Estimating Hydraulic Properties of Alluvial Sand Aquifer in Motloutse River course, Eastern Botswana

Abstract

A single layered alluvial aquifer in Botswana along the Motloutse River catchment was studied to estimate its hydraulic properties. Ground Penetrating Radar Survey (GPR) was used to determine the thickness and porosity of the aquifer. Specific yield of the aquifer was determined in the laboratory. The hydraulic conductivity of the riverbed sediments were estimated using the Alyamani-Sen empirical formula and the hydraulic conductivity of the river bank sediment was measured using the slug test. Geologically, the area is consists of alluvial deposits overlying granite, which is the bed rock of the area. The geophysical survey, GPR, provided information on the range of the thickness of the alluvium (9-12m) and also the porosity of the sediments (40%). The laboratory test resulted in an average specific yield of 13.68% for the riverbed sediments and 8.84% for the river bank sediments. A slug test performed in the riverbank yielded estimates of hydraulic conductivity of 26.43m/day. An average hydraulic conductivity value of 160m/day for the riverbed sediments was determined using the Alyamani-Sen formula. This difference in the hydraulic properties of the riverbed and the riverbank sediments is likely to be a result of sediment heterogeneity as more fine grained sediments is found together with sand in the riverbank sediments. These combined hydraulic properties estimation using empirical formulas, geophysical survey, laboratory tests, and the slug test highly improved the understanding of the hydrologic properties of the single layered alluvial aquifer system in the Motloutse River catchment. In the same area, groundwater resource modelling can be done using these aquifer parameters to determine the groundwater potential of the aquifer.