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Carbonate aquifers

The key to understanding carbonate aquifers is to account for flow in the three permeability elements - the matrix, the fractures, and the channels. The block diagram above shows these three elements.

The matrix is the rock between the fractures. It usually accounts for most of the aquifer's storage capacity for water, but the water moves much slower than in the other two permeability elements.

The fractures were created by mechanical forces such as those associated with plate tectonics or with unloading after the melting of the ice at the end of the last glaciation. Most fractures have apertures of much less than 1 mm.

The channels are enlarged fractures, and have been enlarged by dissolution of the bedrock. Groundwater in carbonates typically has 100 - 300 parts per million of dissolved calcium carbonate. Most of the rock dissolved is close to the surface of the bedrock, but the last few percent of dissolution always take place deeper in the bedrock, often up to tens or hundreds of meters below the water table.

A most important consideration is that the channels form an interconnected tributary network. The smaller upgradient channels, most often in the millimeter to centimeter range, are numerous and are the ones most frequently encountered in water wells. Water wells in carbonates usually interersect one or more of these solutionally-enlarged fractures, which provide most of a well's yield. Velocities in these channels can reach hundreds of meters per day. Channels further downgradient are larger, and are usually called conduits (diameter >1 cm) or caves (diameter >1 m). However, few wells intersect such large channels.

Further information can be found at:

A comprehensive strategy for understanding flow in carbonate aquifers gives an overview of the permeability structure in carbonate aquifers and of suitable testing techniques

Depth of conduit flow in unconfined carbonate aquifers explains why dissolution takes place up to hundreds of meters below the water table.