewater pollution. This section also analysis types of wells. Horizons are horizontal layers of different soils.
Profiles refer to different patterns of soil horizons.
For example, the topsoil may be very thin or even missing in some soil profiles. Humus refers to the organic material in soil 6.