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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 244, part 2
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Hydrostratigraphic Unit Delineation

Hydrostratigraphic units were originally defined by Maxey (1964) as bodies of rock with considerable lateral extent that act as a reasonably distinct hydrologic system. It is clear from Maxey's definition that hydrostratigraphic units were to be hydraulically continuous, mappable, and scale-independent entities. Mappability, in this case, means the subsurface geology can be subdivided according to permeability (Seaber, 1988). Thus, a single hydrostratigraphic unit may include a formation, part of a formation, or a group of formations.

From smallest to largest, the terms used to classify water-yielding rocks are zone, aquifer, and aquifer system (Laney and Davidson, 1986, p. 4). The terms are purposely vague (Freeze and Cherry, 1979, p. 49), so as to be sufficiently flexible to meet a variety of hydrogeologic scales and settings as well as the needs of the project (Jorgensen, 1982). A zone is used to identify a segment of an aquifer with a particular hydrologic characteristic that is not typical of the entire aquifer. An example of a zone is a highly permeable subunit of a regional aquifer that is traceable over an area covering tens to hundreds of square miles. An aquifer was defined by Lohman et al. (1972, p. 2) as "a formation, group of formations, or a part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs." Fetter (1994) suggested that to be an aquifer, the rock unit should have an intrinsic permeability of at least 0.01 darcy or a hydraulic conductivity of about 0.03 ft/day. An aquifer system was originally defined by Poland et al. (1972, p. 5) as "a heterogeneous body of intercalated permeable and poorly permeable material that functions as water-yielding hydraulic unit; it comprises two or more permeable beds separated at least locally by aquitards that impede ground-water movement but do not greatly affect the hydraulic continuity of the system." Laney and Davidson (1986) suggest that the definition could be made more general if the term "aquifers" was substituted for the term "permeable beds." A confining unit was defined in Lohman et al. (1972, p. 5) to be equivalent to the terms aquitard and aquiclude and has been used in U.S. Geological Survey reports since 1972 (Seaber, 1988). Jorgensen et al. (1993, p. B42) described an aquifer system as consisting of "two or more aquifers in the same hydraulic system, which are separated at most locations by one or more confining units." Likewise, they defined a confining system as "two or more confining units separated at most locations by one or more aquifers that are not in the same hydraulic system" (p. B42).

The Laney and Davidson (1986) Guidelines

The guidelines used to designate the proposed names for Kansas aquifer units in this paper are the same guidelines for aquifer nomenclature outlined for U.S. Geological Survey personnel in Laney and Davidson (1986). These guidelines resulted from informal discussions with hydrologists and hydrogeologists with the Water Resources Division and represent a consensus of opinion.

Laney and Davidson (1986) suggested two possible courses of action concerning the naming of aquifer units. The simplest is to leave aquifer units unnamed to avoid the unnecessary coining of new aquifer names. Not naming aquifer units is appropriate in areas where no formal lithostratigraphic units have been identified or where the hydrogeology is poorly known.

If, however, aquifers are to be named, the names should be based on lithologic, lithostratigraphic, or geographic names. Lithology-derived names are useful where there are no formally recognized lithostratigraphic units in the part of the section of interest. The adjectives for these aquifer names are based on lithologic terms (e.g., sand and gravel aquifer) and may be particularly useful for naming aquifers in glacial deposits. Lithostratigraphic names are appropriate for aquifers that may be statewide or more in extent--e.g., the Madison aquifer after the Madison Group of the northern Great Plains. Geographic names could be used as a basis for naming aquifers where no single lithostratigraphic name or combination of lithostratigraphic names would be appropriate, as in the Great Plains aquifer system of the U.S. Geological Survey's Central Midwest RASA project.

Laney and Davidson (1986) did not recommend deriving the names for aquifers or aquifer systems from the following sources: time-stratigraphic names, names based on relative position only, alphanumeric designation for model layers, depositional environment, depth of occurrence, acronyms, or hydrologic condition, such as confined or unconfined. They also did not recommend naming confining units unless such a name would add to the understanding of a complex aquifer system.

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Kansas Geological Survey
Web version December 6, 2000