University of Botswana, Geology Department, Private Bag 0022,. Gaborone, Botswana. Abstract Research is going on in semiarid Botswana to assess.
Regionalization in Hydrology (Proceedings of the Ljubljana Symposium, April 1990). IAHS Publ. no. 191, 1990.
A simple chloride balance routing method to regionalize groundwater recharge: a case study in semiarid Botswana
J. J. DE VRŒS Institute of Earth Sciences, Free University Amsterdam, PO Box 7161, 1007 MC Amsterdam, The Netherlands
A. GIESKE University of Botswana, Geology Department, Private Bag 0022, Gaborone, Botswana
Abstract Research is going on in semiarid Botswana to assess the recharge in a series of small groundwater basins, developed along a fracture system in Precambrian rocks. Multiple recharge sources occur with their individual response to the alternating wet and dry periods of varying time scale. The characterization of groundwater, surface water and soil moisture through their chloride concentration appears to be an excellent tool to quantify the various recharge components. The groundwater is routed through the basin by subdividing the watershed into a number of cells in which complete mixing of chloride is assumed, and for which a mass balance of water and chloride is established. Régionalisation du calcul de la réalimentation des eaux souterraines par la méthode du chlore à partir de l'étude du cas d'un climat semi-aride de Botswana Résumé Des recherches sur l'alimentation des eaux souterraines par différentes méthodes sont actuellement réalisées, dans une série de petits bassins souterrains situés sur une zone de fracture dans une formation rocheuse d'âge Précambrien. Le bilan du chlore offre la possibilité de suivre la circulation des eaux souterraines en tenant compte de la concentration du chlore dans les différents composants du système de réalimentation. Le système hydrogéologique est schématiquement considéré comme formé par des compartiments individualisés estimés homogènes, dans lesquels un mélange complet des eaux est également supposé. Cette méthode nous permet d'étudier des régimes permanents et non permanents en mesurant régulièrement les concentrations en chlore des eaux de pluie, des rivières et des eaux souterraines. INTRODUCTION Botswana is a semiarid country with very limited water resources. The erratic 81
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82
rainfall is restricted to the hot season and varies from 250 mm a year in the extreme southwest to 650 mm in the northwest. The flat surface with sandy soils causes most of the precipitation to infiltrate, but the moisture retention capacity and the high transpiration by dense savanna vegetation impede deep percolation and most if not all of the precipitation surplus eventually evaporates. This condition especially prevails in the vast Kalahari sandy semi-desert, that occupies over 80% of the country. Conditions for groundwater accumulation are more favourable in the eastern part of Botswana which is underlain by older rocks of mainly Precambrian age. Rapid percolation through fractures and weathered material prevent the percolating rainwater from evaporation in the case of suitable morphological and geological structures (De Vries & Von Hoyer, 1988). The aquifers are formed along synforms and fracture zones and constitute groundwater basins of limited size of the order of 10 km2. These basins often form topographical basins too, because of a structural control over geomorphology. The question of the replenishment of these aquifers is of paramount importance to water resources planning. Water balance studies cannot make use of observations of natural discharge because the basins normally do not show concentrated and gaugable outflow through springs or streams. Recharge is balanced by diffuse seepage to lower permeable zones and eventually by evaporation in dry valleys and other depressions. Detailed research on recharge in a few basins in southeastern Botswana was recently started, and includes mass balance studies of water, chloride and isotopes as well as soil moisture transport investigations. The present paper presents preliminary results of the chloride balance component in this research project. The various recharge components are characterized by their chloride content and are routed through a number of subsurface cells, for which a mass balance of water and chloride is established. Similar approaches have been followed by others including Eriksson & Khunakasem (1969), Adar & Neuman (1988) and Dettinger (1989).
STUDY AREA The study area consists of two small interconnected groundwater basins, north of the small town of Lobatse. The basins are formed along a NNE-trending fracture zone in dolomites, shales and quartzites and are morphologically reflected in a small and elongated system of surface catchments (Figs 1 and 2). The axis of these basins is formed by a poorly-defined valley with steep slopes at the western side and slightly sloping plantation surfaces at the eastern margins. The surrounding hills are constituted by ridges and isolated inselbergs reaching heights of 200-300 m above the surrounding pediments. The steep west slopes consist of impervious volcano-clastic sediments of the Ventersdorp Supergroup, whereas the valley and the eastern margins are formed by dolomites, shales and quartzites of the Transvaal Supergroup. The most permeable part of the basins (the aquifer proper) has developed along
83
Chloride balance routing method and groundwater recharge
I - H I Pitsanyane Basin 131 Nnywane Basin '. Morokty,
4050, ,4458
main road
-vXoyj'ï- aquifer
s
observation well
I
®
production well
(9)
/ * \
cell I average chloride content (mg/l) in aquifer
hill >1200 A.S.L
Fig. 1 Location map of Pitsanyane and Nnywane basins the fracture system near the axis of the valley. The valley fill consists of alluvial and colluvial soil and coarse rubble from the slopes, locally up to a depth of 30 m. Weathering on the slopes is shallow. The boundaries of the groundwater basin are delineated through the culminations of the surrounding hills in the west and by a groundwater divide halfway along the eastern pediment. The latter divide is caused by eastward dipping bedding planes in the dolomites, directing groundwater away from the valley (Fig. 2). The southern basin is called the Pitsanyane basin and its surface area is assessed at 7 km2, of which about 2 km2 forms the aquifer proper. Field
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84
Ng. 2 Schematic cross section through Pitsanyane/Nnywane basins. A* = area of basin; A+ = area of aquifer proper; R* = recharge from channelled runoff; R+ = recharge by interflow; I = diffuse recharge; Q = saturated groundwater flow. observations revealed that recharge is concentrated in fan-like colluvial deposits at the foot of the west slope, where small episodic gullies discharge the runoff from the surrounding areas. Main recharge comes from the gauged rivulet (Pitsanyane River) with a catchment of 1.5 km2. In addition to this concentrated percolation, a diffuse infiltration takes place into the alluvial and into the sandy soils on the eastern pediments. This water percolates laterally along the slopes to the central aquifer. Between the concentrated and diffuse percolation, an infiltration of intermediate nature can be identified on the west slopes with shallow soil cover and weathered rock outcrops. This percolation is henceforth indicated as interflow. The Nnywane basin forms the northern part of the aquifer zone. There seems to exist a groundwater barrier between the Pitsanyane basin and the Nnywane basin, impeding groundwater flow in dry periods when the water level falls below a certain threshold. The Nnywane River enters this basin halfway and forms a real alluvial valley. Basin and valley merge into the wider and poorly defined Moroekwe basin. The ephemeral Nnywane River is obstructed by a dam before it enters the Nnywane basin. In wet years the dam may spill and water from the river then percolates into the aquifer. The Nnywane basin thus forms a very small part of the catchment of the Nnywane River. The size of the basin is estimated at 10 km2, of which 2.5 km2 is occupied by the central aquifer. The long-term annual rainfall and potential evaporation are of the order of 550 mm and 2000 mm respectively. The area carries a dense bush and tree savanna vegetation. THE AVERAGE MASS BALANCE OF THE PITSANYANE/NNYWANE BASINS Chloride is an excellent tracer because of its conservative nature. The only source of chloride in the study area with its predominantly arenitic sediments,
85
Chloride balance routing method and groundwater recharge
consists of dry and wet deposition from the atmosphere. Thus shifts in chloride concentration in soil moisture and groundwater in the downstream direction can be attributed to changes in water mass flux. The atmospheric chloride deposition as well as the chloride concentration in the saturated zone has been monitored since 1983. Chloride in soil moisture has been investigated since 1986 in a large number of vertical profiles in which isotope analyses were also carried out (Gieske et al., 1990). The average chloride deposition (CQ) was determined at 500 mg m"2 1 year" . The chloride content in soil moisture (C() is fairly constant over the area, averaging 35 mg l"1 in the Pitsanyane basin (Fig. 3) and 55 mg l"1 in the more silty soils of the Nnywane basin. The distribution of chloride in the saturated zone is indicate in Fig. 1. Concentrations in interflow and surface runoff (C ) were observed to average 6 and 3 mg l"1 respectively.
Average of Profiles : A4.B1.B2.C2.C3,C4,E2.E3 Standard Dev. indicated
0
100
200
300
400
500
Chloride ( m g / l moisture)
Fig. 3 Chloride distribution in the unsaturated zone in the alluvial soils of the Pitsanyane basin. The area is divided into four cells of homogeneous hydrological nature, and the following mass equations apply to any cell: (1) where: Qn
= I A
n n
+ Rn+
G„.a
(2)
/ = diffuse recharge through the soil; R = concentrated recharge at the west slope through interflow (R*) or by runoff (R*); Q = saturated groundwater flux; C = chloride concentration of cell n of the aquifer; A = area of cell; n
86
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= cell index. If we assume a steady state vertical flux in a non-sorbing chloride solute system in the unsaturated zone, then the recharge by diffuse infiltration is given by: / = C0/C(
(3)
which gives: I (Pitsanyane) = 14 mm year"1 I (Nnywane)
= 9 mm year"1
From observations we have the following data:
Celll Cell 2 Cell 3 Cell 4
Area (km1)
C/t (mg l'1 ) i f
R*
2.0 1.5 3.5 10.0
37 16 9 11.5
0 0 ? ?
? ? 0 0
Solving equations (1), (2) and (3) with these data gives in 105 m"3 year"1: Ix = 0.3
F*=0
Q1 = 0.3
I2 = 0.2
R*2 = 1.0
Q2 = 1.5
73 = 0.5
R* = 3.8
0 3 = 5.8
I4 = 0.9
R* = 2.9
Q4 = 9.6
This means a total average annual recharge (sum of components I and R) of 83 mm for the Pitsanyane basin and 38 mm for the Nnywane basin. MASS BALANCE OF THE NNYWANE BASM DURING A DROUGHT A dry spell began in the 1978-1979 rainy season after the 1970s had been relatively wet. The Nnywane Dam became almost depleted in 1982 and public water supply shifted in 1983 from surface water to groundwater with abstractions from the production wells Pv P 4 and Py The average annual rainfall over the 1982-1986 period was only 264 mm and a rather steady groundwater level decline of 1.9 m year"1 was observed from early 1983 until early 1986. At the beginning of 1986 the Nnywane River produced some runoff causing a little rise in the groundwater level (Fig. 4). The total groundwater production during the 1983-1986 period amounted to 11 x 105 m3. The natural depletion of the aquifer by
87
Chloride balance routing method and groundwater recharge Nnywane Basin Wells near river bed Period April 1983 - May 1988
-104132
-20-
-30-
^^-:,.,.,.,
,.--,..
~""~^ -.'. 4129
-40-
1
83
!
1
1
I
85
Nnywane Basin Wells north of Morokwe Period April 1983 - May
4451
-5083
Year
Fig. 4
Hydrographs of boreholes in the Nnywane basin.
groundwater discharge during the period 1980-1982, prior to pumping, caused a groundwater level decline of 1.5 m year"1. This natural discharge stopped early in 1983 because of a reverse of the hydraulic gradient between Pl and borehole 4132 through the abstraction by Pv Pumping tests and drawdown analyses revealed the aquifer to be of phreatic and fractured nature, with an average transmissivity of the order of 25 m 2 day"1 and a storativity between 1 and 10%. The average chloride concentration of the aquifer increased from 11.5 mg l"1 in 1982 to 15.5 mg l"1 in 1986. It is assumed that the diffuse recharge of the aquifer was not affected by the drought and remained steady at 9 mm
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88
year"1 with a chloride concentration of 55 mg F1. This caused a depletion of the shallow groundwater reservoir on the less pervious slopes around the aquifer proper (Fig. 4). Water and chloride mass balances for the periods 1980-1982 and 1983-1986 can be established as follows : 1980-1982:
QoM - A* I - Q-m = A* S* dH1
(4)
1983-1986:
Q0 - A* I - Qin = A+ S*
