AN ANALYSIS OF EARTH-TIDE-INDUCED GROUNDWATER-FLOW IN EASTERN BOTSWANA

Earth-tide induced groundwater level fluctuations with a maximum amplitude of 12 cm have been observed in a fractured shale aquifer in eastern Botswana. Comparison of the oscillations with the tidal variations in the gravity field of the earth, revealed a remarkable phase advance of the groundwater tides relative to the earth tides. A theoretical model is presented that attributes the marked amplitude to a high compressibility of the aquifer, and explains the positive phase shift through a periodic groundwater flow between the shale basin and the surrounding aquifers. The horizontal hydraulic gradient, causing this groundwater movement, is assumed to be induced by the tidal strain differentiation between the compressible shale and the more rigid arenitic and dolomitic adjoining rocks.