Abstract ........................................................ 1
Introduction .................................................... 2
Purpose and Scope ............................................ 4
Description of the Study Area ................................ 4
Geologic Setting .......................................... 5
Groundwater Conditions .................................... 5
Methods of Investigation ..................................... 7
Previous Investigations ...................................... 8
Groundwater-Quality Data ........................................ 9
Data Quality ................................................. 9
Data Distribution ........................................... 10
Data Use and Limitations .................................... 13
Regional Trends in Groundwater Quality ......................... 13
Major Ions .................................................. 14
Spatial Distribution ..................................... 14
Hydrochemical Processes .................................. 15
Saltwater-Transition Zone ................................... 17
Configuration ............................................ 18
Chloride-Concentration Sections ....................... 18
Chloride Iso-Concentration Surfaces ................... 19
Aquifer Chloride Concentrations ....................... 25
Relations to Groundwater Flow ............................ 31
Emplacement of Seawater ............................... 32
Freshwater Flushing ................................... 32
Brine and the Inner Chesapeake Bay Impact Crater ...... 33
Movement ................................................. 34
Rates of Change in Chloride Concentration ............. 36
Patterns of Change in Chloride Concentration .......... 42
Intrusion and Upconing ................................ 45
Monitoring ............................................ 46
Selected Constituents ....................................... 48
Fluoride ................................................. 48
Iron and Manganese ....................................... 51
Nitrate and Ammonium ..................................... 54
Considerations for Groundwater Use .......................... 58
Hardness ................................................. 58
Chloride ................................................. 59
Selected Constituents .................................... 60
Summary and Conclusions ........................................ 61
Acknowledgments ................................................ 62
References Cited ............................................... 62
Attachment 1. - Chemical-constituent values and
related data ......................... CD in pocket
Attachment 2. - Summary of groundwater-quality constituent
values ......................................... 67
Plates
(In envelope)
1-3. Maps showing locations of sampled groundwater wells
and boreholes in:
1. The northern part of the Virginia Coastal Plain
2. The southern part of the Virginia Coastal Plain
3. The Virginia Eastern Shore
4-9. Maps showing the major ion composition of groundwater in:
4. The surficial aquifer
5. The Yorktown-Eastover aquifer
6. The Piney Point aquifer
7. The Aquia aquifer
8. The Potomac aquifer
9. Basement bedrock
10-14.Dissolved chloride-concentration sections:
10. DD-DD'and ED-ED'
11. FD-FD'andGD-GD'
12. ID-ID'and JD-JD'
13. DS-DS'and ES-ES'
14. FS-FS' and GS-GS'
Figures
1. Map showing groundwater-sample locations and chloride-
concentration sections in the Virginia Coatal Plain ....... 3
2. Generalized hydrogeologic section and directions of
groundwater flow .......................................... 6
3. Diagram of stratigraphic relations among hydrogeologic
units ..................................................... 7
4-8. Maps showing approximate altitude and configuration of:
4. The 50 milligram per liter chloride iso-concentration
surface .................................................. 20
5. The 250 milligram per liter chloride iso-concentration
surface .................................................. 21
6. The 1,000 milligram per liter chloride iso-
concentration surface .................................... 22
7. The 10,000 milligram per liter chloride iso-
concentration surface .................................... 23
8. The 19,000 milligram per liter chloride iso-
concentration surface .................................... 24
9-13. Maps showing groundwater chloride concentrations in:
9. The Yorktown-Eastover aquifer ............................ 26
10. The Piney Point aquifer .................................. 27
11. The Aquia aquifer ........................................ 28
12. The Virginia Beach aquifer ............................... 29
13. The Potomac aquifer ...................................... 30
14. Graphs showing rates of change in concentrations of
chloride in groundwater and yearly averaged rates of
groundwater withdrawal ................................... 35
15-19.Maps showing rates of change in concentrations of
chloride in groundwater in:
15. The surficial aquifer .................................... 37
16. The Yorktown-Eastover aquifer ............................ 38
17. The Piney Point aquifer .................................. 39
18. The Aquia aquifer ........................................ 40
19. The Potomac aquifer ...................................... 41
20-21.Graphs showing concentrations of chloride in
groundwater and yearly averaged rates of groundwater
withdrawal in:
20. Well 60C 61 at Portsmouth ................................ 43
21. Selected wells on the central York-James Peninsula ....... 44
22-26.Maps and generalized hydrogeologic sections showing
distributions in groundwater of:
22. Fluoride ................................................. 49
23. Iron ..................................................... 52
24. Manganese ................................................ 53
25. Nitrate nitrogen ......................................... 56
26. Ammonium nitrogen ........................................ 57
Tables
1. Numbers of sampled wells, water-quality samples,
and chemical-constituent values .......................... 11
2. Summary of sequentially collected groundwater samples .... 36
3. Municipalities operating brackish groundwater-
development facilities ................................... 59
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