The Science of Cenotes
Cenotes are found in the Yucatán Peninsula, and are a type of freshwater-filled limestone sinkhole. As cenotes are filled with groundwater, the water that flows through them may be very slow, on the order of 1 to 1000 meters per year. In many cases the cenote is known to connect to an underlying cave system, and the water flow rates through the cave may be much faster, 1 to 1000 meters per day.
Cenote water is often very transparent and fresh, as the water comes from rain water infiltrating slowly through the ground, and therefore there is very little suspended particulate matter. Cenotes around the world attract cave divers since many of them are entrances to underlying flooded cave systems, some of which have been explored for lengths of 100 kilometers or more.
Cenotes are formed by dissolution of rock which creates a subsurface void, which may or may not be linked to a flowing cave system, and the subsequent collapse of the rock ceiling of the void. The rock that falls into the water below will then be slowly removed by further dissolution, creating space for more collapsing rock. The rate of collapse increases during periods when the water table is below the ceiling of the void, since the rock ceiling is no longer supported by the slight buoyancy of water completely filling the void. Cenotes may be fully collapsed, creating an open water pool, or partially collapsed, with some portion of a rock roof overhanging the water below. The stereotypical mature cenotes often resemble small circular ponds, measuring some tens of meters in diameter with sheer drops at the edges.
In coastal locations, notably along the Caribbean and Gulf coasts of the Yucatan Peninsula, the cave systems underlying some cenotes are known to flow out to the ocean, and therefore the cenotes are the entry points into the underground rivers of this area. Where a collapsing cenote has connected caves which penetrate deeply into a coastal aquifer, an interface between the meteoric (infusing) fresh water and the underlying marine salt water coming in from the coast may occur; the contrast in density between fresh and saline water creates a visible blurry swirling effect where these waters are mixed. This interface between the fresh and saline waters is often called the halocline, meaning that there is a sharp change in salt concentration over a small change in depth. It is also more simply referenced as the 'mixing zone'. The depth of the halocline is a function of climate, how much meteoric water recharges the aquifer, and the distance from the coast. Further inland, the halocline occurs at greater depths; in the Yucatan Peninsula this depth is 10 to 20 meters below the water table at the coast, and 50 to 100 meters below the water table in the middle of the peninsula, with saline water underlying the whole of the peninsula.
Information courtesy of Wikipedia.
