U.S. GEOLOGICAL SURVEY CIRCULAR 1099 

Regional Aquifer.-System Analysis Program 

of the U.S. Geological Survey, 1978-1992 
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY 

Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the currentyear issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey 

publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications that are 

listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability 

List" are no longer available. 

Reports released through the NTIS may be obtained by writing to the National Technical Information Service, U.S. Department of 

Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. 

Order U.S. Geological Survey publications by mail or over the counter from the offices given below. 

BY MAIL 

Books 

Professional Papers, Bulletins, Water-Supply Papers, Techniques of Water-Resources Investigations, Circulars, publications 

of general interest (such as leaflets, pamphlets, booklets), single 

copies of Earthquakes & Volcanoes, Preliminary Determination of 

Epicenters, and some miscellaneous reports, including some of 

the foregoing series that have gone out of print at the Superintendent of Documents, are obtainable by mail from 

U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY 

from the 

Superintendent of Documents 

Government Printing Office 

Washington, DC 20402 

(Check or money order must be payable to Superintendent of 

Documents.) 

Maps 

For maps, address mail orders to 

U.S. Geological Survey, Map Distribution 

Box 25286, Bldg. 810, Federal Center 

Denver, CO 80225 

Residents of Alaska may order maps from 

U.S. Geological Survey, Earth Science Information Center 

101 Twelfth Ave., Box 12 

Fairbanks, AK 99701 

OVER THE COUNTER 

Books and Maps 

Books and maps of the U.S. Geological Survey are available 

over the counter at the following U.S. Geological Survey offices, 

all of which are authorized agents of the Superintendent of Documents. 

• ANCHORAGE, Alaska--4230 University Dr., Rm. 101 

• LAKEWOOD, Colorado-Federal Center, Bldg. 810 

• MENLO PARK, California-Bldg. 3, Rm. 3128, 345 Middlefield Rd. 

• RESTON, Virginia-National Center, Rm. 1C402, 12201 

Sunrise Valley Dr. 

• SALT LAKE CITY, Utah-Federal Bldg., Rm. 8105, 125 

South State St. 

• SPOKANE, Washington-U.S. Post Office Bldg., Rm. 135, 

W. 904 Riverside Ave. 

• WASHINGTON, D.C.-Main Interior Bldg., Rm. 2650, 18th 

and C Sts., NW. 

Maps Only 

Maps may be purchased over the counter at the U.S. Geological Survey offices: 

• FAIRBANKS, Alaska-New Federal Building, 101 Twelfth 

Ave. 

• ROLLA, Missouri-1400 Independence Rd. 

• STENNIS SPACE CENTER, Mississippi-Bldg. 3101 
Regional Aquifer-System Analysis Program 

of the U.S. Geological Survey, 1978-1992 

By Ren Jen Sun and Richard H. Johnston 

U.S. GEOLOGICAL SURVEY CIRCULAR 1099 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For sale by 

U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. 

Library of Congress Cataloging-in-Publication Data 

Sun, Reo Jen. 

Regional Aquifer-System Analysis Program of the U.S. Geological Survey, 1978-1992/ by Ren Jen Sun and 

Richard H. Johnston. 

p. em.- (U.S. Geological Survey circular; 1099) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.4/3: 1099 

1. Aquifers-United States. I. Johnston, Richard H. II. National Regional Aquifer Systems Analysis Program (U.S.) 

III. Title. IV. Series. 

GB1199.2.S86 1994 

553.7'9'0973-dc20 94-37846 

CIP 
CONTENTS 

Abstract. ...................................................................................................................... . 1 

Introduction ................................................................................................................ . 1 

Regional aquifer systems .................................................................................. . 3 

Objectives and approach ................................................................................... . 5 

Availability of study results ............................................................................... . 6 

Acknowledgments ............................................................................................. . 7 

Simulation of regional ground-water flow and the effect of scale ...................... . 7 

Part I. Selected descriptions and findings about regional aquifer systems ............... . 10 

Regional descriptions of hydrogeologic frameworks, flow-system 

characteristics, and ground-water chemistry ................................................ . 10 

Hydrogeologic frameworks ..................................................................... . 10 

Areal distribution of hydraulic properties .............................................. . 11 

Flow-system characteristics .................................................................... . 14 

Ground-water chemistry ......................................................................... . 14 

Regional ground-water budgets ........................................................................ . 16 

Aquifer systems of the Atlantic and Gulf Coastal Plains ...................... . 21 

Northern Atlantic Coastal Plain aquifer system ............................ . 21 

Floridan aquifer system ................................................................. . 22 

Gulf Coastal Plain aquifer systems ............................................... . 23 

Aquifer systems of the Central United States ........................................ . 23 

Cambrian-Ordovician aquifer system in the northern Midwest ...... . 23 

Great Plains aquifer system (Dakota Sandstone and associated 

strata) ......................................................................................... . 23 

High Plains aquifer ........................................................................ . 24 

Aquifer systems of the Western United States ....................................... . 25 

Central Valley aquifer system of California ................................. . 25 

Snake River Plain regional aquifer system ................................... . 25 

Evaluation of ground-water development effects .............................................. . 26 

Application of Regional Aquifer-System Analysis Program information ......... . 36 

Part TI. Status of the Regional Aquifer-System Analysis program ........................... . 37 

Fully completed Regional Aquifer-System Analysis studies ............................. . 37 

Floridan aquifer system ........................................................................... . 37 

High Plains .............................................................................................. . 40 

Northern Atlantic Coastal Plain .............................................................. . 42 

Northern Great Plains ............................................................................. . 44 

Southeastern Coastal Plain ...................................................................... . 47 

Southwest Alluvial Basins ...................................................................... . 48 

Completed regional aquifer-system analysis studies; some reports are in 

preparation or in review ................................................................................ . 52 

Caribbean Islands .................................................................................... . 52 

Central Valley, California ........................................................................ . 54 

Central Midwest ...................................................................................... . 57 

Columbia Plateau, Washington, Oregon, and Idaho ............................... . 61 

Great Basin, Nevada and Utah ............................................................... . 63 

Gulf Coastal Plain ....................................................... ~ ........................... . 65 

Northeast Glacial Aquifers ...................................................................... . 69 

Northern Midwest ................................................................................... . 72 

Oahu, Hawaii .......................................................................................... . 74 

San Juan Basin, Arizona, Colorado, New Mexico, and Utah ................ . 77 

Snake River Plain .................................................................................... . 77 

Upper Colorado River Basin .................................................................. . 81 

m 
IV CONTENTS 

Part II. Status of the Regional Aquifer-System Analysis program-Continued 

Ongoing regional aquifer-system analysis studies.............................................. 85 

Appalachian Valleys and Piedmont .. .......... ................... ............ .............. 85 

Edwards-Trinity regional aquifer system, Texas, Oklahoma, and 

Arkansas .. . .......... .. ...... ................ ........... ........ ...................................... 89 

Michigan Basin ..... . .. . ... . . . ........ ...... ............... ...... ............ .......................... 96 

Midwestern Basins and Arches glacial and carbonate regional aquifer 

system.................................................................................................. 102 

Northern Rocky Mountains Intermontane Basins, Montana, and Idaho ..... 106 

Puget-Willamette Lowland, Washington, Oregon, and Canada.............. 109 

Southern California Basins .. .. .. .. .. .. .. .. .. .. .. .. .. . .. .... .... .. .. .. .. .. .. .. .. . .. .. .. .. .. .. .. .. 112 

Ground Water Atlas of the United States ............................................................ 117 

Part III. Conclusions and recommendations............................................................... 117 

Conclusions........................................................................................................ 117 

Recommendations.............................................................................................. 119 

References cited .. ............. ..... ... .. ....... ... . ...... . .. ... .. . ........ ... ............. .................... ........... 119 

FIGURES 

1. Map showing locations of regional aquifer-system studies................................................................................. 2 

2. Map showing States contained in each segment of the Ground Water Atlas of the United States .................... 4 

3. Diagram showing relation oflocal- and regional-scale ground-water flow to discretization in computer flow 

models................................................................................................................................................................... 9 

4. Chart showing rock-stratigraphic and regional hydrogeologic names applied to clastic and carbonate rocks 

of the Southeastern Coastal Plain.......................................................................................................................... 12 

5-11. Maps showing: 

5. Saturated thickness of the High Plains aquifer, 1980 ................................................... , .... .. . .. .. .. .. .. .. ... .. .. .. 13 

6. Porosity and transmissivity of the Devonian and Mississippian carbonate rocks hydrostratigraphic 

unit in the Upper Colorado River Basin, Arizona, Colorado, New Mexico, Utah, and Wyoming........... 15 

7. Transmissivity of the Upper Floridan aquifer, southeastern United States............................................... 16 

8. Approximate predevelopment potentiometric surface for the composite Cambrian-Ordovician 

aquifer system in the Chicago-Milwaukee area......................................................................................... 17 

9. Potentiometric surface for the composite Cambrian-Ordovician aquifer system in the ChicagoMilwaukee area, 1985................................................................................................................................. 18 

10. Hydraulic head difference between the water table and the lower pumped zone, Central Valley 

aquifer system, California, spring 1961 . ... .. .. . .. . .. . .. .. ... .. . .. . .. . .. .. . .. . . . .. .. . .. .... . . . .. .. . . .. .. . . .. . .. .. .. .. .... .. . ... .. . .. .. .. .. .. 19 

11. Hydrochemical facies of ground water in the Mount Simon and St. Peter-Prairie du Chien-Jordan 

aquifers, Cambrian-Ordovician aquifer system in the Northern Midwest ................................................... 20 

12-14. Diagrams showing: 

12. Ground-water budgets before and after development of three major aquifer systems in the Atlantic 

and Gulf Coastal Plains................................................................................................................................ 22 

13. Ground-water budgets before and after development ofthree major aquifer systems in the Central 

United States................................................................................................................................................ 24 

14. Ground-water budgets before and after development of two major aquifer systems in the Western 

United States................................................................................................................................................ 26 

15-18. Graphs showing: 

15. Water-level decline in the High Plains aquifer in Floyd County, Texas, due to irrigation pumping ............. 27 

16. Decline of irrigated acreage per well due to reduction in saturated thickness of the High Plains 

aquifer in parts of Castro, Brosby, Floyd, Hale, Lubbock, Parmer, and Swisher Counties, Texas .......... 28 

17. Changes in ground-water discharge to the Snake River due primarily to changes in irrigation with 

surface water in eastern Snake River Plain, Idaho .. . ...... ..... . .. ........... .............. ......... ......... ................ ........ ... 29 

18. Rises of the water table in parts of Dawson and Lynn Counties, Texas, due to clearing sandy soils of 

native vegetation for cultivation .................................................................................................................. 30 
CONTENTS v 

19-20. Diagrams showing: 

19. Estimated predevelopment discharge from the Upper Floridan aquifer.................................................... 30 

20. Estimated 1980 discharge from the Upper Floridan aquifer...................................................................... 31 

21. Graph showing the relation of hydraulic heads and storage in a compacting aquifer system............................ 32 

22. Map showing land subsidence in the Central Valley, California, 1926-70.. ... . ........ ... .................. .... .. ... . ........ .... 33 

23-25. Photographs showing: 

23. Observed land subsidence of approximately 29 ft at a surveying benchmark, 10 mi southwest of 

Mendota in the San Joaquin Valley, California.......................................................................................... 34 

24. Earth fissures enlarged by erosion, near Apache Junction, central Arizona.............................................. 35 

25. Collapse of land surface due to formation of a sinkhole at Winter Park, Florida, in 1981...................... 35 

26-36. Maps showing: 

26. Extent of the Floridan aquifer system and subareas with descriptions of ground-water hydrology 

in Professional Papers 1403-D through 1403-H ....... .. .............. .. . . . . .. .. . ............ ....... ........... ...... .......... ... ... . 38 

27. Location of the study area of the High Plains regional aquifer-system analysis ...................................... 40 

28. Subdivisions used in modeling the High Plains aquifer............................................................................ 41 

29. Location of the study area, Northern Atlantic Coastal Plain regional aquifer-system analysis................ 43 

30. Location of the study area, Northern Great Plains regional aquifer-system analysis............................... 45 

31. Structural and physiographic features of the Northern Great Plains and vicinity..................................... 46 

32. Location of the study area, Southeastern Coastal Plain regional aquifer-system analysis........................ 47 

33. Location of study area and types of alluvial basins in parts of Colorado, New Mexico, and Texas.............. 49 

34. Location of study area and categories of alluvial basins in southern and central Arizona, and 

parts of California, Nevada, and New Mexico........................................................................................... 50 

35. Location of the study area, Caribbean Islands regional aquifer-system analysis...................................... 52 

36. Thickness of the Central Valley aquifer system......................................................................................... 55 

37. Sections showing the Central Valley aquifer system. A, Concept of a single homogeneous aquifer 

system. B, Structure of computer flow model of the aquifer system.................................................................. 56 

38. Map showing study area of the Central Midwest regional aquifer-system analysis........................................... 58 

39. Geohydrologic section showing regional aquifer systems and confining systems in the central midwestern 

United States .............................................................................. ............................................ ............................... 59 

40-41. Maps showing: 

40. Approximate extent of the Great Plains, Western Interior Plains, and Ozark Plateaus aquifer 

systems........................................................................................................................................................ 60 

41. Location of the study area of the Columbia Plateau regional aquifer-system analysis............................. 62 

42. Diagrams showing basalt interflow structures . . .. ......... ..... .. .. . . . . ... . . ... . . . . .. . . . . .. . . . . ...... .. . . . .......... ......... ... . .. . . . . .. .. . . . . .. . 63 

43-45. Maps showing: 

43. Location of the study area and types of ground-water-flow models, Great Basin regional aquifer 

system.......................................................................................................................................................... 64 

44. Major flow systems and major ground-water discharge areas of the Great Basin regional aquifer 

system ......................................................................... ;................................................................................ 66 

45. Location of the study area and outcrops of regional aquifer systems and confining units, 

Gulf Coastal Plain regional aquifer-systems analysis................................................................................ 67 

46. Chart showing geohydrologic and geologic units of the Gulf Coastal Plain aquifer systems............................ 68 

47-53. Maps showing: 

4 7. Location of the study area and physiographic divisions of the Northeast Glacial Aquifers regional 

aquifer-system analysis................................................................................................................................ 70 

48. Hydrophysiographic terranes in glacial drift of the Northeastern United States....................................... 71 

49. Study area of the Northern Midwest regional aquifer-system analysis and extent of major glaciation 

during the Pleistocene Epoch....................................................................................................................... 73 

50. Dissolved solids distribution in the St. Peter-Prairie du Chien-Jordan aquifer of the 

Cambrian-Ordovician aquifer system in the Northern Midwest................................................................ 75 

51. Location of the study area, Oahu regional aquifer-system analysis and location of aquifers 

underlying the island of Oahu, Hawaii....................................................................................................... 76 

52. Location of the study area, San Juan Basin regional aquifer-system analysis.......................................... 78 

53. Location of the study area, Snake River Plain regional aquifer-system analysis...................................... 79 
VI CONTENTS 

54. Hydrograph showing changes in ground-water levels in the Snake River Plain due to surface-water 

irrigation............................................................................................................................................................... 80 

55-64. Maps showing: 

55. Study areas of the Snake River Plain regional aquifer-system analysis, Phase II .................................... 81 

56. Study area of the Upper Colorado River Basin regional aquifer-system analysis .................................... 82 

57. Principal tectonic features in the Upper Colorado River Basin................................................................. 83 

58. Areal extent of Cenozoic rocks in the Upper Colorado River Basin ............................................... ......... 84 

59. Areal extent of Mesozoic rocks in the Upper Colorado River Basin........................................................ 86 

60. Areal extent of Paleozoic rocks in the Upper Colorado River Basin........................................................ 87 

61. Study area of the Appalachian Valleys and Piedmont regional aquifer-system analysis and location 

of physiographic provinces.......................................................................................................................... 88 

62. Study area of the Edwards-Trinity regional aquifer-system analysis and location of contiguous 

hydraulically connected units in central Texas, southeastern Oklahoma, and southwestern Arkansas .. .. 92 

63. Geographic subareas of the Edwards-Trinity regional aquifer system in west-central Texas................... 93 

64. Aquifers and confining units in the Edwards-Trinity regional aquifer system and contiguous major 

aquifers in west-central Texas..................................................................................................................... 94 

65. Diagram showing simulated flow components to and from the Edwards-Trinity regional aquifer system in 

west-central Texas, predevelopment conditions and winter 1974-75.................................................................... 95 

66. Map showing study area of the Michigan Basin regional aquifer-system analysis ...... .. .. .. ................ .... .. ..... ..... 97 

67. Chart showing the relation between stratigraphic and hydrogeologic units in the Michigan Basin.................. 98 

68-70. Maps showing: 

68. Altitude of base of freshwater in the Michigan Basin .............................................................................. 99 

69. Generalized water table in the glacial drift aquifer of the Lower Peninsula of Michigan........................ 100 

70. Generalized equivalent freshwater head in the Grand River-Saginaw aquifer of the Michigan Basin........... 101 

71. Map and sections showing study area and generalized geology of the Midwestern Basins and Arches 

glacial and carbonate regional aquifer-system analysis....................................................................................... 103 

72-79. Maps showing: 

72. General distribution of glacial deposits, Midwestern Basins and Arches regional aquifer-system 

analysis........................................................................................................................................................ 104 

73. General distribution of dissolved-solids concentration in water from the Silurian-Devonian 

carbonate rock aquifer................................................................................................................................. 105 

74. Study area ofthe Northern Rocky Mountains Intermontane Basins regional aquifer-system analysis .......... 107 

75. Intermontane basins in the northern Rocky Mountains............................................................................. 108 

76. Study areas of the Puget-Willamette Lowland regional aquifer-system analysis...................................... 110 

77. Study area of the Southern California Basins regional aquifer-system analysis and location of 

drainage basins . . . . . . .. . .. .. .. .. . . . . .. . . . . .. . . . . . . . . .. .. . . . . .. . .. ... . . . . .. .. .. .. . . . . .. .. .. .. . .. . .. .. .. .. . . . . .. . .. . .. .. .. .. . . . . .. . .. .. . .. . . .. .. . .. .. .. .. .. .. 113 

78. Santa Clara-Calleguas and Mojave Basin study areas.............................................................................. 114 

79. Chloride concentrations in water from wells on the Oxnard Plain, 1989 ................................................. 115 

TABLES 

1. Coarse-mesh computer models used for simulation of flow in very large regional aquifer systems................. 8 

2. Hydrologic characteristics of hydrogeologic terranes and conceptual flow systems in the 

Appalachian Valleys and Piedmont .. . . . . .... .... . . . . ...... . . . . . ... . . . . .. . . . . .. . . . . .. . ..... . . . . .. . . . . ............ ........ ...... . . .... . . .. .. . . .. .. . . .... .. 90 
CONTENTS 

CONVERSION FACTORS AND VERTICAL DATUM 

Multiply 

foot (ft) 

mile (mi) 

square foot (ft2) 

square mile (mi2) 

acre 

acre-foot (acre-ft) 

gallon per minute (gaVmin) 

million gallons per day (Mgalld) 

billion gallons per day (Bgalld) 

cubic foot per second (ft3

/s) 

inch per year (in/yr) 

foot squared per day (ft2

/d) 

By 

Length 

0.3048 

1.609 

Area 

0.0929 

2.590 

4,047. 

Volume 

1,233. 

Flow 

0.063098 

0.04381 

43.81 

0.02832 

25.4 

0.0929 

To obtain 

meter 

kilometer 

square meter 

square kilometer 

square meter 

cubic meter 

liter per second 

cubic meter per second 

cubic meter per second 

cubic meter per second 

millimeters per year 

meter squared per day 

SEA LEVEL: In this report, "sea level" refers to the National Geodetic Vertical Datum (NGVD) of 1929-a 

geodetic datum derived from a general adjustment of the first-order level nets of both the United States and 

Canada, formerly called Sea Level Datum of 1929. 

VII 

Regional Aquifer-System Analysis Program of the 

U.S. Geological Survey, 1978-1992 

By Ren Jen Sun and Richard H. Johnston 

ABSTRACT 

The major ground-water systems of the United States 

have been investigated by the U.S. Geological Survey 

(USGS) through its Regional Aquifer-System Analysis 

(RASA) Program. During the first 15 years of the program 

(1978-92), 25 regional aquifer systems, including the most 

heavily pumped aquifers in the Nation, were intensively 

studied. As of mid-1992, 18 of the regional aquifer studies 

are completed or nearly so; 7 of the regional aquifer studies 

are ongoing, and compilation of a national ground-water 

atlas is in progress. 

This report summarizes the status of each RASA 

study and briefly describes the hydrology of the 25 regional aquifer systems. Important study results and examples 

of applications of study results are presented for some of 

the completed RASA investigations. 

The major contributions of the RASA Program are 

( 1) assembly of data from numerous local studies and 

long-time data collections into systematic regional data 

bases; (2) comprehensive descriptions of the geologic, hydrologic, and geochemical characteristics of the regional 

aquifer systems; and (3) an understanding of how the regional ground-water-flow systems function under natural 

(predevelopment) and current (developed) conditions. To 

provide the comprehensive system descriptions, many of 

the RASA studies present, for the first time, maps depicting the hydrogeologic frameworks, water chemistry, potentiometric surfaces, and other aspects of entire regional 

aquifer systems. To provide an understanding of how the 

flow systems function, several of the completed RASA 

studies provide, for the first time, hydrologic budgets for 

both predevelopment and developed conditions. 

The results of the RASA Program are contained in 

nearly 900 reports published by the USGS, as well as various State and local agencies, and in articles published in 

scientific journals. A series of U.S. Geological Survey Professional Papers are being published to summarize and 

synthesize the results of each RASA study. 

INTRODUCTION 

Ground water is one of the Nation's most important 

natural resources. According to water-use information compiled by the U.S. Geological Survey (USGS), ground water 

supplied about 74 billion gallons per day in 1985, which was 

about 20 percent of the Nation's total freshwater withdrawals. Ground water is the source of about 40 percent of the 

Nation's public water supply and 30 to 40 percent of water 

used for irrigation. Ground water provides drinking water 

for more than 97 percent of the rural population who do not 

have access to public water-supply systems (Solley and others, 1983, 1988). 

Many problems of ground-water quantity and quality 

that are related to development of the ground-water resources have been documented. To resolve such development-associated problems, information on characteristics 

of aquifers and the functioning of ground-water systems is 

needed. In the past, most ground-water studies were conducted within county or State boundaries. In evaluating 

the effects of development and assessing the potential 

water quality and quantity available for development, a 

question frequently asked is, what will happen beyond the 

extent of the study area? Often, ground-water hydrologists 

begin a local ground-water study only to find limited information on regional geology, hydrology, or water chemistry. These facts underscore the need for a broad regional 

study of the Nation's major aquifers. 

In response to recommendations made by the U.S. 

National Water Commission and U.S. Comptroller General 

and the effects and persistence of the 1977 drought (Sun, 

1986), the Committee on Appropriations of the U.S. House 

of Representatives of the 95th Congress, in a report of the 

appropriations for the Department of the Interior and related agencies for fiscal year 1978, stated that "The Committee is providing funds to initiate a program to identify 

the water resources of the major aquifer systems within the 

United States. This program will establish the aquifer 

boundaries, the quantity and the quality of the water within 

1 
2 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

the aquifer, and the recharge characteristics of the aquifer. 

Although this initiative comes too late to help the present 

drought (1977), the action taken will develop an inventory 

of the major ground-water systems of the United States so 

that in the event of another drought we may be able to 

draw upon the ground-water reservoir system without 

doing irreparable damage to the system. The Committee 

expects the Survey to press this program vigorously." (U.S. 

House of Representatives, 1977, p. 36-37). 

As a result of these specifications in the appropriations 

bill, and in response to needed information for better manag125° 120° 115" 110° 105° 100° 

45° 

35° 

Jo• 

25° 

22·~:6: <:, 155° 

15 ":;;u 

2o• \) 

Hawaii 

500 MILES 

500 KILOMETERS 

ing, planning, and developing of the Nation's ground-water 

resources, the USGS started the Regional Aquifer-System 

Analysis (RASA) Program in 1978; 28 major regional aquifer systems were identified for study under the RASA Program (fig. 1). As of mid-1992, 25 of 28 studies either have 

been completed or are in progress, as shown in figure 1. 

Three aquifer systems (the alluvial basins in California, Nevada and Oregon; the Illinois Basin; and the Pecos River 

Basin in Texas and New Mexico) have yet to be studied. 

Investigation activities of 18 aquifer systems have been 

completed, and 7 aquifer systems are currently under study. 

95° 9Q• 85° so· 75° 7o· 65° 

sa· s1• ss• s5• 

1918° ~ • CJ-··· 16 "" n· 

Caribbean Islands 

EXPLANATION 

c=J Completed studies 

Northern Great Plains (1) 

High Plains (2) 

Central Valley, California (3) 

Northern Midwest (4) 

Southwest alluvial basins (5) 

Floridan (6) 

Northern Atlantic Coastal Plain (7) 

Southeastern Coastal Plain (8) 

Snake River Plain (9) 

Central Midwest (10) 

Gulf Coastal Plain (11) 

Great Basin (12) 

Northeast glacial aquifers (13) 

Figure 1. Locations of regional aquifer-system studies. 

Upper Colorado River basin (14) 

Oahu, Hawaii (15) 

Caribbean Islands (16) 

Columbia Plateau (17) 

San Juan Basin (18) 

Active studies 

Michigan Basin (19) 

Edwards-Trinity (20) 

Midwestern basins and arches (21) 

Appalachian valleys and Piedmont (22) 

Puget-Willamette Lowland (23) 

Southern California alluvial basins (24) 

Northern Rocky Mountain intermontane basins (25) 
IN1RODUCTION 3 

In addition to the major regional aquifer studies, three 

ancillary activities were undertaken to provide support for 

the regional studies and to make the information developed 

during the studies available to potential users with a broad 

range of interests. These activities are the Ground Water 

Atlas of the United States, studies to solve specific hydrologic problems, and basic hydrologic research. 

In 1988 the RASA Program used part of its resources to begin compilation of a Ground Water Atlas of the 

United States. The atlas presents a comprehensive summary of the Nation's ground-water conditions and serves as a 

basic reference for the location, geography, geology, and 

hydrology of the Nation's major aquifers. Information presented in the atlas primarily comes from the RASA studies. However, information and data resulting from many 

decades of ground-water studies by cooperative projects 

between States and the USGS, as well as local and other 

Federal agencies, also are included in the atlas. The atlas 

has a graphical format and is composed of 14 chapters. 

One of the chapters presents an overview of the Nation's 

ground-water conditions and development effects, and 

contains maps showing the locations of the Nation's major 

aquifers. The remaining 13 chapters (segments) collectively cover the 50 States and Puerto Rico and describe geologic and hydrologic conditions of and development effects 

on major aquifers in each segment area. Locations of the 

13 segments are shown in figure 2. The atlas is published 

as the USGS Hydrologic Investigations Atlas HA-730 series, with a letter A-N assigned to each chapter, as shown 

in figure 2. The atlas project is scheduled for completion 

by 1994. 

During the RASA studies, problems with no readily 

available solutions were encountered. For example, determination of the flow from individual aquifers tapped by 

wells open to several aquifers, analyzing flow in aquifers 

containing water with variable density, and estimating the 

permeability of rocks. The RASA staff spent considerable 

time and effort to develop new techniques and methods 

for resolving such problems. For example, a multiaquifer 

well simulation technique was developed by Bennett and 

others (1982), an improved method of simulating movement of ground water with variable density was developed 

by several hydrologists (Weiss, 1982; Kuiper, 1983, 1985; 

Kontis and Mandie, 1988), mass-transfer and reaction 

models were formulated by Plummer and others ( 1990), 

basin geochemical models were constructed by Robertson 

(1991 ), and methods of estimating rock permeability and 

porosity from geophysical logs were developed by Jorgensen (1988). 

The RASA Program has benefited from findings of 

the long-term research efforts of the National Research 

Program (NRP) of the Water Resources Division of the 

USGS. Examples of products from NRP that were available at the beginning of the RASA Program are computerbased models for simulating ground-water flow in three 

dimensions (Trescott, 1975; Trescott and Larson, 1976), 

geochemical models and concepts that have regional applications (Back, 1966; Plummer, 1977), and results from investigations of anomalous fluid pressures in sediments of 

the Gulf Coastal Plain (Bredehoeft and Hanshaw, 1968; 

Wallace and others, 1979). Results from the NRP since the 

beginning of the RASA Program that were particularly 

helpful include development of methods to address data 

uncertainty in ground-water-flow modeling (Cooley and 

others, 1986) and development of a computer model for 

simulating ground-water flow with heat or solute transport 

(Voss, 1984 ). To support the research projects of the NRP, 

the RASA Program contributes about 15 to 20 percent of 

its annual allocations. This contribution is a long-term investment that benefits the RASA Program as well as future programs. 

The RASA Program has produced a vast amount of 

important information on the Nation's major aquifer systems. As of mid-1992, nearly 900 reports were published. 

Additional reports are currently in preparation or in review. 

These reports range from articles in specialized scientific 

journals and contributions to national and international 

meetings and symposia, to map, data, and interpretive reports published by the U.S. Geological Survey and cooperating State and local agencies. 

This circular (1) states the objectives and approaches 

of the RASA Program, (2) describes the aquifer systems 

investigated by the RASA Program, and (3) summarizes 

significant findings and describes the status of the program 

as of mid-1992. 

REGIONAL AQUIFER SYSTEMS 

The concept of regional ground-water flow was formulated in the 1800's when artesian ground-water flow 

was discovered in the central and western parts of the 

United States. 

T.C. Chamberlin (1885) outlined seven prerequisites 

for artesian ground-water flow and described the hydrogeologic properties of water-yielding beds and confining units. 

The seven prerequisites described by Chamberlin are ( 1) a 

pervious stratum to permit the entrance and the escape of 

the water; (2) a water-tight bed below to prevent passage of 

the water downward; (3) a like impervious bed above to 

prevent escape upward, for the water, being under pressure 

from the fountain-head, would otherwise find relief in that 

direction; (4) an inclination of these beds, so that the edge at 

which the waters enter will be higher than the surface at the 

well; (5) a suitable exposure of the edge of the porous 

stratum, so that it may take in a sufficient supply of water; 

(6) an adequate rain-fall to furnish this supply; and (7) an 

absence of any escape for the water at a lower level than the 

surface at the well. Importantly, Chamberlin recognized that 

there are no totally impermeable confining beds: "*** no 
4 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

rock is absolutely impenetrable to water" (Chamberlin, 

1885, p. 137). 

water is also found in smaller amounts in the Niobrara 

Chalk beds above, in the basal beds of the drift formations, 

and in local beds of sand in the great series of clays lying 

above the Dakota formation. The Dakota formation consists 

of sand and sandstones which are particularly favorable in 

their character for the storage and transmission of water. 

The formation is not entirely sand and sandstone, but contains beds of clay irregularly dispersed, and often streaks of 

iron pyrites and iron stone. The texture of its sands and 

sandstones being variable, the capacity for holding water is 

One of the earliest descriptions of a regional aquifer 

system is given by N.H. Darton (1896) of "The great artesian basin of the eastern portion of South Dakota and 

North Dakota." Darton (1896, p. 612-613) described the 

"water horizons" as follows: 

The principal water-bearing bed in the Dakotas appears to 

be the eastward extension of the Dakota sandstones, but 

-..... --·0 

PUERTO 

RICO 

Segment 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

HAWAII D 

EXPLANATION 

Content 

Introductory material 

California, Nevada 

Arizona, Colorado, New Mexico, Utah 

Kansas, Missouri, Nebraska 

Oklahoma, Texas 

Arkansas, Louisiana, Mississippi 

Alabama, Florida, Georgia, South Carolina 

Idaho, Oregon, Washington 

Montana, North Dakota, South Dakota, Wyoming 

Iowa, Michigan, Minnesota, Wisconsin 

Illinois, Indiana, Kentucky, Ohio, Tennessee 

Delaware, Maryland, New Jersey, North Carolina, 

Pennsylvania, Virginia, West Virginia 

Connecticut, Maine, Massachusetts, New Hampshire, 

New York, Rhode Island, Vermont 

Alaska, Hawaii, Puerto Rico 

Atlas chapter 

730-A 

730-B 

730-C 

730-D 

730-E 

730-F 

730-G 

730-H 

730-I 

730-J 

730-K 

730-L 

730-M 

730-N 

Figure 2. States contained in each segment of the Ground Water Atlas of the United States. 
INTRODUCTION 5 

by no means constant. In every well of which I have heard 

that has penetrated the Dakota formation, some water has 

been found in one bed, if not in others, and often several 

horizons of water-bearing sands have been penetrated. In 

many of the wells two or three, and in some as many as 

seven flows have been found, separated either by clays, 

beds of pyrites, or more compact sands or sand rock. 

Later, Darton (1905) presented a potentiometric surface map of the Dakota Sandstone and associated sandstones based on well reports and altitudes of sandstone 

outcrops in several States. This map indicated a west-to-east 

potentiometric gradient and led Darton to conclude that recharge occurred on the west on the flanks of the Black Hills 

and Front Range of the Rockies and discharge occurred 

several hundred miles to the east along the Missouri River. 

The pioneer concepts of Chamberlin and Darton 

about regional aquifers, confining units, and regional flow 

are still valid today. A mapped regional aquifer may contain many locally discontinuous, low-permeability confining beds. On the other hand, a mapped regional confining 

unit may contain locally discontinuous, permeable beds 

that yield significant amounts of water to wells. Aquifers 

and confining units are mapped on the basis of their hy .. 

draulic properties, specifically the degree of contrast in hydraulic conductivity (a parameter used for measuring the 

ease of transmitting water through rocks), rather than on 

geologic age of the rocks. Thus, a regional aquifer or a 

regional confining unit may contain many different geologic formations or time-stratigraphic units or parts of such 

units (for examples, see fig. 4 in Laney and Davidson, 

1986). A regional aquifer system is defined as a body of 

rock containing two or more regional aquifers separated at 

least locally by confining units. Examples include aquifer 

systems underlying the Great Plains, Gulf Coastal Plain, 

and Atlantic Coastal Plain. 

In many parts of the country, individual aquifers are 

present within a single basin or valley and regional aquifers do not exist. Although, each basin or valley aquifer is 

hydrologically independent, many of the basin and valley 

aquifers share common characteristics. Investigations of a 

few of these aquifers can establish common principles and 

hydrogeologic factors controlling the occurrence, movement, and quality of water. Examples of such aquifers are 

found in the alluvial basins of Arizona and New Mexico 

and in the glacial outwash valleys in the Northeastern 

United States. 

Thus, aquifer systems studied in the RASA Program 

are of two general types: (1) an aquifer system comprised 

of an extensive set of aquifers and confining units that 

may be discontinuous locally, but which act hydrologically 

as a single system on a regional scale; and (2) a system 

consisting of a set of independent aquifers that share many 

common characteristics hydrologically. Twenty-eight such 

aquifer systems were selected for study under the RASA 

Program (fig. 1). 

OBJECTIVES AND APPROACH 

The objectives of the RASA Program are to define 

the regional aspects of geology, hydrology, and geochemistry of each studied aquifer system. The two basic approaches used to meet RASA Program objectives are the 

compilation of data and the construction of computer-based 

models to simulate ground-water flow. Study results are 

intended to be used for regional assessment of the groundwater resources and for support of local and detailed studies. Each study is designed to fit particular regional needs. 

However, every study uses computer-based simulation 

models to characterize the flow system and to evaluate the 

effects on the system by ground-water development and 

various types of land uses. 

One of the important aspects of the RASA Program is 

the synthesis of a large volume of data (collected during 

specific studies or obtained from individual wells) into a 

new data set that broadly describes an aquifer system covering an area of tens of thousands of square miles on a 

regional scale. Many detailed aspects that are important for 

local studies therefore are eliminated during the data-synthesizing process. A regional study has a different scope and 

purpose than a local study. This can be illustrated by drawing 

an analogy between the ground-water information required 

for local- and regional-scale studies and the highways and 

streets shown on maps with different scales. Information 

needed for a regional study is analogous to a map showing 

interstate highways and major State roads. Information 

needed for a local ground-water study is analogous to a city 

street map. This concept of scaling should be kept in mind 

when using the RASA results. 

Each RASA study relies primarily on existing data 

that are assembled, analyzed, examined, synthesized, and 

interpreted from available sources. These sources include 

publications, and office files as needed to form a complete 

new data set. This new data set is then analyzed and incorporated into simulation models. If critical data gaps are 

discovered, new data-collection activities such as exploratory drilling and synoptic water-level measurements are 

undertaken. New data collection is kept to a minimum to 

conserve the limited financial and human resources. Examples of new data collection have included drilling test 

wells during studies of regional aquifer systems in the 

Midwest and along the Atlantic Coastal areas (Davis and 

others, 1983; Trapp and others, 1984; Reid and others, 

1986; Sun, 1986, fig. 37) and collection of water samples 

for isotope analyses to support the interpretation of anomalous geochemical data observed in a regional aquifer system in the northern Midwest (Siegel, 1989, 1991). 

The RASA Program took advantage of available opportunities to broaden data bases. For example, during the 

study of the Floridan aquifer system in 1979, four "wildcat" 

oil-test wells were drilled on the Atlantic continental shelf 

offshore from southeast Georgia and northeast Florida. One 
6 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

of the test wells, about 55 mi east of Fernandina Beach, 

Florida, was estimated to be located near the seaward limit 

of freshwater discharge from the Floridan aquifer system. 

The USGS obtained permission from the well owner to 

conduct hydrologic testing before the well was abandoned 

and plugged. With this new information it was possible to 

estimate the offshore location of the freshwater-saltwater 

transition zone in the Floridan aquifer system (Johnston and 

others, 1982). At a fraction of the cost of several million 

dollars needed to drill an offshore test well, the RASA study 

gained valuable information on the Floridan aquifer system. 

Similar efforts are found throughout the RASA Program. 

During the early 1980's, formalized computer-based 

geographic information systems (GIS) were not widely 

available. Therefore, it was necessary for personnel of 

early RASA studies to develop their own systems to handle the vast quantities of data. Some of the studies that 

began later used GIS to handle the data. Most of the early 

RASA data were stored on magnetic tapes (Kontis and 

Mandie, 1980's; Downey, 1982; Luckey and Ferrigno, 

1982). 

A major objective of the RASA Program was to provide a complete description of the hydrogeologic framework, hydraulic properties, regional flow system, and 

geochemistry of regional aquifer systems. To meet this objective, it was necessary to prepare a series of regional 

maps that generally did not exist prior to the RASA studies. Some of the most important are regional potentiometric surface maps of the principal aquifers. Such maps are 

essential for analyzing regional ground-water flow systems. The effect of development can be evaluated using 

maps of estimated predevelopment potentiometric surfaces 

together with a series of potentiometric maps constructed 

for various times since development began. 

Other important regional maps show the altitude and 

configuration of tops of aquifers, the thicknesses of aquifers and confining units, and hydraulic properties such as 

transmissivity and specific yield. Most RASA Projects 

also prepare regional maps describing the ground-water 

chemistry; these maps show concentrations of major dissolved constituents, chemical water types, and other chemical parameters. These maps can be used to interpret the 

evolution of water chemistry from recharge areas to discharge areas. Examples of regional geohydrologic and geochemical maps prepared during the RASA Program are 

presented in the following sections. 

The other major approach of the RASA Program is 

the use of computer-based models to simulate groundwater flow systems using estimated hydrogeologic and hydrologic characteristics as model inputs. The input values 

to the model are then adjusted until the model-simulated 

water levels or flows closely match the observed or estimated water levels or flows of the aquifer system. Simulations of both natural (predevelopment) conditions and 

present-day conditions are generally done. Results of these 

simulations are used to define changes in recharge and discharge conditions brought about by development. In addition, geochemical models are used in some RASA projects 

to examine changes in ground-water chemistry along flow 

paths from recharge areas to discharge areas. 

The regional flow models are intended to provide an 

improved understanding of the major features of regional 

flow systems. However, owing to the large area covered 

by the regional models and the coarse mesh used in them, 

they generally do not provide detailed information on aquifer systems useful to water planners and managers at a 

local level. To provide the greater resolution needed for a 

more detailed analysis of the effects of ground-water development, RASA regional flow models may be further divided into subregional models. Boundary conditions for 

subregional models are supplied by the regional flow 

model. For example, the Northern Atlantic Coastal Plain 

RASA study included a regional flow model that represented an aquifer system underlying an area from the Fall 

Line on the west to the offshore boundary in the Atlantic 

Ocean on the east and from Long Island, New York, in the 

north to Cape Hatteras, North Carolina, in the south. The 

flow system was further simulated by subregional models 

for (1) New Jersey; (2) Delaware, Maryland, and District 

of Columbia; (3) Virginia; and (4) North Carolina. 

AVAILABILITY OF STUDY RESULTS 

Results of RASA studies are being released through 

scientific journals, proceedings of symposiums, and U.S. 

Geological Survey publications such as Open-File Reports, 

Water-Resources Investigations Reports, Hydrologic Investigations Atlases, Water-Supply Papers, Professional 

Papers, and Miscellaneous Maps. Upon completion of 

each RASA study, U.S. Geological Survey Professional 

Papers are being published to summarize the study results. 

A block of Professional Paper numbers between 1400 and 

1428 is reserved for release of the RASA results. A Professional Paper number assigned to a particular RASA 

study is further divided into chapters that are designated 

by letters. Typically chapter A summarizes important aspects of the study, such as the hydrogeologic framework, 

hydrologic properties of aquifers, quality of water, and dynamics of the regional flow system. Succeeding chapters 

B, C, D, and so forth discuss a specific subject or area in 

more detail than given in chapter A. As of mid-1992, nearly 900 reports had been published under the RASA Program. A bibliography of the RASA publications as of May 

1991 has been compiled by Sun and Weeks ( 1991 ). 

Compilations of data used to prepare regional hydrogeologic maps and cross sections, water chemistry maps, 

and potentiometric surface maps were generally released as 

Open-File Reports-for example, Lane (1988) and Miller 

(1988). 
INTRODUCTION 7 

ACKNOWLEDGMENTS 

The summaries of ongoing RASA studies presented 

in this report are based on writeups prepared by the project 

chiefs: Edward Bugliosi, Peter Bush, David Clark, Norman 

Grannemann, Peter Martin, James Miller, Lindsay Swain, 

and John Vaccaro. Thanks are also due the many former 

RASA investigators for providing information used by the 

authors to prepare the summaries of completed RASA 

projects. Thanks also to Peter Bush for rewriting the authors' 

preliminary draft of the section entitled "Simulation of regional ground-water flow and the effect of scale." 

SIMULATION OF REGIONAL GROUND-WATER 

FLOW AND THE EFFECT OF SCALE 

Nearly all of the RASA studies discussed in this report used computer-based models-that is, computerized 

numerical methods to solve mathematical models of 

ground-water flow-to simulate ground-water flow. The 

primary goal of the modeling is to provide a quantitative 

understanding of regional flow in the aquifer systems. 

Most of the flow models are three-dimensional, finite-difference models with modules or groups of modules for 

simulating specific features of the hydrologic system; for 

example, pumping from wells screened in more than one 

aquifer, land subsidence, or stream-aquifer relations. The 

models require definition of conditions at aquifer-system 

boundaries, and input data that typically include transmissivity, storage coefficient, recharge and discharge, measured or estimated heads, and other properties pertaining to 

specific hydrologic features. Once developed, models are 

calibrated by adjusting the input data within physically realistic ranges in a series of simulations until computed 

heads and flows approximate measured values. Most 

RASA model calibrations were achieved by trial-and-error 

processes, although some used computer methods to estimate selected input properties (parameter estimation techniques). The final step associated with model development 

involves simulation of historical hydrologic conditions different from (and independent of) those used for calibration. This process, sometimes referred to as verification, 

commonly requires additional adjustment or refinement of 

the input data until satisfactory simulation of both calibration conditions and verification conditions are obtained. 

Some hydrologists have objected to the use of the terms 

verification or validation to describe this process because 

of the inference that the resulting model is capable of accurately predicting future responses to pumping or other 

.stresses. Konikow and Bredehoeft (1992) state that these 

terms are misleading and suggest using more meaningful 

descriptors of the process such as model testing, history 

matching, and sensitivity testing. For this reason, the terms 

"verification" and "validation" are not used in this report. 

The regional RASA models are not intended to be 

used for management or planning purposes at a local scale. 

The large areal extent of aquifers modeled and a lack of data 

in large areas preclude the use of these models to address 

local or site-specific water-resource issues. RASA model 

studies have drawn attention to the fact that modeling results 

are scale related and have provided insight on the subject, 

but they have not explained all of the complexities, some of 

which are discussed below. 

The computerized numerical methods for simulating 

ground-water flow are based on discretization; that is, dividing the aquifer system into a series of blocks or elemental volumes within which all of the hydrogeologic 

properties are assumed to be equal. In the finite-difference 

approach, a single head value, whether measured or calculated, is associated with a block. Flow enters or leaves a 

block through its faces. However, because the actual distributions of hydrologic properties are continuous rather 

than discrete, numerical methods of simulation are approximate. Even if the distributions of aquifer-system properties were known exactly (which they can never be), 

characterizing each as a series of discrete values associated with finite volumes in the subsurface for modeling purposes would render the simulations approximate. Logic 

indicates that the larger the blocks of equal properties (that 

is, the larger the scale of discretization), the more "approximate" the simulation likely becomes, and the less accurately the simulated system represents the actual system. 

Because the regional RASA models were developed 

to simulate aquifer systems of tens of thousands of square 

miles-for example, those underlying the Atlantic and 

Gulf Coastal Plains, the High Plains, the Great Plains, the 

California Central Valley, and the Snake River Plain-the 

discretization of the systems is relatively coarse. The features of these "coarse-mesh" models are summarized in 

table 1. The area of the blocks in the models ranges from 

16 to 256 mi2. In most terranes where a model includes 

near-surface hydrogeologic units, as the blocks of the 

model become larger, the fraction of the total groundwater flow that the model simulates becomes smaller. 

Water moves under the influence of a gradient in head; in 

the approximated flow system of the model, gradients in 

head exist only between blocks. Thus, water that enters 

and leaves the aquifer system within the area represented 

by the faces of a single block cannot be simulated (except 

by superimposing sources or sinks, a mechanism that cannot be practically used to account for all inflows and outflows). In areas of hills and valleys with numerous small 

streams, large variations in head may exist within volumes 

in the subsurface represented by model blocks (and single 

head values). Much of the recharge in such areas travels 

only short distances before discharging to the streams. 

These local variations in head and the consequent local 

flow are not simulated by regional models, as indicated in 

figure 3. In contrast, in areas of little topographic relief 
Table 1. Coarse-mesh computer models used for simulation of flow in very large regional aquifer systems. 00 

Regional Characteristics Computer RASA 

aquifer Problem of model program report :;tl system used references references 

~ 

1. California Central Valley Simulation of flow in sand-silt- 3-D finite-difference model; 4 Trescott (1975), Trescott and Williamson and others (1989) -0 

z aquifer system clay aquifer system involving active layers and 6-mile grid Larson (1976), Meyer and 

very F: large ground-water spacing Carr (1979), Torak (1982) 

withdrawals, land subsidence, > /:) and recharge by imported c::: 

surface water -ti1 

2. Floridan aquifer system Simulation of flow in carbonate 3-D finite-difference model; 2 Trescott (1975), Trescott and Bush and Johnston (1988) :;tl 

Cn aquifers characterized by large active layers and 8-mile grid Larson (1976) ...::: areal variations of spacing en 

transmissivity, flow activity, 

and pumpage ~ 3. Great Basin aquifer systems Simulation of shallow flow in 3-D finite-difference model; 2 McDonald and Harbaugh (1988) Prudic and others (in press) > z 

basin-fill deposits active layers and 5- by 7.5- > 

superimposed over deeper flow mile grid blocks ~ through fractured carbonate en 

rocks -

en 

;g 4. Great Plains aquifer system (of Simulation of flow in Dakota 3-D finite-difference model; 4 McDonald and Harbaugh (1988) Helgesen and others ( 1993) 0 

the Central Midwest RASA) Sandstone and associated active layers and 14-mile grid 

strata involving withdrawal of spacing ~ > ground water and petroleum ~ 

5. Gulf Coastal Plain aquifer Simulation of variable density 3-D finite-difference model; 10 Kuiper (1983, 1985) Williamson and others (1990) 0 

"TT 

systems flow in very thick sand-silt- active layers and 10-mile grid ~ 

clay 

faulting, 

aquifer 

salt 

systems 

domes, 

with spacing ~ 

c::: geopressured zones at depth, en 

and large ground-water 

withdrawals ~ 

6. High Plains aquifer Simulation of flow in unconfined 2-D finite-difference model; Trescott and Larson (1976), Luckey and others (1986) 

0 

t"" 

aquifer with largest ground- single layer and 10-mile grid Larson (1978) 0 

Q water withdrawals in the U.S. spacing (j 

and large reductions of F: saturated thickness locally en 

7. Northern 

Plain aquifer 

Atlantic 

system 

Coastal Simulation 

heterogeneous 

of flow 

clastic 

in 

aquifers 

3-D 

active 

finite-difference 

layers and 7-mile 

model; 

grid 

10 Trescott (1975), Leahy (1982) Leahy and Martin (1993) ~ trl 

with localized pumping centers spacing :< 

8. Northern Great Plains aquifer Simulation of regional flow 3-D finite-difference model; 5 Trescott (1975), Weiss (1982) Downey (1986) -\0 

-...l 

system system in Paleozoic and active layers and variable grid 00 

I 

Mesozoic bedrock aquifers spacing -\0 

prior to development of \0 

IV 

ground water and petroleum, 

and involving water of 

variable density 
9. Northern Midwest regional Simulation of flow in Cambrian- 3-D finite-difference model; 5 Trescott (1975), Kontis and Mandie and Kontis (1992) 

aquifer system Ordovician sandstone- active layers and 16-mile grid Mandie (1988) 

carbonate aquifers involving spacing 

pumping from multiaquifer 

wells and water of variable 

density 

I 0. Snake River Plain aquifer Simulation of flow in basalt Combined 2-D parameter Garabedian ( 1986) including Garabedian (1993) 

system aquifers of Eastern plain estimation model and 3-D code by R.L. Cooley; 

involving large withdrawals finite-difference model; 4 McDonald and Harbaugh 

and recharge by imported active layers and 4-mile grid (1988) 

surface water spacing 

II. Southeastern Coastal Plain Simulation of deep flow in clastic 3-D finite-difference model; 3 McDonald and Harbaugh ( 1988) Barker and Pernik (in press) 

aquifer system coastal plain aquifers with active layers and 8-mile grid 

locally heavy pumping spacing 

Regional-model 

grid spacing 

Local-model 

grid spacing 

~~~:·~,;f'"m"~T'"m"" '"m'"T'"m""l 

Figure 3. Relation of local- and regional 

scale ground-water flow to discretization 

in computer flow models. 

NOT TO SCALE 

c::=J 

c::=J 

-

+----

3

o..i'\el 

co-(1\\-(\eo 

EXPLANATION 

Aquifer 

Confining unit 

Regional-scale ground-water flow-Simulated in regional flow model 

Local-scale ground-water flow-Simulated only in local-scale models 

! 

8 

~ 

\0 
10 REGIONALAQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

and few streams, variations in head are relatively small, a 

much smaller fraction of the recharge discharges locally, 

and proportionately more of the total ground-water flow is 

simulated by a regional model. 

Although other factors are involved, the most important factors controlling the fraction of total ground-water 

flow simulated by a regional model that includes near-surface units are topography and surface drainage. These characteristics are typically variable across large regional aquifer 

systems; these variations, in turn, cause the fraction of total 

ground-water flow simulated by a coarse-mesh RASA model 

to vary spatially, thus making the flow difficult to quantify for 

the model as a whole. Most RASA modelers have confronted 

this problem by describing its effect on their simulations, but 

rigorous quantitative analysis of the effect is lacking. 

For most of the larger RASA studies in which regional 

models were made, subregional models of parts of the aquifer systems were also constructed. The areas of the blocks in 

the subregional models were correspondingly smaller. Thus, 

more of the local flow, and a larger fraction of the total 

ground-water flow, was simulated by these subregional 

models, although even the model block area (typically 4 to 

16 mi2

) is too large for these models to be used to address 

site-specific management issues. For investigating site-specific ground-water problems, local models (fig. 3) with a 

smaller block size (1 mi2 or less) are generally required. 

In essence, the difference between the amount of recharge and discharge in a real-world aquifer system and 

the amount of recharge and discharge in the corresponding 

simulated system is greatest where the variation in head is 

greatest in the real system. As block size in a model increases, the head variability within a volume of the subsurface represented by a model block likely increases. 

Where there is little head variation in the real system, 

there is less opportunity for discrepancy between the generalized aquifer head representing an entire model block 

and the actual heads in the real aquifer system. Accordingly, block size causes less of a difference between actual 

and simulated flow components in such areas. 

PART I. SELECTED DESCRIPTIONS 

AND FINDINGS ABOUT 

REGIONAL AQUIFER SYSTEMS 

The difficulty in trying to summarize the multiplicity 

of findings from the RASA Program resulted in a twopronged approach for this report. First, examples of selected findings are given (this section of the report). These 

examples serve to highlight the type of regional synthesis 

that has resulted from the RASA Program. They include 

example descriptions of hydrogeologic frameworks, different approaches used to define hydraulic properties, and 

comparisons of regional ground-water budgets. Finally, a 

summary evaluation of ground-water development effects 

is presented and possible and actual uses of results from 

the RASA Program are discussed. The examples are neither the most important nor exhaustive; rather, they are 

those from reports that are in print or those that are most 

illustrative. Abbreviated summaries of each regional aquifer system for which studies had begun by mid-1992 constitute the second major part of this report. 

REGIONAL DESCRIPTIONS OF 

HYDROGEOLOGIC FRAMEWORKS, 

FLOW-SYSTEM CHARACTERISTICS, 

AND GROUND-WATER CHEMISTRY 

A description of the hydrogeologic framework, hydraulic properties of regional aquifers and confining units, 

ground-water chemistry, and flow-system characteristics of 

the Nation's major aquifer systems has been an important 

part of the RASA Program. This aquifer system definition 

generally has been presented as regional maps from compilations of existing data such as drillers' logs, geophysical 

logs, geologic maps, and other ancillary information. However, the availability of existing data tends to be unevenly 

distributed for many of the major ground-water systems 

because most previous hydrologic investigations have been 

conducted in areas of heavy pumping with water quantity or 

quality problems. Elsewhere, data are often sparse. Therefore, an important task of RASA projects has been to fill 

data voids to provide more complete descriptions of a regional aquifer system. This effort to fill data voids has involved some exploratory drilling, conducting synoptic 

measurements of ground-water levels to map regional potentiometric surfaces, and collecting and analyzing water 

samples where water chemistry is unknown. 

The regional system descriptions derived from existing and newly acquired data have been described in several 

hundred reports. A number of these publications present, 

for the first time, maps depicting the hydrogeologic framework, water chemistry, and potentiometric surfaces of entire regional aquifer systems. A few examples of such maps 

are presented in this section. 

HYDROGEOLOGIC FRAMEWORKS 

RASA investigations are concerned with understanding ground-water flow systems on a regional scale, and 

therefore the focus of hydrogeologic framework studies is 

the delineation of regional aquifers and confining units. 

However, most previous hydrogeologic studies of the Nation's major ground-water systems have focused on local 

problems. As a result, the extent, thickness, and hydraulic 

properties of local aquifers and confining beds tend to be 

well known near pumping centers but may be poorly defined 

elsewhere. As noted by Sun (1986), the delineation and 

continuity of aquifers and confining units are dependent on 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 11 

the scale of the problem under study. Thus, a major effort of 

RASA projects has involved the combination of many 

"local" aquifers and confining beds into a small number of 

regional aquifers and confining units. 

The subdivision of a complex sedimentary or volcanic rock sequence into a few regionally extensive aquifers and confining units creates some problems. The use of 

different aquifer or rock-stratigraphic nomenclature in adjacent States for virtually continuous rock units must be 

considered in defining the "new" set of regional hydrogeologic units. In addition, the tops and bases of the newly 

delineated regional units may not coincide with those of 

either time-stratigraphic or rock-stratigraphic units or even 

previously defined aquifers and confining units. 

After a decision has been reached about a tentative 

framework of regional aquifers and confining units, the 

hydrogeologist must define the lateral and vertical extent 

of these "new" units. Such definition generally requires 

preparation of maps and sections showing the top, base, 

and thickness of these new regional hydrogeologic units. 

In addition, the relation of all previously defined aquifers 

and confining units and geologic units to the proposed regional hydrogeologic units need to be explained. 

An example of a terrain where many problems were 

encountered in defining a set of regional hydrogeologic units 

is the thick sequence of clastic and carbonate sediments 

underlying the Coastal Plain of the southeastern United 

States. The clastic rocks were studied under the Southeastern 

Coastal Plain RASA and the carbonate rocks under the 

Floridan RASA (fig. 1). The relations among the regional 

hydrogeologic units defined for the Southeastern Coastal 

Plain RASA and previously defined rock-stratigraphic units 

is shown in figure 4. Note that the regional aquifers generally consist of several formations. On the other hand, a 

formation may be partly in one aquifer and partly in another 

(for example the Eutaw Formation). Renken (1984) noted 

that it is difficult to impossible to apply existing formation or 

local-aquifer nomenclature to the regional aquifers and confining units in this RASA area. For each regional hydrogeologic unit, it was necessary to propose a new name. 

Difficulties can also arise in defining relations at the 

interface of major Coastal Plain aquifer systems. In southeastern Georgia and parts of Florida, the clastic rocks of the 

Southeastern Coastal Plain aquifer interfinger with the carbonate rocks of the Floridan aquifer system and facies 

changes are complex and occur over short distances (Miller, 

1986). The updip limit of the Floridan was arbitrarily placed 

where its thickness is less than 100 ft and where the clastic 

rocks interbedded with the limestone make up more than 50 

percent of the rock column (Miller, 1986, p. 48). 

For both the Southeastern Coastal Plain and Floridan 

aquifer systems, it was necessary to prepare maps showing 

the configuration of the top and thickness of the aquifer 

systems and their component aquifers and confining units. 

For the Floridan, it was not necessary to propose a new set 

of hydrogeologic names. However, as defined by Miller 

(1986, p. 44), the Floridan aquifer system encompasses 

more of the geologic section and extends over a wider geographic area than previous descriptions of the "Floridan" 

in the literature. It is noteworthy that, despite the large 

amount of hydrologic and geologic literature on the Floridan, the maps presented by Miller ( 1986) are the first detailed regional definition of the Floridan aquifer system 

and its component aquifers and confining units. 

The problems involved in defining the framework of a 

regional unconfined aquifer would appear to be less than 

those of the complex Coastal Plain aquifer system just described. The upper surface of an unconfined aquifer is simply the water table; the lower surface is the top of the 

underlying low-permeability rocks. However, mapping of 

these two surfaces must be done accurately because it is the 

basis for determining the volume of ground water in storage. 

The most extensive and heavily pumped unconfined 

aquifer in the United States is the High Plains aquifer, 

which underlies parts of eight western States between 

Texas and South Dakota (fig. 1 ). As described by Gutentag 

and others (1984), the High Plains aquifer is a water-table 

aquifer composed principally of near-surface sand and 

gravel deposits. The aquifer is made up of several geologic units of late Tertiary and Quaternary age. According to 

Gutentag and others (1984), "Hydraulic interconnection 

between geologic units that comprise the High Plains aquifer is sufficient to pemiit contouring a continuous water 

table throughout most of the region." A regional watertable map of the High Plains aquifer was prepared based 

on water-level measurements made during the winter and 

spring of 1980. Because the High Plains aquifer is very 

heavily pumped, this period was selected to minimize the 

effects of seasonal irrigation pumping. 

The High Plains aquifer is underlain by sedimentary 

rocks that range in age from Permian to Tertiary. The configuration of the base of the aquifer has been greatly affected by local faulting and also by dissolution in 

underlying Permian evaporite beds. As with most regional 

aquifers, data required to define the configuration of the 

High Plains aquifer are adequate in some areas; however, 

in other areas, few data exist and mapped contours must 

be generalized (Gutentag and others, 1984). Based on the 

vertical distance between the 1980 water table and the 

base of the aquifer, the saturated thickness of the High 

Plains aquifer in 1980 was mapped as shown in figure 5. 

This map, together with data on the distribution of specific 

yield, was used to estimate the volume of "drainable 

water" in storage in the High Plains aquifer as of 1980. 

AREAL DISTRIBUTION OF HYDRAULIC PROPERTIES 

The principal hydraulic properties that control flow in 

unconfined aquifers are hydraulic conductivity and specific 

yield, and in confined aquifers are transmissivity, storage 
12 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

coefficient, and leakage coefficient (of confining units). Information on the areal distribution of these properties is 

required for the computer-based models used by nearly all 

RASA projects to study regional ground-water flow systems. Prior to the RASA Program, maps showing the distribution of transmissivity, specific yield, and other hydraulic 

properties did not exist for many of the regional aquifer 

Mississippi Alabama 

systems. Publication of these maps are an important product of the RASA Program. 

Different approaches have been used in RASA 

projects to define the distribution of hydraulic properties 

depending on data availability and the local geology. Two 

maps presented here are based on different approaches to 

defining the areal distribution of transmissivity for (1) a 

Georgia South Carolina 

North East-central West East West East Southwest Northeast 

EXPLANATION 

Surficial aquifer 

Upper confining unit of Floridan aquifer system 

Floridan aquifer system 

Southeastern Coastal Plain aquifer system 

Chickasawhay River aquifer 

Pearl River confining unit 

Pearl River aquifer 

[=:J 

~ . 

. 

[=:J 

LJJJ 

~ 

~ 

CJ 

Modified from Miller and Renken, 1988 

Chattahoochee River confining unit 

Chattahoochee River aquifer 

Black Warrior River confining unit 

Black Warrior River aquifer 

Base of Southeastern Coastal Plain aquifer system 

Hydrogeologic unit absent 

Figure 4. Rock-stratigraphic and regional hydrogeologic names applied to clastic and carbonate rocks of the Southeastern Coastal 

Plain. 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 13 

fractured Paleozoic carbonate aquifer in the arid west, characterized by deep burial and slight ground-water development; and (2) a semiconsolidated Tertiary carbonate aquifer 

in the humid southeast, occurring at shallow to moderate 

depth and having large ground-water withdrawals locally. 

The first map was compiled from drill-stem tests and core 

measurements, and the second from aquifer test data, geologic information, and simulation results. 

105' 

WYOMING 

-------

35' 

NEW MEXICO 

100' 

EXPLANATION 

Saturated thickness, in feet 

0 to 100 

100 to 200 

200 to 400 

400 to 600 

600 to 1,000 

Aquifer boundary 

95' 

-------------

IOWA 

------

MISSOURI 

-----~ 

\-------

r--- 100 200 MILES 

100 200 KILOMETERS 

Figure 5. Saturated thickness of the High Plains aquifer, 1980 (from Gutentag and others, 1984). 
14 REGIONALAQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

The distribution of transmissivity for a regional aquifer composed of Devonian and Mississippian carbonate 

rocks in the Upper Colorado River Basin of the Western 

United States is shown in figure 6. The aquifer consists of 

limestone and dolomite that crop out on the flanks of mountain ranges but, for the most part, are deeply buried beneath 

younger Paleozoic and Mesozoic sedimentary rocks. Permeability of the carbonate rocks is derived from intergranular porosity and interconnected fractures and vugs. The 

permeability increases from structural basins to uplifted 

areas as the rocks become increasingly fractured and less 

cemented (Geldon, 1988). Estimates of transmissivity are 

based largely on drill-stem tests and laboratory (permeameter) measurements on cores. The estimated transmissivity 

ranges from less than 0.1 ft2

/d in the deeper parts of the 

basins to more than 10,000 ft2/d in the uplifted areas. 

The transmissivity of the Upper Floridan aquifer in 

the southeastern United States is shown in figure 7. The 

aquifer consists of semiconsolidated limestone and dolomite of Tertiary age in which the water-bearing openings 

range in size from small intergranular openings in coquinas (fossil hash) to large solution cavities. Transmissivity values were obtained from more than 100 multiwell 

aquifer tests, located mostly in areas of large ground-water 

development. Elsewhere, transmissivity was estimated either on the basis of lithology and thickness of the aquifer 

or on simulation results. Estimates of transmissivity based 

on lithology assume that micrites (lime muds) have permeabilities similar to clay and silt and thus low transmissivity. In contrast, pelletal limestones (made up largely of 

loosely cemented fossil fragments and locally containing 

large solution openings) were considered to have permeabilities similar to sand and gravel and thus high transmissivity. Specific capacity was determined to be a poor basis 

for estimating the transmissivity of the Upper Floridan aquifer (Bush and Johnston, 1988). 

The highest transmissivities (greater than 1 million 

ft2/d) are in areas where the Upper Floridan is thick, either 

unconfined or thinly confined, and characterized by extensive karst development. Aquifer tests (involving pumping 

and observation wells) are virtually impossible to conduct 

in the karst areas because of logistical problems (removing 

large volumes of water to prevent recycling) and because 

non-Darcian flow can preclude the use of standard methods 

of aquifer-test analysis. However, simulation suggests that 

transmissivity is in the range of 1 to 10 million ft2/d. The 

areas of low transmissivity (less than 10,000 ft2/d) occur 

either where the aquifer contains large amounts of micritic 

limestone or in updip areas where the aquifer is thin. 

FLOW-SYSTEM CHARACTERISTICS 

The areal distribution of hydraulic heads and the historical changes in water levels in the major aquifer systerns were reasonably well documented prior to the RASA 

Program. However, the available water-level data and potentiometric surface maps tend to be concentrated in areas 

of large ground-water withdrawals. Many water-level contour maps have been prepared for States, counties, or other 

political subdivisions; however, maps rarely are prepared 

that provide complete areal coverage of a regional aquifer. 

A major contribution of the RASA Program has been 

preparation of some of the first aquifer-wide potentiometric surface maps. For example, although synoptic potentiometric surface maps had been prepared individually for the 

Florida and Georgia parts of the Upper Floridan aquifer, 

no such maps had been compiled for the four-State area 

that is underlain by the Floridan. Under the RASA Program, an aquifer-wide synoptic potentiometric surface map 

of the Upper Floridan was prepared on the basis of more 

than 2,700 water-level and pressure-head measurements 

made in May 1980. 

A major objective of the RASA Program is to define 

the effect of development on ground-water-flow systems. 

This task normally involves comparison of predevelopment 

and recent potentiometric surface maps to define the areal 

extent and magnitude of long-term regional head declines 

caused by pumping. Examples of such maps (figs. 8 and 9) 

show the estimated predevelopment and 1985 potentiometric surfaces for the composite Cambrian-Ordovician aquifer system in parts of Illinois, Indiana, and Wisconsin. 

Comparison of the two maps indicates that groundwater withdrawals have created major cones of depression 

around the cities of Chicago and Milwaukee. Maps such 

as these are essential for calibrating computer flow models 

for predevelopment and recent pumping conditions. 

Maps showing head differences between aquifers are 

essential for quantifying vertical flow rates in complex aquifer systems. The head difference between the water table 

and the lower pumped zone (a confined aquifer) in the Central Valley aquifer system of California is shown in figure 

10. Such head data can be used in computer flow models to 

simulate rates of flow between the unconfined and confined 

aquifers if field values of leakage coefficient of the intervening confining units are available or can be estimated. 

Alternatively, if the rates of vertical flow can be estimated 

independently (from water budgets), the head difference 

can be used to compute the values of leakage coefficient for 

the confining unit to deliver the estimated flow rates. 

GROUND-WATER CHEMISTRY 

RASA projects are concerned with describing the 

water chemistry and identifying the geochemical processes 

taking place in regional aquifer systems. For the descriptions of ground-water chemistry, RASA investigators rely 

primarily on analyses of water samples from wells and 

springs in computer files of the U.S. Geological Survey. 
40 " 

PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 

115 " 11 ° 

I 

' 

IDAHO l--

WYOMING 

----- I ·--- --- ' 

I 

I 

I 

I 

' 

I · ·--·--- - ... 

: I 

I 

' ' I 

' 

I 

Bounda ry of 

I 

' ' I study area _j_ 

----------~--

1------------ -" --~--

, 

I 

NEW MEXICO 

50 100 MILES 

50 100 KILOMETERS 

EXPLANATION 

Area where hydrostratigraphic unit is missing 

because of erosion or nondeposition 

Porosity 

Less than 1 to 4 percent 

4 to 8 percent 

8 to 11 percent 

-10- Line of equal estimated transmissivity-Interval, 

in feet squared per day, is variable 

Figure 6o Porosity and transmissivity of the Devonian and Mississippian carbonate rocks hydrostratigraphic unit in the Upper 

Colorado River Basin, Arizona, Colorado, New Mexico, Utah, and Wyoming (from Geldon, 1988). 

15 
16 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

These data are used to prepare maps and diagrams showing 

concentrations of major constituents (such as calcium and 

sulfate), total dissolved-solids concentrations, and hydrochemical facies. Although this approach is similar to that 

traditionally used in county or small-basin studies, description of ground-water chemistry on a regional scale generally requires more explanation of the spatial variations. 

Some of the factors that must be considered in regionalscale studies are the continuity of minor aquifers and 

pumped zones and their relation to regionally extensive aquifers, mixing of ground water among permeable zones in 

open-hole wells, and the geochemical processes that have 

affected the ground-water chemistry through geologic time. 

GULF 

OF 

oV£~ 0 50 100 MILES ('0 

0 100 KILOMETERS 

EXPLANATION 

An example of one type of map used to characterize 

the water chemistry in regional aquifers is shown in figure 

11. This map shows the hydrochemical facies of ground 

water in two aquifers of the Cambrian-Ordovician aquifer 

system in the Northern Midwest United States. The map 

was used by Siegel (1989) to relate ground-water chemistry to aquifer lithology and the major geochemical and hydrologic processes that are occurring in the aquifer system. 

REGIONAL GROUND-WATER BUDGETS 

One of the major contributions of the RASA Program was the calculation of hydrologic budgets for major 

Estimated transmissivity based on 

aquifer tests, geology, and computer 

simulation, in feet squared per day 

C:=J Less than 10,000 

C:=J 10,000 to 50,000 

C:=J 50,000 to 100,000 

.. 100,000 to 250,000 

.. 250,000 to 1,000,000 

.. Greater than 1,000,000 

Figure 7. Transmissivity of the Upper Floridan aquifer, southeastern United States (from 

Johnston and Bush, 1988). 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 

Base modified from U. S. Geological 

Survey Digital line Graph, 1:2.000,000 

87° 

EXPLANATION 

-900- Potentiometric contour-Shows approximate 

altitude of the potentiometric surface of the 

composite Cambrian-Ordovician aquifer 

system prior to its development in the late 

1800's. Contour interval 50 feet. Datum is 

sea level 

• • • • • • • Major potentiometric divide 

.... 

.... 

Hydrology by H. L. Young, 1988 

Illinois part modified from 

Suter and others, 1959 

17 

Figure 8. Approximate predevelopment potentiometric surface for the composite Cambrian-Ordovician aquifer system in the Chicago-Milwaukee area (from Young and others, 1989a). 
18 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Base modified from U. S. Geological 

Survey Digital Line Graph. I :2,000,000 

87 ° 

EXPLANATION 

-900-- Potentiometric contour-Shows altitude of 

the potentiometric surface of the composite 

Cambrian-Ordovician aquifer system, 1985. 

Dashed where inferred. Contour interval, in 

feet, is variable. Datum is sea level 

• • • • • • • Major potentiometric divide 

10 20 MILES 

10 20 KILOMETERS 

Hydrology by H. L. Young 

Illinois part modified from 

Sasman and others, 1986 

Figure 9. Potentiometric surface for the composite Cambrian-Ordovician aquifer system in the Chicago-Milwaukee area, 1985 

(from Young and others, 1989a). 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 19 

ground-water systems in the United States. Several of the 

completed RASA projects provide for the first time hydrologic budgets for both predevelopment and developed 

124' 123' 122' 121' 

40' 

39' 

38' 

37' 

38' 

25 50 MILES 

35' 25 50 KILOMETERS 

conditions. These budgets are based on computer simulations of predevelopment and development conditions and 

utilize the regional hydrologic data bases, information on 

120' 119' 118' 

EXPLANATION 

Hydraulic head difference between 

the water table and the lower 

pumped zone 

CIIJIJ . 0 to 40 feet above the water table 

CJ 0 to 40 feet below the water table 

CJ 41 to 100 feet below the water table 

101 to 200 feet below the water table 

.. 201 to 300 feet below the water table 

.. 301 to 400 feet below the water table 

.. 

More than 400 feet below the water 

table 

~ Not in aquifer system 

Figure 10. Hydraulic head difference between the water table and the lower pumped zone, Central Valley aquifer system, California, 

spring 1961 (from Williamson and others, 1989). 
20 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

boundary conditions, and pumpage data, compiled during 

the RASA studies. 

Some of the ground-water budgets are derived from 

coarse-mesh computer models designed to simulate "regional flow." Such budgets do not include local flow that enters 

Ml NESOTA 

50 100 MILES 

50 100 KILOMETERS 

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970 

and exits regional aquifers after traveling only a few miles, 

or flow in overlying surficial aquifers (fig. 3). The percent 

of the total ground-water flow system represented by "regional flow" is dependent on the mesh size of the model as 

discussed in the earlier section on simulation of regional 

MICHIGAN 

EXPLANATION 

Hydrochemical facies zone boundary-Dashed where 

approximate 

Well sampled for chemical analysis during this study 

e Open to Mount Simon aquifer 

e Open to St. Peter-Prairie du Chien--Jordan aquifer 

Figure 11. Hydrochemical facies of ground water in the Mount Simon and St. Peter-Prairie du Chien-Jordan 

aquifers, Cambrian-Ordovician aquifer system in the Northern Midwest (from Siegel, 1989). 
PART I. SELECfED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 21 

flow. Under natural conditions, the amount of local flow is 

commonly much greater than regional flow, especially in 

humid areas. A common characteristic of both humid and 

semiarid areas is that rates of regional flow are almost always increased by development (Johnston, 1989). In the 

humid East, especially, much "local" flow is diverted into 

the regional flow system as withdrawals from confined aquifers increase. Therefore, the regional ground-water budget 

is characterized by increased recharge and increased groundwater circulation overall. However, the total recharge to the 

ground-water system may not have increased; only that part 

of the total recharge that percolates to the regional flow 

system increases. 

In this section, regional water budgets for eight of the 

most heavily pumped aquifer systems in the United States 

(fig. 1) are discussed. In the humid East and South, regional 

water budgets for the Northern Atlantic Coastal Plain, Gulf 

Coastal Plain, and Floridan aquifer systems are described. 

In the climatically diverse Central United States, budgets for 

the Cambrian-Ordovician aquifer system of the Northern 

Midwest, the Great Plains aquifer system (Dakota Sandstone 

and associated beds), and the High Plains aquifer (southern 

part) are examined. In the semiarid Western United States, 

budgets for the Central Valley aquifer system in California 

and the Snake River Plain aquifer system in Idaho are described. The brief summaries presented for each of the major 

systems emphasize changes in the rates of recharge and 

discharge caused by development. 

AQUIFER SYSTEMS OF THE ATLANTIC AND 

GULF COASTAL PLAINS 

The three major aquifer systems for which groundwater budgets are presented underlie the Eastern, Southeastern, and south-central United States, a region of humid, 

temperate to subtropical climate. The ground-water budgets were derived from simulation and vary somewhat in 

the percent of the total ground-water flow system that is 

represented as follows: 

(1) Northern Atlantic Coastal Plain-budget includes 

all flow (local and regional) in the ground-water system. 

(2) Floridan aquifer system-budget includes only 

flow that enters the Floridan and moves several miles prior 

to discharging from it. The percent of the total groundwater system included in the budget varies areally. In karst 

areas of central and northwest Florida, almost all recharge 

enters the Floridan and moves at least 10 mi prior to discharging to springs-thus, the budget for the aquifer system 

in that area represents nearly all flow in the ground-water 

system. In narrow outcrop belts or confined areas, local 

flow in surficial aquifers or within Floridan outcrops is generally not simulated-thus, the budget for the aquifer system 

in those areas represents only a small percent of the total 

.ground-water flow system. 

(3) Gulf Coastal Plain aquifer systems-budget includes only regional ground-water flow because of the 

coarse mesh used in the flow model (10-mi grid spacing). 

Because ground-water withdrawals have diverted water 

from local flow systems into the regional flow system, the 

water budget for development conditions includes a much 

greater percentage of the total ground-water flow than 

does the predevelopment budget. 

NORTHERN ATLANTIC COASTAL PLAIN AQUIFER SYSTEM 

The Northern Atlantic Coastal Plain aquifer system 

extends from Long Island, New York, to the North Carolina-South Carolina State boundary and includes approximately 55,000 mi2 (fig. 1). The aquifer system consists of 

consolidated and unconsolidated sand, silt, and clay of Jurassic to Holocene age (Trapp, 1993). These sediments 

form an eastward-thickening wedge and contain aquifers 

that are the major source of fresh ground water in the area. 

Ground-water withdrawals, primarily from the confined aquifers, were about 1.2 BgaVd in 1980. Simulation of 

pumping conditions in the coarse-mesh regional model used 

a top boundary condition termed "modified specified-flux 

boundary condition" to simulate withdrawals in the unconfined aquifers (Leahy and Martin, 1993). This boundary 

condition "which included stream-stage elevations, deep 

percolation, and discharge to streams allowed heads in the 

unconfined system to change with time in response to 

changing withdrawals" (Leahy and Martin, 1993). As aresult, the model simulated local flow in the unconfined part 

of the aquifer system in addition to deep (regional) flow in 

the confined aquifers. 

Ground-water budgets for the Northern Atlantic 

Coastal Plain aquifer system for predevelopment and 1980 

conditions are shown in figure 12. These budgets represent 

the total ground-water flow system, not simply deep regional flow. For this reason, the areal recharge rate is the 

same for both predevelopment and 1980 conditions-about 

40 BgaVd. Note that pumpage in 1980 represents only 

about 3 percent of the total recharge entering the flow system. Pumpage, which is primarily from the confined aquifers, is supplied mostly by an increase in downward 

percolation to the confined aquifers and a decrease in upward leakage from them. Less than 2 percent of the pumpage is supplied by water released from aquifer storage. 

However, Leahy and Martin (1993) emphasize that, because withdrawals are not evenly distributed over the system, the local effects of water released from storage can be 

significant. As in predevelopment time, shallow groundwater flow accounts for most of the flow through the aquifer system. However, by 1980, pumpage caused the rate of 

deep percolation into confined aquifers to more than double and the upward leakage from confined aquifers to 

occur at less than one-half the predevelopment rate. Based 
22 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

on simulation, Leahy and Martin (1993) conclude that the 

system rapidly adjusts (less than 3 yr) to changes in withdrawal rates. 

FLORIDAN AQUIFER SYSTEM 

The highly productive Floridan aquifer system underlies all of Florida, southern Georgia, and small parts of 

adjoining Alabama and South Carolina, for a total area of 

about 100,000 mi2 (fig. 1 ). The aquifer system is a sequence of hydraulically connected carbonate rocks (limestone and dolomite) that generally range in age from late 

Paleocene to early Miocene (Miller, 1986). Average rainfall of about 53 in/yr and generally flat topography combine to provide abundant recharge to the Floridan. The 

unconfined and semiconfined parts of the Floridan are 

characterized by high recharge (averaging 10-20 in/yr) 

and a vigorous shallow flow system with most discharge 

occurring at large springs. In contrast, ground-water flow 

is sluggish in parts of the aquifer system that are deeply 

buried and tightly confined. 

Increasing pumpage that reached 3 Bgal/d by 1980 

has resulted in regional water-level declines in three broad 

areas where the Floridan is thickly confined. This withdrawal has not greatly affected the ground-water budget of 

the Floridan, as can be seen in figure 12. Pumpage from 

the Floridan is supplied primarily by reduction of natural 

discharge and by increased recharge rather than by depletion of aquifer storage (Bush and Johnston, 1988). About 

20 percent is from reduced discharge to springs, streams, 

and lakes; about 20 percent is from reduced upward leakage, and about 60 percent is from increased recharge 

(downward percolation from surficial aquifers overlying 

the Floridan). As a result of induced recharge into the 

Floridan, the rate of flow through the aquifer system has 

increased from about 13.9 Bgalld (predevelopment conditions) to about 15.7 Bgal/d in 1980. 

The transient response to changes in withdrawal 

rates dissipates fairly rapidly (days or weeks) in most 

NORTHERN ATLANTIC 

COASTAL PLAIN 

AQUIFER SYSTEM 

FLORIDAN AQUIFER 

SYSTEM 

GULF COASTAL PLAIN 

AQUIFER SYSTEMS 

Predevelopment 

Natural recharge 40.0 

Unconfined· 

0.59. 

• • •• Confi~ed • • . 

.:--...._,. aquifers. ~ 

Discharge to 

streams 39.41 

Diffuse upward 

leakage 0.59 

Development conditions 

(1980) 

Natural 

recharge 40.0 Pumpage 1.21 

Decrease in storage 

0.02 

Discharge to 

streams 38.56 

Diffuse upward 

leakage 0.25 

Source: Leahy and Martin (1993) 

Predevelopment 

Natural recharge 13.9 

Natural 

discharge 

13.9 

Development condition 

(1980) 

Natural and 

induced recharge 

15.7 Pumpage 3.0 

Almost no change 

in storage 

Natural 

discharge 

12.7 

Source: Bush and Johnston (1988) 

Predevelopment 

Natural recharge 1.9 

. -~·· 

• •• Aquifer,· • • 

••• • system. • •• • 

Natural 

discharge 

1.9 

Development conditions 

(1980) 

Natural and 

induced recharge 

9.3 Pumpage 9.5 

Decrease in storage 

0.9 

Natural 

discharge 

0.7 

Source: Williamson and others (1990) 

Figure 12. Ground-water budgets before and after development of three major aquifer systems in the Atlantic and Gulf Coastal 

Plains (values in billion gallons per day). 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 23 

areas (Bush and Johnston, 1988). Thus (excluding seasonal changes), the Floridan aquifer system is considered to 

be approximately at equilibrium, except during periods 

following sustained increases in pumpage. 

GULF COASTAL PLAIN AQUIFER SYSTEMS 

The Gulf Coastal Plain aquifer systems consist mostly of Cenozoic age sediments that underlie approximately 

290,000 mi2 extending from Texas to Florida and including the Mississippi embayment, Gulf Coastal Plain of Texas, and offshore areas beneath the Gulf of Mexico (fig. 1 ). 

These sediments are an interbedded sequence of sand, silt, 

and clay with some gravel, lignite, and limestone (Hosman, 1991 ). The thickness of the sediments increases toward the Gulf of Mexico in a generally wedge shape. 

Ground-water pumpage has steadily increased in recent years to nearly 10 BgaVd and is used mostly for irrigation. The effect of ground-water withdrawals has been 

widespread water-level declines with land subsidence and 

saltwater intrusion locally. 

Williamson and others ( 1990) state that the amount 

of recharge to the regional ground-water flow system is 

not related to average rainfall but is more likely limited by 

the capacity of the aquifer systems to transmit ground 

water away from recharge areas. Because of the very large 

study area, Williamson and others ( 1990) used a very 

coarse-mesh model to simulate regional ground-water 

flow. However, they state that even with their 10-mi horizontal grid spacing, "the regional features affecting flow in 

the aquifer systems are generally preserved." 

Water budgets for the Gulf Coastal Plain aquifer systems for predevelopment and 1980 conditions (based on 

Williamson and others, 1990, p. 107-110) show that the 

rates of regional recharge and discharge have been substantially changed by ground-water pumpage, which by 1980 

was about 9.5 Bgalld (fig. 12). Note that the 1980 pumpage 

is about five times the predevelopment recharge rate. About 

80 percent of the 1980 pumpage was supplied by an increase 

in percolation to the regional flow system. The remaining 20 

percent was supplied by a decrease in natural discharge and 

to a lesser extent by loss of aquifer storage. Williamson and 

·others (1990) noted that some aquifers are "self-supporting," 

that is, most of their pumpage was supplied by increased 

recharge to, or decreased discharge from, the aquifer itself. 

On the other hand, one major aquifer with a very narrow 

outcrop and subcrop band had 80 percent of its pumpage 

provided by downward leakage from an overlying aquifer. 

AQUIFER SYSTEMS OF THE CENTRAL UNITED STATES 

The major aquifer systems described in this section 

underlie a region with a temperate, humid to dry continental 

climate. The Cambrian-Ordovician and Great Plains aquifer 

systems contain sandstone and carbonate rock aquifers that 

receive much of their recharge by leakage across overlying 

less permeable rocks. Simulated ground-water budgets for 

these aquifer systems include only their internal flow and do 

not include flow in overlying surficial, glacial, or sedimentary rock aquifers. In contrast, the High Plains aquifer is a 

regional water-table aquifer with a regional flow system that 

forms a major part of the total ground-water budget of the 

region. 

CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM 

IN THE NORTHERN MIDWEST 

The Cambrian-Ordovician aquifer system consists of 

three major aquifers (sandstone and dolomite) and two internal regional confining units (primarily sandstone and 

shale) that underlie 161,000 mi2 in Illinois, Indiana, Iowa, 

Wisconsin, Minnesota, and Missouri (fig. 1). Simulations 

of regional flow show that the aquifer system functions as 

a leaky confined system in which flow is controlled partly 

by internal confining units. Pumpage from the aquifer system began in 1860 and by 1976-80 averaged 781 Mgal/d. 

Largest withdrawals are in the Chicago/Milwaukee and 

Twin Cities (Minnesota) areas. The largest head declines 

(more than 900 ft; figs. 8 and 9) occur west of Chicago. 

Most recharge and discharge in the Northern Midwest occurs in local flow systems within glacial drift 

(Young and others, 1989b). Most flow in these local systems either does not enter the bedrock aquifers or penetrates only to shallow depths in them. Some water enters 

the confined Cambrian-Ordovician aquifer system and 

moves along flow paths of intermediate length to discharge areas (river valleys and Lake Michigan). Some 

water moves along deep regional flow paths that are as 

much as 400 mi from recharge to discharge areas. 

The predevelopment water budget (fig. 13) indicates 

that flow through this bedrock aquifer system was comparatively small (369 MgaVd), considering the large areal extent of the system. The 1976-80 pumpage of 781 MgaVd 

is more than double the predevelopment recharge rate. 

This pumpage was largely supplied by an increase in recharge (534 MgaVd) and to a lesser extent by a decrease 

in natural discharge ( 103 MgaVd) and release of water 

from aquifer storage (139 MgaVd). A detailed simulation 

of flow in the Cambrian-Ordovician aquifer system in the 

Chicago-Milwaukee area indicates that recharge to the 

system occurs most readily where it subcrops beneath glacial drift (Young, 1992a). 

GREAT PLAINS AQUIFER SYSTEM 

(DAKOTA SANDSTONE AND ASSOCIATED STRATA) 

The Great Plains aquifer system consists of sandstone, 

siltstone, and shale of Early Cretaceous age, including the 
24 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Dakota Sandstone and equivalent beds. As discussed in the 

Introduction, this system was recognized early as a major 

artesian reservoir (Darton, 1896, 1905). The aquifer system 

underlies a 170,000 mi2 area of Nebraska, Colorado, Kan-

·sas, and adjacent States. Withdrawals from the system have 

included both water and petroleum. The aquifer system 

transmits water very slowly, and in the deeper parts water is 

virtually stagnant (Helgesen and others, 1993). Much of the 

deep regional flow system contains saline water; however, 

along its eastern and southeastern margins, local shallow 

flow systems containing freshwater predominate. 

Prior to development, natural recharge to, and discharge from, this very large aquifer system was only about 

220 Mgalld (fig. 13). Simulation indicates that 87 percent 

of the recharge was leakage from overlying units, and 

about 70 percent of the natural discharge was leakage to 

overlying units (Helgesen and others, 1993). 

Oil and gas development has resulted in regional 

head declines of several hundred feet; in contrast, withdrawals of freshwater have resulted in head declines of 

several tens of feet. However, in parts of Nebraska and 

Kansas, much of the head decline is due to pumpage from 

the overlying High Plains aquifer. During the 1970's, 

withdrawals from the Great Plains aquifer system were 

about 517 Mgalld, or more than twice the natural recharge 

rate. Helgesen and others ( 1993) noted that the rate of adjustment to pumping from the aquifer system is extremely 

slow in most areas, and storage depletion (based on head 

declines) may be substantial. As shown in figure 13, simulation indicates that about 30 percent of the withdrawals 

during the 1970's was supplied by a decrease in storage. 

HIGH PLAINS AQUIFER 

The High Plains aquifer is a regionally extensive 

water-table aquifer composed principally of near-surface 

sand and gravel deposits of late Tertiary and Quaternary 

age (Gutentag and others, 1984). The aquifer underlies 

174,000 mi2 in parts of eight States as shown in figure 1. 

CAMBRIAN - ORDOVICIAN 

AQUIFER SYSTEM IN 

THE NORTHERN MIDWEST 

GREAT PLAINS AQUIFER 

SYSTEM (DAKOTA SANDSTONE 

AND ASSOCIATED STRATA) 

HIGH PLAINS AQUIFER 

(SOUTHERN PART) 

Predevelopment 

Natural recharge 0.37 

Natural 

discharge 

0.37 

Development conditions 

(1976-80) 

Natural and 

induced recharge 

0.90 Pumpage 0.78 

Decrease in storage 

0.14 

Natural 

discharge 

0.26 

Source: Mandie and Kontis (1992) 

Predevelopment 

Natural recharge 

(mostly leakage) 0.22 

• • • Aquifer • • 

• • system .' ... ·~· 

Natural 

discharge 

(mostly 

leakage) 0.22 

Development conditions 

(197Q-79) 

Natural and 

intercepted 

leakage 0.59 Pumpage 0.52 

Decrease in storage 

0.15 

Natural 

discharge 

0.22 

Source: Helgesen and others (1993), and 

Helgesen, J.O., written commun. (July 1994) 

Predevelopment 

Natural recharge 0.18 

• •• •• Aquifer· • •• •• • 

·. ·.. . . 

Natural 

discharge 

0.18 

Development conditions 

(1960-80 average) 

Natural and 

irrigation return 

flow 3.79 Pumpage 6.22 

Decrease in storage 

2.5 

Natural 

discharge 

0.07 

Source: Luckey and others (1986) 

Figure 13. Ground-water budgets before and after development of three major aquifer systems in the Central United States 

(values in billion gallons per day). 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 25 

Moderate precipitation combined with high evapotranspiration rates result in low recharge rates-generally less 

than 1 in/yr, except in areas with sandy soils. Water generally flows from west to east in the aquifer in response to 

the slope of the water table at a velocity of about 1 ft/d. 

The High Plains aquifer is the most heavily pumped 

ground-water system in the United States. Pumpage is 

principally for irrigation, and during 1980 about 170,000 

wells pumped almost 18 million acre-ft of water (equivalent to nearly 16 Bgal/d). This pumpage has resulted in 

regionally extensive water-level declines from predevelopment levels. The saturated thickness of the High Plains aquifer has decreased more than 25 percent over an area of 

14,000 mi2, mostly in the High Plains of Texas and in 

western Kansas. As a result, well yields have declined, 

and, in turn, the average number of acres irrigated per well 

has decreased (Weeks and others, 1988). 

Water budgets for the aquifer in the southern High 

Plains of west Texas and eastern New Mexico for predevelopment and 1960-80 conditions show little resemblance (fig. 13). The 1960-80 pumpage exceeds natural 

recharge by more than 30 times, and depletion of aquifer 

storage occurs at a rate about 14 times greater than the 

rate of natural recharge. Most inflow to the aquifer is now 

return flow from irrigation (Luckey and others, 1986). 

AQUIFER SYSTEMS OF THE WESTERN UNITED STATES 

The Central Valley of California and the Snake River 

Plain of Idaho and eastern Oregon have arid to semiarid 

climates and both are underlain by highly productive aquifer systems. Both are areas of intensive agricultural production and are irrigated with a combination of imported 

surface water and large withdrawals from the aquifer systems. The combination of return flow of excess irrigation 

water to the aquifer systems and ground-water pumpage 

has significantly altered the ground-water flow systems in 

both areas. As described in this section, the components of 

the ground-water budgets differ greatly from those that existed prior to development. 

CENTRAL VALLEY AQUIFER SYSTEM OF CALIFORNIA 

The Central Valley of California is a long structural 

trough (20,000 mi2) containing continental deposits of 

Tertiary and Quaternary age that constitute a major aquifer 

system (fig. 1 ). These deposits contain lenses of gravel, 

sand, silt, and clay with the fine-grained sediments making 

up about 50 percent of the sequence (Page, 1986). The 

climate is arid to semiarid with precipitation from 14 to 20 

in/yr in the northern part (Sacramento Valley) and 5 to 14 

in/yr in the southern part (San Joaquin Valley). 

The intensive agricultural development in the Central Valley is supported by ground-water pumpage and 

large imports of surface water. During the 1960's and 

1970's, pumpage from wells averaged about 10 Bgal/d and 

provided nearly 50 percent of the irrigation supply. The 

large ground-water withdrawals have caused water levels 

to decline hundreds of feet in the southern and western 

parts of the San Joaquin Valley. Overall, there was a net 

loss of aquifer storage of 60 million acre-ft from the start 

of development to 1977 (Williamson and others, 1989). 

The loss of storage resulting from inelastic compaction of 

clay beds has produced the largest volume of land subsidence in the world (discussed elsewhere in this report). 

However, importing surface water and decreasing groundwater pumpage has controlled land subsidence in the seriously affected areas in recent years. 

Prior to development, recharge was supplied principally by streams entering the Central Valley from highlands on the northern and eastern sides. Ground-water 

discharge occurred as evapotranspiration or seepage to 

streams in the central part of the valley. The ground-water 

flow system has been completely changed by development, and flow is now largely from areas recharged by 

surface water imported for irrigation to areas where 

ground water is withdrawn for irrigation. The marked 

changes in the water budget for the Central Valley aquifer 

system are shown in figure 14. Recharge to the aquifer 

system has increased nearly six times and is mostly supplied by irrigation return flow (Williamson and others, 

1989). Most discharge is now from pumping wells; natural 

ground-water discharge has sharply decreased. Net depletion of aquifer storage during the 1960's and 1970's occurred at a rate nearly one-half the rate of natural 

recharge. However, decreased ground-water pumpage in 

recent years generally has halted further depletion of 

ground water in storage. 

SNAKE RIVER PLAIN REGIONAL AQUIFER SYSTEM 

The Snake River Plain is an arcuate physiographic 

feature of about 15,600 mi2 in southern Idaho and a small 

part of eastern Oregon (fig. 1). It consists of an eastern 

plain (10,800 mi2) that is a downwarp filled with Quaternary basalt and a western plain (4,800 mi2) that is a graben filled predominantly with fine-grained Tertiary and 

Quaternary sediments (Lindholm, 1986). The Quaternary 

basalt is a highly productive aquifer; simulation suggests 

that the transmissivity of the full thickness of aquifer may 

be as high as 10 million ft2

/d (Garabedian, 1993). 

Diversion of surface water for irrigation began in the 

1880's and initially caused an increase in ground-water recharge due to percolation of excess applied water (Lindholm, in press). The period from 1912 to the early 1950's 

was characterized by accretion of storage; however, there 

was generally more depletion than accretion of storage from 

the 1950's to 1980 (Kjelstrom, in press). The loss of storage 

during the latter period has been attributed to several factors 
26 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

including ( 1) decreased use of surface water for irrigation, 

(2) increased use of ground water, (3) conversion from flood 

to sprinkler irrigation, and ( 4) the occurrence of dry years 

during which ground-water pumpage increased and recharge 

from surface water decreased. 

The ground-water budget for the main part of the 

eastern Snake River Plain has changed markedly since 

predevelopment time owing to imports of surface water 

for irrigation and increased pumping from wells (fig. 14). 

Before irrigation, about 60 percent of the recharge was 

from tributary basins (streamflow and ground-water discharge) and all discharge was to the Snake River. By 

1980, flow through the aquifer system had increased by 80 

percent and over 60 percent of the recharge was percolation of excess surface water on irrigated lands. Groundwater pumpage in 1980 was about 0.7 Bgal/d, or more 

than 10 percent of the total ground-water discharge. The 

decrease in aquifer storage represents about 5 percent of 

the total 1980 budget. However, it should be noted that the 

decrease in storage is simply removal of water that had 

CALIFORNIA CENTRAL VALLEY 

AQUIFER SYSTEM 

Predevelopment 

Natural recharge 1.8 

.. • •• Aquifer ; 

system 

- ·-· 

Natural 

discharge 

1.8 

Development conditions 

(1961-77 average) 

Recharge and 

irrigation return 

flow 9.8 Pumpage 10.2 

Decrease in storage 

0.7 

Natural 

discharge 

0.3 

Source: Williamson and others (1989) 

previously been added to storage during the period from 

1912 to the early 1950's. 

EVALUATION OF GROUND-WATER 

DEVELOPMENT EFFECTS 

Under natural conditions, a ground-water system generally is in a state of dynamic equilibrium; that is, recharge 

to the system is approximately equal to discharge from the 

system. However, when an aquifer system is developed, this 

state of dynamic equilibrium is disturbed. Initially, water 

stored in the pore spaces or fractures in an aquifer near 

pumped wells is removed to sustain pumping rates. This 

creates a hydraulic gradient sufficiently large to allow 

enough water to flow to the wells. The hydraulic gradient 

around a well resembles an inverted cone with the apex 

centered at the pumped well. Hence, it is called a cone of 

depression. The cone of depression expands outward from 

a pumped well as pumping continues. More water will be 

EASTERN SNAKE RIVER PLAIN 

AQUIFER SYSTEM 

Predevelopment 

Recharge from: 

Tributary 

basins 2.1 

Snake River 

losses 0.8 Precipitation 0.6 

-\ 

< -1 

v > -\ 

1-- -\ 

1\ Aquifer system 

1--

1-- -\ 

-1 -\ 

< 

Development conditions 

(1980) 

Recharge from: 

Tributary Snake Irrigation 

basins River return 

Discharge 

to 

Snake River 

3.5 

1.3 losses 0.3 flow 3.7 Precipitation 0.6 

-1 -\ 

1\ 

< \'- > 

-1 -\ 1--

Decrease in storage 

0.4 

Source: Lindholm (in press) 

Pumpage 

0.7 

v Discharge to 

Snake River 

1-- 1\ 5.6 

Figure 14. Ground-water budgets before and after development Of two major aquifer systems in the Western 

United States (values in billion gallons per day). 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 27 

removed from storage until equilibrium is reached or the 

cone of depression reaches a recharge or discharge boundary. An increase in recharge or a decrease in discharge (or a 

combination of both) then occurs until sufficient water is 

obtained to sustain the pumping rate. Thereafter, the growth 

of the cone of depression ceases. If a cone of depression 

does not reach either a recharge or discharge boundary, more 

and more water is removed from aquifer storage and groundwater levels continue to lower until equilibrium is reached. 

Therefore, one of the major effects of development in arid 

and semiarid areas is water-level declines due to pumping. 

Development effects commonly associated with water-level 

decline in various types of geologic terrains are land subsidence, creation of surface earth fissures, induced collapse of 

sinkholes, and saline-water intrusion. The RASA Program 

evaluates these effects using computer-based model simulations. The following are the principal effects of groundwater development: 

1. Changes in water levels-The degree of waterlevel change differs in unconfined (water-table) aquifers 

and confined (artesian) aquifers. In unconfined aquifers, 

the standing water level is a water table which is at atmospheric pressure. In confined aquifers, however, water in 

the aquifer is under pressure greater than atmospheric. In 

wells tapping a confined aquifer, the water level in a well 

is always higher than, or equal to, the top of the aquifer. 

In a water-table aquifer, the hydraulic gradient formed 

around a pumped well is due to the removal of water from 

aquifer storage; the hydraulic gradient is generally steeper 

than in a confined aquifer. Hydraulic gradients around a 

pumped well in a confined aquifer are due principally to 

release of water resulting from compression of the aquifer 

material and expansion of water as a result of the reduced 

pressure. The propagation of a pressure wave in a confined 

aquifer occurs very rapidly and extends far from the 

pumping well. Therefore, the volume of water released 

from the expansion of water and compaction of the aquifer 

in response to the reduction in pressure is sufficient to sustain the pumping rate because the cone of depression extends over a very large area. The result is creation of a 

gentle hydraulic gradient in contrast to the steep gradient 

caused by a well tapping a water-table aquifer with the 

same pumping rate. The cone of depression surrounding a 

pumped well in a confined aquifer is much larger than that 

around a well in a water-table aquifer. Conversely, the 

slope of a hydraulic gradient formed by a well in a confined aquifer is much gentler than that of a well tapping a 

water-table aquifer. 

The lateral growth of a cone of depression stops 

when it reaches a recharge or discharge boundary. For example, in areas with unconfined aquifers and perennial 

streams, when the cone of depression reaches a stream, 

water will be supplied to a well by a reduction in groundwater discharge to the stream, by induced infiltration into 

the aquifer from the stream, or by a combination of these 

processes. In time, a new equilibrium will be reached and 

growth of the cone of depression stops. Under seasonal 

pumping conditions, such as irrigation, all or some of the 

water removed from aquifer storage during the pumping 

season is replenished by precipitation during a wet season. 

If the amount of precipitation is greater than the evapotranspiration, the water level generally recovers after cessation of pumping to the prepumping level. However, in 

arid or semiarid areas, the amount of precipitation generally is less than potential evapotranspiration and water levels that decline during the pumping season generally do 

not recover to prepumping levels. Therefore, depending 

upon the degree of stress, water levels in aquifers in arid 

and semiarid areas become lower from one pumping season to another. Continued water-level decline due to 

pumping eventually reduces the saturated thickness of the 

aquifer and ultimately decreases the yield of wells. Uncontrolled development of ground water in such areas can 

lead to many dry wells and ultimately could result in the 

aquifer being unusable. For example, in Floyd County, 

Texas, pumping for irrigation has resulted in a significant 

water-level decline and reduction of the saturated thickness in the High Plains aquifer (fig. 15). The consequence 

of this reduction of saturated thickness has been a large 

reduction in the number of acres that can be irrigated by a 

well (fig. 16). 

Irrigation not only causes water levels to decline 

near pumping wells, but occasionally it causes water levels to rise. Most of the water-level rises result from excessive irrigation by imported surface water. For example, in 

the eastern Snake River Plain, millions of acre-feet of surface water have been diverted annually for irrigation since 

irrigation started in the 1880's. In one area south of the 

...J 

w 

> w 

...J 3,100 <( 

w 

en 

w 

> 0 

CXl 

<( 3,000 Aquifer Iw 

w 

LL. 

~ 

w' 

Cl 

:::> 2,900 I-

~ <( SedtM 

2,800 

1940 1950 1960 1970 1980 

YEAR 

Figure 15. Water-level decline in the High Plains aquifer 

in Floyd County, Texas, due to irrigation pumping (from 

Luckey and others, 1981). 
28 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Snake River near Twin Falls, Idaho, ground-water levels 

rose as much as 200 ft in 5 yr after the start of irrigation 

(Lindholm, 1986). As water levels rose, ground-water discharge as springflow to the Snake River increased. A longterm trend of increase in ground-water discharge to the 

Snake River from the early 1900's to 1950 occurred in the 

reach from Milner to King Hill, whereas discharge to the 

upstream reach from Blackfoot to Neely remained relatively constant (fig. 17). In the early 1950's, ground-water 

discharge to the Milner to King Hill reach stabilized, indicating the creation of a balance between recharge and discharge. A period of general decrease in ground-water 

discharge followed and continued to 1980. This decrease 

probably was due to a combination of increased withdrawals of ground-water for irrigation, decreased surface-water 

diversions for irrigation, and increased irrigation efficiency. Climatological changes may also be one of the causes 

of the decrease, but this effect probably is negligible. 

Rises of ground-water levels also can be caused by 

increases in recharge following the removal of vegetation 

or less permeable materials covering the land surface. In 

parts of Dawson and Lynn Counties in Texas, rises of 

ground-water levels in the High Plains aquifer (fig. 18) 

were attributed to clearing sandy soils of native vegetation 

for cultivation. The increased rate of recharge from precipitation on dryland crop areas resulted in water-level rises 

(Luckey and others, 1981). 

In summary, land use and ground-water development 

can cause ground-water levels either to decline or to rise 

....1 120 ....1 

w 

~ 

a: 

w 110 a.. 

c 

w 

~ (!) 100 a: 

g; 

C/) 

w 

a: 

u 90 <( 

LL. 

0 

a: 

w 80 a:l 

~ 

::) 

z 

w 70 (!) 

<( 

a: 

w 

~ 60 

50 

1955 1960 1965 1970 1975 1980 

YEAR 

Figure 16. Decline of irrigated acreage per well due to reduction in saturated thickness of the High Plains aquifer in parts 

of Castro, Brosby, Floyd, Hale, Lubbock, Parmer, and Swisher Counties, Texas (from Luckey and others, 1981 ). 

depending on whether development is accompanied by ( 1) 

an increase in ground-water withdrawals, (2) an increase 

in recharge by irrigation return flow using imported surface water, or (3) an increase in recharge by precipitation 

due to removing less permeable soil or vegetation. 

2. Changes in recharge and discharge-As discussed 

above, land use and ground-water development affe.ct 

water levels in aquifers, and such effects alter the characteristics of recharge and discharge from predevelopment 

conditions. In general, increases in ground-water withdrawals will result in increases in recharge rates and the 

size of recharge areas due to areally lowering water levels 

in aquifers. Conversely, withdrawals will decrease discharge rates and the size of discharge areas. For example, 

estimated predevelopment discharge from the Upper Floridan aquifer occurred mostly at springs and to surfacewater bodies (about 80 to 90 percent of recharge); very 

little ground water was discharged as upward leakage 

through confining units and laterally to the sea (fig. 19). 

However, under 1980 conditions with pumpage of 4,170 

ft3/s (about 3 Bgal/d), simulation indicated that most of 

the pumpage was balanced by an increase in recharge (60 

percent of the pumpage) and by a reduction in natural discharge (40 percent of pumpage) (see fig. 20). Simulation 

results also indicated that the total recharge area expanded 

from 67,000 mi2 before development to 76,000 mi2 in 

1980, and that some former discharge areas under predevelopment conditions became recharge areas under 1980 

pumping conditions. There has been almost no decrease in 

aquifer storage. 

In semiarid areas where annual precipitation is less 

than annual potential evapotranspiration, recharge is insufficient to replenish the water removed from aquifer storage. 

Water-level declines due to pumping become permanent, 

resulting in a decrease in saturated thickness of aquifer materials. For example, in the High Plains aquifer, a comparison of predevelopment conditions with 1980 pumping 

conditions indicates that saturated thickness has been reduced by more than 10 percent over an area of about 

44,000 mi2, which is about 25 percent of the areal extent 

of the aquifer. More than 25 percent reduction in saturated 

thickness has occurred over an area of 14,000 mi2 (Weeks 

and others, 1988). Today, the High Plains area is still experiencing water-level declines locally. From 1980 through 

1989, water levels declined more than 15 ft in areas of 

intensive irrigation in Kansas, New Mexico, Oklahoma, 

Colorado, and Texas. A large area in northeastern Colorado 

and southwestern Nebraska is characterized by 7 to 15 ft of 

water-level decline. In the remaining area of the High 

Plains aquifer, water levels have either remained stable or 

risen slightly (Dugan and others, 1990). Except in intensive irrigation areas, the High Plains aquifer is not being 

extensively dewatered at present. 

In summary, in humid areas there generally is sufficient precipitation to replenish the water removed from aquifer storage, and sufficient natural ground-water discharge 
45" 

40" 

a 

z 

0 

u 

UJ 

(/) 

r:r: 

UJ 

Q.. 

tu 

UJ 

u.. 

u 

ai 

:::> 

u 

u.. 

0 

(/) 

a 

z 

< (/) 

:::> 

0 

:I: 

1-

~ 

w' 

(.!) 

r:r: 

< :I: 

u 

(/) 

0 

PART I. SELECfED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 

120" 

WASHINGTON 

OREGON 

-,- ---

::; 

2; 

0:: NEVADA 

0 

u. 

:J 

I 

C5 0 50 100 MILES 

50 100 KILOMETERS 

7 

6 

5 

4 

3 

2 N 0 0 

~ o; N 

(J) 

liS" 

( 

IDAHO vf' 

\ 

110" 

MONTANA 

------ ----- ___.l 

L 

UTAH 

EXPLANATION 

Snake River basin 

Snake River plain 

0 0 :iS ~ 0 (') Cf, " (J) (J) (J) 

YEAR 

WYOMING 

~-

~ 

Figure 17. Changes in ground-water discharge to the Snake River due primarily to changes in irrigation with 

surface water in eastern Snake River Plain, Idaho (from Lindholm, 1986). 

29 
30 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

....J 3,100 w 

> 

~ 

<( 

w 

"' w 

..... Figure 18. Rises of the water table in parts of Dawson and 

Lynn Counties, Texas, due to clearing sandy soils of native vegetation for cultivation (from Luckey and others, 1981) 

> 

0 

al 

~ 3,000 

Water table '-.... 

0 50 100 MILES 

tOO KILOMETERS 

EXPLANATION 

" '-.---- Pe:rc•mt diffuse upward leakage 

'3----t'E!rc•~nt springflow and discharge to 

surface-water bodies 

1 64a------Cubic feet per second total discharge 

8 ---Percent of total discharge-Values 

associated with largest circle are 

regional totals; others are area totals; 

<, less than 

~ 

~ 

w' 

0 

:::l 

>--

5 

<( 2,900 

Boundary of major ground-water areas 

1940 1950 

Bedrock 

1960 

YEAR 

Figure 19. Estimated predevelopment discharge from the Upper Floridan aquifer (from Johnston 

and Bush, 1988). 

Aquifer 

1970 1980 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 31 

can be diverted to sustain ground-water withdrawals. 

Therefore, the effects of development on aquifer systems 

are generally an increase in the size of recharge areas that 

results in an increase of total recharge, a decrease in natural ground-water discharge, or a combination of both. 

However, in arid and semiarid areas, an adverse effect of 

development can be reduction in saturated thickness of aquifers. Under severe conditions of dewatering, this can destroy the usage of parts of the aquifers. 

3. Land subsidence-Another effect of ground-water 

development is subsidence of the land surface due to lowering of ground-water levels. Land subsidence has oc0 50 100 MILES 

~-'-r--,L,----'---' 

0 100 KILOMETERS 

EXPLANATION 

Percent pumpage 

curred in several areas, most notably in the Central Valley 

of California and in Houston, Texas. The principal cause 

of land subsidence is compaction of fine-grained aquifer 

materials (clay and silt). Compaction results from an increase of overburden pressure when the hydraulic head (or 

water level) in an aquifer is lowered by pumping. The mechanics of land subsidence due to compaction of aquifer 

materials is complex and difficult to explain. There are 

two ranges of compressibility. One is elastic compressibility, similar to that of a rubber ball. If the ball is squeezed, 

the ball will deform; however, the ball will return to its 

original shape if pressure is released. In contrast, inelastic 

Percent diffuse upward leakage 

Percent springflow and discharge to 

surface-water bodies 

1•

925__...-Cubic feet per second total discharge 

8 ---Percent of total discharge-Values 

associated with largest circle are 

regional totals; others are area totals 

Boundary of major ground-water areas 

Figure 20. Estimated 1980 discharge from the Upper Floridan aquifer (from Johnston and Bush, 

1988). 
32 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

compression is similar to that of a clay ball, which, if 

squeezed, may rebound to some degree after pressure is 

released; however, the clay ball will never return to its 

original form. Compaction of fine-grained materials having high compressibility in an aquifer progresses slowly. 

On the basis of many years of field investigation on land 

subsidence, it was concluded that the ratio of land subsidence to water-level decline in an aquifer system is small 

until the water-level declines below a critical value 

(termed a preconsolidation level). Then the ratio of land 

subsidence to water-level decline will increase to a large 

constant value. When the water level decline is less than a 

critical value (preconsolidation level), the compaction is 

elastic and the small amount of land subsidence will rebound after the water level in the aquifer rises. However, 

when the water level declines below the preconsolidation 

level, the compression is inelastic, the amount of land subsidence is proportional to the water-level decline, and subsidence is permanent and nonrecoverable. The amount of 

water released from aquifer storage under the elastic condition is derived from the expansion of water and elastic 

compression of aquifer materials, which is much smaller 

than the amount of water released from inelastic compression of the aquifer materials. Under inelastic compaction, 

he, 

he, 

0 

I 

I 

I 

----------------+------ 1 I I I 

water is released from pores that are decreased in size by 

compaction. 

In seasonal pumping areas, ground-water levels recover and compaction of the aquifer materials ceases when 

pumping stops. During the next pumping season, the decline 

of water levels due to pumping initially causes elastic compaction. Inelastic compaction will not occur until water levels are lowered below a critical value, as shown in figure 21. 

In reality, land subsidence is not as simple as illustrated in 

figure 21 because of a time lag during which lowering of 

water levels in fine-grained beds lags behind the lowering 

of water levels in permeable beds. However, the general 

concept of elastic and inelastic compaction in relation to 

hydraulic head declines is illustrated by figure 21. 

Land subsidence is detected by comparison of the 

altitudes of a benchmark after several precise leveling surveys. Land subsidence in the San Joaquin Valley of California was discovered in the mid-1920's; subsidence 

continued until the late 1960's. Beginning in 1968, surface 

water was imported to the valley through canals and aqueducts to replace ground water as the major source of irrigation water, and subsidence stopped shortly thereafter. 

The areal extent of land subsidence in the Central Valley 

of California that exceeded 1 ft is shown in figure 22. 

he,, he,, he,, he, = critical heads 

<( 

UJ I I I 

J: he, 

u 

::::; 

::> 

<( he, a: 

0 

> J: 

----------------~------~-----------~------- 1 I I 

I 1 I I I I I I 

----------------~------ .l_----------:---------f----------t------- I I I 

I I I 

1 I I 

Elastic storage : Elastic storage : Elastic storage : 

1 I I 

I I 

I I 

: Inelastic Inelastic : Inelastic 

I I 

I 

I 

1 Elastic 

: storage 

II 

I 

~Compaction~ (Compaction) :(Compaction) 

1 storage 1 storage 1 storage I I I 

I I I 

I I I 

I I I 

I I I 

I I I 

T, T, T, T, T, T, 

TIME 

Figure 21. Relation of hydraulic heads and storage in a compacting aquifer system (modified from Williamson and others, 

1989). 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 33 

Maximum subsidence was nearly 30ft in 1977 at a benchmark 10 mi southwest of Mendota in the Los Banos-Kettleman City area. The reconstructed land surface altitudes 

surveyed in 1925, 1955, and 1977 are shown in figure 23 

(Ireland and others, 1984). This magnitude of land subsidence is not as spectacular in the field as in the picture because subsidence progressed over tens of years and spread 

over thousands of square miles. However, subsidence has 

created engineering and economic problems, including 

failure of well casings and cracking and loss of carrying 

40' 

39' 

..0 38' ~ 

~ ('> 

37' 

38' 

50 

35' 50 

119' 

CALIFORNIA NEVADA 

I 

I 

() 

0 I 

(QLokeTahoe 1> 

V> 

--\ 

1> 

\ ' ' "' ' ' "' ' ' Mono Lake "' 

"'$) <J, -v~ ' "' ~ <I> 

£. 

~ ~~ 

0 ..t.. 

~ <I> 

~ 

100 MILES 

100 KILOMETERS 

• 

EXPLANATION 

Study area 

Area where land subsidence exceeded 1 foot caused 

primarily by ground-water pumpage 

Location of site southwest of Mendota shown in figure 23 

118' 

' ' "' 

Figure 22. Land subsidence in the Central Valley, California, 1926-70 (modified from Williamson and others, 1989). 

' ' "' ' 
34 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

capacity of irrigation and drainage canals. If land subsidence occurs in small basins surrounded by bedrock, earth 

fissures may be created. These earth fissures initially are 

tiny cracks that are enlarged by erosion and eventually become large crevasses (fig. 24). Such earth fissures have 

developed in Arizona and other places. 

4. Sinkhole collapse-In carbonate rock terrains, the 

development of solution features ranging from widened 

fractures and joints to large caves and sinkholes is a natural 

erosional process. Exposed carbonate rocks or carbonate 

rocks covered with thin unconsolidated sediments can easily develop networks of solution channels. Ground water 

generally flows along paths of least resistance; therefore, 

solution channels are developed more readily along bedding planes, joints, and fractures. As solution channels 

along joints are enlarged and more and more carbonate 

rock is dissolved, large features such as caves, chimneys, 

and sinkholes are formed. Sinkholes have been defined as 

"closed depressions in the land surface that are formed by 

solution of near-surface limestone and similar rocks and by 

subsidence or collapse of overlying surficial material into 

l 

I 

Figure 23. Observed land subsidence of approximately 29 ft at a 

surveying benchmark, 10 mi southwest of Mendota in the San 

Joaquin Valley, California (from Ireland and others, 1984). 

underlying solution cavities" (Beck and Sinclair, 1986). In 

outcrop areas of carbonate rocks, sinkholes can be seen at 

the surface. However, where the carbonate rocks are covered by unconsolidated sediment, sinkholes are concealed 

by sediments that can collapse under certain conditions. 

After collapse of a sinkhole, either a large hole develops or 

a depression is formed at the surface. Collapse of sinkholes 

either can progress slowly for many years, or can occur 

suddenly. One trigger of sinkhole collapse is lowering or 

fluctuation of water levels due to pumping from wells. Before pumping from wells is initiated, unconsolidated sediments overlying enlarged solution channels are supported 

by the static pressure of water in the aquifer. After water 

levels decline due to pumping, the static pressure that supports the unconsolidated sediments is reduced. A close relation between initiation of pumping or increases in 

pumping from high-capacity wells tapping the Floridan aquifer system and collapse of nearby sinkholes has been described by Sinclair (1982). 

Collapse of sinkholes can occur within days, even 

hours. A well-known and spectacular collapse of land surface to form a sinkhole occurred at Winter Park, Florida, in 

1981 (fig. 25). In general, sinkholes were not a focus of 

RASA investigations because they are primarily local phenomena. However, results of RASA studies, particularly the 

data sets developed for ground-water flow models, can be 

useful in studying local sinkhole problems. For example, the 

St. Johns River Water Management District of Florida used 

such data sets to develop a flow model of the Floridan 

aquifer system in an area where sinkholes are common. One 

purpose of the model was to evaluate the effects of heavy 

pumping during overnight periods of freezing weather on 

the potential for sinkhole development. 

5. Saltwater intrusion-Pumping of ground water in 

coastal areas or areas underlain by brackish or saltwater 

can create the potential for saltwater contamination. However, whether or not encroachment of saltwater actually 

occurs depends primarily on two factors: (1) the permeability of rocks separating the pumped wells and the saltwater body, and (2) the direction and magnitude of 

hydraulic gradients in these rocks. Generally, unconfined 

aquifers are more susceptible to saltwater contamination 

than are confined aquifers because: (1) unconfined aquifers 

lack the protective cover of a low-permeability confining 

bed, and (2) the low hydraulic heads common to coastal 

areas preclude the existence of a thick freshwater section 

and the presence of freshwater in the aquifer offshore. 

Many studies of local instances of saltwater encroachment have been conducted in the Nation's coastal 

areas. However, because the RASA Program has focused 

on regional hydrology and regional water problems, investigations of saltwater contamination were limited to problems of regional importance. Two such investigations are 

as follows: (1) Simulation of saltwater intrusion on Hilton 

Head Island, South Carolina, due to the existence of a regional cone of depression in the Upper Floridan aquifer 
PART I. SELECTED DESCRIPTIONS AND FINDINGS ABOUT REGIONAL AQUIFER SYSTEMS 

Figure 24. Earth fissures enlarged by erosion, near Apache Junction, central Arizona (courtesy of Robert L. Laney, USGS). 

Figure 25. Collapse of land surface due to formation of a sinkhole at Winter Park, Florida, in 1981 . 

(Photograph courtesy of Jamal and Associates, Orlando, Florida.) 

35 
36 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

centered around Savannah, Georgia (Bush, 1988), and (2) 

Simulation of freshwater and saltwater movement in the 

most heavily pumped aquifer in Hawaii, which is termed 

the "Honolulu-Pearl Harbor basal water body" (Souza and 

Voss, 1986). 

APPLICATION OF REGIONAL AQUIFER-SYSTEM 

ANALYSIS PROGRAM INFORMATION 

The results of the Regional Aquifer-System Analysis 

Program can be used or have been used in a variety of 

ways. A few examples are presented here to illustrate applications of the RASA study findings: 

1. Ground Water Atlas of the United States-Prior to 

RASA, no detailed maps provided complete nationwide 

coverage of ground-water conditions at a regional scale. 

One previously published map that shows the major aquifers nationwide and withdrawals from wells at a scale of 

1:7,500,000 was included in the National Atlas of the 

United States of America (U.S. Geological Survey, 1970, 

p. 122-123). Brief discussions of ground-water conditions 

in different regions have been presented by Heath (1984 ), 

and Back and others (1988). There was a need for a detailed ground-water atlas that could serve as a basic reference on the location, geology, hydrology, and water quality 

of the Nation's major aquifers. The vast amount of data 

resulting from the RASA studies, together with previously 

published information, provided the information needed to 

compile regional-scale maps showing ground-water conditions of the United States. These maps and explanatory text 

are currently being compiled into the National Ground 

Water Atlas by the U.S. Geological Survey as part of the 

RASA Program. 

2. Planning and development of ground-water resources-An important task in the planning and development of ground-water supplies is to identify areas where 

development can proceed without causing adverse effects. 

Although some regional aquifers have large quantities of 

freshwater remaining for development, parts of the aquifers should be avoided because of poor water quality, large 

current withdrawals and accompanying regional waterlevel declines, or poor water-transmitting properties and 

thus low well yields. Such problem areas are identified 

and described in RASA reports. For ease and use by planners, some RASA reports provide regional aquifer maps 

showing possible future declines in water levels and well 

yields based on simulation of different water-management 

strategies. For example, such maps are presented for the 

High Plains aquifer by Luckey and others (1988). Some 

RASA reports include maps showing potentially favorable 

areas for development of large ground-water supplies. For 

example, a map showing highly favorable areas for development of the Upper Floridan aquifer based primarily on 

minimizing head decline and thereby reducing the likelihood of water-quality deterioration is presented by Bush 

and Johnston (1988, pl. 17). Maps showing potentially favorable areas for constructing wells that would yield more 

than 500 gal/min in the Madison Limestone and Red River 

Formation (Northern Great Plains) are presented by MacCary and others (1983). 

Regional and subregional flow models constructed 

during the RASA studies can be used to estimate the impact of hypothetical pumpage rates on aquifer systems. 

For more detailed studies, local flow models can be constructed on the basis of the regional or subregional flow 

models and supplemented with additional information. The 

regional or subregional model can also be used to determine the appropriate boundary conditions to use for local 

flow models. 

In the southern High Plains, pumpage exceeds the 

natural recharge rate by more than 30 times (fig. 13), and 

most pumpage is supplied by depletion of aquifer storage. 

Under such conditions, withdrawals need to be carefully 

managed to prevent pumping water levels in wells from 

declining to depths near the bottom of wells, making the 

wells unusable. In areas with low recharge and high pumpage, such as the High Plains of Texas, water managers 

need to consider pumping rates, pumping times, and areal 

distribution of wells, so that water-level declines due to 

pumping will not cause wells to go dry. Such planning can 

be aided by the use of model simulations. Calibrated models developed during the High Plains RASA study were 

used to project future water levels that would result from 

various water management strategies. Maps showing projected water-level declines, changes in saturated thickness, 

and probable well yields are presented by Luckey and others (1988). 

As discussed in the previous section, a serious side 

effect of large declines in ground-water levels is subsidence of the land surface. Land subsidence can cause engineering problems, including cracking of irrigation and 

drainage systems and failure of well casings. In low-altitude coastal or estuarine areas, land subsidence can cause 

dry land to become swampy or submerged by seawater, as 

has happened in the Houston area, Texas. Land subsidence 

is caused by large declines in water level in aquifer systems confined by thick units made up of fine-grained sediments. However, not all such aquifer systems are subject 

to land subsidence; only those confining units consisting 

of fine-grained materials that have a high compression 

index (such as montmorillonite clay) are likely to cause 

land subsidence if ground water is extensively pumped. If 

highly compressive materials are present within thick confining units, water managers should consider using landsubsidence models developed by the RASA Program to 

evaluate whether land subsidence will be triggered by proposed pumping. Such models can indicate withdrawal 

rates and pumping durations that will not result in irreversible damage to an area. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 37 

3. Planning drought-resistant water-supply systems-Droughts are natural occurrences. It is impossible 

to prevent a drought; however, with proper planning, the 

problems caused by a drought can be minimized. Water 

managers can use RASA information to plan droughtresistant alternatives. For example, along the California 

aqueducts that supply the Central Valley with surface 

water, a series of wells could be drilled. During wet seasons, these wells could be used as recharge wells to raise 

water levels in the aquifers. During droughts, the wells 

could be pumped to supplement surface-water deliveries in 

the aqueducts. Results of model simulation described in 

many RASA reports should be useful in planning management alternatives during droughts. 

4. RASA information has been used and disseminated by scientific institutions-Much of the information resulting from the RASA Program will not be published by 

the U.S. Geological Survey for many years. As an expedient method of providing timely release of information, the 

American Water Resources Association (AWRA) in 1985 

initiated a series of conferences and symposiums in different parts of the country to discuss regional aquifer systems 

in the United States based on findings of the RASA Program. Almost all papers presented in the symposia were 

authored by RASA investigators. As of mid-1992, eight 

symposia have been held and the papers have been published in the A WRA monograph series. The published 

monographs are: (1) Southwest Alluvial Basins of Arizona 

(Anderson and Johnson, 1986); (2) Aquifers of the Atlantic and Gulf Coastal Plain (Vecchioli and Johnson, 1987); 

(3) The Northeast Glacial Aquifers (Randall and Johnson, 

1988); (4) Aquifers of the Midwestern Area (Swain and 

Johnson, 1989); (5) Aquifers of the Caribbean Islands 

(Gomez-Gomez and others, 1991); (6) Aquifers of the 

Western Mountain Area (McLean and Johnson, 1988); (7) 

Aquifers of the Far West (Prince and Johnson, 1992); and 

(8) Aquifers of the Southern and Eastern United States 

(Hotchkiss and Johnson, 1992). In addition to the publication of RASA findings by A WRA, 20 RASA investigators 

were invited to prepare regional descriptions for the volume on hydrogeology that is part of the Decade of North 

American Geology series of the Geological Society of 

America (Back and others, 1988). Special facets of the 

RASA studies are also described in numerous articles appearing in the various hydrologic, geologic, and geochemical journals. 

5. RASA information has been used directly in the 

management of ground-water resources-The following examples illustrate how RASA infonnation was used for management of ground-water resources: (1) the New Jersey 

part of a regional flow model developed for the Atlantic 

Coastal Plain was used to manage a. coastal-plain aquifer 

in New Jersey in designated critical development areas, 

(2) relations between waste-disposal sites and well locations 

were evaluated using the RASA information in South Carolina, (3) the Internal Revenue Service and the Federal Land 

Bank used the High Plains RASA results to evaluate tax 

depletion allowances and to appraise farmlands, ( 4) Idaho 

water agencies used the information resulting from the 

Snake River Plain RASA study in negotiations of water 

rights between irrigators and hydropower-generating companies, (5) Snake River Plain RASA results also were used 

to help develop Idaho's ground-water-quality management 

plan, and (6) The Water Management Districts of Florida 

have used data sets from RASA models of the Floridan 

aquifer system to develop local models for evaluating the 

effects of ground-water withdrawals on lake levels and on 

potential sinkhole development. 

PART II. STATUS OF 

THE REGIONAL AQUIFER-SYSTEM 

ANALYSIS PROGRAM 

A short summary of the individual RASA studies is 

given in this section of the report. The studies are grouped 

into three categories relative to the status of progress as of 

mid-1992. The three categories are fully completed, completed, and ongoing. The fully completed studies have all 

reports printed or approved for release to the public and 

preparation of the manuscripts for printing was in progress 

as of mid-1992. The completed studies have finished data 

compilation and investigative activities, have printed many 

reports, and final reports are either in review or in preparation. The ongoing studies have data compilation and investigative activities in progress and may have only a few 

reports printed as of mid-1992. 

FULLY COMPLETED REGIONAL 

AQUIFER-SYSTEM ANALYSIS STUDIES 

The six regional studies that were fully completed as 

of mid-1992 are summarized in this part of the report. The 

studies are: the Floridan aquifer system, the High Plains, 

the Northern Atlantic Coastal Plain, the Northern Great 

Plains, the Southeastern Coastal Plain, and the Southwest 

alluvial basins. 

FLORIDAN AQUIFER SYSTEM 

The Floridan aquifer system is one of the major 

sources of ground water in the United States. This highly 

productive aquifer system underlies all of Florida, southern Georgia, and small parts of the adjacent States of Alabama and South Carolina, a total area of about 100,000 

mi2 (fig. 26). Pumpage from the Floridan has increased 

steadily in recent years, from about 0.63 BgaVd in 1950 to 

about 3.5 BgaVd in 1985. In many areas, the Floridan is 

the sole source of freshwater. 
38 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

The Floridan is a thick sequence of carbonate rocks 

that are mostly of late Paleocene to early Miocene age and 

that are hydraulically connected in varying degrees (Miller, 1986). The rocks vary in thickness from a featheredge 

where they crop out to more than 3,500 ft where the aquifer is deeply buried in southwestern Florida. The aquifer 

system generally consists of an upper aquifer and a lower 

aquifer, separated by a regional confining unit of highly 

variable hydraulic properties. Low-permeability clastic 

rocks overlie much of the Floridan aquifer system, and 

these rocks have a controlling effect on the development 

of permeability and ground-water flow in the Floridan. 

The Floridan derives its permeability from waterbearing openings that range from small openings in loosely cemented fossil hashes to large cavernous openings in 

karst areas. For the Upper Floridan aquifer, the transmissivity is highest (greater than 1 million ft2/d) in the unconfined karst area of central and northern Florida. The lowest 

transmissivity (less than 50,000 ft2/d) occurs in the Florida 

panhandle and southernmost Florida, where the Upper 

Floridan aquifer is confined by thick clay sections. 

High rainfall (averaging 53 in/yr) combined with generally flat topography provides abundant recharge to the 

Floridan. The dominant feature of the Floridan flow system, 

"'{t·- ""7'·-V'., 350 soo 

l-:7""-T--;-:------~f:l"'T;~dr=crl"'-=77T.7rl.~¥J "-~ -, --~~ 0 u r ',,,, 

I CAROLIN I 

I 

i • 

i 

as• 

0 50 tOO MILES 

0 tOO KILOMETERS 

EXPLANATION 

E Chapter letter in Professional 

Paper 1403 series * No professional paper is written 

due to little ground-water 

development and limited 

data. However general 

geologic and hydrologic 

conditions of subarea are 

described in Professional 

Papers 1403-B, 1403-C, 

and 1403-1 

Figure 26. Extent of the Floridan aquifer system and subareas with descriptions of ground-water 

hydrology in U.S. Geological Survey Professional Papers 1403-D through 1403-H (from 

Johnston and Bush, 1988). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 39 

both before and after development, is spring discharge from 

the Upper Floridan aquifer. Most of the recharge necessary 

to sustain discharge to springs or streams and lakes occurs 

relatively close to the springs and surface-water bodies. 

The 1980 pumpage (about 20 percent of the predevelopment recharge rate) is supplied primarily by induced 

recharge and diversion of natural discharge rather than 

loss of aquifer storage (Bush and Johnston, 1988). Withdrawal of 3 to 3.5 BgaVd in recent years has not greatly 

affected the ground-water budget of the Floridan, as can 

be seen in figure 12 and discussed in the earlier section on 

regional ground-water budgets. However, pumpage has 

caused long-term head declines of more than 10 ft across 

three broad areas, and locally there are a few deep cones 

of depression where heads have declined more than 100 ft. 

Water chemistry in the Upper Floridan aquifer is 

related to the flow system and proximity to the freshwatersaltwater interface. Generally, dissolved-solids concentrations are low (less than 250 mg/L) in the unconfined or 

semiconfined areas where flow is vigorous, and the concentrations are higher where the Floridan is thickly confined and flow is more sluggish, especially in coastal areas. 

Large quantities of ground water are available from 

the Floridan aquifer system for future development. An 

appraisal of potentially favorable areas for large groundwater withdrawals is presented by Bush and Johnston 

(1988), which is based primarily on minimizing head decline and thereby reducing the chances of water-quality 

deterioration. The favorable areas are largely inland and 

characterized by high transmissivity and minimal development as of the early 1980's. 

Several important contributions to understanding the 

Floridan aquifer system were provided by the Floridan 

RASA study including the following: 

1. The first comprehensive geologic description of 

the Floridan aquifer system, including definition of its 

component aquifers and confining units and their relation 

to previously defined rock and chronostratigraphic units 

(Miller, 1986). 

2. The first quantitative description of the regional 

flow system in the Floridan. This included the first synoptic 

potentiometric surface map covering the entire four-State 

area of the Upper Floridan aquifer and the first regional 

ground-water budgets for both predevelopment and development conditions (Bush and Johnston, 1988). 

3. Definition of the offshore boundary of the freshwater flow system in the Floridan based on head and salinity data collected in an offshore, exploratory well 55 mi 

east of Fernandina Beach, Florida (Johnston and others, 

1982). 

4. Discovery of two large submerged springs based 

on a hydrologic analysis, including calibration of a groundwater flow model that indicated probable sites of groundwater discharge where the springs were subsequently found 

(Tibbals, 1990). 

5. Support for a hypothesis (previously proposed by 

Kohout, 1965) of cyclic circulation in the Floridan in 

southern Florida was provided by new data on hydraulic 

heads, temperature, natural isotopes, and water chemistry 

obtained from a deep exploratory hole drilled as part of 

the RASA project. These data, together with isotope, temperature, and salinity anomalies in the Upper Floridan aquifer, suggest (1) inflow of cold seawater in the Lower 

Floridan from outcrops on the sea floor, (2) heating and 

upwelling of seawater from the Lower Floridan, and (3) 

mixing of the rising seawater with seaward-flowing freshwater in the Upper Floridan (Meyer, 1989). 

The study of the Floridan aquifer system was started 

in 1978 and completed in 1986. Forty-five reports have 

been completed and published. The principal findings of 

the study are presented in nine comprehensive chapters 

published as U.S. Geological Survey Professional Paper 

1403-A through I. 

Professional Paper 1403-A (Johnston and Bush, 

1988) summarizes important aspects of the hydrogeologic 

framework, hydraulic properties of the aquifers, regional 

flow system, effects of ground-water development, and geochemistry, which are described in detail in Professional 

Paper 1403-B through 1403-1. 

Professional Paper 1403-B (Miller, 1986) presents a 

detailed regional description of the hydrogeologic framework of the Floridan aquifer system. The Floridan aquifer 

system was divided into regional aquifers and confining 

units that provide a generalized permeability distribution for 

analyzing the flow system by computer-based simulation 

and for relating the water chemistry to the flow system. 

Professional Paper 1403-C (Bush and Johnston, 

1988) discusses ground-water hydraulics, aquifer and confining-unit properties, and features of the regional flow 

system. On the basis of computer-based simulation, the regional flow system was described and the predevelopment 

flow conditions were contrasted with 1980 development 

conditions. 

To address the ground-water hydrology in more detail, the Floridan aquifer system was divided into five subareas on the basis of predevelopment ground-water divides, 

which are shown in figure 26. Professional Paper 1403-D 

(Krause and Randolph, 1989) discusses the ground-water 

hydrology of the Floridan aquifer system in southeastern 

Georgia and adjacent parts of South Carolina and northeastern Florida. This part of the flow system is characterized by an extensive decline in artesian head due to 

withdrawals in a large coastal area. Differences between 

the relatively sluggish predevelopment flow system and the 

current heavily pumped system were emphasized. Various 

alternative development schemes for the future were evaluated by model simulation and are discussed. 

Professional Paper 1403-E (Tibbals, 1990) summarizes the ground-water hydrology of the Floridan aquifer 

system in east-central Florida. By the late 1970's, pumpage 
40 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

accounted for about one-third of the aquifer discharge; 

however, declines in artesian head occur only in a few 

small areas. Computer simulation was used to evaluate predevelopment and late 1970's flow conditions and also to 

evaluate future increases in pumpage. The large springs 

and drainage wells in this area receive special attention in 

this chapter. 

Professional Paper 1403-F (Ryder, 1985) describes 

the hydrology of the Floridan in west-central Florida. 

Ground-water pumpage in the area was 1 Bgal/d in 1976; 

however, the large springs in the northern part of the area 

are still the dominant feature of the ground-water flow 

system. Effects of future increases of ground-water pumpage were evaluated by computer simulation. 

Professional Paper 1403-G (Meyer, 1989) discusses 

the ground-water hydrology in southern Florida, where the 

Floridan contains saline water and there has been little development for water supply. Due to the lack of deep wells, 

there is little subsurface geologic and hydrologic data. 

Therefore, the study consisted of deep test drilling, geochemical sampling, and offshore seismic surveys. The 

principal uses of the Floridan aquifer system in southern 

Floridan are subsurface storage of treated municipal waste 

water, oil-field brine, and industrial waste water and production of slightly saline water for desalinization plants. 

Professional Paper 1403-H (Maslia and Hayes, 

1988) discusses ground-water hydrology in southwestern 

Georgia, the Florida panhandle, and a small part of Alabama. The study concentrated on two areas: (1) the 15-

county Dougherty Plain area of southwestern Georgia, 

where pumping for irrigation is heavy; and (2) the Fort 

Walton Beach area of the panhandle, where pumping produced a deep widespread cone of depression. 

Professional Paper 1403-I (Sprinkle, 1989) describes 

the geochemistry of the Floridan aquifer system. Concentrations of total dissolved solids in water from the Upper 

Floridan aquifer generally range from less than 25 mg!L 

near outcrop areas to more than 25,000 mg/L along the 

coasts, where freshwater mixes with recent or residual seawater. Water in the Upper Floridan can be categorized into 

four hydrochemical facies, whose distribution is determined by confined or unconfined conditions of the aquifer 

and by chloride concentrations. 

HIGH PLAINS 

The High Plains regional a~uifer-system analysis 

study covers an area of 174,000 mi of flat to gently rolling terrain in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming 

(fig. 27). The High Plains regional aquifer is unconfined 

and consists mainly of near-surface sand and gravel deposits. The Ogallala Formation of Tertiary age, which underlies about 80 percent of the High Plains, is the principal 

geologic unit of the aquifer. In the past it has commonly 

been referred to as the Ogallala aquifer. Saturated thickness of the High Plains aquifer averages about 200 ft; 

maximum thickness is about 1,000 ft in central Nebraska. 

Ground water generally flows from west to east and discharges to streams and springs and by evapotranspiration 

where the water table is near land surface. Infiltration of 

precipitation and seepage from streams are the principal 

sources of recharge to the aquifer. 

Because the High Plains are mostly in a semiarid 

region, the amount of recharge to the aquifer is small. This 

small recharge coupled with large pumpage in parts of 

Texas, New Mexico, and Kansas has resulted in a large 

decline of water levels in these areas and reduction of saturated thickness of the aquifer (fig. 5). In turn, this has 

caused a reduction in well yields (fig. 16) and increased 

pumping costs. Locally wells have gone dry. However, 

these conditions do not occur throughout the entire High 

Plains. Regionally there is enough ground water for development at current rates of withdrawal for the next 40 

years, although withdrawal rates are projected to decrease 

in some areas and increase in other areas. 

The High Plains regional aquifer-system analysis was 

started in 1978 and completed in 1986. The study resulted 

40. 

35. 

NEW 

MEXICO 

. 

'" 

105. 

100 

100. 95. 

SOUTH DAKOTA :MINNESOTA 

200 

r---·---

IOWA 

~----

---~~ ·:.__'$ :~ 

I 'a 

1--

OKLAHOMA \\ 'Cil 

'l> 

L I"' 

'"'\.. .. ,.., : 

!."- ..... ....._ ... --.. -~ .. 

TEXAS 

300 MILES 

100 200 300 KILOMETERS 

Figure 27. Study area of the High Plains regional aquifer-system analysis. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 41 

in 122 reports (Sun and Weeks, 1991), including 5 chapters 

in Professional Paper 1400. Professional Paper 1400....A 

(Weeks and others, 1988) summarizes the geohydrology of 

the aquifer system, effects of development, simulation of the 

regional flow system, and the use of flow models to evaluate 

the effects of future pumping strategies. 

Professional Paper 1400....B (Gutentag and others, 

1984) describes the geohydrology of the High Plains aquifer. The aquifer consists of sediments of late Tertiary or 

Quaternary age, underlain by lower Tertiary to Cretaceous 

sediments of low permeability for which the name Great 

Plains confining system was proposed by the Central Midwest RASA study (Signor and Imes, 1989, fig. 3). The 

sediments that make up the aquifer are mainly alluvial, 

dune-sand, and valley-fill deposits. Hydraulic conductivity 

of the aquifer ranges from less than 25 to 300 ft/d, averaging about 60 ft/d. Specific yield ranges from less than 10 

to 30 percent and averages about 15 percent. Pumpage 

from the High Plains aquifer, principally for irrigation, 

was from about 170,000 wells in 1980. Most of the wells 

are in Kansas, New Mexico, Oklahoma, and Texas. Pumpage in 1978 was 2 to 100 times greater than annual recharge, resulting in large water-level declines in some 

areas. Water levels have declined more than 100 ft from 

the predevelopment level to the 1980 level in an area of 

2,500 mi2 in parts of Kansas, New Mexico, Oklahoma, 

and Texas. Professional Paper 1400....B also briefly discusses the quality of water in the aquifer, which is generally 

acceptable. 

Professional Paper 1400....C (Thelin and Heimes, 

1987) discusses the use of Landsat multispectral-scanner 

data to map irrigated cropland for determination of wateruse from the High Plains aquifer. Information on irrigated 

acreage and water-use for irrigation is critical to evaluate 

the effects of agricultural development on the High Plains 

aquifer because water-use for irrigation accounts for about 

95 percent of all water pumped from the High Plains aquifer. Several methods for determining irrigated acreage 

were evaluated. Digital analysis of Landsat data proved to 

be the most suitable approach. The reliability of irrigatedacreage estimates was tested in the field using a multistage 

random-sampling method. The information on irrigated 

acreage was combined with measurements of applied irrigation water at selected field sites to determine total irrigation pumpage. For the 1980 growing season, it was 

estimated that almost 18 million acre-ft was pumped from 

the High Plains aquifer to irrigate nearly 14 million acres. 

Statistical evaluation of the field data suggested that Landsat data and simple analysis techniques used during this 

RASA study can provide an efficient tool for mapping irrigated cropland on the High Plains. These techniques 

should also be applicable to similar areas of the Western 

United States. 

Professional Paper 1400....D (Luckey and others, 1986) 

discusses the ground-water flow system on the basis of 

model simulation. The High Plains aquifer was divided into 

three parts for simulation. The southern High Plains includes 

the part of the Plains in Texas and New Mexico, south or 

west of Amarillo, Texas (fig. 28). The central High Plains is 

the area north and east of Amarillo and south of approximately 39 degrees latitude. The northern High Plains is the 

area north of approximately 39 degrees latitude. A narrow 

isthmus, approximately 12 mi wide, occurs along the boundary separating the southern and central High Plains models 

in the vicinity of Amarillo, Texas. The isthmus joining the 

central and northern High Plains is about 32 mi wide and 

follows a bedrock high where the aquifer is thin. Probably 

little exchange of water occurs between the three model 

areas (Luckey and others, 1986). The estimated predevelopment recharge for the southern, central, and northern High 

Plains models ranged from 0.086 to 1.03 in/yr, from 0.056 

to 0.84 in/yr, and from 0.075 to 1.52 in/yr, respectively. 

Transient simulations of development conditions were made 

for 194(}....60 and 1960....80. The simulations suggested that 

if net pumpage for irrigation (total pumpage minus irrigation 

return flow that infiltrates to the aquifer) is between 90 and 

100 percent of the estimated irrigation requirements (defined 

as the product of irrigated acreage and composite crop water 

NEW 

MEXICO 

SOUTH DAKOTA 

. . 

. SOUTHERN!. 

1 !HIGH 

PLAINS I 

TEXAS 

:MINNESOTA 

~-------

IOWA 

\ 

. 

~-- · 

OKlAHOMA 

100 100 300 MILES 

100 100 300 KILOMETERS 

Figure 28. Subdivisions used in modeling the High Plains aquifer (from Weeks and others, 1988). 
42 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

demands), then simulated water levels match observed water 

levels better. 

Simulation provided water budgets for the heavily 

pumped southern High Plains. As discussed in the earlier 

section on ground-water budgets, simulation indicated that 

pumpage during 1960-80 exceeded natural recharge by 

more than 30 times. 

Professional Paper 1400-E (Luckey and others, 

1988) discusses the effects of pumpage on water levels in 

the High Plains aquifer under various scenarios of future 

water demand based on model simulation. Pumpage from 

the High Plains aquifer had been projected for 1977, 1985, 

1990, 2000, and 2020 in a study conducted by the Economic Development Administration of the U.S. Department of Commerce; management strategies were proposed 

and recommended (High Plains Study Council, 1982; 

High Plains Associates, 1982). Three of the recommended 

management strategies were evaluated during the RASA 

study, and results are presented in Professional Paper 

1400-E. In the southern High Plains, without significant 

control of pumpage, simulation indicated that saturated 

thickness in more than one-half of the area would be less 

than 25ft due to water-level declines by the year 2000. 

NORTHERN ATLANTIC COASTAL PLAIN 

The Northern Atlantic Coastal Plain regional aquifer 

system underlies about 50,000 mi2 of gently rolling to 

nearly flat topography. The system extends from Long Island, New York, to the North Carolina-South Carolina 

State boundary and from the Fall Line (inner Coastal Plain 

margin) eastward to the Atlantic Ocean (fig. 29). The aquifer system consists of a seaward-thickening wedge of predominantly unconsolidated sediments that thickens from a 

featheredge at the Fall Line in the west to more than 8,000 

ft along the coast of Maryland and 10,000 ft at Cape Hatteras in North Carolina. The sediments are mostly gravel, 

sand, silt, and clay of Jurassic to Holocene age. Limestone 

occurs principally in North Carolina. During this RASA 

study, the sediments were grouped into 10 regional aquifers, consisting principally of sand, gravel, or limestone, 

separated by 9 confining units, consisting principally of 

clay and silt. A regional aquifer may coincide with a recognized local aquifer in one area and comprise several local 

aquifers in another, or it may constitute only a part of an 

aquifer. None of the regional aquifers extend over the entire study area. The delineation of aquifers and confining 

units was based on the predominant lithologies, depositional environment, and hydraulic relations between the 

sediments. Lateral facies shifts that were caused by marine 

transgressions and regressions determined the areal distribution of the aquifers and confining units (Trapp, 1993). 

Recharge to the aquifer system is from precipitation 

and occurs chiefly in upland and interfluvial areas. The recharge rate ranges from 10 to 25 inlyr; most of the recharge 

flows through shallow parts of the aquifer system and discharges to streams that dissect the Coastal Plain sediments. 

A small amount of recharge, generally about 0.5 inlyr, replenishes the deeper confined parts of the aquifer system. 

Water in the deep aquifers discharges upward across confining units into shallow aquifers and ultimately into major 

rivers, coastal estuaries, sounds, bays, and the ocean. Pumpage from the confined part of the aquifer system, principally 

for municipal and industrial use, has increased from 100 

Mgalld in 1900 to about 1,200 Mgalld in 1980, resulting in 

widespread potentiometric head declines that have significantly altered rates and conditions of ground-water recharge, 

discharge, and flow patterns. However, results of model 

simulation indicate that the aquifer system adjusts readily to 

changes in pumpage and reaches a new equilibrium after 

about 5 yr following changes in pumpage. Storage change 

resulting from pumping is minimal (Leahy and others, 1987; 

Leahy and Martin, 1993). A ground-water budget for predevelopment and 1980 conditions is presented in the earlier 

section on regional ground-water budgets (fig. 12). 

Salty ground water underlies freshwater in the eastern 

part of the Northern Atlantic Coastal Plain. Chloride concentrations generally increase with depth within a transition 

zone between the deepest freshwater and the underlying 

saltwater. The zone contains concentrations of chloride 

ranging from 1,000 to 18,000 mg/L. (A 18,000 mg/L chloride concentration is approximately the chloride concentration of seawater.) The saltwater-freshwater transition zone 

generally deepens inland from the coast, except in New 

Jersey and locally in North Carolina, where it is deepest 

along the coast. The transition zone is shallowest in North 

Carolina, particularly in the vicinities of the Cape Fear River 

and Albemarle Sound, and deepens northward to Maryland 

and the coast of New Jersey. However, the transition zone 

is relatively shallow in the vicinity of Delaware Bay. Chloride concentration of less than 5,000 mg/L extends 55 mi 

off the New Jersey coast. The offshore extension of the 

transition zone decreases southward toward southern Virginia. Depth to the saltwater-freshwater transition zone is partly 

controlled by the natural flow pattern of fresh ground water. 

Areas where the transition zone is relatively shallow generally coincide with areas of ground-water discharge, such as 

Delaware Bay, lower Chesapeake Bay, Albemarle Sound, 

and the Cape Fear River. The occurrence of a deep transition 

zone in New Jersey and Maryland and water considerably 

fresher than seawater offshore is attributed to sea-level 

changes in past geologic time. In New Jersey and Maryland, 

past sea level probably was lower than present sea level, 

resulting in wide extension of the transition zone offshore; 

in Virginia and North Carolina, past sea level probably was 

near land surface, resulting in a shallower transition zone 

than in New Jersey and Maryland (Meisler, 1980, 1989). 

The effect of eustatic sea-level fluctuations on the 

location and development of the saltwater-freshwater tran-
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 43 

sition zone in New Jersey was analyzed with a finite-difference cross-sectional flow model. The sedimentary 

wedge from the Delaware River to the Continental Slope 

was simulated as a highly anisotropic porous medium. (A 

ratio of simulated lateral to vertical hydraulic conductivity 

of 30,000: 1 provided the best results in the model.) Simulated sea-level fluctuation was from 50 to 150 ft below 

present sea level. The model was found to be sensitive to 

so· 

PENNSYLVANIA 

OHIO 

WEST VIRGINIA 

SOUTH CAROLINA 

the altitude of sea level and the degree of anisotropy. The 

best match of simulated and observed chloride concentrations was obtained with sea level 50 to 100 ft below 

present sea level. Simulation results indicate that the aquifer system in New Jersey has not reached equilibrium with 

the rising of sea level. The transition zone is probably 

moving slowly landward and upward toward equilibrium 

with present sea level (Meisler and others, 1985). 

Atlantic 

Ocean 

100 MILES 

100 KILOMETERS 

EXPLANATION 

C:=J Study area 

Fall line 

Figure 29. Study area of the Northern Atlantic Coastal Plain regional aquifer-system analysis. 
44 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Anomalously high bicarbonate concentrations occur 

in water from aquifers consisting of nonmarine sediments 

in the Northern Atlantic Coastal Plain. The concentrations 

of bicarbonate are several times higher than could be generated by reactions involving carbon dioxide gas in recharge water entering the aquifers. To explain this anomaly 

it was hypothesized that the carbon dioxide gas must be 

generated within the aquifers. During the RASA study, 

Chapelle and Knobel ( 1985) suggested that such carbon 

dioxide gas must be generated by bacterially mediated 

processes in the aquifer. A test hole was drilled near Waldorf, Maryland, to test this hypothesis. Nineteen cores of 

unconsolidated Coastal Plain sediments from the Aquia aquifer were obtained from 4 7 to 545 ft below land surface 

and analyzed for metabolically active anaerobic bacteria. 

They concluded that the metabolic processes of fermentative and methanogenic bacteria produced carbon dioxide 

gas in the Aquia aquifer in Maryland. This research of microbially mediated oxidation process of sedimentary organic matter has been extended to other areas of Coastal 

Plain sediments (Chapelle and McMahon, 1991; McMahon 

and Chapelle, 1991). 

The Northern Atlantic Coastal Plain regional aquifer-system analysis was started in 1979 and completed in 

1987. As of early 1993, 35 reports had been completed, 

including 13 chapters in Professional Paper 1404 (Sun and 

Weeks, 1991). Seven Professional Paper chapters have 

been printed; six are being processed for printing. 

NORTHERN GREAT PLAINS 

The Northern Great Plains regional aquifer system 

underlies about 250,000 mi2 of the Great Plains and Central 

Lowlands physiographic provinces. It includes North Dakota and parts of South Dakota, Nebraska, Montana, and 

Wyoming (fig. 30). The study area is bounded on the west 

by the central and northern Rocky Mountains, on the east 

by the Red River of the North, on the south by the central 

High Plains, and on the north by the United States-Canadian border. Sedimentary rocks underlying the area originated as erosional products from mountains to the west that 

were deposited to a thickness of more than 16,000 ft in the 

subsiding Williston and Powder River basins and surrounding areas. Subsequently, several hundred feet of sedimentary rocks were eroded, leaving remnants of more resistant 

rocks. 

The principal aquifers, which are areally extensive, 

underlying the northern Great Plains are exposed along the 

flanks of the Bighorn and Powder River basins and other 

structural features. Ground water generally flows northeastward across the study area. The source of recharge is 

precipitation on a topographically high area near the Black 

Hills uplift, the Bighorn Mountains, the eastern flank of 

the northern Rocky Mountains, and other major structurally high areas (fig. 31). Ground water discharges mostly 

into topographically low areas of eastern North Dakota 

and South Dakota and as outflow into Canada. Some 

ground-water discharge occurs as diffuse upward leakage 

into overlying aquifers through confining units. 

Five major aquifer systems were identified during 

this RASA study. In descending order, they are the Upper 

Cretaceous aquifer system, which mostly includes sediments of Late Cretaceous age but includes surficial glacial 

and Tertiary sediments; the Lower Cretaceous aquifer system; the Pennsylvanian aquifer system; the Mississippian 

(Madison Limestone) aquifer system; and the CambrianOrdovician aquifer system. Flow characteristics of the aquifer systems vary significantly between the dominantly 

carbonate aquifers (such as the Madison Limestone) and 

the dominantly clastic aquifers (such as the Dakota Sandstone). The major aquifers also are petroleum reservoirs 

within much of the study area. The hydrodynamic forces 

involved in the regional ground-water flow system have 

contributed to the accumulation of petroleum in the Williston basin (Downey, 1988). 

All aquifer systems crop out and receive recharge in 

the highlands in the western part of the study area. However, a large part of the recharge water in outcrop areas 

discharges within short distances as springs and seeps 

along the flank of the mountains; only a small part of the 

recharge water flows downward to the deep, regional flow 

systems and flows eastward to discharge areas. On the 

basis of model simulation and conceptualized flow patterns in the aquifer systems, it has been deduced that 

water in the Upper Cretaceous aquifer system discharges 

to the Missouri River or upward to overlying surficial aquifers. Discharge from the Lower Cretaceous aquifer system is mainly upward to the overlying Upper Cretaceous 

aquifer system in eastern North Dakota and South Dakota 

and to the subcrop . of the Lower Cretaceous aquifer system in the glacial Lake Agassiz Plain in North Dakota. 

Discharge from the aquifer systems consisting of Paleozoic rocks-the Pennsylvanian aquifer system, the Mississippian aquifer system (Madison Limestone), and the 

Cambrian-Ordovician aquifer system-principally is upward to overlying aquifer systems and to the Red River of 

the North. However, in eastern North Dakota, water in the 

Pennsylvanian aquifer system flows downward into the 

Cambrian-Ordovician aquifer system, then discharges to 

the Red River of the North, or to land surface as springs. 

A body of brine is present in the deeper aquifers 

along the eastern flank of the Williston basin, where water 

movement is sluggish. Three hypotheses have been developed concerning the movement of brine and ground water: 

(1) the brine is static, (2) the brine is moving slowly with 

regional ground-water flow (a sluggish segment of regional flow), and (3) the brine is moving due to the adjustment 

of the flow system following the melting of Pleistocene 

ice. Origin of the brine appears to have been the dissolu-
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 45 

tion of halite. As the density of the saline water increased, 

it caused fresher water to be diverted around the saline 

water body to the north and south, or above it into the 

overlying Cambrian-Ordovician and Mississippian aquifer 

systems. The second hypothesis seems to agree best with 

model simulation and field data; however, the third hypothesis cannot be ruled out completely. The distribution 

of the saline water may still be adjusting in response to 

changes in recharge and discharge at the end of the Pleistocene (Downey, 1988). 

The Northern Great Plains regional aquifer-system 

analysis was started in 1978 as an extension of the Madison 

aquifer (the Mississippian aquifer system) study. The Madison study was started in 1975 to address the problems of 

45 ° 

40 ' 

_... \ 

IDAHO 

water supplies associated with development of coal resources and proposed coal-slurry pipelines in the Fort Union coal 

region. The Northern Great Plains RASA was completed in 

1982 and resulted in 75 reports (Sun and Weeks, 1991), 

including 9 reports completed during the Madison aquifer 

study and 6 chapters of Professional Paper 1402. Professional Paper 1402-A (Downey and Dinwiddie, 1988) summarizes the general geology and hydrology of the regional 

aquifer systems underlying the northern Great Plains. Professional Paper 1402-B (Anna, 1986) discusses the geologic 

framework of the aquifer systems. Professional Paper 1402-

C (Henderson, 1985) discusses geochemistry of water in the 

Kootenai Formation in the Judith basin of Montana, and in 

the Lance Formation and Fox Hills Sandstone in the Powder 

------

NEBRASKA 

I 

UTAH 100 200 MILES ~- ----- --- --- - 100 200 KILOMETERS 

KANSAS 

EXPLANATION 

Fort Union coal region 

Test well drilled by the U.S. Geological Survey 

• Well penetrating the Madison aquifer 

.t. Well penetrating the Pierre Shale 

Approximate study area of the Northern Great Plains 

Regional Aquifer System 

Figure 30. Study area of the Northern Great Plains regional aquifer-system analysis. 
46 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

River basin of Wyoming. These formations consist of two 

important sandstone aquifers in the northern Great Plains 

region. Professional Paper 1402-D (Lohmeyer, 1985) describes the distribution of freshwater heads and groundwater temperatures in Cretaceous and Tertiary aquifers. This 

chapter also describes a method used to convert bottom-hole 

shut-in pressure to equivalent freshwater head by correcting 

for density variations resulting from temperatures observed 

during drill-stem tests. Professional Paper 1402-E (Downey, 

1986) describes the geohydrology, aquifer hydraulic parameters, and regional flow system on the basis of simulation 

results. Professional Paper 1402-F (Busby and others, in 

;" 

' 45 ' \ 

40' 

110 ' 105 ' 

' 

press) discusses the geochemistry of waters in the aquifer 

systems and their related confining units. 

Information obtained during the Madison aquifer study 

is summarized in Professional Papers 1273-A through -G. 

The stratigraphy and sedimentary facies of the Madison 

Limestone and associated rocks are described in Professional 

Paper 1273-A (Peterson, 1984). The correlation of paleo7 

structure and sediment deposition in the Madison Limestone 

and associated rocks is described in Professional Paper 

1273-B (Brown and others, 1984). Thayer (1984) discussed 

the relation of porosity and permeability to the petrology of 

the Madison Limestone in Professional Paper 1273-C. 

100 ' 

UTAH 100 200 MILES ~---- --- --

100 200 KILOMETERS KANSAS 

EXPLANATION 

Mountain ranges of the eastern flank of the northern Rocky Mountains 

1 

2 

3 

4 

5 

.. 

Little Rocky 6 Pryor 11 Uinta 

Little Belt 7 Bighorn 12 Sierra Madre 

Judith 8 Absaroka 13 Medicine Bow 

Big Snowy 9 Owl Creek 14 Laramie 

Beartooth 10 Wind River 

Approximate study area of the Northern Great Plains 

Regional Aquifer System 

Figure 31. Structural and physiographic features of the northern Great Plains and vicinity (from Downey and Dinwiddie, 

1988). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 47 

Apparent water resistivity, porosity, and temperatures of the 

Madison Limestone are discussed in Professional Paper 

1273- D (MacCary, 1984). Potentially favorable areas for 

developing large-yield wells in the Red River Formation and 

Madison Limestone are suggested in Professional Paper 

1273-E (MacCary and others, 1983). The geochemical 

evolution of water in the Madison aquifer is described in 

Professional Paper 1273-F (Busby and others, 1991). Geohydrology of the Madison and associated aquifers is presented in Professional Paper 1273-G (Downey, 1984 ). 

SOUTHEASTERN COASTAL PLAIN 

The Southeastern Coastal Plain aquifer system underlies about 120,000 mi2 in parts of South Carolina, 

Georgia, Alabama, and Mississippi, and adjacent areas of 

northern Florida and southeastern North Carolina (fig. 32). 

The Southeastern Coastal Plain aquifer system is located 

35' 

A RKANSAS 

S'.i 

? 

t; 

<( 

,.5 

...> L __ _ 

TENNESSEE 

among four adjacent regional aquifer systems investigated 

during the RASA Program: the Northern Atlantic Coastal 

Plain regional aquifer system to the northeast, the Floridan 

aquifer system to the south and southeast, the Gulf Coastal 

Plain regional aquifer system to the west, and the Appalachian Valleys and Piedmont regional aquifer system to the 

west of the Fall Line. 

The Southeastern Coastal Plain aquifer system consists of a thick wedge of unconsolidated to consolidated 

clastic and carbonate rocks of Cretaceous to Holocene age. 

These rocks extend and thicken seaward from the inner 

Coastal Plain margin (the Fall Line) to the Atlantic Ocean, 

Gulf of Mexico, or Florida peninsula. Except where covered 

by younger strata, rocks comprising the aquifer system crop 

out in adjacent bands from Mississippi to South Carolina. In 

outcrop areas, most of the water that enters the aquifer 

system discharges to nearby local streams; only a small part 

of the recharge moves downward and enters the deeper, 

confined system. In gulfward areas, deeply buried clastic 

85' 

GULF OF MEXICO 

50 100 KILOMETERS 

Base from U.S. Geological Survey 

National Atlas, 1970 EXPLANATION 

c=J Study area 

Area of outcrop or shallow subcrop 

of the Southeastern Coastal Plain 

aquifer system 

Boundary of physiographic province 

Figure 32. Study area of the Southeastern Coastal Plain regional aquifer-system analysis (from Miller, 1992). 
48 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

and carbonate rocks contain saline water and are not considered as part of the aquifer system. In landward areas, the 

system is underlain by metamorphic, sedimentary, and igneous rocks of Paleozoic and early Mesozoic age that have 

very low permeability. The lower limit of the Southeastern 

Coastal Plain regional aquifer system is designated as the 

top of the freshwater-saltwater interface in the seaward and 

gulfward areas, and at the top of the low-permeability rocks 

of Paleozoic and early Mesozoic age in the landward area. 

Sediments of the aquifer system were grouped into 

four regional aquifers and three confining units. The regional 

aquifers consist of sand, sandstone, gravel, and minor limestone. The regional confining units consist of clay, mudstone, marl, and chalk that separate the regional aquifers. 

The regional aquifers are named after major rivers along 

which they crop out. From shallowest to deepest, they are 

the Chickasawhay River aquifer, Pearl River aquifer, Chattahoochee River aquifer, and Black Warrior River aquifer. The 

regional confining units are named for the regional aquifer 

.the confining unit overlies. They are the Pearl River confining unit, Chattahoochee River confining unit, and Black 

Warrior River confining unit (Miller and Renken, 1988). 

Regional hydrogeologic units are discontinuous within the 

study area. For example, the Black Warrior River aquifer is 

absent in updip areas of South Carolina and eastern Georgia. 

The Chattahoochee River aquifer is absent in a large area in 

western Alabama and southeastern Mississippi. Some of the 

hydrogeologic units of the Southeastern Coastal Plain aquifer system grade into units that are part of adjacent regional 

aquifer systems owing to facies change. For example, there 

is a facies change from clastic rocks of the Southeastern 

Coastal Plain aquifer system into the predominantly carbonate rocks of the Floridan aquifer system. 

Almost all of the recharge is from precipitation that 

falls on the outcrop. areas of the aquifer system; however, 

some recharge does occur in confined parts of the system 

by downward leakage, mostly where the shallower aquifer 

is unconfined or less confined. Flow in outcrop areas is 

predominantly vertical and becomes horizontal in middip 

confined parts of the aquifer system. Farther downdip, discharge occurs as upward leakage to shallower aquifers. 

Upward leakage primarily results from a combination of 

decreased permeability and abrupt increases in concentrations of dissolved solids of the water. Although rates of 

upward leakage are small, leakage occurs over a broad 

area, resulting in a large amount of leakage. 

A simulated water budget of the Southeastern Coastal Plain regional aquifer system is as follows: annual precipitation, 51 in/yr; evapotranspiration, 32 in/yr; overland 

runoff, 12 in/yr; and ground-water recharge, 7 in/yr. More 

than 90 percent of the recharge (6.4 in/yr) moves along 

short flow paths to nearby streams, becoming baseflow; 

the remaining 10 percent (about 0.6 in/yr) percolates to 

deeper (confined) regional aquifers. About 80 percent of 

the regional flow (0.5 in/yr) discharges to large rivers; the 

remaining 10 percent (0.1 in/yr) discharges upward as diffuse leakage into shallower aquifers such as the Floridan 

(Miller and others, 1987). 

Withdrawal of water from the aquifer system started 

in about 1900, and by 1985 pumpage was about 495 

Mgal/d. About 320 Mgal/d was pumped from the Chattahoochee River aquifer, and 113 Mgal/d from the Black 

Warrior River aquifer. Simulation indicated that about 45 

percent of the pumpage was derived from reduction in 

ground-water discharge to streams (reducing baseflow in 

streams), about 31 percent of the pumpage was water removed from aquifer storage, and the remaining 24 percent 

of the pumpage was derived from the adjacent aquifer systems (mostly from the Upper Floridan aquifer). Simulated 

upward leakage to the Upper Floridan decreased in many 

places, and downward leakage from the Upper Floridan 

increased in other places. There was no change in recharge 

because pumping occurred in middip confined parts of the 

Southeastern Coastal Plain aquifer system. Except in a few 

places, water levels in outcrop areas have not been affected by pumping. However, in downdip, confined parts of 

the Chattahoochee River and Black Warrior River aquifers, cones of depression have developed around pumping 

centers, with head declines exceeding 100 ft locally. 

Water chemistry changes along ground-water flow 

paths as a result of water-rock mineral reactions. In outcrop 

areas, concentrations of dissolved solids in ground water are 

less than 50 mg/L, and the water is mostly calcium bicarbonate type. Downgradient from outcrops, water is mostly 

sodium bicarbonate type, and concentrations of dissolved 

solids range from 100 to 200 mg/L. Farther downgradient, 

concentrations of dissolved solids are from 300 to 500 mg/ 

L, and chloride increases with depth. Farthest downgradient, concentrations of dissolved solids are from 1,000 to 

30,000 mg/L, and sodium chloride type water is dominant, 

indicating that freshwater is mixing with saltwater in these 

areas (Miller and others, 1987). 

The Southeastern Coastal Plain regional aquifer-system analysis was started in 1979 and completed in 1988. As 

of May 1991, 61 reports had been completed (Sun and 

Weeks, 1991 ), including 8 chapters in Professional Paper 

1410. Professional Paper 1410-A (Miller, 1992) summarizes the important aspects of the aquifer system, and Professional Papers 1410-B through -D respectively describe the 

regional hydrogeologic framework, regional hydrology and 

simulation of deep ground-water flow, and ground-water 

chemistry; Professional paper 1410-E through -H discuss 

the aquifer system on a subregional basis for South Carolina, 

Georgia, Mississippi, and Alabama, respectively. 

SOUTHWEST ALLUVIAL BASINS 

Hundreds of isolated alluvial basins exist in the southwestern United States. Some of the basins have surface-
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 49 

water outflow, while others do not. It was impossible to 

study all basin aquifers under the RASA Program. Therefore, only some of the basins were selected for study to 

determine the occurrence, movement, and geochemistry of 

ground water in basin aquifers. Study results indicate that 

certain hydrologic and geologic information can be transferred from the studied basin aquifers to other basin aquifers. 

Geographically, the study was divided into two parts. One 

was in parts of Colorado, New Mexico, and Texas; the other 

was in southern and central Arizona and small parts of adjacent States. 

The 70,000 mi2 study of parts of Colorado, New Mexico, and Texas includes 22 basins (fig. 33). Two types of 

basins occur in the study area: (1) open basins that have a 

surface-water outlet and are within the Rio Grande rift, and 

(2) closed basins that have no surface-water outlet and are 

predominantly in southwest New Mexico and west Texas. 

The Rio Grande rift is a fault-bounded structural feature 

with uplifted blocks on the east and west. Uplifted blocks 

east of the basins generally rise several thousand feet above 

the valley floor of the basins. The basins are bounded on the 

north, east, and west mainly by Quaternary and Tertiary 

volcanic rocks, and Mesozoic and Paleozoic rocks. Alluvial 

sediments in the basins were derived from the surrounding 

highlands and mountains. These sediments are composed of 

flood-plain deposits and the Santa Fe Group that consists of 

unconsolidated to moderately consolidated lenticular deposits of gravel, sand, and clay interbedded in places with volcanics. Most wells are completed in the Santa Fe Group and 

individually produce as much as 3,000 gaVmin. 

Precipitation on the mountains on the east and west 

sides of the basins is the principal source of recharge to 

basin aquifers. Surface water runs off from mountainous 

areas and infiltrates the basin aquifers near mountain 

fronts; little surface water flows onto the alluvial fans because of high rates of infiltration in arroyos. The second 

source of recharge in open basins is seepage from the Rio 

Grande and return flow of applied irrigation water from 

surface-water sources. 

Ground water flows from east and west sides of the 

basins toward basin axes and then flows southward, except 

in the Animas basin and the lower part of the Salt basin, 

where ground-water flow is northward. 

Quality of ground water changes areally and vertically. Generally, concentrations of dissolved solids are less 

than 1,000 mg/L in water several hundreds of feet below 

the water table in flood-plain areas. However, concentrations increase and range from 3,000 to 10,000 mg/L in 

water 2,000 ft or more below the water table. 

The study of alluvial basins in parts of Colorado, 

New Mexico, and Texas was started in 1978 and was completed in 1985. As of May 1991, 30 reports had been completed (Sun and Weeks, 1991). As of rnid-1992, chapter C 

of Professional Paper 1407 was published and chapters A 

and B were in press. 

The study of alluvial-basin aquifers in southern and 

central Arizona and parts of California, Nevada, and New 

Mexico covers an area of 82,000 mi2 that contains 72 basins (fig. 34). The area is characterized by sharply rising 

mountains that separate wide and fiat basins. The basins 

are filled with alluvial deposits, ranging from a few thousand feet to more than 10,000 ft in thickness. The basins 

38 " 

34 " 

32 

I 

10 

lo 

'...: 

<{..J :r/~ f-10 

=>,o 

108" 

---t------------__£- «:10 

ZIO o/x Nw 

a::. 

<tl~ 

~~ I 

I I 

I 

I 

I 

I 

I 

I 

50 100 MILES 

50 100 KILOMETERS 

EXPLANATION 

Closed basins (no surface-water outlet) 

//// ;' ' 

~~~~;~~ Open basins (surface-water outlet) 

Study area 

104" 

Figure 33. Study area and types of alluvial basins in parts of 

Colorado, New Mexico, and Texas (modified from Wilkens, 

1986). 
50 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

were formed 10 to 15 million years ago when movement 

along high-angle normal faults downdropped the basins in 

relation to the mountain masses. The result was a series of 

generally northwest-trending basins. The formation of basins was a gradual process that was accompanied by deposition of locally derived sediments. In almost all basins, 

the vertical sequence of sediments, in descending order, 

are stream alluvium, upper and lower basin fill, and sedimentary rocks deposited prior to the formation of the 

Basin and Range topography. 

36' 

34' 

NEVADA 

Las Vegas. 

' ' "' ' 

114' 112' 

I UTAH r---ARizoNA---

25 50 MILES 

0 25 50 KILOMETERS 

The sedimentary rocks deposited before the development of Basin and Range topography consist of moderately to highly consolidated continental deposits, including 

clay to gravel and some volcanic rocks. These sedimentary 

rocks are deeply buried and were faulted and tilted with 

the underlying bedrock. Except near basin margins, very 

few wells have been drilled into these sedimentary rocks. 

The basin-fill sediments, which are the principal aquifers in most basins, were deposited during and after the 

development of Basin and Range topography. The lower 

Gallup 

NEW MEXICO 

• Silver City 

I 

__ _j 

EXPLANATION 

~if~ii ~~~ Modeled basins 

Study area 

Boundary of alluvial basin categories 

Basin boundary 

Figure 34. Study area and categories of alluvial basins in southern and central Arizona, and parts of California, Nevada, and 

New Mexico (from Anderson and others, 1992). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 51 

basin fill is moderately to highly consolidated fine-grained 

materials and contains evaporites. The upper basin fill was 

deposited during a transitional period when the basins 

changed from closed basins to an integrated drainage system. The upper basin fill is less consolidated, generally is 

thinner, and contains less fine-grained materials than the 

lower basin fill. Both the lower and upper basin fill grade 

laterally from coarse-grained sediments at basin margins 

to fine-grained sediments near basin centers. 

The alluvium was deposited by the present surface 

drainage system and ranges from clay to boulders. Where 

saturated, the alluvium is the most productive aquifer. 

Recharge to aquifers occurs mainly from runoff 

along mountain fronts or as seepage from major streams. 

In some basins, subsurface inflow from adjacent basins 

also contributes recharge. ·Ground water generally discharges as springflow, to streams, through evapotranspiration, or to downgradient adjacent basins as subsurface 

outflow. Water quality in the basin aquifers generally is 

good; concentrations of dissolved solids are less than 

1,000 mg/L in most places. Ground-water development 

principally is for irrigation and has resulted in depletion of 

streamflows and water-level declines (from 50 to 400 ft) 

in some heavily developed basins. Pumpage increased 

from 1.7 million acre-ft in 1942 to 4.8 million acre-ft in 

1962. During 1950-80, an average of 4.8 million acre-ft of 

water per year was pumped. 

On the basis of total downvalley flow and lithology 

of basin aquifers, the alluvial basins were grouped into 

five categories by Anderson and others (1992) and named 

geographically as (1) southeast, (2) central, (3) west, (4) 

Colorado River, and (5) highland basins (fig. 34). 

Aquifers in basins of the southeast category consist 

of at least two saturated units, as well as upper basin fill 

and lower basin fill, separated by a low-permeability finegrained confining unit. The major recharge is infiltration of 

streamflow along mountain fronts and along major stream 

channels. Ground-water discharge, before development, 

was mainly through evapotranspiration, to streams, and 

subsurface outflow to downgradient basins. Ground-water 

development has resulted in reduction of natural discharge, 

increase of seepage from streamflow, and a small change 

in aquifer storage. Computer models of basin aquifers in 

the southeast category should be multilayered and designed to allow flow through leaky confining units. 

Aquifers in basins of the central category consist of 

as much as 5,000 ft of lower basin fill and less then 1,000 

ft of upper basin fill. Alluvium along major streams is as 

much as 300 ft thick. Fine-grained sediments commonly 

comprise more than 60 percent of the sediments near the 

centers of the basins. Evaporites typically are found in 

lower basin fill. Recharge consists of small to moderate 

amounts of mountain-front recharge and streamflow infiltration. Subsurface inflow from upstream basins may be 

significant in some basins. Before development, most 

ground-water discharge occurred as evapotranspiration. 

Discharge to streams is small. In some basins, subsurface 

outflow to a downgradient basin may be significant. Development can decrease evapotranspiration and discharge 

to streams. Most pumpage is supplied by a reduction in 

aquifer storage. Computer models of aquifers in the central category should be multilayered. 

Pre-Basin and Range sediments occur at shallow 

depths in basins of the west category and are overlain by 

moderate- to coarse-grained lower basin fill (Anderson and 

others, 1992). Lower basin fill is the principal aquifer in 

this category. Upper basin fill is thin and occurs mostly 

above the water table. Alluvium is present only along the 

lower Gila River. Both recharge and discharge are small. 

Pumpage from wells is almost entirely from aquifer storage. Multilayered computer models are recommended for 

studying basin aquifers in this category. 

Aquifers in the Colorado River category consist of 

fanglomerate overlain by marine estuarine deposits of the 

same age as the lower basin fill. Lower basin fill, in turn, 

is overlain by upper basin fill. Alluvium that overlies and 

occupies channels cut into the basin fill, and the floodplain area is the principal aquifer. Flow in aquifers of this 

category is totally dominated by flow in the Colorado 

River. Seepage of water from the Colorado River is the 

principal source of recharge. Before development, groundwater discharge occurred mainly as evapotranspiration by 

phreatophytes and riparian vegetation. Pumpage has little 

effect on the aquifers because of the immensity of induced 

recharge from the Colorado River. Away from the river, 

small amounts of water may be withdrawn from aquifer 

storage locally. However, when the hydraulic gradient 

reaches the river, eventually all pumpage will be supplied 

by induced flow from the river. Aquifers in this category 

can be simulated with two-dimensional models. 

Aquifers in the highland category consist of thin 

basin-fill deposits of small areal extent that are present in 

flood plains of streams draining the area. These alluvial 

deposits are superimposed on a sequence of older, consolidated sedimentary rocks. Many of the streams are perennial. 

Recharge occurs as underflow from adjacent consolidatedrock aquifers and from infiltration of streamflow. Mountainfront recharge occurs only locally. Ground-water discharge 

occurs as evapotranspiration or to streams. Pumpage effects 

are dependent upon the magnitude and location of the development. Pumpage could be supplied by induced infiltration of streamflow or by reduction in aquifer storage and 

discharge to streams. Aquifers in the h~ghland category can 

be simulated with two-dimensional models. 

During this RASA study, it was observed that in certain basins in southern and central Arizona, earth fissures 

are caused by land subsidence in response to water-level 

declines from pumping (fig. 24). Earth fissures create problems in maintenance of structures, canals, roads, and irrigation and drainage systems. Determination of cause-effect 
52 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

relations between pumping and fissure formation is critical 

to management of ground-water resources. The geochemical evolution of ground-water quality in basins with land 

subsidence has some unique features. Therefore, the study 

of basin aquifers in southern and central Arizona was extended to further investigate earth fissures and water chemistry. Computer models were developed to simulate land 

subsidence related to pumping (Leake, 1990, 1991), and 

earth-fissure movement associated with fluctuations of 

water levels were interpreted by Carpenter (1991). Geochemical models were also developed to determine the 

chemical evolution of water in these basins (Robertson, 

1991). 

The study of basin aquifers in southern and central 

Arizona, and parts of Nevada, New Mexico, and California, was started in 1978 and completed in 1990. As of 

May 1991, 53 reports had been completed (Sun and 

Weeks, 1991). Chapters B and C of Professional Paper 

1406 have been printed and chapters A and D are in press 

as of mid-1992. 

COMPLETED REGIONAL 

AQUIFER-SYSTEM ANALYSIS STUDIES; 

SOME REPORTS ARE IN PREPARATION 

ORIN REVIEW 

The 12 regional studies that were completed as of mid1992 are summarized in this section of the report. The data 

compilation and investigative activities have been completed and final reports are either in review or preparation. 

CARIBBEAN ISLANDS 

The Caribbean Islands regional aquifer-system analysis includes Puerto Rico, its adjacent islands (Vieques, 

Culebra, and Mona), and the U.S. Virgin Islands (St. Croix, St. Thomas, and St. John) as shown in figure 35. The 

most important sources of ground water in Puerto Rico are 

the limestone aquifers underlying the northern coast and 

the alluvial fan-delta aquifers underlying the southern 

coast. The Kingshill aquifer of St. Croix is the only significant aquifer in the U.S. Virgin Islands. Therefore, the 

Caribbean Islands regional aquifer-system analysis focused 

on these aquifers. 

The Island of Puerto Rico is about 3,300 mi2, with a 

central core of east-west mountain ranges flanked by foothills. The mountain ranges have a maximum altitude of 

about 4,400 ft and separate the island into north and south 

sides. Extensive coastal plains, as much as 8 mi wide, are 

present along the north and south sides of the island. Rainfall ranges from 200 in/yr on the rain forests of the northeast to 20 in/yr on the lowlands of the southwest. Annual 

average precipitation on the island is about 75 in. 

The geology of Puerto Rico is complex and varied. 

The central core of the island consists largely of volcanic 

and intrusive rocks of Cretaceous and early Tertiary age. 

Limestone, minor dolomite, and clastic sediments of Oligocene to Pliocene age were deposited north and south of 

the central mountain core. The clastic sediments consist of 

poorly sorted mixtures of gravel, sand, and fine-grained 

materials. On the northern coastal plain, minor clastic sediments grade upward into thick beds of relatively pure 

Figure 35. Study area of the Caribbean Islands regional aquifer-systems analysis. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 53 

limestone. Ground water moving through joints and fractures in the limestone has formed solution channels and 

cavities. In outcrop areas along the north coast, a mature 

karst topography has developed. Along the southern coast, 

conglomeratic gravel, sand, mudstone, and limestone beds 

of Oligocene to Pliocene age are not an important source 

of water. Calcareous cement has effectively reduced the 

porosity of the clastic strata, and the limestone beds have 

not undergone extensive solutioning. Overlying the Tertiary age beds are deltaic deposits of gravel, sand, and silt 

and alluvium of Pleistocene to Holocene age. These deposits compose the principal aquifers along the south 

coast. Permeability distribution in the aquifers is related to 

the depositional patterns of sand and gravel. 

Rocks composing the northern coastal aquifer system of Puerto Rico were grouped into three hydrogeologic 

units: (1) an upper unconfined aquifer, (2) a confining unit, 

and (3) a lower confined aquifer (Rodriquez-Martinez, 

1991 ). The upper unconfined aquifer consists of the Aymamon Limestone and underlying Los Puertos Formation. 

Locally, along the north coast, the upper unconfined aquifer also includes the uppermost rocks of an unnamed 

upper member of the Cibao Formation. In coastal areas 

where this aquifer is confined by surficial deposits, the aquifer contains saline water. The unconfined freshwater aquifer, is 130 to 240 ft thick in the north-central coastal 

area but becomes thinner east of Manati. East of Dorado, 

both the saline water and freshwater sections decrease in 

thickness. The aquifer pinches out in the vicinity of San 

Juan. If it exists locally farther to the east, it is very thin 

and contains brackish water. Near Isabela, the freshwater 

zone gradually decreases to 25 ft in thickness (fig. 35). 

The confining unit of the northern coastal aquifer 

system consists largely of an unnamed member of the 

Cibao Formation, the Quebrada Arenas and Rio Indio 

Limestone Members of the Cibao Formation, a mudstone 

unit, and the lower part of the Los Puertos Formation. The 

confining unit underlies most of the upper confined aquifer 

west of San Juan. Between Manati and Arecibo, the confining unit locally contains a water-yielding zone under artesian conditions (Rodriquez-Martinez, 1991 ). 

The lower aquifer is the principal artesian aquifer of 

the northern coastal aquifer system and attains its maximum transmissivity in the area between Arecibo and 

Manati. The lower aquifer consists of the Lares Limestone, the Montebello Limestone Member, and the lowermost strata of the unnamed upper member of the Cibao 

Formation. Water in the lower aquifer is potable, and well 

yields range from several hundred to several thousand gallons per minute. However, in San Juan and to the east, the 

quality of water varies from fresh to brackish. 

The southern coastal aquifer system consists of six 

fan-deltas that coalesce to form a narrow but continuous 

alluvial plain that extends eastward for about 45 mi from 

Ponce to Patillas. The alluvial plain is bordered on the north 

by intensively faulted Cretaceous and Tertiary highlands. 

Tributaries in the mountains and foothills transport the 

poorly sorted material that comprises the alluvial deposits. 

Fan-delta deposits form a coastward-thickening sequence of 

bedded cobble, gravel, sand, and silt of Quaternary age. In 

the deep subsurface, these deposits may be as old as Miocene age. Bedrock hills protrude through thinner parts of the 

alluvial plain, especially along the northern margin (Renken 

and others, 1991). 

The east-west mountain range effectively cuts off 

much of the precipitation from the northeast trade winds 

and creates a rainshadow on the leeward (southern) side of 

the island. Annual precipitation is less than 40 in. on the 

southern side and is about 20 in. near the coast and at the 

westernmost part of the alluvial plain. Recharge to the 

southern coastal aquifer system is principally by infrequent 

rainfall and by seepage of streamflow near the fan apexes. 

Ground-water discharge occurs as seepage to the sea and to 

streams, or through evapotranspiration where water levels 

are near land surface. The alluvial aquifers are predominantly unconfined at inland locations and confined near the 

coast. Development of water resources in the eastern coastal 

plain in 1914, where water from surface reservoirs is used to 

irrigate sugarcane fields, considerably altered the groundwater flow system. Recharge to the alluvial aquifers is increased by irrigation return flow of surface water used for 

irrigation. About 30 percent of the applied irrigation water 

was estimated to become recharge. In 1964, reported use of 

irrigation water was 68,000 acre-ft. However,_ the irrigation 

demand has declined since 1964. In 1989 the reported use 

for irrigation was 40,000 acre-ft. Recharge to the aquifers 

was reduced accordingly. New industrial development on 

the east coast may require pumping ground water for different water uses. Less profit from raising sugarcane has resulted in a reduction of surface-water irrigation, and thus 

recharge, and may create new problems for management of 

ground water on the east coast. These problems include 

ground-water level declines and possibly saltwater intrusion 

at major pumping centers (Quinones-Aponte, 1991). 

The Kingshill aquifer underlying St. Croix consists 

of the Kingshill Limestone, the Mannings Bay Member of 

the Kingshill Limestone, and the Blessing Formation. The 

most permeable zone of the Kingshill aquifer is the Mannings Bay Member of the Kingshill Limestone and the 

Blessing Formation. Alluvial deposits of Holocene and 

Pleistocene age are associated with stream channels cut 

into the I<ingshill Limestone. Ground water in the Kingshill aquifer mainly is unconfined. Yields to wells are as 

much as 100 gaVmin but usually are about 30 gal/min. 

Ground-water withdrawals increased from 50,000 gaUd in 

the early 1960's to 800,000 gal/d in the late 1980's. Further increase in ground-water withdrawals may induce saline-water intrusion. 

The Caribbean Islands regional aquifer-system analysis was started in 1984 and completed in 1990. As of 
54 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

May 1991, 22 reports had been completed (Sun and 

Weeks, 1991). 

CENTRAL VALLEY, CALIFORNIA 

The Central Valley of California (fig. 36) is a long, 

narrow structural trough containing an aquifer system composed mostly of alluvial deposits of sand, gravel, silt, and 

clay. The valley occupies about 20,000 mi2 of flatland between the Sierra Nevada range on the east and the Coast 

Ranges on the west. The climate is arid, with precipitation 

totals ranging from 14 to 20 in/yr in the northern part (Sacramento Valley) and from 5 to 14 in/yr in the southern part 

(San Joaquin Valley). To support the high level of agricultural production, a combination of intensive ground-water 

pumpage and large imports of surface water is required. 

· The Central Valley is filled with low-permeability 

marine deposits overlain by more permeable continental 

deposits. The continental deposits (of post-Eocene age) 

contain most of the freshwater and constitute a major aquifer system. The base of the aquifer system is considered 

to be the base of the post-Eocene continental deposits; 

however, this is not strictly true everywhere (Bertoldi and 

others, 1991). Thickness of the aquifer system averages 

about 2,400 ft and increases from north to south (fig. 36). 

The continental deposits consist predominantly of lenses 

of gravel, sand, silt, and clay with fine-grained sediments 

making up about 50 percent of the sequence. An important 

contribution of the Central Valley RASA was mapping the 

texture (proportion of fine-grained to coarse-grained sediments) of the continental deposits by Page (1986). He 

noted that the post-Eocene deposits "constitute a heterogenous mixture in which texture differs over short distances 

and depths from chiefly fine-grained to chiefly coarsegrained sediments and vice versa." 

Prior to the RASA study, investigators generally conceptualized the Sacramento Valley as containing one watertable aquifer, and the San Joaquin Valley as containing a 

two-aquifer system separated by a regional clay confining 

unit. However, a slightly different concept of the aquifer 

system was proposed by Williamson and others ( 1989) 

based on an analysis of ground-water levels, texture of the 

sediments, and ground-water flow simulations made during 

the RASA study. The new conceptual model, which is 

shown in figure 37, assumed that "the entire thickness of the 

continental deposits is one aquifer system that has varying 

verticalleakance and confinement depending on the proportion of fine-grained sediments." This conceptual model was 

used as the basis for a regional computer model of the flow 

system that simulates recharge, irrigation return flow, pumpage from wells, and discharge as evapotranspiration and to 

streams (fig. 37). Williamson and others (1989) state that the 

model "simulates the heterogeneity of the aquifer system by 

( 1) varying the aquifer properties from block to block and 

(2) averaging values to represent the aggregate of the heterogeneity within each block." The model utilized a threedimensional finite-difference code (Trescott, 197 5) modified 

to simulate the effects of land subsidence due to inelastic 

compaction of clay beds. 

Ground-water development began in the Central Valley in about 1880, and pumpage for irrigation sharply increased during the 1940's and 1950's. During the 1960's 

and 1970's, ground-water withdrawals averaged about 11.5 

million acre-ft/yr (10 Bgalld) and provided about 50 percent of the irrigation water supply (Williamson and others, 

1989). The intensive pumpage caused ground-water levels 

to decline 200 to more than 400 ft throughout the western 

San Joaquin Valley from predevelopment time to the late 

1960's. However, the importation of surface water beginning in 1968 replaced much of the former ground-water 

pumpage there. 

The combination of large ground-water pumpage 

and large imports of surface water completely changed the 

regional ground-water flow system and ground-water budget (see fig. 14 and discussion in an earlier section). The 

effects of development are summarized in Williamson and 

others (1989) as follows: 

1. Ground-water flow is now largely from areas recharged by imported surface water toward areas of irrigation pumpage from wells. 

2. Most ground-water discharge is now from pumping wells rather than as evapotranspiration or discharge to 

streams. 

3. An estimated 60 million acre-ft loss of aquifer 

storage has occurred since predevelopment time. 

4. The construction of about 100,000 wells, many 

with long intervals of perforated casing, has increased the 

effective vertical leakage of the aquifer system by about 

an order of magnitude. 

5. In· the western and southern San Joaquin Valley, 

the large decline of hydraulic head caused inelastic compaction of fine-grained sediments, resulting in the largest 

volume of land subsidence in the world. 

More than one-half of the San Joaquin Valley (or 

about 5,200 mi2) has undergone land subsidence of more 

than 1 ft (fig. 22). Subsidence due to ground-water withdrawals in the valley has been extensively studied by the 

USGS since the 1950's. The pioneer work by Joseph F. 

Poland and his colleagues of the USGS established many 

of the principles of the mechanics of land subsidence and 

techniques used for field measurements. A brief discussion 

of the mechanics of land subsidence (due to inelastic compaction of fine-grained beds) is presented in the earlier section entitled "Evaluation of Ground-Water Development 

Effects." The maximum subsidence (about 30 ft) has occurred on the western side of the San Joaquin Valley near 

the town of Mendota (fig. 23). Subsidence in the valley has 

been greatest where pumpage from wells is greatest and 
123° 

40° 

39° 

38° 

37" 

36° 

0 

0 

35° 

PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 

Lake 

Almanor 

120° 

EXPlANATION 

119° 

--2,000 -- Line of equal thickness of continental depositsAlluvial 

boundary 

50 100 MILES 

50 100 KILOMETERS 

lntervall ,OOO feet 

55 

118° 

Figure 36. Thickness of the Central Valley aquifer system (largely post-Eocene continental deposits) (from Bertoldi and others, 1991). 
56 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

A. Aquifer system 

West 

Coast Ranges 

Irrigation return 

'-.E11arlollran1spiration 

River 

EXPLANATION 

~ Freshvvater 

• Saline vvater 

~ Clay lenses 

- Arrovvs indicate general direction 

of vvater movement-Longer 

arrows imply larger flows 

B. Model of the aquifer system 

Recharge 

Layer4 f 

Layer3 

Layer2 

Layer1 

NOTTOSCALE 

East 

Verticalleakance ( Tk) values between layers are calculated by dividing the harmonic mean of the vertical hydraulic 

conductivity of the aquifer materials by the thickness between nodes. Tk layers 2 and 3 may be increased by wells that 

are screened in both layers 2 and 3 and less frequently in layers 3 and 4. 

Discharge from all wells in the block simulated at the node. 

Figure 37. Central Valley aquifer system. A, Concept of a single homogeneous aquifer system. B, Structure of computer flow 

model of the aquifer system (from Williamson and others, 1989). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 57 

where the aquifer system contains thick sections of montmorillonite clay. The importation of surface water into the 

western San Joaquin Valley in the late 1960's has largely 

replaced the ground-water pumpage there. However, during the drought of 1976-77, intensive pumpage from wells 

resumed, accompanied by a sharp decline of water levels 

and a short period of renewed compaction. Land subsidence could resume in the future with increased pumpage 

from wells if water levels decline below previous lows. 

Ground-water quality in the Central Valley is influenced by the quality of water in streams that are a major 

source of recharge (Bertoldi and others, 1991). Generally, 

concentrations of dissolved solids are lower in the northern part (Sacramento Valley) than in the southern part 

(San Joaquin Valley). Concentrations of boron, chloride, 

and nitrate in the ground water are large enough locally to 

be a problem to crops and humans. 

Selenium, which is toxic to humans and animals at 

very low concentrations, occurs naturally in soils and 

ground water on the west side of the San Joaquin Valley. 

In 1983, it was determined that selenium in subsurface tile 

drains in the western part of the valley was having toxic 

effects on water fowl in Kesterson Reservoir located there. 

As a result, a phase II RASA was started to investigate the 

sources, distribution, and mobility of selenium in soils and 

ground water of the San Joaquin Valley. It was determined 

that the present areal distribution of selenium in shallow 

ground water reflects the natural distribution of saline 

soils. However, the distribution of selenium in the subsurface results from the long history of irrigation during 

which soluble forms of selenium have been redistributed 

from the soil into ground water. Further concentration of 

selenium in the shallow ground water has occurred owing 

to evapotranspiration. The rising water table required artificial drainage of ground water containing high concentrations of selenium. Gilliom and others (1989) concluded 

that the large volume of high-selenium ground water 

should be left where it is rather than brought to the surface 

and allowed to move elsewhere. 

The study of the Central Valley aquifer system was 

started in 1978, and Phase I (hydrogeologic framework definition and simulation of ground-water flow and land subsidence) was completed in 1982. Phase II (geochemical 

investigations and evaluation of selenium mobility) was 

completed in 1990, and additional reports are still in preparation or review. As of May 1991, 59 reports had been 

completed. The principal findings of Phase I are presented 

in U.S. Geological Survey Professional Paper 1401, which 

consists of the following chapters: Chapter A (Bertoldi and 

others, 1991) summarizes important aspects of the geologic 

framework, regional ground-water flow, and ground-water 

quality in the Central Valley; Chapter B (Hull, 1984) describes the geochemistry of ground water in the Sacramento 

Valley; Chapter C (Page, 1986) describes the geologic 

framework of the Central Valley; Chapter D (Williamson 

and others, 1989) discusses ground-water hydraulics with 

emphasis on an analysis of regional ground-water flow. 

CENTRAL MIDWEST 

The Central Midwest regional aquifer-system analysis 

was an investigation of three major aquifer systems underlying about 370,000 mi2 in parts of Arkansas, Colorado, Kansas, Missouri, Nebraska, New Mexico, Oklahoma, South 

Dakota, Texas, and Wyoming (fig. 38). The study area extends from the foothills of the Rocky Mountains in Colorado 

to the Mississippi River in eastern Missouri, and from South 

Dakota to the Ouachita, Arbuckle, and Wichita Mountains of 

Arkansas and Oklahoma. The study area is informally divided into the Plains subregion and the Ozark subregion. 

The study is limited to sedimentary rocks ranging from 

Cambrian through Cretaceous age and does not include the 

overlying High Plains aquifer, which was studied by the 

High Plains regional aquifer-system analysis project. 

Many of the boundaries of the study area are areas 

of complex geologic structure, and some are characterized 

by several structural deformations. Large structural features bounding the study area include the Rocky Mountains on the west, the Sioux Ridge on the northeast, and a 

series of uplifts on the south. The Mississippi and Missouri Rivers form the eastern and northeastern boundaries. 

The rivers and their associated alluvial aquifers are regional discharge areas of the aquifer systems (Signor and Imes, 

1989). 

The rocks, except those included in the High Plains 

aquifer, are grouped into three aquifer systems and three 

confining units (fig. 39). The three aquifer systems are (1) 

the Great Plains aquifer system, (2) the Western Interior 

Plains aquifer system, and (3) the Ozark Plateaus aquifer 

system. The Western Interior Plains aquifer system is laterally adjacent to the Ozark Plateaus aquifer system. These 

two aquifer systems consist of equivalent geologic strata; 

however, they have distinctly different regional flow systems and have a common boundary along a regional topographic low that parallels the boundary between the Plains 

and Ozark subregions. Except at the upper part of the 

Great Plains aquifer system, both the Great Plains and the 

Western Interior Plains aquifer systems contain mostly saline water. However, the Ozark Plateaus aquifer system 

contains mostly freshwater. The three confining systems 

were named for the underlying aquifer systems; they are, 

in descending order, ( 1) the Great Plains confining system, 

which separates the High Plains aquifer from the underlying Great Plains aquifer system; (2) the Western Interior 

Plains confining system, which separates the Great Plains 

aquifer system from the underlying Western Interior Plains 

aquifer system; and (3) the basement confining unit, the 

lowermost boundary of the Western Interior Plains and 

Ozark Plateaus aquifer systems (figs. 39 and 40). 
58 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

The Great Plains confining system consists of Upper 

Cretaceous shale and extends throughout the northwestern 

one-half of the study area. It increases from near zero 

thickness along its southern and eastern margins to more 

than 8,000 ft in the basin areas of southeastern Wyoming 

and eastern Colorado. Where the High Plains aquifer is 

absent, the confining system generally is the surficial unit. 

The thick shale of the confining system effectively restricts flow between the High Plains aquifer and the underlying Great Plains aquifer system. The small vertical 

leakage through the confining system is from the High 

Plains aquifer (high head) to the Great Plains aquifer system (low head). The known head difference in western 

Nebraska, southeastern Wyoming, and northern Colorado 

exceeds 1,600 ft. Two extensive but minor aquifers, the 

Greenhorn Limestone and Niobrara Chalk, are in the confining system. 

The Great Plains aquifer system consists of two aquifers: the Maha and Apishapa aquifers. In most places 

40' 

35' 

I --------1 

I 

Sioux 

~~Ridge I o':?-

WYOMING 

~-----------------

I 

I NEBRASKA 

-----.1 __ 

(/) 

c 

~ c 

:J 

0 

~ 

~ u 

0 

a: 

COLORADO 

Plains 

subregion 

-------r-L ___________ _ 

:----------, 

I : 

TEXAS I 

NEW MEXICO 

these two aquifers are separated by the Apishapa confining 

unit. Where the confining unit is missing, the two aquifers 

merge into a single aquifer. The Great Plains aquifer system consists mainly of sandstone, siltstone, and shale, with 

wide variations in texture, degree of fracturing, and thickness. Total thickness of the aquifer system generally is 

from 200 to 800 ft, being greatest in north-central Nebraska (Helgesen and Leonard, 1989). The Great Plains aquifer system includes the Dakota Sandstone and equivalent 

beds that were recognized early as a major artesian reservoir by Darton (1905). The Great Plains aquifer system is 

one of the most extensive aquifer systems in North America and extends over most of the northwestern one-half of 

the study area. Ground water generally flows from west to 

east and recharges in large outcrop areas in southeastern 

Colorado, along the front range of the Rocky Mountains in 

southern Colorado, near the Black Hills in southwestern 

South Dakota, and in east-central Kansas. Ground water 

discharges at outcrops where the Missouri River incises 

WISCONSIN 

--------

IOWA 

ILLINOIS 

: Ozark 

-i subreg~~~--f)i" ~------- 8- 1 Boston rJ -

\ Mountains ~ Q ·S .s,~ 

ARKANSAS .$ ~ 

Wichita OKLAHOMA 

01-'i}l'+- .;~ . ~~ 

50 100 MILES 

I I I 

0 50 100 KILOMETERS 

Figure 38. Study area (shaded) of the Central Midwest regional aquifer-system analysis. 

~ 

MISSISSIPPI 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 59 

the aquifer system in southeastern South Dakota, eastern 

Nebraska, and western Iowa. Simulation of regional flow 

in the Great Plains aquifer system suggests that withdrawals during the 1970's were more than twice the natural recharge rate. (See discussion of the aquifer system in the 

earlier section entitled "Regional Ground-Water Budgets.") 

Concentrations of dissolved solids range from 500 mg/L 

or less in the outcrop areas to more than 100,000 mg/L in 

oil-field brines in the Denver basin. In much of the aquifer 

system, water is brackish (1 ,000 to 10,000 mg/L of dissolved solids). 

The Western Interior Plains confining system consists mostly of ( 1) shale with lesser amounts of limestone 

and sandstone of Pennsylvanian age and (2) shale with 

evaporite deposits and sandstone/limestone of Permian 

age. The confining system exists everywhere in the study 

area except along the easternmost part of the border between South Dakota and Nebraska and in the Ozark subregion. The thickness of the confining system ranges from 

near zero in northeastern Nebraska and along the western 

and southern margins of the Ozark subregion to more than 

24,000 ft in southwestern Oklahoma. Locally, sandstone 

and limestone in the confining system are aquifers. 

The Western Interior Plains aquifer system consists 

of aquifers composed of permeable limestone, dolostone, 

and sandstone of Cambrian to Mississippian age. These 

aquifers are separated by shale or dolostone of low permeability; thus, the aquifer system can be subdivided into an 

upper permeable unit and a lower permeable unit. The aquifer system extends from the Rocky Mountains eastward 

to the western boundary of the Ozark Plateaus aquifer system, but it is not present in large areas of Nebraska and 

Colorado. The aquifer system contains mostly brackish 

water (1,000 to 10,000 mg/L of dissolved solids). A potentiometric surface map based on equivalent freshwater 

heads indicates that ground water flows from outcrop 

areas to the east and southeast. 

The Ozark Plateaus aquifer system consists of rocks 

that are laterally equivalent to those of the Western Interior Plains aquifer system. However, the Ozark Plateaus aquifer system contains freshwater and exists as a separate 

regional ground-water flow system. The extent of the 

Ozark Plateaus aquifer system is approximately coincident 

with the extent of the Ozark Plateaus. Rocks making up 

the Ozark Plateaus aquifer system are grouped into three 

aquifers and two confining units. In descending order, they 

are (1) the Springfield Plateau aquifer, (2) the Ozark confining unit, (3) the Ozark aquifer, (4) the St. Francois confining unit, and (5) the St. Francois aquifer. The Ozark 

Plateaus aquifer system consists mainly of limestone, dolostone, and sandstone of Late Cambrian through Late 

Mississippian age. 

The Springfield Plateau aquifer is the uppermost aquifer of the Ozark Plateaus aquifer system and consists of 

Mississippian-age limestone. The aquifer is used only for 

domestic and stock water supply owing to low well yields. 

A A' 

<E<~----==---::---.,------ Plains subregion ---------~:;-------Ozark subregion --->+ /Zone of complex 

~ 

/ geology. 

Geohydrologic 

I 

relations have not 

been defined with 

certainty 

NOT TO SCALE 

0 

ClCJ) 

<(<( 

C:::CJ) 

oz 

0~ u 

Transition zone 

between flow systems 

EXPLANATION 

__. Direction of major ground-water flow 

A -A' Trace of section shown in figure 40 

Figure 39. Geohydrologic section showing regional aquifer systems and confining systems in the central midwestern United States 

(from Signor and Imes, 1989). 
60 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

The thickness of the aquifer ranges from 100 to 400 ft. 

The aquifer acts as a sink and source for the underlying 

Ozark aquifer. The Ozark confining unit, consisting of the 

Maquoketa Shale of Late Ordovician age and the Early 

Mississippian-age Chouteau Limestone, overlies the Ozark 

aquifer, thus separating the Springfield Plateau aquifer 

from the underlying Ozark aquifer. The thickness of the 

confining unit ranges from near 0 to 1,500 ft. However, in 

most of the study area the thickness is less than 100 ft. 

The Ozark aquifer is the most areally extensive and 

widely used aquifer in the Ozark subregion. The aquifer 

consists of dolostone, limestone, and chert of Late Cambrian 

through Middle Devonian age. The thickness of the Ozark 

aquifer ranges from about 800 to 1,500 ft. Yields of deep 

wells may exceed 1,000 gal/min. Dissolution of the mainly 

carbonate rocks along fractures and bedding planes has produced a substantial karst terrane, with hundreds of springs 

in the upper few hundred feet of the aquifer. Water in the 

400 

35° 

WYOMING 

NEW MEXICO 

I 

' I 

I 

0 50 100 MILES 

I 

I I 

100° 

Boundary of Central Midwest 

Regional Aquifer System Anal sis 

0 50 100 KILOMETERS 

aquifer is generally a calcium bicarbonate type with concentrations of dissolved solids ranging from 200 to 500 mg/L. 

The St. Francois confining unit restricts flow between the Ozark aquifer and the St. Francois aquifer. The 

confining unit has significant vertical permeability and is 

considered to be a leaky unit, consisting of shale, siltstone, 

dolostone, and limestone of Late Cambrian age. The confining unit is exposed at land surface in a narrow, nearly 

circular band about 400 mi2 in area that surrounds a central core of older rocks in the St. Francois Mountains. 

Thickness of the confining unit ranges from near zero to 

730ft. 

The St. Francois aquifer is the lowermost aquifer of 

the Ozark Plateaus aquifer system and consists mainly of 

dolostone, siltstone, and sandstone of Late Cambrian age. 

These permeable rocks overlie the Basement confining 

unit. The aquifer crops out in the St. Francois Mountains 

of southeast Missouri. Beyond the 590 mi2 outcrop area, 

MINN 

----[IIIll]] 

~ 

IOWA~ 

D 

go• 

EXPLANATION 

Approximate extent of 

regional aquifer 

systems 

Great Plains aquifer system 

Western Interior Plains 

aquifer system 

Ozark Plateaus aquifer 

system 

Aquifer system absent 

Approximate boundary 

between Plains and 

Ozark subregions 

A -A, Trace of section-Section shown 

in figure 39 

ILLINOIS 

Figure 40. Approximate extent of the Great Plains, Western Interior Plains, and Ozark Plateaus aquifer systems (from Signor 

and Imes, 1989). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 61 

the aquifer dips into the subsurface and is buried beneath 

younger, more permeable rocks. The depth of burial generally is between 800 and 1,400 ft in the central and western 

part of the Ozark subregion and is deeper elsewhere. The 

aquifer is more than 300 ft thick in northern Arkansas; 

however, in western and southwestern parts of the Ozark 

subregion, the aquifer is less than 300 ft thick and thins 

toward the west. 

The St. Francois aquifer is primarily used in and 

near its outcrop area. Wells 200 to 300 ft deep may yield 

less than 10 gaVmin; however, some wells are reported to 

yield 50 gaVmin. Water in the aquifer is a calcium magnesium bicarbonate type with concentrations of dissolved 

solids ranging from 200 to 400 mg/L. 

Underlying the Ozark Plateaus and Western Interior 

Plains aquifer systems is the basement confining unit, consisting mostly of crystalline rocks of Precambrian age. 

These rocks may be fractured and yield water to wells locally, as in east-central Colorado, southeastern South Dakota, and southeastern Missouri, although regionally they 

are assumed to have low permeability and to form the 

base of the aquifer systems in both the Plains and Ozark 

subregions. The top of the basement confining unit ranges 

from more than 1 ,000 ft above sea level in southeastern 

Missouri to about · 34,000 ft below sea level in central 

Oklahoma. 

The Central Midwest regional aquifer-system analysis was started in 1980 and completed in 1989. As of May 

1991, 63 reports had been completed (Sun and Weeks, 

1991). As of mid-1993, chapters B, D, and E of Professional Paper 1414 were approved for publication and 

chapters A and C are in review. 

COLUMBIA PLATEAU, WASIDNGTON, OREGON, AND IDAHO 

The Columbia Plateau is in central and eastern 

Washington, northern Oregon, and a small part of Idaho. 

The plateau covers about 63,200 mi2 entirely within the 

drainage of the Columbia River and is bordered on the 

west by the Cascade Range, on the north and east by the 

Rocky Mountains, and on the south by the Blue Mountains (fig. 41). Extensive hydrologic evaluation and flow 

simulation was done in about 32,700 mi2. The study area 

is underlain everywhere by massive basalt flows with an 

estimated maximum thickness of 16,000 ft in a structural 

low near the center of the plateau. Basalt flows overlie 

sedimentary, igneous, and metamorphic rocks with low 

permeability. This prebasalt rock is considered the lowermost confining unit of the Columbia Plateau regional aquifer system. Over a very large area of the plateau, the 

basalt is overlain by sedimentary rocks of Holocene 

through Miocene age. These sedimentary rocks are the 

major sources of ground water for all uses in the study 

area. During this study, all sedimentary rocks overlying 

the basalt with a thickness of 50 ft or more were collectively named the "overburden aquifer." 

Rocks that comprise the Columbia River Basalt 

Group were emplaced by extrusion of numerous individual 

lava flows through fissures. Individual flows range in thickness from a few inches to more than 300 ft and average 

about 110 ft. The structure of a lava flow generally consists 

of three units (fig. 42): flow top, entablature, and colonnade (Swanson and Wright, 1978). The flow tops also include the basal part (flow bottom) of the overlying flow 

that consists of pillow-palagonite complexes. Groundwater flow is predominantly horizontal within interflow 

zones (the tops of one flow and bottoms of the overlying 

flow), which average about 5 to 30 ft in thickness. In general, the entablature and colonnade are dense with vertical 

fractures. 

Ground-water flow through fractures in the entablature and colonnade is predominantly vertical. Locally, the 

basalt flows are interbedded with sediments that function 

either as confining units or aquifers. The interbeds vary 

from shalely material to sand and gravel, with fine-grained 

materials predominating. Generally, more interbeds are 

present in the younger basalt than in the older basalt. 

The Columbia River Basalt Group has been subdivided into five formations and numerous members. The five 

formations, from oldest to youngest, are the Imnaha Basalt, 

Picture Gorge Basalt, Grande Ronde Basalt, Wanapum Basalt, and the Saddle Mountains Basalt (Swanson and others, 

1979). The Imnaha Basalt is present in northeastern Oregon 

and adjacent Washington and Idaho, where it is exposed 

chiefly in valleys of the Imnaha, Snake, Salmon, and Clearwater Rivers. The Picture Gorge Basalt is exposed only in 

north-central Oregon, mostly within the John Day basin 

south of the Blue Mountains uplift, where it is interlayered 

locally with Grande Ronde Basalt. Because of small areal 

extent and occurrence at edges of the plateau, the Imnaha 

and Picture Gorge Basalts are not considered important aquifers; therefore, they were included with the Grande Ronde 

Basalt. On the basis of these assumptions, the Columbia 

River Basalt Group was subdivided into three basalt aquifers: in descending order, the Saddle Mountains, Wanapum, 

and Grande Ronde Basalt units (Drost and Whiteman, 

1986). These basalt aquifers are separated by two confining 

units, the Saddle Mountains-Wanapum and WanapumGrande Ronde interbed confining units. 

The interbeds are areally extensive but thin compared with the basalt formations. The interbeds locally 

function as aquifers, but regionally they act as confining 

units. The Saddle Mountains-Wanapum interbed confining 

unit consists mainly of clay, silt, claystone, or siltstone. 

Thickness of the confining unit averages about 50 ft but 

may be nearly 0 to 200 ft. The Wanapum-Grande Ronde 

interbed confining unit is most extensive and thickest in 

the northern part of the plateau, where it varies from nearly 

0 to 100 ft in thickness and averages about 25 ft. It consists 
62 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

chiefly of claystone and siltstone, with minor beds of sand, 

gravel, and sandstone. The Wanapum-Grande Ronde thins 

to the south. Where absent, a thin saprolite commonly 

marks the interval. 

Model-derived hydraulic conductivity for the overburden aquifer is from Sxl0-4 to 1.3xlo-2 ft/s with a meI 

..... 

I 

-j 

1 

l 

l. 

c \ '-i 

0 

I 

0 

20 

I 

I 

40 

40 

I 

' ' I 

80 

dian of 5x10-4 ft/s. Hydraulic conductivity of basalt is 

from lxl0-6 to lxl0-4 ft/s, with a median of 1.6xlo-5 ft/s 

for the Saddle Mountains Basalt unit and 3x10-5 ft/s for 

the Wanapum Basalt unit. The Grande Ronde Basalt unit 

was simulated as two zones. Model-simulated median hydraulic conductivity is 1.6x10-5 ft/s for the upper Grande 

'\, ,, 

l 

60 

I 

\ -{J 

~~ l 

I 

t' ' 

0 

80 MILES 

I 

120 KILOMETERS 

l 

J \. 

l \ 

;: 

_o~-­

c 

z 

VALLI~\' 

INDEX MAP OF STUDY AREA 

Figure 41. Study area of the Columbia Plateau regional aquifer-system analysis. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 63 

Ronde basalt unit and 1.4xl0-5 ft/s for the lower Grande 

Ronde Basalt unit. Median vertical hydraulic conductivity 

of the overburden aquifer is about 2xl0-5 ft/s and for the 

basalt units is 1.5xl0-8 ft/s. 

Water in the overburden aquifer flows toward discharge points at surface-water bodies. Water in the overburden aquifer is in direct hydraulic connection with water 

in the immediately underlying basalt unit. The potentiometric surface for the Saddle Mountains Basalt unit indicates that water in the basalt flows primarily toward major 

streams. This flow pattern also exists for the Wanapum 

and Grande Ronde Basalt units where not overlain by 

younger basalts. If they are deeply buried, as in the southcentral part of the plateau, the potentiometric surfaces are 

less influenced by surface-drainage features and consequently have a smooth configuration. Regional discharge 

is toward the Columbia and Snake Rivers. 

Recharge to the aquifers primarily is from precipitation on the plateau. However, this natural condition was 

disturbed by irrigation with surface water imported to the 

plateau under the Columbia Basin Irrigation Project 

(CBIP), starting in the early 1950's. Since then, part of the 

recharge is supplied by excess irrigation water (irrigation 

return flow). 

Ground water was developed mostly after World 

War II, increased rapidly from 1965 to 1979, but has decreased somewhat since then. For example, pumpage was 

about 56,000 acre-ft in 1945, it was 78,000 acre-ft in 

1960, then increased to about 940,000 acre-ft in 1979, and 

decreased to 813,000 acre-ft in 1983. The combination of 

imported surface water and withdrawals of ground water 

for irrigation, and municipal and industrial uses, caused 

ground-water levels in the plateau to rise in some places 

and decline in other places. In the CBIP area, water levels 

rose as much as 300 ft and averaged about 12 ft owing to 

SERIES OF 

LAVA FLOWS 

Broken basalt 

~~~~ Dense flow center 

with vertical joints 

lnterbed, flow top, 

flow bottom 

Dense flow center 

with vertical joints 

Dense flow center 

with vertical joints 

the importation of surface water for irrigation. Water-level 

declines in the Wanapum Basalt units were as much as 

400 ft and averaged about 60 ft due to pumping of ground 

water. Irrigation with imported surface water increased recharge about 50 percent; ground-water discharge increased 

accordingly. During the 1980's, only about 15 percent of 

the pumpage was supplied from aquifer storage. 

Water in the basalt units generally is of good quality 

and suitable for most uses. However, in areas of surfacewater irrigation and where the overburden is less than 200 

ft thick, water from shallow wells drilled into the Saddle 

Mountains Basalt unit may have increased concentrations of 

dissolved solids due to induced recharge from irrigation 

return flow that has high concentrations of dissolved solids. 

Nitrogen concentrations (nitrate plus nitrite as N) of 2 mg/ 

L or more occur in water from the Saddle Mountains Basalt 

unit in surface-water-irrigated areas where the overburden 

is thin or absent. However, nitrogen concentrations are less 

than 2 mg!L in water from wells deeper than 500 ft. 

The Columbia Plateau regional aquifer-system analysis was started in 1982 and completed in 1989. As of 

mid-1992, 24 reports had been completed. Chapter B of 

Professional Paper 1413 has been approved for publication 

and three other chapters are in preparation. 

GREAT BASIN, NEVADA AND UTAH 

The Great Basin regional aquifer-system analysis 

covers an area of about 140,000 mi2 in parts of Nevada, 

Utah, and adjacent states (fig. 43). The area is characterized by generally north-trending mountain ranges 5 to 15 

mi in width, which rise 1,000 to 5,000 ft above adjoining 

valleys. The width of valleys is about the same as the 

width of the adjacent mountain ranges. The valleys are 

STRUCTURE OF 

INDIVIDUAL LAVA FLOW 

locky joints 

Columns 

-~~·j- Piaty joints 

____ fu,.'.:~~c""'-",._vesicular 

base 

Figure 42. Basalt interflow structures (from Swanson and Wright, 1978). 
64 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

35' 

120' 115' 

OREGON 

CALIFORNIA 

50 

50 100 KilOMETERS 

EXPLANATION 

Study area 

Basin flow model by this study-Number 

indicates basin study area listed below 

1 Paradise Valley 

2 Dixie Valley 

3 Smith Creek Valley 

4 Stagecoach Valley 

5 Tule Valley 

6 Milford Area 

Basin flow model by cooperative study 

(Results used by this study)-Number 

indicates basin study area listed below 

7 Carson Valley 

8 Las Vegas Valley 

IDAHO 

Regional models 

Carbonate rock province 

Fish Springs flow system 

White River flow system 

Hydrographic area boundary 

Evapotranspiration site 

.._ __ 

UTAH 

Figure 43. Study area and types of ground-water flow models, Great Basin regional aquifer system (from Harrill, 1986). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 65 

typically elongated, and many extend more than 50 mi in a 

north or northeast direction. The area has a complex geologic history that includes major episodes of sedimentation, 

igneous activity, orogenic deformation, and continental 

rifting. A major tectonic change occurred about 17 million 

years ago with the onset of extensional faulting that 

formed the basin and range topography. The Great Basin 

regional aquifer-system analysis study has grouped 242 hydrographic areas that share many common geologic and 

hydrologic characteristics. Most of the grouped hydrographic areas include one or more structural basins and 

associated basin-fill aquifers. A special situation is found in 

eastern Nevada and western Utah, where permeable carbonate rocks underlie the basin-fill deposits and form a 

complex ground-water flow system that reflects the characteristics of both basin-fill and carbonate aquifers. Selected 

basins were studied and simulated with ground-water flow 

models (fig. 43). · 

The study included investigation of recharge and 

evapotranspiration processes at selected sites. Rechargeprocess studies involved a mass balance of chloride between precipitation and ground water (Dettinger, 1989). 

Evapotranspiration studies used the Bowen-ratio technique 

and eddy correlation measurement (Carman, in press). 

Eight basins were selected for computer-based model simulations, which collectively represent most conditions 

present in the study area. Three regional flow models were 

also made: One simulates flow in the carbonate rock province between the Great Salt Lake Desert and Death Valley, 

and the others are smaller scale models of the Fish Springs 

flow system and the White River flow system (fig. 43). 

The purpose of the regional flow models is to test the validity of the conceptualized flow systems. All models were 

run for steady-state conditions. The carbonate rock province simulation indicates that flow in the carbonate rocks 

can be subdivided into five deep-flow regions. These regions, named after the terminal sinks of the regional flows, 

are (1) Death Valley, (2) Colorado River, (3) Bonneville, 

(4) Railroad Valley, and (5) Upper Humboldt River. Superimposed over the deep-flow regions are 17 shallow-flow 

regions that approximately coincide with flow systems delineated from topography, water-level data, and discharge 

areas. Flow in the carbonate rocks generally is from south 

to north toward the Great Salt Lake Desert (Bonneville region) and the Humboldt River in the eastern and northern 

parts of the province; elsewhere, flows are from north to 

south toward either Death Valley or the Virgin and Colorado Rivers. Recharge is principally precipitation on mountain ranges. About 3 percent of precipitation recharges the 

carbonate rocks. Discharge is mainly evapotranspiration 

and flow to springs and terminal sinks, particularly in the 

Colorado River region (Prudic and others, in press). 

The 242 hydrographic areas were grouped into 39 

major flow systems (fig. 44). Of these, 14 are single-basin 

flow systems, and the remainder are multibasin flow systerns. Large multibasin flow systems outside the carbonate 

rock province are generally coincident with major drainage systems. Large multibasin flow systems within the 

carbonate rock province typically have little surface flow. 

Ground water from multibasins in the carbonate rock 

province, where there is little surface water, discharges to 

large springs and is consumed by evapotranspiration in the 

vicinity of the springs. 

The Great Basin regional aquifer-system analysis 

was started in 1980 and completed in 1988. As of late 

1992, 48 reports had been completed. One chapter of Professional Paper 1409 has been printed, four are in press, 

and others are in review or in preparation. 

GULF COASTAL PLAIN 

The Gulf Coastal Plain regional aquifer-systems 

analysis covers about 230,000 mi2 onshore in parts of Alabama, Arkansas, Florida, Illinois, Kentucky, Mississippi, 

Missouri, Tennessee, Texas, and all of Louisiana (fig. 45). 

The study also includes 60,000 mi2 offshore between the 

coast and the edge of the Continental Shelf. The study is 

limited to the coastal plain sediments of Tertiary and 

younger age except for an area in the Mississippi embayment, where Upper Cretaceous sediments supply water in 

parts of several states. The bottom of the Gulf Coastal 

Plain aquifer systems is specified at either the top of the 

Midway Group of Paleocene age, which consists of lowpermeability sediments (mostly marine clays), or at the top 

of the zone of geopressure. The sediments are thin in and 

near outcrop areas but thicken to several thousand feet 

gulfward in downdip areas. None of the individual aquifers are continuous throughout the study area; some underlie only a few hundred square miles. Some of the aquifers 

in sediments of Eocene age are present in 8 of the 10 

States studied and supply large quantities of freshwater for 

all uses. The unconsolidated to semiconsolidated sediments are complexly interbedded sequences of sand, silt, 

and clay with minor beds of lignite, gravel, and limestone. 

Salt-dome basins occur across the area from southem Texas to southwestern Alabama. The domes penetrate 

most or all Tertiary strata at any given location; however, 

only a few are at or near land surface. Salt domes penetrating aquifers and permeable zones in the Gulf Coastal 

Plain have been mapped by Beckman and Williamson 

(1990). Faulting associated with the domes is localized, 

and the domes typically are 1 to 3 mi in diameter. The 

effects of salt domes on regional freshwater flow are localized. However, the dissolution of salt in the deep part of 

the salt basins may affect ground-water flow in sediments 

that contain highly mineralized water. Regional fault zones 

that parallel the coastline seem to have less influence on 

regional ground-water flow; however, they may have significant impact locally. 
66 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

120' 

40' 

CALIFORNIA 

50 

35' 

50 100 MILES 

100 KILOMETERS 

115' 

' ' 

EXPLANATION 

.. Majorlake 

Area of major ground-water discharge 

as evapotranspiration 

Study area 

Major flow system boundary-Dashed 

where uncertain 

.__ __ 

UTAH 

ARIZONA 

Figure 44. Major flow systems and major ground-water discharge areas of the Great Basin regional aquifer system (from 

Harrill, 1986). 
25. 

PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 

---1 

I 

I 

TEXAS 

95. 

I 

I 

MISSOURI 

--- l 

OKLAHOMA 

\-------

of Mexico 

Gulf 

50 100 MILES 

0 50 100 KILOMETERS 

EXPLANATION 

Midway confining unit 

Texas Coastal Uplands aquifer 

system 

Mississippi Embayment aquifer 

system 

Vicksburg-Jackson confining unit 

Coastal Lowlands aquifer system 

Study area 

KENTUCKY 

TENNESSEE 

\ 

\ 

Figure 45. Study area and outcrops of regional aquifer systems and confining units, Gulf Coastal Plain regional aquifersystems analysis (modified from Mesko and others, 1990). 

67 
68 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

On the basis of differences in regional ground-water 

flow patterns and sediment characteristics, three aquifer systems were delineated during this RASA study: (1) the Mississippi embayment aquifer system, (2) the Texas coastal 

uplands aquifer system, and (3) the coastal lowlands aquifer 

system (fig. 45). These aquifer systems were further divided 

vertically into six regional aquifers, five permeable zones, 

and six confining units (fig. 46). The aquifers range in areal 

extent from about 32,000 mi2 (the Mississippi River Valley 

alluvial aquifer) to about 170,000 mi2 (the middle Wilcox 

aquifer). The average thickness of the Mississippi River 

Valley alluvial aquifer is least (140ft), and permeable zone 

C (lower Pliocene-upper Miocene deposits) has the greatest 

average thickness (2,000 ft). Average percentage of sand 

ranges from 34 percent in the middle Wilcox aquifer to 70 

percent in the Mississippi River Valley alluvial aquifer. 

Water in the aquifer systems is fresh in and near outcrop areas. Except for a few areas in southern Texas, concentrations of dissolved solids are less than 2,000 mg/L in 

outcrop areas. Elsewhere the concentrations of dissolved 

solids of water in outcrop areas are generally less than 

1,000 mg!L. Concentrations of dissolved solids generally 

increase toward the Gulf of Mexico and toward principal 

discharge areas. Increase in concentrations from 10,000 to 

35,000 mg/L typically occur over distances of 10 to 40 mi. 

Aquifers of the Mississippi embayment aquifer system 

contain more freshwater than aquifers of the Texas coastal 

uplands aquifer system primarily because the embayment 

Mississippi Embayment 

aquifer system 

Texas coastal uplands 

aquifer system 

Coastal lowlands 

aquifer system 

Geologic unit Geologic unit 

System Series Group 

Geohydrologic units Geohydrologic units 

System Series 

Geohydrologic units 

Group 

e:- (I) 

c (I) 

co (I) c c U-c (I) Mississippi River Valley B OcU Not present t)co..Q alluvial aquifer co ·- 0 ::::l 

0 £ ::c 

(I) 

(I) c c ~ C"'C (I) <Dcu ]-g~ Vicksburg-Jackson Vicksburg-Jackson 

gcog, uco~ confining unit1 confining unit1 

w :.= co (..) 

0 .., > 

(I) 

c 

(I) 

(..) 

e:- 0 

0 co 

c ::c Permeable zone A 

B f---(I) 

c co (I) ::::l (..) 

0 0 

tl 

·a; 

a: 

(I) Permeable zone B c 

(I) 

(..) 

Upper Claiborne aquifer Upper Claiborne aquifer .Q 

a: 

Middle Claiborne Middle Claiborne 

(I) 

confining unit confining unit 

Permeable zone C 

c 

0 

..c Middle Claiborne aquifer Middle Claiborne aquifer 'Cii 

>- (I) u ;o c Lower Claiborne Lower Claiborne (I) 't (..) confining unit confining unit (I) 0 1- w 

ZoneD (I) confining unit 

e:- c 

(I) 

co (..) 

t 0 Permeable zone D (I) ~ 1-

Lower Claiborne- Lower Claiborne- Zone E 

r---- upper Wilcox aquifer upper Wilcox aquifer confining unit 

X Middle Wilcox aquifer Middle Wilcox aquifer 0 

~ 

~ Lower Wilcox aquifer Not present f---

(I) 

c >- (I) co (..) 

~ Midway confining unit1 Midway confining unit 0 1 (I) "'C 

Permeable zone E (I) 

(I) c c ~ C-c (I) 0 ::::l CI)"'C..C ~cg ~CCI) 

0 co .2l uco~ 

co (..) Vicksburg-Jackson w 6 .., > confining unit1 

Cii 

a.. ~ 

1 The confining units are defined as the massive clay section (with interbedded sands) of the Midway Group and the undifferentiated Jackson and Vicksburg Groups 

that are recognizable on geophysical logs. The recognizable lithologic unit may not be equivalent to the geologic unit as determined by fossils or other means of correlation and dating, because the upper and lower part of either the Midway Group or the undifferentiated Jackson and Vicksburg Groups may be sandy and therefore 

included in the adjacent aquifer or permeable zone. The Midway confining unit was referred to as the Coastal Uplands confining system and the Vicksburg-Jackson 

confining unit was referred to as the Coastal Lowlands confining system by Grubb (1984, p. 11). 

Figure 46. Geohydrologic and geologic units of the Gulf Coastal Plain aquifer systems (from Grubb, 1987). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 69 

aquifers have a greater areal extent updip of the Vicksburg-Jackson confining unit, and perhaps secondarily because there is less rainfall in the western part of the study 

area than in the eastern part. Permeable zones of the coastal lowlands aquifer system in southern Texas are only a 

few hundred square miles in areal extent where concentrations of dissolved solids are less than 1,000 mg/L. 

In 1985, about 9,000 Mgal/d of water was pumped 

from the Gulf Coastal Plain aquifer systems. About 75 percent was for irrigation, 15 percent was for municipal supply, 

and 10 percent was for industrial uses. 

The base of the Mississippi embayment aquifer system 

is the Midway confining unit except along a narrow band, as 

much as 20 mi wide, across southern Louisiana, where the 

top of the geopressure zone is above the top of the sediments 

comprising the confining unit. The confining unit has a maximum thickness of about 3,500 ft in Louisiana and an average 

thickness of about 750 to 850ft. Extensive and massive sand 

beds, especially in the updip freshwater part, are characteristic of the Mississippi embayment aquifer system. Some aquifers are separated by areally extensive confining units; others 

are not separated by areally extensive confining units but are 

defined on the basis of differences in lithology. The resistance to vertical flow among the aquifers is provided by 

interbedded fine-grained sediments (Grubb, 1987). 

The Texas coastal uplands aquifer system consists of 

sediments that are laterally equivalent to those of the Mississippi embayment aquifer system, except that the sediments comprising the Mississippi River Valley alluvial 

aquifer and the lower Wilcox aquifer are missing from the 

Texas coastal uplands aquifer system (Grubb, 1987). The 

other aquifers of the Mississippi embayment aquifer system are also present in the Texas coastal uplands aquifer 

system and are called by the same names (fig. 46). 

The base of the coastal lowlands aquifer system is 

the Vicksburg-Jackson confining unit, except in the Continental Shelf area and a narrow band onshore along the 

coast of Texas and Louisiana where the top of the geopressure zone is above the top of the sediments comprising the 

Vicksburg-Jackson confining unit. The confining unit has a 

maximum thickness of about 6,000 ft in eastern Louisiana 

and an average thickness of about 500 ft. Five permeable 

zones and two confining units were grouped within the 

coastal lowlands aquifer system. Because of the complexity of the interbedded sediments and the absence of areally 

extensive marker beds that have hydrologic significance, 

the permeable zones were delineated on the basis of differences in hydraulic heads between permeable zones (Grubb, 

1987). No names were given to the permeable zones, 

which are labeled by letters from A through E for purposes 

of discussion (fig. 46). 

Regional predevelopment flow in the Mississippi embayment aquifer system was from high-water-level areas 

along the eastern side of the study area to the Mississippi 

Alluvial Plain in Missouri, Arkansas, northwestern Mississippi, and northeastern Louisiana. Some flow paths may be 

as long as 200 to 300 mi, extending from east-central Mississippi southward from aquifer outcrop areas beneath the 

Vicksburg-Jackson confining unit, and then northwestward 

toward large discharge areas in northeastern Louisiana (fig. 

45). Regional flow in the Texas coastal uplands aquifer system was from high-water-level areas between the major 

rivers to narrow discharge areas in the vicinity of the rivers. 

Flow paths in this aquifer system are only tens of miles in 

length. The circulation of freshwater gulfward from the outcrop of the Vicksburg-Jackson confining unit occurred in 

only a small part of the Rio Grande embayment. Regional 

flow in the coastal lowlands aquifer system generally was 

from high-water-level areas between the major rivers and 

adjacent to the outcrop band of the Vicksburg-Jackson confining units to the major rivers. In addition, flow was to 

regional discharge areas that are parallel to the shoreline 

and centered about midway between the recharge areas and 

the shoreline. Discharge of freshwater to the Gulf of Mexico probably occurred only along the coast of Mississippi, 

Alabama, and Florida, where areas of high water levels 

were within a short distance of the shoreline. The extensive 

flat topography with little relief throughout many coastal 

areas in Louisiana and Texas offered little opportunity to 

develop local flow systems (Grubb, 1987). 

Ground-water pumpage for cities or for agriculture 

since the late 1800's has modified the flow system in places. 

For example, most of the onshore predevelopment discharge 

areas have become net recharge areas. As discussed in the 

earlier section on regional ground-water budgets, most of 

the current pumpage is supplied by increased percolation to 

the regional flow system. Due to the abundance of water in 

streams, lakes, and swamps, and their hydraulic connection 

with water-table aquifers, the potential for induced recharge 

to the underlying aquifers is substantial. Therefore, the potential for future development of ground water from the 

three Gulf Coastal Plain aquifer systems is good. Areas with 

potential for further development of ground-water supplies 

have been delineated by Ackerman (1990), Ryder and Ardis 

(1991), and Martin and Whiteman (1991). 

The Gulf Coastal Plain regional aquifer-systems 

analysis study was started in 1980 and completed in 1991. 

As of late 1992, 68 reports had been completed. Two 

chapters of Professional Paper 1416 have been printed, six 

are in press, and three others are either in review or in 

preparation. 

NORTHEAST GLACIAL AQUIFERS 

The Northeast Glacial Aquifers study differs from 

most RASA studies. It involves many sand-and-gravel aquifers deposited by meltwater in the glaciated Northeast 
70 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

that do not constitute a single, continuous aquifer system. 

Almost all glacial aquifers are in valleys bounded by lowpermeability bedrock, thus limiting aquifer width to a few 

thousand feet or less and limiting aquifer length to several 

thousand feet or less. These valley aquifers are independent hydrologic entities; they are related only in a regional 

sense in that they are widely distributed across the glaciated Northeast and their hydrogeologic characteristics are 

similar owing to common depositional conditions. 

The study area includes most of the glaciated Northeast, extending approximately as far west as the edge of 

the glaciated Appalachian Plateau in Ohio. Long Island, 

New York, and Cape Cod, Massachusetts, are excluded 

from the study because the ground-water hydrology in 

these two areas has been studied extensively (fig. 47). The 

study area ranges from mountainous areas, such as the 

CANADA 

45' 

White Mountains of New Hampshire and Maine, the 

Green Mountains of Vermont, and the Adirondack and 

Catskill Mountains of New York, to low-lying areas along 

the Great Lakes, the St. Lawrence River valley, the Hudson and Mohawk River valleys; and the lowlands along 

the Atlantic Coast. 

The study area has been divided into five hydrogeologic terranes for grouping and discussing aquifers that have 

similar characteristics (Randall and Johnson, 1988). These 

divisions or terranes, shown in figure 48, are based on geology of the glacial deposits and physiography. Most of the 

study area and most of the productive glacial aquifers are in 

type A terranes (fig. 48), where aquifers were formed largely 

in valleys that generally sloped or drained away from the ice 

sheets. The stratified deposits were largely laid down in 

lakes of modest extent. Coarse-grained sediment deposited 

75' 

Atlantic 

Ocean 

0 50 100 150 200 MILES 

0 50 100 150 200 KILOMETERS 

EXPLANATION 

IIIII Study area 

Figure 47. Study area and physiographic divisions of the Northeast Glacial Aquifers regional aquifer-system analysis. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 71 

near the ice margin commonly prograded southward as deltas across fine-grained sediments on the bottom of lakes. As 

the ice continued to melt, coarse-grained sediments that had 

been deposited on or against it collapsed and were overlapped by fine-grained lake-bottom sediments that settled 

later as the process was repeated farther north. 

Glacial aquifers in type B terranes were formed in 

broad lowland areas inundated by large Jakes or seawater 

as ice retreated. Some of the early ice-contact deposits are 

productive aquifers, but they are widely scattered. The 

fine-grained sediments deposited later are extensive. Where 

coarse sand caps the fine sediments, the sand commonly is 

45' 

40' 

80' 

111111 

Ill 

11.11 

.. 

.. 

~Hi f:\\\1 

l~f l~f~ 

75' 

CANADA 

Atlantic 

Ocean 

0 50 100 150 200 MILES 

0 50 100 150 200 KILOMETERS 

EXPLANATION 

Most valleys sloped or drained away from ice sheet 

Broad lowlands formerly inundated by large lakes or marine water 

Valleys drained toward the ice sheet; valley fill is chiefly fine-grained 

sediments and till 

Valleys drained toward the ice sheet and contain fine-grained 

sediments, till, and extensive tributary deltas 

Extensive stratified sands bury preglacial topography 

Not recently glaciated; valleys drained away from the 

ice sheet and contain only sandy outwash 

Not recently glaciated; valleys drained toward the ice 

sheet and contain chiefly fine-grained sediment 

Figure 48. Hydrophysiographic terranes in glacial drift of the Northeastern United States (from Randall and Johnson, 1988). 
72 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

thin or so deeply incised by postglacial streams that its 

saturated thickness is small. Coarse-grained stratified drift 

is sparse in type B terranes. 

Glacial aquifers in type C terranes are characterized 

by valleys that drained toward ice sheets. Glacial drift 

commonly is several hundred feet thick and consists largely of lake-bottom silt and clay that are interlayered in 

places with till composed of reworked lacustrine sediments. Coarse-grained sediments, a minor fraction of the 

valley fill, may have originated in two ways: (1) as alluvium or possibly deltaic deposits of north-draining streams 

deposited during intervals between ice readvances, or (2) 

as local outwash deposited by meltwater at the base of an 

ice tongue during advance or retreat of ice. 

In a few areas of low relief, chiefly in southeastern 

New England, broad outwash plains and ice-contact deposits (largely sand) bury the preglacial topography, not 

only filling the valleys but covering most of the ridges. 

These areas are grouped as typeD terranes in figure 48. 

Beyond the southern limit of the last major ice sheet, 

highly permeable, sandy outwash was deposited in numerous valleys that carried meltwater away from the ice. At the 

same time, fine-grained sediments were deposited in lakes 

ponded by ice in a few north-draining valleys. These areas 

are grouped as type E terranes in figure 48. 

Knowing the aquifer geometry and the distribution 

of coarse-grained sediments is important in locating optimum well sites for ground-water development. Several geophysical techniques were evaluated during this study. 

Continuous seismic-reflection techniques, including marine 

seismic reflection, were tested. These surface geophysical 

techniques proved very useful for determining the thickness and lithology of stratified drift in the glaciated Northeast (Haeni, in press). 

Previous workers suggested that most recharge to 

glacial aquifers in the Northeast is from precipitation on 

the valley floor. However, the findings of this RASA 

study suggest that over half of the recharge under natural 

conditions is from upland runoff, infiltration of streamflow, unchanneled runoff, and (or) lateral ground-water 

discharge from the adjacent uplands (Morrissey and others, 1988). Ground water in glaciated valleys mainly discharges to streams. The main streams that follow the 

axes of the larger valleys gain from ground water along 

nearly their entire length. However, ground-water development in the glaciated Northeast can completely disturb 

this natural hydrologic system. Wells drilled near large 

streams can intercept large quantities of water that otherwise would become streamflow. Near small streams, 

pumping from nearby large well fields may reduce 

streamflow significantly. For these reasons, the Northeast 

Glacial Aquifers RASA study used isotopes and temperature measurements to evaluate and observe the interaction between streamflow and water in glacial aquifers 

(Dysart, 1988; Lapham, 1989). 

The Northeast Glacial Aquifers regional aquifersystems analysis was started in 1981 and completed in 

1989. As of May 1991, 27 reports had been completed 

(Sun and Weeks, 1991). The scheduled chapters of Professional Paper 1415 are either in review or in preparation, 

except for chapter C (Haeni, in press), which has been approved for publication. 

NORTHERN MIDWEST 

The Northern Midwest regional aquifer-system analysis was an investigation of the Cambrian-Ordovician aquifer 

system underlying 161,000 mi2 in parts of Illinois, Indiana, 

Iowa, Minnesota, Missouri, and Wisconsin (fig. 49). The 

aquifer system consists of Cambrian and Ordovician rock 

(mainly sandstone and dolomite). The aquifer system is a 

primary source of water throughout most of its area of occurrence, except for Indiana, central and southern Illinois, and 

western Iowa, where the aquifer system contains saline water. 

Rocks of the aquifer system were deposited in shallow 

seas that encroached on the Precambrian rock surface, which 

slopes generally southward from Minnesota and Wisconsin 

but eastward from eastern Wisconsin. Structural basins are 

located in southwestern Iowa, south-central illinois, and 

Michigan. The rock strata of the aquifer system crop out in 

an arc around the Wisconsin arch and thicken toward the 

structural basins, where they are deeply buried by younger 

sedimentary rocks. The aquifer system contains three distinct aquifers-the St. Peter-Prairie du Chien-Jordan aquifer, the Ironton-Galesville aquifer, and the Mount Simon 

aquifer. The aquifers are primarily sandstone, separated by 

shale, shaly dolomite, or siltstone confining units. Where the 

Maquoketa Shale is present, it and the underlying dolomite 

and shale strata of the Galena Dolomite and the Decorah, 

Platteville, and Glenwood Formations form a major confining unit that overlies the three aquifers. 

The Cambrian-Ordovician aquifer system is a leaky 

system, and ground-water flow is partly controlled by confining units. Much of the recharge is in upland areas and 

most discharge is to nearby streams through local flow 

paths. The remainder of the recharge moves slowly downward to deeper formations and downgradient to the regional flow system. Regional ground-water flow generally 

is away from high areas in the north toward low areas in 

the south and east. Principal discharge areas of the regional flow system are the Mississippi and Missouri Rivers, 

the Illinois and Michigan basins, and Lake Michigan. 

Development of the aquifer system began in the 

1860's near Lake Michigan in eastern Wisconsin and 

northeastern Illinois, and along the valleys of the Mississippi River and its nearby tributaries. Wells flowed initially. The initial hydraulic heads were 186 ft above the 

water level of Lake Michigan at Milwaukee and 130ft at 

Chicago (Young and others, 1989b). By 1980, large-scale 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 73 

pumping had caused the hydraulic heads to decline as 

much as 425 ft at Milwaukee and 900 ft at Chicago (Young 

and others, 1989a). Projection of future water needs indicates continuing or increasing demands and, therefore, 

continuing water-level declines. 

Simulation provided ground-water budgets for predevelopment and 1980 conditions (fig. 13) and indicated 

that the 1980 pumpage of 684 Mgal/d was supplied mostly 

by an increase in recharge and to a lesser extent by a decrease in discharge and loss of water from storage (see the 

earlier section on regional ground-water budgets). 

50 100 KILOMETERS 

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970 

The aquifer system contains highly mineralized water 

in several places, especially in its deepest parts, which generally coincide with regional discharge or structurally low 

areas. These areas are mainly in the southwestern, southern, 

and eastern parts of the study area. Water from highly mineralized zones may be introduced into freshwater zones by 

large withdrawals of freshwater, such as those withdrawals 

presently occurring in northeastern Illinois, southeastern 

Wisconsin, and central Iowa. Newly developed simulation 

techniques that account for multiaquifer well effects and 

water with variable density in space (but not with time) 

MICHIGAN 

·- ·--. ·'-.. -. .__ 

'-.\ 

i.? 

Figure 49. Study area of the Northern Midwest regional aquifer-system analysis and extent of major glaciation 

during the Pleistocene Epoch (from Young and others, 1989b). 
74 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

were used in simulation of this aquifer system (Kontis and 

Mandie, 1988). 

During the Northern Midwest RASA study, the hydrologic evidence and results of flow simulation of the 

Cambrian-Ordovician aquifer system indicated that most 

of the water is recharged in outcrop areas in Wisconsin 

and southern Minnesota, or through the leaky Maquoketa 

confining unit. Water then flows eastward to Lake Michigan or southeastward to the Illinois basin. However, the 

concentration of dissolved solids (fig. 50) suggests that 

ground-water flow in central Iowa is southwestward toward Missouri, which is approximately perpendicular to 

the flow direction interpreted from the potentiometric surface or simulated by the flow model. This different interpretation of flow direction in central Iowa was supported 

by isotope analyses of water from wells in Iowa. Siegel 

(1989) hypothesized that Pleistocene recharge probably 

was a factor in the occurrence of a plume of water having 

higher concentrations of dissolved solids (fig. 50). Water 

in the plume is isotopically depleted in o18o (standard expression of the ratio of the less abundant oxygen-18 ion 

with respect to modern precipitation). This observation indicates that water in the aquifer was precipitated in a climate similar to that hundreds of miles north of the study 

area and suggests that the water probably was recharged 

during Pleistocene glaciation. The northeast-to-southwest 

increase in concentration of dissolved solids in water in 

the St. Peter-Prairie du Chien-Jordan aquifer in central 

Iowa suggests that the direction of paleo-ground-water 

flow was perpendicular to present flow. This earlier flow 

path might be due to emplacement of subglacial meltwater 

under high hydraulic gradients from ice loading in areas 

such as the Great Lakes. This hypothetical explanation of 

paleo-recharge is further supported by concentrations of 

sulfur isotopes in water from wells in central Iowa (Siegel, 

1989). 

The Northern Midwest regional aquifer-system analysis was started in 1978 and completed in 1984. As of 

May 1991, 31 reports had been completed, of which 4 are 

Professional Papers 1405-A through -D (Sun and Weeks, 

1991). Professional Paper 1405-A (Young, 1992a) summarizes the Cambrian-Ordovician aquifer system, 1405-B 

(Young, 1992b) discusses the hydrogeologic framework of 

the aquifer system, 1405-C (Mandie and Kontis, 1992) 

describes the simulation of the regional flow system, and 

1405-D (Siegel, 1989) discusses geochemistry of the 

Cambrian-Ordovician aquifer system. 

OAHU, HAWAII 

The Oahu regional aquifer-system analysis investigated the aquifer system underlying 600 mi2 of the island of 

Oahu in Hawaii (fig. 51). Oahu was formed through building 

and subsequent coalescing of two shield volcanoes, Waianae 

and Koolau Volcanoes. The Waianae Volcano forms the 

western part of Oahu, and the Koolau Volcano forms the 

eastern part. A long period of quiescence followed initial 

mountain building. During quiescence, both volcanoes were 

deeply eroded. Waianae Volcano became dormant first, and 

westward-dipping flows of Koolau Volcano overlapped th~ 

eroded surface of the Waianae Volcano in the central part of 

the island. Subsidence of Oahu submerged permeable lava 

flows and placed them in hydraulic connection with the 

surrounding ocean water. Shifts in sea level and erosion of 

the volcanic rocks allowed marine and terrestrial sediments 

to accumulate behind barrier reefs, forming coastal plains in 

some areas. 

Due to the complexity of volcanic rocks and the island environment, local hydrologic terms were developed 

in Hawaii that are not generally used by ground-water hydrologists (Ewart, 1986). A body of ground water floating 

on and in hydrodynamic equilibrium with saltwater is 

termed "basal ground water," or "basal water body," in Hawaii. Ground water stored in lavas between low-permeability dikes is termed a "dike-impounded water body." Where 

water levels are much higher than the water table in a basal 

water body, but the geologic reason for the occurrence is 

not known (flow is either impeded by low-permeability 

dikes, ash beds, or other low-permeability rocks), the 

ground-water body is termed a "high-level water body." 

Ground water in dike-impounded or high-level water bodies eventually seeps through or overflows the low-permeability dikes or barriers and discharges into adjacent basal 

water bodies, leaks to streams in adjacent valleys, or discharges as springs. In places, perched ground water can be 

found. The perched water is unconfined and is separated 

from an underlying basal water body by an unsaturated 

zone owing to the presence of ash beds or other low-permeability materials. On the basis of the terminology used 

by hydrologists in Hawaii, the aquifer system underlying 

the island of Oahu is divided into 10 aquifers as shown in 

figure 51. 

A significant amount of water is stored in the volcanic rocks of Oahu. Water is stored above sea level to an 

altitude of at least 2,000 ft in the Waianae Range, but most 

is stored below sea level. Depths of freshwater storage 

below sea level are from a few feet to 1,000 ft or more in 

the dike-free lavas, and probably to several thousand feet 

in the compartmentalized lavas between dikes. The dikeimpounded water bodies that underlie interior mountainous areas (where rainfall and recharge are greatest) play an 

important role in the recharge, storage, movement, and 

discharge of all ground water on the island of Oahu 

(Takasaki and Mink, 1985). 

A unique method of tapping ground water stored in 

dike-impounded water bodies is the use of tunnels dug 

into the lava formations at high altitudes. The earliest tunnel reported was dug in Waimanalo Valley in about 1888 

for municipal use. About 25 tunnels have been dug in the 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 

95' 

MICHIGAN 

KANSAS 

50 100 MILES 

50 100 KILOMETERS 

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970 

EXPLANATION 

-500-- Line of equal dissolved-solids concentration-Dashed 

where approximate or inferred. Interval, in milligrams 

per liter, is variable 

Aquifer boundary 

• 2,520 Well yielding water with dissolved-solids concentration 

that exceeds the concentrations on which the lines 

are based-Number is dissolved-solids concentration, 

in milligrams per liter 

Figure 50. Dissolved-solids distribution in the St. Peter-Prairie du Chien-Jordan aquifer of the Cambrian-Ordovician aquifer system in the Northern Midwest (from Young, 1992a). 

75 
76 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Koolau Range for irrigation or municipal use, of which 21 

tap water impounded in the main fissure zones and 4 tap 

water in the minor rift zones. The first well to exploit the 

dike-impounded water bodies in the Koolau Range was reportedly drilled in 1892 in Waimanalo Valley. Since then, 

40 wells have been drilled as of 1978. Wells drilled at low 

altitudes in highly weathered rocks in the dike complex 

yield little water. Wells drilled in less weathered rocks, 

mostly in the margins of the dike zone, yield substantial 

quantities of water, ranging from about one to several tens 

of million gallons per day (Takasaki and Mink, 1985). 

Computer models were used to simulate groundwater flow and evaluate the effects of development on 

21. 

45' 

21. 

30 ' 

21. 

15' 

158° 15' 158.00' 

10 MILES 

10 KILOMETERS 

ground-water levels and the movement of saltwater-freshwater interfaces. A finite-element cross-sectional model 

(Souza and Voss, 1986) was used to simulate the saltwater-freshwater transition zone in the aquifers of southern 

Oahu. During the steady-state simulation a regional 

ground-water discharge of 20 Mgal/d per mile of coastline 

was assumed, and constant freshwater recharge was input 

into nodes at the top part of the upstream boundary while 

the bottom of the same upstream boundary was assumed 

to be a no-flow boundary. The downstream sea water 

boundary is a specified-pressure head boundary that corresponds to the depth of the sea water at the respective position. The simulated contours of sea water percentage seem 

0 OKAUAI 

NIIHAU 

157" 45' 

.:tudyarea 

OAHU MOLOKAI 

C::::> ('.--..,_ MAU I 

LANAI'L:, .::\_") 

KAHOOLAWE "" 

50 100 150 MILES HAWAII ~ 0 50 100 150 KILOMETERS v 

Figure 51. Study area of the Oahu regional aquifer-system analysis and location of aquifers underlying the island of Oahu, 

Hawaii (from Ewart, 1986). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 77 

consistent with contours based on observed data along the 

approximate position of the cross section. A cyclic pumping stress of 30 Mgalld per mile of coastline was applied 

during a transient simulation. The cyclic stress involved 

pumping for 6 months, followed by 6 months of recovery. 

This cyclic pumping roughly corresponds to the schedule 

of agricultural and industrial water uses in southern Oahu. 

During the simulated 65 years of cyclic pumping, the 

freshwater lens in the aquifers shrank considerably, and 

the transition zone moved landward and upward to a sig-

~ificant degree (Voss and Souza, 1986; Souza and Voss, 

1986). This conclusion is important for planners and managers concerned with the development of ground-water resources along coastal plain areas in Hawaii. 

The Oahu regional aquifer-system analysis was started in 1982 and completed in 1988. As of May 1991, eight 

reports had been completed (Sun and Weeks, 1991). 

SAN JUAN BASIN, ARIZONA, COLORADO, 

NEW MEXICO, AND UTAH 

Two regional aquifer-system analyses were conducted in the Upper Colorado River Basin: the San Juan Basin 

RASA, and a RASA covering all of the Upper Colorado 

River Basin excluding the San Juan Basin. The San Juan 

Basin RASA in New Mexico, Colorado, Arizona, and 

Utah includes about 21,600 mi2 (fig. 52). The regional aquifer system consists mainly of rocks of Jurassic through 

Tertiary age that underlie about 19,400 mi2 of the San 

Juan Basin. Altitudes in the study area range from 4,500 ft 

above sea level in San Juan County, Utah, to about 11,000 

ft in Cibola County, New Mexico. Annual precipitation on 

mountainous areas along the northern and eastern margins 

of the basin ranges from 20 to 45 in., whereas precipitation on the lower altitude central part of the basin is 10 in/ 

yr or less. Mean annual precipitation is about 12 in. 

The San Juan Basin is a northwest-trending asymmetric structural depression of Laramide age, located at 

the eastern edge of the Colorado Plateau, with a width of 

about 140 mi and a length of 200 mi. The basin contains a 

thick sequence (more than 14,000 ft) of sedimentary rocks 

ranging from Cambrian through Tertiary age. Volcanic 

rocks of Tertiary age and various deposits of Quaternary 

age are also present. The rocks dip from basin margins 

toward the troughlike basin center. Older rocks crop out 

around the basin margins and are successively overlain by 

younger rocks toward the basin center. 

During the RASA study, 10 permeable hydrogeologic units were delineated. Two consist of Tertiary age rocks: 

(1) San Jose, Nacimiento, and Animas Formations; and (2) 

Ojo Alamo Sandstone. Seven units consist of Cretaceous 

age rocks, in descending order: ( 1) Kirtland Shale and 

Fruitland Formation; (2) Pictured Cliffs Sandstone; (3) 

Cliff House Sandstone; (4) Menefee Formation; (5) Point 

Lookout Sandstone; (6) Gallup Sandstone; and (7) Dakota 

Sandstone. The lowermost unit consists of the Morrison 

Formation of Jurassic age. Among the 10 hydrogeologic 

units, the Gallup Sandstone and Morrison Formation are 

the most productive; the Ojo Alamo Sandstone, Kirtland 

Shale and Fruitland Formation, and Cliff House Sandstone 

are the next most productive; the remaining units are less 

productive. 

The principal uses of ground water in the basin are 

for municipal, domestic, and stock supplies. However, industrial water use has increased significantly since the late 

1970's due to increased mining of uranium in the Morrison Formation. Water levels have declined in several areas 

because of dewatering of the Morrison Formation for mining operations. Competition has been great among electric-power companies in recent years for rights to use the 

limited ground-water resources in the basin. 

The San Juan Basin RASA study was the first RASA 

study to use a Geographic Information System (GIS) to 

manipulate geologic and hydrologic data and to formulate 

these data as input for a three-dimensional finite-difference 

ground-water flow model. The model consisted of 12layers 

that included the 10 hydrogeologic units plus the Lewis 

Shale of Cretaceous age, which acts as a thick confining 

unit, and the Entrada Sandstone, which acts as an aquifer. 

The base of the model is the Chinle Formation of Triassic 

age, which has very low permeability. The Entrada Sandstone is highly permeable and productive; however, concentrations of dissolved solids in waters in the Entrada 

Sandstone are very high, more than 10,000 mg/L in most 

areas. Therefore, this unit was not included among the 10 

simulated hydrogeologic units. However, for modeling purposes it is possible that this brackish water may leak upward into the overlying aquifers. 

Confining units separate the 10 hydrogeologic units, 

but they are not areally extensive. In places, the hydrogeologic units may merge together. In the model, confining 

units were treated by specifying a vertical leakance, except 

for the Lewis Shale, which was treated as an active confining layer. 

The San Juan Basin regional aquifer-system analysis 

was started in 1984 and completed in 1990. As of May 1991, 

14 reports had been completed (Sun and Weeks, 1991). 

Atlases of the hydrogeologic units were constructed by 

using GIS (Kernodle and Craigg, 1992). As of mid-1992, 

chapter B (geologic framework) of Professional Paper 1420 

by Craigg (in press) was approved for publication and other 

chapters are either in preparation or in review. 

SNAKE RIVER PLAIN 

The 15,600 mi2 Snake River Plain extends across 

southern Idaho into eastern Oregon. The Snake River descends 2,930 ft along its 502-mi course across the plain 

(fig. 53). The surface of the plain decreases in altitude 

from about 6,000 ft above sea level in the northeast to 
78 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

2,100 ft in the west. Average annual precipitation on much 

of the plain is less than 10 in., one-third to one-half of 

which falls during the growing season from April through 

September. Most of the water available to the plain originates as snow on surrounding mountains, which are as 

much as 12,000 ft above sea level. Annual precipitation on 

the mountains is as much as 60 in. For study purposes, the 

Snake River Plain was divided into eastern and western 

parts on the basis of hydrology and geology, as shown in 

figure 53. 

35° 

39° 

/ 

I 

I 

.; 

I 

I 

I 

I 

' .... , 

\ .... 

113° 

109° 

' \ 

' --, 

---, 

A R l z:o N A 

---, ..... ..... __ _ 

Geophysical studies indicate that the eastern Snake 

River Plain is a downwarp. Most of the fill consists of 

Quaternary basalt. Near the plain margins, unconsolidated 

sedimentary rocks overlie and are intercalated with the basalt. Tops of basalt flows are typica:Jly broken and have 

very high permeability. Consequently, thick sections of basalt, which include many flows, store and yield large 

quantities of water. In places, the basalt is several thousand feet thick; however, the upper 200 to 500 ft of the 

basalt are thought to be the most permeable (Lindholm, 

Study area 

105° 

COLORADO 

San Juan 

structural basin 

--...... NEW 

I 

I 

I 

MEXICO 

I 

- I .... ' ' ........ _,.,.--\ :oundary of 

Colorado Plateau 

50 100 MILES 

50 100 KILOMETERS 

Figure 52. Study area of the San Juan Basin regional aquifer-system analysis. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 79 

1986). An estimated 200 to 300 million acre-ft of water 

are stored in the top 500 ft of the basalt aquifer. In much 

of the eastern plain, the aquifer is unconfined. About twothirds of the ground-water discharge from the eastern plain 

is from a series of springs, most of which issue from basalt forming the north wall of the Snake River canyon between Milner and King Hill (fig. 53). These springs 

include 11 of the 78 springs in the United States that discharge an average of more than 100 ft3

/s. 

The western Snake River Plain is a graben bounded 

by well-defined high-angle faults. Quaternary and Tertiary 

sedimentary rocks of variable thickness are the predominant fill materials. In the Boise River valley, most ground 

water is obtained from unconsolidated alluvial sand and 

gravel. Elsewhere in the western plain, rocks are predominantly fine grained and the included aquifers are largely 

confined. Ground-water discharge to the Snake River in 

118" 116" 

44" 

42" 

25 50 MILES 

0 25 50 KILOMETERS 

the western plain is small compared with ground-water 

discharge to the Snake River in the eastern plain. A geothermal aquifer system underlies much of the western 

plain. 

Irrigation is the largest water use on the Snake River 

Plain. Irrigation started in about 1843 along the Boise River. 

Development was slow until the late 1880's, when Congress 

encouraged reclamation of desert lands and expansion of 

irrigated acreage in the arid West. By 1899, about 550,000 

acres were irrigated with surface water. The Reclamation 

Act of 1902 stimulated further expansion by providing 

funds for construction of major instream storage facilities 

(reservoirs) and transmitting canals. By 1929, about 2.2 

million acres were irrigated with surface water. Increases in 

land irrigated with surface water from 1929 through 1945 

were moderate owing to depressed economic conditions, 

World War IT, and full appropriation of surface-water rights. 

114" 

MONTANA 

I l-------

EXPLANATION 

Eastern Snake River Plain 

Western Snake River Plain 

Area of Snake River Plain 

Boundary of Snake River basin 

t;. Stream gaging station 

0 Well referenced in text and 

figure 54 

Figure 53. Study area of the Snake River Plain regional aquifer-system analysis (modified from Lindholm, 1986). 
80 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

After World War II, demand for irrigation water increased. 

Because surface-water rights were fully appropriated, 

ground water became a logical source of supply. By 1966, 

ground water was used to irrigate about 700,000 acres. In 

1980, about 32 percent of the Snake River Plain, or about 

3.1 million acres of land, were irrigated. In one area south of 

the Snake River near Twin Falls, ground-water levels rose 

as much as 200 ft in 5 yr after the start of irrigation with 

surface water (fig. 54). In much of the plain, water levels 

rose several tens of feet over tens of years as indicated by 

water levels observed in wells at Jerome and near Fort Hall. 

Water levels in the western plain were also affected by 

surface-water irrigation, as observed in wells near Nampa 

(fig. 54). 

As ground-water levels rose, ground-water discharge 

to the Snake River increased. Long-term data on gaining 

reaches of the Snake River that include the major springs 

are the key to understanding changes in the ground-water 

flow system of the eastern plain. Kjelstrom (1986) documented changes in ground-water discharge to reaches of 

the Snake River that include major groups of springs. 

Discharge from the group of springs between Blackfoot and Neeley (fig. 53) has been relatively constant over 

the period of record (1912-80). Upgradient from the 

springs, considerable acreage was irrigated prior to the 

start of data collection. Despite an increase in surfacewater diversion of about 1.5 million acre-ft from 1912 

through 1952, ground-water discharge remained stable. It 

is likely that by 1912, ground-water recharge and discharge affecting the reach from Blackfoot to Neeley were 

approximately in balance (Lindholm, 1986). 

In the reach from Milner to King Hill, some increase 

in ground-water discharge probably occurred between the 

220 

Near Twin Falls 

200 

180 

1-

w 

w 160 ... 

~ 140 

w 

Ul 120 a: 

...J 

w 100 > 

w 

...J 

0:: 80 w 

1-

<l: 60 

5: 40 

Near Fort Hall 

20 

1900 1910 1920 1930 1940 1950 1960 

YEAR 

Figure 54. Changes in ground-water levels in the Snake 

River Plain due to surface-water irrigation (locations of 

wells shown in fig. 53) (from Lindholm, 1986). 

start of irrigation (1880's) and the start of data collection 

(1902). A 40-yr trend of increasing ground-water discharge started in 1912 owing to major increases in surface-water irrigated acreage following the completion of 

reservoirs and diversion structures on the Snake River. In 

the early 1950's, spring discharge stabilized, implying a. 

temporary balance between recharge and discharge that 

lasted for several years (fig. 17). A period of overall decrease in ground-water discharge followed and continued 

to 1980. The decrease probably was due to several factors, 

including an increase in ground-water withdrawal, decrease in surface-water irrigation, and increase in irrigation efficiency (Lindholm, 1986). 

The impact of the long history of surface-water diversions and ground-water withdrawals on the groundwater budgets of the eastern Snake River Plain is shown in 

figure 14 and discussed in the earlier section on regional 

ground-water budgets. In brief, flow through the aquifer 

system in the eastern plain increased by about 80 percent 

from predevelopment to 1980, and most recharge in 1980 

was supplied by irrigation return flow. 

Phase I of the Snake River Plain regional aquifersystem study was started in 1979 and completed in 1984. 

In Phase I, the study was divided into six parts; those parts 

are described in chapters B through G of Professional 

Paper 1408 as follows: (B) geohydrologic framework 

(Whitehead, 1992); (C) ground-water/surface-water relations and ground-water budgets (Kjelstrom, in press); (D) 

solute geochemistry (Wood and Low, 1988); (E) water use 

(Goodell, 1988); (F) hydrology and digital simulation, 

eastern Snake River Plain (Garabedian, 1993); and (G) 

geohydrology, western Snake River Plain (Newton, 1991). 

Chapter A of Professional paper 1408 by Lindholm (in 

press) summarizes Phase I study results. 

During Phase I, several key local areas within the 

Snake River basin were identified as needing more detailed 

analysis than was possible in a regional study. These areas 

required additional data collection and detailed groundwater flow models to improve hydrologic understanding. 

Phase II was initiated in 1985 to study the American Falls 

Reservoir area, the Big Lost River basin, and the Thousand 

Springs area (fig. 55). In the American Falls and Thousand 

Springs areas, large quantities of ground water discharge to 

the Snake River. The Big Lost River basin is a major northside tributary to the plain. Kjelstrom (1986) estimated that 

from 1934 through 1980, the Big Lost River basin annually 

yielded an average of 335,000 acre-ft of water to the Snake 

River Plain. A major objective of the Phase II studies was 

to provide a more accurate water budget for the Snake 

River Plain. Reports summarizing results of the three Phase 

II areal studies are currently in review. 

The Snake River Plain regional aquifer-system analysis was started in 1979 and completed in 1990. As of 

May 1991, 35 reports had been completed (Sun and 

Weeks, 1991). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 81 

UPPER COLORADO RIVER BASIN 

The Upper Colorado River Basin has a drainage area 

of about 113,500 mi2 in western Colorado, eastern Utah, 

southwestern Wyoming, northeastern Arizona, and northwestern New Mexico. The regional aquifer-system analysis of the Upper Colorado River Basin excludes the San 

Juan Basin, which was studied separately (see the previous 

section entitled "San Juan Basin, Arizona, Colorado, New 

Mexico, and Utah"). Therefore, the Upper Colorado River 

Basin RASA is about 92,000 mi2 (fig. 56). The study area 

contains a variety of landforms, including rugged mountains, broad plains, deeply dissected canyons, relatively 

flat flood plains, and many erosional features. The area has 

been subjected to repeated tectonism. The predominant 

tectonic features are numerous basins and uplifts (fig. 57). 

Resulting structural relief is nearly 30,000 ft. 

118" 

44" 

42" 

Tens of thousands of feet of consolidated sedimentary rocks of Paleozoic, Mesozoic, and Cenozoic age underlie the study area. Rocks consisting of fractured limestone, 

dolomite, and sandstone form aquifers. Low-permeability 

limestone, dolomite, shale, and evaporite form confining 

units. Rocks in the Upper Colorado River Basin have been 

classified into three major aquifer systems. They are, in 

descending order: (1) the Tertiary rock aquifer system, (2) 

the Mesozoic rock aquifer system, and (3) the Paleozoic 

rock aquifer system. Within each aquifer system, the rocks 

are subdivided into regional aquifers and confining units 

on the basis of lithology, depositional environment, and 

hydrologic characteristics. Igneous rocks, especially volcanic rocks, are also present in parts of the study area, but 

they are not regional aquifers. 

Tht: Cenozoic rocks are sediments of Tertiary age 

that overlie much of the northern half of the study area 

MONTANA 

I l-------

EXPLANATION 

~ American Falls Reservoir area 

~ Big Lost River basin 

~ Thousand Springs area 

Eastern Snake River Plain 

Western Snake River Plain 

Area of Snake River Plain 

Boundary of Snake River basin 

Figure 55. Study areas of the Snake River Plain regional aquifer-system analysis, Phase II. 
82 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

(fig. 58). The rocks of the Tertiary rock aquifer system 

crop out in the Green River, Great Divide-Washakie, 

Piceance, and Uinta basins (figs. 57 and 58). Hydraulic 

properties of the Tertiary rocks vary markedly from basin 

40' 

115' 

IDAHO 

-----,---------------~ 

: I 

I 

I 

I 

' ' I 

' I 

I 

I 

UTAH 

r---------------

' I 

' 

ARIZONA 

110' 

.Price 

to basin. For example, the Wasatch Formation includes 

water-yielding rocks iit the Green River and Great DivideWashakie basins but transmits little water to we11s in the 

Pic~ance and Uinta basins. Lake deposits in the Green 

105' 

WYOMING 

NEW MEXICO 

50 100 MILES 

50 100 KILOMETERS 

I 

' ' [ __ 

' 

I 

I 

' I 

I 

' I 

EXPLANATION 

Area excluded from study 

c=J Study area 

Figure 56. Study area of the Upper Colorado River Basin regional aquifer-system analysis. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 83 

River Formation are a regional aquifer in the Piceance 

basin where they are fractured; elsewhere, this hydrogeologic unit is confining. In the Uinta basin, the Duchesne 

River Formation of the Tertiary rock aquifer system is the 

most productive unit; however, this unit is limited to the 

northern half of the basin. A well inventory indicates that 

40 ' 

115 ' 

IDAHO 

I -----, --·-----------~ 

: I 

I 

' ' I 

UTAH 

110' 

GREAT 

WYOMING 

{~AWLINS 

\PLIFT 

105 ' 

I 

' [ __ 

I 

' ' I 

' ' I 

COLORADO 

NEW MEXICO 

ARIZONA 

50 100 MILES 

50 100 KILOMETERS 

EXPLANATION 

Area excluded from study 

C:=J Study area 

Approximate boundary of uplifts and basins 

Figure 57. Principal tectonic features in the Upper Colorado River Basin (modified from Grose, 1972). 
84 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978- 1992 

the Uinta Formation is used extensively for water supply 

in the Uinta basin. The Piceance basin consists of one of 

the largest potential areas for oil-shale development in the 

Western United States. The important Tertiary aquifer in 

the Piceance basin consists of the Uinta Formation and 

parts of the Parachute Creek Member of the Green River 

11 ° 

40° 

I 

' I 

' ' I 

' I 

IDAHO 

UTAH 

r--------------

ARIZONA 

110 ° 1 0 

WYOMING 

COLORADO 

50 100 KILOMETERS 

EXPLANATION 

Cenozoic rocks 

Area excluded from study 

C=:J Study area 

I 

' [ __ 

' 

I 

' 

I 

' ' I 

' ' I 

_ _j_ 

Figure 58. Areal extent of Cenozoic rocks in the Upper Colorado River Basin (modified from King and Beikman, 1974). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 85 

Formation. In Wyoming, the Fort Union Formation and 

the Wasatch Formation and their equivalents are the most 

important hydrogeologic units of the Tertiary aquifer system. Recharge to the Tertiary aquifer system occurs near 

the peripheries of basins and in the uplands; discharge is 

to streams in the central parts of basins. In the Great Divide basin, water in the Wasatch Formation discharges to 

salt flats and phreatophytes. The rate of evapotranspiration 

in these areas is large enough to prevent the formation of 

permanent lakes in the Great Divide basin. 

The Mesozoic rock aquifer system consists mainly of 

sandstone sequences that extend across most of the study 

area (fig. 59). Sequences made up mostly of siltstone, claystone, and shale are grouped into confining units. The hydraulic conductivity of sandstone is highly variable, ranging 

from about 0.01 to 3 ft/d. The hydraulic conductivity of 

confining units is from about 0.03 to 0.1 ft/d. Thickness of 

Mesozoic rocks exceeds 10,000 ft over a large part of the 

northern basins; these rock sequences are less than 5,000 ft 

thick in the southern one-half of the study area. Recharge 

occurs mainly in outcrop and subcrop areas, in structural 

uplift areas, and in topographic uplands. Ground water discharges to major rivers-the Colorado, Green, San Juan, 

and Yampa Rivers-by means of springs located on walls 

and bottoms of canyons where aquifers are exposed and by 

upward leakage where aquifers are not exposed. Regionwide development of water in the Mesozoic rock aquifer 

system is negligible because deep drilling is required and 

aquifers contain brackish water. However, some development has occurred in the Navajo Sandstone locally. 

The Paleozoic rock aquifer system consists of sedimentary rocks representing all periods of the Paleozoic 

Era except the Silurian and extends throughout most of the 

study area (fig. 60). The aquifer system includes two regional aquifers and several local aquifers and confining 

units. The two regional aquifers consist of (1) Devonian 

and Mississippian carbonate rocks, and (2) Pennsylvanian 

and Permian sandstone. Geldon (1988) concluded that the 

Devonian-Mississippian carbonate rock aquifer is the most 

permeable unit in the Paleozoic sequence. However, as 

shown previously in figure 6, the estimated transmissivity 

varies by several orders of magnitude. 

Recharge to the Paleozoic rock aquifer system occurs mostly from precipitation on uplift areas where the 

rocks are exposed, leakage from overlying aquifers, and 

from streamflow losses in subcrop and outcrop areas. 

Ground water flows from structurally high areas to structural basins, to the incised canyons of the Colorado and 

Green Rivers and their tributaries, to the northeastern 

Great Divide basin, to the confluence of the Green and 

Yampa Rivers, to the San Juan Basin, to springs, and to 

overlying Mesozoic and Tertiary rocks. In some recharge 

areas, heads in the upper Paleozoic rock aquifer system 

are more than 1 ,500 ft higher than heads in the lower Paleozoic rock aquifer system. 

Well yields from the Paleozoic rock aquifer system 

are small in most areas, generally less than 50 gal/min for 

wells pumped for domestic and stock supplies. However, 

in uplifted areas where the transmissivity is highest (fig. 

6), a few wells yielding 100 to 3,000 gal/min were reported. Most of the ground water is pumped in oil and gas 

fields. In December 1986, the combined daily water production from the Rangely, Lost Soldier-Wertz-Mahoney, 

Ashley Valley, greater Aneth, Upper Valley Brady, and 

Lisbon oil fields and 23 smaller fields was about 63 ft3 Is, 

or 46,000 acre-ft/yr. Based on sparse data, cones of depression several hundred to 1 ,000 ft deep seem to have 

developed around some of these fields (Geldon, 1988). 

The Upper Colorado River Basin regional aquifersystem analysis was started in 1981 and completed in 1989. 

As of mid-1992, 31 reports had been completed. One chapter of Professional Paper 1411 has been printed, three are in 

press, and the others are either in review or in preparation. 

ONGOING REGIONAL AQUIFER-SYSTEM 

ANALYSIS STUDIES 

Seven regional studies and the National Ground 

Water Atlas that were ongoing as of mid-1992 are summarized in this section. Data compilation and investigative 

activities were in progress and only a few reports had been 

printed by mid-1992. The individual project summaries are 

based on writeups prepared by the project chiefs. 

APPALACHIAN VALLEYS AND PIEDMONT 

The Appalachian Valleys and Piedmont regional aquifer-system analysis (APRASA) includes about 142,000 

mi2 in parts of Pennsylvania, New Jersey, Delaware, 

Maryland, the District of Columbia, Virginia, West Virginia, North Carolina, Tennessee, South Carolina, Georgia, 

and Alabama (fig. 61). The eastern and southern boundaries of the study area correspond to the Fall Line, beyond 

which is the Atlantic Coastal Plain. The western boundary 

corresponds to the western limit of thrust faults and folds 

in the Valley and Ridge physiographic province; the rocks 

along this boundary dip steeply. West of this boundary is 

the Appalachian Plateaus physiographic province, which is 

underlain by gently dipping Paleozoic rocks whose hydrologic characteristics differ from the rocks underlying the 

Valley and Ridge physiographic province. The northern 

boundary of the study area is a 1,000-ft-thick diabase 

sheet that exists along the Hudson River; elsewhere, the 

northern boundary is the state line between New York and 

New Jersey. The extreme northern part of the study area is 

locally mantled by glacial deposits. The hydrology of the 

glacial deposits was studied by the Northeast Glacial Aquifers RASA. 
86 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

On the basis of hydrogeologic characteristics, the 

study area was divided into two subareas. The extreme 

western part of the Blue Ridge physiographic province 

and the entire Valley and Ridge physiographic province 

consist of carbonate rocks, sandstone, and shale, forming 

one of the subareas. The Piedmont physiographic province 

40 ' 

115 ' 

IDAHO 

I 

I 

. 

----- I ,------- -- ----~ 

I 

. 

. 

I 

' ' I 

t-----------

' ' I 

I 

I 

. 

[ __ WYOMING 

______________ j_ 

NEW MEXICO 

50 100 MILES 

50 100 KILOMETERS 

EXPLANATION 

Mesozoic rocks 

Area excluded from study 

r=J Study area 

Figure 59. Areal extent of Mesozoic rocks in the Upper Colorado River Basin (modified from King and Beikman, 1974). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 87 

and the central and eastern Blue Ridge physiographic 

province form the other subarea, which consists mostly of 

metamorphic and igneous crystalline rocks. Large rift basins, extending from New Jersey to South Carolina within 

the Piedmont crystalline rocks, are filled with sedimentary 

rocks of early Mesozoic age. 

40 ' 

110' 

IDAHO 

----- I ,---------·-----~ 

: I oo .''(~CJ",::c \,CC! I 

I 

' ' I 

' I 

' ' 

' ' I 

' 

I 

I 

I 

' I 

' ' I 

t-----------

0 

I 

EXPLANATION 

k:,. ·J,o'.:j Paleozoic rocks 

I 

' \ __ WYOMING 

_______ _j_ 

-------·-----·-· 

NEW MEXICO 

50 100 MILES 

50 100 KI LOMETE RS 

Area excluded from study 

c=J Study area 

Figure 60. Areal extent of Paleozoic rocks in the Upper Colorado River Basin (modified from King and Beikman, 1974). 
88 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Regolith that consists of soil, alluvium, and weathered rock overlies most of the rocks throughout the entire 

study area. In places, regolith includes materials that were 

transported and deposited as glacial drift, colluvium, and 

alluvium. In other places, regolith may consist only of 

weathered parent rock, called residuum or saprolite. Thickness of regolith varies greatly, ranging from 0 ft to more 

than 150 ft. Regolith generally is divided into three zones: 

the soil zone, saprolite, and a transition zone separating the 

so· 85° 

EXPLANATION 

Physiographic provinces within 

study area 

LIITJ Valley and Ridge 

40° 

C8 Piedmont 

c=J Blue Ridge and New England 

TENNESSEE 

30° 

saprolite from unweathered rocks. Hydrologically, regolith 

acts as a storage reservoir that receives infiltrating precipitation. Water in regolith then slowly recharges the underlying rock aquifers. 

Aquifers in the study area vary greatly in their hydrogeologic characteristics. For example, in the Valley and 

Ridge physiographic province, karst features such as sinkholes, solution cavities, and large springs occur where 

there are carbonate rocks. Large ground-water supplies can 

so· 75° 

50 100 MILES 

50 100 KILOMETERS 

LL------------------------~----------------------~----------------------~ Base modified from U. S. Geological Survey 

digital data 1:2,000,000, 1972 

Figure 61. Study area of the Appalachian Valleys and Piedmont regional aquifer-system analysis and location of physiographic 

provinces (from Swain and others, 1991). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 89 

be obtained from the carbonate rocks, especially where associated with a thick regolith (50 ft or more). Reported 

well yields from a single well in carbonate rocks exceed 

8,000 gal/min. Ground-water movement in the Valley and 

Ridge physiographic province generally is restricted by 

parallel rock ridges, which act as hydrologic boundaries 

and cause water to discharge to local streams. Well yields 

in the Piedmont and Blue Ridge physiographic provinces, 

which mainly consist of metamorphic and igneous crystalline rocks, vary greatly depending on the thickness of the 

regolith. Yields as great as 1,800 and 2,200 gal/min have 

been reported, respectively, from crystalline and sedimentary rocks of the Piedmont. Once water in the regolith is 

drained, well yield from crystalline rocks may decrease 

significantly. Several hydrogeologic terranes and conceptual flow systems have been identified for the study area, as 

summarized in table 2. These terranes and flow systems 

are described in further detail in Swain and others (1991). 

In 1985, about 1,700 Mgal/d of water was pumped, 

of which 986 Mgalld (about 58 percent of the total) was 

from aquifers in the Piedmont physiographic province, 567 

Mgal/d (about 33 percent of the total) was from aquifers in 

the Valley and Ridge physiographic province, and the remaining 9 percent (about 163 Mgalld) was from aquifers in 

the Blue Ridge physiographic province. Major problems 

with ground-water development in the study area are extremely variable well yields, sinkhole collapses during 

drilling in carbonate rock terranes, reduction in streamflow 

due to pumping from wells, difficulty in estimating sustainable yield of aquifers, and water-quality problems resulting 

from both natural geochemical processes and contamination from human activities. 

One objective of the RASA study is to find possible 

solutions to these development problems. However, the 

principal objective of the study is to develop an understanding of ground-water flow systems in the various hydrogeologic terranes of the Appalachian Valleys and Piedmont. 

The study was started in 1988. As of early 1993, 20 reports 

had been completed, of which 18 are published. 

EDWARDS-TRINITY REGIONAL AQUIFER SYSTEM, 

TEXAS, OKLAHOMA, AND ARKANSAS 

The study area of the Edwards-Trinity regional aquifer-system analysis includes about 95,000 mi2 in parts of 

central Texas, southeastern Oklahoma, and southwestern 

Arkansas (fig. 62). The aquifer system includes three 

major aquifers in rocks of Early Cretaceous age. The 

major aquifers, named the Edwards, the Trinity, and the 

Edwards-Trinity, together comprise about 80,000 mi2. The 

Edwards aquifer is composed mostly of fractured and dissolutioned limestone that yields large quantities of water 

to wells and springs. The Trinity aquifer is composed primarily of dolomitic limestone, with interbedded sand, 

shale, and clay; the Edwards-Trinity aquifer generally is 

composed of limestone and dolomite in its upper part and 

quartz-rich sand in its lower part. These aquifers are laterally adjacent except in the southeastern part of the system, 

where part of the Trinity aquifer is overlain by the Edwards aquifer. 

From Arkansas counterclockwise to the Rio Grande 

(fig. 62), the boundary of the aquifer system is the limit 

beyond which the lower Cretaceous rocks no longer exist 

or do not compose the principal water-yielding unit. Cretaceous rocks extend south into Mexico, but water-level data 

indicate that the Rio Grande is a regional discharge boundary of the aquifer system, and in any case, the hydrology of 

Mexico is beyond the scope of the study. Thus, a segment 

of the Rio Grande serves as part of the boundary of the 

aquifer system. Continuing counterclockwise from the Rio 

Grande into Arkansas, the boundary of the system is the 

updip limit of a freshwater/saline-water transition zone. 

In the southern half of the study area (the area south 

and west of the Colorado River), much of the aquifer system is near the surface and unconfined. However, the 

southeastern fringe of the system is buried and confined. 

In the northern half of the study area, the aquifer system 

crops out in a band along its western and northern boundary and becomes buried and confined in its eastern part. 

During the late 1980's an estimated 1.2 million acreft/yr of water was withdrawn from the study area for all 

uses. More than 80 percent of the withdrawals ( 1 million 

acre-ft/yr) occurred in the southern half of the study area, 

the area south and west of the Colorado Riv~r. Within that 

area, the major withdrawals are from the highly productive 

Edwards aquifer, which underlies an area of approximately 

3,000 mi2. An estimated 547,000 acre-ft of water was 

withdrawn from the Edwards in 1989, primarily to supply 

municipal and industrial water to the San Antonio metropolitan area and irrigation water to farms west of San Antonio. Thus, nearly one-half of the ground-water supplies 

are withdrawn from about 3 percent of the study area. 

Among water issues affecting the aquifer system, 

one issue overshadows all others. That is the long-term 

capability of the Edwards aquifer to continue to supply all 

of the water needs of the greater San Antonio area. Large 

transmissivity values (200,000 to 2,000,000 ft2

/d; Maclay 

and Small, 1986), together with the capacity to accept 

plentiful recharge during wet periods, have thus far kept 

the Edwards from experiencing a long-term loss of water 

from storage. However, the aquifer has become more susceptible to drought in recent years. Increasingly large 

withdrawals have caused short-term water-level declines to 

increase in the San Antonio area. 

The Edwards-Trinity regional aquifer-system analysis was started in 1986 and is scheduled for completion in 

1994. Initially, the study area was divided into two parts 

separated by the Colorado River. The southern study area, 

consisting of the Edwards-Trinity aquifer system and contiguous hydraulically connected units in west-central 
Table 2. Hydrologic characteristics of hydrogeologic terranes and conceptual flow systems in the Appalachian Valleys and Piedmont (from Swain and others, 1991). 

Hydrologic characteristics 

Hydrogeologic 

terranes Type of porosity Depth of flow, in Confined or Regolith Relief Recharge Discharge Type of flow or permeability feet unconfined storage 

Carbonate rocks, Low (residual Mostly precipitation To streams and Intergranular in Diffuse and Shallow to Mostly unconfined Large 

thick regolith regolith) on topographic large springs at regolith; conduit intermediate, 

# High highs and edge of ground- dissolution ::; 600 

i:ia (transported streamflow water basin openings in "0 regolith) rock a 

>. Carbonate rocks Low (carbonate) Precipitation on To springs Dissolution Conduit and Shallow, ::; 300 Mostly unconfined Small .£ 

~ and Low to high topographic highs (carbonate) openings and fracture 

sandstone, thin (sandstone) and valley floors. and as seepage fractures 

regolith Some streamflow to streams 

loss a (sandstone) 

Clay-rich rocks Low (valleys) Precipitation on To streams by Large porosity Diffuse Very shallow, Unconfined Small to 

High (mountain topographic highs uniform at shallow through ::; 150 large 

slopes) and valley floors. seepage. No depth; little clayNo streamflow loss large springs or no filled 

permeability fractures 

Massive or Low to high Precipitation on To streams Fracture Fracture Shallow (mostly) Unconfined Small 

foliated topographic highs to 

crystalline intermediate, 

rocks, thick ::; 500 

regolith 

u Massive or Low to high Precipitation on To streams Fracture Fracture Shallow (mostly) Unconfined Small ! foliated topographic highs to 

l crystalline intermediate, 

lXI rocks, thin ::; 500 

1 regolith 

I Metamorphosed Low to Precipitation on To streams Dissolution Conduit Shallow Unconfined Small to 

carbonate moderate topographic highs openings, fracture moderate 

rocks some 

fractures 

Mesozoic Low to Precipitation on To streams lntergranular, Diffuse, (Mostly) shallow Mostly Small 

sedimentary moderate topographic highs some fracture to unconfined, 

basins fractures intermediate, some confined 

::; 800 

Well yield 

Variable; can be as 

much as 1 ;000 

gallons per 

minute 

Carbonate, 

variable; 

sandstone, less 

variable than 

carbonate 

Very small, less 

variable than 

carbonate 

Proportional to 

regolith 

thickness 

Low 

Variable, some 

very high 

Variable, 

decreasing from 

north to south 

\0 

0 

~ z 

~ 

~ 

~ en 

to<: 

en 

~ > z 

en 

~ 

-

en 

~ ~ 

~ ~ 

!Zl 

I 

en 

m 

~ :< 

~ 

f 

~ \0 

tv 
fr 

~ 

Bg 

0 ~0 • -V'l 

"ea VI ..£: 

tl) 

~ 0 

u 

PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 91 

c:: 

·~ 

.0 

. § 

Q. 

tl) 

j 

·r;; = 

Texas, was to be studied first because most water use and 

water issues are there; the northern study area, consisting 

of the Edwards-Trinity aquifer system in north-central 

Texas, southeastern Oklahoma, and southwestern Arkansas, was to be studied second. As of 1992, most work has 

been in the southern study area. Accordingly, the scope of 

work in the northern study area was reduced so that the 

project can finish on time. 

The aquifer system in the southern study area (fig. 

63) is divided into four geographic subareas: the TransPecos, Edwards Plateau, Hill Country, and the Balcones 

fault zone south of the Colorado River (hereafter referred 

to as the Balcones fault zone). The Trans-Pecos and the 

Edwards Plateau together are coincident with the area of 

the Edwards-Trinity aquifer (figs. 62 and 63). The Hill 

Country coincides with the area where the Trinity is the 

principal aquifer. The Balcones fault zone is coincident 

with the area where the Edwards is the principal aquifer 

and the Trinity is a secondary aquifer. 

Definition of a regional hydrogeologic framework in 

the southern study area is complete. The foundation of the 

framework definition is a series of seven hydrogeologic 

sections constructed primarily from geophysical logs, and 

a regional stratigraphic correlation chart synthesized from 

numerous sources. 

Computer modeling was used extensively to quantify major components of the regional flow system and to 

develop areal distributions of recharge, discharge, and 

transmissivity in the southern study area. The regional 

flow system in the Edwards aquifer is controlled by a series of faults (the Balcones fault zone) that divert flow 

along the strike of the faults (fig. 64). Because of the complex aquifer geometry, a finite-element model was used 

rather than a finite-difference model common to most 

other RASA studies (Kuniansky, 1990). The finite-element 

mesh design has more flexibility to simulate the anisotropic transmissivity associated with faults and the locations of 

streams and springs than the finite-difference mesh design. 

A steady-state, single-layer model of the southern study 

area has been satisfactorily calibrated. Because the Edwards aquifer in the Balcones fault zone is characterized 

by rapid and dramatic changes in water levels in response 

to short-term stresses, and because the water-yielding 

zones are stacked vertically in the Trinity aquifer in the 

adjacent Hill Country, simulation of those parts of the system with a steady-state, single-layer model probably is unrealistic. A transient, multilayer finite-element model of 

the Edwards and the Trinity aquifers in the Balcones fault 

zone and the Hill Country, an area of about 10,000 mi2, is 

nearing completion. 

The geochemistry of the aquifer system in the southern study area is being studied to provide independent information on the nature of the flow system. A data base of "best 

available" existing water analyses has been constructed . 

From this data base, regional maps of hydrochemical facies 
92 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

and concentrations of dissolved solids, sulfate, chloride, and 

fluoride have been made. Recent work involved sampling of 

wells in the updip and downdip parts of the Trinity aquifer 

in the Hill Country to obtain major constituents and isotope 

data, primarily carbon-13 and carbon-14. These data are 

being used in mass-transfer modeling to derive chemical 

reactions that simulate observed water chemistry. 

As of 1992, work in the northern study area consisted of preparation of a series of hydrogeologic sections 

based on geophysical logs that will be used to define the 

1os• 

35. 

NEW 

MEXICO 

hydrogeologic framework there. No computer modeling is 

planned for the northern study area. 

The Edwards-Trinity aquifer system in the southern 

study area also includes two mappable confining units (fig. 

64). The Hammett confining unit occurs within the Trinity 

aquifer in the Hill Country and southeastern parts of the 

Edwards-Trinity aquifer in the Edwards Plateau. The Navarro-Del Rio confining unit directly overlies subcropping 

parts of the Edwards aquifer in the Balcones fault zone. 

The subdivision of the Cretaceous strata into aquifers and 

100• 

I 

~, 

gs• 

OKLAHOMA 

TEXAS 

Jo• 

EXPLANATION 

.. Edwards-Trinity aquifer 

CJ Trinity aquifer 

~ Edwards aquifer 

Contiguous hydraulically connected units 

Base modified from U.S. Geological Survey digital data, 1:2,000,000,1972 

50 100 MILES 

50 100 KILOMETERS 

Figure 62. Study area of the Edwards-Trinity regional aquifer-system analysis and location of contiguous hydraulically connected units 

in central Texas, southeastern Oklahoma, and southwestern Arkansas. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 93 

confining units is based primarily on the relative capacity 

of the rocks to transmit water. This information was derived from well-yield and aquifer-test data and was extrapolated regionally on the basis of stratigraphy. 

The principal units contiguous to and hydraulically 

connected to the aquifer system are the High Plains aquifer and the Cenozoic Pecos alluvium aquifer along the 

Pecos River (fig. 64). Ogallala sediments of the High 

Plains aquifer are essentially indistinguishable from under25" 

25 50 MILES 

EXPLANATION 

Geographic subareas 

C=:J Balcones Fault Zone 

' • J Hill Country 

.. Edwards Plateau 

Trans-Pecos 

Boundary of the EdwardsTrinity aquifer system 

Major fault 

~ Spring-With discharge 

greater than 10 cubic feet 

per second 

0 25 50 KILOMETERS 

Base modified from U.S. Geological Survey, 

National Atlas, 1:7,500,000, 1970 

lying Lower Cretaceous sediments in the northwestern part 

of the study area (Mount and others, 1967, p. 45). Due to 

the dissolution of Permian salt beds and the subsequent 

collapse and erosion of the overlying Cretaceous section, a 

large volume of Edwards and Trinity strata is missing 

along the Pecos River near the northwestern margin of the 

study area. Following the removal of Edwards-Trinity 

rocks, alluvium of Tertiary and Quaternary age filled the 

structural trough to a thickness of as much as 1 ,500 ft. The 

Figure 63. Geographic subareas of the Edwards-Trinity regional aquifer system in west-central Texas. 
94 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Cenozoic Pecos alluvium aquifer is a relatively permeable 

mixture of unconsolidated to semiconsolidated gravel, 

sand, silt, clay, and caliche that is hydraulically continuous 

with Edwards-Trinity rocks in the area. 

Several minor aquifers in rocks of pre- and postCretaceous age are between the aquifer system and the 

Colorado River. Around the northeastern margin of the 

study area just south of the Colorado River, erosion of the 

basal rocks has formed a subtle topographic basin, within 

EXPLANATION 

which a local ground-water regime merges with flow from 

the Edwards-Trinity aquifer. 

The unique hydrogeology of the Balcones fault zone 

is a direct result of large-scale normal faulting during late 

Oligocene through early Miocene time that profoundly 

disrupted rocks in that area (Weeks, 1945). The resulting 

structure and subsequent dissolution of soluble carbonate 

constituents are primarily responsible for the marked contrast between the dynamic, vigorous flow regime that ocEdwards-Trinity aquifer system 

25 50 MILES 

0 25 50 KILOMETERS 

~ Edwards aquifer 

Trinity aquifer 

.. Edwards-Trinity aquifer 

fUD\: \ j Navarro-Del Rio confining unit 

~ Hammett confining unit 

Contiguous major aquifers 

~ Cenozoic Pecos alluvium aquifer 

.. High Plains aquifer 

Boundary of the EdwardsTrinity aquifer system 

Boundary of aquifer or confining 

unit-Dashed where uncertain 

Base modified from U.S. Geological Survey, 

National Atlas, 1:7,500,000, 1970 

Figure 64. Aquifers and confining units in the Edwards-Trinity regional aquifer system and contiguous major aquifers in west-central 

Texas. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 95 

curs in shallow parts of the fault zone and the more 

sluggish flow regime that is typical elsewhere in the study 

area. The faulting created avenues for water to enter, flow 

through, and dissolve the carbonate rocks, which greatly 

enhanced the permeability of the Edwards aquifer (Abbott, 

1975). Compaction and cementation resulting from burial 

and slow-moving ground water have combined to limit the 

permeability in the Trinity and Edwards-Trinity aquifers. 

The striking contrast between the flow regime in the 

Edwards and that in other aquifers and contiguous hydraulically connected units in the southern study area is reflected in the regional distribution of estimated transmissivity. 

Based on the results of aquifer tests, geologic observation, 

and simulation, the transmissivity of the Edwards aquifer 

averages about 750,000 ft2

/d, whereas transmissivity of 

the Trinity aquifer in the Hill Country and of the EdwardsTrinity aquifer in the Edwards Plateau and the Trans-Pecos 

probably averages less than 10,000 ft2

/d. 

Areal 

recharge 

1,840 

Recharge from 

streams 

740 

Comparison of simulated components of the winter 

1974-75 flow system to those of the predevelopment flow 

system gives some insight into how ground-water withdrawals may have affected the natural flow system (fig. 

65). Results of simulations of development and predevelopment conditions indicate that the source of most of the 

water supplied to wells is diverted natural discharge to 

streams and springs, rather than induced recharge. Simulated total recharge to the aquifer system and contiguous 

hydraulically connected units increased about 5 percent, 

from 2.8 to 3 million acre-ft/yr, as the result of groundwater development. This result is compatible with hydrologic conditions-that is, no large source of water exists 

from which additional recharge might be induced. Streambed leakage in the southern part of the Hill Country and 

the northern part of the Balcones fault zone provides the 

largest concentrated volume of recharge to the flow system. The outcropping Edwards rocks are very permeable; 

Discharge 

to streams 

1,770 

Springs 

1,030 

WESTERN 

MOUNTAINS 

AND 

HIGH PLAINS 

AQUIFER 

235 

EDWARDS-TRINITY REGIONAL HIGH PLAINS 

AQUIFER 

15 

WESTERN 

MOUNTAINS 

AND 

HIGH PLAINS 

AQUIFER 

295 

Pumpage 

840 

AQUIFER SYSTEM 

PREDEVELOPMENT CONDITIONS 

Values given in thousands of 

acre-feet per year 

Areal 

recharge 

1,840 

Recharge from 

streams 

830 

Discharge 

to streams 

1,395 

EDWARDS-TRINITY REGIONAL 

AQUIFER SYSTEM 

WINTER 1974-75 

Springs 

710 

HIGH PLAINS 

AQUIFER 

15 

Figure 65. Simulated flow components to and from the Edwards-Trinity regional aquifer system in 

west-central Texas, predevelopment conditions and winter 1974-75. 
96 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

thus, the small water-level changes in the Edwards aquifer 

resulting from well discharge have little effect on the rates 

of streambed leakage. Simulated withdrawals in the study 

area of 840,000 acre-ft/yr reduced simulated discharge to 

streams and springs from 2.8 to 2.1 million acre-ft/yr, a 

reduction of about 25 percent. Discharges of several major 

springs in the Edwards and in the Edwards-Trinity aquifers 

have been reduced or eliminated altogether by groundwater development. 

The major part of the Edwards-Trinity aquifer system contains freshwater (dissolved-solids concentration 

less than 500 mg/L), but sizable parts contain marginally 

fresh water (dissolved-solids concentration 500 to 1 ,000 

mg/L) or slightly saline water (dissolved-solids concentration 1,000 to 3,000 mg/L). The predominant water type is 

calcium bicarbonate; however, one of seven other water 

types characterizes the aquifer system in places. 

The median concentration of dissolved solids among 

water analyses from the Edwards aquifer in the Balcones 

fault zone is 297 mg/L. Water in the Edwards aquifer is 

almost exclusively calcium bicarbonate type. The median 

dissolved-solids concentration among analyses from the 

Trinity aquifer in the Hill Country is 537 mg/L. Four bicarbonate and sulfate water types, spread vertically throughout 

the saturated section, characterize most of the Hill Country 

analyses; calcium bicarbonate predominates. The median 

concentration of dissolved solids in the Edwards-Trinity aquifer in the Edwards Plateau is 379 mg/L. Freshwater is 

nearly everywhere in the southern and northeastern parts of 

the aquifer; mostly slightly saline water is in the northwestem part. Water types are distributed in a similar pattern, 

with bicarbonate water nearly everywhere in the southern 

and northeastern parts of the aquifer. Sulfate and chloride 

water types characterize the northwestern part of the Plateau. The median concentration of dissolved solids among 

analyses from the Edwards-Trinity aquifer in the TransPecos is 929 mg/L. Fresh, calcium bicarbonate water predominates in the southern part, and more mineralized 

mixed and sulfate water types are the most common in the 

northwestern part. 

As of August 1992, nine reports have been completed. 

MICffiGAN BASIN 

The Michigan Basin regional aquifer-system analysis 

includes about 29,000 mi2 of the Lower Peninsula of 

Michigan (fig. 66). The aquifer system consists of Mississippian and younger consolidated and unconsolidated sediments. The Marshall Sandstone (Mississippian), sandstones 

within the Grand River and Saginaw Formations (Pennsylvanian), and glacial deposits (Pleistocene) are major 

sources of ground water for supply in the Michigan basin. 

The contact between the Coldwater Shale (Mississippian) 

and the overlying Marshall Sandstone forms the outer 

boundary of the study area. 

Saline water is present in the aquifer system at 

depths from 50 to 1,000 ft below land surface (Westjohn, 

1989), and encroachment of saline water toward pumping 

centers is a significant problem related to ground-water 

supply in the State. In the center of the basin, the Marshall 

Sandstone and lower parts of the Saginaw Formation contain saline water or brine. In the Saginaw Lowland, saline 

water is present in glacial deposits. In the Lansing area, a 

cone of depression that extends over 100 mi2 has developed in the Grand River-Saginaw aquifer. Water levels 

near the center of the cone have been as much as 160 ft 

below the predevelopment level. Saline water has periodically encroached into municipal supply wells in Lansing, 

Flint, Saginaw, Midland, and Mount Pleasant (fig. 66). 

Increased demand for ground water in Michigan is 

anticipated. To manage and protect ground-water resources 

of the Michigan Basin, it is necessary to understand the 

distribution and relations between saline water and freshwater and to determine relations between saline water and 

past and present ground-water flow conditions. The major 

objectives of the Michigan Basin regional aquifer-system 

analysis study are to ( 1) describe the geology of Mississippian and younger geological units, (2) characterize the geochemistry of ground water, (3) delineate the interface 

between freshwater and saline water, and (4) quantitatively 

describe past and present ground-water flow conditions. 

The aquifer system consists of a glacial-drift aquifer 

and three bedrock aquifers separated by confining units 

(fig. 67). Bedrock aquifers, in ascending order, are the 

Marshall, Parma-Bayport, and Grand River-Saginaw aquifers. The Coldwater Shale, which ranges from 500 to 

1,100 ft in thickness, forms the base of the aquifer system. 

The Marshall aquifer is the basal aquifer in the study 

area. It consists of sandstones that overlie the Coldwater 

confining unit as well as sandstones that form the lower 

part of the Michigan Formation (fig. 67). The Marshall aquifer consists of two or more permeable sandstones in the 

central part of the basin, but where the aquifer subcrops the 

glacial deposits, it consists of one permeable sandstone. 

Where more than one sandstone is present, intercalated 

carbonate, shale, siltstone, and (or) evaporite separate permeable sandstones. The composite thickness of permeable 

sandstone is from 7 5 to 225 ft. 

The Michigan confining unit is an intercalated sequence of thin-bedded limestone, dolomite, shale, gypsum, 

anhydrite, and lenses of sandstone. The unit varies in 

thickness from 100ft near the fringes of the subcrop area 

to about 400 ft in the central part of the study area. 

The Bayport Limestone overlies the Michigan confining unit in some areas but is thin or absent in the central part of the basin (fig. 67). In areas where the Bayport 

is absent, the Parma Sandstone is present and overlies the 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 97 

Michigan confining unit. Geophysical Jogs show that the 

Bayport Limestone consists predominantly of permeable 

limestone and sandstone. The Parma Sandstone Member 

and Bayport Limestone combine to form the Parma-Bayport aquifer, a hydraulically connected and stratigraphically continuous permeable unit over most of the basin. 

The Parma-Bayport aquifer is from 75 to 150ft thick. 

The Grand River-Saginaw aquifer, in most parts of 

the basin, consists of lenticular sandstones whose composite thickness is from 50 to 400ft. The Grand River-Saginaw 

44" OLake Winnebago 

§ 

~ .(: 

.::: 

WISCONSIN ~ 

ILLINOIS 

and Parma-Bayport aquifers are separated by the Saginaw 

confining unit, which consists of shale, siltstone, and thinbedded sandstone intercalated with shale. Locally, this confining unit may yield water to wells; regionally, it acts as a 

confining unit. 

Red beds of Late Jurassic age overlie the Grand 

River and Saginaw Formations in the west-central part of 

the study area. The red beds consist of mudstone and 

poorly consolidated red shale, gypsum, and minor amounts 

of sandstone. The red beds, glacial till, and lacustrine clay 

82" 

Georgian 

Bay 

ONTARIO 

------1No1ANA-, ______ _ 

I of---.--s-'-o-r-----'100 MILES I 0 50 100 KILOMETERS 

Base modified from U.S. Geological 

Survey 1:500.000 map 

Figure 66. Study area of the Michigan Basin regional aquifer-system analysis. 

OHIO 
98 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

form the glacial till-red beds confining unit that locally 

separates the glacial drift aquifer from the underlying 

Grand River-Saginaw aquifer. 

Glacial deposits mantle nearly all of the study area. 

They are from 10 to 200 ft, 200 to 400 ft, and 400 to 

1 ,000 ft thick in the southern, central, and northern parts, 

respectively. Coarse-grained glacial deposits are grouped 

to form the glacial drift aquifer. 

Ground water in glacial drift generally has dissolved-solids concentrations less than 1,000 mg!L. However, dissolved-solids concentrations in water in glacial 

deposits in the Saginaw Lowland commonly exceed 1,000 

mg!L. Water in the Grand River-Saginaw, Parma-Bayport, 

and Marshall aquifers is more saline at the center of the 

basin than at or near subcrops. Dissolved-solids concentrations in water from the Parma-Bayport and Marshall aquifers in the central part of the basin exceed 200,000 and 

300,000 mg!L, respectively. 

Electrical resistivity logs from oil exploration holes, 

chemical analyses of water samples, and surface geophysi-

'tl .c 0 (,) 

cal data were used to construct a map of the altitude of the 

base of freshwater (fig. 68). Freshwater is defined as water 

having concentrations of dissolved solids less than 1,000 

mg!L. Thickness of freshwater varies from about 1,000 ft in 

the northwest to less than 100 ft in the Saginaw Lowland. 

In places where the Marshall aquifer is fractured and 

in hydraulic connection with glaciofluvial deposits, transmissivity is from 130 to 27,000 ft2

/d. Estimated hydraulic 

conductivities are from 10 to 550ft/d. Analyses of tests of 

selected intervals in test holes indicate that hydraulic conductivity of the confined Marshall aquifer is from about 

0.17 to 0.69 ft/d away from the subcrop area. Laboratory 

analyses of Marshall Sandstone cores indicate that intrinsic permeability is from 0.01 to 4 ft/d. These data show 

that the permeability of the Marshall aquifer decreases 

markedly from the subcrop area toward the center of the 

basin. 

The Grand River-Saginaw aquifer is productive 

where lenticular sandstones are thickest, as in the southcentral part of the State near Lansing. Transmissivity val-

«< 

w 'i: 0 Glaciation Stratigraphic unit Hydrogeologic unit Q) Q. 

D. w 

Q) 

c::: 

Q) 

u 

0 

> 0 u ro I Glacial drift ·a c::: N 

~ aquifer 0 

c::: co Q) Q) :J c::: u 0 Q) Wisconsin u 

B Illinoian C/l 

'iii Pre-Illinoian 

0: 

u u Glacial till-red beds '2 'iii 

0 C/l .2l Unnamed confining unit ~ co C/l _J Q) red beds :J 

:::?: --, 

c::: Q) 

co =c Grand River Formation ·c: "C Grand River-Saginaw co 

> ~ aquifer > C/l 1-- c::: 

c::: > Saginaw Formation Q) Saginaw confining unit ;:: a.. co 

UJ Parma Sandstone Member 

Bayport Parma-Bayport 

u .2l Grand Limestone aquifer 

·a co N _J Rapids 0 Michigan confining unit Group Michigan Q) 

co c::: Formation Stray Sandstone Member a.. co 

·a. Napol'f.S?~J~~?stone Marshall aquifer Q. Marshall Sandstone 'iii 

C/l 

'iii 

C/l 

> ~ ;:: 

co 

UJ Coldwater confining Coldwater Shale unit 

Figure 67. Relation between stratigraphic and hydrogeologic units in the Michigan Basin (modified 

from Mandie and Westjohn, 1989). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 99 

ues from aquifer tests range from 300 to 13,500 ft2/d. Estimated hydraulic conductivity is from I to IOO ft/d. 

Transmissivity values from aquifer tests conducted in 

wells completed in glacial deposits are from 650 to 82,000 

ft2/d. Estimated hydraulic conductivity is from less than 

50 to more than 500 ft/d. 

A generalized water-table map of Michigan's Lower 

Peninsula shows the presence of two areas of relatively 

high water-table altitude, which reflect the higher landWISCONSIN 

42° 

surface altitude of the northern and southern uplands (fig. 

69). In the northern uplands, maximum altitude of the 

water table is between I ,200 and I ,300 ft; in the southern 

uplands, maximum altitude is about I,OOO ft. The water 

table slopes away from these areas either toward the Great 

Lakes or toward an elongated northeast-southwest-trending 

depression in the water table. This depression is in the 

pro-glacial Grand River valley, the site of the present-day 

Grand, Maple, and Saginaw Rivers. 

Georgian 

Bay 

ONTARIO 

ILLINOIS MICHIGAN 

; ------INoiANA-~----

1 OI---.--S.LO_,--__ __J100 MILES 

Base modified from U.S. Geological 

Survey 1:500,000 map 

50 100 KILOMETERS 

EXPLANATION 

-500- Base of freshwater contour-Shows 

altitude of base of freshwater. Contour 

interval 100 feet. Datum is sea level 

Figure 68. Altitude of base of freshwater in the Michigan Basin (from Westjohn, 1989). 

OHIO 
100 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

The distribution of equivalent freshwater head in 

the Grand River-Saginaw aquifer (fig. 70) is similar to 

the distribution of head shown on the water-table map 

(fig. 69). However, head values for the Grand River-Saginaw aquifer are lower in the uplands and higher in the 

lowlands than heads in the glacial drift aquifer. This suggests that freshwater recharges the Grand River-Saginaw 

aquifer in the uplands, then flows toward the lowlands 

44' OLake Winnebago 

§ 

.$ 

..c: 

.:: 

WISCONSIN ~ 

where it discharges upward into the glacial drift aquifer. 

Preliminary evaluation of data indicates that heads in the 

Marshall aquifer are greater than those in the Grand 

River-Saginaw and glacial drift aquifers in much of the 

area. It is not likely that the Marshall aquifer is recharged 

by freshwater in the northern uplands area. At and near 

the southern subcrop, local flow systems control groundwater flow. 

82' 

ONTARIO 

Georgian 

Bay 

ONTARIO 

42' ~" ~,ooo~ 

_ ('~ e~I~HIG~~~1 ___ _ ILLINOIS 

: INDIANA ~---

1 Of-- -..,--...1.50-r-----'100 MILES 

Base modified from U.S. Geological 

Survey 1:500,000 map 

50 100 KILOMETERS 

EXPLANATION 

-800- Water-table contour-Shows altitude of 

water table. Contour interval 200 feet. 

Datum is sea level 

OHIO 

Figure 69. Generalized water table in the glacial drift aquifer of the Lower Peninsula of Michigan (from Mandie and Westjohn, 

1989). 
PART II. STATIJS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 101 

A finite-difference computer program (McDonald 

and Harbaugh, 1988) was used to simulate three-dimensional ground-water flow. Modifications to the program 

were made to allow simulation of variable-density flow. 

The modifications and computer program are similar to 

those developed by Kontis and Mandie (1988). The aquifer system was discretized into five layers with four intervening confining layers. The Marshall, Parma-Bayport, 

Grand River-Saginaw, and glacial drift aquifers were each 

represented by one layer in the model. The intervening 

glacial till-red beds, Saginaw, Michigan, and Coldwater 

confining units also were simulated. The modeled area 

44. 

42. 

OLak Winnebago e 

§ 

.$ 

-s 

WISCONSIN ~ 

ILLINOIS MICHIGAN 

------INDIANA-~---

/ OJ---..--...J50c__,-----'100 MILES 

50 100 KILOMETERS 

Base modified from U.S. Geological 

Survey 1:500,000 map 

EXPLANATION 

-900- Line of equal hydraulic head corrected to 

freshwater density (1. 94 slugs per cubic 

foot at 60°F). Interval 100 feet 

Line of contact between Saginaw Formation 

and Bayport Limestone 

82. 

ONTARIO 

OHIO 

Georgian 

Bay 

ONTARIO 

Figure 70. Generalized equivalent freshwater head in the Grand River-Saginaw aquifer of the Michigan Basin (from Mandie 

and Westjohn, 1989). 
102 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

was extended to incorporate the adjacent Great Lakes as 

lateral-flow boundaries. The Traverse aquifer, a bottom 

layer representing the Devonian Traverse Limestone and 

underlying older carbonate rocks, was added to simulate 

the perimeter and lower boundary. This addition allowed 

simulation of ground-water flow across the Coldwater confining unit. The upper boundary for the flow model was 

the water table, which was represented by a constant-head 

boundary for these simulations. 

Preliminary computer simulation produced freshwater 

head distributions for the Grand River-Saginaw aquifer that 

were similar to predevelopment heads. Simulation indicates 

that a downward component of flow exists in upland areas 

and an upward component of flow exists in the pro-glacial 

Grand River valley and in the Saginaw and Michigan Lowlands (Mandie and Westjohn, 1989). These simulated flow 

patterns suggest that the presence of saline ground water in 

the Saginaw and Michigan Lowlands may be the result of 

upwelling of deeper saline water that is part of the regionalflow system. 

This on-going study was started in 1985. As of July 

1992, 11 reports had been completed. 

MIDWESTERN BASINS AND ARCHES GLACIAL AND 

CARBONATE REGIONAL AQUIFER SYSTEM 

The Midwestern Basins and Arches glacial and carbonate regional aquifer-system analysis was called "The 

Ohio-Indiana carbonate-bedrock and glacial-aquifer-system 

analysis" in previous reports. This new name was selected 

because it generally reflects the geological province recognized by many geologists and hydrologists and is in accordance with the nomenclature described by the Committee 

on Stratigraphic Units of North America of the American 

Association of the Petroleum Geologists (AAPG COSUNA). 

The study area also extends beyond the previously delineated 

areas to include parts of Illinois and Michigan and the entire 

ground-water flow system (fig. 71). 

The study area lies between the Appalachian Plateaus physiographic province to the east, the Illinois structural basin to the west, the Michigan structural basin and 

Lake Erie to the north, and the Ohio River to the south 

(fig. 71). The study area is mostly within the Central Lowland physiographic province, east of the Mississippi River, 

and includes about 43,000 mi2. 

The regional aquifer system consists of a single massive Silurian and Devonian carbonate rock aquifer and overlying hydraulically connected aquifers in unconsolidated 

Quaternary glacial deposits. Throughout most of the study 

area, the carbonate rock aquifer subcrops beneath glacial 

deposits (figs. 71 and 72). The lower boundary of the aquifer 

system is in Upper Ordovician rocks that include undifferentiated Cincinnatian rocks in northwestern and west-central 

Ohio and the Maquoketa Group in Indiana. These rocks are 

mostly shale, cherty and argillaceous limestone, and finegrained clastic rocks. They are characterized by low permeability and considered to be the base of the aquifer system. 

The aquifer system extends laterally and downdip from the 

northern, eastern, and western margins of carbonate rock 

subcrop areas. In central Ohio within a relatively short distance downdip from the Silurian-Devonian subcrop area, 

water in the carbonate rocks has a dissolved-solids concentration of more than 10,000 mg/L (fig. 73). The interface 

between fresher ground water and the ground water having 

concentrations of dissolved solids of 10,000 mg/L or more 

is sharp. This abrupt transition occurs because the thickness 

of an overlying Devonian shale increases substantially within a relatively short distance from the rock subcrop, and the 

vertical recharge of freshwater from glacial deposits to the 

carbonate rocks decreases. However, on the western side of 

the study area, the transition zone from freshwater to water 

having dissolved-solids concentration of 10,000 mg/L is 

wider and extends farther from the Silurian-Devonian subcrop limit into the Illinois structural basin than does the 

transition zone on the eastern side of the area (fig. 73). In 

this area (western side) the Devonian shale is not as thick 

as on the eastern side. Along the northern side of the study 

area, the 10,000 mg/L dissolved-solids boundary is intermediate in distance from the Silurian-Devonian subcrop area 

compared with the eastern and western sides. The southern 

hydrologic boundary is the Ohio River. The upper boundary 

of the regional aquifer system is the water table in glacial 

deposits. 

Silurian rocks of the aquifer system are predominantly limestone and dolomite, interbedded with thin beds 

of shale and gypsum. The carbonates are typically massive, crystalline dolomite and grade locally into reef structures. Brecciated dolomites, shaly dolomites, fossiliferous 

and cavernous limestones and dolomites also are present 

as a result of variations in depositional environments and 

postdepositional erosion and weathering. Devonian rocks 

are predominantly limestone that display karst features in 

some places. A unit of thick shale in the Upper Devonian 

rocks marks the upper hydrogeologic boundary of the 

Silurian-Devonian rock aquifer. 

In western Ohio and eastern Indiana, the SilurianDevonian rock aquifer receives recharge from glacial 

deposits in subcrop areas. Elsewhere, water in glacial deposits discharges locally to streams, except for a small 

amount of water that infiltrates to the underlying rock aquifer through confining units. Water in rock aquifers is 

mostly under confined conditions, except where the rocks 

are close to or exposed at land surface. Water in the rock 

aquifer generally flows toward major rivers, to Lake Erie, 

and to subcrop areas along the Scioto Valley and the Illinois structural basin. 

Lateral discontinuity of glacial deposits and variations in secondary permeability of the rocks are major 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 

82° 

37°L-------~--~---------------L--------------~----------------~--------~ Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1912 Geology compiled and modified 

from Bownocker, 1920 and 

Gray and others, 1987 

I r- ~ ~ 

Aquifer-system 

<(I 

boundary ~ 

A ~~~ ~ !2 A' ...J o ali FEET '"I~ ~ o 

SEALt:i~~ '"005 VERTICAL SCALE GREATLY EXAGGERATED #~~z== ~ ~ 

EXPLANATION 

Geologic Units 

8 

0 0 

~II IIZ u 0 0 <( 

~~~ ~~~ z ~ ~ u 

~lo ol~ ~~Aquifer-system boundary---, ~ 

FEET ),( y 

SEA Ll~~~ 2:ooo~~·~ 1000 ,.·,·~·-·- •• ,-_,' ~-'' ... ·."·...... • ••• • •• 

VERTICAL SCALE GREATLY EXAGGERATED 

8' .. Permian c=J Silurian 

~ Pennsylvanian L§] Ordovician -

.. Mississippian ~ Cambrian 

CJ Devonian CJ ' Precambrian - . 

A-A' Line of section 

Boundary of study area .._. Direction of basin slope 

Figure 71. Study area and generalized geology of the Midwestern Basins and Arches glacial and carbonate regional aquifersystem analysis. 

103 
104 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

concerns in planning the development of ground water in 

the study area. Reported transmissivity of the rocks is 

from 70 to 28,000 ft2

/d in Ohio and from 300 to 52,000 

ft2

/d in Indiana. Glacial deposits in the study area have a 

wide range of hydraulic conductivity, depending on their 

texture and lithology. Differences in hydraulic conductivi43" 8~6"--------~84 -----~ 

90" 88" 

41 " 

ILLINOIS 

Lake 

Michigan 

M I CHIGAN 

Base modified from U. S. Geological Survey 

digital data, 1:2,000,000, 1972 

EXPLANATION 

[ill Recessional moraine 

~ Lake deposits 

c=J Ground moraine or till 

Subcrop limit of Silurian and Devonian 

bedrock beneath glacial deposits 

Southern limit of glaciation 

82" 

,...- __ ,. 

/ 

~'-/ 

0 H) I 0 

I 

so 

( 

\ 

I 

/ 

100 MILES 

SO 100 KILOMETERS 

Figure 72. General distribution of glacial deposits, Midwestern Basins and Arches regional aquifer-system analysis. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 105 

ty cause differences in rates of ground-water movement 

and in the amount of recharge to underlying rocks. Locally, the hydraulic conductivity of glacial deposits can differ 

by several orders of magnitude, even within the same type 

of deposits. Generally, transmissivity of outwash is from 

300 to 70,000 ft2/d in Indiana and from I ,300 to 60,000 

ft2/d in Ohio. The vertical hydraulic conductivity of clayrich till is from 3.7xlo-l to 3.0x10-3 ftld in Indiana 

43· 8~6~·---------------;84~·------------7 

so• 

MICHIGAN 

82° 

ILLINOIS 

0 HI 0 

\ 

I 

' Js· f--------+-+_: ----w~--+.:::::~-t---1-~t---n \ 

I 

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972 

\ 

' ' 

KENTUCK 

50 100 MILES 

50 100 KILOMETERS 

EXPLANATION 

Area dominated by CaS04 type 

water-Total dissolved-solids 

concentrations commonly greater 

than 1,000 milligrams per liter 

Area dominated by Ca-Mg-HC03 

type water-Total dissolved-solids 

concentrations commonly less 

than 1,000 milligrams per liter 

Boundary of total dissolved-solids 

concentration-Approximately 

10,000 milligrams per liter 

Subcrop limit of Silurian and 

Devonian rocks beneath glacial 

deposits 

Figure 73. General distribution of dissolved-solids concentration in water from the Silurian-Devonian carbonate rock aquifer. 
106 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

(Fleming, 1989) and from 1.17x1o-3 (Strobel, 1990) to 

4x1o-3 (Norris, 1959) in northwestern Ohio. These values 

are one to five times greater than model-generated hydraulic conductivity values normally reported from other parts 

of the study area. 

Quality of water in the carbonate rock aquifer varies 

spatially and with depth depending on the lithology and 

occurrence of evaporitic deposits, such as gypsum, especially in the northeastern part of the study area. Generally, 

the quality of water in glacial deposits is good. The concentration of dissolved solids in water in the Silurian-Devonian rock aquifer normally is less than 1,000 mg/L in 

the western and central part of the study area and ranges 

from 1,000 to 3,000 mg/L in the northeastern part of the 

area (fig. 73). 

In 1980, more than 280 Mgal/d of ground water was 

pumped to supply industry, agriculture, and a population 

of more than 6.3 million people. Most of the water was 

withdrawn from glacial deposits. The carbonate rock aquifer has not been affected by pumpage from wells, except 

in northwestern Indiana, where irrigation causes seasonal 

water-level declines. 

The study was started in 1988. As of mid-1992, six 

reports were published and five reports were undergoing 

technical review. 

NORTHERN ROCKY MOUNTAINS INTERMONTANE BASINS, 

MONTANA, AND IDAHO 

The Northern Rocky Mountains Intermontane Basins 

RASA is an investigation of hydrogeologic conditions of 

intermontane basin-fill aquifers in western Montana and 

central and northern Idaho. Geographically, these basins 

exist separately; hydrogeologically, they share many characteristics because of their common structural and depositional histories and physiographic settings. The study area 

is about 80,000 mi2 and extends westward from the Northem Great Plains in Montana to the Columbia Plateau in 

western Idaho, and northward from the Snake River Plain 

in Idaho to the United States-Canada border (fig. 74). 

On the basis of stratigraphy and geologic structure, 

about 70 basins were delineated in the study area (fig. 75). 

Basins range in size from 10 to 400 mi2 and are filled with 

as much as several thousand feet of unconsolidated to 

poorly consolidated Tertiary and Quaternary continental 

sediments. 

The Continental Divide (fig. 74) separates the study 

area into two major drainage systems: the Missouri River 

drainage to the east and the Columbia River drainage to 

the west. Major streams in the Missouri River drainage 

include the Big Hole, Jefferson, Madison, and Gallatin 

Rivers. Major streams in the Columbia River drainage include the Snake, Salmon, Clearwater, Spokane, Clark 

Fork/Pend Oreille, Blackfoot, Bitterroot, Flathead, and 

Kootenai Rivers (fig. 74). Annual precipitation on the 

study area ranges from about 8 in. in basins in east-central 

Idaho to about 100 in. in some mountainous parts of Montana. Most valleys receive about 10 to 30 in. of precipitation per year; more than 50 percent of the annual 

precipitation falls during November to April as snow. 

Most of the intermontane basins of the northern 

Rocky Mountains were formed by Basin and Range tectonics during the Miocene Epoch, creating an area of 

broadly distributed crustal extension characterized by hundreds of normal faults. As the crust extended, downthrown 

fault blocks became basins, while upthrown fault blocks 

became the intervening mountains (Eaton, 1979). In general, most of the basins trend northerly to northwesterly, 

roughly parallel to basin-margin normal faults and perpendicular to the regional extension. 

The intermontane basins subsided intermittently 

throughout the Tertiary Period, resulting in continental deposits as much as 16,000 ft thick. In southwestern Montana, these deposits are assigned to the Bozeman Group, 

composed of the Renova Formation and the overlying 

Sixmile Creek Formation. Names for the Tertiary deposits 

vary within the study area; however, the stratigraphy is 

generally correlative on a regional basis. Historically, the 

lower predominantly fine-grained deposits have been 

named the Renova Formation, whereas the overlying predominantly coarse-grained deposits have been named the 

Sixmile Creek Formation. However, recent work on the 

stratigraphy of the Tertiary deposits indicates that fine- and 

coarse-grained deposits occur in both formations (Hanneman and Wideman, 1991 ). Both the Renova and Sixmile 

Creek Formations consist of lacustrine and fluvial sediments that exhibit abrupt lateral and vertical facies changes. The Renova Formation is generally considered to be 

too deep and not sufficiently permeable to be a viable aquifer. In contrast, the uppermost gravels of the Sixmile 

Creek Formation form the most productive deep aquifer in 

the basins with transmissivities of about 1,000 ft2

/d. 

Quaternary fluvial and glacial deposits overlie some 

Tertiary deposits. Quaternary deposits are primarily alluvium along streams and colluvial and alluvial-fan deposits 

near mountain flanks. However, in the north, the basins 

were subjected to repeated glacial advances, inundation by 

glacial lakes, and, lastly, alluvial deposition. Glacial deposits are from a few feet to hundreds of feet thick. 

Unconsolidated Holocene alluvial deposits overlie 

glacial deposits in the north and Tertiary deposits in the 

south. The most significant water-yielding deposits are 

streambed alluvium having thicknesses of 10 to 200 ft 

and, to a lesser degree, alluvial fans along the basin margins having thicknesses of as much as 500 ft. Alluvial aquifers have typical transmissivities of about 10,000 to 

100,000 ft2/d. 

Flow systems in basin-fill aquifers in many intermontane basins are largely isolated with little or no known 

hydraulic connection between unconsolidated deposits of 

adjacent basins (Clark and Kendy, 1992b). However, 

through-flowing streams provide a significant hydraulic 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 107 

connection between basins. Ground water in one basin discharges to a stream, which flows to the next basin where 

the water may recharge the underlying aquifers. Ground 

water may flow between some basins through locally permeable bedrock or along faults having zones of increased 

permeability. 

I 

\ 

EXPLANATION 

Study area 

Drainage basin boundary 

25 

I I 

50 MILES 

25 50 KILOMETERS 

NORTHERN 

GREAT 

PLAINS 

ONTANA 

Figure 74. Study area of the Northern Rocky Mountains Intermontane Basins regional aquifer-system 

analysis. 
108 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Basin-fill aquifers are primarily recharged by stream 

infiltration and subsurface inflow from surrounding bedrock along the mountain fronts, and to a minor extent by 

direct infiltration of precipitation. Infiltration of applied irrigation water and seepage from irrigation canals also provide recharge in some basins. 

The direction of flow in the alluvial aquifers generally follows the topography, with flow components being 

1120 1110 114° 113° __ ---r ,----r- , 

MONTANA 

area boundary 

-1 

I 

\ 

I~ 

~ 1>-

25 

~.)· 1---rl-'-1 ,

1 _ _.I I 

25 50 KILOMETERS 

Figure 75. Intermontane basins in the northern Rocky Mountains (shaded). 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 109 

toward major streams and downvalley. The direction of 

lateral flow in deep aquifers may be different than in the 

shallow aquifers in response to confining strata, faulting, 

or other deep structural controls. Depth to water ranges 

from land surface to hundreds of feet below land surface 

in topographically high areas near basin margins. 

The Northern Rocky Mountains Intermontane Basins 

RASA study was started in 1990. As of early 1993, three 

reports have been published. The study will provide hydrogeologic information on the intermontane basin-fill aquifers in western Montana and central and northern Idaho. 

An important element of the study is to develop a geographically indexed data base of hydrogeology and related 

information including geology, land and water use, soil 

types, climatic data, physical features, water levels, water 

quality, aquifer boundaries, and hydraulic characteristics 

of aquifers (Clark and Kendy, 1992a). The other important 

elements are to develop methods .of evaluating water resources particular to the intermontane basins of the northem Rocky Mountains. The common geologic history 

shared by the basins is the key to their related hydrogeology and to this study. 

PUGET-WILLAMETTE LOWLAND, 

WASHINGTON, OREGON, AND CANADA 

The Puget-Willamette Lowland (PWL) aquifer system is in western Washington, western Oregon, and a small 

part of southwestern British Columbia, Canada (fig. 76). 

The study area extends from near the Fraser River, British 

Columbia, Canada, to just south of Cottage Grove, Oregon. 

The PWL is composed of two distinct areas, the Puget 

Sound Lowland (PSL) and the Willamette Lowland (WL). 

The PSL study includes about 17,600 mi2

; the WL study 

includes about 5,680 mi2. About 2,500 mi2 of the PSL 

study area is covered by seawater. The two study areas 

include Puget Sound and the Willamette River, which drain 

parts of the Cascade Range, the Coast Range, and the 

Olympic Mountains. The extent of the aquifer system to be 

studied in the PSL is defined by the area underlain by Quaternary sediments (7 ,200 mi2); in the WL, it is the area 

underlain by Quaternary sediments and upper Tertiary sediments and volcanics ( 4,100 mi2). Both of the aquifer systems include several structural ground-water basins. 

A complex geologic history has resulted in highly 

variable aquifer lithology. Alluvium, glacial, and interglacial sediments compose the aquifer system in the PSL and 

have been named the Puget Sound aquifer system. These 

deposits consist principally of recent alluvium, recessional 

and advance outwash, till, and other glaciofluvial and interglacial sediments. In the WL, alluvium basin-fill sediments 

and basalt materials compose the aquifer system and have 

been named the Willamette Lowland aquifer system. Older 

Tertiary and pre-Cenozoic sedimentary, volcaniclastic, volcanic, and metamorphic rock units define both the lateral 

and the basal boundaries of the aquifer systems in both 

areas and have been named the basement confining unit. 

About 70 percent of the population of Washington 

and Oregon reside in the study areas, mainly in the metropolitan areas of Bellingham, Everett, Seattle and vicinity, 

Tacoma, and Olympia in the PSL; and Vancouver, Portland 

and vicinity, Salem, and Eugene in the WL. The burgeoning population is increasing the demand for available water. In some areas, available water supplies are already 

fully appropriated. Some supplies are limited due to contamination from anthropogenic sources and to saltwater intrusion from Puget Sound in the PSL and by older brackish 

ground water in the WL. 

The PWL is a structural basin that extends from the 

Coast Mountains just north of the Fraser River in British 

Columbia to about Cottage Grove in central-western Oregon (fig. 76). The PWL is bordered by the Cascade Range 

on the east and the Coast Range and Olympic Mountains 

on the west. North of the Olympic Mountains, the western 

boundary is defined by the Canadian-Washington border 

located in the waters south and east of Vancouver Island, 

British Columbia. The southern boundary of the PSL is 

approximately defined by the extent of Pleistocene glaciation, which is characterized by a series of low hills near 

the divide between the Puget Sound drainage and the Chehalis River basin. The northern boundary of the WL is in 

Cowlitz County, Washington, and is defined by an outcrop 

of the Columbia River Basalt Group that extends northward along the Columbia River from the north end of the 

Portland basin. The southern boundary of the WL is a line 

of foothills where the Coast and Cascade ranges merge. 

Altitudes of the valley floors or lowlands are from 

sea level to about 500 ft. The foothills or uplands have 

altitudes from 500 to about 1,500 ft. The altitude of the 

crest of the Cascade Range averages about 7,500 ft on the 

north to about 4,500 ft on the south; a series of stratovolcanoes in the Cascade Range rise from 8,000 to 14,000 ft 

above sea level. Altitude of the Olympic Mountains in the 

northwestern part of the PWL averages about 3,000 ft. 

In the PSL, lowlands typically are alluvial river valleys with glacial outwash and till plains. These lowlands 

are separated from the bordering mountains by uplands 

with rolling hills and terraces. The transition from uplands 

to mountains generally is abrupt, except near the southern 

boundary. WL topography is interrupted in places by narrow, north- and northwest-trending, continuous basaltic 

ridges, which divide the north-central part of the valley 

into unequal-sized east and west segments. The topography in southwest Portland and northern Clackamas County 

also is interrupted by numerous irregular-shaped, low volcanic hills. Geologic structures in the WL divide the valley into four structural and topographic basins. 

The PWL has a mid-latitude humid, Pacific Coast 

marine climate due to eastward-moving Pacific Ocean derived air masses. The adjacent mountains provide protection 
110 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

from southward moving Canadian cold-air masses and from 

Pacific Ocean winter storms. As a result, both summer and 

winter temperatures are moderated, there is a distinct winter 

precipitation season and summer dry season, and altitude 

and location have a large influence on the distribution of 

precipitation and temperature. 

On lowlands of the PSL, precipitation varies from 

about 15 in/yr in the "rain shadow" of the Olympic Mountains to about 60 in/yr near the Olympic foothills. On lowlands of the WL, precipitation varies from about 37 in/yr 

near Portland, Oregon, to about 55 in/yr near the Cascade 

foothills. Precipitation on mountains bordering each area 

is from 50 to 200 in/yr. Most precipitation falls during the 

winter and is generally rain at altitudes of about 1,000 to 

1,500 ft, rain and snow at altitudes up to about 2,500 ft, 

and snow at higher altitudes. 

In the PSL, annual surface-water runoff from an area 

of about 13,700 mi2 averages about 46,000 ft3/s (45 in/yr). 

100 KILOMETERS 

--- ---------- ___ .. 

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972 

Figure 76. Study areas of the Puget-Willamette Lowland regional aquifer-system analysis. 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 111 

Runoff from islands in northern Puget Sound, which represents less than 3 percent of the total PSL land area, averages about 10 in/yr. Runoff from the Willamette River 

drainage averages about 33,000 ft3/s (40 in/yr) from an 

area of about 11, 100 mi2. Runoff within selected basins in 

both study areas ranges from about 35 to 80 in/yr. 

The Puget Sound aquifer system is composed of unconsolidated deposits as much as 3,300 ft thick and consists 

of materials deposited by ( 1) advancing and retreating continental glaciers, (2) numerous alpine glaciers emanating 

from the adjacent Cascade Range and Olympic Mountains, 

and (3) rivers and relatively quiet water of lakes and bays 

during interglacial episodes, when much of the lowland consisted of saltwater embayments and freshwater lakes. Each 

of these depositional regimes resulted in complex lithostratigraphic patterns and corresponding geologic units that are 

difficult to correlate on a regional scale. The complexity 

within each depositional sequence was compounded by the 

erosive action of continental glaciers. 

In a regional context, three types of deposits are associated with continental glaciers-advance outwash, till, 

and recessional outwash. The lateral and vertical extent of 

each type of deposits is best known for Vashon Drift deposited during Fraser Glaciation. Vashon Drift is relatively 

undisturbed and close to land surface. 

The oldest rocks in the Willamette Lowland are early 

Tertiary marine sediments and subordinate marine volcanics, which crop out chiefly in foothills of the Coast Range 

along the western margin of the valley. These rocks dip 

gradually eastward beneath the younger basin-fill sediments 

and, in part, grade eastward into the Tertiary volcaniclastic 

and volcanic rocks of the Western Cascades. Collectively, 

marine rocks of the Coast Range and volcanic and volcaniclastic rocks of the Western Cascades generally have low 

permeability and are poor aquifers (McFarland, 1983). 

These rocks are thousands of feet thick beneath the valley 

and form the base to the overlying Willamette Lowland 

aquifer system. 

In the central part of the WL, basement rocks are 

overlain by Miocene volcanic rocks of the Columbia River 

Basalt Group. Basaltic lava was erupted from linear vent 

systems in northeastern Oregon, southeastern Washington, 

and western Idaho. The lava was deposited as a series of 

areally extensive thin sheets. Each flow followed topographic or structural lows and topped many intervening 

low hills. As a consequence, some flows in eastern and 

western Oregon are thickest in paleovalleys and thin 

across hills. The basalt underlies younger formations and 

is as much as 1,000 ft thick. 

Overlying the basalt in the northern part of the WL 

is a sequence of Quaternary and Tertiary basin-fill fluvial 

sediments. In the remainder of the lowland, where the basalt 

is absent, the Quaternary-Tertiary sequence lies directly on 

basement rock. The lower part of the basin-fill sediments 

consists of fine-grained clay and silt to mudstone and siltstone, which grade upward into sandy and gravelly units. 

Sediments become finer grained in a westerly and northwesterly direction, away from the eastern valley margins. 

These sediments appear to have been deposited as a series 

of low, broad coalescing alluvial fans issuing primarily from 

the Cascade Range on the eastern side of the valley, with 

lesser fans issuing from the Coast Range on the western 

side of the valley. The combined thickness of these sediments 

is greatest in the northern part of the WL, where geologic 

structures have greater amplitude. 

Valley lowlands below an altitude of about 400 ft 

were inundated during the Pleistocene by floods caused by 

periodic breaching of an ice dam in Idaho (Bretz, 1923; 

Waitt, 1985). Breaching of the ice dam released enormous 

volumes of glacial meltwaters stored in glacial Lake Missoula, an ice marginal lake in Montana (Pardee, 1910). 

The waters flooded large areas adjacent to the present Columbia River, Willamette Valley, and the Columbia River 

valley downstream from present-day Portland. These 

floods spread across the Portland basin, where they 

scoured older deposits, deposited new sheets and deltas, 

and formed backwater lakes. In the Portland basin, sand 

and gravel deltas formed in lakes where currents were sufficiently swift, whereas fine-grained deposits (fine-grained 

sands and silts) were deposited in slack-water areas and in 

the remainder of the area inundated by lakes. 

Youngest deposits in the WL are the shallow alluvial 

deposits along principal streams. Along the flood plain of 

the Columbia River, these deposits may be as much as 300 

ft thick and consist chiefly of sand, with some sand and 

gravel, as well as finer grained overbank deposits consisting of silts and fine sands. Along most of the Willamette 

River flood plain, the alluvial deposits consist chiefly of 

sand and gravel as much as 70 ft thick and averaging 

about 50 ft; in places, they are covered by thin, finegrained overbank deposits. 

In the PSL, ground water generally moves from glaciated uplands fronting the Olympic Mountains and Cascade Range to streams and seawater bodies. Movement is 

predominantly lateral in aquifers and vertically downward 

toward deeper units in high-altitude areas. The gradient is 

upward from deeper aquifers in low-altitude areas and 

near streams and seawater bodies. 

In the upper aquifers in the PSL, lateral hydraulic 

gradient generally is from 30 to 60 ft/mi; however, in 

deeper units it is from 5 to 10 ft/mi. Hydraulic conductivity of glacial aquifers generally is from 30 to 100 ft/d, but 

hydraulic conductivity of coarse-grained alluvial aquifers 

is from 100 to 200 ft/d. 

Quality of ground water in the PSL is suitable for 

most uses. The dominant water types are calcium bicarbonate and magnesium bicarbonate in upper aquifers. 

Deeper aquifers contain water of a sodium bicarbonate and 

sodium chloride type. The sodium chloride water is associated with mixing of seawater. 
112 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

In the WL, most regional ground-water flow occurs in 

the basin-fill sediments and overlying Quaternary deposits; 

significant amount of flow also occurs in the Columbia River 

·Basalt Group. Most water in the basalt occurs in the interflow zones between successive basalt flows. Interflow zones 

consist of very porous, highly fractured basalt and flow-top 

breccias and, sometimes, interbedded coarse-grained sediments. The basalt has high lateral permeability in interflow 

zones and low permeability across flow interiors. Basalt can 

yield moderate to large amounts of water to wells. Increased 

withdrawal of ground water from the basalt has occurred in 

recent years; however, the basalt is still largely undeveloped 

in many places because it is more economical to construct 

wells in the overlying basin-fill sediments. 

Generally, basin-fill sediments are the most important aquifers in the Willamette Lowland aquifer system. 

However, except for the Portland basin, no regional hydrologic information on the characteristics of these basin-fill 

sediments is available. 

The Puget-Willamette Lowland regional aquifer-system analysis was started in 1989. As of mid-1992, three 

reports had been completed. 

SOUTHERN CALIFORNIA BASINS 

The regional aquifer-system analysis study of the 

southern California basins covers an area of 75,000 mi2 

(fig. 77). The study area includes 89 hydrologic units as 

defined by the California Department of Water Resources 

(1969) (fig. 77) and referred to in this report as drainage 

basins. These basins can be grouped according to common 

characteristics and relations into coastal and desert basins. 

The objective of the Southern California Basins 

RASA study is to analyze the major problems and issues 

that affect the use of ground water in southern California 

including ( 1) ground-water overdraft, (2) ground-water 

contamination, (3) seawater intrusion, (4) quantity and distribution of recharge, (5) interaquifer flow, and (6) conjunctive use of ground water and surface water. Because 

of the large size of the study area and the large number of 

basins involved, it is impractical to study these problems 

and issues in each basin. Therefore, selected basins will be 

studied to determine the major geohydrologic processes 

and human activities that control or influence these problems or issues. At least one coastal and one desert basin 

will be intensively studied. 

The studies involve assembling available geohydrologic data into a geographic information system (GIS), defining the regional geohydrology and geochemistry, and 

developing ground-water flow and solute-transport models 

to help understand the ground-water flow systems. Information obtained from these intensive studies should aid in 

the effective management of the ground-water resources of 

these and other basins in southern California. 

The region has a varied climate, ranging from very 

wet to very dry; however, the entire region shares the 

characteristic of dry summers. Precipitation varies markedly from year to year, and the region is subject to relatively 

long periods of time in which precipitation and runoff are 

significantly above or below the long-term mean. Because 

of the variability of precipitation and runoff, ground water 

is an extensively used resource throughout the region. 

The drainage basins include an assemblage of waterbearing and non-water-bearing materials. In coastal basins 

the non-water-bearing materials consist predominantly of 

folded and faulted sedimentary and metamorphic rocks. 

Water-bearing materials are in structural lowlands or valleys and consist of unconsolidated to poorly consolidated 

deposits of gravel, sand, silt, and clay. These deposits are 

from less than 100 ft to more than several thousand feet 

thick and form the major coastal ground-water basins. 

Ground-water movement between adjacent basins is limited because non-water-bearing sedimentary and metamorphic rocks surround and underlie the basins. 

Desert basins include the Mojave Desert, the Colorado Desert, and valleys and ranges of the Great Basin. The 

ground-water basins are filled with unconsolidated to partly consolidated alluvial and basin-fill deposits. Non-waterbearing igneous and metammphic rocks form the hills and 

mountains that surround the ground-water basins. Significant ground-water movement between basins occurs only 

where the non-water-bearing rocks do not extend above 

the water table. 

The coastal basin selected for intensive study is the 

Santa Clara-Calleguas basin; the desert basin selected is 

the Mojave basin (fig. 77). These basins are affected by 

most of the major problems and issues that have been 

identified for study. Both basins have existing geohydrologic data bases-an essential factor in determining the 

major geohydrologic processes and human activities that 

control or influence these problems or issues. In addition, 

water-management agencies in both basins have ongoing 

cooperative programs with the U.S. Geological Survey. 

These cooperative programs will fund the collection and 

analysis of some of the detailed geohydrologic data needed to successfully answer the study objectives. 

The Santa Clara-Calleguas basin extends from the 

Pacific Ocean on the west to the Mojave Desert on the 

east and covers about 2,000 mi2. The topography consists 

of alternating synclinal valleys and anticlinal hills. The 

valleys and hills merge to the west with the Oxnard Plain. 

Almost 90 percent of the drainage-basin surface has rugged topography. This study will concentrate on the waterbearing deposits in that part of the Santa Clara-Calleguas 

basin in Ventura County (fig. 78). 

Agricultural and petroleum production are the chief 

economic activities. As in all of the coastal basins, urbanization since the late 1940's has resulted in the transfer of 

agricultural lands to residential and commercial uses, es-
PART II. STATIJS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 113 

pecially on the Oxnard Plain. The climate of the drainage 

basin varies from a moist Mediterranean type near the Pacific Ocean to a near-desert type at the extreme eastern 

boundary. 

Of the 2,000 mi2 drainage basin, only about 310 mi2 in 

Ventura County is underlain by water-bearing deposits. 

121° 

37° 

33° 

119° 

Central 

Valley 

These deposits are the main source of water for agricultural 

and municipal supplies in the basin. They consist of unconsolidated alluvial deposits of Holocene age, unconsolidated 

alluvial and marine deposits of late Pleistocene age, and 

unconsolidated to partly consolidated marine and continental 

deposits of late Pliocene to mid-Pleistocene age. Orogenesis 

117° 

) 

I 

~ I 

\, 

50 100 MILES 

50 100 KILOMETERS 

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972 EXPLANATION 

Basins selected for intensive study 

Figure 77. Study area of the Southern California Basins regional aquifer-system analysis and location of drainage basins. 
114 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

during the mid-Pleistocene produced intense folding, faulting, and uplift. An erosional unconformity separates the 

folded older deposits from the overlying undisturbed or gen119°00' 118°30' 

tly folded younger deposits. Beneath the Oxnard Plain, this 

unconformity surface has been used to divide the waterbearing deposits into upper and lower aquifer systems. 

EXPLANATION 

Unconsolidated deposits 

Consolidated rocks 

40 MILES 

Santa Clara-Calleguas 

study area 

35° 

00' 

117°30' 117°00' 

Figure 78. Santa C1ara-Calleguas and Mojave basin study areas. 

116°30' 115°30' 

Mojave study area 
PART II. STATUS OF THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 115 

Under predevelopment conditions, ground-water 

movement in both the upper and lower aquifer system was 

from recharge areas along the Santa Clara River to discharge areas near the coast. Ground water discharged as 

underflow through offshore outcrops, to streams near the 

coast, and by evapotranspiration in lowlands along the 

coast. Ground-water pumping near the coast has caused 

water levels to decline below sea level in parts of the 

upper and lower aquifer systems. These water-level declines have resulted in seawater intrusion through outcrop 

areas in submarine canyons near the coast. Seawater intru0 

0 

sion was first suspected in the early 1930's and became a 

serious problem in the 1950's. The most serious seawater 

intrusion occurred in the upper aquifer system. On the 

basis of chloride data collected from monitor wells, the 

County of Ventura Public Works Agency (1990) estimated 

that in 1989 more than 23 mi2 of the Oxnard aquifer (principal water-bearing unit of the upper aquifer system) was 

intruded by seawater (fig. 79). 

To control seawater intrusion, local agencies have 

adopted a management plan to (1) decrease pumping from 

the upper aquifer system, (2) increase pumping from the 

R 22 W R 21 W 

3 MILES 

3 KILOMETERS 

T1N 

T1S 

EXPLANATION 

.. 

Area of Oxnard aquifer where water from monitoring wells exceeds 

100 milligrams per liter chloride (County of Ventura Public Works 

Agency, 1990) 

Area of lower aquifer where water from monitoring wells exceeds 

100 milligrams per liter chloride (Izbicki, 1991) 

-300- Bathymetric contour--Shows depth of water in feet below sea level. 

Contour interval 150 feet 

Figure 79. Chloride concentrations in water from wells on the Oxnard Plain, 1989 (modified from Izbicki, 1991). 
116 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

lower aquifer system, (3) increase recharge of surface water, and (4) reduce ground-water pumping by 25 percent. 

Most of the Santa Clara-Calleguas study will involve analyzing the effectiveness of this management plan. 

The Mojave basin is in the Mojave Desert region of 

southern California and covers about 5,100 mi2 (fig. 77). 

The area of interest for this study includes about 3,340 

mi2 of the basin west of Afton Canyon (fig. 78). 

The Mojave basin is an alluvial plain that slopes 

gently northward and eastward. The plain consists of valleys and closed basins, separated by hills and low mountains. Climate of the basin is characterized by low 

precipitation, low humidity, and high summer temperatures. The proximity of the basin to the highly urban Los 

Angeles area has stimulated recent population growth in 

excess of 5 percent per year. Water supplies in the basin 

are almost entirely from ground water. 

The Mojave River is the principal stream traversing 

the study area and is the main source of recharge to underlying aquifers. Alluvial material beneath the flood plain of 

the Mojave River constitutes the most productive aquifer 

in the basin and yields much of the ground water pumped 

from the basin. The alluvial deposits generally are from X 

to 1 ~ mi in width and are as much as 200 ft thick. The 

deposits consist of boulders, gravel, sand, and silt. Except 

where igneous and metamorphic rocks crop out, older alluvium and fan deposits underlie most of the basin. The 

older alluvium and fan deposits are as much as 1,000 ft 

thick, and contain most of the ground water in storage. 

Partly consolidated to consolidated sedimentary rocks of 

Tertiary age underlie the older alluvium and fan deposits 

in most of the basin. The Tertiary sedimentary rocks include some water-bearing units; however, the water they 

contain is mostly of poor quality and yields from wells 

generally are low. 

About 80 percent of the total ground-water recharge 

in the basin is believed to be from leakage of floodflows 

in the Mojave River along its 100-mi reach between the 

San Bernardino Mountains and Afton Canyon. Some recharge from minor tributary streams occurs along the San 

Bernardino Mountain front. As residential development 

has increased, septic systems have become a source of recharge. Recharge from direct infiltration of precipitation is 

insignificant. 

Ground-water pumping historically has been from 

wells along the Mojave River. Development in recent 

years has moved some of the pumping away from the river. An imbalance between the quantity and distribution of 

pumpage and recharge has created serious overdraft conditions in many areas. Local management agencies would 

like to supplement natural flow to the Mojave River to 

help alleviate the overdraft conditions. A thorough understanding of ground-water and surface-water relations is 

needed to evaluate the effectiveness of this management 

option. This study will concentrate on investigating the exchange of water between the river and the underlying and 

surrounding deposits. 

The Santa Clara-Calleguas basin was the initial 

basin selected for intensive study. Work on this basin 

began in October 1989 and is scheduled for completion in 

September 1993. Work on the Mojave basin began in Oc-: 

tober 1991. This report discusses only the progress and 

results of the Santa Clara-Calleguas basin study because 

work has only recently begun in the Mojave basin. 

Most of the geohydrologic data available are from 

wells that are open only to the upper aquifer or are open 

to several aquifers. Data from these wells are inadequate 

to characterize the three-dimensional hydraulic and waterquality properties of the study area. Since the start of the 

project, the U.S. Geological Survey has constructed multiple monitoring wells at 20 sites in the Santa Clara-Calleguas basin. These monitoring wells make it possible to 

collect aquifer- and depth-dependent data such as hydraulic head, water quality, and hydraulic properties. 

Seawater intrusion is probably the main problem affecting the utilization of ground water in the Santa ClaraCalleguas basin. Agencies responsible for management of 

seawater intrusion presently use 100 mg/L chloride as the 

criterion to determine the position of the seawater front. 

On the basis of this criterion, it was estimated that in 1989 

more than 23 mi2 of the Oxnard aquifer of the upper aquifer system was intruded by seawater (County of Ventura 

Public Works Agency, 1990). Many of the wells used to 

determine the seawater front are abandoned irrigation supply wells or old steel-casing monitor wells. 

Water-quality data from more than 40 wells installed 

as part of this study and results of direct-current resistivity 

soundings completed for this study show that the area affected by seawater intrusion is less than originally believed. 

Trace-element data used in conjunction with stable-isotope 

data revealed that the source of elevated chloride concentrations, in at least some wells, is leakage of seawater from the 

overlying perched aquifer through failed casings or sanitary 

seals (lzbicki, 1991). In other wells, irrigation return may be 

the cause of the elevated chloride concentrations. 

Prior to this RASA study, seawater intrusion into the 

lower aquifer system was not considered a serious threat, 

and ground-water pumping was shifted from the upper to 

the lower aquifer system. Previous investigators believed 

that there was a 200-yr supply of freshwater stored in the 

lower aquifer system offshore. Data from one of the monitoring sites installed at Port Hueneme near the mouth of a 

submarine canyon (fig. 79) indicate that seawater already 

has intruded the lower aquifer system through outcrops in 

the submarine canyon. The lower aquifer system outcrops 

in the submarine canyon less than 2 mi offshore. 

Ground-water pumping from the lower aquifer near 

Point Mugu (fig. 79) also has resulted in elevated chloride 

concentrations in samples from wells. Plots of the ratios of 

chloride to iodide as a function of chloride indicate that 
PART III. CONCLUSIONS AND RECOMMENDATIONS 117 

seawater is not the source of elevated chloride concentrations in this part of the basin (lzbicki, 1991). Data obtained from a monitoring site installed as part of this study 

indicate that high-chloride brines from the underlying Tertiary rocks are the source of the elevated chlorides. 

Surface spreading of water from the Santa Clara 

River has been used to recharge the aquifers in the Santa 

Clara-Calleguas basin for about 65 yr near Saticoy, about 

8 mi from the coast (fig. 78). Previous studies reported 

that the surface spreading recharged the upper and lower 

aquifer systems. In this part of the basin, lenticular clay 

beds that are present in the aquifer system beneath most of 

the Oxnard Plain were thought to be absent or less extensive, thus allowing recharge water to readily move through 

the aquifer system. 

Tritium data collected for this study indicate that 

surface spreading does recharge the upper aquifer system, 

but water recharged since the early 1950,s has not yet 

reached the coast. Water from most wells in the lower aquifer system does not contain tritium, and it must have 

been recharged prior to the early 1950,s. Carbon-14 ages, 

estimated for water from wells in the lower aquifer system 

more than 1 mi downgradient of the recharge site, are 

from 900 to about 25,000 yr before present. These data 

indicate that the lower aquifer system is not effectively recharged by surface spreading. 

The Southern California Basins RASA study was 

started in 1984; however, it was suspended immediately to 

redirect the resources to the Central Valley RASA. The 

study was restarted in 1990. As of mid-1992, nine reports 

had been completed, of which six are published. 

GROUND WATER ATLAS OF THE 

UNITED STATES 

As discussed in the section "Application of Regional 

Aquifer-System Analysis Program Information/, a key 

product of the RASA Program is compilation of an atlas 

describing the ground-water resources of the United 

States. The atlas, which is designed in a graphical format 

that is supported by descriptive discussions, will be composed of 14 chapters. An introductory chapter will present 

an overview of the Nation,s ground-water conditions, describe effects of development such as saltwater encroachment and land subsidence, and include two maps showing 

the distribution of major aquifers on a nationwide scale. 

This introductory chapter will be compiled following completion of 13 descriptive chapters, each of which describes 

ground-water conditions in a particular region (fig. 2). The 

13 chapters (or segments) collectively cover the 50 States, 

Puerto Rico, and the U.S. Virgin Islands, and describe geologic and hydrologic conditions for the major aquifers in 

each region. The scale of the atlas does not allow portrayal 

of local features of the geology and hydrology, nor does it 

include discussion of minor aquifers in a region. However, 

extensive lists of references are included at the end of 

each chapter for readers who seek detailed, local information on specific aquifers. The descriptive atlas chapters 

will be published in the U.S. Geological Survey,s Hydrologic Investigations Atlas HA-730 series, with a letter AN assigned to each segment (fig. 2). 

As of mid-1993, two segments had been published: 

segment 6 describing ground-water conditions in Alabama, 

Florida, Georgia, and South Carolina (Miller, 1990), and 

segment 9 describing ground-water conditions in Iowa, 

Michigan, Minnesota, and Wisconsin (Olcott, 1992). As of 

mid-1993, six segments (segments 1, 2, 4, 7, 10, and 12) 

were in preparation for printing. Five other segments (segments 3, 5, 8, 11, and 13) were either in review or being 

written. The entire atlas is scheduled to be completed in 

1995; however, printing of all segments will require additional time. For additional information, readers may contact J.A. Miller, Project Chief, U.S. Geological Survey 

Regional Office, Norcross, Georgia. 

PART III. CONCLUSIONS AND 

RECO~ENDATIONS 

CONCLUSIONS 

The RASA Program was a successful effort to evaluate the major ground-water systems of the United States. 

During the 15-yr program (1978-92), 25 regional aquifer 

systems, including the most heavily pumped aquifers in 

the Nation, were intensively studied. As of mid-1992, 18 

of the regional aquifer studies are essentially completed; 7 

of the studies and the National Ground Water Atlas project 

are ongoing. 

All RASA projects involved the assembly of existing 

geologic, hydrologic, and geochemical information for the 

purpose of analyzing and understanding a major groundwater system. In addition, most projects had to fill data 

voids to provide a complete description of the ground-water 

system. These efforts in data-deficient areas included exploratory drilling and testing, conducting mass measurements of ground-water levels, collection and chemical 

analysis of water samples, and application of new techniques for determination of water use from heavily pumped 

aquifers. Computer simulation was used extensively to analyze regional ground-water systems under natural conditions, to analyze the hydrologic responses of regional 

aquifers to development, and to examine changes in water 

chemistry throughout regional aquifers. Results of these 

studies are contained in nearly 900 published reports. These 

publications range from compilations of basic geologic and 

hydrologic data to interpretive reports that explain the functioning of complex aquifer systems under natural conditions and under development conditions involving large 
118 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

ground-water withdrawals. A series of U.S. Geological Survey Professional Papers, consisting of several chapters, has 

been or is being prepared for each regional aquifer system. 

Individual chapters describe various aspects of the geology, 

hydrology, and geochemistry of the regional aquifer system 

or address the hydrology and water problems of subareas 

within the system in greater detail. 

Results of the RASA Program were used as a major, 

but not exclusive, source of information for compilation of 

a Ground Water Atlas of the United States. The atlas includes 14 chapters, an introductory chapter and 13 chapters 

(segments), each of which describes the hydrogeology and 

hydrologic conditions for important aquifers in an area of 

several States. Collectively, the completed atlas will cover 

the 50 States, Puerto Rico, and the U.S. Virgin Islands. 

The three major contributions of the RASA Program 

are (1) assembly of data from numerous local studies and 

long-term data-collection programs into systematic regional 

data bases; (2) comprehensive descriptions of the geologic, 

hydrologic, and geochemical characteristics of regional aquifer systems; and (3) an understanding of how regional 

ground-water-flow systems function under natural (predevelopment) and current (developed) conditions. To achieve 

these results, a variety of new and traditional investigative 

approaches were tried, and new concepts of regional groundwater systems were tested. A sampling of some of the new 

concepts that were developed and approaches that were successfully used during the RASA Program are as follows: 

New concept of an aquifer system in the California 

Central Valley. Williamson and others ( 1989) proposed 

that the continental deposits of the Central Valley constitute a single aquifer system with varying vertical leakance 

and confinement rather than a water-table aquifer in the 

northern part and a two-aquifer system separated by a regional confining unit in the southern part, as described by 

previous workers. The new conceptual model was suggested by analysis of water levels and sediment textures and 

by simulated flow conditions. 

New concept of regional ground-water flow in the 

Great Basin of Nevada and Utah and adjacent States. 

Ground-water flow in the Great Basin was conceptualized 

as relatively shallow flow through basin-fill deposits and 

adjacent mountain ranges, superimposed over deep flow in 

Paleozoic carbonate rocks, by Prudic and others (in press). 

Based on model-derived flows, the deep flow system was 

divided into 5 regions; the overlying shallow flow system 

was divided into 17 regions that approximately coincide 

with flow systems delineated from topography, groundwater levels, discharge areas, and water budgets. The delineation of these flow regions differs in part from those of 

previous investigators. 

Hydrologic analysis of the Floridan aquifer system 

led to discovery of two large springs. The existence of a 

depression in the potentiometric surface of the Upper 

Floridan aquifer at Little Lake George in central Florida 

indicated a probable area of ground-water discharge. Calibration of a ground-water flow model during the RASA 

investigation indicated the probable existence of considerable ground-water discharge in the same area. During 

1981, USGS divers entered "Croaker hole" in the bottom 

of the lake (which penetrated into limestone of the upper 

Floridan aquifer) and measured a spring discharge of 80 

ft3/s (Tibbals, 1990). In similar manner, another submerged sprin~ was discovered in central Florida and discharge of 6 ft Is was measured. 

Simulation of pumping from multiaquifer wells in the 

Cambrian-Ordovician aquifer system in the Northern Midwest. Simulation of pumping from the Cambrian-Ordovician aquifer system used a special computer code (based 

on work by Bennett and others, 1982) to calculate the approximate withdrawal rates from each aquifer tapped by a 

multiaquifer well. This method allows the direct use of reported pumpage from such wells, eliminating the need to 

estimate the withdrawal from individual aquifers (Young 

and others, 1989a). Another advantage of this procedure is 

that it allows comparison of simulated composite heads 

with water-level measurements in wells. 

Estimating ground-water pumpage from the High 

Plains aquifer. The large areal extent of the High Plains 

aquifer (174,000 mi2) precluded direct measurement of 

ground-water pumpage for irrigation. An indirect method 

of estimating pumpage was used in which irrigated acreage was first estimated by digital analysis of Landsat data 

(Thelin and Heimes, 1987). The amount of applied irrigation water was measured at several hundred field sites during the 1980 growing season. The volume of ground-water 

pumped from the High Plains aquifer in eight States was 

calculated by combining the field measurements with the 

mapped irrigated acreage information. 

Source of recharge to the Cambrian-Ordovician aquifer system in the Northern Midwest based on geochemistry. 

The direction of increase of total dissolved solids in water 

in the Cambrian-Ordovician aquifer system in central Iowa 

suggests that water moves southwestward, approximately 

perpendicular to the flow direction indicated from the potentiometric surface. The ground water is isotopically depleted in the heavy isotopes oxygen-18 and deuterium. 

Isotope depletion and the dissolved-solids distribution suggests that this ground water originated as recharge during 

Pleistocene time, probably from subglacial meltwater under 

very high hydraulic gradients (Siegel, 1989). 

Identification of the seaward extent of freshwater in 

the Floridan aquifer system based on hydrologic testing in 

an offshore oil well. Prior to RASA, it was known that 

freshwater extended at least 25 mi offshore in the Floridan 

aquifer system. Hydrologic testing was conducted in an 

offshore oil well (located 55 mi east of the northeast Florida coast) during the Floridan aquifer system RASA. Head 

and salinity data obtained from the Upper Floridan aquifer 

indicated that the tested interval is in the transition zone 

between freshwater and seawater in the Upper Floridan 

aquifer (Johnston and others, 1982). 
REFERENCES CITED 119 

RECOMMENDATIONS 

The RASA Program focused on 25 regional aquifer 

systems that provide much of the ground water used for 

irrigation, industrial purposes, and large public water supplies in the United States. These regional ground-water 

systems consist mostly of deep, confined aquifers. In general, the analyses of these deep systems did not include 

detailed investigations of overlying shallow aquifers. For 

the most part, the RASA studies simply treated the shallow aquifers as local sources of recharge to the deep aquifers and sinks for upward discharge from them. However, 

many shallow aquifers are important sources of domestic 

and agricultural water supplies in rural areas and for public water supplies in small cities and towns. 

A logical follow-up to the RASA Program is a more 

detailed analysis of the important shallow, unconfined aq-

.uifers conducted on a regional basis. Some of the shallow 

aquifers have been the subject of intensive hydrologic investigations under the U.S. Geological Survey FederalState Cooperative Program. Generally, these investigations 

have been concerned with local water quality and quantity 

problems. 

Several shallow, unconfined aquifers are sufficiently 

extensive to be major sources of ground water. For example, the Biscayne aquifer, a shallow, highly permeable 

limestone and sandstone aquifer is the principal source of 

water in southernmost Florida. The Biscayne aquifer provides the public water supply of Miami and other cities in 

south Florida because the underlying regional aquifer system (Floridan) contains saline water. 

The major justification for beginning a program of 

shallow aquifer studies is that most contamination of 

ground water has occurred in or has affected shallow, unconfined aquifers and not the deep regional aquifer systems. Shallow, highly permeable aquifers such as those 

consisting of sand and gravel or limestone are highly vulnerable to contamination from land surface or near-surface 

sources (such as herbicides, insecticides, fertilizers, septic 

systems, leaking landfills, and leaking buried storage 

tanks). Most contaminants dissolved in water are moved 

by advective transport-that is, with the ground-water 

flow. Therefore, the ground-water flow system must be defined in order to provide information on the flow paths and 

times of travel of the contaminants. 

It is obviously impractical to investigate all shallow, 

local aquifers in the United States. However, if it can be 

determined that a group of many shallow aquifers in a region share common geologic and hydrologic characteristics, then detailed investigations of a few representative 

aquifers may be sufficient. This approach was successfully 

used during the RASA Program in the investigation of alluvial basins in Arizona and New Mexico. The major objectives of each detailed shallow aquifer study would be 

( 1) to define the hydrogeologic framework of the shallow 

aquifer with emphasis on defining aquifer heterogeneity, 

(2) to define the ground-water budgets, and (3) to describe 

the chemical and physical processes that produced the 

present ground-water quality. As in the RASA Program, 

these studies would utilize computer simulation extensively to analyze the shallow flow systems. 

The shallow aquifer studies would benefit from the 

findings of the RASA Program, especially from water 

budgets developed for regional confined aquifers. These 

budget studies quantified rates of downward leakage from 

shallow aquifers into confined aquifers and upward leakage from the confined. aquifers. Without the budget studies 

from the RASA Program, it would be difficult to determine these leakage rates. 

A second recommended followup to the RASA Program is a long-term program of periodically preparing potentiometric surface maps for the major multistate aquifers 

based on synoptic water-level measurements. As noted 

earlier in this report, an important contribution of the 

RASA Program was preparation of some of the first potentiometric surface maps providing full coverage of multistate aquifers. Such maps were essential for describing 

and analyzing the flow systems in these regional aquifers. 

In particular, the maps were used to calibrate computer 

models of flow in such aquifers. 

Since completion of RASA projects, continued preparation of potentiometric surface maps for multistate aquifers 

has varied depending upon local interest and availability of 

funding. For some aquifers, maps have been prepared in one 

State but not in an adjoining State. As part of the long-term 

basic data collection program of the U.S. Geological Survey, we recommend that synoptic potentiometric surface 

maps be prepared for the Nation's major aquifers at intervals of about 5 yr. 

The need for an ongoing program of potentiometric 

surface mapping of the Nation's aquifers is indicated by a 

congressional directive to the U.S. Geological Survey to 

monitor water levels in the High Plains aquifer. Section 

306, Title ill of Public Law 98-242 (as amended) authorizes and directs the U.S. Geological Survey in cooperation 

with States in the High Plains region to monitor water levels in the High Plains aquifer and report annually to Con· 

gress. In general, the magnitude of water-level declines 

observed in the High Plains aquifer are not occurri~g in 

other regional aquifers. Therefore, a 5-yr interval of synoptic water-level measurements should provide adequate 

definition of changes for most major aquifers. 

REFERENCES CITED 

Abbott, P.L., 1975, On the hydrology of the Edwards Limestone, 

south-central Texas: Journal of Hydrology, v. 24, p. 251-

269. 

Ackerman, D.l., 1990, Hydrology of the Mississippi River Valley 

alluvial aquifer, south-central United States: U.S. Geological 

Survey Open-File Report 90-358, 11 p. 
120 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Anderson, T.W., Freethey, G.W., and Tucci, Patrick, 1992, Geohydrology and water resources of alluvial basins in southcentral Arizona and parts of adjacent States: U.S. Geological 

Survey Professional Paper 1406-B, 67 p. 

Anna, L.O., 1986, Geologic framework of the ground-water system in Jurassic and Cretaceous rocks in the northern Great 

Basins in parts of Montana, North Dakota, South Dakota, 

and Wyoming: U.S. Geological Survey Professional Paper 

1402-B, 36 p. 

Back, William, 1966, Hydrochemical facies and ground-water 

flow patterns in northern part of Atlantic Coastal Plain: U.S. 

Geological Survey Professional Paper 498-A, 42 p. 

Back, William, Rosenshein, J.S., and Seaber, P.R., eds., 1988, 

Hydrogeology: Geological Society of America, The Geology 

of North America, Boulder, Colorado, v. 0-2, 524 p. 

Barker, R.A., and Pernik, Maribeth, in press, Regional hydrology 

and simulation of deep ground-water flow in the Southeastern Coastal Plain aquifer system in Mississippi, Alabama, Georgia, and South Carolina: U.S. Geological Survey 

Professional Paper 141 0-C. 

Beck, B.F., and Sinclair, W.C., 1986, Sinkholes in Florida-An 

introduction: Florida Sinkhole Research Institute Report 85-

86-4, 16 p. 

Beckman, J.D., and Williamson, A.K., 1990, Salt-dome locations 

in the Gulf Coastal Plain, south-central United States: U.S. 

Geological Survey Water Resources Investigations Report 

90:-4060, 44 p. 

Bennett, G.D., Kontis, A.L., and Larson, S.P., 1982, Representation 

of multiaquifer well effects in three-dimensional ground-water 

flow simulation: Ground Water, v. 20, no. 3, p. 334-341. 

Bertoldi, G.L., Johnston, R.H., and Evenson, K.D., 1991, Ground 

water in the Central Valley, California-A summary report: 

U.S. Geological Survey Professional Paper 1401-A, 44 p. 

Bownocker, J.A., Completed 1920 (1981), Geologic map of 

Ohio: Ohio Geological Survey, scale 1:500,000. 

Bredehoeft, J.D., and Hanshaw, B.B., 1968, On the maintenance of 

anomalous fluid pressures-!. Thick sedimentary sequences: 

Geological Society of America Bulletin, v. 79, p. 1097-1105. 

Bretz, J.H., 1923, The channeled scablands of the Columbia Plateau: Journal of Geology, v. 31, p. 617-649. 

Brown, D.L., Blankennagel, R.K., MacCary, L.M., and Peterson, 

J.A., 1984, Correlation of paleostructure and sediment deposition in the Madison Limestone and associated rocks in 

parts of Montana, North Dakota, South Dakota, and Wyoming: U.S. Geological Survey Professional Paper 1273-B, 

24p. 

Busby, J.F., Kimball, B.A., Downey, J.S., and Peter, K.D., in 

press, Geochemistry of ground water in aquifers and confining units of the northern Great Plains in parts of Montana, 

North Dakota, South Dakota, and Wyoming: U.S. Geological Survey Professional Paper 1402-F. 

Busby, J.F., Plummer, L.N., Lee, R.W., and. Hanshaw, B.B., 

1991, Geochemical evolution of water in the Madison aquifer in parts of Montana, South Dakota, and Wyoming: U.S. 

Geological Survey Professional Paper 1273-F, 89 p. 

Bush, P.W., 1988, Simulation of saltwater movement in the Floridan aquifer system, Hilton Head Island, South Carolina: 

U.S. Geological Survey Water-Supply Paper 2331, 19 p. 

Bush, P.W., and Johnston, R.H., 1988, Ground-water hydraulics, 

regional flow, and ground-water development of the Floridan 

aquifer system in Florida and in parts of Georgia, South 

Carolina, and Alabama: U.S. Geological Survey Professional 

Paper 1403-C, 80 p. 

California Department of Water Resources, 1969, Names and 

area code numbers of hydrologic areas in the Southern District: California Department of Water Resources Office Report, 57 p. 

Carman, R.L., in press, Measurement of evapotranspiration in 

phreatophyte areas, Smith Creek Valley and Carson Desert, 

west-central Nevada, 1983: U.S. Geological Survey WaterResources Investigations Report 89-4118. 

Carpenter, M.C., 1991, Earth-fissure movements associated with 

fluctuations in ground-water levels near the Picacho Mountains, south-central Arizona, 1980-84: U.S. Geological Survey Open-File Report 90-561, 64 p. 

Chamberlin, T.C., 1885, Requisite and qualifying conditions of 

artesian wells: U.S. Geological Survey 5th Annual Report to 

the Secretary of the Interior, 1883-84, p. 131-173. 

Chapelle, F.H., and Knobel, L.L., 1985, Stable carbon isotopes of 

bicarbonate in the Aquia aquifer, Maryland-Evidence of an 

isotopically heavy source of carbon dioxide: Ground Water, 

v. 23,no.5,p. 592-599. 

Chapelle, F.H., and McMahon, P.B., 1991, Geochemistry of dissolved inorganic carbon in a coastal plain aquifer-1. Sulfate from confining beds as an oxidant in microbial C02 

production: Journal of Hydrology, 127, p. 85-108. 

Clark, D.W., and Kendy, Eloise, 1992a, Plan of study for the 

regional aquifer-system analysis of the Northern Rocky 

Mountains Intermontane Basins, Montana and Idaho: U.S. 

Geological Survey Water-Resources Investigations Report 

92-4116, 16 p. 

---1992b, Regional analysis of the Northern Rocky Mountains 

Intermontane Basins, Montana and Idaho, in Prince, K.R., and 

Johnson, A.l., eds., Aquifers of the Far West: American Water 

Resources Association Monograph series no. 16, p. 55-64. 

Cooley, R.L., Konikow, L.F., and Naff, R.L., 1986, Nonlinearregression ground-water flow modeling of a deep regional 

aquifer system: Water Resources Research, v. 22, no. 13, 

p. 1759-1778. 

County of Ventura Public Works Agency, 1990, Oxnard Plain of 

Ventura County, 1989 seawater intrusion study, 5 p. 

Craigg, S.D., in press, ·Geologic framework of the San Juan 

structural basin of New Mexico, Colorado, Arizona, and 

Utah, with emphasis on Triassic through Tertiary rocks: U.S. 

Geological Survey Professional Paper 1420-B. 

Darton, N.H., 1896, Preliminary report of artesian waters of a 

portion of the Dakotas: U.S. Geological Survey 17th Annual 

Report to the Secretary of the Interior, 1895-96, pt. 2, 

p. 609-694. 

---1905, Geology and underground water resources of the 

central Great Plains: U.S. Geological Survey Professional 

Paper 32,433 p. 

Davis, M.E., Sparkes, A.K., and Peacock, B.S., 1983, Results of 

a test well in the Nanafalia Formation near Melvin, Choctaw 

County, Alabama: U.S. Geological Survey Water-Resources 

Investigations Report 82-4108, 17 p. 

Dettinger, M.D., 1989, Reconnaissance estimates of natural recharge to desert basins in Nevada, U.S.A., by using chloride-balance calculations: Journal of Hydrology, v. 106, 

no. 1-2, p. 55-78. 
REFERENCES CITED 121 

Downey, J.S., 1982, Machine-readable data files from the Madison Limestone and Northern Great Plains regional aquifersystem analysis project, Montana, Nebraska, North Dakota, 

South Dakota, and Wyoming: U.S. Geological Survey WaterResources Investigations Report 82-4107, 26 p. 

---1984, Geohydrology of the Madison and associated aquifers in parts of Montana, North Dakota, South Dakota, and 

Wyoming: U.S. Geological Survey Professional Paper 1273-

G,47 p. 

--1986, Geohydrology of bedrock aquifers in the Northern 

Great Plains in parts of Montana, North Dakota, South Dakota, and Wyoming: U.S. Geological Survey Professional 

Paper 1402-E, 87 p. 

---1988, Regional bedrock aquifers of the Northern Great 

Plains, north-central United States, in McLean, J.S., and 

Johnson, A.l., eds., Regional aquifer systems of the United 

States, Aquifers of the western mountain area: American Water Resources Association Monograph series no. 14, p. 5-13. 

Downey, J.S., and Dinwiddie, G.A., 1988, The regional aquifer 

system underlying the Northern Great Plains in parts of 

Montana, North Dakota, South Dakota, and WyomingSummary: U.S. Geological Survey Professional Paper 1402-

A,64p. 

Drost, B.W., and Whiteman, K.J., 1986, Surficial geology, structure and thickness of selected geohydrologic units in the 

Columbia Plateau, Washington: U.S. Geological Survey 

Water-Resources Investigations Report 86-4236, 10 sheets. 

Dugan, J.T., Schild, D.E., and Kastner, W.M., 1990, Water-level 

changes in the High Plains aquifer underlying parts of South 

Dakota, Wyoming, Nebraska, Colorado, Kansas, New Mexico, 

Oklahoma, and Texas-Predevelopment through nonirrigation 

season 1988-89: U.S. Geological Survey Water-Resources In· 

vestigations Report 90-4153, 29 p. 

Dysart, J.E., 1988, Use of oxygen-18 and deuterium mass-balance analysis to evaluate induced recharge to stratified-drift 

aquifers in Randall, A.D., and Johnson, A.l., eds., Regional 

aquifer systems of the United States, The Northeast Glacial 

Aquifers: American Water Resources Association Monograph series no. 11, p. 133-154. 

Eaton, G.P., 1979, Regional geophysics, Cenozoic tectonics, and 

geologic resources of the Basin and Range provinces and 

adjoining regions, in Newman, G.W., and Good, H.D., eds., 

1979 Basin and Range Symposium: Rocky Mountain Association of Geologists and Utah Geological Association, p. 11-39. 

Ewart, C.J., 1986, Oahu Island regional aquifer-system study, in 

Sun, R.J., ed., Regional Aquifer-System Analysis Program of 

the U.S. Geological Survey-Summary of projects, 1978-

84: U.S. Geological Survey Circular 1002, p. 195-204. 

Fleming, T.H., 1989, Description and results of the pumping test 

at the Juiletta Landfill [Originally compiled by the Indiana 

Geological Survey, unpublished data on file in Indianapolis 

office of the Indiana Department of Natural Resources, Division of Water, 11 p.] 

Garabedian, S.P., 1986, Application of a parameter-estimation 

technique to modeling the regional aquifer underlying the 

eastern Snake River Plain, Idaho: U.S. Geological Survey 

Water-Supply Paper 2278, 60 p. 

---1993, Hydrology and digital simulation of the regional 

aquifer system, eastern Snake River Plain, Idaho: U.S. Geological Survey Professional Paper 1408-F, 102 p., 10 plates. 

Geldon, A.L., 1988, Hydrostratigraphic characterization of Paleozoic formations in the Upper Colorado River Basin, Arizona, 

Colorado, New Mexico, Utah and Wyoming, in McLean, 

J.S., and Johnson, A.l., eds., Regional aquifer systems of the 

United States, Aquifers of the western mountain area: American Water Resources Association Monograph series no. 14, 

p. 135-159. 

---in press, Hydrologic properties and ground-water flow 

systems of the Paleozoic rocks in the Upper Colorado River 

Basin in Arizona, Colorado, New Mexico, Utah, and Wyoming, excluding the San Juan Basin: U.S. Geological Survey 

Professional Paper 1411-E. 

Gilliom, R.D., Belitz, Kenneth, Deverel, S.J., Dubrovsky, N.M., 

and Fujii, Roger, 1989, Preliminary assessment of sources, 

distribution, and mobility of selenium in the San Joaquin 

Valley, California: U.S. Geological Survey Water Resources 

Investigations Report 88-4186, 129 p. 

Gomez-Gomez, Fernando, Quinones-Aponte, Vicente, and 

Johnson, A.l., eds., 1991, Regional aquifer systems of the 

United States, Aquifers of the Caribbean Islands: American 

Water Resources Association Monograph series no. 15, 113 p. 

Goodell, S.A., 1988, Water use on the Snake River Plain, Idaho 

and eastern Oregon: U.S. Geological Survey Professional 

Paper 1408-E, 51 p. 

Gray, H.H., Ault, C.H., and Keller, S.J., 1987, Bedrock geology 

map of Indiana: Indiana Geological Survey Miscellaneous 

Map 48, scale 1:500,000. 

Grose, L. T., 1972, Tectonics, in Geologic atlas of the Rocky 

Mountain region: Denver, Rocky Mountain Association of 

Geologists, p. 35-44. 

Grubb, H.F., 1984, Planning report for the Gulf Coast Regional 

Aquifer-System Analysis in the Gulf of Mexico Coastal Plain, 

United States: U.S. Geological Survey Water-Resources Investigations Report 84-4219, 30 p. 

---1987, Overview of the Gulf Coast regional aquifer-system 

analysis, south-central United States, in Vecchioli, John, and 

Johnson, A.l., eds., Regional aquifer systems of the United 

States, Aquifers of the Atlantic and Gulf Coastal Plain: American Water Resources Monograph series no. 9, p. 101-118. 

Gutentag, E.D., Heimes, F.J., Krothe, N.C., Luckey, R.R., and 

Weeks, J.B., 1984, Geohydrology of the High Plains aquifer 

in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming: U.S. Geological 

Survey Professional Paper 1400-B, 63 p. 

Haeni, F.P., in press, Application of surface-geophysical methods 

to investigation of sand and gravel aquifers in the glaciated 

Northeastern United States: U.S. Geological Survey Professional Paper 1415-C. 

Hanneman, D.L., and Wideman, C.J., 1991, Sequence stratigraphy of Cenozoic continental rocks, southwestern Montana: 

Geological Society of America Bulletin, v. 103, no. 10, 

p. 1335-1345. 

Harrill, J.R., 1986, Great Basin regional aquifer-system study, in 

Sun, R.J., ed., Regional Aquifer-System Analysis Program of 

the U.S. Geological Survey-summary of projects, 1978-

84: U.S. Geological Survey Circular 1002, p. 146-151. 

Heath, R.C., 1984, Ground-water regions of the United States: 

U.S. Geological Survey Water-Supply Paper 2242, 78 p. 

Helgesen, J.O., and Leonard, R.B., 1989, Geohydrology of the 

Great Plains aquifer system, in Swain, L.A., and Johnson, 
122 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

A.l., eds., Regional aquifer systems of the United States, 

Aquifers of the Midwestern area: American Water Resources 

Association Mooograph series no. 13, p. 179-190. 

Helgesen, J.O., Leonard, R.B., and Wolf, R.J., 1993, Hydrology 

of the Great Plains aquifer system in Nebraska, Colorado, 

Kansas, and adjacent areas: U.S. Geological Survey Professional Paper 1414-E, 80 p., 10 plates. 

Henderson, Thomas, 1985, Geochemistry of ground water in two 

sandstone aquifer systems in the Northern Great Plains in 

parts of Montana and Wyoming: U.S. Geological Survey 

Professional Paper 1402-C, 84 p. 

High Plains Associates, 1982, Six-state High Plains-Ogallala aquifer regional resources study: Austin, Texas, 438 p. 

High Plains Study Council, 1982, A summary of results of the 

Ogallala aquifer regional study, with recommendations to 

the Secretary of Commerce and Congress, 61 p. 

Hosman, R.L., 1991, Regional stratigraphy and subsurface geology of Cenozoic deposits, Gulf Coastal Plain, south-central 

United States: U.S. Geological Survey Open-File Report 91-

66,43 p. 

Hotchkiss, W.R., and Johnson, A.l., eds., 1992, Regional aquifer 

systems of the United States, Aquifers of the southern and 

eastern States: American Water Resources Association Monograph series no. 17, 115 p. 

Hull, L.C., 1984, Geochemistry of ground water in the Sacramento Valley, California: U.S. Geological Survey Professional Paper 1401-B, 36 p. 

Ireland, R.L., Poland, J.F., and Riley, F.S., 1984, Land subsidence in the San Joaquin Valley, California, as of 1980: U.S. 

Geological Survey Professional Paper 437-1, 183 p. 

lzbicki, J.A., 1991, Sources of chloride in a coastal aquifer in 

California: American Society of Civil Engineers 1991 Symposium on Ground Water in the Pacific Rim, Honolulu, Hawaii, July 22-26, 1991, Proceedings, p. 71-77. 

Johnston, R.H., 1989, The hydrologic responses to development 

in regional sedimentary aquifers: Ground Water, v. 27, no. 3, 

p. 316-322. 

Johnston, R.H., and Bush, P.W., 1988, Summary of the hydrology of the Floridan aquifer system in Florida and in parts of 

Georgia, South Carolina, and Alabama: U.S. Geological Survey Professional Paper 1403-A, 24 p. 

Johnston, R.H., Bush, P.W., Krause, R.E., Miller, J.A., and Sprinkle, C.L., 1982, Summary of hydrologic testing in Tertiary 

limestone aquifer, Tenneco offshore exploratory well-Atlantic OCS, Lease-Block 427 (Jacksonville NH 17-5): U.S. 

Geological Survey Water-Supply Paper 2180, 15 p. 

Jorgensen, D.G., 1988, Using geophysical logs to estimate porosity, water resistivity, and intrinsic permeability: U.S. Geological Survey Water Supply Paper 2321, 24 p. 

Kernodle, J.M., and Craigg, S.D., 1992, Using a geographic information system to assist in a regional aquifer-system analysis: 

U.S. Geological Survey Yearbook, fiscal year 1991, p. 12-14. 

King, P.B., and Beikman, H.M., 1974, Geologic map of the United States, exclusive of Alaska and Hawaii: U.S. Geological 

Survey, 3 sheets, scale 1:2,500,000. 

Kjelstrom, L.C., 1986, Flow characteristics of the Snake River 

and water budget for the Snake River Plain, Idaho and eastem Oregon: U.S. Geological Survey Hydrologic Investigations Atlas HA-680, 2 sheets, scale 1:1,000,000. 

---in press, Streamflow gains and losses in the Snake River 

and ground-water budgets for the Snake River Plain, Idaho 

and eastern Oregon: U.S. Geological Survey Professional 

Paper 1408-C. 

Kohout, F.A., 1965, A hypothesis concerning cyclic flow of saltwater related to geothermal heating in the Floridan aquifer: 

New York Academy of Sciences Transactions, ser. 2, v .. 28, 

no. 2, p. 249-271. 

Konikow, L.F., and Bredehoeft, J.D., 1992, Ground-water models 

cannot be validated: Advances in Water Resources, v. 15, 

p. 75-83. 

Kontis, A.L., and Mandie, R.J., 1980, Data-base systems for 

Northern Midwest regional aquifer-system analysis: U.S. 

Geological Survey Water-Resources Investigations Report 

80-104, 23 p. 

---1988, Modifications of the three-dimensional groundwater flow model to account for variable water density and 

effects of multiaquifer wells: U.S. Geological Survey WaterResources Investigations Report 87-4265, 78 p. 

Krause, R.E., and Randolph, R.B., 1989, Hydrology of the Floridan aquifer system in southeast Georgia and adjacent parts 

of Florida and South Carolina: U.S. Geological Survey Professional Paper 1403-D, 65 p. 

Kuiper, L.K., 1983, A numerical procedure for the solution of the 

steady-state variable-density groundwater flow equation: 

Water Resources Research, v. 19, no. 1, p. 234-240. 

---1985, Documentation of a numerical code for the simulation 

of variable density ground-water flow in three dimensions: 

U.S. Geological Survey Water-Resources Investigations Report 84-4032, 90 p. 

Kuniansky, E.L., 1990, A two-dimensional finite-element flow 

model for confined aquifers in steady state: U.S. Geological 

Survey Open-File Report 90-187, 77 p. 

Lane, R.C., 1988, Selected ground-water information for the Columbia Plateau regional aquifer system, Washington and Oregon, 1982-1985: v. 2. Water levels: U.S. Geological Survey 

Open-File Report 88-183, 136 p. 

Laney, R.L., and Davidson, C.B., 1986, Aquifer nomenclature 

guidelines: U.S. Geological Survey Open-File Report 86-

534,46 p. 

Lapham, W.W., 1989, Use of temperature profiles beneath streams 

to determine rates of vertical ground-water flow and vertical 

hydraulic conductivity: U.S. Geological Survey Water-Supply 

Paper 2337, 35 p. 

Larson, S.P., 1978, Direct solution algorithm for the two-dimensional ground-water flow model: U.S. Geological Survey 

Open-File Report 79-202, 25 p. 

Leahy, P.P., 1982, A three-dimensional ground-water flow model 

modified to reduce computer memory requirements and better 

simulate confining-bed and aquifer pinchouts: U.S. Geological 

Survey Water-Resources Investigations Report 82-4023, 60 p. 

Leahy, P.P., and Martin, Mary, 1993, Geohydrology and simulation of ground-water flow in the Northern Atlantic Coastal 

Plain aquifer system: U.S. Geological Survey Professional 

Paper 1404-K, 80 p., 22 plates. 

Leahy, P.P., Martin, Mary, and Meisler, Harold, 1987, Hydrologic 

definition of the Northern Atlantic Coastal Plain aquifer system 

based on regional simulation, in Vecchioli, John, and Johnson, 

A.l., eds., Regional aquifer systems of the United States, Aqui-
REFERENCES CITED 123 

fers of the Atlantic and Gulf Coastal Plain: American Water 

Resources Association Monograph series no. 9, p. 7-24. 

Leake, S.A., 1990, Interbed storage changes and compaction in 

models of regional ground-water flow: Water Resources Research, v. 26, no. 9, p. 1939-1950. 

---1991, Simulation of vertical computation in models of regional ground-water flow, in Johnson, A.l., ed., Land Subsidence: International Symposium on Land Subsidence, 14th 

May 1991, Houston, Texas, International Association of Hydrological Sciences Publication 200, p. 565-574. 

Lindholm, G.F., 1986, Snake River Plain regional aquifer-system 

study, in Sun, R.J., ed., Regional Aquifer-System Analysis 

Program of the U.S. Geological Survey-Summary of 

projects, 1978-84: U.S. Geological Survey Circular 1002, 

p. 88-106. 

---in press, Summary of the Snake River Plain aquifer-system analysis: U.S. Geological Survey Professional Paper 

1408-A. 

Lobmeyer, D.H., 1985, Freshwater heads and ground-water temperatures in aquifers of the Northern Great Plains in parts of 

Montana, North Dakota, South Dakota, and Wyoming: U.S. 

Geological Survey Professional Paper 1402-D, 11 p. 

Luckey, R.R., and Ferrigno, D.F., 1982, A data-management system for areal interpretive data for the High Plains in parts of 

Colorado, Kansas, Nebraska, New Mexico, Oklahoma, 

South Dakota, Texas, and Wyoming: U.S. Geological Survey 

Water-Resources Investigations Report 82-4072, 112 p. 

Luckey, R.R., Gutentag, E.D., Heimes, F.J., and Weeks, J.B., 

1986, Digital simulation of ground-water flow in the High 

Plains aquifer in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S. Geological Survey Professional Paper 1400-D, 57 p. 

---1988, Effects of future ground-water pumpage on the High 

Plains aquifer in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S. Geological Survey Professional Paper 14~E. 44 p. 

Luckey, R.R., Gutentag, E.D., and Weeks, J.B., 1981, Water-level 

and saturated-thickness changes, predevelopment to 1980, in 

the High Plains aquifer in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and 

Wyoming: U.S. Geological Survey Hydrologic Investigations Atlas HA-652, 2 sheets, scale 1:2,500,000. 

MacCary, L.M., 1984, Apparent water resistivity, porosity, and 

water temperature of the Madison Limestone and underlying 

rocks in parts of Montana, Nebraska, North Dakota, South 

Dakota, and Wyoming: U.S. Geological Survey Professional 

Paper 1273-D, 14 p. 

MacCary, L.M., Cushing, E.M., and Brown, D.L., 1983, Potentially favorable areas for large-yield wells in the Red River 

Formation and Madison Limestone in parts of Montana, 

North Dakota, South Dakota, and Wyoming: U.S. Geological Survey Professional Paper 1273-E, 13 p. 

Maclay, R.W., and Small, T.A., 1986, Carbonate geology and hydrology of the Edwards aquifer in the San Antonio area, 

Texas: Texas Water Development Board Report 296, 90 p. 

Mandie, R.J., and Kontis, A.L., 1992, Simulation of regional 

ground-water flow in the Cambrian-Ordovician aquifer system in the Northern Midwest, United States: U.S. Geological Survey Professional Paper 1405-C, 97 p. 

Mandie, R.J., and Westjohn, D.B., 1989, Geohydrologic framework and ground-water flow in the Michigan Basin, in 

Swain, L.A., and Johnson, A.l., eds., Regional aquifer systems of the United States, Aquifers of the Midwestern area: 

American Water Resources Association Monograph series 

no. 13, p. 83-109. 

Martin, Angel, Jr., and Whiteman, C.D., 1991, Hydrology of the 

coastal lowlands aquifer system in parts of Alabama, Florida, Louisiana, and Mississippi: U.S. Geological Survey 

Open-File Report 91-72, 110 p. 

Maslia, M.L., and Hayes, L.R., 1988, Hydrogeology and simulated effects of ground-water development of the Floridan aquifer system, southwest Georgia, northwest Florida, and 

southernmost Alabama: U.S. Geological Survey Professional 

Paper 1403-H, 71 p. 

McDonald, M.G., and Harbaugh, A.W., 1988, A modular threedimensional finite-difference ground-water flow model: U.S. 

Geological Survey Techniques of Water-Resources Investigations, book 6, chap. A 1, 578 p. 

McFarland, W.D., 1983, A description of aquifer units in western 

Oregon: U.S. Geological Survey Open-File Report 82-165, 

35 p. 

McLean, J.S., and Johnson, A.l., eds., 1988, Regional aquifer 

systems of the United States, Aquifers of the western mountain area: American Water Resources Association Monograph series, no. 14, 229 p. 

Meisler, Harold, 1980, Preliminary delineation of salty ground 

water in the Northern Atlantic Coastal Plain: U.S. Geological Survey Open-File Report 81-71, 37 p. 

---1989, The occurrence and geochemistry of salty ground 

water in the Northern Atlantic Coastal Plain: U.S. Geological Survey Professional Paper 1404-D, 51 p. 

Meisler, Harold, Leahy, P.P., and Knobel, L.L., 1985, Effect of 

eustatic sea-level changes on saltwater-freshwater relations 

in the Northern Atlantic Coastal Plain: U.S. Geological Survey Water-Supply Paper 2255, 28 p. 

Mesko, T.W., Williams, T.A., Ackerman, D.J., and Williamson, 

A.K., 1990, Ground-water pumpage from the Gulf Coast aquifer system, 1960--85, south-central United States: U.S. 

Geological Survey Water-Resources Investigations Report 

89-4180, 177 p. 

Meyer, F.W., 1989, Hydrogeology, ground-water movement, and 

subsurface storage in the Floridan aquifer system in southern 

Florida: U.S. Geological Survey Professional Paper 1403--G, 

59 p. 

Meyer, W.R., and Carr, J.E., 1979, A digital model for simulation 

of ground-water hydrology in the Houston area, Texas: Texas 

Department of Water Resources Report LP-103, 131 p. 

Miller, J.A., 1986, Hydrogeologic framework of the Floridan aquifer system in Florida and in parts of Georgia, South Carolina, and Alabama: U.S. Geological Survey Professional 

Paper 1403-B, 91 p. 

---1988, Geohydrologic data from the Floridan aquifer system 

in Florida and in parts of Georgia, South Carolina, and Alabama: U.S. Geological Survey Open-File Report 88-86,678 p. 

---1990, Ground water atlas of the United States, Segment 

6---Alabama, Florida, Georgia, and South Carolina: U.S. 

Geological Survey Hydrologic Investigations Atlas HA730--G, 28 p. 
124 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

---1992, Summary of the hydrology of the Southeastern 

Coastal Plain aquifer system in Mississippi, Alabama, Georgia, and South Carolina: U.S. Geological Survey Professional Paper 1410-A, 38 p. 

Miller, J.A., Barker, R.A., and Renken, R.A., 1987, Hydrogeology of the Southeastern Coastal Plain aquifer system-An 

overview, in Vecchioli, John, and Johnson, A.I., eds., Regional aquifer systems of the United States, Aquifers of the 

Atlantic and Gulf Coastal Plain: American Water Resources 

Association Monograph series no. 9, p. 53-57. 

Miller, J.A., and Renken, R.A., 1988, Nomenclature of regional 

hydrogeologic units of the Southeastern Coastal Plain aquifer system: U.S. Geological Survey Water-Resources Investigations Report 87-4202, 21 p. 

Morrissey, D.J., Randall, A.D., and Williams, J.H., 1988, Upland 

runoff as a major source of recharge to stratified drift in the 

glaciated Northeast, in Randall, A.D., and Johnson, A.I., 

eds., Regional aquifer systems of the United States, The 

Northeast Glacial Aquifers: American Water Resources Association Monograph series no. 11, p. 17-36. 

Mount, J.R., Rayner, F.A., Shamburger, V.M., Jr., Peckam, R.C., 

and Osborne, F.L., Jr., 1967, Reconnaissance investigation 

of the ground-water resources of the Colorado River Basin, 

Texas: Texas Water Development Board Report 51, 107 p. 

Newton, G.D., 1991, Geohydrology of the regional aquifer system, western Snake River Plain, southwestern Idaho: U.S. 

Geological Survey Professional Paper 1408--G, 52 p. 

Norris, S.P., 1959, Vertical leakage through till as a source of 

recharge to a buried-valley aquifer at Dayton, Ohio: Ohio 

Department of Natural Resources, Division of Water, Technical Report no. 2, 16 p. 

Olcott, P.G., 1992, Ground Water Atlas of the United States, Segment 9-Iowa, Michigan, Minnesota, and Wisconsin: U.S. 

Geological Survey Hydrologic Investigations Atlas HA730-J, 31 p. 

Page, R.W., 1986, Geology of the fresh ground-water basin of the 

Central Valley, California, with texture maps and sections: 

U.S. Geological Survey Professional Paper 1401-C, 54 p. 

Pardee, J.T., 1910, The glacial Lake Missoula: Journal of Geology, v. 18,no.4,p. 376-386. 

Peterson, J.A., 1984, Stratigraphy and sedimentary facies of the 

Madison Limestone and associated rocks in parts of Montana, Nebraska, North Dakota, and Wyoming: U.S. Geological Survey Professional Paper 1273-A, 34 p. 

Plummer, L.N., 1977, Defining reactions and mass transfer in 

part of the Floridan aquifer: Water Resources Research, 

~ 13, no. 5,p. 801-812. 

Plummer, L.N., Busby, J.F., Lee, R.W., and Hanshaw, B.B., 

1990, Geochemical modeling of the Madison aquifer in 

parts of Montana, Wyoming, and South Dakota: Water Resources Research, v. 26, no. 9, p. 1981-2014. 

Prince, K.R., and Johnson, A.I., eds., 1992, Regional aquifer systems of the United States, Aquifers of the Far West: American Water Resources Association Monograph series no. 16, 

127 p. 

Prudic, D.E., Harrill, J.E., and Burbey, T.J., in press, Conceptual 

evaluation of regional ground-water flow in the carbonaterock province of the Great Basin, Nevada, Utah, and adjacent States: U.S. Geological Survey Professional Paper 

1409-D. 

Quinones-Aponte, Vicente, 1991, Water-resources development 

and its influence on the water budget for the aquifer system 

in the Salinas to Patillas area, Puerto Rico, in GomezGomez, Fernando, and Johnson, A.I., eds., Regional aquifer 

systems of the United States, Aquifers of the Caribbean Islands: American Water Resources Monograph series no. 15, 

p. 37-55. 

Randall, A.D., and Johnson, A.I., eds., 1988, Regional aquifer 

systems of the United States, The Northeast Glacial Aquifers: American Water Resources Association Monograph series no. 11, 156 p. 

Reid, M.S., Renken, R.A., Wait, R.L., Aucott, W.R., and Lee, 

R.W., 1986, Hydrologic and geologic analysis of two wells 

in Marion County, South Carolina: U.S. Geological Survey 

Water-Resources Investigations Report 86-4102, 20 p. 

Renken, R.A., 1984, Hydrogeology of the Southeastern Coastal 

Plain aquifer system of the United States: U.S. Geological 

Survey Water-Resources Investigations Report 84-4243, 269 p. 

---in press, Geology and hydrogeology of the Southeastern 

Coastal Plain aquifer system of Mississippi, Alabama, Georgia, and South Carolina and adjacent areas of North Carolina and Florida: U.S. Geological Survey Professional Paper 

1410-B. 

Renken, R.A., Gomez-Gomez, Fernando, Quinones-Aponte, Vicente, and Dacosta, Rafael, 1991, Structure and depositional 

patterns and their influence on the hydraulic conductivity of 

fan-deltas in southern Puerto Rico, in Gomez-Gomez, Fernando, and Johnson, A.I., eds., Regional aquifer systems of 

the United States, Aquifers of the Caribbean Islands: American Water Resources Monograph series no. 15, p. 25-36. 

Robertson, F.N., 1991, Geochemistry of ground water in alluvial 

basins of Arizona and adjacent parts of Nevada, New Mexico, and California: U.S. Geological Survey Professional 

Paper 1406-C, 90 p. 

Rodriquez-Martinez, Jesus, 1991, The hydrogeologic framework 

of the Northern Coastal Province aquifer system of Puerto 

Rico, in Gomez-Gomez, Fernando, and Johnson, A.I., Regional aquifer systems of the United States, Aquifers of Caribbean Islands: American Water Resources Association 

Monograph series no. 15, p. 5-16. 

Ryder, P.D., 1985, Hydrology of the Floridan aquifer system in 

west-central Florida: U.S. Geological Survey Professional 

Paper 1403-F, 63 p. 

Ryder, P.D., and Ardis, A.F., 1991, Hydrology of the Texas Gulf 

Coast aquifer systems: U.S. Geological Survey Open-File 

Report 91-64, 127 p. 

Sasman, R.T., Ludwigs, R.S., Benson, C.R., and Kirk, J.R., 1986, 

Water-level trends and pumpage in the Cambrian and Ordovician aquifers in the Chicago region, 1980-1985: Illinois 

State Water Survey Circular 166, 47 p. 

Siegel, D.l., 1989, Geochemistry of the Cambrian-Ordovician aquifer system in the Northern Midwest: U.S. Geological Survey Professional Paper 1405-D, 76 p. 

---1991, Evidence for dilution of deep, confined ground 

water by vertical recharge of isotopically heavy Pleistocene 

water: Geology, v. 19, p. 433-436. 

Signor, D.C., and Imes, J.L., 1989, Geohydrology of regional aquifer systems in Cretaceous and older rocks underlying the 

Central United States, in Swain, L.A., and Johnson, A.I., 

eds., Regional aquifer systems of the United States, Aquifers 
REFERENCES CITED 125 

of the Midwestern area: American Water Resources Association Monograph series no. 13, p. 149-163. 

Sinclair, W.C., 1982, Sinkhole development resulting from 

ground-water withdrawal in the Tampa area, Florida: U.S. 

Geological Survey Water-Resources Investigations Report 

81-50, 19 p. 

Solley, W.B., Chase, E.B., and Mann, W.B., IV, 1983, Estimated 

use of water in the United States in 1980: U.S. Geological 

Survey Circular 1001, 56 p. 

Solley, W.B., Merk, C.F., and Pierce, R.R., 1988, Estimated use 

of water in the United States in 1985: U.S. Geological Survey Circular 1004, 82 p. 

Souza, W.R., and Voss, C.l., 1986, Modeling a regional aquifer 

containing a narrow transition between freshwater and saltwater using a solute transport simulation, Part 11-Analysis 

of a coastal aquifer system: Delft University of Technology 

Salt Water Intrusion Meeting, 9th, Delft, The Netherlands, 

May 12-16, 1986, Proceedings, p. 457-473. 

Sprinkle, C.L., 1989, Geochemistry of the Floridan aquifer system in Florida and in parts of Georgia, South Carolina, and 

Alabama: U.S. Geological Survey Professional Paper 1403-

1, 105 p. 

Strobel, M.L., 1990, Characteristics affecting the vertical flow 

of ground water through selected glacial deposits in Ohio: 

Columbus, Ohio State University, M.S. thesis, 185 

p. [unpublished]. 

Sun, R.J., ed., 1986, Regional Aquifer-System Analysis Program 

of the U.S. Geological Survey-Summary of projects, 1978-

84: U.S. Geological Survey Circular 1002, 264 p. 

Sun R.J., and Weeks, J.B., 1991, Bibliography of Regional Aquifer-System Analysis Program of the U.S. Geological Survey, 

1978-1991: U.S. Geological Survey Water-Resources Investigations Report 91-4122, 92 p. 

Suter, Max, Bergstrom, R.E., Smith, H.F., Emrich, G.H., Walton, 

W.C., and Larson, T.E., 1959, Preliminary report on groundwater resources of the Chicago region, Illinois: Illinois State 

Water Survey and State Geological Survey Cooperative 

Groundwater Report 1, 89 p. 

Swain, L.A., Hollyday, E.F., Daniel, C.C., III, and Zapecza, O.S., 

1991, Plan of study for the regional aquifer-system analysis 

of the Appalachian Valley and Ridge, Piedmont, and Blue 

Ridge physiographic provinces of the Eastern and Southeastem United States, with a description of the study-area geology and hydrology: U.S. Geological Survey Water-Resources 

Investigations Report 91-4066, 44 p. 

Swain, L.A., and Johnson, A.I., eds., 1989, Regional aquifers of 

the United States, Aquifers of the Midwestern area: American 

Water Resources Association Monograph series no. 13, 238 p. 

Swanson, D.A., and Wright, T.L., 1978, Bedrock geology of 

northern Columbia Plateau and adjacent areas in the Channeled Scablands, chap. 3, in Baker, V.R., and Nummedal, D., 

eds., Planetary Geology program: Washington, D.C., National Aeronautics and Space Administration, p. 37-57. 

Swanson, D.A., Wright, T.L., Hooper, P.R., and Bentley, R.P., 

1979, Revisions in stratigraphic nomenclature of the Columbia River Basalt Group: U.S. Geological Survey Bulletin 

1457-G, 59 p. 

Takasaki, K.J., and Mink, J.F., 1985, Evaluation of major dikeimpounded ground-water reservoirs, island of Oahu: U.S. 

Geological Survey Water-Supply Paper 2217, 77 p. 

Thayer, P.A., 1984, Relationship of porosity and permeability to 

petrology of the Madison Limestone in rock cores from 

three test wells in Montana and Wyoming: U.S. Geological 

Survey Professional Paper 1273-C, 29 p. 

Thelin, G.P., and Heimes, F.J., 1987, Mapping irrigated cropland 

from Landsat data for determination of water use from the 

High Plains aquifer in parts of Colorado, Kansas, Nebraska, 

New Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S. Geological Survey Professional Paper 1400-C, 38 p. 

Tibbals, C. H., 1990, ·Hydrology of the Floridan aquifer system in 

east-central Florida: U.S. Geological Survey Professional 

Paper 1403-E, 98 p. 

Torak, L.J., 1982, Modification and corrections to the finite-difference model for simulation of three-dimensional ground-water 

flow: U.S. Geological Survey Water-Resources Investigations 

Report 82-4025, 67 p. 

Trapp, Henry, Jr., 1993, Hydrogeological framework of the 

Northern Atlantic Coastal Plain, in parts of North Carolina, 

Virginia, Maryland, Delaware, New Jersey, and New York: 

U.S. Geological Survey Professional Paper 1404-G. 

Trapp, Henry, Jr., Knobel, L.L., Meisler, Harold, and Leahy P.P., 

1984, Test well DO-CE88 at Cambridge, Dorchester County, 

Maryland: U.S. Geological Survey Water-Supply Paper 

2229,48 p. 

Trescott, P.C., 1975, Documentation of finite-difference model 

for simulation of three-dimensional ground-water flow: U.S. 

Geological Survey Open-File Report 75-438, 104 p. 

Trescott, P.C., and Larson, S.P., 1976, Supplement to Open-File 

Report 75-438, Documentation of finite-difference model for 

simulation of three-dimensional ground-water flow: U.S. 

Geological Survey Open-File Report 76-591, 21 p. 

U.S. Geological Survey, 1970, The National atlas _of the United 

States: Washington, D.C., U.S. Geological Survey, 417 p. 

U.S. House of Representatives, 1977, Report of Committee on 

Appropriations for the Department of the Interior and related 

agencies for fiscal year 1978: Washington, D.C., 95th Congress, First Session, House Report no. 95-392. 

Vecchioli, John, and Johnson, A.l., eds., 1987, Regional aquifer 

systems of the United States, Aquifers of the Atlantic and 

Gulf Coastal Plain, American Water Resources Association 

Monograph series no. 9, 173 p. 

Voss, C.l., 1984, A finite-element simulation model for saturatedunsaturated fluid-density-dependent ground-water flow with 

energy transport or chemically-reactive single-species solute 

transport: U.S. Geological Survey Water-Resources Investigations Report 84-4369, 409 p. 

Voss, C.l., and Souza, W.R., 1986, Modeling a regional aquifer 

containing a narrow transition between freshwater and saltwater using a solute transport simulation, Part 1-Theory 

and methods: Delft University of Technology Salt Water Intrusion Meeting, 9th, Delft, The Netherlands, May 12-16, 

1986, Proceedings, p. 493-514. 

Waitt, R.B., Jr., 1985, Case for periodic, colossal jokulhlaups 

from Pleistocene Glacial Lake Missoula: Geological Society 

of America Bulletin, v. 96, p. 1271-1286. 

Wallace, R.H., Jr., Kraemor, T.F., Taylor, R.E., and Wesselman, J.B., 

1979, Assessment of the geopressured-geothermal resources in 

the northern Gulf of Mexico basin, in Muffler, L.J.P., ed., 

Assessment of geothermal resources of the United States, 

1978: U.S. Geological Survey Circular790, p. 132-155. 
126 REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM OF THE U.S. GEOLOGICAL SURVEY, 1978-1992 

Weeks, A.W., 1945, Balcones, Luling, and Mexia fault zones in 

Texas: American Association of Petroleum Geologists Bulletin, v. 29, no. 12, p. 1733-1737. 

Weeks, J.B., Gutentag, B.D., Heimes, F.J., and Luckey, R.R., 

1988, Summary of the High Plains regional aquifer-system 

analysis in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming: U.S. 

Geological Survey Professional Paper 1400-A, 30 p. 

Weiss, Emanuel, 1982, A model for the simulation of flow of 

variable density ground water in three-dimensions under 

steady-state conditions: U.S. Geological Survey Open-File 

Report 82-352, 66 p. 

Westjohn, D.B., 1989, Application of geophysics in the delineation of the freshwater/saline-water interface in the Michigan 

Basin, in Swain, L.A., and Johnson, A.I., eds., Regional aquifer systems of the United States, Aquifers of the Midwestem area: American Water Resources Association Monograph 

series no. 13, p. 111-134. 

Whitehead, R.L., 1992, Geohydrologic framework of the Snake 

River Plain regional aquifer system, Idaho and eastern Oregon: U.S. Geological Survey Professional Paper 1408-B, 

32p. 

Wilkens, D. W., 1986, Southwest alluvial-basin regional aquifersystems study in parts of Colorado, New Mexico, and Texas, 

in Sun, R.J., ed., Regional Aquifer-System Analysis Program 

of the U.S. Geological Survey-Summary of projects, 1978-

84: U.S. Geological Survey Circular 1002, p. 107-115. 

Williamson, A.K., Grubb, H.F., and Weiss, J.S., 1990, Groundwater flow in the Gulf Coast aquifer system, south-central 

United States-A preliminary analysis: U.S. Geological 

Survey Water-Resources Investigations Report 89-4071, 

124 p. 

Williamson, A.K., Prudic, D.E., and Swain, L.A., 1989, Groundwater flow in the Central Valley, California: U.S. Geological 

Survey Professional Paper 1401-D, 127 p. 

Wood, W.W., and Low, W.H., 1988, Solute geochemistry of the 

Snake River Plain regional aquifer system, Idaho and eastem Oregon: U.S. Geological Survey Professional Paper 

1408-D, 79 p. 

Young, H.L., 1992a, Summary of ground-water hydrology of the 

Cambrian-Ordovician aquifer system in the Northern Midwest: 

U.S. Geological Survey Professional Paper 1405-A, 55 p. 

---1992b, Hydrogeology of the Cambrian-Ordovician aquifer system in the Northern Midwest: U.S. Geological Survey 

Professional Paper 1405-B, 97 p. 

Young, H.L., MacKenzie, A.J., and Mandie, R.J., 1989a, Simulation of ground-water flow in the Cambrian-Ordovician aquifer system in the Chicago-Milwaukee area of the Northern 

Midwest, in Swain, L.A., and Johnson, A.I., eds., Regional 

aquifer systems of the United States, Aquifers of the Midwestern area: American Water Resources Association Monograph series no. 13, p. 39-72. 

Young, H.L., Mandie, R.J., Kontis, A.L., and Siegel, D.I., 1989b, 

The Cambrian-Ordovician regional aquifer system in the 

Northern Midwest-A summary, in Swain, L.A., and Johnson, 

A.I., Regional aquifer systems of the United States, Aquifers 

of the Midwestern area: American Water Resources Association Monograph series no. 13, p. 5-37. 

*'U.S. ~ PIUNl'IR; OFFICE: 1994-683-048/10010 


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS 

Periodicals 

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly). 

Technical Books and Reports 

Professional Papers are mainly comprehensive scientific reports of wide and lasting interest and importance to professional 

scientists and engineers. Included are reports on the results of 

resource studies and of topographic, hydrologic, and geologic investigations. They also include collections of related papers addressing 

different aspects of a single scientific topic. 

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the 

results of resource studies and of geologic and topographic investigations, as well as collections of short papers related to a specific topic. 

Water-Supply Papers are comprehensive reports that present 

significant interpretive results of hydrologic investigations of wide 

interest to professional geologists, hydrologists, and engineers. The 

series covers investigations in all phases of hydrology, including 

hydrogeology, availability of water, quality of water, and use of 

water. 

Circulars present administrative information or important scientific information of wide popular interest in a format designed for 

distribution at no cost to the public. Information is usually of shortterm interest. 

Water-Resource Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS 

publications series. Copies are reproduced on request unlike formal 

USGS publications, and they are also available for public inspection 

at depositories indicated in USGS catalogs. 

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may 

be cited in other publications as sources of information. 

Maps 

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 7 I/2- or I5-minute quadrangle formats (scales 

mainly I :24,000 or I :62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps 

include structure and columnar sections only. 

Geophysical Investigations Maps are on topographic or planimetric bases at various scales; they show results of surveys using 

geophysical techniques, such as gravity, magnetic, seismic, or 

radioactivity, which reflect subsurface structures that are of economic or geologic significance. Many maps include correlations 

with the geology. 

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; 

they present a wide variety of format and subject matter. The series 

also includes 7 I/2-minute quadrangle photogeologic maps on planimetric bases that show geology as interpreted from aerial photographs. Series also includes maps of Mars and the Moon. 

Coal Investigations Maps are geologic maps on topographic 

or planimetric bases at various scales showing bedrock or surficial 

geology, stratigraphy, and structural relations in certain coal-resource areas. 

Oil and Gas Investigations Charts show stratigraphic information for certain oil and gas fields and other areas having petroleum 

potential. 

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or 

irregular areas at various scales. Pre-I97I maps show bedrock 

geology in relation to specific mining or mineral-deposit problems; 

post-I97I maps are primarily black-and-white maps on various 

subjects, such as environmental studies or wilderness mineral investigations. 

Hydrologic Investigations Atlases are multicolor or black-andwhite maps on topographic or planimetric bases presenting a wide 

range of geohydrologic data of both regular and irregular areas; 

principal scale is I :24,000, and regional studies are at I :250,000 scale 

or smaller. 

Catalogs 

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under the conditions indicated below from the U.S. Geological 

Survey, Books and Open-File Reports Sales, Federal Center, Box 

25286, Denver, CO 80225. (See latest Price and Availability List.) 

"Publications of the Geological Survey, 1879-1961" may be 

purchased by mail and over the counter in paperback book form and 

as a set of microfiche. 

"Publications of the Geological Survey, 1962-1970" may be 

purchased by mail and over the counter in paperback book form and 

as a set of microfiche. 

"Publications of the Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche. 

Supplements for I982, I983, I984, I985, I986, and for subsequent years since the last permanent catalog may be purchased by 

mail and over the counter in paperback book form. 

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by 

mail and over the counter in paperback booklet form only. 

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback 

booklet form only. 

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" are available free of charge by mail or may 

be obtained over the counter in paperback booklet form only. Those 

wishing a free subscription to the monthly catalog "New Publications 

of the U.S. Geological Survey" should write to the U.S. Geological 

Survey, 582 National Center, Reston, VA 22092. 

Note.--Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, and publications.