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U.S. GEOLOGICAL SURVEY

Water-Resources Investigative Report 01-4029


Hydrogeology and Ground-Water Quality of the Piney Point-Nanjemoy and Aquia Aquifers, Naval Air Station Patuxent River and Webster Outlying Field, St. Marys County, Maryland

By Cheryl A. Klohe and C. Erin Feehley

 

Abstract

Rapid population growth in southern Maryland has brought ground-water quality and groundwater quantity issues to the forefront. Historical and current water-quality data, ground-water withdrawal data, and water-level data were compiled to help water managers at the Naval Air Station Patuxent River understand how water-management practices affect regional ground-water resources, in particular the Piney Point-Nanjemoy and the Aquia aquifers.

Lithologic and borehole geophysical data were interpreted to create a local hydrogeologic framework of the Piney Point-Nanjemoy and Aquia aquifers. Ground-water withdrawal data and personnel data were compared to ground-water levels in both aquifers. Water samples were collected and analyzed to determine basic ground-water quality of both aquifers, and to compare these data with data collected from the 1940s through the early 1960s. The data were analyzed to determine changes in water quality over the past 50 years, and to investigate indications of saltwater intrusion.

The Piney Point-Nanjemoy aquifer is 50 to 70 feet thick, with a top altitude of 220 to 255 feet below sea level and a hydraulic conductivity of 2 feet per day at Naval Air Station Patuxent River. This aquifer is 65 feet thick, with a top altitude of 260 feet below sea level and a hydraulic conductivity of 2 feet per day at Webster Outlying Field. Ground-water withdrawal from the Piney Point-Nanjemoy aquifer has decreased and water levels have remained constant since the 1950s. The water is calcium-magnesium-sodium-bicarbonate rich, and shows no evidence of saltwater intrusion.

The Aquia aquifer is 100 to 145 feet thick, with a top altitude of 450 feet below sea level and a hydraulic conductivity of 6 to 10 feet per day at Naval Air Station Patuxent River. This aquifer is 50 feet thick, with a top altitude of 470 feet below sea level and a hydraulic conductivity of 6 to 10 feet per day at Webster Outlying Field. Although ground-water withdrawal in the Aquia aquifer has decreased, water levels continue to decline at Naval Air Station Patuxent River and Webster Outlying Field due to increased municipal withdrawal near the air station. The water is sodiumbicarbonate rich at Naval Air Station Patuxent River and at Webster Outlying Field; at Webster Outlying Field, however, the specific conductance, acid-neutralizing capacity, and concentrations of sodium, sulfate, boron, and iodide are noticeably different from the water at the main facilities. There is no evidence of saltwater intrusion at either site, although the specific conductance and the concentrations of some constituents are higher at Webster Outlying Field, which could indicate early stages of saltwater intrusion.

Contents

Abstract

Introduction

Background

Purpose and scope

Study area

Previous investigations

Acknowledgments

Hydrogeologic framework

Surficial aquifer and the upper confining unit

Piney Point-Nanjemoy aquifer

Middle confining unit

Aquia aquifer

Ground-water withdrawal and water levels

Piney Point-Nanjemoy aquifer

Aquia aquifer

Ground-water quality

Water quality of the Piney Point-Nanjemoy aquifer

Water quality of the Aquia aquifer at Naval Air Station Patuxent River

Water quality of the Aquia aquifer at Webster Outlying Field

Comparison of water types in the Piney Point-Nanjemoy and Aquia aquifers

Saltwater intrusion

Summary and conclusions

References cited

Appendixes

A. Selected geophysical logs with generalized hydrologic units for:

A1. Well SM Df 93

A2. Well SM Df 96

A3. Well SM Dg 9

A4. Well SM Ff 21

A5. Well SM Ff 35

A6. Well SM Ff 64

 

B. Laboratory-analyzed constituents and methods

 

C. Microorganisms and analytical methods for sample analyses from wells SM Df 61 and SM Df 98

Figures

1-5. Maps showing:

1. Location of Naval Air Station Patuxent River and Webster Outlying Field study areas, and outcrop areas of the Piney Point-Nanjemoy and Aquia aquifers, southern Maryland

 

2. Well locations at Naval Air Station Patuxent River and Webster Outlying Field

 

3. Altitude of the top of the Piney Point-Nanjemoy aquifer, Naval Air Station Patuxent River, Maryland

 

4. Thickness of the Piney Point-Nanjemoy aquifer

 

5. Location of hydrogeologic sections A-A'and B-B'

 

6. Hydrogeologic sections A-A' and B-B'

 

7-8. Maps showing:

7. Altitude of the top of the Aquia aquifer

 

8. Thickness of the Aquia aquifer at selected locations

 

9. Plot showing ground-water withdrawal from the Piney Point-Nanjemoy and Aquia aquifers at Naval Air Station Patuxent River, ground-water withdrawal from the Aquia aquifer at Webster Outlying Field, and personnel at Naval Air Station Patuxent River, 194699

 

10. Map showing altitude of the potentiometric levels of the Piney Point-Nanjemoy aquifer on April 20, 2000

 

11. Hydrographs for selected wells screened in the Piney Point-Nanjemoy aquifer, 19962000

 

12. Map showing altitude of the potentiometric surface of the Aquia aquifer on January 12, 2000

 

13. Hydrographs for selected wells screened in the Aquia aquifer at Naval Air Station Patuxent River, and Webster Outlying Field, 19502000

 

14. Graph showing average daily ground-water withdrawal from the Aquia aquifer for large users in parts of Calvert and St. Marys Counties, Maryland, 198099

 

15. Piper diagram showing the chemistry of water from selected wells at Naval Air Station Patuxent River, and Webster Outlying Field

Tables

1. Records for selected wells at Naval Air Station Patuxent River and Webster Outlying Field, Maryland

 

2. Hydrogeologic units and corresponding geologic units at Naval Air Station Patuxent River

 

3. Ground-water withdrawal from the Piney Point-Nanjemoy and Aquia aquifers at Naval Air Station Patuxent River and the Aquia aquifer at Webster Outlying Field, 19462000

 

4. Concentrations of the major inorganic constituents, selected minor constituents, and radionuclides of ground water from selected wells in the Piney Point-Nanjemoy and Aquia aquifers at Naval Air Station Patuxent River and Webster Outlying Field, 1999

 

5. Historical comparison of major inorganic constituents, and selected minor constituents in ground water from selected wells in the Piney Point-Nanjemoy and Aquia aquifers, Naval Air Station Patuxent River, 194362

 

6. Select national primary drinking-water regulations: contaminants and their respective maximum contaminant level goals (MCLGs), and maximum contaminant levels (MCLs) as regulated by the U.S. Environmental Protection Agency

 

7. Select secondary drinking-water regulations

 

8. Comparison of average sea water and average connate water to ground-water samples collected in 1999 from the Piney Point-Nanjemoy aquifer, the Aquia aquifer at Naval Air Station Patuxent River, and the Aquia aquifer at Webster Outlying Field


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MD-DE-DC Water Science Center
U.S. Geological Survey
8987 Yellow Brick Road
Baltimore, MD 21237

 

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Fax: (410) 238-4210

 

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