The underground drainage of the Mendip Hills is focused on a series of large springs along the foot of the Mendips. These generally occur at the lowest points in the Carboniferous Limestone or the overlying Triassic Dolomitic Conglomerate aquifers.
The links between the stream sinks on the Mendip plateau and the springs has been proved by many experiments using a series of different ‘tracers’ including chaff, ink, copper sulphate, fluorescent dyes and optical brighteners (the chemical which makes your clothes appear whiter than white).
The chemistry of the spring water can also give an indication of its source. For example, springs with a very high calcium carbonate content are often fed by a diffuse input rather than point sources such as stream sinks.