Simulation of Arid Multi-Layer Aquifer System to Evolve Optimal Management Schemes: A Case Study in Shashe River Valley, Okavango Delta, Botswana

A multi-leaky aquifer system in Shashe river valley (Okavango delta), Botswana was simulated to study the aquifer response and thereby evolve optimal pumping schemes. The river valley falls in an arid region and has three aquifers separated by two aquitards. The upper alluvial aquifer below the narrow riverbed is being exploited to augment drinking water supply to the nearby wildlife tourist center, Maun. The bottom aquifer is saline. Continuous increase of abstraction from the upper unconfined and middle semi-confined aquifers has been causing progressive decline of water levels in these aquifers, as well as migration of saline water to the middle aquifer. In order to quantify the upward flow from the saline unit, a multi-layer model was constructed and calibrated for both steady state and transient condition. The calibration was reasonably good even with the given limited water level history for the middle (semi-confined) aquifer system. The calibration has shown that vertical conductivity (K¹) of the confining layers has a considerable influence on the hydrodynamics of multi-aquifer system. As a natural corollary, groundwater movement to the middle (semi-confined) aquifer is found to be dependent on the vertical downward leakage from the upper phreatic aquifer and upward leakage from the lowermost saline unit. As the river-derived recharge to the phreatic aquifer has been negligible due to no-flow condition existing in the riverbed for nearly a decade, even a moderate pumping rate of 3150m³/day may cause severe damage to the middle aquifer. The upper reaches of the middle semi-confined aquifer are likely to become unconfined and the upward leakage from the underlying saline aquifer will increase the concentration of total dissolved solids (TDS). The situation may be improved by reconfiguration of well fields. The longevity of wellfields will increase if the pumping rate from the lower reach is reduced and that from middle and upper reaches is increased.