Aquitards are geologic formations in Earth's sub-surface that can store groundwater, but can not transmit them at fast enough rate to serve as viable groundwater sources. Aquitards are composed of clay, silt or rocks and act as barriers to groundwater flow from one aquifer to another. Unlike aquitards, aquifers are defined as sub-surface geologic formations that can store and transmit water at appreciable rates. When we bore wells in the sub-surface for irrigation purposes, we aim for the aquifers and hope to reach a large one, so that sustainable water extraction is possible. Aquitards are important for two very important reasons. They can act as storehouses of contaminants and serve as long-term sources of contaminants. They are also being explored for their potential to store carbon dioxide for underground CO2 sequestration.
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