Ground-Water Supplies in 

Shale and Sandstone in 

Fairfax, Loudoun,and Prince 

William Counties, Virginia

GEOLOGICAL SURVEY 

CIRCULAR 424
Ground-Water Supplies in Shale and 

Sandstone in Fairfax, Loudoun, and 

Prince William Counties, Virginia

By Paul M. Johnston

GEOLOGICAL SURVEY CIRCULAR 424

Washington, 7960
United States Department of the Interior

FRED A. SEATON, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on applkation to the U. S. Geological Survey, Washington 25, 0. C.
CONTENTS

Page Page

Abstract---------------------------- 1 Ground water Continued

Introduction----. _--_--_ ------ - 1 Recent developments.--.--.-. ---- 3

Previous work _------------.-_-_-- 1 Chemical quality of the water- -_-_ 4

Geology .-.-. ---------- -- _ -- 1 Future development-- ---. -- 6

Ground water -_-_----_--_-__-.__---- 2 Summary----------_-__----------_-- 6

Previous developments- -_- ----- 2 References cited --- - ------------ 6

Present municipal supplies--------- 3

ILLUSTRATIONS

Page 

Figure 1. The Triassic rocks of Northern Virginia------------------------------------- IV

TABLES

Page 

Table 1. Municipal wells of Leesburg, Va-_-__--_-__-__-__-_--__--__-----_---_ ----- 4

2. Municipal wells of Herndon, Va-__-__-__ ---_---_-----_-_-_--- ___--_-__-- 4

3. Construction wells for Dulles International Airport --_-____-------_--_-------- 4

4. Chemical analyses of water from wells in Triassic rocks of Fairfax,

Loudoun and Prince William Counties, Virginia _----__-------_------------- 4

III
IV

EXPLANATION

10 MILES

DULLES INTERNATIONAL >A' DISTRICT \

Figure 1. The Triassic rocks of northern Virginia
Ground-Water Supplies in Shale and Sandstone in 

Fairfax, Loudoun, and Prince William Counties, Virginia

By Paul M.Johnson

ABSTRACT

The Triassic rocks of northern Virginia may be a potential 

source of moderately large supplies of ground water for municipal and industrial use if the performance of two deep 

wells drilled at the site of the new Dulles International Airport is a criterion. These two wells produced 327 and 600 

gpm (gallons per minute) from depths of 8601 and 955 feet in 

sedimentary rocks in an immediate area where the previous 

maximum depth reported was 180 feet and the maximum yield 

12 gpm.

Chemical analyses of the water indicates that it is extremely hard 533 and 500 ppm (parts per million) in the two 

wella and would require treatment to be satisfactory for 

domestic and some industrial uses. However, water of better 

quality may be present at greater depths, and it may be possible to case off the more highly mineralized water. Further 

exploration and sampling of water from various depths will be 

necessary for efficient development of the Triassic groundwater reservoir.

INTRODUCTION

Water-bearing characteristics of Triassic 

rocks, as shown by two wells drilled at the 

site of the new Dulles International Airport 

at Chantilly, indicate that these rocks may be 

of substantial importance as sources of moderately large water supplies for municipal 

and industrial use, in addition to their traditional role as sources of small domestic and 

farm water supplies.

Since the Chantilly area was selected as the 

site for Washington's new international airport, much interest has developed regarding 

the ground-water resources of the area.

The Dulles International Airport is about 

20 air-line miles west of Washington, D. C., 

just northwest of the town of Chantilly, Va. 

It lies north of U. S. Route 50 and straddles 

the Fairfax-Loudoun County line.

PREVIOUS WORK

The most comprehensive geologic study of 

the Triassic of Virginia made to date was that 

by Joseph K. Roberts (1938). Most of the 

geologic description included here is adapted 

from that report.

A geologic map covering a part of the area 

at a scale of about 1:125,000 is available for 

consultation at the U. S. Geological Survey, 

18th and F. Streets N. W., Washington. The 

map covers the Fairfax quadrangle and the 

part of the Seneca quadrangle lying south of 

the Potomac River. The geology was mapped 

by A. P. Bennison and Charles Milton, U. S. 

Geological Survey.

A geologic map of Virginia, scale 1:500,000 

published by the State in cooperation with the 

U. S. Geological Survey in 1928, is now out of 

print but is available in some public libraries.

GEOLOGY

The Triassic of northern Virginia is part 

of a long southwestward-trending belt which 

enters the Commonwealth from Maryland at 

the Potomac River between Point of Rocks 

and Seneca Creek. It is a thick wedge of 

westward-dipping sedimentary rocks and associated intrusive igneous rocks, in contact 

on the east with the Wissahickon formation 

and on the west with the Loudoun formation 

and the Catoctin greenstone. The ages of 

some of the older rocks are somewhat uncertain. The Catoctin is considered to be of Precambrian age and the Wissahickon, possibly 

early Paleozoic. Yet, the two may be contemporaneous in part, and the Catoctin in part
GROUND-WATER SUPPLIES, NORTHERN VIRGINIA

may be even younger than the Wissahickon. 

The Loudoun formation is regarded as of Early 

Cambrian age. The formations are listed below without reference to age.

Wissahickon formation. Schist, Phyllite, quartzite, and granitic and basaltic intrusive rocks.

Catoctin greenstone. "Altered diabase, with 

lenses of epidote and quartz, and associated 

with great masses of....granite" (Keith,1894).

Loudoun formation. "...dark-colored slates and 

shales grading into sandstones containing 

lenses of slate and limestone, all more or 

less metamorphosed" (Roberts, 1928, p. 65).

Newark group. Border conglomerate of Roberts, 1923. Includes limestone conglomerate, 

quartz conglomerate, arkose conglomerate, 

schist conglomerate, trap conglomerate, and 

quartz arkose. The quartz conglomerate is 

common on the eastern margin of the Triassic 

between Centerville and Herndon, where it is 

colored gray to black. North of Herndon lightgray schist conglomerate is exposed. The 

limestone conglomerate is exposed along the 

western margin of the Triassic in Loudoun 

County. It has been called Potomac marble 

or calico marble and has been quarried in 

Virginia and Maryland. About 90 percent of 

the pebbles are limestone, the rest quartz 

and feldspar, all in a red or white calcareous 

matrix.

Manassas sandstone of Roberts, 1923. Includes 

coarse to very fine-grained sandstone, but 

grades from conglomerate to shale. Colors 

are gray, red, and yellow. The gray sandstone 

is generally coarser than the red. In some 

places the sandstone was altered by the heat 

associated with intrusion of diabase.

Bull Run shales of Roberts, 1923. The youngest 

Triassic sedimentary rocks; alternating fineto coarse-grained shale colored red, gray, 

blue and black, altered in places by intrusive 

diabase.

Diabase. Diabase or "trap rock" in flat 

bodies called "dikes" (cutting across beds), 

"sills" (intruded between beds), and irregular 

bodies called "stocks" are intruded into the 

Triassic sedimentary rocks and into the 

underlying Wissahickon formation.

The widest section of the Triassic in Virginia is exposed between Herndon and Leesburg, a distance of about 15 miles. Roberts

(1928) estimated the thickness in this area to 

be 1,000 to 1,500 feet.

In general the structure of the Triassic 

sedimentary rocks in monoclinal. The strike 

ranges from N. 15° W. to N. 45° E. The dip 

is westward to northwestward and ranges 

from 0° (horizontal) to 45°. Locally the rocks 

are faulted, or the dip is altered as a result 

of the intrusion of diabase.

There are two sets of joint cracks in the 

sedimentary rocks. The most conspicuous 

set strikes parallel to the bedding (N. 15° W. 

to N. 45° E.) and dips steeply. The second 

set, less conspicuous, strikes at right angles 

to the first and also has a steep dip.

Two sets of joints are present in the diabase. One set strikes northerly, the other 

westerly; both dip steeply.

The most accurate delineation of the outcrops of diabase is shown on the map byBennison and Milton (1950), Other available maps 

are too small in scale to show the outcrops in 

detail.

Beneath the surface the location and extent 

of the intrusives are largely conjectural. The 

diabase body whose outcrop surrounds Herndon is thought to be a saucer-shaped sill of 

which only the edges are exposed. An exploratory hole was drilled in the east rim of 

this body in 1955 by Jack Ange on Annadale, 

Va. This hole is about 0.3 mile east of Chestnut Grove cemetary, which is on State Route 

28 about a mile north of the Washington and 

Old Dominion tracks in Herndon. The hole 

was 1,000 feet deep and was still in diabase 

at that depth, but the rock was softer and 

coarser, than above probably indicating that 

the well was nearing a contact. A scanty 

supply of highly mineralized water was produced. (See analysis.)

Faulting is common in the Triassic rocks, 

both in sedimentary rocks and in the diabase. 

Reverse and normal faults both are.present, 

but they are difficult to locate and trace because of deep weathering and the lack of extensive distinctive beds.

GROUND WATER

PREVIOUS DEVELOPMENT

The following summary of ground-water 

conditions in the Triassic rocks of northern
GROUND-WATER

Virginia is based in part on data reported by 

Cady (1938). They, of course, represent 

ground-water development at that time, but the 

conditions and principles are still applicable.

Most of the wells in the Triassic rocks are 

drilled. Dug wells are generally unsatisfactory, as they have low yields, tend to fail 

in droughts, and are subject to pollution. The 

diabase is very resistant to weathering and 

in many places is very hard at the surface. 

For this reason it is very difficult to dig and 

impossible to bore (auger) a well in the 

diabase.

In the sandstone and shale, wells are generally less than 100 feet deep and yield less 

than 10 gpm (gallons per minute.) The deepest wells in these rocks in Fairfax and Loudoun Counties as reported by Cady (1938) were 

at Herndon and near Sterling. The Herndon 

well penetrated 300 feet of red shale but 

struck diabase at the bottom and produced 

only ^ gpm of water. The Sterling well also 

was 300 feet deep in red shale and produced 

3^ gpm. On the other hand, two wells half a 

mile west of Floris in red shale, 96 and 100 

feet deep, produced 55 and 50 gpm. Cady 

pointed out that the comparatively shallow 

wells commonly furnished adequate supplies 

for domestic use and there was no reason to 

drill deeper. However, he thought that if 

large supplies were needed they might be 

obtained by drilling "several hundred feet 

down..."

In the diabase most of the water is in the 

upper weathered zone, which is commonly 

less than 100 feet thick. Yields are generally 

low, but at least one well produced as much 

as 60 gpm. This well, only 36 feet deep, was 

l£ miles south of Chantilly. Another well in 

diabase near Ashburn was only 18 feet deep 

but yielded 10 gpm. In contrast, a well 802 

feet deep at Goose Creek and Leesburg turnpike, where the rock was comparatively fresh 

near the surface, produced only 3 gpm.

No records are available for wells in the 

conglomerate at the eastern margin of the 

Triassic but these rocks seem to be tight and 

probably are not very productive. On the 

western margin the limestone conglomerate 

has yielded j to 60 gpm to wells 28 to 360 

feet deep. A few holes were dry.

PRESENT MUNICIPAL SUPPLIES

Leesburg, the county seat of Loudoun 

County, and Herndon in Fairfax County both 

obtain public water supplies from the Triassic rocks. (See tables 1 and 2.)

Leesburg has a population of about 2,800 

and is supplied by three wells and a spring, 

all presumably in the limestone conglomerate. 

About 73 percent of the water comes from the 

wells, the rest from the spring. The following 

records were obtained from the town authorities:

Date

Jan. 1-31, 1959.. 

July 1-31, 1959..

Number of 

connections

759

771

Total consumption 

(gallons)

12,708,000

15,192,400

The town of Herndon has a population of 

about 2,100, supplied by three wells in the 

Triassic sandstone and shale. These records 

were furnished by town authorities:

Dste

Jan. 1-31, 1959..........

July 1-31, 1959.........

Number of 

connections

440

446

Total consuoption 

(gallons)

3,820,000

3,473,000

RECENT DEVELOPMENT

During 1956 two wells were drilled in the 

Triassic rocks near Manassas by the Hagmann 

Co. of Vienna, Va. One of these wells, at 

Manassas Park, was 1,000 feet deep and was 

pumped for 46 hours at 327 gpm, which was 

the maximum capacity of the pump. This 

well passed through 996 feet of shale, into 

sandstone. The static water level was 56 

feet below the land surface. The other well 

was 612^ feet deep and produced 260 gpm 

with a drawdown of 283 feet after 51 hours.

In December 1958, for the construction of 

the Dulles International Airport, it was proposed to establish a concrete-mixing plant 

southeast of the intersection of State Routes 

607 and 608 (the community of Willard). To 

furnish water for concrete mixing and for 

fill compaction, two wells were drilled for 

C. J. Langenfelder, the contractor, by the 

Hagmann Co. in the spring of 1959. These 

wells, numbered 1 and 2, are reported to be 

entirely in shale. Pertinent data are shown 

in table 3.
GROUND-WATER SUPPLIES, NORTHERN VIRGINIA 

Table 1. Municipal veils of Leesburg, Va.

Town 

well 

no.

1. 

0

3--...

Driller

Wortman, Ashburn, Va ---------------

Hagmann, Vienna, Va ----------------

o * *

Thomas, Berryville, Va --------------

Date 

Completed

Before 

1912 

10-28 

10-54

Depth 

(feet)

150

360 

350

Water Level (feet)

Static

30

35 

28

Pumping

145

250 

200

Yield 

(gpm)

40

65 

165

Table 2. Municipal wells ofHemdon, Va.

Town

well

no.

1 __

2 __

3i---

Driller

Zoll Brothers. - ___________--_______-____---

The Hagmann Co., Vienna, Va ___-_-_____-___

Date

Completed

1931

1954

1958

Depth

(feet)

200

400

420

Water

Level

(feet)

165

150

40

Yield

(gpm)

50

100

25

1 Used only in emergencies.

Table 3. Construction wells for Dulles International Airport

Airport 

well 

no.

1------

2 __ ._

Driller

The Hagmann Co,, 

Vienna, Va.

.-.--do.. -.___---

Date 

completed

1959 

1959

Depth 

(feet)

860 

955

Casing

diameter 

(inches)

8 

8

Length 

(feet)

8 

7

Water 

level 

'feel below 

land 

surface

8

4

Yield 

(gpm)

327 

600

Remarks

Drawdown to 52 

feet after 48 

hours at 327 gpm.. 

Drawdown to 155 

feet after 52 

hours at 600 gpm.

CHEMICAL QUALITY OF THE WATER

Chemical analyses of the water from the 

airport wells indicate that it is extremely 

hard (533 and 500 ppm for wells 1 and 2) and 

that maximum limits recommended by the 

Public Health Service for certain constituents 

are exceeded.

Water from the airport wells is satisfactory 

for concrete mixing, fill compaction, and irrigation.but water for most other uses will require treatment. Mixing equal parts of water 

from wells 1 and 2 would bring the iron and 

manganese within the acceptable limit, but 

not the sulfate and dissolved solids.

Constituent;

Iron and

manganese 

Sulfate--...

Dissolved 

solids __ «.

Concentration, in parts per million

Well No. 

1

0.38 

462

942

Well No. 

2

0.21 

327

856

Public Health 

Service 

maximum

0.3 

250

*500

1 If such water is not available a total solids content of 1,000 

ppm may be permitted.

It may be possible to obtain water of better 

quality from the Triassic sedimentary rocks
Table 4. Chemical analyses of water from wells hi Triassic rocks of Fairfax, Loudoun, and Prince William Countie

[Analyses by U. S. Geoligical Survey. Results in parts per million except as indicat

Well No- ___ - -- __ ---

Rock type -_-__---------_--_ 

Depth (feet) _____ __ __ __,

Date of collection ____________

Silica (SiO 2)-_- -- -

Aluminum (Al)__ _____________

Iron (Fe)__-------_--__------

Manganese (Mn) _____________ ,_» \ *

Copper (Cu)____- __________

Zinc (Zn)_-____--------_---_-

Calcium ( Ca) ________________

Magnesium (Mg) _____________ o \ o^

Sodium (Na) _________________

Potassium (K) _______________

Lithium (Li)__ _______

Bicarbonate (HCO3) __________

Carbonate ( CO3) _____

Sulfate (SO4 ) _ ___ ___

Chloride (Cl)___ ______ __ _

Fluoride (F)_--_-_--_-___-___

Nitrate ___, ._ _______ ______

Phosphate (PO4) _____________

Residue on evaporation at 

180°C _ ___ _______ ___ _

Hardness as CaCO3 __________

Noncarbonate hardness as 

CaCO3 _ __ __ __,_ __ __

Alkalinity as CaCOo- _______ 

i/ O

Free Carbon Dioxide (CO 2) 

(Cs^lc\

Specific conductance 

(micromhos at 2 5°) _________

pH _________________________

Color. ______________________

Temnerature (°F) ______ ______

Airport

No. 1 

Shale

860

7-59

25

.0

.33

.05

.00

.00

175

21

64

1.2

.1

194

0

462

14

.3

12

.1

942

533

374

R Q

1,200

7.4

2

64

Airport

1 No. 2 

Shale

Q^

5-59

67

.2

.15

.06

.00

.00

131

42

54

1.3

.0

275

0

327

13

o

.4

.0

856

500

274

226

1,010

7.8

8

62

102 11

Shale

65

1931

35

212

3

6.0

1.8

176

13331

Shale

196

1931

24

162

4

10

.57

122

10461

Sand T

stone 

(?) 

70

1931

72

17

15

204

67

14

31

330

250

1 1 1 ^1

Sandis tons 

297

1 Q31

50

,25

39

11

12

1.6

181

5.4

10

4.6

212

143

1116 1

Sandstone 

505

1 Q31

38

1.1

31

23

12

1.4

219

5.8

5.0

1.8

202

172

6841

Limestone

Conglomerate 

R3

10^1

50

15

3.5

206

5

5.0

16

209

186

7pcl

Limeston

Conglomerat

Qcn

1 QQ1

17

.1

68

19

19

4.

239

19

29

50

qq i

248

1 Well numbers from Cady (1938).
GROUND-WATER SUPPLIES, NORTHERN VIRGINIA

by locating and casing off the mineralized 

water if it enters the well from thin zones. 

Careful sampling during or after completion 

of the wells is necessary to obtain representative samples.

Table 4 gives analyses of water from various rock types at different depths. Water 

from the Ange well in the diabase contains a 

large amount of calcium chloride, suggesting 

that connate water (deposited with the sediments) is present in the sedimentary rocks 

below.

much water unless it is broken by faulting. 

Where examined at the eastern margin of the 

Triassic rocks it is a massive, tightly cemented rock without visible bedding or joint 

planes. However, at the base of the conglomerate, which is presumably in contact with 

metamorphic basement rocks, some water 

may be available.

Until more is known about the waterbearing characteristics of the Triassic formations it is suggested that deep wells be 

kept at least 1,000 feet apart to reduce 

mutual interference.

FUTURE DEVELOPMENT

In locating the wells at Dulles International 

Airport it was assumed that water under artesian pressure would be found in the Manassas sandstone beneath the Bull Run shales. 

However, the driller's well logs show shale 

for the entire depth of both wells. Therefore, 

if the sandstone is present it must lie still 

lower in the section. If Roberts' estimate 

(1928) of 1,000 to 1,500 feet as the thickness 

of the Triassic rocks is correct, not more 

than 500 feet of sandstone could underlie the 

shale at the site of Well 2.

If future high-yield wells are to be drilled, 

the structure of the Triassic rocks must be 

taken into account. Although not enough deep 

drilling has been done as yet to give more than 

rudimentary data on subsurface conditions, 

the most promising areas appear to be 3 to 4 

miles west of the eastern contact of the Triassic. Furthermore, wells should be located 

as far as possible from diabase bodies. If 

wells must be located near diabase bodies they 

should be on the updip (east) side of the intrusive. In the few places where the writer has 

seen the contacts of the intrusive, adjacent 

sedimentary rocks have been baked and the 

contact virtually sealed, forming an effective 

barrier to the movement of ground water. 

Even if outcropping diabase bodies are avoided 

consealed ones may be encountered at any 

depth.

The effect of faulting on the movement of 

ground water is not known and probably will 

not be known until more wells or test holes 

are.drilled. It is probable, however, that 

faults produce barriers to ground-water 

movement in the shale and sandstone bodies.

It appears unlikely that the quartz conglomerate at the base of the Triassic contains

SUMMARY

Records of two wells drilled at the site of 

the new Dulles International Airport in the 

spring of 1959 lead to the conclusion that 

moderately large quantities of water are 

probably available elsewhere in the Triassic 

rocks of Virginia. Both these wells are in the 

Bull Run shales of Roberts (1923). Well 1 is 

860 feet deep and produces 327 gpm. Well 2 

is 955 feet deep and produces 600 gpm.

Analyses of water from these wells shows 

that the water is extremely hard and moderately mineralized. Although the water is 

suitable for concrete mixing it will need 

treatment for domestic and for most industrial 

uses.

Careful sampling and analysis of water from 

separate water-bearing zones in deep wells 

may reveal the source of the mineralized water, which then perhaps could be cased off. 

Also, the sandstone that presumably lies beneath the shale should be explored for water 

of better quality.

REFERENCES CITED

Bennison, A. P., and Milton, Charles, 1950, 

Preliminary geological map of the Fairfax 

quadrangle, Virginia, and part of the Seneca 

quadrangle, Virginia-Maryland; scale about 

1:125,000: U.S. Geol. Survey open-file map.

Cady, R. C., 1938, Ground-water resources 

of northern Virginia: Virginia Geol. Survey 

Bull. 50, 200 p.

Keith, Arthur, 1894, Description of the 

Harpers Ferry quadrangle [Maryland]: 

U.S. Geol. Survey Geol. Atlas, Folio 10:
GROUND-WATER SUPPLIES, NORTHERN VIRGINIA

Roberts, J. K., 1923, The Triassic of northern U. S. Public Health Service, 1946, Drinking 

Virginia: Pan Am. Geologist, v. 39, p. 185 water standards: 14 p. 

200. Virginia Geological Survey, 1928, Geologic

 1928, The geology of the Virginia Tri- map of Virginia: Scale 1:500,000. 

assic: Virginia Geol. Survey Bull. 29, 205 p.

INT.our.,B.C.60- 7*a sa