Stable-isotope geohydrology of the Lower Maner Basin, Andhra Pradesh, India

Journal of Hydrology, 59 (1982) 315--330

315

Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands [21

STABLE-ISOTOPE GEOHYDROLOGY OF THE LOWER MANER BASIN, ANDHRA PRADESH, INDIA

B. KUMAR, R.N. A T H A V A L E and K.S.N. SAHAY

National Geophysical Research Institute, Hyderabad 500 007 (India) (Received April 8, 1982; accepted for publication April 29, 1982)

ABSTRACT Kumar, B., Athavale, R.N. and Sahay, K.S.N., 1982. Stable-isotope geohydrology of the Lower Manet Basin, Andhra Pradesh, India. J. Hydrol., 59: 315--330. Hydrogen (D/H) and oxygen (Ots/O 16) isotopic ratios were measured in precipitation-, lake-, stream- and well-water samples collected from the Lower Maner Basin, Andhra Pradesh, India, during 1976 and 1977. Granites, sandstones, quartzites and shales comprise the main lithological units exposed in the basin, which has a semi-arid climate and an average annual rainfall o f 1000 ram. The basin is characterized by lakes and tanks of varying capacities. Isotopic data on precipitation samples gave a meteoric water line for the Lower Manet Basin, having a slope which is slightly different from that of the characteristic global meteoric water line. This marginal deviation may be due to the local evaporation effect on rain drops. Isotopic ratios o f November 1976 and May 1977 lake-water samples show a considerable relative enrichment in the latter samples due to evaporation. Similar sample pairs from wells show no such enrichment trend, thus indicating absence o f the evaporation effect on water which has reached the zone o f saturation. Groundwater samples show enrichment with respect to the weighted mean isotopic ratios for precipitation. The extent of enrichment was found to vary with the recharge characteristics of various soil and rock types in the basin. The groundwater samples could be c l a r i f i e d , upon the basis o f their isotopic ratios, into two types. This classification agrees approximately with the lithological classification. Only one of the various tributaries o f the Maner River, namely the Maruvancha Stream, is perennial. Isotopic data indicate that this perennial nature is due to return flow o f lake waters through irrigation. The relative contribution of surface and groundwater to the discharge in this stream, during a low-flow month, was calculated from isotopic ratio values and found to be 80% and 20%, respectively.

INTRODUCTION

The increasing trend in exploitation of groundwater resources in arid and semi-arid areas has created a general awareness of the need for groundwater management. In 1974, a collaborative project on Exploration and Managem e n t Studies of Groundwater Resources was initiated between the National Geophysical Research Institute, Hyderabad, India, and the Federal Institute

316 of Geosciences and Natural Resources, Hannover, F.R.G. The Lower Maner Basin, located in the vicinity of the Warangal and Karimnager districts of the state of Andhra Pradesh, India, was selected as a pilot basin for integrated geohydrological investigations comprising photogeological, hydrogeological, hydrogeochemical, geophysical, nuclear, and modelling studies. The present study forms a part of these investigations. It deals with results of stable oxygen and hydrogen isotopic ratio measurements on precipitation, surface, and subsurface water samples collected in the Lower Maner Basin during 1976 and 1977, and discusses some specific hydrogeological problems such as interconnection of water bodies, isotopic types of groundwaters, and their recharge characteristics, in the light of isotopic data variations found in the samples.

GEOLOGY, HYDROGEOLOGY, BASIN A R E A

DRAINAGE AND CLIMATE OF THE LOWER MANER

The Lower Maner Basin forms a part of the Godavari Rift Valley and extends between latitudes 18005 , and 18°42'N and between longitudes 79032 ' and 80°00'E, covering an area of ~ 1 6 0 0 km 2 . Seven different geologic formations, ranging in age from Jurassic to Archean, are exposed in the basin (Fig. 1). These formations are: (1) Kota shales and sandstones (Jurassic); (2) Kamthi sandstones (Triassic); (3) Barakar sandstones (Permian); (4) Talchir tillites and boulder beds (Upper Carboniferous); (5) Sullavai sandstones and quartzites (Proterozoic); (6) Pakhals shales and dolomites (Proterozoic); and (7) granites and gneisses (Archean). The sedimentary sequence strikes generally NW--SE and dips gently NE. The major stream draining the basin is the Manet River which joins the Godavari River at the NE boundary of the basin. The Manet River, in the area under study, has three main tributaries, namely, (1) the Maruvancha Stream; (2) the Ara Stream; and (3) the Bogula Stream. The Manet River and Maruvancha Stream are perennial; the Ara and Bogula streams as well as some other minor streams are ephemeral in nature. The area has m a n y small and large tanks and two large lakes, the Ramappa and Ghanpur lakes (Fig. 2). The climate of the basin, in general, is tropical semi-arid with hot summers and mild winters. The m a x i m u m air temperature reaches as high as 48 ° + 2°C in summer and the m i n i m u m one reaches to as low as 14 ° +2°C in winter. The mean annual temperature for the entire basin is around 28 ° +4°C. The mean annual average rainfall of ~ 1 0 0 0 m m is mainly confined to the monsoon months of June--September. Hydrogeological studies by Radhakrishna et al. (1976, 1978, 1979) indicate that groundwater occurs under both unconfined and confined conditions in the basin. The confined groundwater potential, in general, is poor except in the case of the sub-basins of the Ara and Bogula tributaries draining the Kamthi formations. Well hydrograph data show that the upper aquifer is replenished through monsoon precipitation in about two months time.

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Water samples for isotopic studies were collected from six rain-gauge stations and surface and subsurface water bodies located in the basin. Rain waters were collected during the 1977 m o n s o o n season, as m o n t h l y composite precipitation samples. The surface and subsurface waters were collected

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Fig. 2. Drainage map of the Lower Maner Basin, showing sampling sites. in t w o phases, first in November 1 9 7 6 (post-monsoon) and then in May 1 9 7 7 (pre-monsoon). All samples were stored in 2 0 0 ml screw-top stoppered glass bottles. The sampling sites are shown in Fig. 2. Isotopical measurements were performed on all samples, using standard techniques. The oxygen isotopic composition of the water samples was determined by use of the CO2 equilibration method and the hydrogen isotopic measurement was carried out on hydrogen gas produced by reduction of water over uranium at 750°C. The mass spectrometers used for isotope analyses are the 602-C and 602-D units manufactured by VG-Isotopes Ltd., Winsford, U.K. Details of the laboratory setup and measurement procedure are given elsewhere (Kumar et al., 1980).

319 T h e s u r f a c e a n d s u b - s u r f a c e w a t e r s a m p l e s w e r e a n a l y s e d in d u p l i c a t e r u n s a n d p r e c i p i t a t i o n w a t e r s a m p l e s in single runs. I n t o t a l , t h i r t y w a t e r s a m p l e s w e r e a n a l y s e d f r o m rain-gauge stations, t w e l v e s a m p l e s e a c h f r o m s t r e a m s a n d t a n k s , t w e n t y - t w o f r o m d u g wells, a n d o n e f r o m an artesian borewell. I s o t o p i c r a t i o s are e x p r e s s e d in p e r mil vs. V-SMOW ( V i e n n a S t a n d a r d M e a n O c e a n Water}. T h e m e a s u r i n g p r e c i s i o n f o r t h e 180/160 a n d D / H ratios is + 0.1°/0o a n d + l°/0o, r e s p e c t i v e l y .

RESULTS T h e d e u t e r i u m a n d 180 c o n c e n t r a t i o n s in m o n t h l y c o m p o s i t e p r e c i p i t a t i o n w a t e r s w i t h s a m p l e details are given in T a b l e I a n d are p l o t t e d in Fig. 3. T h e TABLE I Isotopic values of monthly composite precipitation samples of the Lower Manet Basin, collected during the 1977 monsoon season Serial No.

Sample No.

Location of rain gauge

Month

stations

Amount of

8D

81SO

Seasonal

(%)

(%)

weighted mean (°/0o)

rainfall (mm)

8D

8180

1 2 3 4 5

T-1 T-2 T-3 T-4 T-5

Tundla Buzurg

July Aug. Sep. Oct. Nov.

211.7 235.4 96.8 28.7 39.1

--25.5 --14.5 --35.3 --50.1 --77.1

--4.33 --2.30 --5.75 --7.61 --10.76

--27.3

--4.34

6 7 8 9

C-1 C-2 C-3 C-4

Chinpak

--16.7 --9.3 --61.1 --34.9 --48.0

--2.70 --2.36 --8.78 --5.32 --7.82

--4.26

C-5

302.8 146.3 119.8 5.2 57.1

--26.4

10"

July Aug. Sep. Oct. Nov.

11 12" 13 14 15"

N-1 N-2

Nasaram

334.6 242.9 114.4 27.3 32.8

--17.2 --21.7 --58.1 --50.8 --69.6

--2.74 --3.01 --8.20 --7.53 --8.63

--28.4

--4.08

N-4 N-5

July Aug. Sep. Oct. Nov.

16

B-1

Bhopalpalli

451.1 150.0 93.3 34.3 60.1

--13.7 --20.8 --49.3 --35.4 --85.8

--3.04 --3.79 --6.95 --5.43 --12.40

--4.46

B-2 B-3

July Aug. Sep. Oct. Nov.

--25.7

17

Kamalapur

July Aug. Sep. Oct. Nov.

383.4 250.9 109.6 18.4 35.0

--15.2 --15.1 --52.9 --36.2 --77.80

--3.04 --3.27 --8.20 --5.65 --10.85

--23.6

--4.23

N-3

18 19 20

B-4 B-5

21 22 23 24 25

K-1 K-2 K-3 K-4 K-5

320 TABLE I (continued)

Serial No.

Sample No.

26 27 28 29 30*

Location of rain gauge stations

Month

Salagiri

S-1 S-2 S-3 S-4 S-5

July Aug. Sep. Oct. Nov.

Amount of rainfall (ram)

8D (%)

286.3 144.9 140.4 18.3 51.3

--13.70 --21.40 --74.40 --53.80 --57.80

Seasonal weighted mean

81SO (%0)

(°/oo) --2.16 --3.56 --10.65 --7.80 --9.04

8D

8180

--33.4

--5.05

* Excluded from the best-fit straight-line correlation.

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isotopic ratios for the July and August samples are enriched compared to those of the later months. The weighted mean isotopic ratio values for all stations are more or less similar, except for the value of the station at Salagiri, which was somewhat more depleted than the means for the other stations. Tables II and III give oxygen and hydrogen isotopical results of surface

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and subsurface water samples of November 1976 and May 1977, respectively. Isotopic data are plotted in Fig. 4. The data of Tables II and III demonstrate that, in general, groundwaters are depleted in 5D- and 5 laO-values compared with surface waters. The surface waters of May 1977 are significantly enriched in their 5D- and 5 laO-values compared to those of November 1976, except in the case of the Bogula Stream sample of May 1977 (sample No. 8 in Table III). Groundwater samples of November 1976 and May 1977 of the same wells have more or less identical isotopic ratios (see, for example, isotopical results of samples 22, 26, 27 and 34 in Tables II and III).

DISCUSSION

Precipitation waters The deuterium and 180 concentrations in precipitation waters from different parts of world display a linear relationship (Craig, 1961; Dansgaard, 1964) which is represented by a line having the equation: ~D = 8~180 + 10

324

This line is known as the meteoric water line for world-wide precipitation. In order to establish a meteoric water line for the Lower Manet Basin, the data given in Table I were c o m p u t e d for fitting a straight line, using a computer programme based on those by York {1966, 1969). The best fit gave a line represented by the relationship: ~iD = (7.6 +0.4)61SO + (6.3 +2.7)

(2)

The errors shown in eq. 2 are l a errors. The values for four samples (serial Nos. 10, 12, 15 and 30 in Table I) were not taken into account while computing the best fit. The line represented by relationship (2) is very similar to that represented by relationship (1) {see Fig. 3). The slight departure in slope and intercept from those of the line represented by eq. 2 may be due to insufficient data, or due to partial evaporation of rain droplets during their fall from clouds in the semi-arid region (Ehhalt et al., 1963; Dansgaard, 1964; Gat, 1971). From the weighted mean isotopic ratio for each station (see Table I), the composite weighted mean isotopic values for rain water in the Lower Manet Basin have been calculated and these are ~D =--27.3°/00 and ~lSO =--4.39°/00 . These values represent the mean average isotopic ratios for rainfall over the basin during 1977. Fig. 5 shows a plot of m o n t h l y variations of 8D- and ~ 1SO-values of pre-

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Fig. 5. ~ D- and ~ 180-values of monthly composite precipitation samples from the hydrometeorological station at Bhopalpalli.

325

cipitation samples at station Bhopalpalli. The isotopic values of precipitations at other rain-gauge stations follow more or less the same trend, i.e. enriched values for July and August samples as compared to later months. One of the possible explanations for this trend could be the following: in July and August, the contribution of the southwest monsoon to the local atmospheric moisture is much more than that of moisture due to evaporation of local water bodies. On the other hand, in September--November the monsoon is receding and the relative amounts of monsoon and locally evaporated moisture are comparable (Athavale et al., 1967). Monsoon vapours originate from oceans and are enriched in 8D- and 8]SO-values as compared to the locally evaporated moisture. The July and August precipitations originate mainly from the monsoon and therefore are enriched in their 8D- and 81SO-values.

Surface and subsurface waters During evaporation there is an enrichment of heavy isotopes in the water. In the dry period, i.e. from November to May, surface water bodies get less input from rainfall while evaporation is taking place. The large surface reservoirs in the basin are subjected to less isotopic enrichment, i.e. less evaporation, as compared to small tanks, see for example the November and May isotopic values for samples 15 (Ghanpur lake) and 9 (Kamalapur tank) in Tables II and III. Isotopic values of waters of the Maruvancha and Bogula streams of both seasons are plotted in Fig. 6. A comparison of November 1976 and May 1977 40

LEGEND

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Maruvoncha samples[

30

--

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~ I'

BI I I '

I0 A

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f

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L

I

-5

-3

-I

0

I

I I

3

6 mO (%°) Fig. 6. Isotopic values of Maruvancha and Bogula stream-water samples.

326

data shows that the Maruvancha Stream water is enriched in its D and ~aO contents during the dry season but, on the other hand, the Bogula Stream water isotopic ratios are depleted. The Maruvancha is a perennial stream and the Bogula is non-perennial. The Maruvancha Stream passes through areas irrigated by the Ramappa and Ghanpur lakes, and the isotopic composition of its May 1977 sample near Ghandhinagar is very much similar to those of the Ramappa and Ghanpur lake waters of May 1977 (see Table III, samples 6, 15 and 16). This suggests that the Maruvancha Stream, during the lowflow season, is sustained by contributions made by return flow of irrigation water, released from the lakes. The Bogula Stream dries up during the dry season, and the May sample of the stream had to be collected after digging ~ 3 0 c m deep in the dry bed of the stream. The isotopic contents of this sample are similar to the mean isotopic contents of groundwater in the Kamthi formation, as discussed on p.327.

Recharge characteristics of groundwaters and their types The oxygen and hydrogen isotopic ratios of all groundwater samples given in Tables II and III are plotted in Fig. 7. The data were computed for fitting a straight line and the best fit gave a line represented by: ~D = (5.47 +0.2)~180 + (--2.7 +0.6)

10

(3)

LEGEND Weighted mean isotol~w: value tar rain waters

Nov. '76

Groandwaters

May '77

Groundwaters

o

A

-20

-30

L 5

-4

1

-3

L -2

6 '8o ( °/00)

I

-I

L

0

Fig. 7. ~ D vs. ~ lSO plot of groundwater samples from the Lower Maner Basin.

327 Eq. 3 has a lower slope as compared to eq. 2, possibly due to partial evaporation of precipitation during its percolation through the unsaturated zone. The values of the intercept of the line represented by eq. 3 with the line represented by eq. 2, i.e. the meteoric water for the Lower Manet Basin line, are: 5180 = --4.2°/00 and ~D = --25.6°/00 . These intercept values can be considered as the long-term mean isotopic composition of precipitation water prior to being subjected to evaporation during infiltration, and are more or less similar to the weighted mean isotopic ratio values calculated for rain waters sampled in this study. T w o types of groundwaters can be distinguished upon the basis of isotopic ratios: T y p e I: Waters having mean isotopic ratios close to 5D = --19.7 + 1.6°/00 and 81So = - - 3 . 1 4 + 0.27°/00 . These waters are from the wells located in the Kamthi formation, which covers ~ 3 0 % of the area of the basin. Type II: Waters having mean isotopic ratios of 5D = - - 1 5 . 4 + 1.0°/00 and 5180 = - - 2 . 1 9 +0.14~/00. These waters are from the wells located in the Sullavai and Pakhals formations, which cover ~ 4 5 % of the area of the basin. The weighted mean isotopic ratios for rain waters in the basin are 5D = --27.3°/00 and 8180 = --4.39°/00 . Comparing these values with those of type-I and type-II waters and the isotopic contents of surface waters of May 1977, it can be concluded that the groundwaters of both types are recharged by direct percolation of precipitation water, and contribution from surfacewater bodies is generally negligible in the case of the wells sampled in this study. The enrichment in 5D- and ~180-values of these groundwaters, in comparison to the mean isotopic ratios for rain waters, appears to be mainly due to partial evaporation of these waters during their infiltration through the unsaturated zone. Once the percolating waters reach the saturated zone, there is apparently no further loss of moisture due to evaporation. This is indicated by the fact that the November and May samples from the same wells show very similar ratio values. Recharge measurement studies carried o u t by Athavale et al. (1980) in the L o w e r Manet Basin, using the tritium injection method, show that the Kamthi formation has a higher infiltration rate, i.e. better permeability, as compared to the Sullavai and Pakhals formations. Less evaporation can, therefore, be expected during percolation of precipitation waters through the unsaturated zone in the Kamthi formation as compared to that in the Sullavai and Pakhals formations, which are comparatively hard and compact. Groundwaters in the Kamthi formation have lower 5D- and 51SO-values relative to groundwaters in the Sullavai and Pakhals formations. The isotopic data thus corroborate the results of recharge measurement studies. Miscellaneous results

Sample 25 (Fig. 7) is from the Adwisrirampur weU and its D / H and 180/ 160 ratios are similar to weighted mean isotopic ratio for rain waters. This

328 suggests that the well is probably recharged from direct inflow of precipitation water through fractures. Samples 18, 19 and 21 are from the Manthani, Dhanwada and Baswapuram wells, respectively. The Dhanwada and Baswapuram wells are in Kota clays which have a poor permeability. The Manthani well is located in the Kamthi formation but the local soil is clayey. The enrichment in 5D- and 81SO-values of these wells relative to groundwaters of types I and II, may be due to higher evaporation during infiltration. Isotopic ratios for the Narsapur well (sample 33) located in the vicinity of Ramappa Lake show that the well is probably receiving partial contribution from seepage of the lake water. Isotopical balance studies

The relative contribution of groundwater and Ramappa and Ghanpur lake waters to the Maruvancha Stream during May 1977 has been estimated. Assuming that the contribution of lake water to the Maruvancha Stream is Mk and of groundwater is Mg, we obtain: Mg +

M k

=

1

(4)

Taking the mean isotopic value for groundwater as g and for lake water as k, and of the Maruvancha Stream, 2 km upstream from the confluence with the Maner, as c, the isotope balance equation is then given by: (5)

gMg + kMk = c

The Maruvancha Stream flows mostly through the Sullavai formation. In the Sullavai formation the mean isotopic ratio value for groundwaters is found to be equivalent to type-II waters, i.e. close to 8D =--15.5°/00 and 8180 = - - 2 . 2 % . The isotopic ratios of the Ramappa and Ghanpur lake waters have been measured (see Table III, samples 15 and 16 of May 1976). The mean of 5D- and 51SO-values for lake waters is: ~iD = + 1 3 . 5 % and 8180 = + 3 . 4 % . The isotopic ratios for Maruvancha Stream water 2 k m upstream from the confluence with the Manet River are 5D = + 8 . 9 % and 8180 = + 2 . 2 7 % (sample 7 of May 1977, Table III). The isotopical balance equation for 5D therefore is: --15.5Mg + 13.5Mk = 8.9

(6)

and for 51So is: --2.2Mg + 3.4Mk = 2.27

(7)

from eqs. 4 and 6 we obtain: Mg = 16%

and

M k

=

84%

and from eqs. 4 and 7: Mg = 20%

and

M k = 80%

329 This shows that the Maruvancha Stream, during the month of May, receives ~ 80% contribution from the Ramappa and Ghanpur lake waters and 20% from groundwaters. CONCLUSIONS Stable-isotopic analysis of precipitation-, surface-, and subsurface-water samples from the Lower Manet Basin, has revealed the following: (1) The meteoric water line for the Lower Maner Basin, which is located inside a continent, in the semi-arid belt of the India peninsula, is similar to the global meteoric water line. (2) Discharge in the Maruvancha Stream, which is the only perennial tributary to the Maner River, consists largely of return flow of irrigation water. The relative contribution of surface and groundwater to the discharge during a low-flow month (May 1977) was found to be approximately 80% and 20%, respectively. (3) Isotopic ratios of'well waters show different degrees of enrichment vis-a-vis the weighted mean isotopic ratios for precipitation waters. This observation was found to be in qualitative agreement with infiltration rates measured by others in various soil types present in the basin. (4) The post- and pre-monsoon samples collected from the same wells, having a time gap of about six months, have generally identical isotopic ratio values. This observation indicates that there is no substantial loss of groundwater due to evaporation from the zone of saturation. (5) Isotopic ratios of most of the well samples indicate percolation of precipitation as the main source of recharge. One case of direct recharge through a fracture and another case of a well partially recharged by lake water could be identified from isotopic ratio values. (6) The groundwater of the basin can be classified into two isotopical types. This classification was found to be consistent with lithology and degree of compactness of the formations. ACKNOWLEDGEMENTS The authors thankfully acknowledge the assistance of Shri S.D. Deshmukh and Shri M.V. Nanda Kumar in the collection of surface and groundwater samples. We are also thankful to the Director, Andhra Pradesh State Groundwater Department for providing rain-water samples. The Micromass® 602 mass spectrometers and sample preparation setup were gifts of the Federal Institute of Geosciences and Natural Resources (F.I.G.N.R.), Hannover, F.R.G., under an Indo-German collaborative project on Exploration and Management Studies of Ground Water Resources. Our sincere thanks are due to Drs. W. Stahl and E. Faber of F.I.G.N.R., for help and advice in establishing the stable-isotope laboratory.

330 Dr. K a b i r R o y C h o w d h a r y p r o v i d e d t h e c o m p u t e r p r o g r a m m e , DF1TKABIR- 7 7 1 0 , f o r the least-squares fit. We are g r a t e f u l t o h i m a n d t o Dr. A.K. Baksi f o r useful discussions. T h e assistance r e n d e r e d b y Shri G . G . Kingi in t h e l a b o r a t o r y w o r k is t h a n k f u l l y a c k n o w l e d g e d . T h e a u t h o r s wish t o e x p r e s s t h e i r special t h a n k s t o Dr. S. B a l a k r i s h n a , D i r e c t o r , N a t i o n a l G e o p h y s i c a l R e s e a r c h I n s t i t u t e , H y d e r a b a d , f o r his k e e n i n t e r e s t a n d active s u p p o r t in c a r r y i n g o u t t h e studies a n d f o r his p e r m i s s i o n t o p u b l i s h this p a p e r .

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