Assessing The Influence Of Groundwater Recharge Mechanism In Non-Perennial River Systems, Tankwa Karoo, South Africa

In South Africa and neighboring countries such as southern Zimbabwe, Botswana, southern Angola and Namibia, most river systems are non-perennial due to semi-arid/arid climatic characteristics. In such river systems, the interaction between groundwater and surface water is of significance in terms of developing appropriate methods for determining ecological water requirements among others. However, the interaction is not well understood in terms of the influence on the volume and quality of water on the gaining and losing water bodies. In past years, research on non-perennial rivers (NPRs) has not been widely published for various reasons. In certain cases, NPRs experience extended periods of water ponding within their channels. This could possibly be caused by groundwater seepage that is sufficient to maintain pools but insufficient to generate channel flow (gaining stream) and overcome evaporation losses. The opposite can also occur, whereby some reaches of the river channel are recharging the underlying aquifer (losing stream). Abstraction of either groundwater or surface water thus impacts on both water resources.

The objectives of the study are to investigate the role of the Tankwa River in recharging the underlying aquifer and the role of the aquifer in recharging the Tankwa River. Preliminary findings through literature review and field observations seem to suggest that the groundwater flow in terms of the regional perspective is driven by recharge in the far upland TMG Mountains. However, on a local scale, field observations seem to suggest that there is some aquifer-river interaction, whereby the aquifer is maintaining the pools in some parts of the channel whereas some parts of the river are dry. These findings suggest that the river contains both losing and gaining reaches thereby providing indication of an exchange of water between the water resources. This has implication on the quantity and quality of water in gaining and losing water bodies in aquifers and rivers. Future work will involve installing piezometers at points where permanent pools are located and along the river riparian zone. Monitoring of groundwater levels and the river will be carried through the different seasons. Samples will be collected from the aquifer, surface water bodies and rain gauges to integrate with the groundwater chemistry. The overall purpose of the present study is to develop a regional hydrogeological conceptual model of recharge for the Karoo in order to improve understanding of the recharge mechanism in non-perennial river systems especially in the semi-arid environment, using the Tankwa River as a case study.

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