14C ages of confined groundwater from the Gwandu aquifer, Sokoto Basin, northern Nigeria

14C ages of confined groundwater from the Gwandu aquifer, Sokoto Basin, northern Nigeria

Journal of Hydrology, 48 (1980) 281--288 281 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [4] 14C AGES OF CONFIN...

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Journal of Hydrology, 48 (1980) 281--288

281

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

[4]

14C AGES OF CONFINED GROUNDWATER FROM THE GWANDU AQUIFER, SOKOTO BASIN, NORTHERN NIGERIA

MEBUS A. GEYH and KLAUS WIRTH

Nieders~chisches Landesam t fi~r Bodenforschung, Hannover (Federal Republic of Germany) Institut fi~r Bodenkunde und Walderniihrung, Universitiit Gbttingen, Gbttingen (Federal Republic of Germany) (Received January 29, 1980; accepted for publication March 26, 1980)

ABSTRACT Geyh, M.A. and Wirth, K., 1980. 14C ages of confined groundwater from the Gwandu aquifer, Sokoto Basin, northern Nigeria. J. Hydrol., 48 : 281--288. Groundwater samples from the confined aquifer of the Gwandu Formation, Sokoto Basin, northern Nigeria were dated by the 14C method. Conventional ages from 3000 to 30,000 yr. B.P. were determined. Hydroisochrons (i.e. lines of equal age) were constructed. F r o m these the velocity of the groundwater was calculated and the direction of flow determined. A low rate of recharge from 10,000 to 20,000 yr. ago, comparable to observations in the Central Sahara, was established. Because values for recently recharged groundwater are not available it is not possible to calculate the present recharge.

INTRODUCTION

The interest of hydrogeologists in the natural recharge and m o v e m e n t of fossil groundwater has rapidly increased during the last decade, mainly because these waters are being increasingly used for drinking water and irrigation. Especially in semi-arid and arid regions, with their limited groundwater reserves, an assessment of present-day recharge should be done before these aquifers are exploited. Groundwater ages of up to 40,000 yr. are determined by means of the radiocarbon m e t h o d (Miinnich, 1957, 1968; Knetsch et al., 1962; Andres and Geyh, 1970; Klitzsch et al., 1976). As the 14C c o n t e n t of the biosphere has been rather constant during at least the last 100,000 yr., the initial ~4C content of the newly recharged groundwater should also be the same as in the past. The present ~4C c o n t e n t of groundwater in comparison to the initial ~4C c o n t e n t may be a measure for the age of the groundwater if it has had no contact with the atmosphere during the time of its m o v e m e n t within the 002 2 - 1 6 9 4 / 8 0 / 0 0 0 0 - - 0 0 0 0 /$ 0 2 . 5 0 © 1980 Elsevier Scientific Publishing Company

282 aquifer. This concept excludes hydrochemical (Wigley et al., 1978) and hydraulic changes (Geyh and Backhaus, 1978) in the 14C c o n t e n t of the groundwater.

HYDROGEOLOGY The study area represents the southeastern part of a flat basin (Sokoto Basin) on the African Shield (Fig. 1). The center of this basin is in Niger, where it is called the Illumeden Basin. The sediments are of Cretaceous and Tertiary age, the youngest strata (continental terminal) may represent an Eocene stage (or younger} (Kogbe, 1973). They consist mainly of unconsolidated clay, silt and sand, dipping gently towards the center of the basin (Fig. 2). A layer of limestone and some thin layers of lignite and ironstone interrupt the m o n o t o n o u s series. Most of the sandy beds of this basin are good aquifers and are tapped by wells. The youngest series, the Gwandu Formation {continental terminal), consist of an unconfined aquifer on top, separated by a layer of clay from the confined aquifer, which is the (Anderson and Ogilbee, 1973): " . . . most important part of the Gwandu Formation with respect to groundwater . . ." A basal clay, too thin to be shown in Fig. 2, seals the confined part of the Gwandu Formation from the underlying limestone of the Kalambaina Formation. Only during the rainy season (May--September) is the rate of precipitation ( ~ 7 5 0 m m / y r . at Sokoto Airport) higher than the potential evapotranspiration so that recharge can take place by direct infiltration of rainwater within area A as shown in Fig. 2. In addition, both surface and subsurface r u n o f f above the clay stratum may reach area A and increase the rate of recharge. The Sokoto and Zamfara rivers have contact with the aquifer where they intersect the area of outcrop; a seepage of river water into the aquifer is also very likely. The main area of discharge seems to be the valley of the Niger River. The low, but perennial r u n o f f of the Sokoto and Zamfara rivers is due to the discharge of recently recharged groundwater into these rivers. Peak r u n o f f during the rainy season is due to the surface runoff.

RESULTS OF THE GROUNDWATER DATING Groundwater dating in the area of investigation was restricted to the confined aquifer of the Gwandu Formation. The objectives were to determine the groundwater velocity, to estimate the rate of recharge, and to confirm the direction of flow. Water samples were taken from nine wells scattered over the Nigerian

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part of the productive area of the Gwandu confined aquifer. The calcium bicarbonate in the groundwater samples was precipitated by addition of barium hydroxide. Contamination of the samples by atmospheric CO2 did not occur as maximum 14C ages were found (Table I).

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The conventional radiocarbon ages of the groundwater samples range from 3000 yr. (Tangaza) to 31,000 yr. (Yeldu). The absence of 3 H reveals that the samples do n o t contain recently recharged groundwater. The pH values of ~ 6 (determined with indicator paper) are not comparable with previously determined values of pH = 7. The narrow range of calcium bicarbonate concentrations between 200 and 4 0 0 p p m reflects a rather undisturbed TABLE I Results of t h e i s o t o p e analysis No.

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3H content (TU)

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--18.1 --16.1 ---16.8 --15.6 --16.2 - - 18.2 --18.3 --16.4

7 , 1 3 0 -+ 2 8 0 26,650-+1,850 >26,350 3 1 , 3 0 0 -+5 , 1 5 0 20,800-+ 8 9 0 9,050-+ 235 2 3 , 5 5 0 -+1 , 3 5 0 2 , 8 2 5 + 120 5,930 + 70

41.2-+1.4 3.6-+0.8

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p.m.c. = p e r c e n t m o d e r n c a r b o n .

2.0 -+1.0 7.4-+0.8 32.2-+0.9 5.3 -+0.8 70.3-+1.1 47.8-+0.4

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Fig. 4. Relationship between groundwater recharge and velocity in the confined Gwandu aquifer (schematic). h y d r o c h e m i s t r y . Correction of the data in Table I was therefore n o t necessary (Wigley et al., 1978). Isotope exchange between limestone in the aquifer and the dissolved calcium bicarbonate as discussed by Miinnich (1968) is unlikely as the rock of the aquifer does n o t contain carbonate and most of the limestone layer at the base is separated from the aquifer by clay. From the geological situation in the c a t c h m e n t area alone, an initial 14C c o n t e n t of 85% m o d e r n or m or e may be assumed (Geyh, 1972). With this assumption, the actual groundwater ages differ from the conventional radiocarbon ages (Table I) by - - 1 3 0 0 yr. The areal distribution of the conventional ~4C ages of the groundwater samples allows the c ons t r uc t i on of hydroi sochrons (lines of equal age) which mo r e or less follow the isopotential lines (Fig. 3). Radiocarbon ages are low close to the area of recharge, increasing with the direction of flow. G r o u n d w a t e r velocity during the Holocene from east to west may have a m o u n t e d to a b o u t 3--4 m/yr. A reliable estimate for the time before 20,000 yr. B.P. c a n n o t be made because of the distances between the wells. A lower velocity of the g r oundw a t e r (i.e. the hydr oisochrons are closer to each other) west of the area of recharge m a y be due to the increasing thickness of the aquifer. Th e k-values derived from this velocity and an assumed porosity of p = 20% are in the range of 10 -4- -10 -s m/s. The g r o u n d w at er velocity also allows an estimate of the recharge R. The ratio between the average thickness of the aquifer near the o u t c r o p and the width o f the o u t c r o p is in the order of q = 1 : 5 (Fig. 4). R can therefore be calculated by:

P R = Vqlo0 We obtained an R-value of 0 . 1 6 0 m / y r . which seems to be quite high; m ore exact data for q and p may yield m or e reliable results. Of course, this calculation is valid only for the time between 5,000 and 10,000 yr. B.P. The close distance between the h y d r o i s o c h r o n s for 10,000 and 20,000 yr. B.P. may reflect the k n o w n world-wide palaeoclimatic event. Analogous to the actual Sahara desert (Klitzsch et al., 1978), groundwater recharge might have been

287 c o n s i d e r a b l y d i m i n i s h e d or even i n t e r r u p t e d d u r i n g this p e r i o d , as has b e e n d e m o n s t r a t e d in o t h e r p a r t s o f t h e g l o b e {Andres et al., 1 9 7 0 ; G e y h , 1972}. R e c h a r g e f o r t h e last d e c a d e or e v e n t h e last c e n t u r y c a n n o t be derived f r o m t h e s e d a t a since the y o u n g e s t w a t e r i n v e s t i g a t e d is a l r e a d y s o m e t h o u sand y e a r s old.

CONCLUSION T h e g r o u n d w a t e r o f t h e c o n f i n e d G w a n d u a q u i f e r is fossil w a t e r in m o v e m e n t . R e c h a r g e o f a p p r o x i m a t e l y 150 m m / y r , in t h e p a s t m a y n o t be t r u e f o r the last f e w d e c a d e s or c e n t u r i e s d u e to the climatic d e t e r i o r a t i o n . Bec a u s e o f t h e n a r r o w o u t c r o p o f t h e a q u i f e r o n l y a few million c u b i c m e t e r s o f w a t e r c a r be r e c h a r g e d w i t h i n t h e Nigerian s e c t o r u n d e r g o o d c o n d i t i o n s . B e f o r e t h e d e v e l o p m e n t o f this a q u i f e r is b e g u n , t h e w a t e r level has to be o b s e r v e d p r i m a r i l y n e a r t o t h e area o f r e c h a r g e to avoid e x p l o i t a t i o n o f t h e g r o u n d w a t e r a n d a decline o f t h e p i e z o m e t r i c level. A l o w e r level o f g r o u n d w a t e r w i t h i n t h e u n c o n f i n e d p a r t o f t h e a q u i f e r will also a f f e c t t h e dryseason f l o w o f t h e S o k o t o a n d Z a m f a r a rivers, w h i c h c o u l d cause h e a l t h problems.

ACKNOWLEDGEMENTS The authors appreciate the cooperation of the Geological Survey of Nigeria in K a d u n a a n d also t h e h e l p b y local A u t h o r i t i e s in S o k o t o d u r i n g this field investigation.

REFERENCES Anderson, H.R. and Ogilbee, W., 1973. Aquifers in the Sokoto Basin, Northwestern Nigeria, with a description of the general hydrogeology of the region. U.S. Geol. Surv., Water-Supply Pap. 1757-L, 79 pp. Andres, G. and Geyh, M.A., 1970. Untersuchungen iiber den Grundwasserhaushalt im iiberdeckten Sandsteinkeuper mit Hilfe yon 14C- und 3H-Wasseranalysen. Wasserwirtschaft, 8 : 259--263. Geyh, M.A., 1972. Basic studies in hydrology and 14C and 3H measurements. Proc. 24th Int. Geol. Congr., Montreal, Que., 11: 227--.234. Geyh, M.A. and Backhaus, G., 1978. Hydrodynamic aspects of carbon-14 groundwater dating. Proc. Syrup. Isotope Hydrology 1978, II, I.A.E.A., Vienna, pp. 631--643. Klitzsch, E., Sonntag, Ch., Weistroffer, K. and E1 Shazly, E.M., 1976. Grundwasser der Zentralsahara: fossile Vorrhte. Geol. Rundschau, 65: 264--287. Knetsch, G., Shata, A., Degens, E., Mfinnich, K.O., Vogel, J.C. and Shazly, M.M., 1962. Untersuchungen an Grundw~issern der Ost-Sahara. Geol. Rundsch., 52: 587--610. Kogbe, C.A., 1973. Geology of the Upper Cretaceous and Tertiary sediments of the Nigerian sector of the Illumeden Basin (West Africa). Geol. Rundsch., 62: 197--211.

288 Miinnieh, K.O., 1957. Messung des 14C-Gehaltes von hartem Grundwasser. Naturwissenschaften, 44 : 32--33. Miinnich, K.O., 1968. Isotopen-Datierung yon Grundwasser. Naturwissenschaften, 55: 158--163. Wendt, I., Stahl, W., Geyh, M.A. and Fauth, F., 1967. Model experiments for the 14C water age determination. Proc. Symp. Isotopes in Hydrology, I.A.E.A., Vienna, pp. 321--326. Wigley, T.M.L., Plummet, L.N. and Pearson, Jr., F.J., 1978. Mass transfer and carbon isotope evolution in natural water systems. Geochim. Cosmochim. Acta, 42: 1117-1139.