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USGS Current Conditions for Missouri
Aquifer--a geologic formation(s) that is water bearing; a geological formation or structure that stores and/or transmits water, such as to wells and springs. Use of the term is usually restricted to those water-bearing formations capable of yielding water in sufficient quantity to a well or spring to constitute a usable supply.
Local aquifers in the USGS water resources data base are identified by a USGS geohydrologic unit code. This code consists of a three-digit number which relates to the age of the formation, followed by a 4 or 5 character code which is an abbreviation for the geologic unit or aquifer name.
Generally, the smaller the number, the younger the geohydrologic unit. The 100 series refer to geohydrologic units from the CENOZOIC ERA, the 200 series refer to geohydrologic unit from the MESOZOIC ERA, and the 300 series refer to geohydrologic units from the PALEOZOIC ERA.
For example, a code of 210DKOT can be interpreted as follows:
The 210 series indicates the geohydrologic unit is from the CRETACEOUS SYSTEM of the MESOZOIC ERA.
The code DKOT refer to the DAKOTA SANDSTONE OR FORMATION OR GROUP.
Aquifer names and the definition of an aquifer can be very subjective. One rock unit may be called different aquifer names by different people. Local aquifers and layered aquifers are often grouped into larger named regional aquifers or aquifer systems. For example, the Northern Atlantic Coastal Plain aquifer system consists of five layered regional aquifers. Each regional aquifer is divided into two or more aquifers which may have a different name in each of the states in which the aquifer is found.