Abstract: In order to evaluate groundwater resources in heterogenous aquifer in Palapye wellfield, Botswana, Agroundwater model was constructed which integrates all available knowledge in order to arrive at an estimate ofrecharge and an understanding of the functioning of the aquifer. The recharge rate is the single most importantquantity required as a basis for sustainable aquifer management. This figure is especially difficult to determine inarid and semi arid environments. The fractured nature of the aquifer and an ephemeral river required specialmodeling solutions. Rechange was initially characterized using environmental isotope as well as chloride measurements. The simulated chlorofluorocarbon (CFC) distribution in the aquifer satisfactorily reproduced the measurements and confirmed the model concept.
The aquifer is bordered to the north by the Lotsane River and to the south by the Tswapong Hills. The Lotsane andthe 'Tsw apong formations form the confining and waterbearing layers of the axquifer. 'The main axqpuifer is the 'Tswapongformation, which is fractured and consists maimly of sandstones. Its groundwater potential is highly variable, whichstrongly depend on the connections to the system of large fractures. Structural features acting as barriers to flow weremodeled by assigning horizontal flow barriers to the edges of the cells along those features.
The Lotsane River is ephemeral and was modeled by establishing an automatically adapting dual boundaryconsisting of a drain and an evaporation boundary :Stable isotopes in water are affected by meteorological processes that provide a characteristic fingerprint of their origin-It is displayed against for samples taken in the well-field, which signifies that the groundw ater in the 'Tswapong layer is subject to little or no evaporation beforereaching the water table, despite the usually strong evaporation in semi arid regions. 'T'he measurements all lievery close together, which means that the groundwater was recharged at a similar altitude and temperature, i-e. in the outcrop of the 'T'swapong formation and recharge is only from precipitation.Recharge was estimated bymeasuring the chloride content in groundwater samples from the 'Tswapong outcrop. The value is to lie between 8.3 mm/a and 29. 8 mm/a, which is 2% to 10% of the mean annual rainfall T'hey are in line with the estimateof 5% of the mean annual rainfall by Neumann-Redlin (1984) and Smoor (1988).
Apparent ages in the groundwater can be obtained by converting measured CFC concentrations in ground-water to equivalent air concentrations using know n solubility relationships and the recharge temperature. 'T'he result shows that the age of groundw ater in Palapye wellfield is between 25 to 50 years, wtich is proved by theirpresence in the production boreholes that fault zones are preferential flow paths that conduct most of the availableflow. 'The alternative hypothesis of a homogenous flow through the bulk rock would not allow CFC to reach theboreholes in many years to come. Effective porosity values in the faults and in the background of the two layerswere adjusted to obtain the best result for the modeled CFC distribution.
The model satisfactorily reproduces measurements both qualitatively and quantitatively. The high degree of (qualitative reproduction indicates that the model reflects the major characteristics of the wellfield. The functioning of the wellfield seems to be understood.

Key words: Botswana, environmental tracers（δ¹⁸O, δ²H, CFC）, Double porous media method, Chloride mass balance