groundwater exploration in hard rock terrain: an...

The Hydrological Basis for Water Resources Management (Proceedings of the Beijing Symposium, October 1990). IAHS Publ. no. 197,1990.

Groundwater exploration in hard rock terrain: an experience from eastern India

G. K. DEV BURMAN* Central Ground Water Board, Kasal Subamarekha (UNDP) Project, Jamshedpur, Bihar, India

P. K. DAS CGWB, Eastern Region, 24 Park Street, Calcutta 700016, WB, India

Abstract The results of systematic groundwater exploration have clearly indicated that normally the weathered mantle is the principal water bearing zone within PreCambrian crystalline rocks. It acts as a primary source of recharge to the underlying aquifers under natural conditions and also during pumping with characteristic time variation. The productivity of the saprolite formations and the deeper fractures depends on the degree and manner of hydraulic connections with the top saturated section of the weathered mantle. The yield generally decreases with depth, though initially the saturated deeper fractures produce high discharge during the drilling operation. The yield from the saprolite zone, meagre during drilling, improves considerably during regular pumping. Organizations engaged in tapping groundwater in such terrain are easily misled by the preliminary yield picture of such structures, often resulting in fruitless expenditure.

Considérations pratiques concernant la prospection en eau en terrains rocheux: une expérience de l'Inde de l'Est

Résumé Les résultats de sondages symstématiques pour la recherche d'eau souterraine, ont clairement indiqué que le manteau d'altération est la principale zone aquifère dans les roches cristallines datant du précambrien. Ce manteau agit comme source primaire de recharge pour les nappes d'eau souterraines, dans des conditions naturelles ainsi que lors de pompages, avec des variations caractéristiques dans le temps. La productivité des formations de saprolite et des fractures profondes dépendent du degré et du type de connexions hydrauliques avec la couche supérieure saturée de la couche d'altération. Le rendement diminue généralement avec la profondeur, bien qu'initialement les fractures saturées inférieures aient un haut rendement durant les opérations de forage. La production des

* Now at: Central Ground Water Board, North Central Region, E-5/40, Arera Colony, Bhopal 462016, MP, India.

19

G. K. Dev Burman &P.K. Dus 20

zones saprolite, faible durant les forages s'améliore considérablement lors du pompage. Les organismes chargés de la prospection en eau dans ces terrains sont souvent trompés par la production initiale de telles structures, ce qui conduit souvent a des dépenses improductives.

INTRODUCTION

The main objectives of groundwater exploration in the PreCambrian hard rock terrain of the Kasai-Subarnarekha basins in east India were to determine the terrain's groundwater resource and develop the most suitable abstraction techniques for its optimal utilization. In order to achieve the goal the study of the depth, persistence and pattern of the different saturated weathered and fracture aquifers/layers and their hydraulic characteristics under different geomorphic and geological setups was undertaken down to 300 m. For this purpose 14 well fields, each having one or two observation wells, were drilled in different hydrogeological setups. This paper deals with different hard rock aquifers at different depths and their hydraulic behaviour.

GENERAL GEOLOGICAL SETUP

The area is roughly bounded by coordinates: 22°00 ' -23°30 ' N and 85°05'-87°00'E, occupying parts of Bihar, Orissa and Bengal states in eastern India, and is underlain by granites, metasediments and metavolcanics of PreCambrian age (Fig. 1).

Stratigraphically and also to some extent geographically, the metasediments are sandwiched between the older and younger PreCambrian granitic rocks. Underlying these metasediments is the older Singhbhum granite, the basement unit of the area. It is intruded by numerous basic dykes in criss-cross fashion. The metasediments of different groups include the talc-chlorite schist, garnetiferous mica-schist, quartzite, phyllite, chlorite-biotite-muscovite schist etc., and are associated with metavolcanics of basic composition. The youngest crystalline rock is the Chotanagpur granite. The other granite exposures (Chakradharpur, Kuilapal etc.) are also present within the metasediments.

BEHAVIOUR OF THE AQUIFER SYSTEM

Persistence with depth

A close examination of the drilling data indicates that the occurrence of saturated aquifers varies from a depth of 12.66 to 230 m but is generally restricted to 140 m. The presence of unsaturated fractures has been detected down to 285 m depth (Fig. 2). The unsaturated fractures are of two types -one is non-productive and the other completely dry. The non-productive ones are the fractures which bear the mark of the passage of water and air

21 Groundwater exploration in hard rock terrain

86

Km 10 0 10 20 Km

index ft- + + + \ -f "Vr i

CHOTANAGPUR GRANITE " \ . "*• + + l + f

KOLHAN FORMATION

S s t . L i l . Phyl iK)

KOIRA GROUP OF META SEDIMENTS

' . % CHAKRAOHARPUR GRANITE,KUILAPAL GRANITE

PROJECT AREA '

H +

+

DALMA.DHANJORI, Oogarbiro metavolcanics DHANJORI /SIMUPALjGHATSILA ond BAOAMPAHAR GROUP OF METASEDIMENTS

StNGHBHUM GRANITE

EXPLORATORY DRILL SITE

86 I O

Fig. 1 General geological setup.

8 7

(oxidation marks), but raising no recoverable discharge. Another type of fracture, though rare in occurrence, is encountered at depth, which indicates more or less a perched type of isolated situation. When such' fracture zones are encountered, they yield only for a short initial period after which they tend to be dewatered.

The general range of occurrence of saturated aquifers has been discussed

G. K Dev Burman &P.K. Das 22

DORANDA

- 126 321

: 35-1 0-25_ 1-67 "

: ms

PALANOU •6-

0 8 3 , 0 8 3

CHAKRAOHARPUR -fr

BUNDU -6.

. 732 ' 77-0 • 92-7 .

•1220 1330

-1440

1620

8795

MANBAZAR

HESEU -A-

4 8 0 0 . 2 5 . 6 4 0

• - 8 4 0 0 - 3 4 -• - 9 4 0

2 I 0 _ 5-40-

1770-1790

20942

KUDADA -6-

— 2 6 0 —• 39-0 — 59-0 — 70-0 — 83-0

-105-0 0-60-J 3 I 0 | i . s o _ - 1 4 0 0 ~

-1600

-56-0 - 7 6 0

~ z W e 3 " » 2

MAHUUA(ULDA) -A-

• 41-90

- 66-70

MATGODA -A-

0-75 = = 4 8-36-49-36 -- 6 3 0

3 0 0 0 - 3 1 0 0

3 7 0

- — 9 9 - 0 10-0

/»)»w 198-75

— 4— 9 4 0

— 1170

-105-0 -1170 9 . l 6

-1370 -139-2

145-41

285-0 7777^300 0

EXPLORATORY WELL (EW1

SATURATED FRACTURE W W 3 7 - 9 7 DISCHARGE (Ips) 1 DEPTHlmbgll

UN-SATURATEO FRACTURE | ~ - OEPTH(mbql)

-1290

,/,),» TOTAL OEPTH(mbgl)

^ * î ^ 2 0 4 - 7 5

Fig. 2 Total fracture system encountered by exploratory wells at depth.

above. But the Iithological characteristics, antiquity of rock mass, and their structural features may have significant control over < the behaviour of the saturated aquifers. The total hard rock area may be broadly divided into two groups: granitic and non-granitic metasedimentaries. The depth persistence of the hard rock aquifers occurring within the granites and metasediments is discussed below.

The granites and the aquifers The granitic rocks may be divided into the upper PreCambrian Chotanagpur and Chakradharpur granites and the older (Singhbhum) granites of the lower PreCambrian.

It has been observed that in the younger granites saturated zones are restricted down to 110 m while in the older granites, saturated fractures are encountered between 100 and 140 m depth. Depending on the depth range, the total aquifer system, besides the top weathered mantle, may be divided into two groups, the saprolites and the underlying saturated fractures in the bedrock.

The saprolite zone is partly weathered but retains its original features, like joints, fractures etc. Actually it is the transitional zone between the weathered mantle and the fresh bedrock. The average depth of occurrence of this.zone is 16 m in younger granites while in older granite it is 12 m below ground level (b.g.L). The maximum thickness of the zone is 8 m with an average thickness of 4 m. Normally the deep and prolonged process of weathering facilitates formation of a thick saprolite zone. Chemical composition of the rocks may also determine the extent of rock


decomposition and thickness of the saprolite zone. Drilling data have clearly revealed that the younger granites which are more acidic in nature, with an abundance of free silica such as quartz, always form thicker (5-8 m) saprolite zones. While the older Singhbhum granite with calc-afinity and much less free silica, forms thin zones of saprolites. The prominent saprolites in younger granites have good water-bearing zones and the yield sometimes goes as high as 2.66 1 s"1, which is about 60% of the total yield of a successful borewell in this terrain.

Bedrock fractures are divided into shallow and deep fractures, depending on the depth range. Shallow fractures occur below the saprolite zone and are restricted to a depth of 50 or 60 m. Fractures, which are tapped by shallow borewells fitted with hand pumps have been termed as shallow fractures. In the younger granites, these shallow fractures are quite prominent and form good water-bearing zones. Deep fractures occur below 50 or 60 m. These saturated fractures are more or less restricted down to 110 m depth with a maximum individual discharge of 3 1 s"1 in the younger granites. In the older granites, the prominent zones are generally encountered from 100 to 140 m depths with maximum individual yield of 6 1 s"1.

The occurrence of deep fractures in the Singhbhum (older) granitic mass may be related to numerous dyke intrusions along the macrofractures in this area, because all the saturated deep fractures have been found along the dyke-granite contact plains. The dykes being deep seated and having numerous primary fractures, at least in the upper parts and along the contacts with the country rocks, may have allowed deeper infiltration of groundwater. In this area the dyke itself acts as an aquifer system. Dykes have been observed to sustain a number of good dugwells and shallow borewells located on or adjacent to the dykes. Hence the deep exploratory wells located near dykes, as in the Hesel test well site, may have encountered deep saturated fractures as evident from the yield (3.68 1 s"1) obtained from the test well.

This type of field conditions, i.e. profusion of intruded dyke, are generally absent in the younger granites, though some isolated cases of dyke intrusions have also been found in this area.

The metasediments and the aquifers The disposition of aquifers at depth in the metasediments is quite similar to that of granitic terrains. But within the metasediments the depth persistence of deeper aquifers is more clearly controlled by the structural features and relative hardness of the adjacent lithological units (Das & Dev Burman, 1986). It has been observed that the hard formations like quartz-feldspar-mica schists, gneisses and the quartzites when affected by folding, offer better scope for the formation of deep seated saturated fractures. All the saturated fractures between 50 and 139 m depth at Kudada, Mahuliya and Dugni sites are of these formations which are affected by folding. In Salboni and Matgoda sites the saturated fractures are limited down to 50 m only and are underlain by soft formations like chlorite-muscovite-biotite schists. The Salboni wellfield occurs within the deep seated major fault zone (Manbazar thrust). The fracture conduits appear to be better developed and preserved in the hard formations than in the low grade softer schists etc. In low grade schist, softness generally results

G. K. Dev Burman &P.K. Das 24

in extensive weathering. Abundance of clay minerals tend to choke the fractures at shallow depth resulting in clogging and hinderance of water passage to deeper fractures. For this reason probably the deep fractures at Salboni and Matgoda sites have been found dry as they were deprived of recharge from the top saturated zones.

Hydraulic behaviour

The hydraulic behaviour of these aquifers has been studied by conducting step drawdown tests (SDT), aquifer performance tests (APT) and preliminary yield tests (PYT) in exploratory and observation wells. The salient feature of different aquifers are shown in Table 1.

The behaviour of deep fractures has been studied from the exploratory well at Kudada, Hesel-1 and Chakradharpur (Fig. 3). All these wells pierced prominent saturated aquifers below 100 m depth. These wells exhibit the following characteristics: (a) lower discharges during pumping tests than

Table 1 Aquifer details and aquifer performance test results of some wells

Wells Rock type Aquifer Drill time Duration Pumping Drawdown discharge of discharge

4 pumping , (I s~l) (min) (l s'1) (m)

Kudada EW

Hesel-1 EW

Chakradharpur EW

Salboni EW

Hesel-2 EW

Palandu EW

Doranda EW

Bhaludi EW

Chakradharpur OW

Metasediments

Singhbhum granite

Chakradharpur granite

Metasediments

Singhbhum granite

Chotanagpur granite

Chotanagpur granite

Singhbhum granite

Chakradharpur granite

Deep fracture

Shallow and deep fracture

Deep fracture

Shallow fracture

Shallow and deep fracture

Saprolite and deep fracture

Saprolite, shallow and deep fracture

Saprolite and shallow fracture

Saprolite and shallow fracture

12.51

10.0

4.60

2.25

1.30

4.66

4.66

1.25

5.6

1620

1560

720

1680

1460

1450

1440

1440

1310

8.83

3.68

1.2

3.22

1.50

6.28

5.93

2.50

8.5

18.21

45.84

34.46

19.10

24.68

12.28

18.03

10.43

17.85

EW = exploratory well. OW = observation well.


A W D 0 W N

(m)

'

h .

M i l l 1 1 Mll l l SHAKRADHARPUREW

^mp ig Rate = i-2ips

Hquifer(mbgl) withDri l iT

1

i i until

me Discharge! |ps)

,

•

#

1000 TIME(min)

Fig. 3 Drawdown and recovery trend of wells with potential deep fractures.

during drilling; (b) no stabilization of water level during controlled pumping; (c) heavy drawdown and very slow rate of recovery; (d) initially low drawdown for a few minutes, afterwards higher rate of drawdown.

The behaviour of shallow fractures have been studied from the exploratory wells at Salboni and Hesel-2 (Fig. 4). Both the wells have tapped shallow aquifers within 50 m. The aquifer at Hesel-2 is a fractured quartz vein 9-40 m below ground level. These wells show the following phenomena: (a) little increase in discharge over that during drilling; (b) no stabilization of pumping water levels; (c) heavy drawdown, although somewhat less than that of deep fractures; (d) recovery slow, but faster than deep fractures.

It is difficult to study the behaviour of the saprolite zone separately because the wells have pierced shallow and deep fractures underlying the saprolites. Hence the behaviour of saprolites has been inferred from pumping

D R A W D 0 W N

(m)

5"

10

15

20

25

.

i

1 )

<

> 1 :t ' >

• •

"1

: ) le ; o i

.. .. ,

fl it

p

:

1 |

i

HESEL Pumping Q =1-5Ips

9 - 4 0 m , l Ips Aquifer 2 3 0 m , 0-301

H i ---rrrrm— 1 n u " BHALUDI

) PumpingO = 2-5 Ips 1 . ., IB-0m( Saprolite),0-75lps.

Aqui fer2 3 .0 m . . . . ; 0-'5lps ""

\ * x

< i o

_._L

PS

X )

•

1 <

X x \t

y*1

. o

< <>

, i

i ,

^ x

,

Ï O

in iii

,i i

i .,

SA

LBO

NI

Pum

ping Q

= 3-2

2 Ip

s 2

3-4

0m

, 0-5

0lp

s A

quifer 3

10

2m

, 0-7

5lp

s 46-2

6 m

, |-2

5lp

s

I 10 100 1000 10000 TIME Cmin]

Fig. 4 Drawdown trend of wells with shallow fractures.

G. K Dev Burman &P.K. Das 26

test results at Palandu, Doranda and Bhaludi exploratory wells and the Chakradharpur observation well (Fig. 5). The salient features are: (a) an increase of discharge during pumping, as high as two to three times that during drilling; (b) stabilization of pumping water levels are generally achieved; (c) drawdown is relatively less than shallow and deep fractures; (d) recovery much faster than in fractures; (e) the time-drawdown curve is characteristically different from that in fractures.

D R A W D 0 W N

:EW

> _i :

OW

> *>

•

i

h X *

1

i

::>x»

> I 1 ) x>

> : • Ï *F 1

Pumping Rote = 5 • 93lps ( EW) Aquifer(mbgl) with Drill Time Discharge(lps)

EW OW 12-66 0 - 2 5 18-0 1-66

35-11 58-97 0 63-59 0

107-31 0 lp.9-,3,1 0

• 25 •33 •50 •83 •83,

100 1000 TIME(min)

Fig 5 Drawdown and recovery trend of wells with prominent saprolite zones.

DISCUSSION OF HYDRAULIC BEHAVIOUR

Perusal of the test results suggests that leakage from the top saturated weathered mantle, the nature, thickness and extension of weathering in the saprolites and the chemical composition of rock types are important features affecting the relative potential of aquifers. The dimensions and density of the fractures along with their interconnection with the weathered mantle are also significant.

Leakage from the weathered mantle

The leakage from the weathered mantle has been studied in detail for the Palandu well field. The Palandu exploratory well pierced aquifers at 15 (saprolite), 79.20 and 92.70 m (deep fractures) depth with individual yields of 2.66, 1.0 and 1.0 1 s"1 respectively - totalling 4.66 1 s"1 of cumulative discharge. During regular testing, however, the net yield of this well has been found to increase substantially. The rate of increase in regular yield is also variable from season to season. The data of the three step drawdown tests is presented in Table 2 to illustrate this behaviour. The results, besides showing the seasonal change in discharge characteristics, also indicate certain unusual trends in the specific capacity values. The data indicate that during July and November the weathered mantle was fully saturated and a considerable


Table 2 Step drawdown tests at the Palandu exploratory well

Date Steps Duration

(min)

Discharge

(l s'*)

Cumulative drawdown (m)

Specific capacity

(I mm m )

9/7/87 (mid-monsoon)

25/11/87 (just post-monsoon)

30/3/87 (pre-monsoon)

1 2 3 4

1 2 3 4

1 2 3 4

60 60 60 60

60 60 60 60

100 100 100 100

4.50 6.28 8.83

11.00

4.50 6.28 7.85

10.46

3.63 4.00 4.43 5.00

4.848 7.487 9.447

11.448

5.21 8.028

10.280 12.80

4.788 6.019 6.725 7.865

55.59 49.41 56.10 57.65

51.82 46.96 45.81 49.06

45.53 39.87 39.55 38.14

amount of leakage took place from this zone to the underlying aquifers, due to which the specific capacity values registered much higher values with progressive stages. During March, the contribution from such leakage is less due to the decline in the overlying water table. An increase in specific capacity values during November was also observed at Doranda. The test results are shown in Table 3.

Leakage from the weathered mantle is also evident from the decline of the water table in the nearby dug well at Palandu site during step drawdown and aquifer performance. During a long (1450 min) aquifer performance test

Table 3 Step drawdown test results

SI.

1

2

3

4

Wells

Kudada OW

Kudada OW

Ulda EW

Hesel-2 EW

Date

16/1/88

5/10/89

6/9/89

21/11/89

Steps

1 2 3 4

1 2 3 4

1 2 3 4 1 2 3 4

Duration

(min)

120 120 120 120 60 60 60 60 60 60 60 60

100 100 100 100

Discharge

(l s"1)

3.93 5.5 6.87 8.0 4.0 5.71 7.55 8.83 7.0 9.0

11.0 12.5 1.02 1.50 1.75 2.00

Cumulative drawdown

(m)

3.775 7.25

10.47 13.21 3.275 6.577 9.63

13.004 3.215 6.485 9.425

12.315 9.205

14.165 21.355 38.205

Specific capacity

(I mirfim~1)

6154 45.51 39.86 3633 73.28 5215 47.04 40.76

130.63 83.26 70.62 60.92 7.82 6,353 4.916 3.140

G. K Dev Burtnan &P.K. Das 28

(APT) in Palandu exploratory well (6.28 1 s"1 discharge) the dug well (5.28 m depth) located 25 m away recorded a drawdown of 0.87 m. The two other deep observation wells located 89 and 92 m away also recorded 0.41 and 0.35 m of drawdown. The reactions in the observation wells, including the dug well, are not instantaneous and the drawdown continued after pumping was stopped. While discussing the flow mechanisms of low yielding hard rock aquifers, similar observations were made by Powel et al. (1983). Afterwards recovery starts at a very slow rate. Similar drawdown and recovery patterns were also observed in other well fields during testing. Even the Hesel-1 and Chakradharpur exploratory wells, which tap only the deep potential fractures, have similar drawdowns in their observation wells. The decline of water levels recorded in the observation wells in a few well fields is presented in Table 4.

Table 4 Decline of water levels in observation wells

Pumping well

Palandu EW Palandu EW Hesel-1 EW Hesel-2 EW Chakradharpur EW Chakradharpur OW Mahuliya-2 OW Mahulya-2 OW

Duration of pumping

(min)

1450 1450 1560 1450

720

1310

270 270

Distance of observation well

(m)

89.0 92.0 89.0 35.0 50.0

210.0

480.0 520.0

Time lag commencement of decline after pumping starts (min)

18.0 28.0 50.0 15.0 35.0

Not observed 25.0 35.0

Drawdown

(m)

0.41 0.35 2.483 1.89 2.14

0.12

2.195 2.074

Recouperation rate in m/min

0.26/370 0.164/370 1.79/1380 0.545/310 0.58/600

Not observed

0.685/300 0.62/300

It is evident from the above discussion that considerable leakage from weathered mantle to saprolites and in fractures always takes place under normal as well as induced conditions. The saprolite being the shallowest zone and occurring just below the weathered mantle receives maximum recharge. On the other hand this is limited to deep fractures, as seen from step drawdown test results in Table 4, due to the lack of wide and extensive interconnections with the weathered mantle. Probably the leakage is larger vertically than horizontally as indicated by the step drawdown test results of Hesel-2. This well taps a fractured quartz vein at shallow depth (9-40 m) and the step drawdown test was conducted just after monsoon, showing no increase in specific capacity values at later stages. The major part of this quartz vein is exposed, except for some limited portion in and around the well. For this reason most of the recharge may have taken place horizontally, resulting in a very limited contribution from the adjoining weathered mantle.


Chemical composition of rocks

The behaviour of fractured aquifers at shallow depth, particularly when there is limited scope of leakage from the adjoining upper weathered mass, depends also on the chemical composition of aquifer material which ultimately determines its response to weathering. Quartz veins being highly resistant to weathering may show different nature than fractured basic intrusions. Part of the fractured mass generally suffers weathering and creates better aquifers than a highly resistant fractured quartz vein. At Manbazar well field a basic dyke occurs in a similar fashion to the quartz vein at Hesel-2 exploratory well and was encountered at 13 m depth with 1.41 1 s"1 of discharge. The varying pictures of the preliminary yield test of this basic dyke and the quartz vein at Hesel-2 support this idea (Table 5). The tests were carried out by deploying a rig compresser at 160 p.s.i. for 100 min in both cases.

Table 5 Preliminary yield test results

Well Aquifer Depth range Static water Discharge Drawdown material of aquifer level .

(m) (m b.g.l.) (I s1) (m)

Manbazar-2 Fractured 13.0-16.94 1.35 1.41 14.27 EW and semi-

weathered dolerite dyke

Hesel-2 Fractured 9.0-40.0 3.44 1.0 24.71 EW quartz vein

b.g.l. = below ground level.

Behaviour of the saprolite zone

The preliminary yield tests at Chakradharpur-1 observation well were conducted to study the behaviour of the saprolite zone and also the cumulative effect of saprolite and the underlying shallow fracture. The tests were conducted by deploying an air compresser of the rig (at 160 p.s.i.) for 100 minutes. The test results are presented in Table 6.

The test data highlight the characteristic small drawdown and comparatively fast recovery within the saprolite and indicates that saprolite is better than the cumulative zone. The well also recorded a higher yield (8.5 1 s"1) during the pumping test in comparison to the discharge during the initial drilling (5.6 1 s"*).

The increase in discharge as observed in wells having potential saprolite aquifers may not be due only to the leakage from the top weathered mantle. Probably the saprolites, which are a mixture of weathered and fractured material are less permeable than fractures, and could not react properly during drilling. High air pressure within boreholes during drilling may have partially hindered the response from the saprolite zone. During pumping, in

G. K. Dev Burtnan &P.K Das 30

Table 6 Preliminary yield test results, Chakradharpur-1 observation well

Type of Depth range Static water Discharge Drawdown Remarks aquifer of aquifer level .

(m) (m b.g.l.) (I O (m)

Saprolite 16.99-25.61 4.007 1.83 5.345 Well recovered in 35 min

Saprolite 16.99-25.61 4.16 5.6 13.273 80% recoupment and and in 70 min. shallow 33.23-35.47 Afterwards slow fracture rate

b.g.l. = below ground level.

the absence of high pressure compressed air in circulation within the borehole, the saprolites respond properly and contribute their full yield capacity.

CONCLUSION

Within the hard rock domain, weathered mantle is the primary water-bearing zone. The saprolites and fractures receive recharge from the weathered mantle under normal as well as induced conditions. The low drawdown in the initial few minutes in deep fracture wells may be due to stored water (dead storage) in fractures. The performance of wells tapping fractures depends on the degree of interconnection with the top saturated weathered mantle. Deep fracture wells in metamorphic terrain (comparatively hard formation) may perform better than those in granitic terrain as better interconnection is possible through foliation, schistosity etc. By and large saprolite behaves similarly to unconsolidated formations.

Acknowledgement The authors are grateful to Shri B. P. C. Sinha, Chief Hydrogeologist and Member of the Central Ground Water Board, for his constant encouragement during the preparation of the paper and for his approval to publish the paper.

REFERENCES

Das, P. K. & Dev Burman, G. K. (1986) Groundwater exploration in hard rocks of Singhbhum District, Bihar. Some feld observation on relevant geomorphic and geological features. Presented at the Mineral Exploration Seminar at Patna, India.

Powel, S., Rushton, K. R & Dev Burman, G. K. (1983) Groundwater resources of low yielding aquifers. In: Ground Water in Water Resources Planning (Proc. Koblenz Symp., August-September 1983), vol. I, 461-470. IAHS Publ no. 142

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