khartoum hydorology

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Application of Tracers in Arid Zone Hydrology (Proceedings of the V ienna Symposium,August 1994). IAHS Publ. no. 232 , 1995. 5 7

Isotope hydrogeology and water balance assessmentnear the Nile in SudanM. A. GEYHNiedersachsisch.esLandesamt fur Bodenforschung,PO Box S10153,D-30631 H annover, GermanyK. FROEHLICHIsotope Hydrology Section, Department of Research and Isotopes, InternationalAtomic Energy Agency, Wagramerstrasse 5, PO Box 100, A-1400 Vienna, AustriaB. TH. VERHAGENSchonland Research Centre, Witswatersrand University, Johannesburg,South AfricaAbstract Hydrogeological studies with numerical modelling of theNubian sandstone aquifer near Khartoum have been conducted for futuregroundwater development, proposing considerable infiltration of Nileriver water. This concept appeared to be confirmed by piezometric levelsand environmental tritium gradients. It was concluded that present daygroundwater pumping, plus projected increases, are balanced by thisinfiltration. Isotope hydrological and hydrochemical observations led,however, to a different postulate. The high tritium values observed havebeen interpreted as a result of downward seepage of irrigation w ater. Onthis basis a simplified transport model was devised assuming that duringthe last pluvial some 6000 years ago, groundwater level stood at aboutthe present land surface. Following this, it declined due to evaporationand regional drainage into the Nubian sandstone aquifer. The subsequentmore modest infiltration from the river is roughly in balance with lateralgroundwater drainage. Planned increases in groundwater exploitation atsome distance from the Nile will therefore mine "fossil" groundw ater.

HYDROGEOLOGICAL SITUATIONIn cooperation with the Geological and Mineralogical Resources Department, Khartoum,the Federal Institute for Geosciences and Natural Resources, Hannover, Germany,carried out hydrogeological studies including isotope hydrological analyses, in theKhartoum Province from 1976 to 1978. The Institute of Environmental Physics of theUniversity of Heidelberg , G ermany, previously carried outpaleohydrogeological investigations in this area (Sonntag, 1985).

The investigation area is located along the eastern side of the Nile near K hartoum.The southern part adjacent to the Blue Nile east of Khartoum extends about 40 km eastward. The northern part abuts the Nile north of the confluence of the Blue and WhiteNiles and covers an area 10 km wide and 30 km long (Fig. 1).In the investigation area , the major aquifer lies in the Nubian sandstone. It reachesa thickness of about 400 m and overlies the basement which consists of crystalline rocks.Pleistocene deposits of the Nile ("Gezira formation") cover the Nubian sandstone near
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58 M. A. Geyh et al.the river (Fig. 2).The groundwater of the Gezira formation is in hydraulic contact with the Nubiansandstone. Near the Nile the gradient of the groundwater table varies between 1:500 and1:1500. Five k ilometres further away the gradient approximates to 1:1500 (Fig. 1).The groundwater table reaches a level of 375 m a.s.l. near the Nile. Thirty to fortykilometres away from the Nile there are two depressions where the groundwater tabledecreases to 345 m a.s.l. (Fig. 1; Civita et al., 1975). The groundwater can only flowwest from these depressions as in the eastern and northern direction there is the

70

50

55,3 1 4 C - v a l u e ( p i769 we l l No.. -36S~ p iezom et r i c l ine (m a .s . l .)

5 k mFig. 1 Piezometric map of the upper part of the phreatic groundwater zone north ofKhartoum and 14C values of various sampling sites (Profile A-B, Fig. 2).


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Isotope hydrology and water balance assessment near the Nile in Sudan 59

after Krampe et al . 1979|-;:o.i;.:?:'| sand or sandstone193^ 1 clay or mudstone0 2 4 kmL 1 1

Fig . 2 Geological profile north of Khartoum (A-B, Fi g. 1).

impermeable basement. Thus, it was proposed that groundwater reverses its flow atdepth, crossing underneath the Nile ("Nile-roll"; Bender et ah, 1983) and, finally,discharges in the marshlands of Dongola, situated some 450 km northwest of Khartoum.The concept of "Nile-roll" is based on the observation of different transmissivities(950 and 80 m day"1, respectively) in the lower and upper part of the aquifer. The intermediate layers are less permeable, especially near the Nile where the water levels ofneighbouring dug wells and boreholes can differ by up to 2 m.In the study area the mean annual precipitation is about 150 mm (IAEA , 1981). Inview of the high potential evaporation, recent recharge seems to be negligible but cannotbe completely excluded (Verhagen, 1985).ISOTOPE HYDROLOGICAL AND HYDROCHEMICAL RESULTSThe 2H/180 plot for the groundwater samples followed 2H = 8 180 + 8. Almost allthe values lay between the locally relevant precipitation lines of Addis Ababa andKhartoum. The mean isotopic compositions of the White and the Blue Nile overlappedthe scattering range of the groundwater, reflecting mixing of water from these twosources (Verhagen et al., 1991).

Groundwater close to the Nile exhibited contemporary tritium values in a rangetypical for bomb-produced tritium in the Nile water. About 4-5 km further away, thetritium values became undetectable (Fig. 3).Near the Nile, groundwater contained 14C up to 114 pMC in 1986 (Fig. 1). This wasin accordance with the tritium results and indicates surface water infiltration. In addi-


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60 M. A. Geyh et al.

1 0 03 H (TU)

o o

50

\\Q\A . oo o o i O o2 3 4 5distance from Nile river (km)

Fig. 3 3H values of groundwater samples versus distance from the Nile (broken curveindicates the boundary of N ile water infiltration by 1976).

tion, it gave evidence that in the study area the pre-bomb 14C initial content could beassumed to be about 100 pM C.In the southern part of the study area, the distance of the sampling points from theBlue Nile reaches m ore than 10 km and the 14C values dropped to below 50 pM C. Thelowest 14C value was observed at the 136 m deep well No 912. This groundwater samplewas probably pumped from the lower aquifer zone with a more depleted 14C value(Fig. 4).With a few exceptions, the mineralization of the groundwater was rather low . The

AV VB - profile sectionO 74.4 - 1 4C (p m c)^3ss_, - piezometric lines (m

Ia.s.l.)

Fig. 4 Profile section AB of the isohypsic map (according to Krampe et al. (1979) eastof Khartoum with 14C values (pMC ).


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Isotope hydrology and water balance assessment near the Nile in Sudan 61bicarbonate content ranges from 140 to 400 mg l"1. Cl~ and SO J ions reach values ofup to about 600 and 2500 mg l"1, respectively. The NO J content of shallow groundw aterranges up to 90 mg l"1 and correlates with the electric conductivity (EC) of the watersamples (Fig. 5).

INTERPRETATION OF THE RESULTSTritium, stable isotopes and hydrochemistryThe high tritium values observed in samples collected in 1976 were interpreted as beingindicative of infiltration of Nile water into the upper aquifer (Krampe et al, 1979) witha maximum distance from the Nile of about 4 km since 1963 (3H detection limit). F romthis, a mean tracer velocity was estimated to be vT = 4000 m /1 3 year = 308 m year"1.Assuming an effective porosity ne = 0.1-0 .15, one obtains aflowrate of 31-46 m year"1.However, the corresponding value derived from Darcy's law calculated with the adoptedregional hydraulic conductivity K of 5 x 10"5 m s"1 and a hydraulic gradient h/l =0.007 was vDL = K x dh/dl = 11 m year"1. The disagreement between the two resultsis evident.The discrepancy disappears with the assumption (Verhagen et al., 1991) that thehydrochemical and isotopic composition of the groundwater of the upper part of thephreatic aquifer at distances of 1.5-4 km from the Nile is mainly determined by theinfiltration of irrigation and rainwater. The arguments are :(a) Samples exhibiting high tritium values were taken from dug wells which tap onlythe uppermost layer of the aquifer.(b) Clear evidence of infiltration of mineralized irrigation water is obtained by the plotof EC and NO J vs the distance from the Nile (Fig. 5). Considering the goodcorrelation between EC and the NO3 contents, fertilizers seem to be the major

4 0 0 - (6000 BPend of pluvial period)level of Nile

' " - . 4 3 7 0 0 BPJ..,- 375 m

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