摘要
为评价博茨瓦纳帕拉匹村非均质裂隙含水层的地下水资源 ,采用双重介质法及增加阻水水平屏障来模拟含水层 ,用排水和蒸发双重边界来模拟季节性河流 ,用溶质运移模型来再现示踪剂氟氯烃化合物 (CFC)在地下水中的分布 ,用环境同位素氢和氧分析地下水的来源 ,用氯离子质量平衡法估计入渗量。对双重介质的不同有效空隙度的确定则采用了地下水年龄示踪剂CFC的实测值分析得出。这些为地下水模型的真实、准确提供了几种独立。
In order to evaluate groundwater resources in heterogenous aquifer in Palapye wellfield, Botswana, A groundwater model was constructed which integrates all available knowledge in order to arrive at an estimate of recharge and an understanding of the functioning of the aquifer. The recharge rate is the single most important quantity required as a basis for sustainable aquifer management. This figure is especially difficult to determine in arid and semi arid environments. The fractured nature of the aquifer and an ephemeral river required special modeling solutions. Recharge 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 and the Tswapong formations form the confining and water bearing layers of the aquifer. The main aquifer is the Tswapong formation, which is fractured and consists mainly of sandstones. Its groundwater potential is highly variable, which strongly depend on the connections to the system of large fractures. Structural features acting as barriers to flow were modeled 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 boundary consisting 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 wellfield, which signifies that the groundwater in the Tswapong layer is subject to little or no evaporation before reaching the water table, despite the usually strong evaporation in semi arid regions. The measurements all lie very close together, which means that the groundwater was recharged at a similar altitude and temperature, i.e. in the outcrop of the Tswapong formation and recharge is only from precipitation. Recharge was estimated by measuring 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, They are in line with the estimate of 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 groundwater to equivalent air concentrations using known solubility relationships and the recharge temperature. The result shows that the age of groundwater in Palapye wellfield is between 25 to 50 years, wtich is proved by their presence in the production boreholes that fault zones are preferential flow paths that conduct most of the available flow. The alternative hypothesis of a homogenous flow through the bulk rock would not allow CFC to reach the boreholes in many years to come. Effective porosity values in the faults and in the background of the two layers were 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.
出处
《干旱区地理》
CSCD
北大核心
2001年第4期347-352,共6页
Arid Land Geography
