iii

CONTENTS

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

Introduction ............................................................................................................................................................................2

Purpose and scope .........................................................................................................................................................2

Previous investigations ..................................................................................................................................................6

Acknowledgments .........................................................................................................................................................6

Geologic setting......................................................................................................................................................................6

Sykesville Formation.....................................................................................................................................................6

Biotite-hornblende tonalite member of the Georgetown Intrusive Suite.......................................................................7

Structural controls on ground-water flow....................................................................................................................12

Hydrogeology.......................................................................................................................................................................12

Saprolite.......................................................................................................................................................................12

Bedock aquifer.............................................................................................................................................................14

Bedrock boreholes ..............................................................................................................................................17

Borehole geophysics ...........................................................................................................................................18

Hydraulic heads and hydraulic gradients............................................................................................................18

Hydraulic properties of the bedrock aquifer .......................................................................................................24

Single-hole hydraulic tests .........................................................................................................................25

Multiple-hole hydraulic test .......................................................................................................................25

Hydrogeologic controls on ground-water discharge to the subway tunnel near the Medical Center Station

and Crossover................................................................................................................................................................30

Summary and conclusions....................................................................................................................................................31

References cited ...................................................................................................................................................................32

Figures

 1a–b. Maps showing:

1a. Location of the Washington METRO Red Line and Medical Center Station and Crossover,

Montgomery County, Maryland.............................................................................................................3

1b. Location of the Medical Center Station and Crossover, the National Institutes of Health,

and the National Naval Medical Center, Montgomery County, Maryland............................................4

2. Photographs showing (A) the interior of the Medical Center Station and Crossover, and (B) close-up

along a tunnel wall of the “aluminum tents” installed to divert water leaking from the ceiling ........................5

3. Map showing regional geology near the Medical Center Station and Crossover, Montgomery 

County, Maryland, and locations of geologic cross sections A–A′ , B–B ′ , and C–C ′ .....................................8

4. Geologic cross sections A–A′ , B–B ′ , and C–C ′ .................................................................................................9

5. Photographs showing fracture patterns in outcrops of the Sykesville Formation ......................................10

6. Photographs showing (A) fracture patterns in outcrops of the biotite-hornblende tonalite (Ogh)

member of the Georgetown Intrusive Suite, and (B) seepage of ground water along a fracture .....................11

7. Geologic map and orientation of foliation and joints in the Sykesville Formation and the

biotite-hornblende tonalite (Ogh) member of the Georgetown Intrusive Suite................................................13

8. Conceptual hydrologic model for the Piedmont Region showing a water table in the saprolite

and relative hydraulic heads in bedrock and saprolite wells ............................................................................14

9. Map showing location of National Naval Medical Center well field, seismic-refraction lines, 

and 2-D resistivity lines ....................................................................................................................................14
iv

Figures—Continued

10. Map showing location of regional wells, National Oceanic and Atmospheric Administration (NOAA)

precipitation station 2325, and pumping station A6.......................................................................................16

11. Hydrograph for saprolite observation well MO Eg 28 and monthly rainfall hyetograph for

National Oceanic and Atmospheric Administration (NOAA) precipitation station 2325 at

Dalecarlia Reservoir in Washington, D.C. .....................................................................................................17

12. Best-fit type-curve match to the drawdown data from the air-pressurized slug test conducted

 in observation well MO Ff 28, using an applied air pressure of 3.8 pounds per square inch .......................17

13. Hydrograph for bedrock observation well MO Eg 27 and monthly rainfall hyetograph for

National Oceanic and Atmospheric Administration (NOAA) precipitation station 2325 at

Dalecarlia Reservoir in Washington, D.C. .....................................................................................................18

 14–17. Selected geophysical and driller’s logs for:

14. Well MO Ff 27 ..........................................................................................................................................19

15. Well MO Ff 29 ..........................................................................................................................................19

16. Well MO Ff 30 ..........................................................................................................................................20

17. Well MO Ff 31 ..........................................................................................................................................20

18. Orientation of mapped foliation and joints in the Sykesville Formation and biotite-hornblende

tonalite (Ogh) member of the Georgetown Intrusive Suite, and fracture orientations in wells at the

National Naval Medical Center (NNMC) well field ......................................................................................21

19. Histogram and statistics (mean and standard error) of the dip of the fractures mapped by the

acoustic televiewer in wells at the National Naval Medical Center (NNMC) well field...............................21

20. Map showing regional hydraulic heads in the bedrock (measured November 28, 2000) and 

general direction of ground-water flow..........................................................................................................22

21. Hydraulic head determined from packer tests in National Naval Medical Center (NNMC)

wells MO Ff 27, MO Ff 29, MO Ff 30, and MO Ff 31..................................................................................23

22. Hydrograph for saprolite observation well MO Ff 28 and open bedrock observation

wells MO Ff 22, MO Ff 27, MO Ff 29, MO Ff 30, and MO Ff 31 at the National Naval

Medical Center (NNMC) well site .................................................................................................................24

23. Transmissivity determined from fluid-injection tests in National Naval Medical Center

(NNMC) wells MO Ff 27, MO Ff 29, MO Ff 30, and MO Ff 31 ..................................................................26

24. Graph showing pumping rate measured at well MO Ff 30 during the hydraulic test started on

December 10, 2001, conducted at the National Naval Medical Center (NNMC) well site ...........................27

25. Graph showing measured drawdown in saprolite well MO Ff 28 and bedrock observation wells

MO Ff 29, MO Ff 30, MO Ff 31 during the hydraulic test started on December 10, 2001,

conducted at the National Naval Medical Center (NNMC) well site.............................................................28

26. Graph showing time divided by radius squared plot for the hydraulic test started on December 10, 2001,

conducted at the National Naval Medical Center (NNMC) well site.............................................................29

27. Hydrograph showing comparison of hydraulic heads in observation well MO Eg 27 and 

hourly pumpage at pumping station A6 .........................................................................................................31

Tables

1. Generalized geologic column for parts of Washington West and Kensington Quadrangles, 

Washington, D.C., Maryland, and Virginia.........................................................................................................7

2. Data for wells used for borehole geophysics, water-level measurements, and aquifer testing.............................15
v

Conversion Factors and Vertical Datum

_____________________________________________________________________________________________________________________________________

Multiply By To obtain

_____________________________________________________________________________________________________________________________________

acre 4,047 square meter (m 2)

acre-foot (acre-ft) 1,233 cubic meter (m3)

cubic foot per second (ft 3/s) 0.02832 cubic meter per second (m3/s)

foot (ft) 0.3048 meter (m)

1 foot squared per day (ft 2/d) 0.0929 meter squared per day (m2/d)

gallon (gal) 3.785 liter (L)

gallon per minute (gal/min) 0.06308 liter per second (L/s)

mile (mi) 1.609 kilometer (km)

square mile (mi 2 ) 2.590 square kilometer (km2 )

_____________________________________________________________________________________________________________________________________

1 The standard unit for transmissivity is cubic foot per day per square foot [(ft3/d)/ft 2

] times the aquifer thickness. In this 

report, the mathematically reduced form, foot squared per day (ft2/d), is used for convenience.

Vertical Datum: In this report, “sea level ” refers to the National Geodetic Vertical Datum of 1988—a geodetic datum 

derived from a 1988 update of the reference geoid and first-order level nets of the United States and Canada.

Altitude refers to the height above mean sea level and Elevation refers to the height above a defined reference datum. The 

reference datum used throughout this report is mean sea level, so in this instance, the terms are interchangeable.
vi
Abstract 1

Hydrogeologic Controls on Ground-Water Discharge to the

Washington METRO Subway Tunnel Near the Medical Center

Station and Crossover, Montgomery County, Maryland

By Earl A. Greene, Allen M. Shapiro, and Andrew E. LaMotte

Abstract

Excessive water intrusion has been observed 

inside several of the Washington Metropolitan 

Area Transit Authority subway tunnels, with the 

worst leakage occurring along the Red Line tunnels and stations north of Dupont Circle in 

Washington, D.C. These tunnels were constructed 

in bedrock that contains permeable (water-bearing) joints and fractures. Excessive water leakage 

through the walls and water inside the underground facilities has damaged mechanical and 

electrical components in the tunnel, and has escalated the deterioration rate of the rail system. The 

U.S. Geological Survey and the Washington 

Metropolitan Area Transit Authority have worked 

cooperatively on a study from 2000–03 to describe 

and quantify the factors controlling ground-water 

flow into the Red Line subway tunnel near the 

Medical Center Station and Crossover in Montgomery County, Maryland.

The Red Line near the Medical Center Station 

and Crossover passes through or beneath the 

gneissic Sykesville Formation and the biotitehornblende tonalite member of the Georgetown 

Intrusive Suite, both of which contain numerous 

fractures. The mapped foliation and joints of the 

Sykesville Formation in the vicinity of the Medical Center Station and Crossover are generally orientated north-south. Fractures in the Sykesville 

Formation in outcrops appear to be poorly connected. In the biotite-hornblende tonalite member 

of the Georgetown Intrusive Suite, the general orientation of the mapped foliation and joints is eastwest. In contrast to the fractures in the Sykesville 

Formation, the fractures in the Georgetown 

Intrusive Suite in outcrops appear to be more 

numerous and have a greater degree of connectivity. Fractures intersecting four bedrock wells near 

the Medical Center Station and Crossover that 

were drilled into the biotite-hornblende tonalite 

member of the Georgetown Intrusive Suite show 

an east-west orientation matching the foliation and 

joints shown on geologic maps. The excessive 

water intrusion at the Medical Center Station and 

Crossover could be the result of its location within 

the Georgetown Intrusive Suite. The abrupt 

changes in the mapped directions of ground-water 

flow based on the hydraulic heads at the contact 

between the Sykesville Formation and biotitehornblende tonalite member of the Georgetown 

Intrusive Suite could also be the result of the 

change in fracturing between these two lithologies.

Saprolite, a residual of soft, red/brown to gray 

clay from decomposed crystalline rock, overlies 

the bedrock and varies from about 20 to 55 feet 

thick, depending on location. On the basis of a 

slug test conducted in the lower part of the saprolite near the Medical Center Station and Crossover, transmissivity and storativity of the saprolite 

were estimated to be 10 feet squared per day and 

10-6 , respectively.

The transmissivity of fractures intersecting 

bedrock boreholes drilled in the biotite-hornblende 

tonalite member of the Georgetown Intrusive Suite 

varies over five orders of magnitude, from a maximum of approximately 10 feet squared per day to 

the detection limit of the in situ testing apparatus, 

which is approximately 10-4 feet squared per day. 

In general, the transmissivity of fractures intersecting the boreholes increases with depth. The 

low transmissivity of bedrock fractures in close 

proximity to the saprolite is likely to be caused by 

the fractures being filled with byproducts of rock 

weathering, resulting in reduced permeability. 
2 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel

The bulk transmissivity of the bedrock aquifer is 

approximately 3.7 feet squared per day, as determined from an aquifer test conducted by pumping 

a 240-foot-deep borehole and monitoring the 

drawdown over 3 days in the pumped borehole 

and several observation boreholes.

In general, the hydraulic head decreases with 

depth in bedrock boreholes, indicating the potential for downward ground-water flow. Based on 

hydraulic head values measured in the boreholes 

and this potential for downward ground-water 

flow, the saprolite may be recharging the bedrock 

aquifer.

Introduction

Between 1966 and 2001, the Washington Metropolitan 

Area Transit Authority (WMATA) designed and built a 

103-mile rail system that services Washington, D.C. and 

parts of Maryland and Virginia; approximately one-half of 

the rail system is underground. Excessive water intrusion 

has been observed inside several of the tunnels with the 

worst leakage occurring along the Red Line (figs. 1a–1b). 

The Red Line tunnels, stations, and crossovers north of 

Dupont Circle were constructed in bedrock that contains

permeable (water-bearing) joints and fractures. 

The Red Line tunnels were constructed prior to the early 

1980s, when no proven water-sealing technologies were 

available to effectively mitigate water intrusion by sealing 

the tunnel walls. In addition, tunnel-boring machines are 

incapable of constructing the shape of a crossover and 

station. A crossover is a cavernous structure, approximately 

40 ft (feet) high and 100 ft wide, utilized for the “crossing 

over” of trains from track to track. Drilling and blasting 

techniques were utilized in the construction of the station 

areas and crossovers in the fractured bedrock along the

Red Line, and the walls were supported with steel ribs and 

pneumatically applied concrete (shotcrete).

Tunnels are typically designed to permit a limited 

amount of leakage through the lining. This water is usually 

collected in a drain system and then pumped back to the surface for disposal. WMATA design specifications to prevent 

water intrusion into the Red Line were based on a Hydrostatic Pressure Relief System (HPR). This system would 

convey water from outside of the tunnel lining to the track 

drains. The tunnel lining was allowed to conduct a small 

amount of leakage—about 0.1 gal/min (gallons per minute) 

per 100 ft of tunnel. Mineral deposits clogged many of the 

relief ports soon after construction, and about 1 out of every 

10 ports are in good working order (Capital Transit Consultants, 2001).

Shortly after construction, noticeable amounts of water 

began to leak from cracks, voids, and construction joints in 

the tunnel linings. Excessive water leakage through the 

walls and the presence of water inside the underground facilities has damaged mechanical and electrical components in 

the tunnel, and has escalated the deterioration rate of the rail 

system. This leakage will eventually reduce the life span of 

the structure and rail system. Current water mitigation practices include the application of sealants to cracks that are 

leaking or diverting flow from the ceiling to the drain system 

with small “aluminum tents” (fig. 2). 

Water intrusion inside the crossovers along the Red Line 

is more severe than in other parts of the subway system. 

These crossovers are supported by steel ribs with a thin layer 

of shotcrete and do not contain a waterproofing membrane. 

In general, the structural performance of the crossovers has 

been satisfactory up to now, but due to excessive water leaks, 

the life expectancy of the electrical and mechanical infrastructure and structural members in the proximity of the 

crossovers may be less than in other locations (Capital Transit Consultants, 2001). In order for the engineers and managers of WMATA to develop a strategy to prevent or reduce 

water intrusion inside the underground facilities, an understanding of the hydrogeologic controls on water intrusion is 

needed. The U.S. Geological Survey (USGS) and WMATA 

worked cooperatively on a hydrogeologic investigation to 

characterize ground-water flow into the tunnel system from 

2000–03. The results of this study will provide a methodology for investigating similar water intrusion problems at 

other sites constructed in fractured bedrock along the subway system, and provide information useful in the design of 

methods to divert or reduce the volume of water entering the 

subway tunnels.

Purpose and Scope

This report describes an investigation to determine the 

hydrogeologic factors controlling ground-water inflow into 

the Medical Center Station and Crossover on the Red Line. 

This part of the Red Line was chosen for investigation by 

WMATA and the USGS because of the severity of the water 

intrusion at this location. The Medical Center Station and 

Crossover is located in Bethesda, Maryland, adjacent to the 

National Institutes of Health (NIH) and the National Naval 

Medical Center (NNMC) (fig. 1b). The Medical Center 

Station and Crossover is approximately 48 ft high and was 

constructed approximately 140 ft below land surface (190 ft 

above mean sea level).

Several methods of investigation were used to characterize the hydrogeology, and investigate ground-water discharge from bedrock to the Medical Center Station and 

Crossover. Previously published geologic maps, surface 

geophysics, and data from existing wells in the area were 

used to evaluate the extent of the saprolite and bedrock. 

Water levels in the bedrock were mapped and correlated to 

structural features. In addition, temporal water-level fluctuations were analyzed at observation wells completed in the 

saprolite and bedrock.

Four bedrock and one saprolite test wells were drilled as 

part of the study and were used to investigate and character-
Introduction 3
4 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel
Introduction 5
6 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel

ize the physical properties of the bedrock. Analysis of borehole geophysical logs provided information on the location 

and orientation of the fractures.

Aquifer tests consisting of a slug test in the saprolite and 

single and multiple well tests in the bedrock were used to 

determine the transmissivity and storativity of the formations. An air-pressurized slug test conducted in the saprolite 

gave information on the hydraulic properties of this formation. Single well tests (packer tests) conducted in the bedrock provided hydraulic information on fractures within an 

isolated portion of the borehole. A multiple-hole hydraulic 

test was conducted over a longer period of time to infer the 

bulk transmissivity of the bedrock.

Inflow of ground water into the tunnel near the Medical 

Center Station and Crossover was measured by recording 

discharge at the A6 pumping station. Discharge was then 

correlated with observed hydraulic head data measured in 

the bedrock.

Previous Investigations

WMATA personnel and various engineering and consulting firms have conducted several investigations to characterize water intrusion inside the tunnels since they were 

constructed. The first major investigation was conducted by 

Mueser Rutledge Consulting Engineers (1988), who looked 

at the reasons for the leakage. They concluded that the 

intensity of leakage in the tunnel was a result of various factors including the calcification of the HPR system, the permeability of the surrounding ground, ground-water pressure 

conditions, acid water from soils, and cracks in the cast-inplace concrete.

Additional investigations to characterize water intrusion 

into the Red Line, specifically at the Medical Center Station 

and Crossover, have been conducted by Capital Transit 

Consultants, Inc. (2001) and DMJM/Deleuw, Cather & EBA 

Engineering, Inc. (2001). These firms concluded that the 

blasting utilized to construct the crossover and station areas 

may have intensified fracturing of the bedrock near the tunnel. Other factors they identified that could be affecting 

water intrusion may be shrinkage cracks along the tunnel and 

construction joints in the concrete.

Acknowledgments

The authors would like to thank the employees of 

Mueser Rutledge Consulting Engineers, Bechtel Associates 

Inc., and others who researched their files for information 

regarding the original construction notes and subsequent 

water-intrusion investigations of the Red Line. In addition, 

the authors would like to thank Harry Lupia, John Rudolf, 

and James Darmody of WMATA for their insights on water 

intrusion into the tunnel, historical and engineering knowledge of tunnel construction practices, and local geologic 

information. Colleague reviewers Forest Lyford and Richard Yager of the USGS are thanked for their thoughtful comments and suggestions. Special thanks go to Valerie Gaine, 

Jean Hyatt, Donna Knight, and Timothy Auer of the USGS 

for editorial reviews, report preparation, and report graphics. 

Lastly, the authors would like to acknowledge the assistance 

of U.S. Navy personnel, including Patrick Whittington and 

William Freeman, for their logistical support and assistance 

in obtaining needed permissions to work at the National 

Naval Medical Center.

Geologic Setting

The study area is located in the Maryland part of the 

Piedmont and Blue Ridge Region (Heath, 1984). The 

Piedmont part of this region is characterized by low, rounded 

hills, with long, rolling northeast-southwest trending ridges 

whose summits range from about 300 ft along its eastern 

boundary with the Coastal Plain to about 1,500 to 1,800 ft 

along its western boundary with the Blue Ridge. The 

Piedmont is underlain by bedrock of Precambrian and 

Paleozoic age consisting of igneous and metamorphosed 

igneous and sedimentary rocks (Heath, 1984). The rock 

types include granite, gneiss, schist, quartzite, slate, marble, 

and phyllite. The near-surface material is saprolite, which is 

composed of clay-rich, unconsolidated material derived 

from in situ weathering of the underlying bedrock. In many 

valleys, flood plains are underlain by thin, moderate- to wellsorted alluvium deposited by streams. When the distinction 

between saprolite and alluvium is not important, the unconsolidated material covering the bedrock is generally referred 

to as regolith.

Near the NIH and NNMC and beneath Wisconsin 

Avenue in Bethesda, Maryland, the subway system and 

Medical Center Station and Crossover pass through or 

beneath the Sykesville Formation and the biotite-hornblende tonalite (Ogh) member of the Georgetown Intrusive 

Suite (table 1, fig. 3). The spatially dominant unit is the 

Sykesville Formation, which is intruded by igneous rocks 

of various ages. The Red Line tunnel north of the Medical 

Center Station and Crossover (cross section A–A′ ) is constructed in the Sykesville Formation. At the Medical Center 

Station and Crossover (cross section B–B ′ ), the tunnel is in 

the biotite-hornblende tonalite and south of the Medical 

Center Station and Crossover (cross section C–C ′ ) , the tunnel is constructed in the Sykesville Formation (figs. 3 and 4).

Sykesville Formation

The Sykesville Formation is a gneiss that is light to 

medium gray, lower Cambrian in age, and locally shows a 

well-developed foliation produced by the alignment of muscovite grains set in a quartzofeldspathic matrix (Fleming and 

others 1994; Drake 1998). The matrix is composed of 

quartz, feldspars, and fragments of other metamorphosed 

rocks (phyllonite, metagraywacke, migmatite, serpentinite, 

amphibolite, and schist) (Fleming and others, 1994). The 

spatial organization of the matrix mineralogy imparts a 

significant mechanical anisotropy to the rock. 

There are no exposures of the Sykesville Formation in 

the vicinity of the Medical Center Station and Crossover, 

though large exposures within the Washington, D.C. area 

occur along the banks of the Potomac River in the vicinity 
Geologic Setting 7

of the C & O Canal (fig. 1a). These exposures show a 

moderately well-developed foliation within the rock mass 

that does not coincide with the planes of jointing and other 

fracturing. The dominant joint families are orthogonal to the 

foliation and cut each other at high angles. Much of the 

jointing appears to lack a larger-scale three-dimensional 

interconnectivity (fig. 5). 

Biotite-Hornblende Tonalite Member of the Georgetown 

Intrusive Suite

The biotite-hornblende tonalite (Ogh) member of the 

Georgetown Intrusive Suite that occurs in the vicinity of the 

Medical Center Station and Crossover is medium- to coarsegrained, early Ordovician in age, and varies from massive to 

foliated (fig. 3). Fleming and others (1994) reported that the 

biotite-hornblende tonalite has “strong reflect (igneous) flow 

structures... and contains many ultramafic and mafic xenoliths, as well as xenoliths of other metasedimentary rocks.” 

At the Medical Center Station and Crossover area, the 

biotite-hornblende tonalite is an inclusion or inlier within the 

larger expanse of the Sykesville Formation (figs. 3 and 4; 

cross section B–B′ ). The mean east-west width of this unit 

is about 1 mile, and within this unit is a smaller north-south 

elongated inclusion of muscovite-biotite monzogranite (Odl 

on fig. 3) about 300 ft in width (Fleming and others, 1994). 

Neither of these igneous bodies is exposed near the Medical 

Center Station and Crossover.

Outcrops of the biotite-hornblende tonalite can be found 

along the Potomac River near Lock 8 of the C & O Canal 

(figs. 1a and 6). Observations of the tonalite and joint orientations at the outcrops indicate that internal fracturing is pervasive. Where foliated, the foliation plane has acted as a 

plane of weakness, localizing the development of subsequent 

joints. Orthogonal to this plane, two additional mutually 

orthogonal planes of joints are well developed. All three 

planes of jointing are coated with a widespread brown ironoxide staining, suggesting that paleo-ground-water flow has 
8 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel
Geologic Setting 9
10 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel
Geologic Setting 11
12 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel

taken place. Collectively, the joints define an orthotropic 

symmetry, and subdivide the rock into a crude boxwork. 

The geometry of the three joint set populations as well as the 

observations of actively percolating ground water indicate 

that an extensive three-dimensional network of fractures has 

been established in the biotite-hornblende tonalite member 

of the Georgetown Intrusive Suite.

Structural Controls on Ground-Water Flow

The regional structural geology of the Sykesville Formation and biotite-hornblende tonalite (Ogh) member of 

the Georgetown Intrusive Suite may control and influence 

the direction and magnitude of ground-water flow. Geological structures (foliation, faults, joints, folds, anticlines, and 

synclines) may be significant hydraulic conductors of 

ground-water flow if they are connected to form networks of 

fractures. Planar features (foliation and joints) near the 

Medical Center Station have been mapped by Fleming and 

others (1994), Drake (1998), and Drake and Froelich (1997) 

and are shown in figure 7. The control these planar features 

have on ground-water flow in the study area is not well 

defined, although they may be an expression of the fracture 

orientations and may exert an influence on the direction and 

rate of ground-water flow in each geologic unit. 

Statistical rose diagrams of mapped foliations and joints 

(146 in the Sykesville Formation and 41 in the biotite-hornblende tonalite) were developed to determine the orientation 

(direction) of these features. The mapped orientations of 

the foliation and joints of the Sykesville Formation are generally in a north-south direction with a mean orientation of 

about 358 degrees and a standard error of plus or minus 

10 degrees. In the biotite-hornblende tonalite intrusion, the 

general orientation of the foliation and joints is shifted 

90 degrees to an east-west direction, with a mean orientation 

of 88 degrees and a standard error of plus or minus 

34 degrees (fig. 7).

Hydrogeology

The ground-water-flow system of the Maryland part of 

the Piedmont and Blue Ridge Region (Heath, 1984) consists 

of saturated unconsolidated regolith (saprolite, alluvium) 

overlying saturated fractured bedrock. Regolith will contain 

water in the pore spaces (intergranular porosity) between the 

rock particles. The thickness of the regolith ranges from 

about 30 to 60 ft and can be as thick as 300 ft on some ridge 

tops, whereas on other ridge tops, the regolith may be very 

thin or missing and fractured bedrock may be exposed in 

outcrops. Regolith in the study area can be subdivided 

between saprolite (deposits on ridges and midslopes) and 

alluvium (deposits in valleys and streams).

Beneath the regolith is a massive fractured bedrock that 

has little intergranular porosity, but contains water in permeable fractures (fig. 8). In general, the bulk hydraulic conductivities of the regolith and bedrock are similar and range 

from about 0.003 to about 3 ft per day (Heath, 1984). The 

major difference between the regolith and bedrock is porosity, with the regolith having a saturated porosity of about 

20 to 30 percent and the bedrock about 0.01 to 2 percent 

(Heath, 1984).

The saprolite-bedrock contact is a zone of gradation from 

unconsolidated saprolite (weathered rock) to unweathered 

fractured bedrock, and is referred to as a transition zone (fig. 

8). This transition zone is at the top of the bedrock and 

although it may not be geologically different from the fractured bedrock, it may be acting very differently hydraulically 

because of the extensive clay infilling of the fractures. The 

clay infilling of fractures may lower the hydraulic conductivity and slow vertical leakage from the unconsolidated saprolite to the unweathered fractured bedrock.

In general, ground water will move as in a typical 

regional flow system, from recharge in the topographic highs 

(ridges) to discharge in areas of topographic lows (streams 

and valleys). Depending on location, the bedrock may 

receive areal recharge from the regolith and may also discharge water to the regolith. Ground-water systems function 

as reservoirs (storage of water) and as conduits that transmit 

water from recharge areas to discharge areas. The groundwater system in the Piedmont can be viewed as two separate 

and interconnected flow systems (regolith and bedrock), 

because of their difference in storage (porosity) (fig. 8). 

Because of its larger porosity, the regolith can be considered 

to be a reservoir that slowly feeds water into the lower bedrock fractures. The fractures act as a network of conduits 

that transmit water to wells, springs, or openings in the bedrock, such as tunnels.

Saprolite

Bedrock at the Medical Center Station and Crossover is 

covered by saprolite. The saprolite (approximately 20–55 ft 

thick) is a soft, red/brown to gray, earthy, weathered porous 

material formed from decomposed crystalline rock in which 

constituent minerals, other than quartz, have been altered to 

clays. Permeability of the saprolite is variable, but commonly is highest just above the unweathered bedrock (Nutter 

and Otton, 1969). The upper part of the saprolite is generally 

a few feet thick, massive, and grades upward into a soil. As 

the saprolite grades downward, it retains the structural characteristics of the unweathered bedrock. At the saprolite-bedrock contact is a transition zone, whose thickness is highly 

variable.

Engineering consultants for WMATA (DMJM/Deleuw, 

Cather & EBA Engineering, Inc., 2001) performed surface 

geophysical investigations near the Medical Center Station 

and Crossover area to determine the thickness of the saprolite and explore any unconformities in the saprolite-bedrock 

contact, which might indicate preferential pathways (faults, 

joints, folds) for ground-water flow. The location of seismic 

refraction lines and 2-D resistivity lines is shown in figure 9. 

The seismic-refraction profiles show that near the Medical 

Center Station and Crossover, the saprolite thickness ranges 

from 20–55 ft and the 2-D resistivity profiles identified 

several possible anomalies in the saprolite-bedrock contact 

that may indicate areas of intensive fracturing (DMJM/
Hydrogeology 13
14 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel

Deleuw, Cather & EBA Engineering, Inc., 2001). Test wells 

(MO Ff 29, MO Ff 30, and MO Ff 31) were located and 

drilled based on the information from the seismic refraction 

and 2-D resistivity geophysical surveys. 

Two observation wells (MO Eg 28 and MO Ff 28) near 

the Medical Center Station and Crossover are finished in the 

saprolite. Well-construction data for these wells are listed in 

table 2 and the locations are shown in figures 9 and 10. Well 

MO Eg 28 (fig. 10) is an existing saprolite monitoring well 

with a 25-ft screen. MO Ff 28 ( fig. 9) was drilled as part of 

this study and has a 10-ft screen in the lower part of the 

saprolite. 

Response of water levels in the saprolite to precipitation 

at well MO Eg 28 is shown in figure 11. The hydrograph is 

fairly constant for the period of record, even though significant rainfall events were recorded for several months (May–

August) at the nearby National Oceanic and Atmospheric 

Administration (NOAA) Station 2325 (Dalecarlia Reservoir, 

Washington, D.C. ). Based on the hydrograph (fig. 11), 

water levels in the saprolite at this site do not have a strong 

response to recharge. This is probably because porosity and 

storage capacity in the saprolite are very high. The high 

storage capacity of the saprolite is essentially keeping it fully 

saturated, and the response to recharge and droughts is 

minor.

An air-pressurized slug test (Shapiro and Greene, 1995; 

Greene and Shapiro, 1995) was conducted in observation 

well MO Ff 28 to estimate transmissivity, hydraulic conductivity, and storativity of the saprolite. This type of test only 

stresses a limited volume (few feet) of geologic material 

around the well. The data were analyzed graphically and 

transmissivity (T) is 10 ft2/d (feet squared per day), hydraulic conductivity (K) is 1 ft/d (foot per day), and storativity 

(S) is 10-6 (fig. 12).

Bedrock Aquifer

The hydraulic properties of the fractured bedrock aquifer 

are controlled by the fractures; therefore, a detailed knowledge of the geometric and hydraulic properties of the fractures is required to characterize the ground-water-flow 

system. Test wells were drilled into the biotite-hornblende 

tonalite (Ogh, on fig. 3) member of the Georgetown 

Intrusive Suite at the NNMC, adjacent to the Medical Center 

Station and Crossover, to determine the hydraulic properties 

of the fractured bedrock (fig. 9). Characteristics of the fractures in the bedrock were investigated using borehole-geophysical logging techniques. These techniques can be used 

to infer the location and orientation of fractures intersecting 

the borehole. The hydraulic properties of the fractures were 

determined using single-hole and multiple-hole hydraulic 

tests. Hydraulic heads measured in the bedrock boreholes 
Hydrogeology 15

 

 

 

Table 2. Data for wells used for borehole geophysics, water-level measurements, and aquifer testing

[Location of wells shown in figures 9 and 10; °, degree; ', minute ; '', second; msl, mean sea level; --, no data]

U.S. Geological Survey

well number

State of Maryland

well permit number/

identification

Owner Latitude

( ° ' '' )

Longitude

( ° ' '' )

Altitude of 

land surface,

(feet msl)

Casing

depth

(feet)

MO Eg 27 AM-41U WMATA 1 39 00 08 77 05 48 268.9 147

MO Eg 28 MO-94-0540 NNMC 2 39 00 08 77 05 27 301.1 15

MO Eg 29 MO-81-2116 Private 39 00 07 77 04 26 281.2 38

MO Ff 12 -- NNMC 38 59 56 77 05 37 274.4 --

MO Ff 19 MO-69-0225 CCC 3 38 59 52 77 05 08 322.8 30

MO Ff 21 A-74 WMATA 38 59 37 77 05 47 329.9 132

MO Ff 22 A-79 WMATA 38 59 51 77 05 47 320.7 137

MO Ff 23 MO-73-3188 KGC 4 38 58 28 77 06 53 342.4 43

MO Ff 24 MO-93-0418 CCCC 5 38 58 19 77 04 58 307.5 24

MO Ff 25 MO-92-0268 RCC 6 38 59 08 77 05 12 323.0 24

MO Ff 26 A-71 WMATA 38 59 25 77 05 43 352.0 147

MO Ff 27 MO-94-1755 WMATA 38 59 51 77 05 47 328.0 60

MO Ff 28 MO-94-1759 WMATA 38 59 55 77 05 45 318.9 35

MO Ff 29 MO-94-1762 WMATA 38 59 55 77 05 45 318.9 60

MO Ff 30 MO-94-1757 WMATA 38 59 55 77 05 45 315.7 60

MO Ff 31 MO-94-1756 WMATA 38 59 55 77 05 45 322.0 60

A6 7 -- WMATA 39 00 39 77 05 51 260.0 --

U.S. Geological Survey

well number

Length of

open hole

(feet)

 Total

 depth

 (feet) 

Thickness of 

overburden

 (feet)

Year

drilled

Lithology Type of well

MO Eg 27 3 150 25 1974 Bedrock Observation

MO Eg 28 25 40 28 1998 Saprolite Observation

MO Eg 29 162 200 14 1998 Bedrock Water Supply

MO Ff 12 -- 275 -- 1951 Bedrock Water Supply

MO Ff 19 125 155 35 1961 Bedrock Irrigation

MO Ff 21 3 135 25.5 1970 Bedrock Observation

MO Ff 22 3 140 45.5 1970 Bedrock Observation

MO Ff 23 457 500 10 1981 Bedrock Irrigation

MO Ff 24 276 300 5 1995 Bedrock Irrigation

MO Ff 25 20 44 35 1992 Bedrock Observation

MO Ff 26 3 150 41 1970 Bedrock Observation

MO Ff 27 91.5 151.5 34.3 2001 Bedrock Observation

MO Ff 28 10 45 -- 2001 Saprolite Observation

MO Ff 29 180 240 45 2001 Bedrock Observation

MO Ff 30 180 240 50 2001 Bedrock Observation

MO Ff 31 180 240 55 2001 Bedrock Observation

A6 -- -- -- -- -- Pump Station

1 Washington Metropolitan Area Transit Authority 5 Chevy Chase Country Club

2 National Naval Medical Center 6 Riviera of Chevy Chase

3 Columbia Country Club 7 Pumping Station for the Medical Center Area

4 Kenwood Golf Club
16 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel
Hydrogeology 17

were used to infer the hydraulic gradients in the bedrock and 

the direction of ground-water flow. In addition, sources and 

sinks of water to the bedrock were investigated by considering responses in saprolite and bedrock wells to precipitation 

and ground-water discharge measured at pumping station A6 

(fig. 10). 

Response of water levels in the bedrock to precipitation 

measured at nearby NOAA Station 2325 (Dalecarlia Reservoir, Washington, D.C.) observation well MO Eg 27 is 

shown in figure 13. Water levels in the bedrock are very 

responsive to recharge and drought because the bedrock does 

not have the high porosity or storage capacity of saprolite. 

The introduction of recharge to the bedrock can result in 

large changes of fluid pressure in the bedrock. 

Bedrock Boreholes Four 6-inch bedrock test wells

(MO Ff 27, MO Ff 29, MO Ff 30, and MO Ff 31) and one

2-inch saprolite test well (MO Ff 28) were drilled for this 

investigation using an air-rotary drilling method (fig. 9). 

The bedrock wells were drilled through the unconsolidated 

material and/or weathered rock to about 5 ft into competent 

rock. Casing was set through the saprolite and/or weathered 

rock and grouted into the competent bedrock. The bedrock 

was then drilled as an open hole to about 240 ft below land 

surface for boreholes MO Ff 29, MO Ff 30, and MO Ff 31 

and to a depth of about 150 ft below land surface for 
18 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel

MO Ff 27. In addition, drill cuttings were collected and 

inspected to help define the lithology of the bedrock. Geophysical logs from the boreholes are shown in figures 14–17. 

To assist with interpretation, the driller’s log and the waterbearing zones (fractures that produce water) from the on-site 

driller’s reports are included for each well. Generally, waterbearing zones in each well were limited, and found mostly in 

the biotite-hornblende tonalite where biotite was dominant. 

Even though there are numerous fractures in the hornblendebiotite, the driller did not report significant water production 

in these fractures (figs. 14–17). Well MO Ff 29 (fig. 15) was 

drilled into mostly hornblende-biotite and had the least 

amount of water compared to the other wells. Wells MO Ff 

27, MO Ff 30, and MO Ff 31 all had zones where biotite 

dominated and all these wells produced significant amounts 

of water (figs. 14–17). 

Borehole Geophysics Geophysical well logs are an 

important tool in characterizing aquifers. Geophysical logs 

provide a continuous depth sampling record of undisturbed 

material that can be associated with numerous types of measurements (Keys and MacCary, 1981; Paillet and Crowder, 

1996). Geophysical-logging surveys of the wells at the 

NNMC site (fig. 9) included: caliper, natural gamma, resistivity, short- and long-normal resistivity, and acoustic televiewer.

The acoustic televiewer logs were used in each well to 

map the fractures intersecting the boreholes. Fracture location and orientation (strike and dip) were identified in each 

borehole (figs. 14–17). This information was used to compare to the regional fracture statistics for the Sykesville 

Formation and biotite-hornblende tonalite member of the 

Georgetown Intrusive Suite. In addition, the information 

obtained by the acoustic televiewer logs was also used to 

determine where to conduct single-hole hydraulic tests 

within the borehole. The east-west strike of the fractures in 

the boreholes is nearly in the same direction as the regional 

distribution of structural features mapped for the biotitehornblende tonalite (fig. 18). The dip of the fractures (angle 

of inclination from horizontal) determined from the acoustic 

televiewer logs shows that they are steeply dipping with a 

mean dip angle of about 65 degrees (fig. 19).

Hydraulic Heads and Hydraulic Gradients The Bethesda 

area of Montgomery County, Maryland, is highly urbanized 

and much of the water supplied for business and private 

residences is provided by city water utilities. Therefore, 

only a few wells in the bedrock were available to develop an 

areal bedrock hydraulic head map in the Medical Center 

Station and Crossover area (table 2, fig. 20). Even though 

there are very few head measurements for the bedrock, a 

hydraulic head map is approximated for most of the study 
Hydrogeology 19
20 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel
Hydrogeology 21

area (fig. 20). Heads measured in the bedrock provide 

insight into areal ground-water flow. The general direction 

of ground-water flow is from the drainage divides to minor 

streams and Rock Creek at lower elevations. The hydraulic 

heads are approximated on the west side of the Red Line due 

to the sparse number of bedrock wells. The Medical Center

Station and Crossover and adjacent tunnel act as a drain, 

routing ground-water flow into the subway. 

The abrupt directional change in ground-water flow at 

the contact between the Sykesville Formation and the 

biotite-hornblende tonalite may be a result of different fracture orientations in these units. In addition, it is likely that 

the east-west oriented fractures in the biotite-hornblende 

tonalite may intersect the north-south oriented Medical 

Center Station and Crossover and tunnel with a high frequency, resulting in increased water intrusion into the tunnel.

Single-hole hydraulic tests were conducted in hydraulically isolated intervals in observation wells MO Ff 27,

MO Ff 29, MO Ff 30, and MO Ff 31 at the NNMC site using 

equipment described in Shapiro (2001). Prior to conducting 

each single-hole test, the ambient hydraulic head was measured (fig. 21). The ambient hydraulic head in the open 

borehole is a composite hydraulic head; the hydraulic head 

associated with each fracture may differ from the open hole 

hydraulic head because of the local hydrologic conditions 

and the connectivity of the fractures (Shapiro, 2002). Information about the variability of the hydraulic head in fractures over the length of the borehole is important in understanding the local flow regime in the fractured bedrock.

The estimate of the ambient hydraulic head in each test 

interval was made using a fluid-pressure transducer placed 
22 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel
Hydrogeology 23
24 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel

in the test interval of the apparatus. After inflating the two 

borehole packers to isolate the section of the borehole for a 

single-hole fluid-injection test, the fluid pressure was 

allowed to equilibrate prior to initiating the fluid-injection 

test. The hydraulic head in the test interval prior to the start 

of the fluid-injection test is assumed to represent the ambient 

hydraulic head in the test interval. The length of the test 

interval where the hydraulic head was measured is shown in 

figure 21. 

In general, hydraulic head decreases with depth in bedrock wells MO Ff 29, MO Ff 30, and MO Ff 31, indicating 

the potential for downward ground-water flow. The potential for downward ground-water flow implies that the water 

in the saprolite is recharging the bedrock aquifer. The 

hydraulic head in saprolite well MO Ff 28 at the NNMC is 

higher than in wells MO Ff 29, MO Ff 30, and MO Ff 31 

(fig. 22), which further indicates that ground water is flowing downward from the saprolite to the bedrock.

The hydraulic head in the upper three test intervals below 

the bottom of the surface casing in well MO Ff 29 appears to 

be unusually high in comparison to the hydraulic head at 

similar elevations in wells MO Ff 30 and MO Ff 31. This is 

likely the result of not allowing the hydraulic head in the test 

interval to equilibrate for a sufficient period of time prior to 

conducting the single-hole fluid-injection test; the time 

required to achieve the ambient hydraulic head in test intervals with low transmissivity is longer than in high-transmissivity test intervals.

Hydraulic heads in well MO Ff 27 also showed potential 

for downward ground-water flow in the bedrock. The lower 

hydraulic heads with depth in this borehole show that 

hydraulic head in this well is greatly affected by the nearby 

subway tunnel. The hydraulic head is more than 10 ft lower 

in well MO Ff 27 than at similar elevations in wells MO Ff 

29, MO Ff 30, and MO Ff 31 (fig. 22). The atmospheric 

pressure in the subway tunnel causes the gradient in the 

hydraulic head in the bedrock to be in the direction of the 

tunnel. Thus, the hydraulic heads measured in the test intervals of MO Ff 27 cannot be interpreted as only showing the 

potential for downward ground-water flow. Additional wells 

in the vicinity of the subway tunnel would be needed to 

provide more detail on the nature of the hydraulic gradient 

and the directions of ground-water flow in the vicinity of the 

subway tunnel. The hydraulic heads in the bedrock observation well MO Ff 22 (33 ft from the tunnel) are about 50 ft 

lower than hydraulic heads in well MO Ff 27 (81 ft from the 

tunnel ) (fig. 22).

Hydraulic Properties of the Bedrock Aquifer The ease 

with which water moves through a geologic material is 

defined by the formation transmissivity (Bear, 1979). Estimates of transmissivity are made by conducting hydraulic 

tests in which a known hydraulic perturbation is imposed by 
Hydrogeology 25

either injecting or withdrawing water while measuring the 

associated fluid pressure response. In situ hydraulic tests 

can be conducted in a single borehole, where the hydraulic 

perturbation and pressure response are measured in the same 

borehole, or in multiple boreholes, where the pressure 

response to the hydraulic perturbation is measured at multiple locations. Single-hole hydraulic tests in bedrock aquifers 

are usually conducted for short durations (minutes to hours) 

to infer the transmissivity of the fractures in the immediate 

vicinity of the borehole being tested, and provide insight into 

the range and spatial variability of hydraulic properties of 

fractures in the bedrock. Multiple-hole tests are often conducted over longer durations than single-hole tests (hours to 

days) to infer the bulk transmissivity over much larger volumes of rock. This bulk transmissivity in bedrock aquifers is 

of significance in understanding the ease with which water 

can move through complex networks of interconnected fractures. Single-hole and multiple-hole hydraulic tests were 

conducted at the bedrock wells installed in the vicinity of the 

Medical Center Station and Crossover (fig. 9).

Single-Hole Hydraulic Tests Single-hole hydraulic tests 

were conducted using the apparatus described in Shapiro 

(2001). In general, the apparatus consists of two inflatable 

borehole packers that can form a hydraulic seal against the 

borehole wall to isolate a short section of a bedrock borehole 

(the test interval) for either fluid injection or pumping. 

Fluid-injection tests, as described in Shapiro and Hsieh 

(1998), were conducted in bedrock wells MO Ff 27, 

MO Ff 29, MO Ff 30, and MO Ff 31. The test apparatus was 

set up with borehole packers separated by 33.78 ft. The 

fluid-injection rate and the associated increase in fluid pressure were measured in the test interval. The transmissivity 

of the test interval was estimated using a steady-state flow 

approximation (Shapiro and Hsieh, 1998). The fluid pressure response above and below the test interval in the borehole was also monitored during fluid-injection tests to ensure 

that borehole packers formed suitable hydraulic seals against 

the borehole wall.

The estimates of the transmissivity from the fluid-injection tests in wells MO Ff 27, MO Ff 29, MO Ff 30 and 

MO Ff 31 are shown in figure 23. The length of the test 

intervals in each borehole is denoted by the vertical thickness of the estimate of the transmissivity. The location of the 

test intervals in each borehole was based on borehole conditions, including the roughness of the borehole wall as interpreted from the caliper log, and the presence of fractures, as 

interpreted from the acoustic televiewer log. An estimate of 

the transmissivity of the fractures immediately below the 

bottom of the surface casing in well MO Ff 27 could not be 

made because the water level in the borehole was slightly 

below the bottom of the casing at the time that the testing 

was conducted in December 2001. 

The transmissivity of the tested intervals in the bedrock 

boreholes varies over five orders of magnitude, from a 

maximum of approximately 10 ft 2/day to the detection limit 

of the in situ testing apparatus, which is approximately

10-4 ft 2 /day. In general, the transmissivity of the tested 

intervals shows an increase in transmissivity with depth in 

the bedrock boreholes. Fractures immediately below the 

surface casing in the bedrock at wells MO Ff 30 and 

MO Ff 31 are likely filled with byproducts of weathering 

rock. This may have the effect of reducing transmissivity in 

this zone as indicated from the tests. In contrast, fractures in 

the unweathered rock at depth are likely to be unfilled and 

have a higher transmissivity.

Zones of intense fracturing in the bedrock are not necessarily correlated with high transmissivity (figs. 14–17, 23). 

The density of fracturing, as interpreted from the acoustic 

televiewer logs, appears to decrease with depth in the boreholes, whereas the transmissivity appears to increase with 

depth. Transmissivity, however, appears to correlate with 

the resistivity log; low resistivity correlates with low transmissivity. Low resistivity is usually an indicator of high clay 

content (Keys and MacCary, 1981), which could be an indication of fracture-fill material due to rock weathering.

The single-hole hydraulic tests do not provide sufficient 

information to hypothesize on the spatial continuity between 

boreholes of the intervals of high or low transmissivity. 

Additional hydraulic tests, which isolate selected intervals of 

adjacent boreholes and monitor the fluid pressure response 

in these isolated intervals due to a known hydraulic perturbation, would be needed to identify the spatial continuity of the 

intervals of high and low transmissivity (Hsieh and Shapiro, 

1996).

Multiple-Hole Hydraulic Test A multiple-hole hydraulic test 

was conducted in the fractured bedrock in the vicinity of the 

Medical Center Station and Crossover by pumping from 

well MO Ff 30, while monitoring the change in the water 

level in the pumped borehole, the bedrock wells (MO Eg 27, 

MO Ff 27, MO Ff 29, and MO Ff 31), and the piezometer in 

the saprolite well MO Ff 28. The pump was placed in the 

bottom of well MO Ff 30, and the pump discharge rate was 

measured using a totalizing flow meter. Water-level measurements in the bedrock wells and saprolite piezometer 

were made using pressure transducers with digital records 

stored in data loggers; check measurements using electric 

water-level sounders also were made periodically. The 

multiple-hole test was started on December 10, 2001, at 

2:00 p.m.; pumping continued for 3 days, ending on

December 13, 2001, at 2:00 p.m. Following the termination 

of pumping, water levels were recorded for an additional

5 days to monitor the water-level recovery.

The multiple-hole hydraulic test was conducted by 

pumping at an average rate of approximately 1 gal/min. 

Based on estimates of the transmissivity from the singlehole tests, a pumping rate less than 1 gal/min was originally 

selected for this multiple-hole test to maintain the water level 

above the bottom of the surface casing in the pumped well 

(MO Ff 30). This was done to avoid dewatering fractures in 

the bedrock during the hydraulic test, which could result in a 

reduction of the transmissivity and lead to erroneous estimates of the bulk transmissivity of the fractured bedrock. 

The pumping rate during the hydraulic test, however, had to 

be maintained at 1 gal/min or higher because the pump could 
26 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel
Hydrogeology 27

not be operated at a rate lower than 1 gal/min for an extended 

period of time. After approximately 2 hours of pumping at 

approximately 1 gal/min, the water level in well MO Ff 30 

was lowered below the bottom of the surface casing. When 

the data were interpreted, however, it appeared that the dewatering of these fractures in the weathered rock immediately below the surface casing in well MO Ff 30 did not 

affect the overall transmissivity estimate of the rock. Based 

on the single-hole hydraulic tests, the fractures immediately 

below the surface casing have a much lower transmissivity 

than the unweathered rock. 

The pumping rate in well MO Ff 30 during the hydraulic 

test is shown in figure 24. The drawdown in saprolite well 

MO Ff 28 and bedrock wells MO Ff 29, MO Ff 30, and MO 

Ff 31 during the hydraulic test is shown in figure 25. The 

drawdown measured in these wells is the difference between 

the time-varying depth to water in the well during pumping 

and the depth to water at the onset of pumping (Bear, 1979). 

MO Ff 29 and MO Ff 31 were the only bedrock wells other 

than the pumped well to show a hydraulic response to pumping. The observation well finished in the saprolite showed 

only a small response (less than 0.1 ft change in water level) 

to the 3 days of pumping in well MO Ff 30. 

The measured drawdown in MO Ff 29, MO Ff 30, and 

MO Ff 31 is plotted against the logarithm of time on 

figure 25. After approximately 1.5 days (approximately 

2,160 minutes), there is a sharp increase in the drawdown 

measured in each well, which is an artifact of the increased 

pumping rate toward the end of the multiple-hole hydraulic 

test. Furthermore, the drawdown measured in the pumped 

well and two observation wells does not show appreciable 

leakage from the saprolite to the fractured bedrock, indicating that the saprolite has a limited vertical hydraulic connection to the bedrock. 

The formation transmissivity of the bedrock can be estimated from the straight-line portion (slope) of the plot of the 

drawdown against the logarithm of time in figure 25 (Fetter, 

2001; Lohman, 1979). The transmissivities estimated from 

the drawdown recorded in the pumped well (MO Ff 30) and 

the two observation wells (MO Ff 29 and MO Ff 31) are all 

equal to 3.7 ft 2 /day. This estimate of transmissivity is 

approximately one-half an order of magnitude less than the 

maximum transmissivity estimated from single-hole hydraulic tests conducted in MO Ff 30, and approximately one-half 

an order of magnitude less than the maximum transmissivity 

estimated from the single-hole hydraulic tests conducted in 

all other bedrock wells. The multiple-hole hydraulic test 

was conducted without borehole packers to isolate discrete 

intervals in any of the bedrock wells, whereas the single-hole 

hydraulic tests were conducted on short hydraulically isolated intervals in the wells. Because transmissivity estimated from the multiple-hole hydraulic test is one-half an 

order of magnitude less than the estimate from the singlehole hydraulic tests, this implies that the intervals of high 

transmissivity detected by the single-hole hydraulic tests are 

not areally extensive. The intervals of high transmissivity 

detected in each borehole are likely connected by fractures 

with slightly lower transmissivity, resulting in a bulk transmissivity of the fractured bedrock that is less than the maximum transmissivity estimated from the single-hole hydraulic 

tests.

The formation storativity of the bedrock aquifer can also 

be estimated from the straight-line portion of the plot of the 

drawdown against the logarithm of time for the two observation wells, MO Ff 29 and MO Ff 31 (Lohman, 1979). The 

formation storativity is the volume of water released from 

compressive storage over the thickness of the bedrock aquifer per unit decline in the hydraulic head (Bear, 1979). The 

storativity associated with the drawdown measured in boreholes MO Ff 29 and MO Ff 31 is 1.8 x 10-4 and 1.0 x 10-5, 

respectively. The conceptual model used to estimate the 

storativity is based on the assumption of an aquifer with 

homogeneous hydraulic properties. The difference in the 

storativity between these two observation wells is likely the 

result of aquifer heterogeneity. A further indication of heterogeneity in aquifer properties is the fact that plots of drawdown against time divided by radius squared (t / r w2 ) for 

wells MO Ff 29, MO FF 30, and MO Ff 31 do not overlay 

each other (fig. 26), where “ t ” is the elapsed time since the 

start of pumping and r w2 is the square of the radial distance 

from the pumped well to the observation borehole (Lohman, 

1979). Additional hydraulic tests, which isolate selected 

intervals of adjacent boreholes and monitor the fluid pressure response in these isolated intervals due to a known 

hydraulic perturbation, would be needed to identify the 

aquifer heterogeneity (Hsieh and Shapiro, 1996).
28 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel
Hydrogeology 29
30 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel

Hydrogeologic Controls on Ground-Water 

Discharge to the Subway Tunnel Near the 

Medical Center Station and Crossover

The major hydrogeologic controls on ground-water flow 

into the Medical Center Station and Crossover include geologic factors, hydraulic heads and gradients, leakage from 

saprolite to bedrock, and permeability of the fractures in the 

bedrock. In the vicinity of the Medical Center Station and 

Crossover, the two major geologic units are the Sykesville 

Formation and the biotite-hornblende tonalite (Ogh) 

member of the Georgetown Intrusive Suite (Fleming and 

others, 1994; Drake, 1998). The mapped structural planar 

features (foliation and joints) of the areally extensive 

Sykesville Formation are oriented in a predominantly 

north-south direction, which is aligned with the tunnel (fig. 

7). The Medical Center Station and Crossover are in the 

biotite-hornblende tonalite member of the Georgetown 

Intrusive Suite where the mapped foliation and joints are 

predominantly in an east-west direction (perpendicular to the 

tunnel). If it is inferred that these orientations (strike) are the 

preferred direction of ground-water flow, then there may be 

greater inflow of ground water to the tunnel when the fractures are in an east-west orientation, which is perpendicular 

to the tunnel. Analysis of fracture orientations with depth 

(up to 240 ft below land surface) in test wells drilled near the 

Medical Center Station and Crossover shows that the strike 

of the fractures is also in this east-west orientation, which is 

consistent with the regionally mapped orientations of the 

foliation and joints. This fracture orientation of the biotitehornblende tonalite may increase the potential for groundwater flow into the tunnel. Ground-water flow directions 

based on regional hydraulic heads in the bedrock (Sykesville 

Formation and biotite-hornblende tonalite) show an abrupt 

change in direction at the contact between the Sykesville 

Formation and biotite-hornblende tonalite.

Based on data from five observation wells, hydraulic 

heads in the bedrock near the Medical Center Station and 

Crossover are responsive to precipitation. The monitoring 

well in the saprolite near this area shows little response to 

precipitation; the data from this well indicate that water levels in the saprolite are fairly constant with time.

 In general, the hydraulic head in the bedrock decreases 

with depth in the test wells at the NNMC site, and the 

hydraulic head is higher in the saprolite than in the bedrock, 

indicating the potential for downward flow. This potential 

for downward ground-water flow implies that water in the 

saprolite is recharging the bedrock aquifer. The significance 

of this connection is unknown at this time; however, pumping in the bedrock for 3 days at the NNMC site resulted in a 

negligible hydraulic response in the saprolite. This implies 

that there is a low vertical hydraulic conductivity between 

the saprolite and the unweathered bedrock. The fractures at 

the top of the competent bedrock in wells at the NNMC site 

have transmissivities below the detection limit of the in situ

testing equipment (less than 10-4 ft 2 /day). The fractures are 

likely filled with weathering products, reducing their ability 

to transmit ground water. In contrast, the transmissivity of 

the fractures at greater depths in the boreholes at the NNMC 

site were at or near 10 ft 2/day.

The potential or driving force for water intrusion into the 

Medical Center Station and Crossover is due to the difference between the hydraulic head in the fractures in the surrounding rock and the tunnel, which is at atmospheric 

pressure. Because the tunnel is at atmospheric pressure, an 

increase in hydraulic heads in the bedrock results in an 

increased hydraulic gradient and thus a larger volume of 

water intruded into the tunnel. The volume of water entering 

the Medical Center Station and Crossover was measured by 

instrumenting the A6 pumping station with a flow meter that 

measures hourly discharge. This station collects water from 

a large section of tunnel from just north of the Bethesda 

Station to the portal north of the Medical Center Station 

(fig. 1b).

The discharge hydrograph of daily pumping at A6 (fig. 

27) is correlated with heads measured near the tunnel in bedrock observation well MO Eg 27 (table 2, fig. 27). More 

water is pumped from the tunnel when the hydraulic heads in 

the bedrock increase. The tunnel pressure remains at atmospheric pressure; thus, a higher hydraulic head in the bedrock implies a larger hydraulic gradient and a larger driving 

force for ground-water flow into the tunnel. 
Summary and Conclusions 31

Summary and Conclusions

Excessive water intrusion has been observed inside several of the Washington Metropolitan Area Transit Authority 

subway tunnels, with the worst leakage occurring along the 

Red Line. The Red Line tunnels and stations from Dupont 

Circle north were constructed in bedrock that contains permeable (water-bearing) joints and fractures. Excessive water 

leakage through the walls and the presence of water inside 

the underground facilities has damaged mechanical and electrical components in the tunnel, and has escalated the deterioration rate of the rail system. This leakage will eventually 

reduce the life span of the structure and rail system. Current 

water mitigation practices include the application of sealant 

to cracks that are leaking or diverting flow from the ceiling 

to the drain system with small “aluminum tents.” In order 

for the engineers and managers of the Washington 

Metropolitan Area Transit Authority to develop a strategy 

to prevent or reduce water intrusion inside the underground 

facilities, an understanding of the hydrogeologic controls 

on water intrusion was needed. To meet this need, the 

U.S. Geological Survey and the Washington Metropolitan 

Area Transit Authority conducted a cooperative hydrogeologic investigation from 2000–03 to characterize groundwater flow into the tunnel system. Results of this study provide a methodology for investigating similar water intrusion 

problems in subway tunnels constructed in fractured bedrock, and may provide helpful information in the design of 

methods to divert or reduce the volume of water entering 

subway tunnels.

The Red Line near the Medical Center Station and 

Crossover passes through or beneath two fractured bedrock 

lithologic units, the Sykesville Formation and the biotitehornblende tonalite member of the Georgetown Intrusive 

Suite. Overlying the unweathered bedrock is a transition 

zone of varying thickness that consists of decomposed rock. 

Overlying the transition zone is saprolite, a residual deposit 

of soft, red/brown to gray, earthy, weathered porous material 

formed from decomposed crystalline rock in which constituent minerals, other than quartz, have been altered to clays. 

Seismic refraction and 2-D resistivity profiles indicate that 

the saprolite thickness ranges from 20 to 55 feet, and identi-
32 Hydrogeologic Controls on Ground-Water Discharge to the Washington METRO Subway Tunnel

fied several possible anomalies in the saprolite-bedrock contact that may indicate intensive fracturing. Slug tests 

conducted in the saprolite near the Medical Center Station 

and Crossover estimated a transmissivity (T) of 10 feet

squared per day, hydraulic conductivity (K ) of 1 foot per 

day, and storativity (S) of 10-6 .

The mapped foliation and joints of the Sykesville 

Formation in the vicinity of the Medical Center Station and 

Crossover are generally oriented north-south, with a mean 

orientation of about 358 degrees and a standard error of plus 

or minus 10 degrees. In the biotite-hornblende tonalite, the 

general orientation of the structural features is shifted 

90 degrees to an east-west direction, with a mean orientation 

of 88 degrees and a standard error of plus or minus 

34 degrees. The orientation of the foliation and joints may 

influence the direction and rate of ground-water flow.

Fracture analysis in the four bedrock boreholes at the 

National Naval Medical Center well site shows the fractures 

are in an east-west orientation, which matches the orientation of the mapped foliation and joints in the biotite-hornblende tonalite. The mean dip of the fractures at the 

National Naval Medical Center well site is about 65 degrees; 

thus, the majority of fractures are steeply dipping. The 

abrupt directional change in ground-water-flow directions 

based on mapped hydraulic heads at the contact between the

Sykesville Formation and biotite-hornblende tonalite coincides with a change in fracture orientation in these two geologic units. The east-west fracture orientation of the biotitehornblende tonalite may have an influence on ground-water 

flow and could explain some of the increased water intrusion 

at the Medical Center Station.

Monthly precipitation records at nearby National

Oceanic and Atmospheric Administration Precipitation 

Station 2325 (Dalecarlia Reservoir in Washington, D.C.) and 

the hydrographs from the bedrock wells and the one saprolite 

observation well were correlated to investigate ground-water 

recharge in the vicinity of the Medical Center Station and 

Crossover. Although the single well in the saprolite showed 

little response to precipitation events, the heads in the bedrock wells were quite responsive to monthly precipitation.

The transmissivity of the tested intervals in the bedrock 

boreholes varies over five orders of magnitude, from a maximum of approximately 10 feet squared per day to the detection limit of the in situ testing apparatus, which is 

approximately 10-4 feet squared per day. In general, the 

transmissivity of the tested intervals shows an increase with 

depth. The low transmissivity immediately below the surface casing in the bedrock is likely due to a gradation in rock 

weathering. Fractures in close proximity to the saprolite are 

likely to be filled with byproducts of rock weathering, resulting in reduced transmissivity; in contrast, fractures in the 

unweathered rock at depth are probably unfilled.

In general, hydraulic head decreases with depth in the 

bedrock boreholes, indicating the potential for downward 

ground-water flow. The head difference between the saprolite and bedrock also indicates a potential for downward 

ground-water flow, and implies that the water in the saprolite 

is recharging the bedrock aquifer.

Transmissivities estimated from the drawdown recorded 

in the pumped well and the two observation wells are equal 

to 3.7 feet squared per day. This estimate is approximately 

one-half an order of magnitude less than the maximum transmissivity estimated from the single-hole hydraulic tests conducted in well MO Ff 30, and approximately one-half an 

order of magnitude less than the maximum transmissivity 

estimated from the single-hole hydraulic tests conducted in 

all other bedrock wells. The multiple-hole hydraulic test 

was conducted without borehole packers to isolate discrete 

intervals in any of the bedrock wells, whereas the single-hole 

hydraulic tests were conducted on short, hydraulically isolated intervals in the boreholes. Because transmissivity estimated from the multiple-hole test is one half an order of 

magnitude less than the transmissivity estimated from the 

single hole tests, this implies that the intervals of high transmissivity detected by the single-hole hydraulic tests are not 

areally extensive. The intervals of high transmissivity 

detected in each borehole are likely connected by fractures 

with slightly lower transmissivity, resulting in a bulk transmissivity of the fractured bedrock that is less than the maximum transmissivity estimated from the single-hole tests.

References Cited

Bear, J., 1979, Hydraulics of groundwater: New York, 

McGraw-Hill, 567 p.

Capital Transit Consultants, Inc., 2001, Washington 

Metropolitan Area Transit Authority: Preliminary 

water intrusion investigation Red Line Tunnel:

Washington, D.C., 62 p.

DMJM/Deleuw, Cather & EBA Engineering, Inc., 2001, 

Interim report on water intrusion at Medical Center 

Crossover: Geological, geophysical, and hydrological 

investigations: WMATA–AECS, Task Order # 37, 68 p.

Drake, A.A., 1998, Geologic map of the Kensington 

quadrangle, Montgomery County, Maryland: U.S. 

Geological Survey Geologic Quadrangle Map GQ–1774, 

scale 1:24,000.

Drake, A.A., and Froelich, A.J., 1997, Geologic Map of the 

Falls Church Quadrangle, Fairfax and Arlington 

Counties, and the City of Falls Church, Virginia, and 

Montgomery County, Maryland: U.S. Geological Survey 

Geologic Quadrangle Map GQ–1734, scale 1:24,000.

Fetter, C.W., 2001, Applied hydrogeology (4th ed.):

Upper Saddle River, NJ, Prentice Hall, 597 p.

Fleming, A.H., Drake, A.A., and McCartan L., 1994, 

Geologic map of the Washington West quadrangle, 

District of Columbia, Montgomery and Prince Georges 

Counties, Maryland, and Arlington and Fairfax Counties, 

Virginia: U.S. Geological Survey Geologic Quadrangle 

Map GQ-1748, scale 1:24,000.
References Cited 33

Greene, E.A., and Shapiro, A.M., 1995, Methods of 

conducting air-pressurized slug tests and computation of 

type curves for estimating transmissivity and storativity: 

U.S. Geological Survey Open-File Report 95–424, 43 p.

Heath, R.C., 1984, Ground-water regions of the United 

States: U.S. Geological Survey Water-Supply Paper 

2242, 78 p.

Hsieh, P.A., and Shapiro, A.M., 1996, Hydraulic 

characteristics of fractured bedrock underlying the FSE 

well field at the Mirror Lake site, Grafton County, NH, 

in Morganwalp, D.W. and Aronson, D.A., eds., 

U.S. Geological Survey Toxic Substances Hydrology 

Program—Proceedings of the Technical Meeting, 

Colorado Springs, CO, September 20–24, 1993: 

U.S. Geological Survey Water-Resources Investigations 

Report 94–4015, p. 127–130.

Keys, W.S., and MacCary, L.M., 1981, Application of 

borehole geophysics to water-resources investigations: 

Techniques of Water-Resources Investigations of the 

U.S. Geological Survey, book 2, chap. E1, 126 p.

Lohman, S.W., 1979, Ground-water hydraulics: U.S. 

Geological Survey Professional Paper 708, 70 p.

Mueser Rutledge Consulting Engineers, 1988, Water 

intrusion in underground structures: Final report: 

UMTA-DC-06-0374-888-2, U.S. Department of 

Transportation, Urban Mass Transport Administration, 

[variously paged].

Nutter, L.J., and Otton, E G., 1969, Ground-water 

occurrence in the Maryland Piedmont: Maryland 

Geological Survey Report of Investigations No. 10, 56 p.

Paillet, F.L., and Crowder, R.E., 1996, A generalized 

approach for the interpretation of geophysical well logs 

in ground-water studies—theory and application: 

Ground Water, v. 35, no. 4, p. 883–898.

Shapiro, A.M., 2001, Characterizing ground-water 

chemistry and hydraulic properties of fractured-rock 

aquifers using the Multifunction Bedrock-Aquifer 

Transportable Testing Tool (BAT3 ): U.S. Geological 

Survey Fact Sheet FS–075–01, 4 p.

_____ 2002, Cautions and suggestions for geochemical 

sampling in fractured rock: Ground Water Monitoring 

and Remediation, v. 22, no. 3, p. 151–164.

Shapiro, A.M., and Greene, E.A., 1995, Interpretation of 

prematurely terminated air-pressurized slug tests: 

Ground Water, v. 33, no. 4, p. 539–546.

Shapiro, A.M., and Hsieh, P.A., 1998, How good are 

estimates of transmissivity from slug tests in fractured 

rock?: Ground Water, v. 36, no. 1, p. 37–48.