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Vlei Rehabilitation Study illustrates Importance of Groundwater

28 Oct 2008 Posted by in Environment | Comments

From South African Groundwater Division – Western Cape Branch

Coastal vleis in and around the Cape Town, under rising pressure from urbanisation, have received a welcome boost with the completion of a multidisciplinary study aimed at restoring Zeekoevlei to a healthily functioning ecosystem. “It is important, from both ecological and social points of view, to restore the Zeekoevlei system,” said Ms Julia Wood of the South Peninsula Administration, which funded the project. “This action will play a role in helping to develop tourism and upgrade surrounding communities.”

The project, led by leading freshwater ecologist and wetland specialist, Dr Bill Harding of Southern Waters Ecological Research & Consulting, found that sustained addition of nutrients had resulted in the vlei becoming hypertrophic. This is a condition of extreme nutrient enrichment that affects the ecology of the vlei and disturbs the natural balance of the system. Previous shallow lake and vlei studies led by Southern Waters had confirmed the need to understand the role played by groundwater in the hydrological and ecological dynamics of these systems.

Thus the Zeekoevlei rehabilitation study also included a groundwater investigation, an aspect not previously addressed for any of the Cape Flats vleis. “For many years it was generally believed the vleis were fed by surface water,” said Harding. “From work we have done previously, we recognised groundwater could play an important role in supporting the vlei. So we decided to included this aspect in the study.”

Work by Parsons & Associates Specialist Groundwater Consultants showed about 15% of the total volume of water discharged into Zeekoevlei is derived from groundwater.

“More importantly, groundwater is the sole source of water during dry summer months,” revealed Parsons & Associates’ Roger Parsons, who undertook the groundwater work. “Groundwater inflows reduce the effect of evaporation losses and ensure water in the vlei during this period. Our work showed groundwater plays a critical role in sustaining the system during summer, while heavy downpours during winter help flush the system. This is a good example of the interdependent roles played by surface and groundwater.”

A further important finding of the study was that about one third of the nutrient loading into Zeekoevlei results from seepage from the adjacent Cape Flats sewage works. “The main contributors to nutrient loading in the vlei are the sewage works, nutrients trapped in the sediments in the vlei and inflows from the surrounding catchment,” explained Harding. “To be able to improve the state of the vlei we need to address all three issues. Problems related to the sewage works and sediments can be addressed in the short term, while catchment issues such as litter and other forms of pollution in the Lotus River catchment are longer term problems.”

The Zeekoevlei project is the first of a series of vlei rehabilitation studies committed to find workable solutions rather than calls for further research. From the outset, the project team strove to converge existing information and expert assessment into a suite of realistic management scenarios that, if implemented, will vastly improve the grossly impaired ecological condition of Zeekoevlei. “Zeekoevlei and the other vleis of the Cape Flats are an important asset to Cape Town,” emphasized Julia Wood of the South Peninsula Administration. “A management action plan has been devised and costed, and we are now assessing the proposals made by the project team. The intention is to implement recommendations of the study without delay.”

Project Sponsor: South Peninsula Administration
For more information contact Roger Parsons (rogerpasgc.co.za)

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