Ground water of the Piedmont and Blue Ridge provinces in the Southeastern States

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GEOLOGICAL SURVEY CIRCULAR United States Department of the Interior

ROGERS C. B. MORTON, Secretary

Geological Survey

V. E. McKelvey, Director Introduction ____ ------ __ -------______ Evaluating sites--------------------- Yield ------------------------------ Depth of wells----------------------- Fractures in the rock---------------- Water table-------------------------

2. Concentrations of chemical constituents and their characteristic effects on
3. Topographic map and profiles of ground surface showing rating in
4. Graph showing rating in points for soil thickness--------------------------
5. Graph showing probability of getting a certain yield from a well at
6. Photograph of countryside in the Blue Ridge province showing approximate
7. Photograph showing area where soil zone is likely very thin _______________ _
8. Curve showing relation of yield to drawdown ------------------------------
9. Diagram showing yield of a well at two different pumping rates ____________ _
10. Diagram showing extent to which deepening of average well increases yield __
11. Sketches of sixctypes of ground conditions showing distribution of fractures __
12. Hydrograph shows that the water table generally declines in summer
13. Sketch of dry zone lifted up to show water table ---------------------------

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

water use in the region-- ________________ --- _________________ ----------- points for topographic positions ---- ____ --------- _______________ ------ _

different sites - ___ ---- _ --------- ____ ------------ _ -------------------- ratings for topography __________________________________ ----- ________ _

TABLES Chemical quality of the water ________ _ Contamination of ground water _______ _ General statements about ground water in the region-------------- __ Sources of information ______________ _

By H. E. LeGrand

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 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.

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 Table 1.-Use of numerical rating of well site to estimate the percent chance of success of a well

A 4A'

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 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

Figure 5. -countryside in the Blue Ridge province showing approximate ratings for topography.

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.

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 betwee n 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.

~ 60f---+~h Well 220ft deep~----------

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.

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 w

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 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.

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 veg-

etation. 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

c

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

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.

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

Silica (Si0 2) ------------ Rarely less than 15 or more

Iron (Fe)--------------- Commonly less than 0.3 in

Calcium (Ca) and magnesium (Mg)

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

Chloride (Cl) ----------- Rarely less than 1 or more

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

Dissolved solids -------- Total of all mineral matter

Hardness as equivalent CaCOs --------------- Rarely less than 10 or more

pH -------------------- Rarely less than pH of 5.5 or parenthesis after concentrations]

than 45, commonly 20 to 35.

natural water, but corrosion of iron pipes from water 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).

than 150. commonly 30 to 100.

than 100. commonly 1 to 40.

than 40, commonly 1 to 20.

0.0 to 0.6.

monly less than 10.

rarely exceeds 2 50. commonly 70 to 150.

than 150 (commonly·1o to 50 in water beneath lightcolored soilc:: and 40 to 200 in water beneath dark-colored soils).

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).

Characteristic effects on water use

Forms hard Scale in pipes and boilers but not normally a serious problem in the region. More than 0.3 ppm stains laundry, utensils, and fixtures reddis~ brown.

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

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

Concentrations in region are not generally high enough to cause trouble. Salty taste to water having mere than a few hundred parts per minion.

in water retards decay of te:~th. but amounts in excess of 1.5 ppm may 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. Water containing more than 1,000 ppm of dissolved solids is unsuitable for most purposes.

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.

CONTAMINATION OF GROUND WATER

3. The water table has a hill and valley re-

lation 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 ap - preciable 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 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 ground-water 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 water-bearing characteristics, but, if so, it is also likely to show distinctive topographic and soil-mantle features. Topography and soil-mantle features are readily observed and may be used as criteria for predicting the water-yielding 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 under-

lying 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.

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 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.

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