' ' /1---,-

r'' L r~' "11/1 

( 

\, I 

n. ,__,) 

, , ' ,_,., .. 

--- .,.,/ -- ,.-- --------'~---- --------

, 

I 

I 

I 

GEOLOGICAL SURVEY CIRCULAR 538 

Ground Water of the Piedmont and 

Blue Ridge Provinces in the 

Southeastern States 

By H. E. LeGrand 

GEOLOGICAL SURVEY CIRCULAR 538 

Wcultington 1967 
United States Department of the Interior 

ROGERS C. B. MORTON, Secretary 

Geological Survey 

V. E. McKelvey, Director 

First printing 1967 

Second printing 1973 

Free on appli;;;ation to the U.S. Geological Survey, Washington, D.C. 20244 
CONTENTS 

Page 

Introduction ____ ------ __ -------______ 1 Chemical quality of the water ________ _ 

Evaluating sites--------------------- 1 Contamination of ground water _______ _ 

Yield ------------------------------ 4 General statements about ground 

Depth of wells----------------------- 5 water in the region-------------- __ 

Fractures in the rock---------------- 5 Sources of information ______________ _ 

Water table------------------------- 6 

ILLUSTRATIONS 

Figure 1. Generalized geologic map ________ ------ __ -------------------------------

2. Topographic map and profiles of ground surface showing rating in 

points for topographic positions ---- ____ --------- _______________ ------ _ 

3. Graph showing rating in points for soil thickness--------------------------

4. Graph showing probability of getting a certain yield from a well at 

different sites - ___ ---- _ --------- ____ ------------ _ -------------------- 5. Photograph of countryside in the Blue Ridge province showing approximate ratings for topography __________________________________ ----- ________ _ 

6. Photograph showing area where soil zone is likely very thin _______________ _ 

7. Curve showing relation of yield to drawdown ------------------------------ 8. Diagram showing yield of a well at two different pumping rates ____________ _ 

9. Diagram showing extent to which deepening of average well increases yield __ 

10. Sketches of sixctypes of ground conditions showing distribution of fractures __ 

11. Hydrograph shows that the water table generally declines in summer 

and fall-------------------------------------------------------------

12. Sketch of dry zone lifted up to show water table ---------------------------

TABLES 

Table 1. Use of numerical rating of well site to estimate the percent chanc~ 

Page 

8 

8 

8 

11 

Page 

1 

2 

2 

3 

3 

4 

4 

5 

5 

6 

6 

9 

Page 

of success of a well _____________ ----- _______________ ----_----- ______ --- 2 

2. Concentrations of chemical constituents and their characteristic effects on water use in the region-- ________________ --- _________________ ----------- 7 

III 

Ground Water of the Piedmont and Blue Ridge Provinces 

in the Southeastern States 

By H. E. LeGrand 

INTRODUCTION 

This circular summarizes the underground 

water conditions in the Piedmont and Blue 

Ridge provinces of the Southeastern Statesthe region shown on the geologic map (fig. 1). 

Ther e are several ways of developing water 

from the ground in this region. In earlier days 

springs were used because they are common 

in coves or on lowland slopes. Almost all 

springs in the region yield between t to 3 gallons per minute an,d rarely show a significant 

decline in yield during dry weather. Dug wells 

were common in the past, but they are being 

replaced by bored and drilled wells. Bored 

wells, like dug wells, are as much as 2 feet in 

EXPLANATION 

Igneous and 

metamorphic rocks 

Triassic 

sedimentary 

rocks 

---------

0 50 100 200 300 MILES 

Figure 1, -Generalized geologic map. Areas underlain by igneous 

and metamorphic rocks are better suited to numerical rating of 

well sites than areas underlain by Triassic sedimentary rocks, 

diameter and are commonly lined with concrete or terra cotta pipe; these wells do not 

extend into hard rock and go dry if the water 

table falls below the bottom of the well. Drilled 

wells, which are now the most common source 

of ground-water supply and which are the chief 

concern of this report, are cased to the hard 

rock and extend as open holes into the rock. 

Although some drilled wells are as small as 

2'inches in diameter and others are as large 

as 10 inches, the most common size is about 

5 or 6 inches. Almost every well in recent 

years has been properly constructed to prevent water on the ground from running down 

the outside of the casing into the well. 

EVALUATING SITES 

A special attempt is made to help those who 

are interested in the yields of wells. Because 

yields of individual wells in the region vary 

greatly within distances as short as 100 feet, 

estimates of potential yields of prospective 

wells are difficult to make. This fact has led 

frequently to water shortages, excessive costs, 

inconveniences, or undue anxiety in many 

cases. As the yield of a well is unpredictable, 

the next best approach is to attempt to show, 

on a percentage basis, the chance for a certain 

yield from a well for different conditions. 

Although many factors determine the yield 

of a well, two ground conditions, when used together, serve as a good index for rating a well 

site. These conditions are topography and soil 

thickness. The ratings are based on the following statement: High-yielding wells are 

common where thick residual soils and relatively low topographic areas are combined, 

and low-yielding wells are common where thin 

soils and hilltops are combined. By comparing 

conditions of a site according to the topographic and soil conditions one gets a relative 

1 
2 GROUND WATER OF THE PIEDMONT AND BLUE RIDGE PROVINCES IN SOUTHEASTERN STATES 

rating value . For example, the following topographic conditions are assigned point values: 

Points 

0 . . ...... ...... ... .. .. . 

2 .. .. .. .............. .. 4 .................... .. 

5 .......... .. ......... . 

7 .................... .. 

8 .. ... ......... ...... .. 

9 ................. ... .. 

12 ............ ...... .. 15 .... .... ........... . 

18 .................. .. 

Topography 

Steep ridge top 

Upland steep slope 

Pron01mced rounded upland 

Midpoint ridge slope 

Gentle upland slope 

Broad flat upland 

Lower part of upland slope 

Valley bottom or flood plain 

Draw in narrow catchment area 

Draw in large catchment area 

Figure 2 shows values for certain topographic conditions. Figure 3 shows rating 

values for soil thickness. The soil zone in 

this report includes the normal soils and also 

the relatively soft or weathered rock. The 

topographic conditions and soil conditions are 

separately rated, and the points for each are 

added to get the total points which may be 

used in table 1 to rate a site. 

Using two well sites, A and .B, as examples, 

we can evaluate each as to the potential yield 

of a well. Site A, a pronounced rounded upland 

(4-point rating for topography in fig. 2) havin g 

a relatively thin soil (6-point rating for soil 

characteristic in fig. 3), has a total of 10 

points. In table 1 the average yield for site A 

is 6 gpm (gallons per minute). This site has a 

65-percent chance of yielding 3 gpm and a 40-

percent chance of yielding 10 gpm. Site B, a 

A 4A' tL:: 12~ 

B 4 B 

I 

Figure 2. -Topographic map and profiles of ground surface showing 

rating in points for various topographic positions, 

~ IJJ 

~ t:i30 

(.)IJJ 

:I:"-

~~60 

...J 

~ 90 

POINT VALUE FOR SOIL THICKNESS 

6 8 10 12 1.5 

POINT 

VALUE 

0-2 

2-6 

6-9 

9-12 

12-15 

CHARACTER OF SOIL 

AND ROCK 

Bare rock-almost no soil 

Very thin soil-some rock outcrops 

Soil thin-a few rock outcrops 

Moderately thick soil-:no fresh outcrops 

Thick soil-no rock outcrops 

,Figure 3, -Rating in points for various conditions of soil thiclmess. 

Table 1.-Use of numerical rating of well site to estimate 

the percent chance of success of a well 

[Data are based on maximum depth of 300 feet 

or maximum drawdown of water level of 

about 200 feet. No interference from pumping is assumed. Numerical rating is obtained by adding rating in points for topography 

and soil thickness] 

Total !Aver Chance of success, in percent, for 

points age a well to yield at leastof a yield 

site (gpm) 3gpm ~Ogpm 25gpm 50gpm 75gpm 

5 2 48 18 6 2 ------ 6 3 50 20 7 3 ------ 7 3 55 25 8 3 ------ 8 4 55 30 11 3 ------ 9 5 60 35 12 4 ------ 10 6 65 40 15 5 ------ 11 7 70 43 19 7 ------ 12 9 73 46 22 10 ------ 13 11 77 50 26 12 ------ 14 12 80 52 30 14 ------ 15 14 83 54 33 16 ------ 16 16 85 57 36 18 ------ 17 17 86 60 40 20 12 

18 20 87 63 45 24 15 

19 23 88 66 50 25 18 

20 26 89 70 52 27 20 

21 28 90 72 54 30 22 

22 31 91 74 56 35 24 

23 34 92 76 58 38 26 

24 37 92 78 60 40 29 

25 39 93 80 62 43 32 

26 41 93 81 64 46 36 

27 43 94 82 66 48 40 

28 45 95 83 68 50 42 

29 46 95 84 71 53 44 

30 50 96 87 73 56 47 

30 50 97 91 75 60 50 
EVALUATING SITES 3 

LIJ 

1-

::J 

~50~--~---4-+--++--~~ r+~~T-~ :IE 

a: 

LIJ 

~40~---+----~--~~~~~--~~~~~ 

(/) 

~ ~30~--~--~~~--~---~r,~_,~--~~ <3 

z 

ci20~---+~~4-~~~~~~--~~~~~ 

...J 

LIJ 

> 

Example: 

25 30 

A site with 16 points has 3 chances in 10 

of yielding at least 30 gallons per minute 

and 6 chances in 10 of yielding 10 

gallons per minute 

Figure 4, -Probability of getting a certain yield from a well at 

different sites having various total-point ratings, 

draw or slight sag in topography (18-point 

rating) having a moderately thick soil (12-

point rating), has a total of 30 points, an average yield of 50 gpm, and a 73-percent chance 

of yielding 25 gpm. Referring to figure 4, we 

see that the 10-point site has less than 1 

chance in 10 of yielding 40 gpm whereas the 

30-point site has better than an even chance 

of yielding 40 gpm. 

Some topographic conditions of the region 

and a few topographic ratings are shown in 

figure 5. Wells located on concave slopes are 

commonly more productive than wells on convex slopes or straight slopes. Broad but slight 

concave slopes near saddles in gently rolling 

upland areas are especially good sites for potentially high-yielding wells. On the other 

hand, steep V- shaped valleys of the gully type 

may not be especially good sites, and they 

should be avoided if surface drainage near the 

well is so poor that contamination is possible. 

More difficulty is likely to occur in rating 

character of soil and rock than in rating 

Figure 5. -countryside in the Blue Ridge province showing approximate ratings for topography. 
4 GROUND WATER OF THE PIEDMONT AND BLUE RIDGE PROVINCES IN SOUTHEASTERN STATES 

f igure 6.-The soil zone is likely very thin near these rock outcrops 

(soil-thickness rating 0 to 4 points). 

topography. Everyone should be able to determine by observation if the soil is thin (less 

than 7 soil and rock points as shown in figure 

6) and if the soil is fairly thick (more than 10 

soil and rock points), but the intermediate 

ratings are difficult to make. If the observer 

is unsure of the soil and rock rating above the 

6-point (thin soil) value he may choose a 10-

point value for the site with assurance that he 

is fairly correct. White quartz of flint, which 

occurs as veins and as rock fragments on the 

ground, is not considered a true rock in this 

report because it persists in the soil zone; a 

quartz vein in many cases is considered to be 

a slightly favorable indication of a good well 

site. 

The numericalrating system is not intended 

to be precise. One person may rate a particular site at 15 points, whereas another person 

may rate it at 17 points; such a small difference in rating would not be misleading. Almost everyone's rating will be within 5 points 

of an average rating for a site. 

YIELD 

The term "yield" is not definite but is the 

reported capacity of a well to produce water, 

generally during a short pumping test. The 

water level in a well will stabilize if a certain 

limited yield or withdrawal of water is maintained; however, a greater withdrawal or yield 

will cause the water level to fall. In ru.any 

cases the water level continues to fall until 

the pumping stops so that continuous pumping 

would result in a smaller yield than that estimated earlier. The percentage oi' relative 

yield is not directly proportionate to the percentage of drawdown of the water level, but the 

greater percentage of yield is reached before 

the greater percentage of drawdown. "Figure 7 

shows an approximate relation of drawdown to 

yield for an average well in the region. Note 

that the yield- drawdown relationships of all 

wells lie within the shaded zone and that aver -

age conditions occur on or near the heavy 

line. As an example of the relation between 

yield and drawdown, we may consider a well 

220 fe et deep having a static water level of 20 

feet below land surface. (See fig. 8.) This well 

yields 40 gpm with a pumping level at a depth 

of nearly 220 feet; the pump might better be 

set at 120 feet (50 percent of drawdown or half 

the thickness of the water) where about 36 gpm 

or 90 percent of the relative yield could be 

realized. It is unnecessary and uneconomical 

to lower the water level of a well to a position 

near the bottom unless the yield is so poor 

that the water stored in the well is needed. 

There is no simple definition of the yield of 

a well-especially in the Blue Ridge and Piedmont provinces. Yields for various levels of 

the water in the pumped well are rarely 

known. The yields in this report are considered to be standard for wells about ;wo feet 

deep which are pumped about 12 hours each 

day and in which drawdown of the water level 

is about 200 feet; it is assumed that there is 

no interference by pumping from other wells, 

which would increase drawdown. 

0 d 80~-+--~-7~+ 

:;:: 

UJ 

> 

~ 60f---+~h UJ 

ct: 

o~~---L--~--L-~--~--~--L-~--~ 

0 20 40 60 80 100 

PERCENTAGE OF DRAWDOWN OF WATER LEVEL 

Figure 7. -The curve shows that an increase in yield of a well is 

not directly proportionate to an increase in drawdown of the water level. A yield of nearly 80 percent of the total capacity of 

a well results from lowering the water level only 40 percent of 

the available drawdown. 
FRACTURES IN TiiE ROCK . 5 

Well 220ft deep~----------

Static water level 20 ft 

.£e~~a:__ 

Pumping level for 

/'about 36 gpm / ; 

/ 

Pumping level for 

,..-' 40 gpm 

" 

Figure 8. -Yield of a well at two different pumping rates. 

DEPTH OF · WELLS 

How deep should a well be drilled? This 

question is not easy to answer for an individual well. In most places fractures in the rock 

get smaller and fewer with depth and deep 

drilling may not be economical. Figure 9 

shows the percentage of total yield for certain 

depths in an average well. 

The following table shows the percentage of 

wells that reach their maximum yields at certain depths below which drilling is useless. As 

Depth 

(feet) 

75 •••••••••.••.•••.•.•••.•••.•....•.•••.•••••••••••••• 

100 •••••..•••••••••••.•••••••••.••••.••••.••••••••.•• 

150 •••••••••••••··•···•••••••·•••·••••·•••••••••••••· 

200 ·•·••·•••••· ··••·••••••••••••••••••••••••••••••••• 300 .•••.•.•••••..••••••••••••••••••••••.••••••••••••• 

400 ••..••.••••.•••••••••••••••••••••••••••••••••••••• 

Percentage 

of weiis 

25 

30 

50 

70 

85 

93 

Land surface 

1- 100 w 

w 

LL. 

135 

z 

:i 160 Ill. 

w 

0 200 

300 

...J 

c( ...J 1- ...J 45 0 0 w t-...J3: 

LL~W 500>-C!l 

ww< ...Ja: 

60 ~!!! ~ t-en< 

zen 

WO::!: 

85 () Cl. 0 a: a: 

~ LL. 

90 

Figure 9. -Extent to which deepening of average well increases the 

yield. 

most of the interconnecting fractures occur in 

a zone no deeper than 150 feet below the land 

surface, it may be wise to drill no deeper than 

150 feet if the yield is very poor, or no deeper 

than 300 feet in almost all cases. 

FRACTURES IN THE ROCK 

Figure 10 illustrates six different fracture 

patterns in rocks penetrated by wells. To 

simplify the illustrations the water table and 

soil thickness are considered uniform, and 

each well, cased to 50 feet, is 250 feet deep. 

The approximate number of times each general pattern of fractures occurs in 100 wells 

is shown in percentage beneath each type. 

Well A penetrates no fractures below the casing; therefore, the well yields no water. Well 

B penetrates a fracture zone in which two or 

more fractures occur a few feet below the 

casing. This type of well is common. It may 

yield as much as 10 to 20 gpm for a period of 

several minutes until the fractures are drained. Then its yield will likely decline suddenly, and the amount of decline will depend upon 

the amount of water transmitted to the well by 

the soil and the underlying thin zone of fractured rock. That part of the well below the 

fracture zone contributes no water and acts 

only as a storage reservoir into which water 

drains. The yield of this well does not increase with increased drawdown. Well C penetrates only one fracture, a large one near the 
6 GROUND WATER OF THE PIIDMONT AND BLUE RIDGE PROVINCES IN SOUTHEASTERN STATES 

A B 

3 percent 20 percent 

/ ,!/ 

;/ 

/ 

// 

I 

/ / 

c D / 

/ 

percent 

E 

25 pe-rcent 

Figure 10. -Six types of ground conditions showing distribution of 

fractures that influence the yields of wells. The stippled pattern 

represents soil and soft rock; the dashed line is the water table. 

The degree of frequency of the different types is shown in percentage. 

top of the fresh rock. This well is similar to 

well B. It may yield considerable water for a 

few minutes until the stored water in the frac-

·mre is drained. The perennial yield, under 

continuous pumping, will depend on the permeability of the soil and weathered rock and 

on the amount of water that is released to the 

fracture. Well D penetrates several fractures, 

which contribute small amounts of water, and 

a large fracture at a depth of about 90 feet. 

WellE penetrates several small- to mediumsized fractures. These fractures are larger 

and more closely spaced in the upper part of 

the bedrock. Well_ F penetrates only one fracture-a large one below a depth of 200 feet. 

WATER TABLE 

The water table, or upper surface of the underground reservoir, continuously fluctuates 

and reflects changes in underground storage. 

During droughts we see evidence of a falling 

water table when many shallow wells go dry. 

We also can dete~t a lowering of the water 

table locally around wells from which water 

is pumped. There is a continual discharge of 

ground water by seepage into streams, by 

evaporation, and by transpiration through vegetation. The discharge causes a gradual lowering of the water table except for periods 

during and immediately after significant precipitation when recharge to the underground 

reservoir exceeds the discharge from it and 

the water table rises. Figure 11 shows the 

trends of water-level fluctuation in a well at 

Chapel Hill, N. C. The water level in this well 

is controlled entirely by natural conditions, 

and its fluctuation is typical of that in the region. There is a characteristic seasonal 

change in the water table, which begins to decline in April or May owing to the increasing 

amount of evaporation and transpiration of 

plants. In November or December, when much 

of the vegetation has become dormant, the 

precipitation first makes up the summertime 

soil-moisture deficiency and then again becomes effective in producing recharge, and 

the water table begins to rise. In a year of 

normal rainfall the recharge to the underground reservoir is approximately equal to 

the discharge from it, so that the water table 

U~t± 5 1960 1961 :I:HfJ 1962 1963 1964 1965 

Figure 11. -The water table generally declines in summer and fall 

but rises to a high level in early spring, as shown by the record of 

of this well in Chapel Hill, N. C. 
WATER TABLE 7 

Table 2.-Concentrations of chemical constituents and their characteristic effects on water use in the 1egion 

[Concentration in parts per million except as indicated. Occurrence, where noted. is given in 

parenthesis after concentrations] 

Constituents Concentration Characteristic effects on water use 

Silica (Si02) ------------ Rarely less than 15 or more Forms hard 1Scale in pipes and boilers 

than 45, commonly 20 to 35. but not normally a serious problem 

in the region. 

Iron (Fe)--------------- Commonly less than 0.3 in More than 0.3 ppm stains laundry, 

natural water, but corrosion utensils, and fixtures reddis~ brown. 

of iron pipes from water 

Calcium (Ca) and 

magnesium (Mg) 

with pH less than 6.8 causes 

a fairly common iron problem. 

Rarely less than 5 or more 

than 60 (commonly 5 to 20 

in water beneath lightcolored soils and 15 to 50 in 

water beneath dar k.-colored 

soils). 

Bicarbonate (HCOs) ----- Rarely less than 15 or more 

than 150. commonly 30 to 

100. 

Cause most of the hardness and scaleforming properties of water. (See 

hardness below.) 

Concentrations in region are not generally high enough to cause trouble. 

Sulfate (S04 ) ____________ Rarely less than 1 or more Concentrations in region are not genthan 100. commonly 1 to 40. erally high enough to cause trouble. 

Chloride (Cl) ----------- Rarely less than 1 or more Salty taste to water having mere than 

than 40, commonly 1 to 20. a few hundred parts per minion. 

Fluoride (F) _ _ _ _ _ _ _ _ _ _ _ _ Rarely more than 1. commonly Concentration between 0. 6 anc~ 1. 7 ppm 

0.0 to 0.6. in water retards decay of te:~th. but 

amounts in excess of 1.5 ppm may 

Nitrate (N03) ----------- Rarely more than 20. commonly less than 10. 

cause mottled enamel of tee:-4:h. 

Where concentration is greate:r than 

20 ppm, contamination frorr. sewage 

may be suspected. Water of concentrations greater than 45 ppm 

may be harmful to babies. 

Dissolved solids -------- Total of all mineral matter Water containing more than 1,000 ppm 

rarely exceeds 2 50. commonly of dissolved solids is unsuitable for 

70 to 150. most purposes. 

Hardness as equivalent 

CaCOs --------------- Rarely less than 10 or more 

than 150 (commonly·1o to 

50 in water beneath lightcolored soilc:: and 40 to 200 

in water beneath darkcolored soils). 

pH -------------------- Rarely less than pH of 5.5 or 

more than 7. 5 (commonly 

5.5 to 6.8 in water beneath 

light -colored soils and 6. 8 

to 7. 5 in water beneath 

dark-colored soils). 

Causes consumption of soap bo.fore 

lather will form. Hard water forms 

scale in boilers and hot water 

heaters. Water whose hard'less is 

less than 60 ppm is considered soft; 

61 to 120 ppm. moderat~ly hard; 

121 to 180 ppm. hard; more than 

180 ppm, very hard. 

Values less than 7.0 indicate acidity, 

and corrosiveness of water generally increases with decreasing 

pH. 
8 GROUND WATER OF THE PIEDMONT AND BLUE RIDGE PROVINCES IN SOUTHEASTERN STATES 

at the end of the year is at about the same 

level as at the beginning of the year. Wells 

drilled into rock may, when pumped at full 

capacity, yield slightly less during the driest 

part of the year when the water table is low. 

Yet there appears to be no evidence to support 

the general belief that the water table has 

been declining during recent years. 

CHEMICAL QUALITY OF THE WATER 

In comparison with ground water in widely 

scattered regions of the world, the water in 

the Piedmont and Blue Ridge provinces ranks 

among the best in chemical quality. (See table 

2.) Most of the water is low in total dissolved 

solids and is generally soft, but some is moderately hard. 

Iron in water is the most common complaint. As little as 0.4 ppm (parts per million) 

will cause a red stain on plumbing fixtures. 

About 5 of every 10 wells yield water with 

less than 0. 3 ppm of iron. About 4 of 10 wells 

yield water with just enough iron to cause a 

slight stain, and about 1 of 10 wells yields water that has considerable iron. Some iron 

problems result when iron is dissolved from 

rocks, and other problems result when water, 

moving through iron pipes, consequently picks 

up a brown iron stain by corrosion. It is important to determine the source of the iron, 

whether dissolved from the rocks or from the 

pipes, before methods for its removal are 

employed. Most of the water is satisfactory 

for use without any type of treatment (table 2). 

Yet an analysis of the water should be made 

as soon as a well is drilled to determine if 

treatment is necessary. It is not possible to 

determine the quality of water before a well 

is drilled. 

CONTAMINATION OF GROUND WATER 

In view of the many hundreds of thousands 

of wells that are interspersed with about an 

equal number of septic tanks and other waste 

sites, it is proper to give serious attention to 

the possibility of contaminating an individual 

water supply. The tendency for ground waterand contaminants that might be in it-to move 

naturally from upland areas toward stream 

valleys offers help in planning wells and waste 

sites to avoid contamination. A well that is 

pumped may modify the natural movement of 

water and draw contaminated water toward it; 

this condition is more likely where the soil is 

thin or absent than where it is thick. Care 

should be taken to see that no water from the 

land surface can seep easily into the well 

around the casing. Not only is thP. well site 

important but so is the waste site. In most 

casec the chances of contaminated water from 

a waste site moving into a well are not easy 

to predict, but a few general statements can 

be made. For example, at a waste site (1) a 

deep water table is safer than a shallow water 

table, (2) thick soil is safer than thin soil or 

rock outcrops, (3) sandy soil with some clay 

may be better than a clean sandy soil or a 

sticky clay soil, and (4) a slope of both the 

land surface and the water table away from a 

well is better than one toward it. 

The soil and weathered rock ar~ generally 

effective in preventing waste materials from 

passing through to underlying rock fractures, 

but the combination of (1) certain types of 

wastes, (2) excessive quantities of disposed 

wastes, and (3) thin soils may res·.tlt in contaminated water reaching bedrock fractures. 

Once in the bedrock fractures tho. contaminated water may move easily to water supplies. Only a small percentage of wells have 

been contaminated, but proper care in locating 

and constructing wells and waste sites must 

be taken to minimize the risk of contamination. Minimum standards specified by health 

officials, such as those relating to permeability of the soil, distance between a well and a 

waste site, and depth of the water table, must 

be followed. 

GENERAL STATEMENTS ABOUT GROUND WATER 

IN THE REGION 

1. Ground water may be considered as occurring in an underground reservoir, the water being held in the open spaces of the rock 

materials. The water table, representing the 

top of the reservoir, generally lies in the clay, 

or disintegrated rock materials. In the lower 

part of the reservoir, water occurs in interconnecting fractures in bedrock; the fractures 

diminish in number and size with increasing 

depth. Water enters the fractures by seeping 

through the overlying clay, and drilled wells 

draw water from these fractures. The source 

of this water is precipitation in the general 

area of a well and not in some rer::wte place. 

2. A layer of residual soil and weathered 

rock lies on the fresh rock in most places;' 

the thickness of the soil and weatl'o.red rock 

ranges from zero to slightly moro. than 150 

feet. 
GENERAL STATEMENTS ABOUT GROUND WATER IN THE REGION 9 

Figure 12. -Dry zone (above water table) lifted up to show water 

table or surface of saturated zone. Movement of water (arrows) 

tends to be downward in the dry zone and lateral toward streams 

in the saturated zone. 

3. The water table has a hill and valley relation that approximately conforms with surface topography, although the water table is 

somewhat flatter. (See fig. 12.) For example, 

a creek or river is the surface expression of 

the water table in a valley, but beneath a hill 

the water table may be 30 to 70 feet below the 

ground surface. Ground water, like surface 

water, has the tendency to drain away from 

the hills to the valleys. This tendency helps 

in planning the location of wells in relation to 

other wells and to sources of possible contamination. 

4. A close network of streams prevails, and 

in most places on an upland area a perennial 

stream is less than 1 mile away. 

5. Toward the streams is a continuous flow 

of ground water. Some of the outflowing 

ground water is used up by evaporation and by 

transpiration of plants in the valley areas; the 

remainder of the water discharges as small 

springs and as bank and channel seepage into 

the streams. 

6. The natural movement of ground water is 

relatively short and is almost everywhere restricted to the zone underlying the gross topographic slope extending from a particular 

land-surface divide to the adjacent streams. 

7. In ideal cases the pumping of a well 

causes the water table to be depressed smoothly in the shape of an inverted cone, the apex of 

the cone being in -the w~ll; however, the erratic distribution of rock fractures and the 

contrasting nature of permeability between 

rock fractures and overlying soils cause the 

depressed part of the water table to extend 

unevenly around a well. Where two heavily 

pumped wells are within a few hundred feet of 

each other, there is a strong likelihood of 

some interference of pumping level between 

the two, but in most cases there is not any appreciable interference between low-yielding 

wells a few hundred feet apart. From a pumped well the depressed part of the water table 

rarely extends beneath a perennial stream or 

beneath a hilltop to a slope on the opposite 

side. Well interference is local, and there is 

no regional lowering of the water table because of pumping. 

8. The relation of the depth of a well to 

yield of the aquifer is not simple. In spite of 

some beliefs, water already available to a well 

is rarely lost by drilling deeper; therefore, 

there is always a chance of getting a larger 

supply by increasing the depth of the well. Yet 

this chance becomes poorer as the well deepens because the interconnecting fractures and 

the ability of the rocks to store and transmit 

water decrease significantly with depth. More 

than 90 percent of all ground water occurs in 

the first 100 feet below the water table. Generally two wells 200 feet deep each will yield 

more water than one well 400 feet deep. 

9. The relationship of topography to yield 

is emphasized. The great majority of wells 

are located on hills or smooth upland slopes 

because of convenience and because these locations appear safe from sources of contamination. Yet the percentage of low-yielding 

wells is much greater on hills and upland convex slopes than in lowlands or draws (concave 

slopes that lead upward from a valley to a 

saddle or sway-backed position in a ridge). 

Steep-sided depressions, such as gullies and 

ravines, should not be considered acceptable 

sites for wells. 

10. In general, wells are more productive 

and tend to have a more stable year-round 

yield where there is a thick mantle of soil than 

where bare rock crops out. The presence of a 

soil cover and the absence of rock outcrop 
10 GROUND WATER OF THE PIEDMONT AND BLUE RIDGE PROVINCES IN SOUTHEASTERN STATES 

suggest that water moves downward into the 

rock and is not readily shunted toward the adjacent valley; in fact, the soil cover suggests 

that interconnecting rock fractures are available to store water and totransmit it to wells. 

Where there is a good soil cover, the water 

table generally lies in it; therefore, the storage capacity in the vicinity is much greater 

than where bare rock is exposed and where 

the only water in storage is in the rock fractures that might be quickly drained. 

11. Simple clear- cut statements about the 

water-yielding properties of the various types 

of rocks are not easy to make. There are 

many varieties of igneous and metamorphic 

rocks, but for a discussion of their groundwater properties they may be grouped as follows: (1) Somewhat massive igneous rocks, 

such as granite, and (2) metamorphic rocks, 

such as schists, gneisses, and slates, which 

may show an alinement of minerals or an 

alinement of cleavage planes or openings 

along which water may move. In some places 

a type of rock may have distinctive waterbearing characteristics, but, if so, it is also 

likely to show distinctive topographic and soilmantle features. Topography and soil-mantle 

features are readily observed and may be 

used as criteria for predicting the wateryielding potential of a well site, whereas the 

water-bearing characteristics of a type of 

rock by itself may be obscure. At any rate, 

there are too many complex factors involved 

to justify generalizations about the yield of 

wells in individual types of rock. 

12. Whenever water is pumped from a well, 

the water level is lowered in and around the 

well. The drawdown increases with an increase in the rate of pumping, although this 

relation is not simple. For example, a well 

yielding 20 gpm with a drawdown of 50 feet 

will not double its yield by increasing the 

drawdown to 100 feet. Instead, it will yield 

less than 40 gpm and perhaps no more than 25 

to 30 gpm with a drawdown of 100 feet. 

13. Some wells that are pumped heavily 

tend to decline gradually in yield. This fact 

may be due to the following circumstances. 

The size and setting of a pump are determined 

from a short bailer or pumping test when the 

well is completed. Such a short test may not 

indicate the long-term yield of the well because the first water is withdrawn form storage in the rock materials, and many hours, 

days, or even months may pass before there 

is a stable adjustment between the amount of 

water that the frac:tures can feed· into the well 

and the amount of water available to drain 

through the overlying clay into the fractures 

feeding the well. Failure to have knowledge of 

water-level fluctuations as a result of pumping is the cause of many well problems and of 

the erroneous conclusion that well supplies 

are not dependable. If a well tends to have an 

unstable yield, it is probably overpumped. A 

reduction in the rate of pumping and consequently a raising of the water level will result 

in a perennially safe yield. Constant pumping 

at a moderate rate does not damage a well. 

14. There is a tendency for rocks underlying a light- colored. soil to yield water that is 

low in dissolved mineral matter and is soft. 

On the other hand, rocks underlying darker 

soils (dark red, brown, and yellow) tend to 

yield water that is slightly hard, or hard, and 

that may contain objectionable amounts of 

iron. 

15. Many people think that a shallow depth 

to the water table is an indication of a good 

yield of a potential well, but this is not a rule 

to follow. In fact, where the water table is 

only a few feet beneath the land surface on an 

upland area, the rock fractures may be so 

scarce that water may not be able to move 

downward in the rock; it is held near the 

ground surface and perhaps is shunted out to 

the land surface as a wet seepage spot on a 

steep slope. 

16. There are many mistaken notions about 

the availability of ground water in the region. 

These notions arise from lack of knowledge of 

the occurrence and movement of ground water 

and of the behavior of wells. The commonerroneous statement that a certain town in the 

region could not depend on well water stems 

from the existence of a limited number of 

wells; never has the underground reservoir 

beneath any town or city in the region been 

completely depleted of its water. There has 

been a tendency for towns of about 2, 000 

people to convert from well supplies to a 

treated surface-water supply; such conversion 

commonly occurs when the town requires 

more than 500,000 gallons of water per day, 

an amount which only a few wells in aggregate 

may not produce. Few towns have the experienced persons with diversified knowledge of 

wells and ground-water conditions to provide 

the good management comparable to that of 

municipal surface-water supplies. 
GROUND WATER OF THE PIFDMONT AND BLUE RIDGE PROVINCES IN SOUTHEASTERN STATES 11 

SOURCES OF INFORMATION 

There are many sources of information 

about ground-water conditions in specific 

parts of the region. At least one agency in 

each State has cooperated financially with the 

U.S. Geological Survey, and these agencies 

GP0920·198 

have contributed in some way to the ref\ults of 

this report. Further information about reports 

published or work in progress may be obtained from the district offices of the Geological 

Survey in each State or from the res":lective 

State cooperating agencies.