U .S . GEOLOGICAL SURVEY PROFESSIONAL PAPER 1404--F
Conceptualization and Analysis of


Ground-Water Flow System in the 

Coastal Plain of Virginia and Adjacent


Parts of Maryland and North Carolina


By JOHN F. HARSH and RANDELLJ. LACZNIAK 

REGIONAL AQUIFER-SYSTEM ANALYSISNORTHERN ATLANTIC COASTAL PLAIN 

U .S . GEOLOGICAL SURVEY PROFESSIONAL PAPER 1404-F 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1990

U.S . DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary 

U.S . GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

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 

Harsh, John F. 

Conceptualization and analysis of ground-water flow system in the Coastal Plain of Virginia and adjacent parts of Maryland and 

North Carolina . 

(U.S. Geological Survey professional paper ; 1404-F) 

Bibliography : p. 

Supt. of Does . no . : I 19 .16:1404-F 

1. Groundwater flow-Virginia. 1. Laczniak, Randell J. II . Title. III. Series . 

GB1197.7 .H37 1990 551.49'09755 87-600217 

For sale by the Books and Open-File Reports Section, U.S . Geological Survey,

Federal Center, Box 25425, Denver, CO 80225 
FOREWORD 

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM 

The Regional Aquifer-System Analysis (RASA) Program was started in 

1978 following a congressional mandate to develop quantitative appraisals of 

the major ground-water systems of the United States. The RASA Program represents a systematic effort to study a number of the Nation's most 

important aquifer systems, which in aggregate underlie much of the country

and which represent an important component of the Nation's total water 

supply. In general, the boundaries of these studies are identified by the 

hydrologic extent of each system and accordingly transcend the political

subdivisions to which investigations have often arbitrarily been limited in the 

past. The broad objective for each study is to assemble geologic, hydrologic,

and geochemical information, to analyze and develop an understanding of the 

system, and to develop predictive capabilities that will contribute to the 

effective management of the system. The use of computer simulation is an 

important element of the RASA studies, both to develop an understanding of 

the natural, undisturbed hydrologic system and the changes brought about in 

it by human activities, and to provide a means of predicting the regional 

effects of future pumping or other stresses. 

The final interpretive results of the RASA Program are presented in a series 

of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number, 

and where the volume of interpretive material warrants, separate topical 

chapters that consider the principal elements of the investigation may be 

published . The series of RASA interpretive reports begins with Professional 

Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available. 

Dallas L. Peck 

Director 
CONTENTS


Page 

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III 

Abstract . 

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 Ground-Water Use. . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

General Setting and Location of Study Area . . . . . . . . . . . . . . . . . . . . . 2
 Analysis of the Ground-Water Flow System. . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

Previous Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 Model Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

Conceptualization of Ground-Water Flow System. . . . . . . . . . . . . . . . . . . . . 4
 Grid Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Hydraulic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 Hydraulic Characteristics and Stresses . . . . . . . . . . . . . . . . . . . . . . 17 

Description of Hydrogeologic Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 Transmissivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Columbia Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
 Storage Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

Yorktown-Eastover Aquifer and Yorktown Confining
 Vertical Leakance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
 Ground-Water Recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

St . Marys-Choptank Aquifer and St . Marys Confining
 Streambed Leakance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Chickahominy-Piney Point Aquifer and Calvert
 Withdrawal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

Confining Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
 Lateral Boundary Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

Aquifer 5 and Confining Unit 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8


Strategy of Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

Aquifer 4 and Confining Unit 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8


Prepumping Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

Brightseat-Upper Potomac Aquifer and Confining
 Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 

Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
 Limitation and Application of the Flow Model . . . . . . . . . . . . 82 

Middle Potomac Aquifer and Confining Unit . . . . . . . . . . . . . . . 9
 Summary and Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 

Lower Potomac Aquifer and Confining Unit . . . . . . . . . . . . . . . . 10
 References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 

Page 

Conceptualization of Ground-Water Flow System-Continued 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F1 

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


Hydrogeologic Effects of Pleistocene Erosion . . . . . . . . . . . . . . . . . . . F10 

Ground-Water Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Time Discretization and Ground-Water 

Aquia Aquifer and Nanjemoy-Marlboro Confining


Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8


Simulation of the Ground-Water Flow System . . . . . . . . . . . . . . . . . . 40 

Pumping Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 

ILLUSTRATIONS 

Page 

FIGURE 1. Map showing location of study area within the northern Atlantic Coastal Plain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F3 

2. Generalized hydrogeologic section of eastward-thickening wedge of alternating sand and clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


3,4. Maps showing: 

3. Locations of core holes and aquifer test sites in the Virginia Coastal Plain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6


4. Approximate locations and depths of Pleistocene channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


5. Schematic diagram of generalized prepumping ground-water flow system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12


6,7. Graphs showing: 

6. Estimated annual ground-water withdrawal from Coastal Plain aquifers, 1891-1980. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13


7. Average annual withdrawal from the lower, middle, and upper Potomac aquifers in the Franklin area, 

Virginia, 1891-1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14


8. Map showing locations of major ground-water users. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15


9. Schematic diagram of model conceptualization of prepumping ground-water system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16


10 . Conceptual and model representations of aquifers and confining units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17


11 . Map showing finite-difference grid and model boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18


12 . Cross section of finite-difference grid and boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

VI CONTENTS 

Page 

FIGURES 13-21. Maps showing transmissivity used in model simulations: 

13. Columbia aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F20 

14. Yorktown-Eastover aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21


15. St . Marys-Choptank aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22


16 . Chickahominy-Piney Point aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23


17 . Aquia aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24


18 . Aquifer 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25


19 . Brightseat-upper Potomac aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26


20 . Middle Potomac aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27


21 . Lower Potomac aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28


22 . Schematic diagrams of the conceptual and simulated aquifer contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29


23-30. Maps showing vertical leakance used in model simulations : 

23 . Yorktown confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31


24 . St. Marys confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32


25 . Calvert confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33


26 . Nanjemoy-Marlboro confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34


27 . Confining unit 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35


28 . Brightseat-upper Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36


29 . Middle Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37


30 . Lower Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38


31 . Graph showing estimated annual withdrawal and average withdrawal calculated for simulated pumping periods . . . . . . . . . . 39


32 . Schematic diagram showing model conceptualizations of the water-table aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41


33-110 . Maps showing: 

33 . Simulated prepumping potentiometric surface of the Yorktown-Eastover aquifer and measured water 

levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42


34 . Simulated prepumping potentiometric surface of the St. Marys-Choptank aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43


35. Simulated prepumping potentiometric surface of the Chickahominy-Piney Point aquifer and measured water 

levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44


36. Simulated prepumping potentiometric surface of the Aquia aquifer and measured water levels . . . . . . . . . . . . . . . . . . . . 45


37. Simulated prepumping potentiometric surface of aquifer 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46


38. Simulated prepumping potentiometric surface of the Brightseat-upper Potomac aquifer and measured water 

levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47


39. Simulated prepumping potentiometric surface of the middle Potomac aquifer and measured water levels . . . . . . 48


40. Simulated prepumping potentiometric surface of the lower Potomac aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49


41-49. Maps showing direction of simulated prepumping flow : 

41 . Across the Yorktown confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50


42. Across the St . Marys confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51


43 . Across the Calvert confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52


44 . Across the Nanjemoy-Marlboro confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53


45 . Across confining unit 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54


46 . Across the Brightseat-upper Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55


47 . Across the middle Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56


48 . Across the lower Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57


49 . Into and out of the confined system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58


50-57. Maps showing: 

50 . Simulated potentiometric surface of the Yorktown-Eastover aquifer and measured water levels, 1980 . . . . . . . . . . 60


51 . Simulated potentiometric surface of the St . Marys-Choptank aquifer, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61


52 . Simulated potentiometric surface of the Chickahominy-Piney Point aquifer and measured water levels, 1980 . 62


53 . Simulated potentiometric surface of the Aquia aquifer and measured water levels, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63


54 . Simulated potentiometric surface of aquifer 4, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64


55 . Simulated potentiometric surface of the Brightseat-upper Potomac aquifer and measured water levels, 1980 . 65


56 . Simulated potentiometric surface of the middle Potomac aquifer and measured water levels, 1980 . . . . . . . . . . . . . . . 66


57 . Simulated potentiometric surface of the lower Potomac aquifer, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67


58-61. Graphs showing simulated and measured change in water levels for history of ground-water development: 

58 . Wells 62C 4, 59H 1, 55B 25, and 59J 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68


59 . Wells 55D 3, 56N 1, 57C 12, and 58K 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68


60 . Wells 55B 22, 55B 45, 54C 1, and 52G 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69


61 . Wells 52J 12, 55A 3, 55B 36, and 59K 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

CONTENTS VII 

Page 

FIGURE 62. Map showing locations of observation wells used in model calibration for pumping conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F70 

63-71. Maps showing direction of simulated flow, 1980 : 

63 . Across the Yorktown confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71


64 . Across the St . Marys confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72


65 . Across the Calvert confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73


66 . Across the Nanjemoy-Marlboro confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74


67 . Across confining unit 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75


68 . Across the Brightseat-upper Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76


69 . Across the middle Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77


70 . Across the lower Potomac confining unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78


71 . Into and out of the confined system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79


72, 73 . Hydrographs showing effects on simulated heads of varying the calibrated value of transmissivity of the middle


Potomac aquifer: 

72 . Wells 54C 1 and 52G 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80


73 . Wells 55B 22 and 55B 45 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80


74, 75 . Hydrographs showing effects on simulated heads of varying the calibrated value of vertical hydraulic conductivity of 

the middle Potomac confining unit: 

74 . Wells 55B 22 and 52G 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81


75 . Wells 55B 45 and 54C 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81


76, 77 . Hydrographs showing effects on simulated heads of varying the calibrated value of storage coefficient : 

76 . Wells 59H 1 and 56N 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81


77 . Wells 55B 25 and 54C 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81


78, 79 . Hydrographs showing effects on simulated heads of transient leakage:


78. Wells 58K 7 and 54C 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82


79. Wells 55B 22 and 52G 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82


TABLES 

Page 

TABLE 1. Relation of stratigraphic formations and hydrogeologic units of the Virginia Coastal Plain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F89 

2. Correlation of hydrogeologic units of Maryland, Virginia, and North Carolina and corresponding layers used in the flow 

model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90


3. Transmissivities and storage coefficients determined for the lower and middle Potomac aquifers and the Brightseat-upper 

Potomac aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91


4. Vertical hydraulic conductivities of confining units determined by laboratory methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92


5. Major withdrawals by aquifer, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93


6. Average estimated and model-calibrated values of lateral and vertical hydraulic conductivity for aquifers and confining 

units, respectively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94


7. Minimum and maximum values of transmissivity for aquifers and vertical leakance values derived by model calibration . . . . . . . . . 95


S. Average withdrawal from each aquifer used in the calibrated model, by pumping period from 1891 to 1980 . . . . . . . . . . . . . . . . . . . . . . . . . 96


9. Computed lateral boundary fluxes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97


10 . Computed leakage rates across confining units into and out of the confined flow system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98


11 . Model-computed ground-water budgets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99


12 . Summary of sensitivity tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100


13 . Specific storage and computed storage coefficients of confining units used for sensitivity tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

VIII CONTENTS 

METRIC CONVERSION FACTORS 

For readers who wish to convert measurements from the inch-pound system of units to the 

metric system of units, the conversion factors are listed below: 

Multiply inch-pound unit By To obtain metric unit 

foot (ft) 0 .3048 meter (m) 

mile (mi) 1 .609 kilometer (km) 

square mile (mil) 2 .590 square kilometer (km2) 

foot per day (ft/d) 0 .3048 meter per day (m/d) 

foot squared per day (ft 2/d) 0 .0929 meter squared per day (m2/d) 

gallon per minute (gal/min) 0.06309 liter per second (L/s) 

gallon per day (gal/d) 3.78540 liter per day (L/d) 

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s) 

inch per year (in/yr) 25.4 millimeter per year (mm/yr) 

ALTITUDE DATUM 

Sea level : In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 

(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. 
REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

CONCEPTUALIZATION AND ANALYSIS OF GROUND-WATER FLOW 

SYSTEM IN THE COASTAL PLAIN OF VIRGINIA AND ADJACENT 

PARTS OF MARYLAND AND NORTH CAROLINA 

By JOHN F. HARSH and RANDELL J. LACZNIAK 

ABSTRACT 

This report presents the results of a study of the ground-water flow 

system in the Coastal Plain of Virginia and adjacent parts of Maryland 

and North Carolina . The ground-water flow system consists of a 

water-table aquifer and an underlying sequence of confined aquifers 

and intervening confining units composed of unconsolidated sand and 

clay . Water levels have declined steadily, and cones of depression have 

expanded and coalesced around major ground-water withdrawal 

centers A digital flow model was developed to enhance knowledge of 

the behavior of the ground-water flow system in response to its 

development. Transmissivity and vertical leakance maps were developed for each aquifer and confining unit . The model was calibrated to 

simulate ground-water flow within the system under both prepumping and pumping conditions . Simulated prepumping potentiometricsurface maps indicate that regional movement of ground water was 

from the Fall Line toward coastal areas and that local movement of 

ground water was from interfluves toward major river valleys. Maps of 

simulated prepumping flow across confining units show that most 

recharge occurred in narrow bands approximately parallel to the Fall 

Line and under interfluves and that discharge was toward major river 

valleys and coastal water. Simulated prepumping rates of recharge 

into the confined aquifer system from the water-table aquifer varied 

up to 3 .2 inches per year (in/yr), and rates of discharge out of the 

confined system varied up to 2.8 in/yr. 

Ten pumping periods covering 90 years (yr) of withdrawal 

simulated the history of ground-water development. Simulated 

potentiometric-surface maps for 1980 show lowered water levels and 

the development of coalescing cones of depression around the cities of 

Franklin, Suffolk, and Williamsburg and the town of West Point, all in 

Virginia . The largest simulated decline in water level, about 210 feet 

(ft), was near Franklin . Water budgets indicate that over the period of 

simulation (1891-1980) (1) pumpage from the model area increased 

by about 105 million gallons per day (Mgal/d), (2) lateral boundary 

outflow increased by about 5 Mgal/d, (3) ground-water flow to streams 

and coastal water decreased by about 107.5 Mgal/d, (4) lateral boundary inflow increased by about 0.7 Mgal/d, and (5) water released from 

aquifer storage increased by about 1 .6 Mgal/d . The difference 

Manuscript approved for publication July 18, 1986. 

between total inflow and total outflow is the numerical truncation 

error of the digital simulation . Analysis of water budgets for individual 

confined aquifers shows that the major source of water supplied to 

wells was vertical leakage induced through confining units by pumping. Simulated rates of recharge into the confined aquifer system at 

the end of the final pumping period (1980) varied up to 3.8 in/yr, and 

simulated rates of discharge out of the confined system varied up to 

2.2 in/yr. Results of simulations show an increase of about 110 Mgal/d 

into the confined system from the unconfined system over the period 

of simulation . This increase in flow into the confined system affected 

local discharge of ground water to streams and regional discharge to 

coastal water. Withdrawal of ground water from the confined aquifers 

also induced brackish water from Chesapeake Bay into the confined 

system . 

Results of sensitivity analyses indicate that simulated water levels are 

more sensitive to decreases in aquifer transmissivity and confining unit 

vertical hydraulic conductivity than to increases in these properties . 

Lowering the storage coefficient of an aquifer had minimal effect on 

simulated water levels, whereas increasing the storage coefficient had 

a much more significant effect . The effect of confining unit storage is 

shown to be insignificant if it is assumed that the water released from 

confining unit storage is attributable to the compressibility of water 

only . 

INTRODUCTION 

Ground water is an important source of industrial, 

municipal, domestic, and agricultural water supplies in 

the northern Atlantic Coastal Plain. The continued withdrawal of ground water has caused a steady decline of 

water levels and the expansion and coalescence of cones 

of depression centered at major pumping centers. This 

decline concerns ground-water users and those responsible for the study and management of the resource. More 

hydrologic information is needed to better understand 

ground-water flow in the aquifers of the northern Atlantic Coastal Plain. 

In 1978, the U.S . Geological Survey began a comprehensive program of regional investigations, known as the 

F1 
F2 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

Regional Aquifer-System Analysis (RASA), to describe 

the hydrogeology of major aquifers in the United States. 

The study of the northern Atlantic Coastal Plain aquifer 

system began in 1979. The northern Atlantic Coastal 

Plain was divided into five subregional projects extending from Long Island, N.Y., through North Carolina 

(Meisler, 1980) . One of the five subregional projects 

defines the hydrogeologic framework and analyzes 

ground-water flow in the multiaquifer system of the 

Virginia Coastal Plain. Two reports have resulted from 

the subregional project: a report by Meng and Harsh 

(1988) that describes the hydrogeologic framework, and 

this report, which provides the results of an analysis of 

ground-water flow in the multiaquifer system of the 

Coastal Plain of Virginia and adjacent parts of Maryland 

and North Carolina . 

PURPOSE AND SCOPE 

The purposes of this report are to describe the groundwater flow system in the Coastal Plain of Virginia and to 

provide an analysis of the response of the ground-water 

flow system to past and present ground-water withdrawals through the use of a digital flow model. Specifically, 

the report describes (1) the conceptualization of the 

ground-water flow system, (2) the development of the 

subregional digital flow model, (3) the simulation of 

the ground-water flow system, and (4) the sensitivity of 

the digital flow model to changes in selected hydraulic 

characteristics of the ground-water flow system. 

Available hydrologic data provided most of the necessary information for the interpretation and conceptualization of the multiaquifer system. The physical 

boundaries of individual aquifers and confining units are 

presented in hydrogeologic maps by Meng and Harsh 

(1988). Hydraulic characteristics of aquifers and confining units were initially estimated from (1) analysis of 

geophysical and lithologic logs of water wells and geologic test holes, (2) laboratory tests of core samples, (3) 

data on specific capacity of wells, and (4) available 

selected aquifer tests. The ground-water flow system 

was simulated through the use of a digital flow model. 

Hydraulic characteristics of the ground-water flow system were adjusted to calibrate the model to measured 

water levels throughout the history of ground-water 

development (1891-1980). Sensitivity of modelgenerated water levels to selected variations in hydraulic 

characteristics was tested. The information presented is 

intended to assist those involved in the management of 

the ground-water resource in the Coastal Plain aquifers 

of Virginia. 

Data used to develop the subregional digital flow 

model were also used to develop a regional digital flow 

model of the northern Atlantic Coastal Plain aquifer 

system (Meisler, 1980). The regional model analyzed the 

entire ground-water flow system of the northern Atlantic 

Coastal Plain and provided lateral boundary flows to the 

individual subregional models. 

GENERAL SETTING AND 

LOCATION OF STUDY AREA 

The study area is located within the Atlantic Coastal 

Plain physiographic province and includes the entire 

Coastal Plain of Virginia and adjacent parts of the 

Coastal Plain of Maryland and North Carolina (fig . 1) . 

The area covers about 17,000 square miles (mil) and is 

characterized by a gently seaward sloping land surface 

and a dissected lowland with a series of broad, seawardfacing, ocean-cut terraces trending north-south. The 

study area is underlain predominantly by unconsolidated 

elastic sediments of Early Cretaceous to Holocene age. 

PREVIOUS INVESTIGATIONS 

Important sources of data on the geology and hydrogeology of the Virginia Coastal Plain include reports by 

Richards (1945, 1948), Spangler and Peterson (1950), 

Bick and Coch (1969), Brown and others (1972), Johnson 

(1972), Teifke (1973), the Virginia Division of Mineral 

Resources (1980), and Meng and Harsh (1988) . Darton 

(1896), Sanford (1913), Cederstrom (1945, 1957), Leggette and others (1966), Geraghty and Miller (1967, 

1978a, 1978b, 1979a, 1979b), Sinnott (1968), the Virginia 

State Water Control Board (1973, 1974), Cushing and 

others (1973), Lichtler and Wait (1974), Brown and 

Cosner (1974), Siudyla and others (1977), Newton and 

Siudyla (1979), Harsh (1980), Siudyla and others (1981), 

and Fennema and Newton (1982) describe the geology 

and water resources in specific areas of the Coastal Plain 

of Virginia. Converse and others (1981) provide a comprehensive water-supply study for the City of Virginia 

Beach, Va. Brown and Silvey (1977) evaluate the feasibility of injecting freshwater into Cretaceous-age sand 

containing saline water at Norfolk, Va. Meisler (1981) 

documents the occurrence and distribution of salty 

ground water in the northern Atlantic Coastal Plain 

aquifer system. Larson (1981) describes the occurrence 

of salty ground water in the Coastal Plain aquifers of 

Virginia. Cosner (1975), Bal (1977, 1978), and Faust and 

others (1981) studied, by means of digital flow models, 

the movement of ground water in specific areas of the 

Virginia Coastal Plain. Layne-Western Company (1983) 

developed a steady-state electric analog model to simulate flow in the Cretaceous-age aquifers of Virginia and 

North Carolina . 
INTRODUCTION 

80° 75° 

NEW YORK 

I 

I CONNECTICUT I 

PENNSYLVANIA [l~New York,; Pi 

(: (NEW 

OHIO Trenton 

Philadelphia 

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.'J NORTH CAROLINA Raleigh 

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SOUTH CAROLINA 

SCALE 1 :71 500 ;000 

100 200 MILES 

0 200 KILOMETERS 

Base from U .S . Geological Survey EXPLANATION National atlas, 1970 

STUDY AREA 

ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Shows location of water that contains 

chloride concentrations of 10,000 milligrams per liter 

or less 

FIGURE 1 .-Location of study area within the northern Atlantic Coastal Plain. 
F4 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

ACKNOWLEDGMENTS Not all aquifers are continuous over the entire study 

area. The Black Creek and Peedee aquifers of North 

Acknowledgment is given to the Virginia State Water 

Control Board (VSWCB), the Virginia State Health 

Department, and the many water users in Virginia for 

furnishing information on ground-water withdrawals and 

water-supply wells. The authors wish to thank the 

VSWCB for providing core samples and completing an 

observation well in Gloucester County, Va. Thanks also 

is extended to the many drillers and consultants active in 

the area for providing access to well information. 

CONCEPTUALIZATION OF 

GROUND-WATER FLOW SYSTEM 

The ground-water flow system in the Coastal Plain of 

Virginia is a multiaquifer system consisting of an 

eastward-thickening wedge of unconsolidated sand and 

clay that unconformably rests on an uneven, eastwardsloping surface of crystalline rocks, referred to as the 

"basement." The Fall Line is the westernmost extent of 

these unconsolidated sediments and delineates their contact with the igneous and metamorphic rocks of the 

Piedmont physiographic province . The sediments attain 

a maximum thickness in northeastern Virginia; 

Onuschak (1972) reports that sediments are about 6,200 

feet (ft) thick beneath the northern part of Virginia's 

Eastern Shore Peninsula. The wedge generally consists 

of a thick sequence of nonmarine deposits overlain by a 

thinner sequence of marine deposits. The sediments are 

mostly undeformed except for slight warping and tilting 

with associated minor faulting; they range in age from 

Early Cretaceous to Holocene and have a complex history of deposition and erosion (Meng and Harsh, 1988, 

p. C11). 

The sediments are subdivided into a sequence of 

discrete lithologic layers that form a regionally correlative geohydrologic framework of aquifers and confining 

units (fig . 2) (Meng and Harsh, 1988). The framework 

includes an unconfined, or water-table, aquifer underlain 

by a series of confined aquifers separated by intervening 

confining units. The subsurface correlations of aquifers 

and confining units are based primarily on analyses of 

geophysical and lithologic logs of wells. Table 1 (all tables 

at end of report) shows the relation between stratigraphic formations and hydrogeologic units defined for 

the Coastal Plain of Virginia. Table 2 summarizes the 

correlation of the hydrogeologic units of the Virginia 

Coastal Plain by Meng and Harsh (1988) with those ofthe 

adjoining States of Maryland (D .A. Vroblesky, U.S . 

Geological Survey, written commun., 1984) and North 

Carolina (M. D. Winner, U.S . Geological Survey, written 

commun., 1984) . 

Carolina and the Matawan and Severn aquifers of southern Maryland (aquifers 4 and 5, table 2) are missing for 

the most part in the Coastal Plain of Virginia. The 

Brightseat aquifer, not present in North Carolina, is 

combined with the upper Potomac aquifer in the digital 

flow model (aquifer 3, table 2) because of the absence of 

a continuous intervening confining unit and similarities in 

hydraulic properties. The areal extent of aquifers and 

confining units is shown on maps of aquifer transmissivity and confining unit leakance presented in later sections 

of this report and in areport by Meng and Harsh (1988) . 

HYDRAULIC PROPERTIES 

Transmissivity and storage coefficient are the hydraulic properties used to describe the ability of an aquifer to 

transmit and store water. Most hydraulic properties of 

aquifers in the Coastal Plain of Virginia have been 

determined from aquifer tests. Drawdown and recovery 

data generally are collected from an observation well 

positioned near a high-capacity production well that 

penetrates more than one aquifer. Other estimates of 

aquifer properties are determined from specific capacity 

(yield per unit of drawdown) and single-well tests of 

production wells that penetrate more than one aquifer. 

Because most wells penetrate more than one aquifer, 

direct application of these tests to determine hydraulic 

properties of an individual aquifer is difficult. Table 3 

lists the type of data and method of analysis used to 

compute transmissivity . Locations of aquifer test sites 

are shownin figure 3. Applying results from aquifer tests 

over large areas is difficult because values represent only 

the test area and because of the assumptions inherent in 

the methods-that an aquifer is homogeneous and that 

test wells penetrate the entire aquifer. 

Data on the hydraulic properties of individual confining units are sparse . Some vertical hydraulic conductivities have been estimated from laboratory tests of core 

samples (table 4) . The locations of core holes are shown in 

figure 3. Laboratory values should be used with caution, 

because undisturbed core samples are difficult to obtain 

and typically represent only a small interval of a highly 

complex hydrogeologic unit . 

DESCRIPTION OF HYDROGEOLOGIC UNITS 

Each aquifer and overlying confining unit is assigned 

an identification number for model simulation, from 10 

through 1 in descending order from land surface (table 2) . 

The following sections summarize the lithology and 
CONCEPTUALIZATION OF GROUND-WATER FLOW SYSTEM F5 

A' 

1500 VERTICAL SCALE 

GREATLY EXAGGERATED 

CLAY 

SAND 

BASEMENT 

SCALE 1 :1,000,000 0~ 10 20 MILES 

, 

0 10 20 KILOMETERS 

EXPLANATION 

CONTACT-Dashed where approximately located 

5309 

WELL AND LOCAL WELL NUMBER (Location shown in figure 8) 

E SINGLE-POINT ELECTRIC-RESISTIVITY LOG 

A-A' LINE OF SECTION SHOWN IN FIGURE 8 

FIGURE 2.-Generalized hydrogeologic section of eastward-thickening wedge of alternating sand and clay . 

hydraulic properties of aquifers and confining units of the 

study area. The reader is referred to Meng and Harsh 

(1988) for a more detailed description of the age, lithologic characteristics, stratigraphic position, depositional 

history, and areal extent of each hydrogeologic unit, 

except where otherwise referenced . 

COLUMBIA AQUIFER 

The Columbia aquifer, designated aquifer 10, is made 

up primarily of Holocene- and Pleistocene-age sediments 

that were deposited as channel fill and fluvial-marine 

terraces. The aquifer is composed of interbedded gravel, 

sand, silt, and clay and is unconfined throughout the 

study area. The aquifer is a major source of recharge to 

the underlying confined flow system and supplies water 

to rural and domestic users. 

The saturated thickness of the Columbia aquifer 

ranges from about 15 ft near its western extent to about 

80 ft in the southeastern part of the study area. Spatial 

variation in the hydraulic properties of the aquifer are 

not adequately defined by available data. Results from 

an aquifer test conducted at Northwest River Park in 

Chesapeake, Va., indicate a transmissivity of 250 feet 

squared per day (ft2/d) (Siudyla and others, 1981). A 

specific yield of 0.15 was estimated by Cushing and 

others (1973) from analysis of aquifer-test data collected 

on the Eastern Shore Peninsula of Virginia. 
F6 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

�75° 78° 77° 76 0 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

�20 State base maps, 1 :1,000,000 0 1 0 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

0 55-10 CORE HOLE AND NUMBER SHOWN IN TABLE 4 

'Wp AQUIFER TEST SITE-Letters identify test site shown 

in table 3 

0 58H 4 LOCAL WELL NUMBER FROM WHICH CORE SAMPLES 

WERE COLLECTED 

FIGURE 3 .-Locations of core holes and aquifer test sites in the Virginia Coastal Plain. 
CONCEPTUALIZATION OF GROUND-WATER FLOW SYSTEM F7 

YORKTOWN-EASTOVER AQUIFER AND 

YORKTOWN CONFINING UNIT 

The Yorktown-Eastover aquifer, designated aquifer 9, 

includes sediments of the Pliocene Yorktown Formation 

and the upper part of the Miocene Eastover Formation, 

which were deposited in a shallow marine to deltaic or 

estuarine environment (Rasmussen and Slaughter, 1955, 

p. 43; Ward and Blackwelder, 1980). The aquifer is both 

confined and unconfined, depending on location, and is 

made up of eastward-thickening, interfingering, fine to 

coarse sand interbedded with clay, shell, and sandy clay . 

The aquifer is unconfined in a broad belt almost parallel 

to the Fall Line. Thickness ranges from about 6 ft near 

its western limit to about 100 ft in the southeastern part 

of the Virginia Coastal Plain. The aquifer has been 

incised by streams in areas along the middle and upper 

reaches of major river valleys and tributaries, particularly north of the James River. It is an important source 

of water in the southeastern and northeastern parts of 

the study area. Where the aquifer is unconfined, it is a 

major source of recharge to the confined flow system. 

Transmissivity of the Yorktown-Eastover aquifer, 

compiled from Geraghty and Miller (1979b), Converse 

and others (1981), and Siudyla and others (1981), ranges 

from 200 to 3,000 ft 2/d in the southeastern part of the 

Virginia Coastal Plain. Storage coefficients from these 

sources range from 6.0 x 10 -3 to 7.7x 10-4. Reported yields 

from residential, commercial, and light industrial wells in 

these areas range from 20 to 250 gallons per minute 

(gal/min) (Siudyla and others, 1981; Fennema and Newton, 1982). 

The Yorktown confining unit, designated confining 

unit 9, overlies the Yorktown-Eastover aquifer, except 

near the Fall Line, where the aquifer is unconfined or is 

in direct contact with the Columbia aquifer. The confining unit is a blue-gray to green-gray clay interbedded 

with massive silty clay, fine sand, and calcareous shell 

fragments. Thickness of the confining unit ranges from 

about 10 ft near its western limit to about 80 ft along the 

Atlantic Coast. The confining unit is incised by streams 

along the major river valleys and tributaries, particularly north of the James River. 

Vertical hydraulic conductivity of the Yorktown confining unit, determined from laboratory tests, ranges 

from 5.9 x 10 -4 to 3.9 x10-3 feet per day (ft/d) (table 4) . 

Siudyla and others (1981) estimated that leakance (the 

ratio of vertical hydraulic conductivity to thickness of the 

confining unit) of the Yorktown confining unit from 

aquifer tests using the Hantush-Jacob method of analysis 

(Hantush and Jacob, 1955) ranges from 1 .3X 10-4 to 

1.8X 10 -3 per day. 

ST. MARYS-CHOPTANK AQUIFER AND 

ST. MARYS CONFINING UNIT 

The St. Marys-Choptank aquifer includes sediments of 

the Miocene Choptank Formation and the lower part of 

the Miocene St. Marys Formation and is designated 

aquifer 8. These sediments are light-yellow to greenishgray, very fine to fine quartzose sand interbedded with 

shell and silty clay . The sediments were deposited in a 

shoaling, midshelf setting, possibly during a climatic 

change (Hansen, 1981, p. 128) . The aquifer is not a 

primary source of water because adequate supplies are 

more easily obtained from overlying aquifers. 

The aquifer is confined and consists of sand units 

separated by noncontinuous clay layers . It attains a 

maximum thickness of about 160 ft in the northeastern 

part of the study area. Values of transmissivity and 

storage coefficient have not been determined in Virginia, 

but are estimated from aquifer tests for the same aquifer 

in southern Maryland (Cushing and others, 1973). Transmissivity ranges from 200 to 4,000 ft2/d, and storage 

coefficient ranges from 1 .0 x10-4 to 6.0 x 10 -3 . 

The St. Marys confining unit, designated confining 

unit 8, overlies the St. Marys-Choptank aquifer. The 

confining unit includes sediments ofthe upper part of the 

St. Marys Formation and the lower part of the Eastover 

Formation. The unit forms an eastward-thickening 

wedge of light-gray, shelly to laminated clay interbedded 

with very fine sand; it attains a maximum thickness of 

about 300 ft in the northeastern part of the study area 

and 150 ft in the southeastern part. Vertical hydraulic 

conductivity determined from laboratory tests ranges 

from 2.8 x10-s to 2.0 x10-5 ft/d (table 4) . 

CHICKAHOMINY-PINEY POINT AQUIFER AND 

CALVERT CONFINING UNIT 

The Chickahominy-Piney Point aquifer includes sediments of the Miocene and Oligocene Old Church Formation and the Eocene Chickahominy and Piney Point 

Formations, and is designated aquifer 7. The aquifer is a 

fine to medium, well-sorted, glauconitic sand interbedded with dark-green micaceous clay and calcareous shell 

fragments. The sediments were deposited in a shallow 

neritic environment (Hansen, 1971, p. 139) . The aquifer 

is an important source of water for residential, agricultural, and light industrial users in the Northern Neck, 

Middle, and York-James Peninsulas of Virginia. Figure 3 

shows the location of these general geographic areas. 

The confined aquifer contains two continuous sand 

bodies separated by clayey material; these sediments 

function as asingle aquifer north of the James River. The 

aquifer consists of a single lenticular body interbedded 
F8 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

with clay and shell fragments south of the James River. 

The aquifer averages about 90 ft thick in the northern 

part of the study area and 85 ft thick in the southern 

part. Composition of the aquifer changes from predominantly sand west of Chesapeake Bay to silt and clay 

beneath the bay. The aquifer is unconfined where it crops 

out near the Fall Line. 

Interpretation of reported specific capacities from the 

Middle Peninsula of Virginia (fig. 3) indicates that the 

transmissivity of the aquifer ranges from 150 to 2,000 

ft 2/d (Siudyla and others, 1977). No transmissivity data 

on the aquifer are available for other parts of this study 

area. Data on storage coefficients in Virginia are lacking; 

however, Hansen (1972) reports values for the same 

aquifer in southern Maryland ranging from 3.0X10-4 to 

4.0X 10-4 . Reported well yields range from 20 to 250 

gal/min (Siudyla and others, 1977; Newton and Siudyla, 

1979). 

The Calvert confining unit, designated confining unit 

7, overlies the Chickahominy-Piney Point aquifer, except 

where the aquifer crops out near the Fall Line. The 

confining unit includes sediments of the Miocene Calvert 

Formation and forms an eastward-thickening wedge of 

dark-green clay interbedded with fossiliferous sandy 

clay, marl, and diatomite. The unit attains a maximum 

thickness of about 300 ft in the northern part ofthe study 

area. Laboratory analysis of a core from a test hole in 

Isle of Wight County, Va., indicates that vertical 

hydraulic conductivity is 9.2X 10-6 (table 4) . 

AQUIA AQUIFER AND NANJEMOY-MARLBORO 

CONFINING UNIT 

The Aquia aquifer consists primarily of sediments of 

the Paleocene Aquia Formation and is designated aquifer 

6. The sediments are marine in origin and typically are 

well-sorted, fine to medium, dark-green, glauconitic 

sand interbedded with marl and shell fragments. These 

marine sediments were deposited in a shallow, nearshore 

environment, perhaps below wave base. The aquifer is 

an important source of water for residential and light 

industrial users north of the James River. 

The sand thickness of the confined aquifer averages 

about 75 ft in the northern part of the study area and 95 

ft in the southern part. The aquifer contains sand layers 

that are elongate and lenticular in shape and thin slightly 

to the west and considerably to the east . Composition of 

the aquifer changes from predominantly sand west of 

Chesapeake Bay to silt and clay beneath and east of the 

bay. The aquifer is unconfined where it crops out near 

the Fall Line. 

Results from single-well aquifer tests and specific 

capacity tests indicate that transmissivity ranges from 

125 to 1,000 ft2/d in the Middle and Northern Neck 

Peninsulas of Virginia (Siudyla and others, 1977; Newton 

and Siudyla, 1979). As expected, transmissivities are 

high where the sand is thickest, as in the southern part 

of the study area. No data on storage coefficient for the 

Aquia aquifer in Virginia are available; however, Hansen 

(1972) reports storage coefficients of the Aquia aquifer in 

southern Maryland of 1.0x10-4 to 4.0x10-4 . Reported 

well yields range from 20 to 110 gal/min (Siudyla and 

others, 1977; Newton and Siudyla, 1979) . 

The Nanjemoy-Marlboro confining unit, designated 

confining unit 6, overlies the Aquia aquifer, except 

where the aquifer crops out in the northwestern part of 

the study area. The confining unit, in its lower part, is a 

red to gray, kaolinite-illite clay of the Paleocene and 

Eocene Marlboro Clay, and, in its upper part, a thickbedded, argillaceous and calcareous green sand of the 

Eocene Nanjemoy Formation. The unit varies in thickness from about 15 ft to about 300 ft. The confining unit 

crops out in a narrow belt along the upper reaches of 

major rivers and tributaries from the James to Potomac 

Rivers. Results of laboratory tests to determine the 

vertical hydraulic conductivity are listed in table 4. 

AQUIFER 5 AND CONFINING UNIT 5 

Aquifer 5 includes sediments of the Upper Cretaceous 

Peedee Formation in North Carolina and the Severn 

Formation in Maryland. It is designated aquifer 5. The 

sediments are marine in origin and consist of fine to 

medium, gray to green sand, with varying amounts of 

glauconite and shell material interbedded with gray to 

black silt and clay . Thin beds of consolidated calcareous 

sandstone and impure limestone are present locally. The 

aquifer is confined and consists of interfingering sand and 

clay that function as a single aquifer. The aquifer ranges 

in thickness from a featheredge at its western limit to 

about 300 ft along the Atlantic Coast (M.D. Winner, U.S. 

Geological Survey, written commun., 1984). The estimated transmissivity of the aquifer ranges from about 

300 to about 1,240 ft 2/d in the study area. 

Confining unit 5 overlies aquifer 5. The sediments of 

the confining unit are composed of clay, silty clay, and 

sandy clay ; they thicken to the east, where they attain a 

maximum thickness of about 60 ft along the Atlantic 

Coast (M. D. Winner, U.S. Geological Survey, written 

commun., 1984). A typical vertical hydraulic conductivity of the confining unit is estimated to be 8.64x 10-6 ft/d 

(G. L. Giese, U.S . Geological Survey, written commun., 

1984). 

AQUIFER 4 AND CONFINING UNIT 4 

Aquifer 4 includes sediments of the Black Creek and 

Middendorf Formations and is present only in North 

Carolina and the extreme southeastern part of the Virginia Coastal Plain. The sediments typically are gray to 
CONCEPTUALIZATION OF GROUND-WATER FLOW SYSTEM F9 

tan in color and consist of fine to coarse sand with thin to 

laminated clay. Lignitized wood, shell material, and 

glauconite are common in the sample cuttings. Tan and 

red to white kaolinitic clay balls or fragments are scattered throughout the sand. The aquifer is confined and 

consists of fluvial-marine deposits that thicken seaward 

and function as a single water-bearing unit. These deposits are characterized by well-defined beds of sand and 

clay or lens-shaped bodies of sand. The aquifer attains a 

maximum thickness of 300 to 400 ft at the Atlantic Coast 

(M. D. Winner, U.S . Geological Survey, written commun., 1984). The estimated transmissivity of the aquifer 

ranges from about 210 to about 3,320 ft 2/d in the study 

area. 

Confining unit 4 completely covers aquifer 4; the unit 

consists of a series of clay, silty clay, and sandy clay beds 

ranging in thickness from less than 10 ft near its updip 

limit to more than 150 ft along the Atlantic Coast (M. D. 

Winner, U.S . Geological Survey, written commun., 

1984). A typical vertical hydraulic conductivity of this 

confining unit is estimated to be 3.2X 10-5 ft/d (G. L. 

Giese, U.S . Geological Survey, written commun., 1984). 

BRIGHTSEAT-UPPER POTOMAC AQUIFER AND 

CONFINING UNIT 

The Brightseat-upper Potomac aquifer includes sediments of the upper part of the Cretaceous Potomac 

Formation and the Paleocene Brightseat Formation and 

is designated aquifer 3 . Typically, the sediments of the 

aquifer are white to gray, medium to very fine, quartzose 

sand interbedded with dark-colored micaceous clay, 

varying amounts of shell material, lignite, and glauconite . The sediments are marine in origin and represent 

either a marginal outer-delta or nearshore intertidal 

environment. Theaquifer is confined and consists ofsand 

layers separated by thin clay beds. The layers are either 

interbedded sheet-form sand and silty clay or lensshaped bodies ofsand. The aquifer is aprincipal source of 

ground water for municipal, industrial, and agricultural 

use in the York-James, Middle, and Northern Neck 

Peninsulas of Virginia. 

The sands form an eastward-thickening wedge that 

attains amaximumthickness of 425 ft in the northeastern 

part ofthe study area. The lens-shaped sand is more than 

150 ft thick beneath Chesapeake Bay. Transmissivity 

decreases toward the Atlantic Coast, where the sand 

thins and silt and clay predominate. Transmissivities 

determined from aquifer test data are listed in table 3. 

Average transmissivities range from 1,500 to 13,000 ft2/d 

in southeastern Virginia. Locations of aquifer test sites 

are shown in figure 3. Other estimated values of transmissivities contained in ground-water data reports by 

Siudyla and others (1977) and Newton and Siudyla (1979) 

range from less than 150 ft2/d in northern Westmoreland 

County, Va., to 2,000 ft 2/d near West Point, Va. 

Reported yields from wells completed in the Brightseatupper Potomac aquifer range from 25 to 350 gal/min 

(Cederstrom, 1957; Siudyla and others, 1977; Newton 

and Siudyla, 1979). 

The Brightseat-upper Potomac confining unit, designated confining unit 3, overlies the Brightseat-upper 

Potomac aquifer. The sediments are dark-green to black, 

highly micaceous silty clay with oxidized red to yellow, 

thin clay . Hydraulic properties of this confining unit in 

Virginia are not available. Hansen (1977) reports a 

vertical hydraulic conductivity of 8.64X 10-4 ft/d and a 

specific storage of 7.4x 10-5 ft-1 (specific storage is 

defined as storage coefficient per unit thickness of confining unit sediment) for the same confining unit in 

southern Maryland. These values were determined from 

laboratory analyses of a core. 

MIDDLE POTOMAC AQUIFER AND 

CONFINING UNIT 

The middle Potomac aquifer includes sediments of the 

middle part of the Cretaceous Potomac Formation and is 

designated aquifer 2. These sediments are continental in 

origin and were deposited in fluvial-deltaic environments. The aquifer typically consists of interfingering 

lenses of medium sand, silt, and clay of differing thickness. A small part of the aquifer is unconfined where it 

crops out near the Fall Line. The aquifer is a principal 

source of ground water for large users throughout the 

Coastal Plain of Virginia. 

The aquifer thickens to the east and attains a maximum thickness of more than 1,000 ft in the northeastern 

part of the Eastern Shore Peninsula of Virginia. Transmissivity of the aquifer decreases seaward because of 

increased amounts of silt and clay . A summary of transmissivities is given in table 3. Average transmissivities 

range from 2,000 ft 2/d in Westmoreland County, Va., to 

19,000 ft 2/d in the southeastern part of Virginia. Other 

values of transmissivity based on aquifer and specific 

capacity test data, reported by drillers, are summarized 

in ground-water data reports by the Virginia State 

Water Control Board (1973, 1974), Siudyla and others 

(1977), Newton and Siudyla (1979), and Siudyla and 

others (1981) . Storage coefficients for this aquifer from 

these references range from 2.0X 10-4 to 1.5 x10-3 . Well 

yields from this aquifer in southeastern Virginia are 

reported to exceed 750 gal/min (Brown and Cosner, 

1974). 

The middle Potomac confining unit, designated confining unit 2, overlies the middle Potomac aquifer, except 

where the aquifer crops out in a narrow belt along the 

Potomac River south of Washington, D.C. The confining 

unit consists predominantly of montmorillonitic red clay 

and is typically massive and thick bedded, but it is finely 
F10 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

laminated in some places . The confining unit thickens 

from the Fall Line and attains a maximum thickness of 

more than 200 ft in the northeastern part of the study 

area. The confining unit also thickens to the south along 

the Virginia-North Carolina border . The vertical 

hydraulic conductivity of the confining unit, determined 

from laboratory analysis of a core from a test hole at 

Sewells Point in Norfolk, Va., is 3.4x10-6 ft/d (table 4) . 

LOWER POTOMAC AQUIFER AND CONFINING UNIT 

The lower Potomac aquifer is the lowermost confined 

aquifer in the Coastal Plain and lies unconformably on 

basement; it is designated aquifer 1 . The sediments are 

the lower part ofthe Cretaceous Potomac Formation and 

typically consist of coarse, arkosic quartz sand with 

intervening clay . Individual sand bodies within the aquifer suggest a continental origin and deposition by lowgradient meandering streams in a broad alluvial plain. 

These sand bodies tend to be thick, interbedded 

sequences of angular to subangular coarse sand, clayey 

sand, and clay . The aquifer is a principal source of ground 

water for large users throughout the western and central 

parts of the Virginia Coastal Plain. 

The aquifer thickens to the east and reaches a maximum thickness of more than 3,000 ft in the northeastern 

part of the study area . Even though the average thickness of the aquifer increases seaward, transmissivity 

decreases because of a facies change to finer grained 

marine sediments having lower permeability. The aquifer begins to thin to the south at the Virginia-North 

Carolina border . The aquifer contains saltwater along 

the coast in North Carolina and in an adjoining area of 

the Virginia mainland . Average transmissivity ranges 

from 12,000 to 19,000 ft2/d . A summary of transmissivities is given in table 3. Other transmissivity values 

interpreted from aquifer and specific capacity test data 

from production wells, reported by drillers, can be found 

in ground-water data reports by the Virginia State 

Water Control Board (1973, 1974), Siudyla and others 

(1977), Newton and Siudyla (1979), and Siudyla and 

others (1981) . Average storage coefficients range from 

5.0x10-4 to 1 .5x 10 -3 (table 3). Well yields from this 

aquifer are reported to be as much as 700 gal/min (Brown 

and Cosner, 1974). 

The lower Potomac confining unit, designated confining unit 1, overlies the lower Potomac aquifer. The 

confining unit is composed of thick sequences of finely 

laminated, usually brown, gray, or dark-green carbonaceous clay interbedded with thin, sandy clay . The unit 

thickens from the Fall Line and attains a maximum 

thickness of more than 175 ft in the northeastern part of 

the study area. Vertical hydraulic conductivity, determined from laboratory analysis of a core from a test hole 

near Pughville in the City of Suffolk, is 1.9X10-6 ft/d 

(table 4) . 

HYDROGEOLOGIC EFFECTS OF 

PLEISTOCENE EROSION 

Considerable erosion by river channels occurred in the 

study area during the Pleistocene when Chesapeake Bay 

estuary-a drowned river system-approached equilibrium with a sea level 300 to 400 ft below present sea level 

(Hack, 1957). Chapelle and Drummond (1982, 1983) 

identified areas in southern Maryland where rivers 

eroded through a sequence of interbedded sand and clay 

corresponding to aquifers and confining units 9 through 

6. Hack (1957) constructed geologic sections at six bridge 

sites in Virginia from examination of borehole data 

gathered during construction . These sections showed 

that the Pleistocene channels closely coincided with the 

present-day channels of the major rivers . The data were 

used to estimate the longitudinal profiles of selected 

Pleistocene channels and major tributaries. Figure 4 

shows the approximate locations and estimated depths of 

these channels based on data from Hack (1957) . 

As a result of Pleistocene channel incision, aquifers 

and confining units were partially or completely eroded 

and replaced by material more permeable than the 

confining units but less permeable than the aquifers . This 

condition increased the hydraulic connection between 

surface water in the major river channels and ground 

water in the underlying aquifers . Because of the 

increased connection, the lowering of the water level in 

an aquifer below river stage results in more rapid 

movement of river water into the aquifer until the water 

level in the aquifer again approaches that of the river. 

Chapelle and Kean (1985) described the occurrence and 

movement of salty river water into the Patuxent (lower 

Potomac) aquifer near Baltimore, Md. 

GROUND-WATER MOVEMENT 

Prior to ground-water development, an approximate 

state of hydraulic equilibrium prevailed in the multiaquifer system (fig . 5) . Precipitation infiltrated downward to 

recharge the water-table aquifer. Once in the watertable aquifer, water either moved laterally in the direction of decreasing hydraulic head (water level) and was 

ultimately discharged to the surface as seepage to 

streams, swamps, and coastal water or moved downward 

through confining units to recharge the confined flow 

system . Most downward movement of water into the 

confined aquifers occurred along a narrow band almost 

parallel to the Fall Line and under interfluves (areas of 

high elevation between the major river valleys) . Some 
CONCEPTUALIZATION OF GROUND-WATER FLOW SYSTEM F11 

78° 77° 76° 75° 

Base from U.S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 ' 20 30 40 50 KILOMETERS 

EXPLANATION 

INFERRED LOCATION OF PLEISTOCENE CHANNEL 

0 -11 DATA POINT-Number is estimated depth of Pleistocene 

channel in feet below sea level . Based on 

longitudinal profiles of Hack (1957) 

FIGURE 4. -Approximate locations and depths of Pleistocene channels . 
F12 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

J 

West 

NOT TO SCALE 

Stream 

Ocean 

East 

EXPLANATION 

IDEALIZED INTERFACE OF FRESHWATER AND 

SALINE WATER 

DIRECTION OF GROUND-WATER FLOW 

SEDIMENT-Predominantly channel deposits 

SEDIMENT-Predominantly clay 

SEDIMENT-Predominantly sand 

SEDIMENT CHANGE-Predominantly silt and clay 

BASEMENT 

FIGURE 5 .-Generalized prepumping ground-water flow system. 

water moved upward from the confined flow system and 

recharged the water-table aquifer in areas of low elevation beneath and adjacent to major river channels . 

Movement of ground water in the confined flow system 

was predominantly lateral in the direction of decreasing 

hydraulic head. The general direction of flow, defined by 

the hydraulic gradient, was from the Fall Line toward 

coastal areas and from interfluves toward major river 

valleys. Flow through the intervening confining units 

was predominantly vertical-downward between major 

river valleys and along the Fall Line and upward toward 

major river valleys and coastal water. Rates of flow 

through the confining units were small compared with 

rates of flow within the aquifers. However, the total 

volume of water moving across confining units was a 

significant component in the ground-water budget of the 

aquifers, because the confining units extend over very 

large areas. 

The withdrawal of large amounts of ground water from 

the confined aquifers lowered water levels and resulted 

in the formation of regional cones of depression around 

major pumping centers. These withdrawals disturbed 

the natural balance between recharge and discharge and 

captured a large part of the water previously discharged 

from the ground-water flow system to surface water 

during prepumping conditions . The lowering of water 

levels changed the directions of ground-water flow 

toward the major pumping centers. Withdrawals 

increased vertical leakage through confining units, 

reduced the volume of water stored in the ground-water 
CONCEPTUALIZATION OF GROUND-WATER FLOW SYSTEM F13 

flow system, increased flow from the water-table aquifer 

into the confined flow system, and decreased local 

ground-water discharge to streams and regional discharge to coastal water. 

GROUND-WATER USE 

Use of ground water from confined aquifers in the 

Virginia Coastal Plain began in the late 1800's. Sanford 

(1913) and Cederstrom (1945) reported the presence of 

many flowing wells along major rivers and tributaries 

throughout the study area. Estimates of discharge from 

these individual flowing wells range from a few gallons 

per minute to about 50 gal/min. The estimated aggregate 

annual discharge from these wells ranges from 4 to 10 

million gallons per day (Mgal/d) for the period 1891 

through 1945 . Flowing wells continued to be the major 

source of ground water until about 1935; after that time, 

water levels in the deeper confined aquifers declined 

below land surface. As demand for water increased, 

users had to depend on pumped wells for adequate water 

supplies. Water levels continued to decline, with an 

accompanying decrease in the number offlowing wells. A 

few wells open to shallow confined aquifers located near 

coastal areas still flow today, though most contain salty 

water. 

Data on ground-water use were compiled to provide 

the history of ground-water development in the study 

area. Withdrawal data for industrial, commercial, and 

public-supply wells were obtained from individual users 

and the Virginia State Water Control Board, and for 

commercial and public-supply systems from the Virginia 

State Health Department. Complete records on domestic 

use are not available. Per capita domestic use has been 

estimated to be between 50 and 80 gallons per day 

(gal/d), but because most ground water withdrawn for 

self-supplied domestic use is returned through septic 

tank discharge and because overall domestic use represents only a small percentage of total ground-water use 

from the confined flow system, domestic withdrawals 

were not included as part of the estimated history of 

development. Most industrial, commercial, and publicsupply users maintained complete and consistent 

records; however, significant gaps of as much as 25 years 

(yr) in the historical record have occurred between the 

beginning of ground-water development and the date 

that withdrawal records were first reported . Withdrawal 

values were estimated for these data-gap periods from 

pump capacities and from a few reported withdrawals. 

Withdrawal rates were reported as metered or estimated averages or totals by month or year. Reports 

describing water use in areas of the Virginia Coastal 

Plain by Sanford (1913), Cederstrom (1945), Sinnott 

(1967), Cederstrom (1968), and Cosner (1975) were used 

FIGURE 6.-Estimated annual ground-water withdrawal from 

Coastal Plain aquifers, 1891-1980. 

to supplement the withdrawal data compiled during this 

study. Withdrawal rates were interpolated for time 

intervals for which data were not available. Withdrawals 

from wells in Maryland and North Carolina were provided by W.B . Fleck and G.L. Giese, respectively (U.S. 

Geological Survey, written commun., 1983). 

Data on ground-water use include the user name, 

latitude and longitude, annual withdrawal rate, aquifers 

from which water is withdrawn, and percentage ofwater 

withdrawn from each aquifer. These data are stored on a 

computer file at the U.S. Geological Survey in Richmond, Va., and are available to the public upon request. 

Maps of aquifer tops and confining unit thicknesses 

(Meng and Harsh, 1988) were compared with the waterintake depth interval for each screen or open-end well to 

identify the source aquifer. Formost multiaquifer wells, 

detailed transmissivity data were not available to determine the rates of flow from individual aquifers; therefore, withdrawal was apportioned to aquifers by the 

percentage of the total screen present within each 

aquifer. 

Estimated annual ground-water withdrawal (including 

withdrawal for industrial, commercial, and public-supply 

uses) from the study area for the period 1891 through 

1980 is shown in figure 6. Withdrawal in 1980 is estimated to have been about 110 Mgal/d. The figure also 

shows estimated annual withdrawal from wells in only 

the Virginia Coastal Plain. Withdrawal from the Virginia 

Coastal Plain in 1980 is estimated to have been about 100 

Mgal/d. Estimated withdrawal includes discharge from 

both pumped and flowing wells. 

Major ground-water-pumping centers in the Virginia 

Coastal Plain are located near the cities of Franklin, 

Williamsburg, Suffolk, and Alexandria and the towns of 
F14 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

50 

r 

a 

a 

a 40 

Z

0

J 

a 

0 30 

0

z

J

J 

2 

J 

a 

0

1891 

­

1900 1910 1920 1930 1940 1950 1960 1970 1980 

YEAR 

FIGURE 7.-Average annual withdrawal from the lower, middle, and 

upper Potomac aquifers in the Franklin area, Virginia, 1891-1980. 

West Point and Smithfield . Estimated withdrawal from 

these centers in 1980 totaled about 65 Mgal/d . The 

largest center is near Franklin, where withdrawals from 

the lower, middle, and upper Potomac aquifers have 

increased to more than 40 Mgal/d (fig. 7) . 

The principal sources of ground water in the Virginia 

Coastal Plain are the lower and middle Potomac, 

Brightseat-upper Potomac, Aquia, Chickahominy-Piney 

Point, and Yorktown-Eastover aquifers . These aquifers 

supply more than 90 percent of the ground water withdrawn, and three aquifers, the lower and middle Potomac and the Brightseat-upper Potomac, provide approximately 90 percent of this withdrawal. Table 5 gives 

ground-water withdrawals for the major users in the 

study area in 1980. Locations of the major users are 

shown in figure 8. 

ANALYSIS OF THE 

GROUND-WATER FLOW SYSTEM 

The ground-water flow system in the Virginia Coastal 

Plain was analyzed using a digital flow model. Digital 

flow models are used to test a conceptualization of the 

ground-water flow system, to improve or verify estimates of the flow components in the ground-water budget, and to estimate regional aquifer properties and 

water levels in areas where data are sparse . Once 

calibrated, the model can analyze the response of an 

aquifer system to past and present ground-water withdrawals, estimate the effects of future ground-water 

development, and test effects of alternative waterdevelopment plans on a regional scale. 

MODEL DESCRIPTION 

The ground-water flow system was simulated using 

the quasi-three-dimensional approach describedby Bredehoeft and Pinder (1970) . The computer code used was a 

version of the three-dimensional finite-difference model 

of Trescott (1975) as modified and documented by Leahy

(1982) . This approach uses a layered sequence of twodimensional aquifers coupled to simulate vertical flow 

through intervening confining units without considering 

the release of water from storage within the confining 

unit (fig . 9, table 2) . This assumption of two-dimensional 

flow within the aquifer is valid if the horizontal hydraulic 

conductivities of the aquifers are more than two orders of 

magnitude greater than those of the intervening confining units (Neuman and Witherspoon, 1969). The error 

due to neglecting storage in the confining unit is minimal 

if simulated pumping periods are long enough to establish a constant hydraulic gradient between the individual 

aquifers . The shortest simulated pumping period was 3 

yr, which was considered long enough to avoid any errors 

resulting from this assumption . In this approach, confining units are not represented as individual layers but as 

vertical conductors of flow between adjacent aquifers 

and are defined by leakance values (fig . 10). Horizontal 

flow in the confining units is assumed to be negligible because of the low hydraulic conductivity of the 

sediments. 

BOUNDARIES 

Model boundaries were selected and located to approximate the natural hydrologic boundaries of the groundwater flow system . The westernmost boundary of the 

model coincides with the Fall Line and is considered 

impermeable to flow. This assumption is supported by 

the large difference in hydraulic conductivity between 

the rocks of the Piedmont physiographic province and 

the unconsolidated sediments of the Coastal Plain physiographic province. 

The easternmost boundary approximates the seaward 

limit of the freshwater system and is defined by the 

location of 10,000 milligrams per liter (mg/L) chloride 

concentration delineated by Meisler (1981) . Movement 

across this boundary is assumed to be negligible because 

of density effects. This condition is considered stable 

over time. The spatial stability ofthis boundary throughout the history of ground-water development has been 

suggested by Larson (1981) and by recent research of 

P.P. Leahy (U.S. Geological Survey, oral commun., 

1985). Significant lateral movement of this boundary 

could occur as a result of greatly increased pumping and 

would thus require modification of the model conceptualization to incorporate boundary movement. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

78° 77° 76° 75° 

Base from U .S. Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

1­

0 10 20 30 40 50 KILOMETERS

EXPLANATION 

o02s LOCATION AND IDENTIFICATION NUMBER OF WATER 

USER SHOWN IN TABLE 5 

A-A, LINE OF SECTION SHOWN IN FIGURE 2 

054P 3 WELL AND LOCAL WELL NUMBER SHOWN IN 

FIGURE 2 

FIGURE 8 .-Locations of major ground-water users . 
F16 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

NOT TO SCALE 

EXPLANATION 

NO-FLOW BOUNDARY 

GENERALIZED FLOW LINE 

SIMULATED MOVEMENT OF GROUND WATER 

GROUND-WATER RECHARGE 

WATER-TABLE AQUIFER 

CONFINING UNIT 

CONFINED AQUIFER 

BASEMENT 

FIGURE 9.-Model conceptualization of prepumping ground-water system. 

The lowermost boundary coincides with the contact 

between the lower Potomac aquifer and the underlying 

basement rocks. The contact is considered a no-flow 

boundary. This assumption is supported by the large 

difference in hydraulic conductivities between the two 

rock types. 

The uppermost boundary simulates surface water and 

is approximated as the average stage in surface-water 

bodies. The boundary is simulated as a constant-head 

boundary and allows for vertical flow between it and the 

underlying water-table aquifer. The relative consistency 

of water levels within surface-water bodies over the time 

and scale of simulation supports the use of this boundary 

condition (Cederstrom, 1945; Johnston, 1977). 

Aquifers continue beyond the northern and southern 

limits of the study area. Continuity of the aquifers across 

these lateral model boundaries is simulated using lateral 

boundary fluxes that were calculated by the regional flow 

model of the entire northern Atlantic Coastal Plain. 

Lateral boundaries are located to include, within the 

modeled limits, those areas outside the Virginia Coastal 

Plain that are affected by withdrawals from within the 

Coastal Plain of Virginia. The northern flux boundary is 

north of the Potomac River and extends across southern 

Maryland. The southern flux boundary coincides with 

Albemarle Sound and extends westward to the Fall Line 

across North Carolina . The locations of lateral flow 

boundaries are shown in figure 11 . The method of 

estimating lateral boundary fluxes is discussed later in 

the section "Lateral Boundary Flow." 

GRID DESIGN 

A three-dimensional grid of blocks is superimposed on 

a map of the study area in the finite-difference method of 

solving the ground-water flow equation . The discretiza­
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F17 

CONCEPTUAL REPRESENTATION 

NOT TO SCALE 

MODEL REPRESENTATION 

NOT TO SCALE 

EXPLANATION 

AQ2 1 AQUIFER 

CONFINING UNIT 

: BASEMENT v 

INACTIVE BLOCK REPRESENTING AQUIFER 

PINCH OUT 

AQUIFER DIRECTLY CONNECTED TO ANOTHER 

AQUIFER (Confining unit is missing) 

CONFINING UNIT (Leakance) 

NO-FLOW BOUNDARY 

FIGURE 10 .-Conceptual and model representations of aquifers and 

confining units . 

tion defines the geographic limits and spatial variation of 

hydraulic properties of each of the 10 aquifers. 

The grid consists of 68 rows and 50 columns for a total 

of 3,400 blocks per layer (fig . 11). Each aquifer is 

represented by a layer of blocks (fig . 12). Each block is 

assigned values representative of the average aquifer 

characteristics within the block. The block size of 12.25 

square miles (mil) and the grid orientation are the same 

as for models of adjoining States, allowing for transfer of 

input parameters and continuity of aquifers across model 

boundaries . The grid orientation is compatible with the 

regional model of the northern Atlantic Coastal Plain. 

HYDRAULIC CHARACTERISTICS AND STRESSES 

The hydraulic characteristics and hydrologic stresses 

that influence the flow of ground water in the digital flow 

model are aquifer transmissivity and storage coefficient, 

vertical leakance of confining units, streambed leakance, 

lateral boundary flow rates, and ground-water recharge 

and withdrawal. Maps presented in the following sections describe the spatial variation in the hydraulic 

characteristics. Values of hydraulic characteristics and 

hydrologic stresses used for the simulations are stored on 

computer files at the U.S . Geological Survey, Richmond, 

Va., and are available to the public upon request. 

TRANSMISSIVITY 

Movement of water within the multiaquifer system is 

characterized by values of transmissivity. The value of 

transmissivity for each block was calculated by multiplying an estimated sand thickness for each block by the 

lateral hydraulic conductivity of the sand. Average 

hydraulic conductivities initially (table 6) were estimated 

using lithologic logs and aquifer test data and were 

assumed constant for each aquifer. Transmissivity was 

assumed isotropic, and values were adjusted during 

model calibration. In areas where aquifer material was 

eroded and replaced by less permeable deposits, lower 

values of hydraulic conductivity were used. The range of 

transmissivity for each aquifer is given in table 7. 

Figures 13 through 21 show the estimated transmissivity of the major aquifers. Aquifer 5 and confining unit 

5 are not present in the Coastal Plain of Virginia; 

therefore, illustrations for these units are not included in 

this report. In general, transmissivity of the Brightseatupper Potomac, middle Potomac, and lower Potomac 

aquifers (figs. 19, 20, 21) increases eastward from the 

Fall Line. Values begin to decrease near the 10,000 mg/L 

chloride concentration. This decrease is attributed to a 

facies change from continental to marine deposits (Meng 

and Harsh, 1988) and a thinning of the aquifer thicknesses caused by the presence of saltwater (greater than 

10,000 mg/L chloride concentrations) in the lower part of 

the aquifers. Areas of high transmissivity are present 

south and west of Chesapeake Bay. 

Transmissivity maps for the Chickahominy-Piney 

Point and Aquia aquifers and aquifer 4 (figs. 16, 17, 18) 

show zones of higher transmissivity landward of coastal 

water bodies . In general, values increase in a southeastwardly direction into North Carolina, where these aquifers are a major source of water for industrial and 

municipal users in North Carolina. 

Transmissivity maps for the Columbia and YorktownEastover aquifers (figs. 13, 14) show an increase in an 

eastwardly direction. Lower values are present where 
F18 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

79* 78° 77° 

38° 

37° 

36° 

NORTH CAROLINA 

35° 

v 

fl -

SCALE 1 :3,000,000 

0 20 40 60 80 MILES i ­

0 20 40 60 80 KILOMETERS 

EXPLANATION 

®FINITE-DIFFERENCE GRID 

MODEL BOUNDARIES 

No-flow boundary 

-°------ Flux boundary 

FIGURE 11 . -Finite-difference grid and model boundaries . 

STORAGE COEFFICIENT these aquifers were incised by ancient river systems

which eroded the original aquifer sediment and replaced The storage coefficient defines the ability of an aquifer it with less permeable material. to store water. A value of 1 .0 x 10-4 was used in the model 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F19 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


Q1 

AQ5 

EXPLANATION 

VERTICAL LEAKANCE OF CONFINING UNIT 

VERTICAL LEAKANCE OF STREAMBED UNIT 

NO-FLOW BOUNDARY 

AQ2 AQ9 AQUIFER 

TU F_1

AQ1 

AQUIFER BLOCK 

. 

STREAM BLOCK 

OCEAN BLOCK NOT TO SCALE 

FIGURE 12.-Cross section of finite-difference grid and boundary conditions. 

for all confined aquifers . This value is slightly lower than 

those given in table 3, but it is assumed reasonable 

because most values in table 3 were determined from 

standard nonleaky methods. This value is in close agreement with that estimated by Hopkins (U.S. Geological 

Survey, written commun., 1984) from compactionrecorder data and determined by model calibration (Cosner, 1975; Chapelle and Drummond, 1983). A value of 

0.15 was used to simulate storage for the unconfined 

(water-table) parts of aquifers . 

VERTICAL LEAKANCE 

Vertical leakance controls the vertical flow of ground 

water through confining units. It defines the degree of 

hydraulic connection between aquifers and is dependent 

on the physical properties of the sediment that makes up 

the confining unit. Vertical leakance is defined as the 

average vertical hydraulic conductivity of the confining 

unit sediment divided by its thickness. 

In the model, vertical leakance controls the degree of 

hydraulic connection between two vertically adjacent, or 

sequential, aquifer blocks and represents the intervening 

confining unit (fig. 22A). Confining unit thicknesses for 

blocks were estimated from confining unit thickness 

maps reported by Meng and Harsh (1988) . Vertical 

hydraulic conductivities of confining units, estimated 

from laboratory cores, are given in table 6. The range 

of vertical leakance for each confining unit is given in 

table 7. 

Aquifers and confining units are not continuous over 

the entire study area (Meng and Harsh, 1988). In some 

areas, the confining unit between two sequential aquifers 

pinches out or is not present (fig . 22B). In the model, 

high vertical leakance values were used to represent the 

hydraulic connection between two sequential aquifers 

not separated by a confining unit . Vertical leakance 

between two such blocks was assumed to be four orders 

of magnitude greater than the vertical hydraulic conductivity of the missing confining unit . A value of this 

magnitude is considered more representative of the 

vertical leakance of aquifer material. 

In some areas, two nonsequential aquifers are connected hydraulically through a single confining unit, 

because the overlying sequential hydrologic units are 

missing (fig . 22C); blocks representing the missing aquifers are eliminated from the ground-water flow system 
F20 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

Base from U .S. Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

5oo- LINE OF EQUAL TRANSMISSIVITY-Approximately located . Interval is 500 feet squared per day 

APPROXIMATE LIMIT OF COLUMBIA AQUIFER 

FIGURE 1 3 .-Transmissivity of the Columbia aquifer used in model simulations . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F21 

78° 77° 76° 75° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-500---- LINE OF EQUAL TRANSMISSIVITY-Approximately 

located . Interval, in feet squared per day, is variable 

APPROXIMATE LIMIT OF YORKTOWNEASTOVER AQUIFER 

FIGURE 1 4 .-Transmissivity of the Yorktown-Eastover aquifer used in model simulations . 
F22 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 1 0 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-500- LINE OF EQUAL TRANSMISSIVITY-Approximately

located . Interval is 500 feet squared per day 

APPROXIMATE LIMIT OF ST. MARYSCHOPTANK AQUIFER 

FIGURE 15.-Transmissivity of the St . Marys-Choptank aquifer used in model simulations . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F23 

78° 77° 76° 75 0 

Base from U .S. Geological Survey SCALE 1 :2,000,000 

state base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 4,0 50 KILOMETERS 

EXPLANATION 

-1,000- LINE OF EQUAL TRANSMISSIVITY-Approximately

located . Interval is 2,000 feet squared per day 

APPROXIMATE LIMIT OF CHICKAHOMINYPINEY POINT AQUIFER 

FIGURE 1 6.-Transmissivity of the Chickahominy-Piney Point aquifer used in model simulations. 
F24 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U.S. Geological Survey SCALE 1 :2,000,000

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES I

10 20 30 40 50 KILOMETERS 

EXPLANATION 

-1,000-1- LINE OF EQUAL TRANSMISSIVITY-Approximately

located . Interval is 1,000 feet squared per day 

APPROXIMATE LIMIT OF AQUIA AQUIFER 

FIGURE 17.-Transmissivity of the Aquia aquifer used in model simulations . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

�76° 78° 77 0 75° 

Base from U .S. Geological Survey 

�40 State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

1 0 20 30 50 MILES 

F 

0 10 20 30 40 50 KILOMETERS

EXPLANATION 

-1,000-- LINE OF EQUAL TRANSMISSIVITY-Approximately 

located . Interval is 1,000 feet squared per day 

---- APPROXIMATE LIMIT OF AQUIFER 4 

. . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams per liter chloride 

FIGURE 18 .-Transmissivity of aquifer 4 used in model simulations . 
F26 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES F 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-1,000- LINE OF EQUAL TRANSMISSIVITY-Approximately . . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

located . Interval is 1,000 feet squared per day SYSTEM-Less than 10,000 miligrams per liter chloride 

APPROXIMATE LIMIT OF BRIGHTSEAT-UPPER 

POTOMAC AQUIFER 

FIGURE 1 9.-Transmissivity of the Brightseat-upper Potomac aquifer used in model simulations . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F27 

78° 77° 76° 75° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-12,000- LINE OF EQUAL TRANSMISSIVITY-Approximately

located . Interval is 4,000 feet squared per day 

-- APPROXIMATE LIMIT OF MIDDLE POTOMAC AQUIFER 

. . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams per liter chloride 

FIGURE 20. -Transmissivity of the middle Potomac aquifer used in model simulations . 
REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

Base from U .S . Geological Survey 

State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS

EXPLANATION 

-2,00o- LINE OF EQUAL TRANSMISSIVITY-Approximately

located . Interval is 2,000 feet squared per day 

APPROXIMATE LIMIT OF LOWER POTOMAC AQUIFER 

ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams per liter chloride 

FIGURE 21 .-Transmissivity of the lower Potomac aquifer used in model simulations. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F29 

CONCEPTUAL SIMULATED 

AQ3 

CU2 

AQ2 

GU 

A01 

A. Vertical leakance between two sequential aquifers seperated by a confining unit. 

AQ3 

AQ3 

._ CU2: AQ2 . 

AQ2 

-- CU A01 

AQ1 

B. Vertical leakance between two sequential aquifers in direct contact . 

C. Vertical leakance between two nonsequential aquifers separated by a confining unit . 

EXPLANATION 

VERTICAL LEAKANCE OF CONFINING UNIT BETWEEN _Elf2 . CONFINING UNIT TWO SEQUENTIAL AQUIFERS 

VERTICAL LEAKANCE OF AQUIFER MATERIALS 

WHERE CONFINING UNIT BETWEEN AQUIFERS IS FAQ2 AQUIFER 

MISSING . TWO ADJACENT AQUIFERS ARE IN 

DIRECT CONTACT AQUIFER BLOCK 

. . . . . . . VERTICAL LEAKANCE BETWEEN TWO NONSEQUENTIAL AQUIFERS WHERE THE INACTIVE BLOCK (Horizontal transmissivity is assumed SEQUENTIAL AQUIFER AND (OR) THE 

to be zero) CONFINING UNIT ARE MISSING 

FIGURE 22.-Schematic diagrams of the conceptual and simulated aquifer contacts . 

by assigning azero transmissivity. To allow vertical flow between the nonsequential aquifers. This procedure 

between two nonsequential blocks, leakance values are allowed vertical flow between the nonsequential aquifer 

needed to connect the nonsequential aquifers. Leakance blocks and simulated the true vertical leakage between 

values for missing confining units were computed by aquifers. 

multiplying the leakance value calculated for the existing In areas where confining unit material was eroded and 

confining unit by the number of simulated confining units replaced by more permeable stream deposits, vertical 
F30 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

leakance was increased one or two orders of magnitude 

(Chapelle and Drummond, 1983). An increase of two 

orders of magnitude was used if a confining unit was 

completely eroded and replaced by stream deposits. An 

order-of-magnitude increase was used for a confining 

unit partially eroded and replaced by stream deposits. 

The degree of hydraulic connection between the stream 

and the aquifer underlying the confining unit was 

increased with this procedure. Figures 23 through 30 

show the vertical leakance used for simulation of the 

confining units present in the study area. In general, 

vertical leakance decreases toward the east as confining 

units thicken. 

GROUND-WATER RECHARGE 

Recharge entering the water-table aquifer was estimated by the following equation : 

QRE=P-OF-ET (1) 

where 

QRE =rate of ground-water recharge, in inches per 

year; 

P =precipitation, in inches per year; 

OF = overland flow, in inches per year; and 

ET =evapotranspiration, in inches per year. 

Average annual precipitation in the study area is about 

43 inches per year (in/yr) (National Oceanic and Atmospheric Administration, 1980). A study by Cushing and 

others (1973, p. 35) shows that average overland flow on 

the Eastern Shore Peninsula, which is part of the study 

area, is about 6.5 in/yr. This value is assumed to represent the average hydrologic condition in the study area. 

About 50 percent of the average annual precipitation 

(21.5 in/yr) in the study area is estimated to be evaporated and transpired by vegetation (Geraghty and Miller, 

1978b; Harsh, 1980). Hence, the average rate of areal 

recharge to the water-table aquifer system is about 15 

in/yr. This value was assigned to blocks representative of 

the water-table aquifer. The water-table aquifer includes 

the Columbia aquifer and those parts of underlying 

aquifers that crop out. A digital-flow-model study in the 

Coastal Plain of central and southern Delaware 

(Johnston, 1977) shows that 14 in/yr is a good estimate of 

the long-term recharge rate for the water-table aquifer 

in the Coastal Plain of Delaware. Undoubtedly, groundwater recharge rates vary spatially throughout the 

model area; however, data are insufficient to define these 

local variations . The recharge rate was assumed constant 

during all model simulations. 

STREAMBED LEAKANCE 

Streambed leakance, as used in this report, controls 

the movement of water between streams and the watertable aquifer. It is defined as the ratio of the vertical 

hydraulic conductivity of the streambed sediment to its 

thickness. The rate and direction of flow through the 

streambed are calculated by multiplying streambed leakance by the head difference between the water-table 

aquifer and stage in the stream and the area through 

which flow is occurring. This assumes that the aquifer 

material adjacent to the streambed is fully saturated. 

Few quantitative data are available that define the 

physical properties of the streambed sediment. However, an alternative method was developed to calculate 

streambed leakance in order to simulate flow between 

the water-table aquifer and streams. This method calculates streambed leakance from the simulated flow to the 

underlying confined flow system, the estimated groundwater recharge, and the estimated hydraulic gradient 

between the water-table aquifer and streams. The 

method equates two equations describing stream base 

flow. The first equation, based on conservation of mass 

for steady-state, prepumping conditions, is of the form 

BF=QRE-DP (2) 

where 

BF =base flow per unit area, in feet per second; 

QRE =volumetric rate per unit area of ground-water 

recharge to water-table aquifer, in feet per second; and 

DP=deep percolation or volumetric rate per unit 

area of flow into (positive) or out of (negative) 

underlying confined aquifer system, in feet per 

second. 

The second equation, based on Darcy's law, states 

BF=M(h,,-hs)=SL (h¢-hs) (3) 

where 

BF =base flow per unit area, in feet per second; 

K' =vertical hydraulic conductivity of streambed, in 

feet per second; 

M=thickness of streambed, in feet ; 

h,, =altitude of water table, in feet ; 

hs =elevation of stream stage, in feet ; and 

SL =streambed leakance, in seconds- ' . 

Equating the two expressions for base flow results in 

the following expression : 

SL=QRE-DP 

(h,,-h,) 

The method requires calculation of deep percolation 

(DP), the volumetric rate of water per unit area moving 

between the confined flow system and the water-table 

aquifer. Block values of deep percolation were computed 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

�76°77 0 75° 78° 

Base from U .S. Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-t o x 1o= APPROXIMATE LINE OF EQUAL LEAKANCE-Vertical 

hydraulic conductivity / confining unit thickness . 

Interval is 1 .0 x 10-5 day- ' 

APPROXIMATE LIMIT OF YORKTOWN 

CONFINING UNIT 

FIGURE 2 3 .-Vertical leakance of the Yorktown confining unit used in model simulations . 
F32 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 1 0 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LINE OF EQUAL LEAKANCE-Vertical _- APPROXIMATE LIMIT OF YORKTOWN 

hydraulic conductivity / confining unit thickness . CONFINING UNIT - Interval is 1 .5 x 10 b day- ' AREAS REQUIRING MODIFICATION OF THE 

CALCULATED VALUE OF LEAKANCE BECAUSE APPROXIMATE LIMIT OF ST . MARYS UPPER CONFINING UNIT MISSING-Discussed in the CONFINING UNIT section "Vertical Leakance" 

FIGURE 24.-Vertical leakance of the St . Marys confining unit used in model simulations. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

78° 77° 76° 75° 

Base from U .S . Geological Survey 

State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LINE OF EQUAL LEAKANCE-Vertical AREA REQUIRING MODIFICATION OF THE hydraulic conductivity/confining unit thickness . Interval CALCULATED VALUE OF LEAKANCE BECAUSE is 0.4 x 10- 'day- ' UPPER CONFINING UNIT MISSING-Discussed in the 

APPROXIMATE LIMIT OF CALVERT CONFINING UNIT section "Vertical Leakance" 

APPROXIMATE LIMIT OF ST. MARYS CONFINING UNIT 

FIGURE 25.-Vertical leakance of the Calvert confining unit used in model simulations . 
F34 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

�77°5 78 0 76 0

39 ° 

38° 

37° 

36° 

Base from U.S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 5 0 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-3.5x10= APPROXIMATE LINE OF EQUAL LEAKANCE-Vertical - - APPROXIMATE LIMIT OF CALVERT 

hydraulic conductivity / confining unit thickness . CONFINING UNIT 

Interval is 2.0 x 10- ' day - ' AREAS REQUIRING MODIFICATION OF THE 

CALCULATED VALUE OF LEAKANCE BECAUSE APPROXIMATE LIMIT OF NANJEMOY-MARLBORO UPPER CONFINING UNIT MISSING-Discussed in the CONFINING UNIT section "Vertical Leakance" 

FIGURE 26 .-Vertical leakance of the Nanjemoy-Marlboro confining unit used in model simulations . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

75 0 78° 77° 76° 

39 ° 

38° 

37° 

36° 

Base from U.S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 1­

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LINE OF EQUAL LEAKANCE-Vertical AREAS REQUIRING MODIFICATION OF THE 

hydraulic conductivity / confining unit thickness . CALCULATED VALUE OF LEAKANCE BECAUSE 

Interval is 8.0 x 10- ' day- ' UPPER CONFINING UNIT MISSING-Discussed in the 

APPROXIMATE LIMIT OF CONFINING UNIT 4 section "Vertical Leakance" 

APPROXIMATE LIMIT OF CONFINING UNIT 5 

FIGURE 2 7.-Vertical leakance of confining unit 4 used in model simulations . 
F36 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78' 77° 

37° 

Base from U.S . Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LINE OF EQUAL LEAKANCE-Vertical APPROXIMATE LIMIT OF CONFINING UNIT 4

hydraulic conductivity / confining unit thickness . Interval is .03 x 10' AREAS REQUIRING MODIFICATION OF THE day-' CALCULATED VALUE OF LEAKANCE BECAUSE

APPROXIMATE LIMIT OF BRIGHTSEAT-UPPER UPPER CONFINING UNIT MISSING-Discussed in POTOMAC CONFINING UNIT the section "Vertical Leakance" 

FIGURE 28.-Vertical leakance of the Brightseat-upper Potomac confining unit used in model simulations . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F37 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S. Geological Survey SCALE 1 :2,000,000 

�40State base maps, 1 :1,000,000 0 10 20 30 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LINE OF EQUAL LEAKANCE-Vertical APPROXIMATE LIMIT OF THE BRIGHTSEAThydraulic conductivity / confining unit thickness . UPPER POTOMAC AQUIFER 

Interval is 2.0 x 10- ' day- ' AREAS REQUIRING MODIFICATION OF THE 

CALCULATED VALUE OF LEAKANCE BECAUSE 

APPROXIMATE LIMIT OF MIDDLE POTOMAC UPPER CONFINING UNIT MISSING-Discussed in the 

CONFINING UNIT section "Vertical Leakance" 

FIGURE 29 .-Vertical leakance of the middle Potomac confining unit used in model simulations . 
F38 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 �30 0 10 2 0 40 50 MILES I 

0 to 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LINE OF EQUAL LEAKANCE-Vertical 

hydraulic conductivity / confining unit thickness . Interval is 2.0 x 10 - ' day - ' 

APPROXIMATE LIMIT OF LOWER POTOMAC 

CONFINING UNIT 

FIGURE 30.-Vertical leakance of the lower Potomac confining unit used in model simulations . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

120 

¢

0 100 w 

a 

rn 

Z 

0

J 80 J ¢ 

¢ 40 Withdrawal used in 

3 model simulation 

= 20 Pumping period 

3 

0 ~ 

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 

YEAR 

FIGURE 31 .-Estimated annual withdrawal and average withdrawal calculated for simulated pumping periods. 

with a steady-state simulation of the prepumping 

ground-water flow system. This simulation treated the 

water-table aquifer as a constant-head boundary. 

Constant-head values for each block were interpolated 

from historic water-level measurements and 1:24,000 

U.S . Geological Survey topographic maps. Given knowledge of block values of QRE, equation 2 calculated 

stream base flow values for each block in the study area. 

Stream stage elevation for each grid block was estimated 

from surface-water altitudes on 1:24,000 U.S. Geological 

Survey topographic maps. Streambed leakance for each 

block was calculated from equation 4. Streambed leakance was assumed to be constant over the simulated 

history of ground-water development. Because DP 

depends on prepumping water-level distribution, values 

of DP were recalculated for each change in a model input 

value during model calibration. 

TIME DISCRETIZATION AND GROUND-WATER WITHDRAWAL 

The history of ground-water development is divided 

into pumping periods-time intervals during which withdrawals are represented by a constant average pumping 

rate. The length and number of pumping periods were 

based on availability of water-level data and on significant changes in withdrawal trends in the northern Atlantic Coastal Plain. Ten pumping periods covering 90 yr 

were used to simulate the period of ground-water development: 1891-1920, 1921-39, 194015, 1946-52, 1953-57, 

1958-64, 1965-67, 1968-72, 1973-77, and 1978-80. Each 

pumping period begins on January 1 and ends on DecemF39 

ber 31 for the years listed . Figure 31 compares estimated 

annual withdrawal with average withdrawal calculated 

for each pumping period. The regional model uses identical pumping periods in order to provide flux values 

along the lateral boundaries of its component subregional 

models. 

Withdrawals for each pumping period were estimated 

from the average annual withdrawal rates for individual 

water users discussed in the section "Ground-Water 

Use." Users within a nodal block were added to determine the total withdrawal rate from that block. Table 8 

lists the withdrawal rates for each aquifer by pumping 

period. The general trend in withdrawal is a steady 

increase through pumping period 8. Total withdrawal 

stabilizes at about 105 Mgal/d for pumping periods 9 and 

10. The middle Potomac aquifer has the largest withdrawal for pumping periods 2 through 10 . The table 

includes only those water users reporting withdrawal of 

more than 10,000 gal/d; therefore, rates do not necessarily indicate the relative importance of aquifers supplying 

water for domestic purposes. 

LATERAL BOUNDARY FLOW 

Lateral boundary flow is the movement of ground 

water across a vertical cross section of the aquifer 

designated the lateral model boundary. At this boundary, the aquifer continues beyond the limits of the model. 

The use of this boundary reduces the size of the model 

grid by eliminating the need to include all of an aquifer in 

the model simulation. 

Lateral boundary flow for each pumping period was 

approximated with flux values. Fluxes were simulated 

through recharge or discharge wells placed in blocks 

located along lateral model boundaries and were calculated with the regional flow model, which extends 

beyond the lateral limits of its component subregional 

models (P.P. Leahy, U.S . Geological Survey, oral commun., 1984). Flux values were computed from Darcy's 

law, which states that flux is proportional to the simulated head gradient across the twoblocks adjacent to the 

lateral flow boundary and to the harmonic mean of their 

transmissivity. Fluxes were computed for, and were 

assumed constant throughout, each pumping period. The 

regional model grid correlates with the model grid of this 

study, and flux values were assigned to the appropriate 

model-grid blocks. Because the regional model is made 

up of the hydraulic properties of the individual subregional models, boundary fluxes were recalculated each 

time a subregional model updated an aquifer and confining unit characteristic in model calibration. Table 9 gives 

the lateral boundary flow into and out of each aquifer 

computed for each pumping period. 
F40 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

SIMULATION OF THE GROUND-WATER FLOW SYSTEM 

STRATEGY OF CALIBRATION 

Calibration of the digital flow model involves areally 

adjusting hydraulic characteristics until the simulated 

response is similar to the observed response of the 

ground-water flow system both prior to and throughout 

the history of ground-water development. Success of the 

model simulation is evaluated through comparisons 

between model-generated and measured water levels at 

selected observation wells. Simulated water level at an 

observation well was interpolated from model-generated 

water levels in the three nearest blocks. 

Historic water-level measurements began about 1860 

and are summarized in reports by Darton (1896) and 

Sanford (1913) . Prepumping potentiometric maps constructed by Siudyla and others (1977), Bal (1978), Newton and Siudyla (1979), and Cosner (1975) supplemented 

early water-level data. Reports on ground-water availability by Cederstrom (1945, 1968), Sinnott (1967), and 

files of the U. S. Geological Survey and the Virginia State 

Water Control Board provided additional water-level 

data for the period of ground-water development. 

Although numerous measurements of water level were 

available, only those that represent water levels in an 

individual aquifer were used for calibration. 

The calibration procedure began by comparing measured water levels with those simulated by the model 

using the initial estimates of the hydraulic characteristics . The hydraulic characteristics were adjusted to 

minimize differences between model-simulated and measured water levels . The procedure was repeated using 

revised values of hydraulic characteristics until simulated water levels closely approximated measured levels . 

The model was first calibrated to simulate the prepumping ground-water flow system. These results provided hydraulic characteristics and initial water levels 

for simulation of pumping conditions. Because the simulation of pumping conditions is dependent on hydraulic 

characteristics and initial water levels from prepumping 

simulations, calibration involved alternating prepumping 

and pumping simulations until hydraulic characteristics 

were acceptable in both simulations. 

PREPUMPING CONDITIONS 

Simulation of prepumping conditions is based on the 

assumption that no major withdrawals occurred in the 

Coastal Plain of Virginia and adjoining States and that 

the system was in an approximate state of hydraulic 

equilibrium. Therefore, the prepumping flow system was 

simulated under a steady-state condition. 

Two conceptualizations were used to simulate the 

water-table aquifer under prepumping conditions (fig . 

32). In the first conceptualization, the water-table aquifer and coastal water were represented as a constanthead boundary defined by the average altitude of the 

water table or freshwater equivalent elevation of the 

coastal water surface (fig. 32B) . The simulation was used 

to quantify the flow into or out of the underlying confined 

aquifer system, previously referred to as DP (deep 

percolation) . In the second conceptualization, a constanthead boundary, representing elevations of stream stage, 

was placed above the blocks representing the watertable aquifer (fig . 32C) in order to allow lateral flow and 

fluctuation of water levels in the water-table aquifer. 

Streambed leakance values, calculated using DP values 

computed from the first conceptualization, controlled the 

vertical flow of water between the water-table aquifer 

and streams. 

The simulated potentiometric-surface maps shown in 

figures 33 through 40 represent the steady-state solution 

of prepumping conditions . The maps include measured 

water levels available for each aquifer. Differences 

between the simulated potentiometric-surface maps and 

the prepumping maps constructed by Cosner (1975), 

Siudyla and others (1977), Bal (1978), and Newton and 

Siudyla (1979) are minor. Model-generated water levels 

in the Chickahominy-Piney Point, Aquia, Brightseatupper Potomac, and middle Potomac aquifers (figs. 35, 

36, 38, 39) are in close agreement with measured water 

levels . The hydraulic gradients determined from the 

prepumping potentiometric surfaces of aquifers define 

flow directions ; figures 33 through 40 indicate a regional 

movement of water from the Fall Line toward coastal 

water and local movement from interfluves toward major 

river valleys. The bending of potentiometric contours 

upstream, especially in the deeper confined aquifers 

under major river valleys, is an effect of erosion into the 

aquifer by ancient and present-day streams. 

The direction of simulated flow across confining units 

into or out of the underlying confined aquifer under 

prepumping conditions is shown in figures 41 through 48; 

water moves upward across confining units toward 

major river valleys and coastal water, and downward 

under interfluves. Recharge to the deeper confined aquifers is concentrated along a band adjacent to the Fall 

Line. 

The direction of simulated flow into or out of the 

confined flow system under prepumping conditions is 

shown in figure 49. Simulated rates of recharge and 

discharge varied up to 3.2 and 2.8 in/yr, respectively. 

The highest rates of recharge into the confined flow 

system are concentrated along the Fall Line. Table 10 

summarizes the computed volumetric leakage rates 

across each confining unit . Themiddle Potomac aquifer is 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F41 

A. Schematic of water-table aquifer system. 

AQ9 

B. Water- table aquifer simulated as a constant-head boundary . 

C. Water-table aquifer simulated as a confined aquifer with an overlying 

constant-head boundary . 

EXPLANATION 

----- WATER TABLE GENERALIZED FLOW LINE 

CONSTANT-HEAD BOUNDARY LATERAL GROUND-WATER FLOW 

~1I9­ MODEL CONFINING UNIT 9 11 DEEP PERCOLATION 

AQ9 MODEL AQUIFER LAYER 9 1 STREAM LEAKAGE 

STREAMBED SEDIMENTS 4 4 GROUND-WATER RECHARGE 

FIGURE 32.-Model conceptualizations of the water-table aquifer. 

the only aquifer that received net recharge from the sources of water into the confined flow system were 

overlying aquifer -a gain ofless than 1 MgaUd . Flow into lateral boundary flow and lateral flow from unconfined 

the aquifer is attributed to direct recharge along the Fall parts of aquifers. 

Line from the overlying water-table aquifer. Table 10 Prepumping lateral boundary fluxes for each aquifer, 

also gives the computed prepumping vertical volumetric computed with the regional model, are summarized in 

leakage rate into and out of the confined flow system and table 9. The values in the table indicate that flow across 

shows that approximately 124 MgaUd of water moved out lateral boundaries was not a significant component in the 

of and 119 Mgal/d moved into the confined system. Other overall ground-water budget of individual aquifers. Low 
F42 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S. Geological Survey SCALE 1 :2,000,000

�40 State base maps, 1 :1,000,000 0 1 0 20 30 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-20- POTENTIOMETRIC CONTOUR-Shows altitude of 0s WELL-Number is measured altitude of water level, in feet, potentiometric surface . Contour interval is 20 feet above sea level . Letter F indicates flowing well

with supplemental contour at 10 feet . Datum is sea level 

APPROXIMATE LIMIT OF YORKTOWNEASTOVER AQUIFER 

FIGURE 33.-Simulated prepumping potentiometric surface of the Yorktown-Eastover aquifer and measured water levels . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

78° 77° 76° 75° 

Base from U.S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

POTENTIOMETRIC CONTOUR-Shows altitude of 

potentiometric surface . Contour interval is 10 feet with 

supplemental contour at 5 feet . Datum is sea level 

APPROXIMATE LIMIT OF ST. MARYSCHOPTANK AQUIFER 

FIGURE 34.-Simulated prepumping potentiometric surface of the St. Marys-Choptank aquifer . 
F44 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S. Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-20- POTENTIOMETRIC CONTOUR-Shows altitude of *12 WELL-Number is measured altitude of water level, in feet 

potentiometric surface . Contour interval is 20 feet . above sea level 

Datum is sea level 

APPROXIMATE LIMIT OF CHICKAHOMINYPINEY POINT AQUIFER 

FIGURE 35.-Simulated prepumping potentiometric surface of the Chickahominy-Piney Point aquifer and measured water levels . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

�40State base maps, 1 :1,000,000 0 10 20 3 0 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

'I--20- POTENTIOMETRIC CONTOUR-Shows altitude of ,25 WELL-Number is measured altitude of water level, in feet, 

potentiometric surface . Contour interval is 20 feet . above sea level 

Datum is sea level 

APPROXIMATE LIMIT OF AQUIA AQUIFER 

FIGURE 36.-Simulated prepumping potentiometric surface of the Aquia aquifer and measured water levels . 
F46 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

39 ° 

38° 

37° 

36` 

Base from U .S. Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

POTENTIOMETRIC CONTOUR-Shows altitude of . . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER

potentiometric surface . Contour interval is 5 feet . SYSTEM-Less than 10,000 milligrams per liter chloride Datum is sea level 

APPROXIMATE LIMIT OF AQUIFER 4 

FIGURE 3 7 .-Simulated prepumping potentiometric surface of aquifer 4 . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

POTENTIOMETRIC CONTOUR-Shows altitude of . . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

potentiometric surface . Contour interval is 10 feet . SYSTEM-Less than 10,000 milligrams per liter chloride 

Datum is sea level 

APPROXIMATE LIMIT OF BRIGHTSEAT-UPPER 038 WELL-Number is measured altitude of water level, in feet, 

POTOMAC AQUIFER above sea level 

FIGURE 3 8 .-Simulated prepumping potentiometric surface of the Brightseat-upper Potomac aquifer and measured water levels . 
F48 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES I 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

_20- POTENTIOMETRIC CONTOUR-Shows altitude of - " " . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

potentiometric surface . Contour interval is 20 feet . SYSTEM-Less than 10,000 milligrams per liter chloride

Datum is sea level 

APPROXIMATE LIMIT OF MIDDLE 014 WELL-Number is measured altitude of water level, in feet, POTOMAC AQUIFER above sea level . Letter F indicates flowing well 

FIGURE 39.-Simulated prepumping potentiometric surface of the middle Potomac aquifer and measured water levels . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F49 

36° 

Base from U .S . Geological Survey 

State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

. . . . . . -40- POTENTIOMETRIC CONTOUR-Shows altitude of ESTIMATED SEAWARD LIMIT OF FRESHWATER 

potentiometric surface . Contour interval is 4 feet . SYSTEM-Less than 10,000 milligrams per liter chloride 

Datum is sea level 

APPROXIMATE LIMIT OF LOWER POTOMAC AQUIFER 

FIGURE 40.-Simulated prepumping potentiometric surface of the lower Potomac aquifer . 
F50 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

37° 

SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 2 0 30 40 50 MILES . 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF CONFINED PART OF

YORKTOWN-EASTOVER AQUIFER DOWNWARD-Into confined part of YorktownEastover aquifer 

UPWARD-Out of confined part of YorktownEastover aquifer 

FIGURE 41 .-Direction of simulated prepumping flow across the Yorktown confining unit. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F51 

78° 77° 76° 75° 

Base from U.S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF ST. MARYSCHOPTANK AQUIFER 

UPWARD-Out of St . Marys-Choptank aquifer 

DOWNWARD-Into St. Marys-Choptank aquifer 

FIGURE 42.-Direction of simulated prepumping flow across the St . Marys confining unit. 
F52 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

�77° 78 0 76° 75° 

39 ° 

SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 1~0 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF CHICKAHOMINY-PINEY 

POINT AQUIFER 

UPWARD-Out of Chickahominy-Piney Point aquifer 

DOWNWARD-Into Chickahominy-Piney Point aquifer 

FIGURE 43.-Direction of simulated prepumping flow across the Calvert confining unit . 
780 

ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F53 

77° - 76 75° 

39 ° 

37° 

Base from U.S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF CONFINED PART OF 

AQUIA AQUIFER 

UPWARD-Out of Aquia aquifer 

DOWNWARD-Into Aquia aquifer 

FIGURE 44.-Direction of simulated prepumping flow across the Nanjemoy-Marlboro confining unit . 
F54 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

�77° 78 0 76° 75° 

Base from U .S . Geological Survey 

�40 State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 50 MILES I 

0 1o 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF CONFINED PART OF

AQUIFER 4 UPWARD-Out of Aquifer 4 

ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams 

per liter chloride DOWNWARD-Into Aquifer 4 

FIGURE 45.-Direction of simulated prepumping flow across confining unit 4. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF BRIGHTSEAT-UPPER UPWARD-Out of Brightseat- upper Potomac aquifer POTOMAC AQUIFER 

ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams DOWNWARD-Into Brightseat- upper Potomac aquifer per liter chloride 

FIGURE 4 6 .-Direction of simulated prepumping flow across the Brightseat-upper Potomac confining unit. 
F56 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

75° 

Base from U .S. Geological Survey

State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES I 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF MIDDLE 

POTOMAC AQUIFER UPWARD-Out of middle Potomac aquifer 

ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams 

per liter chloride DOWNWARD-Into middle Potomac aquifer 

FIGURE 47.-Direction of simulated prepumping flow across the middle Potomac confining unit. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

�757' 76 0 0 78 0

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

F 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF LOWER POTOMAC AQUIFER UPWARD-Out of lower Potomac aquifer 

. . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams DOWNWARD-Into lower Potomac aquifer per liter chloride 

FIGURE 48. -Direction of simulated prepumping flow across the lower Potomac confining unit . 
F58 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75 0 

39 ° 

38° 

37° 

36° 

Base from U .S 

State base map

. 

s

Geological Survey 

, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS

EXPLANATION 

UPWARD-Out of confined system 

DOWNWARD-Into confined system 

FIGURE 4 9 .-Direction of simulated prepumping flow into and out of confined system. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F59 

values for the prepumping simulation indicate that the 

selected positions of the lateral flow boundaries approximately coincide with flow lines or no-flow boundaries . 

PUMPING CONDITIONS 

The digital flow model simulates 90 yr of pumping 

beginning January 1, 1891, and ending December 31, 

1980, under transient conditions . The 10 pumping periods previously discussed were used for transient simulations. The water-table aquifer was simulated without 

consideration for dewatering of the aquifer material . 

Therefore, the transmissivity of the water-table aquifer 

was constant over the entire period of transient simulation . This assumption is considered reasonable because 

regional drawdown in the water-table aquifer is negligible. However, the storage coefficient was assigned a 

value that represents a more reasonable storage condition of a water-table aquifer. Other hydraulic characteristics and initial water levels were equivalent to those 

used to simulate prepumping conditions . 

Two methods were used to compare model-simulated 

and measured historic water levels : (1) simulated 

potentiometric-surface maps, constructed for each aquifer and pumping period, were compared with measured 

water levels, and (2) simulated hydrographs were compared with measured hydrographs at 89 observation 

wells. Figures 50 through 57 show the simulated 

potentiometric-surface maps of the major aquifers at the 

end of the final pumping period (1980) . The maps include 

water levels measured at different times during 1980. 

Hence, simulated water levels are expected to differ 

slightly from those measured, because the simulated 

potentiometric surfaces represent the water-level distribution in each aquifer on December 31, 1980. Overall, 

measured water levels agree with levels simulated by the 

model. 

A comparison of prepumping potentiometric-surface 

maps (figs. 33 through 40) with the 1980 potentiometricsurface maps shows the effect of ground-water development on the water-level distribution in each aquifer. The 

maps of the simulated ground-water flow system in 1980 

show lower water levels and cones of depression around 

major pumping centers. The potentiometric-surface 

maps of the Aquia, Brightseat-upper Potomac, middle 

Potomac, and lower Potomac aquifers show that cones of 

depression developed and coalesced near the cities of 

Williamsburg, Franklin, and Suffolk and the town of 

West Point (figs. 53, 55 through 57). The simulated 

potentiometric surface of the Chickahominy-Piney Point 

aquifer shows a decline in water levels near the town of 

West Point and the City of Williamsburg (fig. 52). The 

hydraulic gradients, determined from the potentiometric 

surfaces of 1980 in the major aquifers, indicate that flow 

directions changed considerably from those simulated for 

prepumping flow conditions and that the direction of flow 

in 1980 was toward the major pumping centers. 

Measured and simulated hydrographs show the agreement between measured and model-generated water 

levels for the history of ground-water development. 

Figures 58 through 61 show hydrographs for 16 of the 89 

observation wells used to calibrate the model. The locations of these 16 wells are shown in figure 62. Most are 

near major ground-water users (fig . 8) . The middle 

Potomac aquifer, near Franklin, shows the largest simulated water-level decline from prepumping flow conditions, about 210 ft in well 55B 22 (fig . 60). 

In addition to simulating water-level changes, the 

model provides a water budget, which quantifies the 

individual components of flow into and out of the groundwater flow system. The relative magnitudes of the 

individual flow components define their significance during a simulated pumping period and over the entire 

period of simulation. Table 11 summarizes the individual 

flow components into and out of the ground-water flow 

system at the end of each simulated pumping period. A 

comparison of the prepumping period and the final 

pumping period (1978-80) water budgets indicates that 

(1) pumpage from the model area increased by about 105 

MgaUd, (2) lateral boundary outflow increased by about 5 

MgaUd, (3) ground-water flow to streams and coastal 

water decreased by about 107.5 Mgal/d, (4) lateral 

boundary inflow increased by about 0.7 Mgal/d, and (5) 

water released from aquifer storage increased by about 

1.6 MgaUd. The slight difference between total inflow 

and total outflow is attributed to the numerical truncation error of the digital simulation. The most significant 

effect of ground-water development over the period of 

simulation was the decrease in ground-water flow to 

streams and coastal water. The increase in lateral boundary outflow is attributed to large withdrawals from 

outside the model area. 

Tables 8, 9, and 10 can be used to evaluate water 

budgets for the confined aquifers. Pumpage (table 8) and 

lateral boundary flow (table 9) are averaged over the 

length of each pumping period . Flow into and out of an 

aquifer across the overlying confining unit is calculated 

at the end of each pumping period. Acomparison of the 

water budgets of individual confined aquifers indicates 

that the major source of water replacing water pumped 

was increased vertical flow into the aquifers through the 

intervening confining units and decreased vertical flow 

out of the aquifers (table 10). 

The direction of simulated flow across confining units 

into or out of the underlying confined aquifers in 1980 is 

shown in figures 63 through 70. Comparison with figures 

41 through 48 shows the change in the direction of 

vertical flow across confining units that resulted from the 
F60 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

�75° 78° 77° 76 0 

39 ° 

38° 

37° 

36° 

Base from U .S. Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-20- POTENTIOMETRIC CONTOUR-Shows altitude of ,3 WELL-Number is measured altitude of water level, in feet 

potentiometric surface . Contour interval is 20 feet . above or below sea level 

Datum is sea level 

APPROXIMATE LIMIT OF YORKTOWNEASTOVER AQUIFER 

FIGURE 50 .-Simulated potentiometric surface of the Yorktown-Eastover aquifer and measured water levels, 1980. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

78° 77° 76° 75° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-to- POTENTIOMETRIC CONTOUR-Shows altitude of 

potentiometric surface . Contour interval is 10 feet with 

supplemental contour at 5 feet . Datum is sea level 

APPROXIMATE LIMIT OF ST. MARYSCHOPTANK AQUIFER 

FIGURE 5 1 .-Simulated potentiometric surface of the St. Marys-Choptank aquifer, 1980 . 
F62 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

�75° 78° 77° 76 0 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES F 

0 1~0 20 30 40 50 KILOMETERS 

EXPLANATION 

-20- POTENTIOMETRIC CONTOUR-Shows altitude of itt WELL-Number is measured altitude of water level, in feet, potentiometric surface . Contour interval is 20 feet . above sea level 

Datum is sea level 

APPROXIMATE LIMIT OF CHICKAHOMINYPINEY POINT AQUIFER 

FIGURE 52.-Simulated potentiometric surface of the Chickahominy-Piney Point aquifer and measured water levels, 1980 . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

�75 78° 77° 76 0 0

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

POTENTIOMETRIC CONTOUR-Shows altitude of ,42 WELL-Number is measured altitude of water level, in feet

potentiometric surface . Contour interval is 30 feet . above or below sea level 

Datum is sea level 

APPROXIMATE LIMIT OF AQUIA AQUIFER 

FIGURE 53.-Simulated potentiometric surface of the Aquia aquifer and measured water levels, 1980 . 
F64 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

75` 

Base from U.S . Geological Survey

�30 State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 2 0 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

POTENTIOMETRIC CONTOUR-Shows altitude of . . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER

potentiometric surface . Contour interval is 8 feet . SYSTEM-Less than 10,000 milligrams per liter chloride Datum is sea level 

APPROXIMATE LIMIT OF AQUIFER 4 

FIGURE 5 4 .-Simulated potentiometric surface of aquifer 4, 1980. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

78° 77° 76° 75° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-30- POTENTIOMETRIC CONTOUR-Shows altitude of . . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

potentiometric surface . Contour interval is 30 feet . SYSTEM-Less than 10,000 milligrams per liter chloride 

Datum is sea level 

APPROXIMATE LIMIT OF BRIGHTSEAT-UPPER "8 WELL-Number is measured altitude of water level, in feet 

POTOMAC AQUIFER above or below sea level 

FIGURE 55.-Simulated potentiometric surface of the Brightseat-upper Potomac aquifer and measured water levels, 1980 . 
F66 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S. Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES I 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-40- POTENTIOMETRIC CONTOUR-Shows altitude of . . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

potentiometric surface . Contour interval is 40 feet . SYSTEM-Less than 10,000 milligrams per liter chloride

Datum is sea level 

APPROXIMATE LIMIT OF THE MIDDLE 028 WELL-Number is measured altitude of water level, in feet 

POTOMAC AQUIFER above or below sea level 

FIGURE 56.-Simulated potentiometric surface of the middle Potomac aquifer and measured water levels, 1980. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

78° 77 0 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

-30- POTENTIOMETRIC CONTOUR-Shows altitude of . . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

potentiometric surface . Contour interval is 30 feet . SYSTEM-Less than 10,000 milligrams per liter chloride 

Datum is sea level 

APPROXIMATE LIMIT OF LOWER POTOMAC AQUIFER " 25 WELL-Number is measured altitude of water level, in feet 

above or below sea level 

FIGURE 57.-Simulated potentiometric surface of the lower Potomac aquifer, 1980 . 
------------------

F68 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

30 

Simulated 20 

10 

0 Measured 

-10 

-20 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

A. Well 62C 4 screened in Yorktown-Eastover aquifer 

20 

15 J 

W 10 W 

J 5 

W 0 

O -5 

1920 1930 1940 1950 1960 1970 1980 m 

m 

O YEAR 

W B. Well 59H 1 screened in Chickahominy-Piney Point aquifer 

O 

m 

a 

w 40 W 

20 

Simulated 

W 0 

-20 Measured 

W -40 

3 -60 

1920 

J 

1930 1940 1950 1960 1970 1980 

YEAR 

C. Well 55B 25 screened in Aquia aquifer 

25 

0 

-25 

-50 

-75 

-100 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

D. Well 59J 3 screened in Aquia aquifer 

FIGURE 58 .-Simulated and measured change in water levels for 

history of ground-water development. (Location of wells shown in 

fig. 62 .) 

withdrawal of ground water from the confined aquifers. 

For example, during prepumping flow conditions the 

lower Potomac aquifer was recharged over about 25 

percent of the total area of the lower Potomac confining 

unit (fig . 48). In 1980, the lower Potomac aquifer was 

recharged over about 93 percent of the total area of the 

lower Potomac confining unit (fig . 70). The remaining 7 

percent of the lower Potomac confining unit recharged 

the middle Potomac aquifer because of large withdrawals 

from this aquifer near the cities of Williamsburg and 

Suffolk. 

60 

40 

20 

0 

-20 

-40 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

A . Well 55D 3 screened in Brightseat-upper Potomac aquifer 

30 

20 

J 

j

W 

10 

QW 

rn 

0 

-10 

O - 20 

J 920 1930 1940 1950 1960 1970 1980 m 

¢ YEAR O 

W B. Well 56N 1 screened in Brightseat-upper Potomac aquifer 

O 

m 

a 

w 40 

0 z 

J -40 W 

- 80 J 

-120 F 

-160 

920 1930 1940 1950 1960 1970 1980 

YEAR 

C . Well 57C 12 screened in Brightseat-upper Potomac aquifer 

25 

0 

-25 

-50 

-75 

-100 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

D Well 58K 7 screened in Brig htseat-upper Potomac aquifer 

FIGURE 59.-Simulated and measured change in water levels for 

history of ground-water development. (Location of wells shown in 

fig. 62 .) 

The development of ground water affected the direction of flow of water between the confined and watertable aquifer systems . Table 10 gives the quantity of 

water entering and leaving the confined flow system at 

the end of each pumping period . In 1980, about 177 

Mgal/d of water flowed into the confined flow system 

from the water-table aquifer and about 73 Mgal/d flowed 

out of the confined flow system into the water-table 

aquifer. Comparison of the prepumping and final pumping period (1978-80) net leakage values shows an 

increase of about 110 Mgal/d into the confined flow 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F69 

75 

0 

-75 

-150 

-225 

-300 1970 1980 1920 1930 1940 1950 1960 

YEAR 

A . Well 558 22 screened in middle Potomac aquifer 

50 

0 

J 

-50 w 

J -100 

w 

-150 

0 

w 

-200 

1920 1930 1940 1950 1960 1970 1980 

m 

B. Well 55B 45 screened in middle Potomac aquifer 

tw 50 w 

w 25 z 

J 0 w

7 

w -25 J 

Q

w -50 

-75 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

C . Well 54C 1 screened in middle Potomac aquifer 

30 

20 

10 

0 

-10 

-20 1920 1930 1940 1950 1960 1970 1980 

YEAR 

D. Well 52G 1 screened in middle Potomac aquifer 

FIGURE 60 .-Simulated and measured change in water levels for 

history of ground-water development. (Location of wells shown in 

fig. 62.) 

system over the entire period of simulation. This change 

in leakage affected the local discharge of ground water to 

streams and the regional discharge of ground water to 

coastal water. 

The simulated direction of flow between the confined 

flow system and the water-table aquifer in 1980 is shown 

in figure 71. Simulated rates of flow entering the confined flow system varied up to about 3.8 in/yr. Simulated 

rates of flow leaving the confined flow system varied up 

to about 2.2 in/yr. Comparison of figures 49 and 71 

60 

40 

20 Simulated 

0 w l`'~-.~ 

Measuredi '_ -20 

-40 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

A . Well 52J 12 screened in lower Potomac aquifer 

50 

0 

J 

j -50 w 

m -150 

0 -zoo 1920 1930 1940 1950 1960 1970 1980 

m 

Q

O YEAR 

B. Well 55A 3 screened in lower Potomac aquifer 

O 

m 

YEAR 

C . Well 558 36 screened in lower Potomac aquifer 

40 

20 

0 

-20 

-40 

-60 1970 1980 1920 1930 1940 1950 1960 

YEAR 

D. Well 59K 1 screened in Brig htseat-upper Potomac aquifer 

FIGURE 61 .-Simulated and measured change in water levels for 

history of ground-water development. (Location of wells shown in 

fig. 62 .) 

suggests that the withdrawal of ground water from the 

confined aquifers increased the area of recharge into the 

confined flow system by about 33 percent. Prior to 

withdrawal, water in the confined flow system discharged into Chesapeake Bay (fig. 49). Withdrawal from 

the confined aquifers resulted in the movement of water 

from Chesapeake Bay into the confined flow system (fig . 

71). This movement could affect the water quality of 

both Chesapeake Bay and the underlying confined flow 

system. 
F70 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

75° 

Base from U .S. Geological Survey 

State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

0 58K 7 OBSERVATION AND LOCAL WELL NUMBER SHOWN 

IN FIGURES 58 - 61 AND FIGURES 72 - 79 

FIGURE 62 .-Locations of observation wells used in model calibration for pumping conditions . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

Base from U .S . Geological Survey 

State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF CONFINED PART OF 

YORKTOWN- EASTOVER AQUIFER 

UPWARD-Out of Yorktown-Eastover aquifer 

DOWNWARD-Into Yorktown-Eastover aquifer 

FIGURE 63 .-Direction of simulated flow across the Yorktown confining unit, 1980 . 
F72 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

�776° 78 0 0 75° 

39 ° 

38° 

37° 

36° 

Base from U .S. Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES . 0 1 0 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF ST. MARYSCHOPTANK AQUIFER 

UPWARD-Out of St . Marys-Choptank aquifer 

DOWNWARD-Into St . Marys-Choptank aquifer 

FIGURE 6 4 .-Direction of simulated flow across the St. Marys confining unit, 1980 . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F73 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 10 20 -.30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF CHICKAHOMINYPINEY POINT AQUIFER 

UPWARD-Out of Chickahominy-Piney Point aquifer 

DOWNWARD-Into Chickahominy-Piney Point aquifer 

FIGURE 6 5 .-Direction of simulated flow across the Calvert confining unit, 1980 . 
F74 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTICCOASTAL PLAIN 

�76° 78° 77 0 75° 

Base from U .S. Geological Survey SCALE 1 :2,000,000

�30 State base maps, 1 :1,000,000 0 10 20 40 50 MILES 

0 1~0 20 30 40 50 KILOMETERS 

EXPLANATION 

APPROXIMATE LIMIT OF CONFINED PART OF 

AQUIA AQUIFER 

UPWARD-Out of Aquia aquifer 

DOWNWARD-Into Aquia aquifer 

FIGURE 66 .-Direction of simulated flow across the Nanjemoy-Marlboro confining unit, 1980 . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F75 

78° 77° 76° 75° 

Base from U .S 

�4State base map

. 

s

Geological Survey 

0

, 1 :1,000,000 0 1 0 

SCALE 1 :2,000,000 

20 30 50 MILES F 

0 10 1 20 30 40 50 KILOMETERS

EXPLANATION 

APPROXIMATE LIMIT OF AQUIFER 4 UPWARD-Out of Aquifer 4 

ESTIMATED SEAWARD LIMIT OF FRESHWATER = -­

SYSTEM-Less than 10,000 milligrams _---'

! ._ __ i DOWNWARD-Into Aquifer 4 per liter chloride 

FIGURE 67.-Direction of simulated flow across confining unit 4, 1980 . 
F76 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

78° 77° 76° 75 0 

Base from U .S . Geological Survey 

State base maps, 1 :1,000,000 0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

--- APPROXIMATE LIMIT OF BRIGHTSEAT-UPPER 

POTOMAC AQUIFER UPWARD-Out of Brightseat- upper Potomac aquifer 

. . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams 

per liter chloride DOWNWARD-Into Brightseat- upper Potomac aquifer 

FIGURE 6 8 .-Direction of simulated flow across the Brightseat-upper Potomac confining unit, 1980 . 
78 0 

ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

77' 76° 75° 

39 ° 

38° 

37° 

36° 

Base from U .S . Geological Survey SCALE 1 :2,000,000 

State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES 

0 to 20 30 40 50 KILOMETERS 

EXPLANATION 

-- APPROXIMATE LIMIT OF MIDDLE UPWARD-Out of middle Potomac aquifer POTOMAC AQUIFER 

. . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams DOWNWARD-Into middle Potomac aquifer per liter chloride 

FIGURE 69.-Direction of simulated flow across the middle Potomac confining unit, 1980. 
F78 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

Base from U .S. Geological Survey SCALE 1 :2,000,000 State base maps, 1 :1,000,000 0 10 20 30 40 50 MILES . 

0 1 0 20 30 40 50 KILOMETERS 

EXPLANATION -- APPROXIMATE LIMIT OF LOWER POTOMAC AQUIFER UPWARD-Out of lower Potomac aquifer 

. . . . . . . ESTIMATED SEAWARD LIMIT OF FRESHWATER 

SYSTEM-Less than 10,000 milligrams 

per liter chloride DOWNWARD-Into lower Potomac aquifer 

FIGURE 70 .-Direction of simulated flow across the lower Potomac confining unit, 1980 . 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM 

�76°77 0 75° 76° 

Base from U .S. 

State base maps, 1 :1,000,000 

Geological Survey 

0 

SCALE 1 :2,000,000 

10 20 30 40 50 MILES 

0 10 20 30 40 50 KILOMETERS

EXPLANATION 

UPWARD-Out of confining system 

DOWNWARD-Into confining system 

FIGURE 7 1 .-Direction of simulated flow into and out of the confined system, 1980. 
F80 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

40 

0 

J 

1 -40 

J -80 a 

-120 Modeled 50-percent decrease 

O -160 

W 1920 1930 1940 1960 1960 1970 1980 m 

O YEAR 

w A . Well 54C 1 screened in middle Potomac aquifer

O 

m

Q 

r

W 40 W odeled 100-percent increase uZ - _Calibrate 20 d_-_~ef,. .l~,h IKAInkhdi__ 

J 0 -Measured hydrograph _-_--_-

w -20 -----------­

0= -40 \Modeled 50-percent decrease -

3 -60 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

B. Well 52G 1 screened in middle Potomac aquifer 

FIGURE 72.-Effects on simulated heads of varying the calibrated 

value of transmissivity of the middle Potomac aquifer. (Location of 

wells shown in fig. 62 .) 

SENSITIVITY ANALYSIS 

Sensitivity analysis determines the accuracy of model 

results in accordance with the degree of uncertainty 

associated with the simulated value of hydraulic characteristics. In the procedure, the value of an individual 

calibrated hydraulic characteristic is changed over a 

specific range, while other values of calibrated characteristics remain unchanged. The magnitude of the change 

in water levels defines the sensitivity ofthe model to that 

hydraulic characteristic and is measured by comparing 

water-level hydrographs simulated by the calibrated 

model with those simulated with the changed value of the 

hydraulic characteristic . 

Hydraulic characteristics varied to test model sensitivity were (1) transmissivity of the middle Potomac 

aquifer, (2) vertical hydraulic conductivity of the middle 

Potomac confining unit, (3) storage coefficient of the 

confined aquifers, and (4) specific storage of the confining 

units. The first two were selected because it was found 

during calibration that these two hydraulic characteristics had the greatest effect on simulated water levels . 

Theaquifer storage coefficient wastested to evaluate the 

response of the ground-water flow system to changes in 

aquifer storage. Storage properties of confining units 

were tested to determine the significance of neglecting 

storage in confining units during transient simulations. 

Table 12 summarizes these sensitivity tests and the 

results. Lateral flow across model boundaries was not 

recalculated for any of the sensitivity simulations. 

Figures 72 and 73 compare water levels in the middle 

Potomac aquifer at four observation wells resulting from 

150 

Modeled 100-percent increase 0 

.Calibrated -'--

1 -150 Measured hydrograph -'--­ J -300 a 

-450 Modeled 50-percent decrease 

p -600 J 1920 1930 1940 1950 1960 1970 1980 w 

m 

¢ YEAR O 

A . Well 55B 22 screened in middle Potomac aquifer

O 

m

Q 

r 75 Modeled 100-percent increase 

0 Calibrated 

-75 w 

w -150 Modeled 50-percent decrease _ " 

J 

- 225 r Measured hydrograph 

a - 300 1920 1930 1940 1950 1960 1970 1980 

YEAR 

B. Well 55B 45 screened in middle Potomac aquifer 

FIGURE 73 .-Effects on simulated heads of varying the calibrated 

value o£ transmissivity of the middle Potomac aquifer. (Location of 

wells shown in fig . 62.) 

a 50-percent decrease and a 100-percent increase in the 

calibrated values of transmissivity. The locations of the 

wells are shown in figure 62. Water levels within this 

same aquifer resulting from a 50-percent decrease and a 

100-percent increase in the calibrated value of vertical 

hydraulic conductivity of the overlying confining unit are 

compared in figures 74 and 75. The sensitivity analysis 

showed that simulated water levels are much more 

sensitive to decreases in the selected values of transmissivity and vertical hydraulic conductivity than to 

increases in the values tested. 

Four hydrographs (figs. 76, 77), representing observation wells in the Chickahominy-Piney Point, 

Brightseat-upper Potomac, and middle Potomac aquifers, compare water levels resulting from an increase of 

and a decrease of one order of magnitude in the calibrated storage coefficient (1 .0x10-4). The locations of 

these wells are shown in figure 62 . The general agreement between hydrographs simulated with the calibrated and the lower value of storage coefficient 

(1 .0x10-5 ) indicates that the system is near equilibrium 

at the end of each pumping period and suggests that the 

calibrated storage coefficient is a reasonable value for 

simulation. The water levels, which result from an 

increase in storage coefficient of one order of magnitude, 

show that the system is sensitive to the higher value 

(1 .0x 10-3 ) . The increase in the amount of water released 

from storage had a significant effect on simulated water 

levels . The higher value of storage coefficient is not 

reasonable for confined aquifers in the study area except 

in a few local areas. 
ANALYSIS OF THE GROUND-WATER FLOW SYSTEM F81 

75 

0 

w -75 Calibrated 

wJ -150 

Measured hydrograph 

-225 

3

O 

w - 300 1920 1930 1940 1950 1960 1970 1980 

m 

O YEAR 

A . Well 55B 22 screened in middle Potomac aquifer 

O 

m 

w 30 

20 z 

10 w 

w 0 J 

w -10 Measured hydrograph' Modeled 50-percent decrease 

F 

3 -20 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

B. Well 52G 1 screened in middle Potomac aquifer 

FIGURE 74 .-Effects on simulated heads of varying the calibrated 

value of vertical hydraulic conductivity of the middle Potomac 

confining unit . (Location of wells shown in fig. 62 .) 

The calibrated model was developed on the assumption 

that the effects of storage released from the confining 

units are negligible over a pumping period. The sensitivity of the model to confining unit storage was tested 

simulating two values of storage coefficient for each 

confining unit. One value of storage coefficient was 

calculated by multiplying the average thickness of each 

confining unit by a specific storage of 1 .0x10-6 ft-1 

(Lohman, 1979). This method assumes that all water 

50 -_-_ . . . __ odeled 100-percent increase 

0 J Calibrated w -50 w 

-100 Modeled 50-percent decrease 

W 

3 -150 Measured hydrograph

O

J - 200 

1920 1930 1940 1950 1960 1970 1980 m 

O YEAR 

> A . Well 55B 45 screened in middle Potomac aquifer O 

m 

F

W 80 

Modeled 100-percent increase 

z 40 Measured hydrograph ­

0 - Calibrated 

W

J -40 

w - 80 r 

-120 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

B. Well 54C 1 screened in middle Potomac aquifer 

FIGURE 75 .-Effects on simulated heads of varying the calibrated 

value of vertical hydraulic conductivity of the middle Potomac 

confining unit . (Location of wells shown in fig. 62 .) 

20 One order magnitude increase 

15 

Calibrated ~' -----. 

w 10 J 5 One order magnitude decrease 

w Measured hydrograph 

3 0 _ 

w 

m - 5 1920 1930 1940 1950 1960 1970 1980 

O YEAR 

> A . Well 59H 1 screened in Chickahominy-Piney Point aquifer

O 

m 

H 30 w One order magnitude u . z 20 increase 

Calibrated 

w 10 One order magnitude decrease w

J 0 

-10 

a

3 -20 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

B. Well 56N 1 screened in Brightseat-upper Potomac aquifer 

FIGURE 76 .-Effects on simulated heads of varying the calibrated 

value of storage coefficient. (Location of wells shown in fig. 62 .) 

released from storage results only from the compressibility of water. The other set of values tested was computed 

by multiplying the average confining unit thickness by a 

specific storage of 1 .0x10-4 ft-I . Storage coefficients 

tested are shown for each aquifer in table 13. 

A specific storage of 1.0X10-6 ft -1 and the calibrated 

value, which neglects water released from confining unit 

storage, resulted in approximately the same water levels 

(figs. 78, 79). These simulations indicate that the effect of 

40 One order magnitude 

~-~- :-= increase J 20 

w Calibrated I w 0 One order magnitude decrease J -20 r Measured hydrograph 

3 - 40 

-60 w 1920 1930 1940 1950 1960 1970 1980 

m 

O YEAR 

creened in middle Potomac aquifer > A . Well 55B 25 O 

m 

50 w 

z 25 Calibrated J

W 0 >

W

J -25 

w 

F - 50 

-75 1970 1980 1920 1930 1940 1950 1960 

YEAR 

B. Well 54C 1 screened in middle Potomac aquifer 

FIGURE 77 .-Effects on simulated heads of varying the calibrated 

value of storage coefficient . (Location of wells shown in fig. 62 .) 
F82 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

40 -_-_-----_~~ Specific storage=

1.0=10 - 4foot ­ J 

W -40 - Specific storage= 0.0/foot and 

J Specific storage= 1 .0 x 10 - /foot 

a - 80 w

N 

OJ 

m 

-120 

-160 1920 1930 1940 1950 1960 1970 1980 

O YEAR 

w A . Well 58K 7 screened in Brightseat-upper Potomac aquifer >O 

m 

a

F 

LLJ 50 

z 25 

w 0 Specific storage= 0 .01foot and 

Specific storage =1 .0 x 10 -1/foot w -25 

w -50 F3 -75 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

B. Well 54C 1 screened in middle Potomac aquifer 

FIGURE 78 .-Effects on simulated heads of transient leakage . (Location of wells shown in fig . 62 .) 

confining unit storage probably is not significant if the 

water released from confining unit storage is due to the 

compressibility of water only. Hydrographs generated 

from confining units having a specific storage of 

1.0 x10-4 ft -1 show significantly higher water levels (figs. 

78, 79; table 12) . The calibrated model does not include 

confining unit storage because quantitative data defining 

the storage properties of confining units are not available, and actual storage coefficients probably are closer 

to the values computed assuming a specific storage of 

1. 0 x 10_6 ft -I . 

LIMITATION AND APPLICATION OF THE FLOW MODEL 

The calibrated model is suitable for analyzing the 

regional flow of ground water in the confined flow system 

and for evaluating the long-term effects of large-scale 

withdrawal from the confined aquifers. The model can 

project the regional impact of proposed large-scale withdrawal scenarios. The large grid scale limits the model's 

ability to provide detailed analysis of local flow effects. In 

this study, the accuracy of the model is governed by 

estimates of hydraulic characteristics, grid spacing 

(12.25-mil blocks), and time intervals of the 10 pumping 

periods (3 to 30 yr). Further refinement of the model grid 

and the values ofhydraulic characteristics ofaquifers and 

confining units is needed to analyze local ground-water 

flow and the short-term effects of ground-water withdrawal and recharge. 

The method used to simulate ground-water flow in the 

water-table aquifer provides an upper boundary condition for the model. The simulation allows water levels to 

75 

-----_ Specific storage= 1 .0 x 10 -'/foot 

J 0 

W 

W -75

J 

W -150 _ Specific storage= 0.0/foot and 

Specific storage= 1 .0 x 10 -/foot m 

O -225 

J -300 

m 1920 1930 1940 1950 1960 1970 1980 

A . Well 558 22 screened in middle Potomac aquifer 

F

w 40 

I Specific storage= 1 .0 x 10 - '/foot z 20 

w 0 

-20 Specific storage =0 .01foot and 

¢ ~ Specific storage= 1 .0x10 - °/foot Measured hydrograph 

i -40 r ¢

3 -60 

1920 1930 1940 1950 1960 1970 1980 

YEAR 

B . Well 52G 1 screened in middle Potomac aquifer 

FIGURE 79.-Effects on simulated heads of transient leakage . (Location of wells shown in fig . 62.) 

fluctuate in the water-table aquifer; however, it is not 

intended to provide a detailed analysis of flow in the 

water-table aquifer or of flow between the water-table 

aquifer and streams. Additional data defining streambed 

leakance, stream base flow, and withdrawal from the 

water-table aquifer are needed to simulate the watertable aquifer more accurately and to quantify flow 

between the water-table aquifer and streams locally. 

More detailed definition of the time-dependent 

stresses acting on the ground-water flow system and the 

transient effects of confining unit storage is needed to 

simulate short-term effects of droughts, seasonal variations in precipitation, and withdrawal. Data needed are 

storage coefficients of individual confining units and 

ground-water withdrawal and recharge rates for time 

intervals smaller than the average annual value. 

SUMMARY AND CONCLUSIONS 

The unconsolidated sand, silt, and clay of the Coastal 

Plain aquifer system are an important source of industrial, municipal, domestic, and agricultural water supplies in the Coastal Plain of Virginia. This fresh-groundwater flow system is bounded below and on the west by 

bedrock, and on the east by salty water. These unconsolidated sediments form a multiaquifer system consisting of a water-table aquifer and an underlying sequence 

of confined aquifers and intervening confining units that 

unconformably rest on basement. Near major river channels, aquifers and confining units have been partially or 

completely eroded and replaced by material more permeable than the confining unit but less permeable than 

the aquifer. 
REFERENCES CITED F83 

Use of ground water from confined aquifers began in 

the Virginia Coastal Plain in the late 1800's and had 

increased to about 100 Mgal/d in 1980. The continued 

withdrawal of large quantities of water has resulted in a 

steady decline of water levels . The decline has changed 

the direction of ground-water flow toward major pumping centers. These centers are located near the cities of 

Franklin, Williamsburg, Suffolk, and Alexandria and the 

towns of West Point and Smithfield . Total withdrawal 

from these centers is estimated to have been 65 Mgal/d in 

1980. The largest center is near Franklin, where withdrawals exceeded 40 Mgal/d in 1980. 

A digital flow model was developed to simulate the 

response of the ground-water flow system to groundwater development. Withdrawal data for each confined 

aquifer were compiled for the period of simulation, 

1891-1980. The middle Potomac aquifer is the most 

important source of ground water in the Virginia Coastal 

Plain and supplied an average of about 56 MgaUd during 

the period 1978-80. The transmissivity distribution was 

defined for each aquifer; in general, transmissivity 

increases from the Fall Line eastward but decreases 

farther eastward near the freshwater-saltwater interface. The lower and middle Potomac aquifers are the 

most transmissive aquifers; estimated transmissivity 

ranges from 410 to 18,145 ft2/d for the middle 

Potomac aquifer and from 250 to 12,440 ft2/d for the 

lower Potomac aquifer. Vertical leakances simulated 

the effects of confining units on vertical flow between 

aquifers. 

Maps showing simulated prepumping potentiometric 

surfaces indicate regional movement of water from the 

Fall Line toward coastal areas and local movement of 

ground water from interfluves toward major river valleys . Maps showing simulated flow across confining units 

indicate that most recharge occurred in narrow bands 

approximately parallel to the Fall Line and under interfluves and that discharge was toward major river valleys 

and coastal water. Simulated prepumping rates of 

recharge into the confined flow system varied up to 3.2 

in/yr, and rates of discharge varied up to 2.8 in/yr. The 

highest rates of simulated recharge are concentrated 

along the Fall Line. 

The simulated potentiometric-surface maps of the 

major aquifers for 1980 show the lower water levels and 

the cones ofdepression that are developing around major 

pumping centers. The largest simulated decline, about 

210 ft, is near Franklin . Waterbudgets indicate that over 

the period of simulation (1891-1980) (1) pumpage from 

the model area increased by about 105 MgaUd, (2) lateral 

boundary outflow increased by about 5 Mgal/d, (3) 

ground-water flow to streams and coastal waters 

decreased by about 107.5 MgaUd, (4) lateral boundary 

inflow increased by about 0.7 Mgal/d, and (5) water 

released from aquifer storage increased by about 1 .6 

MgaUd. Changes in the direction of vertical leakage 

toward major pumping centers resulted from groundwater withdrawal. The major source of recharge replacing the water pumped from confined aquifers was vertical leakage. 

Simulated rates of flow into the confined aquifer 

system in 1980 varied up to 3.8 in/yr, and rates of flow 

out of the confined flow system varied up to 2.2 in/yr. 

Simulations show a net increase of about 110 Mgal/d into 

the confined from the unconfined flow system over the 

period of simulation. This change in leakage affected the 

local discharge of ground water to streams and the 

regional discharge of ground water to coastal water. The 

withdrawal of ground water from the confined aquifers 

increased the area of recharge into the confined flow 

system by about 33 percent and resulted in the movement of brackish water from Chesapeake Bay into the 

confined flow system. 

Sensitivity analysis shows that simulated water levels 

are more sensitive to decreases in aquifer transmissivity 

and confining unit vertical hydraulic conductivity than to 

increases for the values tested. Lowering the storage 

coefficient has a negligible effect on simulated water 

levels ; however, increasing the storage coefficient has a 

significant effect . Sensitivity simulations also indicate 

that the effect of confining unit storage is not significant 

if the water released from storage in the confining unit is 

from the compressibility of water only. 

The calibrated model is suitable for analyzing the 

regional flow of ground water through the confined 

aquifers. The large grid scale limits the capability of the 

model to provide a detailed local analysis of the groundwater flow system. The adequacy of the model is governed by estimates of hydraulic characteristics, grid 

spacing, and time intervals of the 10 pumping periods. 

The method developed for simulating flow in the watertable aquifer provides an adequate upper-boundary condition for this study. Additional data on streambed 

leakance, stream base flow, and withdrawal from the 

water-table aquifer are needed to simulate water levels 

in the water-table aquifer more accurately and to quantify flow between the water-table aquifer and streams 

locally. More detailed data are needed to define the 

time-dependent stresses and the transient effect due to 

the release of water from storage in the confining units 

that is neglected in the model developed for this study. 

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the Eastern Shore of Virginia: Virginia State Water Control Board 

Planning Bulletin 309, 73 p. 
F84 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

1978, A three-dimensional computer simulation model for 

ground-water flow in the York-James-Middle Peninsula, Virginia : 

Virginia State Water Control Board Planning Bulletin 313, 45 p. 

Bick, K.F ., and Coch, N.K., 1969, Geology of the Williamsburg, Hog 

Island, and Bacons Castle Quadrangles, Virginia : Virginia Division 

of Mineral Resources Report o£ Investigations 18, 28 p. 

Bredehoeft, J.D ., and Pinder, G.F ., 1970, Digital analysis of areal flow 

in multiaquifer ground-water systems-A quasi-three-dimensional 

model: Water Resources Research, v. 6, no . 3, p. 883-888. Brown, D.L ., and Silvey, W.D ., 1977, Artificial recharge to a 

freshwater-sensitive brackish-water sand aquifer, Norfolk, Virginia: U.S . Geological Survey Professional Paper 939, 53 p. 

Brown, G.A ., and Cosner, O.J ., 1974, Ground-water conditions in the 

Franklin area, southeastern Virginia : U.S . Geological Survey

Hydrologic Investigations Atlas HA 538, 3 sheets . 

Brown, P.M., Miller, J.A ., and Swain, F.M ., 1972, Structural and 

stratigraphic framework, and spatial distribution of permeability

of the Atlantic Coastal Plain, North Carolina to New York: U.S . 

Geological Survey Professional Paper 796, 79 p. 

Cederstrom, D.J., 1945, Geology and ground-water resources o£ the 

Coastal Plain in southeastern Virginia : Virginia Geological Survey

Bulletin 63, 384 p. 

1957, Geology and ground-water resources of the York-James 

Peninsula, Virginia : U.S . Geological Survey Water-Supply Paper

1361, 237 p. 

1968, Geology and ground-water resources of the Middle Peninsula, Virginia : Virginia Division of Mineral Resources Bulletin, 

221 p. 

Chapelle, F.H ., and Drummond, D.D., 1982, Modeling chloride in the 

Patuxent aquifer in the Baltimore industrial area: The influence of 

Pleistocene erosion channels : EOS, Transactions of the American 

Geophysical Union, v. 63, no . 18, p. 317. 1983, Hydrogeology, digital simulation, and geochemistry of 

the Aquia and Piney Point-Nanjemoy aquifer system in southern 

Maryland: Maryland Geological Survey Report of Investigations

38, 100 p. 

Chapelle, F.H ., and Kean, T.M., 1985, Hydrogeology, digital solute 

transport simulation, and geochemistry of the Lower Cretaceous 

aquifer system near Baltimore, Maryland: Maryland Geological 

Survey Report of Investigations 43, 220 p. 

Converse, Ward, Davis, Dixon, Consulting ground-water geologists, 1981, Hydrogeologic investigation, ground-water development 

phase, Virginia Beach fresh ground-water project for City of 

Virginia Beach, Final report : Caldwell, N.J ., 107 p. Cooper, H.H ., Jr ., and Jacob, C.E ., 1946, A generalized graphical

method for evaluating formation constants and summarizing wellfield history: Transactions of the American Geophysical Union, 

v. 27, no. 4, p. 526-534. 

Cosner, O.J ., 1975, A predictive computer model of the Lower Cretaceous aquifer, Franklin area, southeastern Virginia: U.S . Geological Survey Water-Resources Investigations 51-74, 62 p. 

Cushing, E.M ., Kantrowitz, I.H ., and Taylor, K.R ., 1973, Water 

resources of the Delmarva Peninsula: U.S. Geological Survey

Professional Paper 822, 58 p. Darton, N.H ., 1896, Artesian-well prospects in the Atlantic Coastal 

Plain region : U.S . Geological Survey Bulletin 138, 232 p. 

Faust, C.R ., Mercer, J.W., and Miller, W.J ., 1981, Quantitative 

evaluation of ground-water resources in the Virginia Beach area, 

Virginia, Final report : Geotrans, Inc., 91 p. 

Fennema, R.J ., and Newton, V.P., 1982, Ground-water resources of 

the Eastern Shore of Virginia: Virginia State Water Control Board 

Planning Bulletin 332, 94 p. 

Geraghty and Miller, Consulting ground-water geologists, 1967, The 

status of ground-water resources, 1967, Nansemond County and 

Isle of Wight County, Virginia : Port Washington, N.Y ., 44 p. 

1978a, Availability of ground waterfor public supply in the City

of Virginia Beach, Virginia, Final report : Tampa, Fla., 57 p. 

--1978b, Availability of ground water in the southeastern Virginia ground-water management area, Final draft: Annapolis, 

Md., 108 p. 

1979a, Assessment of availability of brackish ground water for 

desalination in the City of Virginia Beach, Virginia: Annapolis, Md., 97 p. 1979b, Evaluation of pumping test on Yorktown aquifer, City of 

Virginia Beach, Virginia : Annapolis, Md., 53 p. 

Hack, J.T ., 1957, Submerged river system o£ Chesapeake Bay

(Maryland-Virginia): Geological Society of America Bulletin, v. 68, 

no. 7, p. 817-830. 

Hansen, H.J ., 1971, Transmissivity tracts in the Coastal Plain aquifers 

of Maryland : Southeastern Geology, v. 13, no . 3, p. 127-149. 

1972, A user's guide for the artesian aquifers of the Maryland

Coastal Plain, Part 2-Aquifer characteristics: Maryland Geological Survey, 123 p. 

1977, Geologic and hydrologic data from two core holes drilled 

through the Aquia Formation (Eocene-Paleocene) in Prince 

Georges and Queen Annes Counties, Maryland : Maryland Geolog­ ical Survey Miscellaneous Open-File Report, 77 p. 1981, Stratigraphic discussion in support of a major unconformity separating the Columbia Group from the underlying upper

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Hantush, M.S ., 1960, Modification of the theory of leaky aquifers : Journal of Geophysical Research, v. 65, no. 11, p. 3713-3725. 

Hantush, M.S., and Jacob, C.E ., 1955, Nonsteady radial flow in an 

infinite leaky aquifer: Transactions of the American Geophysical Union, v. 36, no . 1, p. 95-100 . Harsh, J.F ., 1980, Ground-water hydrology of James City County,

Virginia: U.S . Geological Survey Open-File Report 80-961, 73 p. 

Johnson, G.H ., 1972, Geology of the Yorktown, Poquoson West and 

Poquoson East Quadrangles, Virginia : Virginia Division of Mineral 

Resources Report of Investigations 30, 57 p. 

Johnston, R. H., 1977, Digital model of the unconfined aquifer in central 

and southeastern Delaware : Delaware Geological Survey Bulletin 

15, 47 p. 

Larson, J.D ., 1981, Distribution of saltwater in the Coastal Plain 

aquifers of Virginia: U.S . Geological Survey Open-File Report

81-1013, 25 p. Layne-Western Company, Inc., 1983, Virginia/North Carolina analog model study, V. 1: Kansas City, Kans ., 131 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, 59 p. 

Leggette, Brashears, and Graham, Consulting ground-water geologists, 1966, Ground-water supply potential of the West Point area, 

Virginia, Final report: NewYork, 31 p. 

Lichtler, W.F ., 1974, Report on the development of a ground-water

supply at George Washington Birthplace National Monument : 

National Park Service Administrative Report, 12 p. 

Lichtler, W.F ., and Wait, R.L ., 1974, Summary of the ground-water 

resources of the James River basin, Virginia : U.S . Geological Survey Open-File Report 74-139, 54 p. 

Lohman, S.W., 1979, Ground-water hydraulics: U.S . Geological Survey Professional Paper 708, 70 p. 

Meisler, Harold, 1980, Plan of study for the northern Atlantic Coastal 

Plain Regional Aquifer System Analysis : U.S . Geological Survey 

Water-Resources Investigations 80-16, 27 p. 

1981, Preliminary delineation of salty ground water in the 

northern Atlantic Coastal Plain: U.S . Geological Survey Open-File Report 81-71, 34 p. 
REFERENCES CITED F85 

Meng, A.A., III, and Harsh, J.F ., 1988, Hydrogeologic framework of 

the Virginia Coastal Plain: U.S . Geological Survey Professional 

Paper 1404-C, 81 p. 

National Oceanic and Atmospheric Administration, Climatological 

data, Virginia, 1940-80: National Climatic Center monthly report . 

Neuman, S.P ., and Witherspoon, P.A., 1969, Applicability of current 

theories of flow in leaky aquifers : Water Resources Research, v. 5, 

no . 4, p. 817-829. 

Newton, V.P ., and Siudyla, E.A ., 1979, Groundwater of the Northern 

Neck Peninsula, Virginia : Virginia State Water Control Board 

Planning Bulletin 307, 110 p. 

Onuschak, E., Jr ., 1972, February 1972 deep test in Accomack County, 

Virginia : Virginia Division of Mineral Resources, Virginia Minerals, v. 18, no . 1, p. 1-4. 

Rasmussen, W.C ., and Slaughter, T.H ., 1955, The ground-water 

resources, in The water resources of Somerset, Wicomico, and 

Worcester Counties : Maryland Department of Geology, Mines, 

and Water Resources Bulletin 16, 469 p. 

Richards, H.G ., 1945, Subsurface stratigraphy of the Atlantic Coastal 

Plain between NewJersey and Georgia: American Association of 

Petroleum Geologists Bulletin, v. 29, no . 7, p. 885-955. 

1948, Studies on the subsurface geology and paleontology of the 

Atlantic Coastal Plain: Academy of Natural Science, Philadelphia, 

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Sanford, Samuel, 1913, The underground water resources of the 

Coastal Plain province of Virginia : Virginia Geological Survey 

Bulletin 5, 361 p. 

Sinnott, Allen, 1967, Records of wells on the Northern Neck Peninsula, 

Virginia : U.S . Geological Survey open-file report, 152 p. 

1968, Results of aquifer tests in sands of the Potomac Group in 

the Franklin area, southeastern Virginia (1949-1950): U.S . Geological Survey Open-File Report 69-260, 80 p. 

Siudyla, E. A., Berglund, T.D ., and Newton, V.P., 1977, Ground water 

of the Middle Peninsula, Virginia: Virginia State Water Control 

Board Planning Bulletin 305, 45 p. 

Siudyla, E.A., May, A.E ., and Hawthorne, D.W., 1981, Ground water 

resources of the four cities area, Virginia : Virginia State Water 

Control Board Planning Bulletin 331, 168 p. 

Spangler, W.B ., and Peterson, J.J ., 1950, Subsurface geology of 

Atlantic Coastal Plain in New Jersey, Delaware, Maryland and 

Virginia: American Association of Petroleum Geologists Bulletin, 

v. 34, no . 1, 99 p. 

Teifke, R.H ., 1973, Geologic studies, Coastal Plain of Virginia: Virginia 

Division of Mineral Resources Bulletin 83 (Part 1), 101 p. 

Theis, C.V ., 1935, The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using 

ground-water storage: Transactions of the American Geophysical 

Union, v. 16, p. 519-524. 

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, 32 p. 

Virginia Division of Mineral Resources, 1980, Geology of the Oak Grove 

core : Publication 20, 88 p. 

Virginia State Water Control Board, 1973, Ground water of the 

York-James Peninsula, Virginia: Basic Data Bulletin 39, 129 p. 

1974, Ground water of southeastern Virginia: Planning Bulletin 

261-A, 33 p. 

Ward, L.W., and Blackwelder, B.W., 1980, Stratigraphic revision of 

upper Miocene and lower Pliocene beds of the Chesapeake Group, 

middle Atlantic Coastal Plain: U.S . Geological Survey Bulletin 

1482-D, 71 p. 
TABLES 1-13

TABLES 

Geologic age 

Period Epoch 

Holocene 

Quatern ry 

Pleistocen e 

Pliocene 

Miocene 

Tertiary Oligocene 

Eocene 

Paleocene 

Late 

Cretaceous 

Cretaceous 

Early 

Cretaceous 

F89 

TABLE 1 .-Relation of stratigraphic formations and hydrogeologic units of the Virginia Coastal Plain 

Hydrogeologic unit 

Columbia aquifer 

Yorktown confining unit 

Yorktown-Eastover 

aquifer 

St . Marys 

confining unit 

St . Marys-Choptank aquifer 

Calvert confining unit 

Chickahominy-Piney Point 

aquifer 

Nanjemoy-Marlboro Clay 

confining unit 

Aquia aquifer 

Brightseatupper Potomac 

confining unit 

Brightseatupper Potomac 

aquifer 

Middle Potomac 

confining unit 

Middle Potomac aquifer 

Lower Potomac 

confining unit 

Lower_ Potomac aquifer 

[Modified from Meng and Harsh, 1988] 

Stratigraphic 

formation 

Holocene deposits 

Undifferentiated 

deposits 

I Yorktown Formation 

Eastover Formation 

I St . Marys Formation 

Choptank Formation 

Calvert Formation 

JOld Church Formation 

Chickahominy Formation 

I Piney. Point Formation 

Nanjemoy__Formation 

Marlboro Clay 

Aquia Formation 

Brightseat Formation 

Potomac Formation 
F90 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

TABLE 2 .-Correlation of hydrogeologic units of Maryland, Virginia, and North Carolina and corresponding layers used in the 

flow model 

[AQ, aquifer; CU, confining unit] 

Maryland

hydrogeologic unit 

Virginia

hydrogeologic unit 

North Carolina 

hydrogeologic unit Flow-model layer 

Surficial aquifer Columbia aquifer Surficial aquifer AQ10 

Upper Chesapeake Yorktown confining unit Confining unit CU9 

confining unit 

Upper Chesapeake aquifer Yorktown-Eastover aquifer Yorktown aquifer AQ9 

St . Marys confining unit St . Marys confining unit Confining unit CUB 

Lower Chesapeake aquifer St . Marys-Choptank aquifer Pungo River aquifer AQ8 

Lower Chesapeake 

confining unit 

Calvert confining unit Confining unit CU7 

Piney Point-Nanjemoy

aquifer 

Chickahominy-Piney Point 

aquifer Castle Hayne aquifer AQ7 

confining 

Nanjemoy-Marlboro 

unit 

confining 

un t 

Nanjemoy-Marlboro Confining unit CU6 

Aquia-Rancocas aquifer Aquia aquifer Beaufort aquifer AQ6 

Upper Severn 

confining unit Confining unit CU5 

Severn aquifer Correlative units not Peedee aquifer AQ5 

present in Virginia

Coastal Plain 

Lower Severn 

confining unit Confining unit CU4 

Matawan aquifer Black Creek aquifer AQ4 

Matawan and Brightseat Brightseat-upper Potomac 

confining units confining unit Confining unit CU3 

Magothy and Brightseat

aquifers 

Brightseat-upper Potomac 

aquifer Upper Cape Fear aquifer AQ3 

Patapsco confining unit Middle Potomac confining unit Confining unit CU2 

Patapsco aquifer Middle Potomac aquifer Lower Cape Fear aquifer AQ2 

Potomac confining unit Lower Potomac confining unit Confining unit CU1 

Patuxent aquifer Lower Potomac aquifer Lower Cretaceous aquifer AQ1 
TABLES 

TABLE 3.-Transmissivities and storage coefficients determined for the lower and middle Potomac aquifers and the Brightseatupper Potomac aquifer 

[ft2/d, feet squared per day] 

Aquifer names Aquifer names Source of data 

used In used In Selected area and method of TransmIssi_vit_y (f_t 2_/d_) __Storage coefficient 

.ice=rlrir" ,2_~orts this report or test site analysis Low High Average Low High Average 

Franklin (F) 6 1,500 

Brightseatupper Lake Prince (LP) 6 1,500 

Potomac 

aquifer 

Upper West Point (WP) 10 13,000 5x10-4 

Artesian Burton Station 11 3,800 4,500 

and Principal (BS) 

(Sludyla and 

others, 1977, 1,2,3, and 4 19,000 55,000 1 .1x103 1 .5x10'3 

Newton and Franklin (F) 5 19,000 1 .0x)0 -4 6 .0x10 -4 

Sludyla, 1979) ; 6 12,000 

Mattaponi and 8 19,000 

Potomac 9 6,000 24,000 

(Cederstrom, 

1945, 1957) 

Middle Lake Prince (LP) 2 20,000 27,000 1 .5x10'3 

Potomac 5 19,000 1 .0x10 -4 6 .0x10-4 

aquifer 6 8,000 12,000 

7 20,000 23,000 

Washington's 

Birthplace (WB) 12 2,000 2 .0x10 -4 

West Point (WP) 10 15,000 5 .0x10-4 

Ferry Slip (FS) 11 2,600 4,200 

1,2,3, and 4 19,000 55,000 I .Ix10'3 1 .500-3 

Franklin (F) 5 19,000 I .Ox10-4 6 .0x10' 4 

6 12,000 

8 19,000 

9 6,000 24,000 

Lower 

Potomac 2 20,000 27,000 1 .5x10-3 

aquifer Lake Prince (LP) 5 19,000 1 .0x10-4 6 .0x10 -4 

6 8,000 12,000 

7 20,000 23,000 7 .8x10-4 

West Point (WP) 10 15,000 5 .0x10-4 

Explanation : 

1 . Aquifer test recovery data Sinnott (1968), Cooper and Jacob (1946) . 

2 . Aquifer test drawdown data Sinnott (1968), Cooper and Jacob (1946) . 

3 . Aquifer test recovery data Sinnott (1968), Theis (1935) . 

4 . Aquifer test drawdown data Sinnott (1968), Theis (1935) . 

5 . Cosner (1975), model calibration . 

6 . Layne-Western (1983), analog model . 

7 . Aquifer test drawdown data Geraghty and Miller (1967), Hantush (1960) . 

8 . Cosner (1975), circumference method . 

9 . Cosner (1975), potentlometric-slope method . 

10 . Aquifer test drawdown data Leggette and others (1966), Cooper and Jacob (1946) . 

11 . Aquifer test drawdown data Geraghty and Miller (1979b), Cooper and Jacob (1946) . 

12 . Aquifer test drawdown data Lichtler (1974), Cooper and Jacob (1946) . 

Letters In parentheses appear on location map of test sites, figure 3 . 

7 .800 -4 
F92 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERNATLANTIC COASTAL PLAIN 

TABLE 4.-Vertical hydraulic conductivities of confining units determined by laboratory methods 

[ft, feet ; ft/d, foot per day] 

U .S . Depth of 

Name of Geological sample below Hydraulic

City or County confining unit Survey No . land surface (ft) conductivity Cft/d) 

Suffolk Lower Potomac 1 25-3 978 .5-979 .5 1 .9x10-6 

Norfolk Middle Potomac 1 24-2 1034-1035 3 .4x10-6 

Accomac Nanjemoy- 1 55-10 949-951 1 .6x10- 5 

Marlboro 

Northumberland Nanjemoy- 1 59-12 485-486 2 .2x10-6 

Marlboro 

Gloucester Nanjemoy- 1 58H4 609 2 .0x10 -5 

Marlboro 

Isle of Wight Calvert 1 26-5 267-268 9 .2x10 -6 

Norfolk St . Marys 1 24-1 538 .5-540 2.8x10-6 

Gloucester St . Marys 2 58H4 248 2.0x10-5 

James City Middle Potomac 3 56H20 523 2,3x10-5 

Suffolk Yorktown 3 58B250 42-44 .5 3 .9x10 -3 

Suffolk Yorktown 3 58B259 60-62 5 .9x10 -4 

lAnalysis performed by Corps of Engineers, Cincinnati, Ohio . Samples remolded and tests 

conducted at a series of overburden pressures, with highest pressure equal to or greater 

than in situ pressure . 

2 Analysis performed by Core Laboratories, Inc., Dallas, Texas. 

3Analysis performed by Corps of Engineers, Cincinnati, Ohio . 
TABLES 

TABLE 5.-Major withdrawals by aquifer, 1980 

[Mgal/d, million gallons per day; do ., ditto. Locations of water users shown in fig. 81 

_ - - -1986 

Water user Geographic withdrawal 

number location Aquifer (Mgal/d) 

020 Franklin Lower Potomac 10 .29 

025 West Point do. 3 .79 

020 Franklin Middle Potomac 25 .21 

023 Williamsburg do. 1 .95 

025 West Point do. 6.57 

038 Franklin do . 1 .44 

039 Franklin do . 3.66 

045 Tidewater do . 4 .96 

048 Tidewater do . 2.29 

068 Henrico County do . 1 .96 

071 Alexandria do . 1 .12 

016 Smithfield Brightseat-upper 1 .12 

Potomac 

018 Smithfield do . 1 .38 

023 Williamsburg do . 1 .33 

025 West Point do . 2 .61 

028 Urbanna do. 1 .65 

045 Tidewater do . 2 .71 

054 Williamsburg do. 1 .70 

025 West Point Aquia .71 

434 Southern Maryland do . .39 

445 Southern Maryland do. .21 

024 James City Chickahominy-Piney Point .35 

025 West Point do . 2 .37 

309 Edenton do . .68 

006 Delmarva Peninsula Yorktown-Eastover 1 .55 

031 Delmarva Peninsula do . .78 

300 Elizabeth City do . 1 .30 
F94 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

TABLE 6.-Average estimated and model-calibrated values of lateral and vertical hydraulic conductivity for aquifers and confining units, respectively 

[In feet per day] 

Average l ateral - hydraulic 

conductivity of aquife rs 

Model Initial Modellayer Aquifer name estimated value calibrated value 

AQ1 Lower Potomac 25 .0 41 .4 

AQ2 Middle Potomac 25 .0 51 .8 

AQ3 Brightseat-upper Potomac 25 .0 32 .8 

AQ4 Aquifer 4 15 .0 25.9 

AQ5 Aquifer 5 15 .0 23.3 

AQ6 Aquia 40 .0 26 .9 

AQ7 Chickahominy-Piney Point 35 .0 25 .1 

A08 St . Marys-Choptank 10 .0 14 .7 

AQ9 Yorktown-Eastover 20.0 14 .7 

AQ10 Columbia 15 .0 18 .1 

Average vertical hydraulic 

conductivity of confiningunits 

Model Initial Modellayer Confining unit name estimated value calibrated value 

CU1 Lower Potomac 8.50x10-4 3 .28x10-5 

CU2 Middle Potomac 8.50x10 -4 4 .06x10-5 

CU3 Brightseat-upper Potomac 1 .30x10-4 4 .41x10-5 

CU4 Confining unit 4 1 .12x10 -6 3 .46x10-5 

CU5 Confining unit 5 8 .64x10' 6 7 .78x10-5 

CU6 Nanjemoy-Marlboro 8 .64x10 -5 5 .36x10-5 

CU7 Calvert 8 .64x10' 5 1 .12x10-4 

CUB St . Marys 4 .32x10-3 4 .15x10-4 

CU9 Yorktown 3 .46x10-3 8 .64x10-4 
TABLES F95 

TABLE 7 .-Minimum and maximum values of transmissivity for aquifers and vertical leakance 

values for confining units derived by model calibration 

[ft2/d, feet squared per day; 1/d, inverse day] 

Transmissivity (ft2/d) 

Model 

layer Aquifer name Minimum Maximum 

AQ1 Lower Potomac 250 12,440 

AQ2 Middle Potomac 410 18,145 

AQ3 Brightseat-upper Potomac 330 4,175 

AQ4 Aquifer 4 210 3,320 

AQ5 Aquifer 5 300 1,240 

AQ6 Aquia 100 3,830 

AQ7 Chickhominy-Piney Point 65 7,640 

AQ8 St . Marys-Choptank 210 2,600 

AQ9 Yorktown-Eastover 10 4,650 

AQ10 Columbia 15 3,000 

Vertical leakance ( 1 /d) 

Model 

layer Confining unit name Minimum Maximum 

CUi Lower Potomac 1 .01x10 -7 1 .64x10 -5 

CU2 Middle Potomac 2.54x10-7 4 .06x10 -3 

CU3 Brightseat-upper Potomac 3 .90x10-7 4 .41x10-3 

CU4 Confining unit 4 1 .30x10-7 3 .84x10-6 

CU5 Confining unit 5 4 .89x10-7 7 .78x10-6 

CU6 Nanjemoy-Marlboro 8 .25x10-8 2 .68x10-3 

CU7 Calvert 2 .67x10-7 5 .60x10-3 

CU8 St . Marys 1 .14x10-6 3 .19x10-3 

CU9 Yorktown 4 .80x10-6 1 .08x10-3 
F96 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERNATLANTIC COASTAL PLAIN 

TABLE 8.-Average withdrawal from each aquifer used in the calibrated model, by pumping period from 1891 to 1980 

[In million gallons per day] 

- Pumping Poriod 

Model 1 2 3 4 5 6 7 8 9 10 layer Aquifer 1891-1920 1921-1939 1940-1945 1946-1952 1953-1957 1958-1964 1965-1967 1968-1972 1973-1977 1978-1980 

1 Lower Potomac 0 .01 0 .29 2 .14 3 .69 6 .13 9 .19 11 .55 14 .56 14 .91 14 .22 

2 Middle Potomac 5 .34 8 .38 12.73 15 .30 20 .34 31 .06 38 .78 51 .09 54 .48 55 .91 

3 Brightseat-upper Potomac 5.46 6 .06 11 .43 11 .99 10.59 13 .14 17 .28 20 .76 19 .26 19 .42 

4 Aquifer 4 .01 .2S .26 .26 .25 .24 .22 .56 .20 .20 

5 Aquifer 5 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 

6 Aquia .06 .28 1 .39 1 .70 1 .61 1 .51 2 0S 2 .52 2 .85 2 .82 

7 Chickahominy-Piney Point .16 .90 1 .91 2 .28 3 .01 3 .52 4 .44 4 .15 3 .84 4 .19 

8 St . Marys-Choptank .00 .00 .00 .00 .00 .00 .00 .00 .02 .1 6 

9 Yorktown-Eastover .03 .32 .50 .93 1 .16 1 .54 2 .59 5 .81 8 .46 8.25 

10 Columbia .00 .00 .00 .00 .00 .01 .02 .02 .03 .05 

Totals 11 .07 16 .48 30 .36 36.15 43 .09 60 .21 76 .93 99 .47 104 .05 105 .22 
TABLES F97 

TABLE 9.-Computed lateral boundary fluxes 

[Values, in million gallons per day, are not intended to imply accuracy to the precision shown. do., ditto] 

BrlghtseatMiddle Potomac upper Potomac 

aquif er aquife Aqu i fer 4 Aquifer 5 

Flux Flux Flux Flux 

Simulated conditions Into Out of Net I n to Out of Net Into Out of Net Into Out of Net Into Out of Net 

Prepumping 0 .36 0 .04 0.32 0 .17 1 .21 -1 .14 0 .27 0.30 -0 .12 0 .05 0 .24 -0 .19 0 .00 0 .08 -0 .08 

Pumping 1891­

period 1 1920 .36 .04 .32 .17 1 .21 -1 .14 .27 .39 - .12 .05 .24 - .19 .00 .08 - .08 

1921­

do . 2 1939 .00 1 .32 -1 .32 .21 .91 - .70 .27 .38 - .11 .06 .22 - .16 .00 .08 - .08 

1940­

do . 3 1945 .00 2 .97 -2 .97 .26 .81 - .55 .28 .39 - .11 .14 .21 - .07 .00 .08 - .08 

1946­

do . 4 1952 .00 4 .82 -4 .82 .44 .55 - .11 .28 .35 - .07 .14 .20 - .06 .00 .07 - .07 

1953­

do . 5 1957 .00 3 .17 -3 .17 .44 .63 - .19 .26 .38 - .12 .14 .20 - .66 .00 .07 - .07 

1958­

do . 6 1964 .00 2 .70 -2 .70 .50 .71 - .21 .23 .44 - .21 .12 .21 - .09 .00 .07 - .07 

1965­

do. 7 1967 .00 2 .28 -2 .28 .75 .75 - .00 .22 .55 - .33 .10 .20 - .10 .00 .06 - .06 

1968­

do. 8 1972 .00 2 .83 -2 .83 1 .00 .69 .31 .22 .63 - .41 .08 .21 - .13 .00 .05 - .05 

1973­

do. 9 1977 .00 3 .85 -3 .85 1 .36 .94 .42 .20 .75 - .55 .09 .23 - .14 .00 .05 - .05 

1978­

do. 10 1980 .00 4 .27 -4 .27 1 .37 1 .17 .20 .19 .86 - .67 .09 .26 - .17 .00 .04 - .04 

Chickahomlny- St . Marys- YorktownAquia Piney Point Choptank Eastover Columbia 

aquifer aquifer aquifer aquifer Aquifer 

Flux Flux Flux Flux Flux 

Simulated conditions Into Out of Net Into Out of Net Into Out of Net Into Out of Not Into Out of Net 

Prepumping 0 .15 0 .78 -0 .63 0 .20 1 .60 -1 .40 0 .14 0 .39 -0 .25 0 .33 0 .90 -0 .67 0.08 0 .67 -0.59 

Pumping 1891­

period 1 1920 .15 .78 - .63 .20 1 .60 -1 .40 .14 .39 - .25 .33 .90 - .57 .08 .67 - .59 

1921­

do. 2 1939 .15 .76 - .61 .20 1 .57 -1 .37 .14 .38 - .24 .33 .91 - .58 .08 .67 - .59 

1940­

do. 3 1945 .15 .73 - .58 .20 1 .54 -1 .34 .15 .39 - .24 .32 .90 - .58 .08 .67 - .59 

1946­

do. 4 1952 .12 .75 - .63 .17 1 .55 -1 .38 .15 .39 - .24 .32 .90 - .58 .08 .67 - .59 

1953­

do. 5 1957 .12 .75 - .63 .16 1 .55 -1 .39 .15 .39 - .24 .32 .90 - .58 .08 .67 - .59 

1958­

do. 6 1964 .10 .80 - .70 .15 1 .60 -1 .45 .15 .38 - .23 .32 .90 - .58 .08 .67 - .59 

1965­

do. 7 1967 .07 1 .00 - .93 .13 1 .62 -1 .47 .16 .38 - .22 .32 .90 - .58 .08 .66 - .58 

1968­

do. 8 1972 .05 .90 - .85 .12 1 .60 -1 .48 .17 .37 - .20 .32 .89 - .57 .08 .66 - .58 

1973­

do. 9 1977 .05 1 .03 - .98 .11 1 .63 -1 .52 .19 .37 - .18 .40 .88 - .48 .08 .67 - .59 

1978­

do. 10 1980 .04 1 .02 - .98 .11 1 .61 -1 .50 .20 .37 - .17 .38 .88 - .50 .08 .67 - .59 
F98 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

TABLE 10.-Computed leakage rates across confining units into and out of the confined flow system

[Values, in million gallons per day, are not intended to imply accuracy to the precision shown. do., ditto] 

BrightseatLower Potomac Middle Potomac upper Potomac 

confining-unit confining-unit confining-unit Confining unit 4 Confining unit 5 

Volumetric Volumetric Volumetric Volumetric Volumetric 

Simulated Conditions 

leakage rats 

Into Out of Net 

leakage rate 

Into Out of Net Into 

leakage rate 

Out of Net 

leakage rate 

Into Out of Net 

leakage rate 

Into Out of Net 

Lower Potomac Middle Potomac Brightseat-upper Potomac Aquifer Aquifer 

aquifer aquifer aquifer 4 5 

PrepunpIng 1.96 1 .71 -0 .25 31 .38 30 .56 0.82 15 .65 19 .31 -3 .66 1.55 3.40 -1 .85 0.00 0.06 -0 .06 

Pumping 1891­

Period 1 1920 1.96 2.45 -.49 33 .48 27 .76 5.72 18 .34 15.21 3.13 1 .65 2.97 -1 .32 .00 .05 -.05 

1921­

do . 2 1939 2 .91 1.50 1 .41 35 .43 25 .12 10 .31 19 .94 13.79 6.15 1 .80 2.64 -.84 .00 .05 -.05 

1940­

do . 3 1945 5.65 .95 4.68 40 .18 22 .76 17 .42 25 .54 11 .39 14.15 2.20 1.92 .28 .00 .04 -.04 

1946­

do . 4 1952 8.77 .54 8.23 43 .58 20 .45 23 .13 27.93 10.62 17 .31 2.01 1.98 .03 .00 .04 -.04 

1953 

do . 5 1957 9.36 .23 9.13 47 .96 18 .63 29 .33 31 .28 9.77 21 .51 2.20 1.65 .55 .00 .03 -.03 

1958­

do. 6 1964 11 .81 .21 11 .60 58 .56 15 .97 42.59 41 .24 8.51 32.73 2.88 1.03 1.85 .01 .02 - .01 

1965­

do. 7 1967 13 .30 .39 12 .91 66 .39 15 .87 50.12 49.50 7.97 41 .53 3.48 .77 2 .71 .01 .02 - .01 

1968­

do. 8 1972 17 .08 .42 16 .56 80 .02 13.50 66.52 63.43 7.24 56.19 5.16 .40 4.76 .03 .00 .03 

1973 

do . 9 1977 18 .65 .25 18 .40 83 .80 11 .71 72 .09 66 .14 6.99 59.15 5.17 .42 4.75 .02 .00 .02 

1978­

do. 10 1980 18 .59 .20 18 .39 84.89 10.85 74.04 66 .89 6.46 60.43 5.33 .39 4.94 .02 .00 .02 

Nan)emoyMarlboro Ca Ivert St . Mary. Yorktown 

confining unit confining unit confining unit confining unit 

volumetric Volumetric Volumetric Volumetric Volume+ric 

leakage rate leakage rate leakage rate leakage rate leakage rate 

Simulated Conditions Into Out of Net Into Out of Net Into Out of Net Into Out of Net Into Out of Net 

Aqula ChIdkahominy-Piney Point St . Merys-Choptank Yorktown-Eastover Confined system

aquifer aquifer aqu ifer aquifer 

Prapumping 21 .30 34.52 -13.22 29 .61 35 .21 -5 .60 8.12 9.40 -1 .28 92 .23 109.71 -17.48 118.73 124.03 -5 .30 

Pumping 1891­

Period 1 1920 24.10 28 .68 -4 .58 31 .90 31 .26 .62 8.37 8.93 -.56 96 .66 103.68 -7 .02 123.30 117.68 5.62 

1921­

do . 2 1939 26.00 26 .15 .15 33 .47 29 .36 4 .11 8.66 8.65 .01 99 .12 100.55 -1 .43 126.37 113.80 12 .57 

1940 

do . 3 1945 32 .06 21 .88 10 .17 37 .55 25 .28 12 .27 8.92 8.14 .78 103.56 92 .93 10 .63 132.08 104.85 27 .23 

1946 

do . 4 1952 34 .80 19 .74 15 .06 39 .23 23 .62 15 .61 9.06 7.88 1 .17 107.31 90 .71 16 .61 136.66 101 .38 35 .28 

1953­

do . 5- 1957 37 .37 18 .74 18 .63 41 .06 22 .37 18 .69 9.22 7.62 1 .60 108.64 88 .14 20 .50 138.36 98 .77 39 .61 

1958­

do . 6 1964 45 .43 15 .85 29 .58 45 .75 19 .49 26 .26 9.83 6.84 2.99 117.65 82 .46 36 .19 149.45 91 .10 58 .35 

1965­

do . 7 1967 50 .60 17 .08 33 .53 52 .97 14 .22 38 .75 10 .27 6.45 3.82 121 .74 77 .06 44 .68 154.74 84 .83 69 .91 

1968­

do. 8 1972 66 .44 11 .45 54 .99 58 .99 14 .57 44 .42 11 .22 5.91 5 .31 136.96 69 .54 37 .42 171 .50 76 .24 95 .26 

1973 

do . 9 1977 69 .90 10 .68 59 .22 60 .81 13 .63 47 .18 11 .61 5.80 5.81 140.22 68 .33 71 .89 175.48 74 .48 101 .00 

1978­

do. 10 1980 70 .31 10 .38 59 .93 61 .15 13 .42 47 .73 11 .83 5.69 6.14 140.77 67 .15 73 .62 177.02 72.74 104.28 
TABLE 11 .-Model-computed ground-water budgets 

[Values, in million gallons per day, are not intended to imply accuracy to the precision shown] 

,%oM_-ca+PD)EV WLUft,x)U_FiUW HATE5 

Sources FOR PREPUPING SIMULATION 

Recharge from precipitation 9,237.81 

Lateral boundary Inflow 1 .76 

Ground-racer flaw from sirens end coastal *star bodies .00 

Discharges 

Lateral boundary outflow 6.31 

Ground-voter flow Into strews and coast.sl-water bodies 9,233.93 

FOOEL-CCMPUTED VOL(METRIC FLOW RATES 

FOR PUMPING SIMULATION 

1 

(10957 .2 days) 

1891-1920 

2 

(6939.8 days) 

1921-1939 

3 

(2191 .0 days) 

1940-1945 

4 

(2556.7 days) 

1946-1952 

5 

(1826.4 days) 

1953-1957 

6 

(2556.7 days) 

1958-1964 

7 

(1095.1 days) 

1965-1967 

B 

(1826.4 days) 

1968-1972 -

9 

(1826.4 days) 

_ 1973-1971 

10 

(1095.7 days) 

1978-1980 

Sources 

Water released from aquifer storage 0.00 0.09 2.92 1 .39 1.82 2.62 7.26 6.57 2.80 1.60 

Lateral boundary Inflow 1.76 1.45 1 .58 1 .71 1.61 1 .67 1.85 2.05 2.49 2.46 

Recharge from precipitation 9,237.81 9,237.81 9,237.81 9,237.81 9,237.81 9,237.81 9,237.81 9,237.81 9,237.91 9,231.91 

Ground-water flow from streass 

and coastal water bodies .00 .00 .00 .00 .00 .00 .07 .26 .48 .53 

0(scharges 

Water entering aquifer storage .00 .00 .02 .00 .03 .00 .00 .00 .10 .11 

Lateral boundary outflow 6.30 7.22 8.69 10.23 8 .12 6.48 8.45 8.86 10 .41 11 .50 

Ground-water withdrawal from wells 10.88 15 .49 29 .94 35 .05 42.52 60.23 76 .75 99.42 104.02 105.13 

Ground-water flow Into strews 

and coastal voter bodies 9,224.24 9,217.13 9,204.65 9,197.09 9,191.28 9,174.47 9,162.84 9,139.51 9,129.88 9,127.29 

Note: The difference between tote( sources and discharges Is due to wow leaf truncation errors In the digital simulation . 
F100 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN ATLANTIC COASTAL PLAIN 

TABLE 12.-Summary ofsensitivity tests 

[ft/d, foot per day; ft, foot] 

Change in water eves n feet 

ydrographs o se ected 

Range in deviation confined aquifers shown in hydrographs of in figures 76-79 ; range

middle Potomac aquifer in deviation from Hydraulic Actual from calibrated hydro- calibrated hydrographs characteristic Range of change value graphs in 1980 in 1980 

Transmissivity of 

middle Potomac Increase 100% variable +20 to +75 Not applicable aquifer Decrease 50% -15 to -125 

Vertical hydraulic 

conductivity of 

middle Potomac Increase 100% 8.12x10-5 ft/d +10 to +30 Not applicable confining unit Decrease 50% 2.03x10- 5 ft/d -30 to -60 

Storage coefficient Increase 1 order 

of all confined of magnitude 1 .0x10 -3 +5 to +15 

aquifers Decrease 1 order Not applicable 

of magnitude 1 .0x10 - 5 Less than 5 

Specific storage 

coefficient of all 1 .0x10 -4 /ft Not applicable +15 to +40 

confining units 1 .0x10 -6/ft Less than 5 1 

1 Hydrographs for calibrated model which neglected water released from storage in the confining units during transient simulations (specific storage - 0/ft) and the assumed specific storage of the confining unit (1 .0x10-6 /ft) are shown as same line in figures 78 and 79 . 

TABLE 13.-Specific storage and computed storage coefficients of confining units used for 

sensitivity tests 

Estimate average Computed storage coefficient dimension mss 

layer Counit in9 thickness (feet) p I .Ox10 -4tfta l e p ICOx10 - t ft- I e 

CUI Lower Potomac 25 2 .50x10 - 3 2 .50x10 -5 

CU2 Middle Potomac 40 4 .00x10-3 4.00x10 -5 

CU3 Brightseat- 35 3 .50x10-3 3.50x10-5 

upper Potomac 

CU4 Confining 25 2 .50x10 -3 2 .50x10 - 5 unit 4 

CU5 Confining 25 2 .50x10 -3 2 .50x10 -5 unit 5 

CU6 Nanjemoy- 100 1 .00x10 - 2 1 .00x10 -4 Marlboro 

CU7 Calvert 125 1 .25x10- 2 1 .25x10- 4 

CU8 St . Marys 90 9 .00x10 - 3 9 .00x10 -5 

CU9 Yorktown 50 5 .00x10 -3 5 .00x10 - 5