89˚ SANDSTONE AQUIFERS— Continued 88˚ 100 The ground-water flow system of the Cambrian–Ordovi- western and southern Iowa, north-central Illinois, and along the 50 OZAUKEE 50 cian aquifer system is summarized in figure 25. Water from northwestern shore of Lake Michigan. Where the aquifer is DODGE WASHINGTON 100 precipitation moves downward through surficial deposits of deeply buried and ground-water movement is almost stagnant SCALE 1:2,500,000 100 glacial drift, shallower aquifers, and the Maquoketa confining in parts of Missouri, concentrations are greater than 10,000 02550 MILES 200 unit into the St. Peter–Prairie du Chien–Jordan aquifer. Some milligrams per liter. The lobe of water with low dissolved-sol- WAUKESHA Milwaukee JEFFERSON of the water moves horizontally along short flow paths to lo- ids concentrations that extends southward in central Iowa is 43˚ 375 02550 KILOMETERS 350 L cal streams, such as the Little Cannon River, where it moves thought to represent unflushed subglacial meltwater that 300 a upward to discharge to the streams as base flow. Some water moved into the aquifer during the Pleistocene Epoch from an WISCONSIN k RACINE e moves in a similar fashion along flow paths of intermediate area of extremely high hydraulic head created by the weight ROCK 50 WALWORTH M length and discharges to larger streams, such as the Cannon of the glacial ice. Dissolved solids in water from the Mount

WISCONSIN 300 KENOSHA i River. Some of the water continues to percolate downward Simon aquifer show the same trends as those in water from c through successively deeper confining units into successively the St. Peter–Prairie du Chien–Jordan aquifer. ILLINOIS h LAKE i deeper aquifers. Water in these deeper aquifers moves later- Other layered sandstone aquifers that are exposed adja- WINNEBAGO 400 BOONE MC HENRY EXPLANATION g ally over long distances and eventually discharges to major cent to domes and uplifts or that extend into large structural 500 a Water-level decline in Cambrian– n rivers, such as the Mississippi River. A small part of the water basins or both are the Colorado Plateaus aquifers, the Denver Ordovician aquifer system, 500 moves down the regional hydraulic gradient, perpendicular to Basin aquifer system, the lower Tertiary aquifers, the Upper OGLE in feet ILLINOIS 42˚ 600 Chicago the plane of the section shown in figure 25 and toward deeply Cretaceous aquifers, the Lower Cretaceous aquifers, the Wyo-

KANE DU PAGE 800 Less than 100 buried parts of the aquifer system. This deep, regional flow dis- ming Tertiary aquifers, the Mississippian aquifer of Michigan, 100 DE KALB 100 to 300 charges by upward leakage to shallower aquifers or is captured and the New York sandstone aquifers (fig. 4). The Rush LEE 700 300 to 500 by pumping wells. Springs, Central Oklahoma, and Ada–Vamoosa aquifers of

KENDALL The chemical quality of the water in large parts of the Oklahoma are small aquifers that contain water largely under 50 800 500 to 700 100 aquifer system is suitable for most uses. The water is not highly unconfined conditions and generally yield small amounts of

200 Greater than 700 WILL LA SALLE mineralized in areas where the aquifers crop out or are buried water. The Jacobsville aquifer on the Upper Peninsula of Michi- GRUNDY 300 Line of equal water-level decline, to shallow depths, but mineralization generally increases as the gan is also small, mostly unconfined, and yields little water; 1864-1980—Dashed where water moves downgradient toward the structural basins. The however, the glacial deposits that cover this aquifer store pre- approximately located. Contour KANKAKEE interval, in feet, is variable distribution of dissolved-solids concentrations in the St. Peter– cipitation and release it slowly into the underlying sandstone. Prairie du Chien–Jordan aquifer (fig. 26) shows this increase. The Pennsylvanian aquifers that cover large parts of the east- 41˚ Base modified from U.S. Ground-water divide Where the aquifer is at or near the land surface in southeast- central United States cap hills and plateaus, are poorly per- Geological Survey digital Modified from Young, 1992a data, 1:2,000,000, 1972 ern Minnesota, northeastern Iowa, southern Wisconsin, the meable, and yield water mostly from shallow fracture systems Upper Peninsula of Michigan, and central Missouri, it contains and interbedded, cleated coals. The early Mesozoic basin Figure 24. Water levels in the Cambrian– water with dissolved-solids concentrations of less than 500 mil- aquifers of the eastern part of the Nation occupy titled grabens Ordovician aquifer system declined more than 800 ligrams per liter (the limit recommended for drinking water by or half-grabens, are commonly interbedded with fine-grained feet at Chicago as a result of large withdrawals the U.S. Environmental Protection Agency) or less. Concen- sediments and intruded by traprock, and generally yield only from 1864 to 1980. trations increase to more than 1,000 milligrams per liter in small amounts of water.

95°

Figure 26. Dissolved-solids concentrations in water from the St. Peter–Prairie du Chien– Jordan aquifer generally are less than 500 milligrams per liter 90° where the aquifer crops out, but increase greatly downdip where the aquifer is deeply buried.

Figure 25. Water in the stacked ° Duluth 85 sandstone aquifers moves vertically ● downward, then horizontally toward major MINNESOTA rivers, where it moves upward to discharge as base flow. Regional flow, not shown here, MINNESOTA is perpendicular to the section and into deep, confined parts of the aquifers.

45° Minneapolis A' ● ★ Saint Paul

A A' Green Bay ● Feet A N. Branch Middle WISCONSIN 1,400 Fork River N. Fork Zumbro River 1,200 1200 Little Cannon River 1100 Cannon River MICHIGAN 1,000 1000 Madison Mississippi River ★ ● Milwaukee 950 950 ★ 750 800 Lansing 850 800 ● 1000 900 750 Sioux City Detroit ● 700 Rockford 600 ● IOWA 650 850 ●Chicago 400 700 850 750 700 Des Moines 950 900 800 ★ Davenport ● 200 800

Sea level

200 VERTICAL SCALE GREATLY EXAGGERATED Modified from Delin and Woodward, 1984 INDIANA 0 5 10 15 MILES 40° ILLINOIS 015 015 KILOMETERS Springfield★ Indianapolis★

EXPLANATION

●Kansas City

Surficial aquifer system yyEau Claire confining unit Jefferson City Saint Louis Upper carbonate aquifer Mount Simon aquifer ★ ●

Cambrian–Ordivician aquifer system Crystalline-rock aquifer MISSOURI yy

Maquoketa confining unit Water table SCALE 1:7,500,000 St. Peter–Prairie du Chien–Jordan aquifer 900 Line of equal hydraulic potential—Interval 50 and 100 feet 0 2550 MILES St. Lawrence–Franconia confining unit Base modified from U.S. Geological Survey digital 0 2550 KILOMETERS Direction of ground-water movement data, 1:2,000,000, 1972 Ironton–Galesville aquifer A A' Line of hydrogeologic section Modified from Young, 1992b EXPLANATION

Dissolved-solids concentration in water from the St. Peter–Prairie du Chien–Jordan aquifer, in milligrams per liter. Dashed where appoximately located

500 1000 5000 10,000

Aquifer absent CARBONATE-ROCK AQUIFERS

Aquifers in carbonate rocks are most prominent in the carbonate rocks is dissolution. Carbonate rocks are readily central and southeastern parts of the Nation, but also occur dissolved to depths of about 300 feet below land surface where in small areas as far west as southeastern California and as they crop out or are covered by a thin layer of material. Pre- far east as northeastern Maine and in Puerto Rico (fig. 4). The cipitation absorbs some carbon dioxide as it falls through the rocks that comprise these aquifers range in age from Precam- atmosphere, and even more from organic matter in the soil brian to Miocene. Most of the carbonate-rock aquifers consist through which it percolates, thus forming weak carbonic acid. of limestone, but dolomite and marble locally are sources of This acidic water partially dissolves carbonate rocks, initially water. The water-yielding properties of carbonate rocks are by enlarging pre-existing openings such as pores between highly variable; some yield almost no water and are consid- grains of limestone or joints and fractures in the rocks. These ered to be confining units, whereas others are among the most small solution openings become larger especially where a vig- Carbonate- productive aquifers known. orous ground-water flow system moves the acidic water Most carbonate rocks form from calcareous deposits that through the aquifer. Eventually, the openings join as networks accumulate in marine environments ranging from tidal flats to of solution openings, some of which may be tens of feet in rock aquifers reefs to deep ocean basins. The deposits are derived from cal- diameter and hundreds to thousands of feet in length. The end careous algae or the skeletal remains of marine organisms that result of carbonate-rock dissolution is expressed at the land range from foraminifera to molluscs. Minor amounts of carbon- surface as karst topography, characterized by caves, sinkholes, ate rocks are deposited in fresh to saline lakes, as spring de- and other types of solution openings, and by few surface posits, geothermal deposits, or dripstone in caves. The origi- streams. Where saturated, carbonate-rock aquifers with well- nal texture and porosity of carbonate deposits are highly vari- connected networks of solution openings yield large volumes able because of the wide range of environments in which the of water to wells that penetrate the solution cavities, even deposits form. The primary porosity of the deposits can range though the undissolved rock between the large openings may from 1 to more than 50 percent. Compaction, cementation, be almost impermeable (fig. 27). Because water enters the and dolomitization are diagenetic processes which act on the Figure 27. Dissolution along joints and bedding planes in carbonate-rock aquifers rapidly through large openings, any carbonate rocks forms extremely permeable conduits that conduct carbonate deposits to change their porosity and permeability. contaminants in the water can rapidly enter and spread large volumes of water through the almost impermeable The principal post-depositional process that acts on through the aquifers. undissolved rock.

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