WATER-BEARING CAPACITY AND WELL YIELD

2.2.3.1 Introduction

The site-specific occurrence of ground water in hard rocks is often difficult to predict. The ground-water production potential from such rocks is considered to be generally low, but production can vary from very low (or nil) to very high sustained production rates, that is, greater than 68 m3 /hr (Ellis, 1909; Stewart, 1962 b). Limited and incomplete knowledge of those geological conditions that have combined to yield ground water at relatively high production rates have been major constraints to cost-effective ground-water exploration and development in such rocks.

Table 2.2.3.1 Reported yields from wells in hard rocks of Africa (United Nations, 1973)

Ethiopia 1000-2000 Sidamo Granite, gneiss Weathered layers poorly developed

and schist LOW Yields from fractured zones low

Africa 400-15ao basement area and schists 0.5-10 Detected by electrical resistivity

surveys. About 50 % of the bore-

Zambia 500-1500 Kalomo-Choma Weathered granite- 3-5.5 35

gneiss, quartz veins

Table 2.2.3.2 Reported yields from wells in hard rocks of the Western hemisphere, India and Korea drilling, augmented occasionally by preliminary geological evaluations. Drilling has been conducted in many areas underlain by hard rocks, and voluminous data resulting from such drilling indicate a broad range in the productivity of economic drilling depth. This principle is followed in many countries.

2.2.3.2 Typical well yields in hard rock areas

Typical well yields in hard rock areas of Africa, India, Korea and parts of the Western Hemisphere are indicated in Tables 2.2.3.1 and 2.2.3.2. When significant production was encountered, the geological conditions (fractures, joints, faults, etc.) that permitted such productivity were considered too complex to be evaluated quickly and were con- sidered simply a matter of good fortune. The general impression was that such productivity could not have been fore- cast because of the complexity of the geological conditions. However, the type of systematic investigations discussed in the present work demonstrates that the regional and local structural history of igneous and metamorphic rocks in selected regions, combined with various surface geophysical and geochemical techniques, can serve to identify permeable zones potentially capable of significant ground-water production.

These methods could increase the rate of success and consequently the cost effectiveness of such programmes. The ramifications of the investigations discussed in the present work are significant in that vast areas of the earth’s surface are underlain directly by hard rocks.

2.2.3.3 Types of fractures

Unweathered hard rocks generally have porosity of less than one per cent (Davis and Dewiest, 1966) and frequently this is discontinuous or ineffective pore space. Their permeability is therefore low as well.

Fracturing, either associated with regional deformation as discussed in Section 2.1.2, or weathering (Section 2.1.3) may create significant porosity and permeability in these rocks and is alone responsible for their ground-water potential.

Based on investigations in the United States, the frequency of occurrence of fractures in crystalline rocks has generally been found to decrease with depth (Davis and Dewiest, 1966; Davis and Turk, 1964; Landers and Turk, 1973; and Legrand, 1967). However, underground mines have encountered heavy flows of ground water hundreds of metres beneath the surface, indicating that some fractures extend to great depths ((Hurr and Richards, 1966; Snow, 1968a, 1968b; Wahlstriim and Horback, 1962). Robinson (1976) suggests that two zones are present, that is, an active and an underlying passive zone. Therefore two types of fractures have been combined in previous evaluation programmes of ground-water productivity in ingneous and metamorphic rocks: (1) fractures related to weathering and unloading, and (2) fractures related to regional tectonics. Marine (1966, 1967) has also noted two types of fractures in hard rocks of Georgia, that is, “fine” and “open” fractures.

The direction of ground-water movement in saturated fractures is frequently difficult to establish because of the nature of these openings and their possible relationship to fracture pattern in general.

It should be emphasized that igneous and metamorphic rocks having a hard crystalline texture in the outcrop do not always extend into the subsurface in the same condition. A variety of structural and compositional differences of the

Figure 2.2.3.1 Vertical intersection of joints for various dips (Karanth, 1973).

Horizontal spacing of joints is assumed to be 1 metre.

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rock appear at depth. These differences are generally associated with structural features such as faults, or other chemical or contact features which can occur between two rock units of dissimilar composition. Since secondary mechanical disintegration produced by tectonic phenomena, horizontal or sub-horizontal sheet-jointing produced by unloading, and chemical decomposition produced by diagenetic alteration are associated with such factors, the result may be permeable zones ranging from a few centimetres to a few metres in thickness (Jahns, 1943; LeGrand, 1949 and 1952; Reed et al., 1963). However, Lachenbruch (1961) indicates that many joints or fractures caused by tension at or near the ground surface die out rapidly with increasing depth. This is apparently related to unloading and is a near-surface phenomenon to which most occurrences of ground water in igneous and metamorphic rocks have been related.

For a given spacing of fractures, the probability of intersecting a fracture while drilling a well decreases with the increase in fracture dip magnitude, being maximum in rocks with sheet fractures and minimum in vertically fractured rock (Karanth, 1973). The chances of intersecting fractures decrease rapidly when the fracture dip exceeds 70” (Figure 2.2.3.1). To optimize well yield, drilling, ideally, should be at right angles to the attitude of the principal fracture system in the area of the greatest fracture frequency.

2.2.3.4 Effect of fracturing on ground-water production

In order to evaluate the effect of near-surface fracturing on ground-water production, welldepth relationships are here reviewed for a number of rock types and locations in the United States and Korea. Davis and Turk (1964) present the results of an evaluation of productivity from granite and schist in the eastern United States; granodiorite or closely related igneous rocks in the Sierra Nevada area of California; amphibolite and granite from injection tests at the Oroville dam site and other sites in California; and for wells in miscellaneous metamorphic and igneous rocks in California (Figures 2.2.3.2 - 2.2.3.6). Summers (1972) also presents similar data for wells in a variety of rock types from a 15.5 km2 area in the Rotschild region of Wisconsin which includes nepheline syenite, quartz syenite, granite, gabbro- diorite, “greenstone,” felsite, schist and rhyolite. (See Figure 2.2.3.7 and 2.2.3.8.)

2.2.3.5 Relationship between well yield and well depth

Figures 2.2.3.2 to 2.2.3.6 show the relationship between well yield per unit length of well penetration in the aquifer, or the water injection rate per vertical unit length of well as functions of well depth. All the graphs shown indicate that production generally decreases with depth. It should be noted that the scatter of the points plotted is large and may be due to one or more of the following reasons:

(1) number of fractures penetrated by the individual wells;

(2) differences in the type of fractures encountered;

(3) inaccurately reported data;

(4) variations in rock type.

Well yield in myh per meter of aquifer

Welllyield in GPM perfoot of aquifer

Figure 2.2.3.2 Yield of wells in crystalline (hard rocks of eastern United States. Open circles represent mean yields in granite rocks for 814 wells. Black dots represent yields in schistose rocks for 1522 wells.

(Modified after Davis and Turk, 1964.)

Well yield in myh per meterof aquifer

1

\

\

\

101 ' I I I I

0.01 0.05 0.l 0.5 1.0 - 3.0

Well yield in GPM per foot of aquifer

Figure 2.2.3.3 Yield of wells in crystalline (hard,) rock of Sierra Nevada, California.

Open Circles represent mean yields. Black circles represent median yields both in granodiorites or related rocks (modified after Davis and Turk, 1964).

Watertake in m3/h per meterof aquifer

1000 0.007 0.04 0.07 0.36 0.76 3.70

' '\ '\I 1, I I , (304.8

90% tests have

water take below

Figure 2.2.3.4

101 1 I I '\ , , .'\ '\ 1 3.0

0.01 0.05 0.1 0.5 1.0 5.0

Watertake in GPM perfoot of aquifer

Water injection data from Oroville Dam site, California. Based on re- sults of 385 injection tests in amphibolite or related rocks (modified after Davis and Turk, 1964). ’

Water take in mj’h per meter of aquifer 1000 “.“,“’ 0.04 0.07 0.36 0.76 3.70

'\ I x

\' I I 304.8

'\ \

500- \ \ \ '\ - 1524

0.05 0.l 0.5 1.0

Watertake in GPM perfoot of aquifer

Figure 2.2.3.5 Water injection data from California. Based on results of 4 12 injection tests in granitic rocks (modified after Davis and Turk, 1964).

55

Watertake in myh per meterof aquifer

0.007 0.04 0.07 0.36 0.76 3.70

Watertake in GPM perfoot of aquifer

Figure 2.2.3.6 Water injection data from California. Based on results of 494 injection tests in serpentine, slate, phyllite, gabbro and other miscellaneous rock types (modified after Davis and Turk, 1964).

2.2.3.6 Effect of rock type

The effect of the rock type on the relationship between well yield and well depth in the Llano area of Texas is shown in Figure 2.2.3.9. A comparison of the well yield and well depth in granite rocks in other regions of the United States is shown in Figure 2.2.3.10. In the former graph, the overlying weathered material (“grus”) is shown to be a significant contributor. In the latter, the plot of the Llano area declines more rapidly with depth than the plots of the other regions.

Landers and Turk (1973) suggest that climate may have a significant role in the occurence of ground water in crystalline rocks. Different rates of formation of weathered material (grus) may be related to rainfall (Bannermann,

1973). The Llano area has the driest climate followed by the Sierra Nevada and eastern United States (see Figure 2.2.3. IO). Th,e effects of climate on ground water have been discussed earlier in Section 2.1.3.

IOOO- --- Grus -3048

.-.-.-.-. F,oct”red granite -- Other fractured rocks -66nelss

0 SChlS,

0.007 , 0’07 017

V&It y&d m miyh per meter of aquifer

Figure 2.2.3.9 Yield of wells versus depth of well in crystalline rocks of the Llano area, Texas. Based on 848 wells in grus, granite, gneiss, schist and other fractured rocks (modified after Landers and Turk, 1973).

Callahan and Choi (1,973) present a comprehensive review of a drilling programme in crystalline rocks, undertaken in Korea during the period 1966- 197 1, Figures 2.2.3.1 - 2.2.3.14 summarise the result of that programme.

Well yield in GPM per foot of aqwfer

001 01

1

p/’

lo#O- - 304.8

--LLlono oreo,Texos

--.- -Slerro Nevado,Col,forn,o - -O- ---Eastern Ul>lted States

Figure 2.2.3.10 Yield of wells versus depth.of well in granitic rock aquifers of various regions in the United States (modified after Landers

and Turk, 1973). ₅₇

Yield in myhour per meter of aquifer

Yield in myhour per meterof aquifer