GUIDELINES FOR NAMING AQUIFERS 

T HE FOLLOWING GUIDELINES for naming 

aquifers are based on many discussions among 

Survey hydrologists (Laney and Davidson, 1986). An 

essential requirement for evaluating the hydrologic 

properties of in-place earth materials is to define and 

map hydrogeologic units-aquifers and confining 

units-on the basis of relative permeability. Hydrogeologic units are determined indirectly through 

studies of the geologic materials (geologic mapping, 

surface geophysical surveys, borehole geophysical 

logs, drill cuttings and core descriptions, and so forth) 

and through hydrologic testing (aquifer tests, laboratory permeability tests on core samples, and so forth). 

The physical properties of all rock units change 

laterally and vertically. Unconformities and faults 

may affect the flow of ground water. The process of 

designating and naming aquifers and confining units, 

therefore, is somewhat subjective and can lead to confusion if not thoroughly documented. Guidelines for 

naming aquifers can help you avoid confusion and 

problems associated with hydrogeologic studies if the 

guidelines are straightforward and flexible and apply 

to varied scales from site specific to regional. The 

guidelines that follow include (1) discussions of the 

aquifer nomenclature, (2) the definition of the hydrogeologic framework, (3) the recommended procedures 

for naming aquifers, and (4) examples of naming 

aquifers. In this section the terms "rock-stratigraphic" 

and "time-stratigraphic" are used (see p. 45). 

scale, an aquifer system could be defined totally within a single formation, but at the regional scale an 

aquifer system could consist of several formations. 

The guidelines must be flexible to meet a variety of 

hydrogeologic scales and settings. 

A brief discussion of the terms "aquifer," "aquifer 

system," "zone," and "confining unit" provides a 

common reference base. Agreement on definitions is 

not complete, but the terms are adequate to transfer 

knowledge to readers of reports. These guidelines are 

not intended to formally redefine the terms or to 

define new terms to take their place. 

The term "aquifer" probably has more shades of 

meaning than any other term in hydrology (Freeze 

and Cherry, 1979, p. 47). It can mean different things 

to different people and different things to the same 

person at different times. Meinzer (1923, p. 52-53) 

defined an aquifer as follows: 

A rock formation or stratum that will yield water in sufficient 

quantity to be of consequence as a source of supply is called an 

"aquifer," or simply a "water-bearing formation," "water-bearing 

stratum," or "water-bearer." * It is water bearing not in the 

sense of holding water but in the sense of carrying or conveying 

water. 

Lohman and others (1972) refined Meinzer's defini. 

tion as: 

A formation, group of formations, or part of a formation that contains sdficient saturated permeable material to yield significant 

quantities of water to wells and springs. Because aauifers do not lend themselves to neat 

and simple definitions, a flexible hierarchy of terms is 

used for largest (regional) to smallest (local) wateryielding units, as follows: 

Aquifer system (Poland and others, 1972), 

Aquifer (Lohman and others, 1972), and 

Zone (R.H. Johnston, written commun, 1985; 

Miller, 1986). 

Parallelism should be avoided in the hierarchy of 

terms for water-yielding rocks and rock-stratigraphic 

terms-aquifer system (group), aquifer (formation), 

and zone (member)-because water-yielding rocks can 

cross geologic boundaries or can constitute only part 

of a geologic unit. The scale of the study also may 

determine the best usage; for example, at the local 

Both definitions imply that the aquifer is bounded 

by or is included within the formation(s) (or stratum), 

but the concept of the aquifer extending across formational boundaries is not indicated explicitly. In 

many local study areas covering a few tens to a few 

hundreds of square miles, the aquifer and the formation may be the same, and defining the aquifer in 

such areas may present few problems. Since the late 

1970's, however, regional aquifers that cover hundreds of thousands of square miles have been studied 

under the Regional Aquifer System Analysis (RASA) 

Program, and results of the these studies have shown 

that (1) regional aquifers may include many formations and rock types and (2) the aquifers may cut 

across formational and lithologic boundaries so that 

Guidelines for naming aquifers 65 
no one formation is completely representative of the 

aquifer. In studies of regional scope, the shapes and 

the boundaries of the permeable rocks that form the 

aquifer have greater importance to understanding the 

flow system than do the individual formational boundaries. A definition that places less emphasis on the 

formal term "formation" (see North American Commission on Stratigraphic Nomenclature, 1983) and 

more on "permeable rocks" has merit. For example, 

aquifer is defined in the "Glossary of Geology" (Bates 

and Jackson, 1980) as follows: 

A body of rock that is sufficiently permeable to conduct ground 

water and to yield economically significant quantities of water to 

wells and springs. 

Regardless of the fine points in any definition, 

delineating permeable rocks should be a major goal 

of hydrologists mapping and describing an aquifer. 

By the same token, detailed knowledge of the stratigraphic units and postdepositional processes (such as 

solution, cementation, folding, and faulting) are essential in determining the boundaries of the aquifer and 

in understanding the flow system. In addition, 

hydraulic properties (hydraulic conductivity and 

storage coefficient) are usually estimated indirectly 

by aquifer tests, analyses of drill cuttings and cores, 

borehole geophysical logging, and surface geophysical 

surveys. In many situations, hydrologic estimates and 

extrapolations can be based on rock type alone without any determination of hydrologic properties. For 

example, a widespread, thick clay separating two sand 

units could be designated tentatively as a confining 

unit on the basis of geologists' logs and borehole 

geophysical logs alone without any hydrologic data. 

Poland and others (1972) defined "aquifer system' 

as follows: 

A heterogeneous body of intercalated permeable and poorly 

permeable material that functions regionally as a water-yielding 

hydraulic unit; it comprises two or more permeable beds [aquifers] 

separated at least locally by aquitards [confining units] that impede 

ground-water movement but do not greatly affect the regional 

hydraulic continuity of the system. 

The definition of Poland and others would be more 

general if the term "aquifers" were used in place of 

"permeable beds." "Bed" implies a single stratigraphic unit, whereas the individual aquifer could include or 

cross many "beds." "Confining unit" should be used 

instead of "aquitard" because the definition of confining unit is broad enough to include varying degrees of 

"leakiness." 

The hierarchy of aquifer and aquifer-system names 

may not always be consistent in practice. Because 

of differences in scales of investigations, different 

authors may combine individual aquifers into a single 

aquifer system, which may be only a part of yet 

another even larger aquifer system of a larger area. 

Authors are responsible for explaining these relationships clearly by means of comparison charts and text 

descriptions. 

The term "zone" may be used to subdivide an 

aquifer to delineate a hydrologic characteristic that is 

not typical of the entire aquifer. For example, the 

"Fernandina permeable zone" is a highly permeable 

subunit of the Lower Floridan aquifer (Miller, 1986, 

p. B70). The zone consists of vuggy, locally cavernous 

limestone that is traceable for as far as 100 miles in 

coastal Georgia and Florida. The permeability of the 

zone greatly exceeds that of most of the Lower 

Floridan aquifer. 

Lohman and others (1972) defined the term "confining bed" as a body of "impermeable" material 

stratigraphically adjacent to one or more aquifers. In 

nature, however, its hydraulic conductivity may range 

from nearly zero to some value distinctly lower than 

that of the aquifer. Its conductivity relative to that of 

the aquifer it confines should be specified or indicated 

by a suitable modifier, such as "slightly permeable" 

or "moderately permeable." 

Although the term "confining bed" (Lohman and 

others, 1972) is descriptive and could be used, the 

term "confining unit" is more appropriate, especially 

if more than a single bed makes up the confining 

unit. The term "bed" is incorrect usage for a thick 

sequence of stratigraphic units that could be of member or formation rank. Bed is particularly inappropriate when used for intrusive igneous rocks beneath 

an aquifer. The term "bed" has a formal definition 

(North American Commission on Stratigraphic 

Nomenclature, 1983, art. 26) and should not be 

used in definitions of aquifer nomenclature. 

Many confining units are leaky under natural conditions and may contribute significant amounts of water 

to the aquifers they confine, or they may contribute 

even larger quantities of water if heads are lowered 

in the aquifer by pumping. Where withdrawals from 

aquifers have caused large declines in head, considerable water may be derived from storage in the confining unit. Poland and others (1972, p. 2) retained the 

terms "aquiclude" and "aquitard" in their definitions 

66 Aquifer system 
related to the mechanics of aquifer systems and land 

subsidence caused by fluid withdrawal. An aquiclude 

is defined as a body of saturated but relatively impermeable material that has very low values of "leakance" (the ratio of vertical hydraulic conductivity to 

thickness) and that allows negligible interaquifer 

flow. An aquitard is a saturated, poorly permeable 

bed whose values of leakance range from relatively 

low to relatively high. Where an aquitard is sufficiently thick, it may form an important ground-water 

storage unit. 

For Survey reports, the general term "confining 

unit" is preferable to "aquitard," "aquiclude," and 

"aquifuge". The "leakiness" of the confining unit 

should be discussed if it can be estimated. 

Terms intended to be synonymous with "aquifer" or 

"aquifer system" should be avoided. Terms such as 

"hydrofer" or "aquiformation" should not be used. 

"Aquigroup" should not be used in place of "aquifer 

system." The term "aquifer" may lack precision, but 

it has wide use and acceptance in the hydrologic literature. Coining new terms that are synonyms of 

aquifer and aquifer system or that propose slightly 

different meanings only creates confusion, especially 

among nonhydrologists. Use of the term "aquiformation," moreover, infers an equivalence between 

aquifer and formation that is not always correct. 

DEFINITION OFTHE 

In hydrogeologic studies, as in purely geologic 

investigations, the orderly, consistent designations of 

pertinent parts of the framework are essential to a 

clear reporting and understanding of the study 

results. In ground-water studies these designations 

involve defining and correlating water-yielding 

units and relating those units to established rockstratigraphic units. Survey authors of reports on 

ground-water resources must follow the North 

American Stratigraphic Code for designating rockstratigraphic units, just as authors of purely geologic 

reports do. The authors of ground-water reports also 

must identify significant water-yielding parts of the 

geologic framework. Commonly, the water-yielding 

parts do not correspond exactly to named geologic 

units and, therefore, do present additional nomenclatural problems. Exhaustive systematic guidelines 

for naming geologic units have been developed over 

several decades by the North American Commission 

on Stratigraphic Nomenclature, but comparable 

guidelines have not been developed for naming wateryielding units. The proper designation of hydrogeologic units involves the consistent use of ground-water 

terms as well as the actual naming of the units. 

One of the first considerations in describing an 

aquifer in a report is mappability. The aquifer should 

be mappable at the map scale used in the report of 

the study area, but thin, highly transmissive aquifers 

that cannot be easily mapped at the principal map 

scale may still be important hydrologically. The report 

also should contain comparison sharts; maps of the 

tops, thicknesses, and geographic extents of the 

aquifers; and hydrogeologic sections. Hydraulic 

characteristics should be discussed to show how the 

aquifers differ from the underlying and overlying 

confining units. 

If additional information is needed to clanfy the 

characteristics of an aquifer in the third dimension, a 

"type area" or "type locality" and (or) a "type well" 

can be described. Several surface exposures and wells 

may be required to characterize the aquifer if its 

characteristics change greatly vertically and laterally. 

If so, selected surface exposures and wells can be 

used to illustrate important aspects. For example, 

grain size, bedding thickness, faulting, folding, and 

effects of fracturing or solution may affect movement 

and storage of ground water. Borehole geophysical 

logs, drill cuttings and core descriptions, and drillers' 

and geologists' logs for wells can illustrate hydrologic 

properties in the subsurface. 

A comparison chart is an essential part of a report 

that describes a ground-water flow system and 

aquifer names, and it should consist of three major 

components: 

1. A correlation chart that shows rock- and timestratigraphic (geologic) units for the water-bearing 

materials described in the report. 

2. A comparison of hydrogeologic units to layers 

used in a digital flow model (if one is used). 

3. A comparison of hydrogeologic units of the report 

with those in previous reports. 

The amount of detail in the comparison chart depends 

on the scale and complexity of the investigation. If the 

report contains only a few geologic and hydrogeologic 

units, the comparisons may be shown in one illustration. The comparison chart should make clear to the 

reader the relationships of the hydrogeologic units to 

the geologic units (and to equivalent layers in the 

computer flow models if flow models are included in 

the report). For complicated investigations that involve 

many geologic and hydrogeologic units, two or three 

illustrations may be needed to show the comparisons. 

Definition of the hydrogeologic framework 67 
GEOLOGIC UNIT 

usaged6ureauofGeologl.FloridaDepar 

2Cda~mupof~of~eology.Florida 

3 Crystal RNBT Fonnaimn of Oda Gmup. 

4 lnglii Formabon and WlHion Formatan (d 

.- 

Z 

a 

2 

-1 

rmahe W Md. unamd&W and generally w>add. 

0-23 Ako. some Matile and bastrine sand. day, marl. and peal depo& 

Nonmarine interbedded day. sand. md sllndy day; much d unil is ' .h I drdmzd4~dpnopl*lOb.andsiI'WI~~mina~ I 

u5 

E 

Ib " * 

;; 

and green phosphatic day mabix 

Non~nne ftuviatik sand, white to gray, vecregiacrd orange, pnple. and red in upper part. 

0-30 fins lo coarsegrained to pebbly, dayey, crossbedded 

w 

and 

Lomn 

htacems 

rods 

Marine hwbddd sand. aeam. whine, and gray, ph&phmk commonly pyey: day. 

green 

m, 

to gray and white, phosphatic. often sandy: dolomiie, cream to whne and gray. 

sandy, clayey: and some limestone, hard. dense. in part sandy and 

phosphatic. Tends to be sandy In upper pan and dolomi and limy in lowef part 

-u-nr0-46 Merim limestone. my pale orange. thly crystalline, anall amounls of sin and day 

Marine linmtotte, seam to wMe. dl granular. hlghly porws. coquinal; often consists 

almost entirely of tests of fwaminifefs; cherty in phs 

m-, part at places is dobmiie. gray and bmwt-~, crystalline. ~~roidal. I 

I or peaty; gypsum is pesenl in small amounts I 

ILL ~arina rcnes)ona, ligM kom, to dlaky, white. porous, lossiliferous, with inmbddd I -. porous. crystalline dolomite; minor amounts of anhydrite and gypsum I 

Marine ddomii. ligM gray. hard, slightly porws to porws. crystalline, in part I ----- 

I-l / tossiliierws. with comideraMe anhydrite and gypsum, some limestone 

BasemaR Mafhm Devonien. Sihntan, and ' ' qwmose sendrnone and dark eMe. lawsr PabdcV) 

, r I arFmcambnan(?)~e,M~~b~e 

C 

451-7 

nent 04 Nalural Aeswrces. 

leparbnentc4NaluralReswroes. 

~marineL)pper~~andevapaate~.sandsmdshales;tMnLomn 

Cretacsous dasik section in some of arm 

BT to younger) d Ocala Gmp 

Figure 17. A chart comparing geologic units, hydrogeologic units, and equivalent units in a digital ground-water 

flow model (from Tibbals, in press). 

68 Definition of the hydrogeologic framework 
PRINCIPAL EQUIVALENT LAYER 

HYDROGEOLOGIC IN DIGITAL 

I Lower confining unit I 

A comparison chart that interrelates geologic units, 

hydrogeologic units, and model layers is shown in 

figure 17. Figure 18 compares geologic and hydrogeologic units with those in previous reports. A chart 

such as figure 18 is especially important in reports 

that redefine and rename aquifers. Figure 19 summarizes hydrogeologic units made up of many rockstratigraphic units. Unlike figure 17, figure 19 places 

the hydrogeologic units on the left side of the chart 

and combines the rock-stratigraphic units on the right 

side. This chart emphasizes hydrogeologic units 

primarily and rock-stratigraphic units secondarily, 

although considerable analysis of rock-stratigraphic 

data from throughout the study area was required to 

develop the chart. Such an analysis of time- and rockstratigraphic units in a correlation chart should be 

shown as a separate illustration because of the great 

number of rock-stratigraphic units to be considered. 

To prepare a comprehensive comparison chart, you 

must search the literature for all previous studies that 

contain rock-stratigraphic names and aquifer names in 

the project area. Your comparison chart should contain the following items: 

Headings entitled erathem, system, series, rockstratigraphic unit, thickness, lithology, hydrogeologic unit, and hydrologic characteristics. 

The geologic units that are pertinent to the 

hydrology. 

The hydrogeologic units that you are using and 

how they relate to geologic units and previously 

named hydrogeologic units. 

A column that shows relations of hydrogeologic 

units to layers in the flow model, if one is included in the study. 

Only the geology that pertains to the hydrology 

under study should be discussed and shown in detail. 

Your discussion should be limited mainly to what 

affects the movement and storage of ground water. 

An exception could be made if details of the stratigraphy were not well known before your hydrologic 

study, and hence by describing the hydrogeologic 

units you have clarified an understanding of the 

stratigraphy. 

Differences of opinions as to what should constitute 

aquifer@) and confining units(s) may still exist among 

hydrologists after your report is published. However, 

no uncertainty should exist as to what you include in 

your definition of the aquifer(s) and confining unit@) 

and their relation to geologic and hydrogeologic units 

in previous investigations. 

Aquifer names used currently within the Survey, 

rightly or wrongly, are derived from the following 

sources: 

Rock-stratigraphic terms (Sparta aquifer). 

Geographic features (High Plains aquifer, Floridan 

aquifer). 

Time-stratigraphic terms (Cambrian-Ordovician 

aquifer). 

Lithology (limestone aquifer). 

Deriving aquifer names 69 
Depth of occurrence ("500-ft" sand in the Memphis area). 

Depositional environment (shallow marine aquifer, 

glacial aquifer). 

Alphanumeric designations for model layers (A1 

aquifer layer, C1 confining layer). 

Relative position (upper carbonate aquifer). 

Unusual locations (Clinton Street-Ballpark 

aquifer). 

Unusual geologic features of rock exposures 

(bird's-nest aquifer). 

The many ways in which aquifers have been named 

and the varied scales of hydrologic investigations are 

among the causes of confusion regarding aquifer 

nomenclature. Until the advent of the RASA Program, few ground-water studies were areally large 

enough to face the problems of extending local 

aquifer and stratigraphic nomenclature to a regional 

scale. The gradational changes that are commonplace 

in geologic materials complicate the work of hydrologists who are trying to define aquifers and related 

confining units. At a study scale of a few tens to a 

few hundred square miles, gradations in the physical 

properties of the rocks are often not obvious, and 

because of the relative uniformity of the rocks within 

such an area, where a stratigraphic unit may make up 

an entire aquifer, rock-stratigraphic names can be 

applied to aquifers. At the scale of many of the 

RASA studies, however, differentiating regionally 

extensive units of relatively high or relatively low 

permeability becomes a problem within a sequence of 

rock units whose relations and variability are frequently complex, and whose names may change at 

political boundaries. 

If your report involves hydrogeology, the Survey 

recommends that you first consider not naming aquifers. If aquifers are already named, or if the extent of 

an aquifer is reasonably well known, aquifer names 

should be derived from the following sources: 

Lithologic terms (sand and gravel aquifer). 

Rock-stratigraphic names (Sparta aquifer, after 

the Sparta Sand). 

Geographic names (High Plains aquifer, for the 

permeable parts of the Ogallala Formation and 

overlying and underlying hydrologically continuous 

deposits in parts of eight States; Floridan aquifer 

system, for permeable parts of several Tertiary 

carbonate formations in the Southeastern United 

States). 

The Survey recommends that aquifer or aquifersystem names not be derived from the following 

sources (although some have been so-derived in the 

past): 

Time-stratigraphic names (Cretaceous aquifer). 

Relative position names (upper carbonate aquifer). 

Alphanumeric designations for model layers (A1 

aquifer layer, C1 confining layer). 

Depositional environment (shallow marine aquifer, 

glacial aquifer). 

Depth of occurrence ("500-ft" sand). 

Figure 18. A chart comparing geologic and hydrogeologic units with those in previous reports (from Miller, 1986). 

70 Deriving aquifer names 
Acronyms (the first letter of each formation in a 

multiaquifer system). 

Hydrologic condition ("principal artesian aquifer"). 

Each of these preceding sources of aquifer names is 

discussed in the following sections. 

Authors of reports on hydrogeology can handle 

aquifer nomenclature in the following ways: (1) Do 

not name the aquifers or (2) name the aquifers after 

lithologic terms, rock-stratigraphic names, or geographic names. 

Water-bearing properties of rocks can be described 

without naming aquifers. Each rock unit and its 

water-bearing properties can be described in comparison charts and tables. Phraseology would be the 

principal difference between a report of this kind and 

one describing named aquifers. This approach could 

be used in studies involving both formal and informal 

rock-stratigraphic names, but it would apply particularly to areas where no formal rock-stratigraphic units 

had been designated or where the stratigraphy and 

the hydrology of the particular rocks are poorly 

known. Not cluttering the literature with aquifer 

names is advantageous if the hydrogeology of an 

area has not been studied in great detail, if a study 

describes an area in a cursory or reconnaissance 

fashion, or if a study area is so small that only a 

small part of an aquifer is investigated. 

If aquifers must be named, use lithologic or rockstratigraphic names to the extent permitted by 

permeability distribution and hydrologic continuity. 

Use geographic names for larger areas where lithologic or rock-stratigraphic names are inappropriate. 

For example, where an aquifer consists of a single 

rock-stratigraphic unit, the rock-stratigraphic name 

may be used for the aquifer. If a later study encompasses a larger area, judgment would be needed to 

determine if the rock-stratigraphic name remains 

appropriate. If the aquifer in the larger area consists 

of the same rock-stratigraphic unit as in the smaller 

area, the earlier name could be retained. However, a 

geographic name should be used if the aquifer consists of several units, none of which has an appropriate name, or if the aquifer extends across rock-unit 

boundaries. Such relations should be shown clearly in 

the comparison charts of the report. 

If an aquifer is named for a rock-stratigraphic unit 

or a geographic feature, rules of priority should be 

followed, and a thorough literature search should be 

made to avoid name duplication. The name should be 

cleared through the Reston Geologic Names Unit and 

should not be preempted by a rock-stratigraphic 

name. 

Lithologic Names for Aquifers 

Lithology-derived names of aquifers are useful in 

defining water-bearing materials where formal rockstratigraphic names do not exist. The adjectives for 

lithologic names of aquifers may be based on lithologic terms-"sand and gravel aquifer,'' "granite 

aquifer," "limestone aquifer9'-but if lithologic consistency throughout the extent of the aquifer is uncertain, a geographic name should be used. Lithologic 

names are especially useful for naming aquifers in 

glacial deposits. If several aquifers discussed in a 

report are in glacial deposits, however, lithologic 

terms for each might be similar, and local geographic 

names may be more appropriate. 

Rock-Stratigraphic Names for Aquifers 

Rock-stratigraphic names may be used for aquifer 

names in studies that cover one State or parts of a 

State and an adjacent State. At the scale of such 

studies, the rock-stratigraphic unit and the aquifer 

commonly are equivalent. In addition to criteria for 

defining the hydrologic framework of a report, the 

following guidelines should be used, as appropriate, 

to assign names or to modify existing aquifer names 

based on rock-stratigraphic names: 

1. Through the use of comparison charts, maps, and 

cross sections, show clearly how much of the 

rock-stratigraphic unit is included in the aquifer. 

In some areas, aquifers have been named for 

rock-stratigraphic units but consist of parts of 

the units only. In the Atlantic and Gulf Coastal 

Plains, sediments generally thicken oceanward 

and become progressively less permeable because 

of increasingly fine grain. Thus, an aquifer may 

thin as a formation thickens-the Tuscaloosa Formation or Group and the Tuscaloosa aquifer of 

Alabama, for example. A similar lack of agreement between an aquifer and a corresponding 

rock-stratigraphic unit of the same name can 

exist at any scale if the formation name is automatically used for an aquifer name without due 

consideration as to how much of the formation 

actually constitutes the aquifer. 

2. Shorten the binomial name of the rockstratigraphic unit for use as the aquifer name: 

A. Madison aquifer, after the Madison Group. 

B. Edwards aquifer, after the Edwards 

Limestone. 

C. Sparta aquifer, after the Sparta Sand. 

Rock-stratigraphic names for aquifers 71 
Figure 19. A chart showing comparison of hydrogeologic units, rock-stratigraphic units, and time-stratigraphic units 

(modified from Jorgensen, written commun., 1986, and Jorgensen and others, in press). 

The argument has been made that including the 

full rock-stratigraphic name would provide additional information (Edwards Limestone aquifer), 

but if an aquifer is adequately described in the 

comparison table, the text, and the maps, a full 

name is redundant (and is incorrect if additional 

rock types are included in the aquifer). Including 

all the modifiers, moreover, makes for awkward 

names. For existing, entrenched names of aquifers, lithologic modifiers should not be capitalized 

(Burnam limestone aquifer, not Burnam Limestone aquifer). 

Hydrogeologic 

unit 

I 

Western Interior Plains 

confining unit 

3. Do not use the name of a rock-stratigraphic unit 

for an aquifer name if the unit is not part of the 

aquifer. 

4. Aquifer names based on multiple stratigraphic 

units: 

A. If an aquifer includes all or part of two rockstratigraphic units, one on the other, both 

unit names are used, separated by a hyphen, 

and the name of the younger unit name 

comes first. For example, the lower Hell 

Creek-Fox Hills aquifer consists of the lower 

part of the Upper Cretaceous Hell Creek 

Mostly igneous and metamorphic rocks. Rocks are fractured and 

locally will yield small quantities of water to wells. No known 

aquifers beneath these rocks; thus, unit is the basal confining unit. 

Thickness 

(feet) 

:,,.. 

(M,000 

., 

\. 

0-1,500 

*I 

0-1.500 

. 

.-- 

0-4,OOo 

I' 

,- 

0-730 

0-500 

4 

E 

3 

(I) +. 

V) 

4 

3 

(II . $. * 

(d 

3 

L 

A4 

2 

0 

i,ij 

- 

72 Rock-stratigraphic names for aquifers 

Lithology and hydrologic 

characteristics 

a* 

Shale layer of very low permeability separated by permeable 

limestones and sandstones. Leakage through shale is slow. 

Permeable limestone, fractured and solutioned locally. Well yields 

range from 1 to 300 gallons per minute, but typical yields are 5-10 

gallons per minute. 

Shale of very low permeability; however, at most locations, 

thickness of shale is less than 20 feet. Thus, unit is moderately 

leaky. * - 

Mostly dolostone with limestone and sandstone layers. Dolostone 

highly fractured with very permeable zones of fractured and 

solutioned dolostone. Well yields range from 2 to 2,000 gallons 

per minute, but typical yields are 200400 gallons per minute. 

Shale, siltstone, dolostone, and limestone, all of low permeability. 

Unit is leaky to slightly leaky. 

Fractured and permeable dolostone and sandstone. Well yields 

range from 1 to 500 gallons per minute, but typical yields are 

50-200 gallons per minute. 

r i 

Springf~eld ,- 

Plateau 

aquifer 

&ark 

contining 

unit 

i. a " Ozark 

aquifer e 

A*! . ?, *, 

St. Francois 

confining 

unit 

St. Francois 

aquifer - 

YA 

r 
Note.-Although measurements in this example are given in inchlpound units, metric (SI) units are now preferred 

(See "Metric System," p. 122, for USGS policy.) 

Rock-stmligrr~pliic unj! 

Formation and the underlying Fox Hills 

Sandstone. This usage conforms to that of 

map explanations, tables, cross sections, and 

the computerized Water Data and Storage 

Retrieval System (WATSTORE). 

However, an aquifer name designated from 

units in order of decreasing age is acceptable 

if it is entrenched in local usage or has been 

used in legal terminology. For example, the 

oldest- to youngest-named Potomac-RaritanMagothy aquifer in the Cretaceous Potomac 

Group and the overlying Raritan and 

Magothy Formations has longtime usage 

in New Jersey. 

Timc-slotigrnpttic unir 

B. If an aquifer includes three or more superimposed rock-stratrigraphic units, the aquifer 

name may include all units youngest to oldest (hyphenated) or only the youngest and 

oldest units. For example, the GalenaPlatteville aquifer of local Wisconsin usage is 

in the Galena Dolomite (youngest), Decorah 

Formation, and Platteville Formation. An 

appropriate geographic name would be a 

desirable alternative of a cumbersome 

hyphenated rock-stratigraphic name. 

C. If the middle rock-stratigraphic unit is the 

primary aquifer, its name may be used, provided the overlying and underlying stratiRock-stratigraphic names for aquifers 73 

I 

Msrmslnn Gmrrp, Chrrnkce Craup. A~ohnn rcrk.~. Bluyd Shale, Hd3c 

Formarion. Momwan mkq. PITLID Irrnaran~. Faycrtrvillc Sb;lCp, and 

Hnrcsvlklt: Sandstarrc 

;5Tmmficld Formalion. SI. Lnuiq bmertnnc, Sxlem Limcsrr~nc, W:rrsnu 

Lamrctanc, Bnone Formalinn, rncludlnp St. JW Lime~ln~ic Mrnilrr. Kcnkuk 

L~mc<tonr, Hurl~netn L~nic%ronr. and Fcrn Glen Llmrstnnc 

Il'ltouteiiu Gmup [Limcsroncl nnd CX+h;rrranwr~r Shnlc 

Clrrly bimcst[~nrr, Pcnrcr. Ckn, lalimy Limrrrnne, 31. aoir Lrrncs~nnc. 

Rrussficld L~rne~toiic, Casm Shale. F~rnvulc Limccto~ic, Kinim~wick 

L~tncstmc. !'t;lt~iri Lirnmtuna. booch~m Dolnmrlc, SI. Pe~tr S~ndqrone. 

Evrnan F%matian, hu-rll h!orn~rr. Sttril htlllc Fiimi~ri~n. c~rrrcr 

hlnmitc, leflcr$nn Clry Dt~ltrrnirc. R~5i1h1(!!1ilh Fi>;i)flh.atlfm. G&cnnadr 

Unl~rni[c, rncludlng rbc Gu~i~cr [i~ntlrrurrc bfrmlwr, El~hjnener LMlmltr. 

ant1 Pnrnsi L>lc~rnllc. 

Elvifts, Gmilp: 

Dm Run Dodamirc. 

Derby Dnlttni r~r. 

Davis Ftrni~olron 

Ronnc~cm Rdnmirt wd Lmnkutte Snndstonc 

Mnstlg igneous md rnct~rnnqhrc wks 

Mlddk Penns~[vsni;rn 

thmru~h 

Upw 511-isi.t~ipp1an 

?Chr\lert;w) 

llpp ?+TI-~~~L\IPP tan 

i~nrl 

hwtr Mrwissippiu~i 

Lnwrr M1~91nipp[m 

and 

I:plxr Dcvnniarr 

hlirldlc IXvn~~ian 

rhmu~h 

rrgprmorl 

C:irnhrr;~n 

llpprr C,rrnbrinri 

Prccurn bnan 
graphic units are clearly identified. For 

example, the Edwards aquifer in Texas is in 

the Georgetown Limestone (youngest), Edwards Limestone, and Comanche Peak 

Limestone. 

D. An aquifer that includes many water-bearing 

rock-stratigraphic units that are hydraulically 

connected vertically and laterally should not 

be named for any of the individual rockstratigraphic units. A geographic name 

would be appropriate. For example, the 

Floridan aquifer system includes the Tampa 

Limestone, Suwannee Limestone, Ocala 

Limestone, Avon Park Formation, Oldsmar 

Formation, and part of the Cedar Keys 

Formation. 

5. An abandoned rock-stratigraphic name should 

not be used for an aquifer name; the newly 

assigned stratigraphic name should be used 

instead. If, however, the abandoned name is 

entrenched in local usage or is a legal term in 

State regulations, the name may be used, but the 

stratigraphic change should be described in the 

introduction of the report and should appear on a 

correlation chart. 

Using rock-stratigraphic names for aquifers. 

The use of rock-stratigraphic names for aquifers 

carries some risk of confusion because you must distinguish the rock-stratigraphic unit from the aquifer 

throughout the text and in the illustrations of your 

report. After having first used a name in full, you can 

shorten both rock-stratigraphic and aquifer names in 

text by dropping the rank or descriptive part of the 

name if doing so improves readability and causes no 

doubt in the mind of the reader. For example, if the 

Baker aquifer makes up a large part of the Baker 

Formation, but not all of it, confusion might result if 

you say, "The Baker is 450 feet thick south of the 

Possum River." (The Baker Formation or the Baker 

aquifer?) If such doubt might arise, be sure to use the 

term "aquifer" when discussing the aquifer. 

To avoid unnecessarily long names and to clarify 

the distinction between the aquifer and the rockstratigraphic unit, do not use lithologic modifiers of 

rock-stratigraphic names for aquifers. Thus, an aquifer made up largely of the Jacob Sand Member of the 

Blackjack Formation should be called the Jacob aquifer, not the Jacob Sand Member aquifer. Lithologic 

modifiers have been used in aquifer names by authors 

who believed the modifiers added useful information 

to the aquifer names, but if the aquifer is clearly 

defined in the comparison charts, a reader should 

have no difficulty learning its composition. In addition, a single lithologic modifier of a name may be 

incorrect if more than one rock type makes up the 

aquifer. 

Descriptions of aquifers and rock-stratigraphic units 

should be clearly separated or distinguished in the 

text and illustrations. For example: 

1. Information on the potentiometric surface, storage 

coefficient, and specific yield describes the aquifer, not the rock-stratigraphic unit. 

2. Information on dip, strike, plunge, and deposition 

of sediments describes the rock-stratigraphic unit, 

not the aquifer. 

Terms such as porosity and permeability could refer 

to either the aquifer or the rock-stratigraphic unit. 

Geographic Names for Aquifers 

Geographic names can be the basis for aquifer 

names where (1) no rock-stratigraphic names are 

available, (2) no single rock-stratigraphic name or 

combination of rock-stratigraphic names (or lithologic 

names) would be appropriate, or (3) the use of previously named aquifers in small-area studies would not 

be appropriate. Geographic names are appropriate for 

aquifers of subregional extent where the location of 

the aquifer might provide more meaningful information than its physical characteristics. Geographic 

names include the names of cities, towns, districts, 

hills, mountains, lakes, rivers, creeks, and physiographic regions or subregions. In addition to geographic names, a regional aquifer name could be 

derived from a geologic structural feature (a basin, 

for example) that has relevance in the area underlain 

by the aquifer. Physiographic names should be from 

well-known sources, such as Fenneman's map (1946), 

"Physical Divisions of the United States." Geographic 

names should be from standard topographic quadrangles of the Geological Survey. The "High Plains 

aquifer" and the "Floridan aquifer system" are 

examples of regional aquifer names derived from 

physiographic and geographic names. 

NONRECOMMENDED SOURCES FOR AQUIFER NAMES 

Timestratigraphic Names 

Time-stratigraphic boundaries do not necessarily 

coincide with rock-stratigraphic boundaries or other 

physical changes in the hydrologic characteristics of 

rocks and should not be used as a basis for placing 

aquifer boundaries or for naming individual aquifers. 

Aquifers have been named for time-stratigraphic 

74 Using rock-stratigraphic names for aquifers 
terms, but later studies and more detailed mapping 

have shown that some parts of an aquifer may be 

older or younger than the time-stratigraphic unit in 

the aquifer name. For example, several years after 

the "Tertiary limestone aquifer" in the Southeastern 

United States was named, it was found to contain Upper Cretaceous rocks. Another possible complication is 

that longstanding time-stratigraphic boundaries have 

been changed in this country to agree with boundaries 

established under international geologic agreements 

(for example, the change in the Miocene-Pliocene 

boundary from 10 million years ago to 5 million). 

Also, terms such as "Cretaceous aquifers" are not 

strictly correct. The aquifer is not of Cretaceous age; 

it consists of rocks of Cretaceous age whose hydrologic properties are not the same as they were when 

the rocks were formed. "Aquifers in rocks of Cretaceous age" is correct and should be used instead. 

Some aquifer names based on time-stratigraphic 

names are in the literature and are commonly usedthe "Cambrian-Ordovician aquifer" of the North Central United States, for example. Other aquifers in the 

country have similar time-stratigraphic names that 

are entrenched in local usage. These names should be 

phased out if possible. Time-stratigraphic nomenclature should not be used for newly named aquifers, 

and existing time-stratigraphically based aquifer 

names should not be extended from local use to 

aquifers of regional scale. 

Relative Position 

If a layer of saturated permeable rock overlies 

another layer of saturated permeable rock-regardless 

of differences in lithology-the two layers form one 

aquifer and should not be designated "upper and 

lower" aquifers. If they are mostly separated by mappable, distinctly less permeable confining units, they 

are two separate aquifers. 

The terms "upper" and "lower" may be used where 

parts of an aquifer are separated by confining units if 

the full extent of the aquifer or aquifer system is 

reasonably well known. For example, the Floridan 

aquifer system was described as the "Upper Floridan 

aquifer" and "Lower Floridan aquifer" in the part of 

the area where the two units are separated by a 

regional confining unit. Where the confining unit is 

nonexistent, the term "Floridan aquifer system" is 

used, but in fact, the term "Floridan aquifer system" 

is correct throughout the area, including places where 

the two parts are separated by the confining unit. If 

you are referring to parts of the same aquifer that 

have some distinctive difference, the term "zone" is 

preferred. For example, use "upper zone of the 

Chicot aquifer" (not "upper Chicot aquifer"). 

Alphanumeric Designations 

Alphanumeric designations, such as "A1 aquifer 

layer" and "C1 confining layer," are useful in discussing layers of a numerical ground-water flow model, 

but they should not be used as aquifer names. A clear 

distinction must always be made in a report between 

the real flow system and the simulated flow system. 

Illustrations such as figure 17 help differentiate these 

distinctions and relations. 

Depositional Environment 

Names based on depositional environment can be 

misleading and should not be used for aquifer names. 

For example, "shallow marine aquifer" may be totally 

unclear as to what is meant and included. Even if 

the aquifer were described as consisting of "sand 

deposited in a shallow sea," problems and additional 

confusion may arise if the rocks of the aquifer grade 

into hydrologically continuous deposits from a different depositional environment or grade into different 

rocks in a similar depositional environment. Likewise, 

a "glacial aquifer" may contain or be hydrologically 

continuous with other deposits or rocks that are not 

of glacial origin. Lithologic or geographic terms would 

be more appropriate. 

Depth of Occurrence 

Aquifers should not be named after depth of occurrence. The aquifer named after the "2,000-ft" sand 

may well be present at a depth of about 2,000 ft at a 

given location where it was named in a local study, 

but on a regional scale the sand may be present 

elsewhere at a greater or lesser depth and may have 

no relation to the name derived from the local study. 

Established local usage may require the continued use 

of a name, but the name should not be extended to 

regional use. 

Acronyms 

Aquifers or aquifer systems should not have acronyms for names, such as an aquifer name derived 

from the first letter of each rock-stratigraphic unit 

that makes up the aquifer. If many rock-stratigraphic 

units make up an aquifer, a geographic name unrelated to any of the rock-stratigraphic names should 

be used. 

Hydrologic Condition 

Terms such as "water-table aquifer" and "artesian 

aquifer" are not recommended, because they are 

based on hydrologic conditions that can change as 

outside stresses change (pumping, climate). Hydrologic conditions also can vary from place to place. 

For example, an artesian aquifer can be dewatered by 

pumping, and an aquifer that is under artesian condiNonrecommended sources for aquifer names 75 
tions within one place may be under water-table 

conditions in another. 

RECOMMENDATIONS 

FOR NAMING 

CONFINING UNITS 

Confining units should not be named unless a 

clear-cut need exists for understanding a complex 

aquifer system. If several aquifers and confining 

units are discussed, the confining units could be given 

individual names, but a hierarchy of terms for confining units comparable to aquifer system, aquifer, 

and zone is not necessary. If names are applied to 

confining units, they, like aquifer names, should be 

derived from lithologic terms, rock-stratigraphic 

names, or geographic names. If a confining unit 

consists of one rock-stratigraphic unit, the confining 

unit may be named after the rock-stratigraphic unit. 

If a confining unit consists of several rockstratigraphic units, it could have the hyphenated 

name of the youngest and oldest units, or preferably, 

a geographic name. 

A confining unit could be named after the aquifer it 

confines, but two possible situations may cause confusion. First, what name would be given if the confining 

unit separates two aquifers? It confines both. It could 

be named, however, for aquifers it overlies. If crystalline basement or other rock having low hydraulic conductivity forms the lowest confining unit, a name 

unrelated to an aquifer name should be chosen, such 

as "basal confining unit." 

Second, if an aquifer is named for a rockstratigraphic unit that forms all or most of the 

aquifer, the same name should not be used for a 

confining unit. In other words, the confining unit 

should not be named for a rock-stratigraphic unit that 

is not part of the confining unit. For example, in 

western South Dakota, the upper part of the Minnelusa Formation is an aquifer named the Mimelusa 

aquifer. This aquifer is overlain by a confining unit 

that consists of six formal rock-stratigraphic units. 

The confining unit should not be called the "Minnelusa confining unit," because the Minnelusa Formation is not a part of the confining unit. The options 

are (1) do not name the confining unit, (2) name it 

after an appropriate combination of rock-stratigraphic 

units that are in the confining unit, or (3) name the 

confining unit after a geographic feature. However, if 

the lower part of the Minnelusa Formation is a confining unit, the name "Minnelusa confining unit" 

could be used. 

GENERAL PROCEDURES, STYLE, 

AND EXPRESSION 

REDEFINING AND RENAMING 

PREVIOUSLY NAMED AQUIFERS 

A previously named aquifer can be redefined and 

renamed, and the approach is the same as that for 

naming an aquifer for the first time. Redefining and 

renaming an aquifer should not be done casually or 

just to change the name. The guidelines in the previous pages apply. Comparison charts are important, 

especially charts that show the relation of renamed 

aquifers to previously named ones. No hard, fast rules 

can be given for redefining and renaming an aquifer, 

but justification should result from a thorough analysis of the hydrogeology and should represent an improvement in the understanding of the hydrology. 

Technical review should be used to judge the merit of 

the nomenclature changes. The work of Miller (1986) 

is an example of a detailed hydrogeological analysis 

that resulted in redefining and renaming the waterbearing units of the Floridan aquifer system. In 

reality, all aquifer names are informal names (North 

American Stratigraphic Code, 1983, art. 26) that 

might be changed with additional study. To represent 

clearly the hydrology of a particular area is more important than to retain old names or introduce new 

ones. 

FORMAT CONVENTIONS FOR AQUIFER NAMES 

The following format conventions are recommended 

for reports that name aquifers or contain discussions 

of aquifer names: 

) The terms "aquifer," "aquifer system," "zone," 

and "confining unit" are not capitalized. 

Terms such as sand and gravel aquifer and limestone aquifer are not capitalized or hyphenated. * Adjective modifiers, except parts of formal geographic names, are not capitalized: Mississippi 

River alluvial aquifer. 

Relative-position terms-upper, middle, and 

lower-are not capitalized. However, the terms 

may be capitalized if they represent parts of a 

regional aquifer system that are separated by a 

major confining unit. For example, Miller (1986) 

formally divided the Floridan aquifer system into 

an Upper Floridan aquifer and a Lower Floridan 

aquifer in all Florida and parts of adjacent States. 

Quotation marks are not used for aquifer names 

unless the term is a misnomer. The term "500-ft" 

sand is set in quotes only because the sand is not 

76 Recommendations for naming confining units 
at 500 ft below land surface everywhere. (As mentioned before, depth of occurrence should not be 

used to name aquifers.) 

Hydrologic and geologic terms should be clearly 

distinguished. 

A. Water from the Madison aquifer-not Madison 

water. 

B. Wells completed in Madison Limestone (or 

aquifer)-not Madison wells. 

EXAMPLES OF DESIGNATING AND

NAMING AQUIFERS 

Examples of designating and naming aquifers are 

shown in figure 20. The examples are hypothetical 

and generalized for convenience, but they illustrate 

characteristic hydrologic settings throughout the 

country. Even though most of the examples use rockstratigraphic names, the options for naming aquifers 

in order of consideration are (1) do not name the 

aquifer, (2) use a lithologic name, and (3) use a 

rock-stratigraphic or geographic name, whichever 

is appropriate. 

Example 1: Aquifer and rock-stratigraphic unit 

coincide. Example 1 shows an aquifer that coincides 

with a rock-stratigraphic unit and is confined above 

and below by much less permeable material. The aquifer probably would be named the Johnsville aquifer 

even though the full lateral extent of the aquifer may 

not be known. 

Example 2: Aquifer consists of one rockstratigraphic unit and part of an adjacent rockstratigraphic unit. The aquifer shown in example 2 

is made up of the lower two-thirds of the Whiskey 

Creek Formation (sandy silt and clayey sand) and the 

moderately cemented Devils Lake Sandstone. Hydrologically, the two units are continuous and form a 

single aquifer. The aquifer is confined above and 

below. The name of the aquifer could be taken from 

the rock-stratigraphic name "Whiskey Creek-Devils 

Lake aquifer." Likewise, a prominent geographic 

feature near the place where the aquifer was 

described (by wells or outcrops) could be the basis 

of the aquifer name. The description of the aquifer in 

the text, comparison chart, and illustrations should 

carefully state how the upper and lower boundaries of 

the aquifer were selected. In addition, the description 

should make clear that the upper formation and 

aq-ifer are not totally coincident. 

Example 3: Aquifer consists of a small part of 

two major rock-stratigraphic units. The aquifer in 

example 3 consists mostly of the Murphy Member of 

the Ringer Formation and probably would be called 

the Murphy aquifer. If the Murphy Member had not 

been named, the aquifer might be called the BellRinger aquifer. However, the aquifer makes up only a 

small part of each formation, especially the Bell Formation, so a local geographic name might be more 

appropriate. 

Example 4: Aquifer and aquifer system. Example 4 represents an aquifer system consisting of three 

permeable carbonate formations and the sand facies 

of a clastic formation. The clay facies forms a confining unit over part of the area. If the study had 

included only the area east of the State line, two 

separate aquifers could have been defined-the 

Beckville-Jonesville aquifer and the Riley aquifer (or 

two aquifers named for geographic locations). If the 

study had included only the area west of the State 

line, the following options could be considered: (1) The 

aquifer might be called the Lewis aquifer if the sand 

were siflicantly more permeable than the limestone 

units, or (2) the aquifer might be called the BeckvilleRiley aquifer if the permeability of the four units 

were not greatly different, or (3) the aquifer could be 

named after an appropriate geographic feature. 

If the study area included all the units shown on 

the cross section, no individual rock-stratigraphic unit 

would be representative everywhere, and a geographic name should be used to name the aquifer system. If 

the sketch represented the full extent of the aquifer 

and the aquifer were given a name, say the Williamsburg aquifer, the parts above and below the confining 

unit could be named the Upper Williamsburg aquifer 

and the Lower Williamsburg aquifer in a manner 

similar to the naming of the Floridan aquifer system 

of Miller (1986). For local studies on either side of the 

State line, the local aquifer name could still be used if 

the names were entrenched in usage, but the authors 

of local reports should clearly show and explain the 

broader relation. 

Example 5: Aquifer system in a coastal area. 

Example 5 illustrates an aquifer system (A-B) in a 

coastal area where the tendency has been to give 

separate aquifer names of hydrologically contiguous 

rock-stratigraphic units. For example, in a study area 

represented by section A-B, the aquifers from youngest to oldest are: surficial (the sand unit), Ford, Bass, 

Wilks, and Dade. In reality, all these named units 

form a single aquifer system that should be named 

after a physiographic or geographic feature. In a 

local-scale study represented by section C-D, the 

surficial deposits and the Bass Sand form one aquifer 

that should have a single name. It could be called the 

Bass aquifer as long as the designation clearly included the surficial deposits. The second aquifer 

under C-D would be the Wilks-Dade aquifer. 

Examples of designating and naming aquifers 77 
EXPLANATION 

Low-permeability rock Permeable rock, water- 1 bearing, fresh 

} Aquifer 

Permeable rock, waterbearing, saline 

Example 1. -Aquifer and rock-stratigraphic unit coincide. Example 2.-Aquifer consists of one rock-stratigraphic unit and part 

County lin[m[q line of an adjacent rock-stratigraphic unit. 

Lost Creek Confin~ng unit uq Format~on - unit -* wdt 

Johnsville bmafbn 

Sandstone { 

Example 3. - 

Bell 

Formation 

Ringer 

Formation 

.Aquifer consists of a small part of two major 

rock-stratigraphic units. 

Example 5.-Aquifer system in a coastal area. Example 6.-Aquifer system in a large structural basin. 

Figure 20. Examples of designating and naming aquifers. 

78 Examples of designating and naming aquifers 
Example 7. - -Aquifer crosses boundaries of rock-stratigraphic units 

and time-stratigraphic units. 

- - 

Example 9.-Use of aquifer terminology where rock-stratigraphic Example 10.-Designation of aquifers and confining units for 

units are discontinuous. different purposes and scales of investigations. 

Well A Well B Well C Well D -, n . . , . . - 1 ,,I ! \,,r1,,, .. , . I 

is- Boxwood -1 1 a Boxwood 111 --: sandnon, 

I 

Roa Rw Fommbon 

(91. Sad) 

Pobov Shak 

Examples of designating and naming aquifers 
Aquifer materials that contain saline water are part 

of the same aquifer that contains fresh water. Interfaces between saltwater and freshwater are subject to 

movement, depending on the hydrologic conditions of 

the area, and should not be used as aquifer boundaries. However, the boundary between the salt water 

and fresh water and its apparent stability (or instability) should be defined as clearly as possible in the 

report. 

Example 6: Aquifer system in a large structural 

basin. Example 6 depicts an aquifer system in a 

large structural basin. The aquifer system should be 

named after a physiographic or geographic feature, or 

as here, after a structural basin-the Lion aquifer 

system. If the tops and bottoms of the Capitol Formation, Thompson Sandstone, and Baxter Sandstone are 

all well defined, and if the boundaries of these units 

correspond largely to the boundaries of the aquifers 

in the system, rock-stratigraphic names could be used 

for individual aquifers. If the subsurface extent and 

boundaries of the rock-stratigraphic units are not well 

known, however, or if the individual aquifers consist 

of several rock-stratigraphic units, names unrelated to 

rock-stratigraphic terms should be assigned to the 

individual aquifers. If considerable uncertainty exists 

in defining the boundaries of the aquifers, the uncertainty should be indicated in the comparison charts 

and text. If the aquifer is well defined, it could be 

subdivided into the Upper, Middle, and Lower Lion 

aquifers, as was done for the Floridan aquifer system 

of Miller (1986). For local studies preceding the 

regional evaluation, as in the area represented by 

section A-B, individual aquifers might have been 

designated the Capitol aquifer, the Thompson 

aquifer, and the Baxter aquifer. 

For subsequent local studies, the Lion aquifer 

system names could be used for individual aquifers 

unless the rock-stratigraphic names were entrenched 

or otherwise advantageous. If the rock-stratigraphic 

names are used as the basis for aquifer names, their 

corresponding equivalents in the regional aquifer 

system should be discussed and shown in the comparison table of the report. 

Example 7: Aquifer crosses boundaries of rockstratigraphic units and time-stratigraphic units. 

Example 7 shows an aquifer that crosses the boundaries of four rock-stratigraphic units and consists of 

parts of them. East of the State line the aquifer could 

be named the Jones-Smith aquifer; west of the State 

line it could be called the Toad-Wood aquifer. The 

aquifer boundaries bear no relation to the timestratigraphic boundaries. In studies of the entire 

aquifer, a single rock-stratigraphic name is not appropriate; a geographic name should be used. Of course, 

a geographic name rather than a rock-stratigraphic 

name could be selected for the aquifer name at the 

local scale also. 

Example 8: Aquifers in an alluvial basin of the 

West or Southwest. The sedimentary units shown 

in example 8 are representative of closed-basin 

deposits. In such a setting, the grain size generally 

decreases basinward from the source areas, and 

cementation increases at depth. Hydraulic conductivity likewise decreases downward and basinward. Even 

though the hydraulic conductivity generally is lower in 

the deeper units, many deposits in the upper part of 

the basin are hydraulically connected and consist of 

one aquifer. Most of the deposits do not have formal 

rock-stratigraphic names but may have informal 

names, such as basin fill, valley fill, cemented gravel, 

playa deposits, or lake deposits. Other rock units, 

such as volcanic flows interbedded with the basin 

deposits, may complicate the relationships. Welldefined confining clays in some basins subdivide the 

materials into two or more aquifers. In other basins, 

however, well-defined clay layers may be absent, or 

clay deposits may form "plugs" at depth in the 

centers of the basins. Your first option is to name 

no aquifers but to describe the water-bearing characteristics of the deposits. Informal rock names could 

be used (for example, valley-fill aquifer), or the aquifer could be named for a geographic feature, such as 

a basin or valley. Zones could be designated for 

hydraulic features that require emphasis or 

separation. 

Example 9: Use of aquifer terminology where 

rock-stratigraphic units are discontinuous. The 

aquifer depicted in example 9 could be called the 

Boxwood-Rose River aquifer. Its upper boundary 

coincides with an erosional discontinuity, and although 

the Boxwood Sandstone is not present in the eastern 

part of the area, the name "Boxwood-Rose River 

aquifer" would be appropriate. Use of the aquifer 

name is illustrated by the wells in the sketch: 

Well A completely penetrates the Boxwood-Rose 

River aquifer, well B partly penetrates it, well C 

completely penetrates it, and well D partly penetrates it. 

If a report were prepared on the area between 

wells C and D, the aquifer could be called the Rose 

River aquifer because the Boxwood Sandstone is not 

present in that area. However, if the entire sketch 

area were studied and if the Boxwood-Rose River 

aquifer were already named, the study report must 

contain text statements and comparison charts to 

show that the Rose River aquifer thickens west to 

include the overlying Boxwood Sandstone and there 

becomes the Boxwood-Rose River aquifer. 

80 Examples of designating and naming aquifers 
Aquifer materials that contain saline water are part 

of the same aquifer that contains fresh water. Interfaces between saltwater and freshwater are subject to 

movement, depending on the hydrologic conditions of 

the area, and should not be used as aquifer boundaries. However, the boundary between the salt water 

and fresh water and its apparent stability (or instability) should be defined as clearly as possible in the 

report. 

Example 6: Aquifer system in a large structural 

basin. Example 6 depicts an aquifer system in a 

large structural basin. The aquifer system should be 

named after a physiographic or geographic feature, or 

as here, after a structural basin-the Lion aquifer 

system. If the tops and bottoms of the Capitol Formation, Thompson Sandstone, and Baxter Sandstone are 

all well defined, and if the boundaries of these units 

correspond largely to the boundaries of the aquifers 

in the system, rock-stratigraphic names could be used 

for individual aquifers. If the subsurface extent and 

boundaries of the rock-stratigraphic units are not well 

known, however, or if the individual aquifers consist 

of several rock-stratigraphic units, names unrelated to 

rock-stratigraphic terms should be assigned to the 

individual aquifers. If considerable uncertainty exists 

in defining the boundaries of the aquifers, the uncertainty should be indicated in the comparison charts 

and text. If the aquifer is well defined, it could be 

subdivided into the Upper, Middle, and Lower Lion 

aquifers, as was done for the Floridan aquifer system 

of Miller (1986). For local studies preceding the 

regional evaluation, as in the area represented by 

section A-B, individual aquifers might have been 

designated the Capitol aquifer, the Thompson 

aquifer, and the Baxter aquifer. 

For subsequent local studies, the Lion aquifer 

system names could be used for individual aquifers 

unless the rock-stratigraphic names were entrenched 

or otherwise advantageous. If the rock-stratigraphic 

names are used as the basis for aquifer names, their 

corresponding equivalents in the regional aquifer 

system should be discussed and shown in the comparison table of the report. 

Example 7: Aquifer crosses boundaries of rockstratigraphic units and time-stratigraphic units. 

Example 7 shows an aquifer that crosses the boundaries of four rock-stratigraphic units and consists of 

parts of them. East of the State line the aquifer could 

be named the Jones-Smith aquifer; west of the State 

line it could be called the Toad-Wood aquifer. The 

aquifer boundaries bear no relation to the timestratigraphic boundaries. In studies of the entire 

aquifer, a single rock-stratigraphic name is not appropriate; a geographic name should be used. Of course, 

a geographic name rather than a rock-stratigraphic 

name could be selected for the aquifer name at the 

local scale also. 

Example 8: Aquifers in an alluvial basin of the 

West or Southwest. The sedimentary units shown 

in example 8 are representative of closed-basin 

deposits. In such a setting, the grain size generally 

decreases basinward from the source areas, and 

cementation increases at depth. Hydraulic conductivity likewise decreases downward and basinward. Even 

though the hydraulic conductivity generally is lower in 

the deeper units, many deposits in the upper part of 

the basin are hydraulically connected and consist of 

one aquifer. Most of the deposits do not have formal 

rock-stratigraphic names but may have informal 

names, such as basin fill, valley fill, cemented gravel, 

playa deposits, or lake deposits. Other rock units, 

such as volcanic flows interbedded with the basin 

deposits, may complicate the relationships. Welldefined confining clays in some basins subdivide the 

materials into two or more aquifers. In other basins, 

however, well-defined clay layers may be absent, or 

clay deposits may form "plugs" at depth in the 

centers of the basins. Your first option is to name 

no aquifers but to describe the water-bearing characteristics of the deposits. Informal rock names could 

be used (for example, valley-fill aquifer), or the aquifer could be named for a geographic feature, such as 

a basin or valley. Zones could be designated for 

hydraulic features that require emphasis or 

separation. 

Example 9: Use of aquifer terminology where 

rock-stratigraphic units are discontinuous. The 

aquifer depicted in example 9 could be called the 

Boxwood-Rose River aquifer. Its upper boundary 

coincides with an erosional discontinuity, and although 

the Boxwood Sandstone is not present in the eastern 

part of the area, the name "Boxwood-Rose River 

aquifer" would be appropriate. Use of the aquifer 

name is illustrated by the wells in the sketch: 

Well A completely penetrates the Boxwood-Rose 

River aquifer, well B partly penetrates it, well C 

completely penetrates it, and well D partly penetrates it. 

If a report were prepared on the area between 

wells C and D, the aquifer could be called the Rose 

River aquifer because the Boxwood Sandstone is not 

present in that area. However, if the entire sketch 

area were studied and if the Boxwood-Rose River 

aquifer were already named, the study report must 

contain text statements and comparison charts to 

show that the Rose River aquifer thickens west to 

include the overlying Boxwood Sandstone and there 

becomes the Boxwood-Rose River aquifer. 

80 Examples of designating and naming aquifers 
Example 10: Designating aquifers and confining 

units for different purposes and different scales of 

investigations. Example 10 depicts a highly permeable deposit of gravel and sand in a valley occupied 

by a major perennial stream. The bedrock is granite 

and is several orders of magnitude less permeable 

than the gravel and sand. 

Because of the large permeability contrast, the 

deposit of gravel and sand is the aquifer, and the 

granite is the confining unit. If you were evaluating 

the potential for developing ground-water supplies 

from the gravel and sand, or were evaluating the 

interaction between ground water and surface water, 

you might consider the granite to be effectively 

"impermeable," and you could ignore the flow in the 

granite. If, however, you were evaluating the potential for storing high-level radioactive wastes in the 

granite, the designations of the aquifer and confining 

unit would not necessarily change, but you would 

have to consider the flow system through both units. 

The rate of flow through the granite into the gravel 

and sand would be slow, but you could not ignore it in 

evaluating minimum travel times of radionuclides that 

the ground water might transport through the granite. The depicted situation is similar to that of an 

aquifer overlain by a confining unit (such as clay over 

sand) that contributes water to the aquifer by leakage. A small to large part of the water withdrawn 

from the aquifer could come from the confining unit, 

but the designations of the aquifer and confining unit 

would not change. Therefore, the purpose of an investigation in a given area should not alter the designations of aquifers and confining units. 

Aquifers and confining units may be designated 

differently in two or more investigations because of 

differences in scale or extent of the study areas. In 

an investigation of just the granitic terrane in the 

sketch (such as an evaluation of ground-water availability for domestic use), the granite would be the 

aquifer because it is the only water-bearing unit 

in the study area. A report on a larger area that 

included the gravel should mention the previous 

report and should describe how the various hydrogeologic units were selected. Similarly, a unit of low 

hydraulic conductivity might be utilized locally as an 

aquifer, whereas a regional evaluation might show it 

as a regional confining unit. You as author should 

explain the apparent anomaly by means of the 

comparison charts and text. 

Example 11: Aquifers and confining units in 

thick lava-flow sequences. Thick lava-flow sequences, as on the Columbia Plateau (Heath, 1984), 

require special consideration in the designation of 

aquifers and confining units. These sequences may be 

hundreds to thousands of feet thick and may contain 

individual flows a few feet thick to a few hundred 

feet thick. The most permeable parts are interflow 

zones of lava-rock rubble a few feet thick at the tops 

of flows and thinner rubbly zones at the bases of 

overlying flows. The interflow zones are interrupted 

laterally, or they terminate; continuous aquifers, 

therefore, are identifiable for only a few miles (Newcomb, 1969). The part of a flow between interflow 

zones-the flow center-consists of dense, vertically 

jointed lava rock that cooled slowly. The interflow 

zones may account for only 1-30 percent of the 

volume of the rock, but their lateral hydraulic conductivity may be several orders of magnitude greater 

than the vertical hydraulic conductivity of the dense, 

vertically jointed rock, unless the top of the flow 

had been subjected to long subaerial weathering 

before burial. If the top of a flow were extensively 

weathered before being covered by another lava flow, 

clay minerals in the altered lava rock might reduce 

the permeability of the interflow zone. The interflow 

zones may contain discontinuous deposits of finegrained sediment that have little hydrologic effect 

on the flow sequence or that may grade into wider 

spread sedimentary deposits. The hydraulic conductivity of widespread sedimentary deposits varies but 

usually is much less than that of a rubbly interflow 

zone. 

Designating aquifers may be governed by the scale 

of the study and the thickness of the individual lava 

flows. Where individual flows are several hundred 

feet thick (the middle and lower part of the sketch), 

the interflow zones are easily recognized as individual 

aquifers, and the dense lavas between interflow zones 

are obvious confining units. The part of the flow 

sequence consisting of several permeable interflow 

zones separated by dense, much thicker lava would be 

an aquifer system. At the other extreme, the designation of aquifer versus aquifer system may not be as 

clear cut in a sequence where the individual flows are 

only a few feet thick (the upper part of the sketch). 

At some point the ratio of interflow zone to dense 

zone may become large enough that the multiple thinflow sequence could be considered a single aquifer. A 

comparison can be made with a sequence of interbedded sandstone and shale: Taken as a whole, the 

sequence might behave hydrologically as a single 

aquifer and not an aquifer system, even though thin 

continuous "confining units" are part of the aquifer. 

Other information, such as head measurements versus 

depth in areas where the aquifer is under stress, 

might be used to determine whether the sequence 

under study behaves as a single aquifer or as several 

aquifers separated by confining units. 

Examples of designating and naming aquifers 81 
If you assume that the thin-bedded flows in the 

upper part of example 11 behave as a single aquifer, 

the hypothetical lava-flow sequence consists of an 

aquifer and two aquifer systems, all of which constitute an even larger aquifer system. A higher 

category than aquifer system might appear to be 

needed in the hierarchy of nomenclature to classlfy 

the water-bearing rocks in this example, but the term 

"aquifer system" adequately encompasses the example shown. An appropriate geographic name should be 

used for the entire hydrologic system represented by 

the sketch, such as the "Rome River aquifer system," 

after a major river in the area. The individual parts 

of the system could be called "the Upper," "Middle," 

and "Lower Rome River aquifer," as in the Floridan 

aquifer system (Miller, 1986). An alternate method of 

naming would give the upper, middle, and lower parts 

individual names based on rock-stratigraphic units (or 

appropriate geographic names) that make up the 

aquifers, as follows: 

Cornwall aquifer (after Cornwall Basalt) Rome River aquifer Lancing aquifer (after Lancing Basalt) system Blanding aquifer (after Blanding Basalt) 

As in any other aquifer description, the characteristics of the dense, less permeable parts of the aquifer 

versus the very permeable interflow zones must be 

carefully described in the comparison tables and text. 

82 Examples of designating and naming aquifers