Is Groenvlei really fed by groundwater discharged from the Table Mountain Group (TMG) Aquifer?

R Parsons

doi:10.4314/wsa.v35i5.49425

Abstract

Vankervelsvlei is a unique wetland located in the stabilised dunes east of Sedgefield. Groenvlei is one of a series of 5 brackish coastal lakes along the Southern Cape coast of South Africa, but is the only one disconnected from the sea. It has been hypothesised that discharge from the underlying Table Mountain Group Aquifer sustains Vankervelsvlei, which in turn discharges into Groenvlei. This paper critically reviews the conceptual model and information on which the hypothesis was based. It is argued that the conceptual model is flawed as it does not take account of topographical and geohydrological conditions prevalent in the area. Analysis of limited hydrochemical data did not explore other possible water sources, and the electrical conductivity characteristics used to confirm the link between the wetlands and the deeper secondary aquifer also apply to 56.3% of boreholes located in a variety of aquifer types across the Western Cape Province. No information is available that supports a link to the Table Mountain Group. Rather, it appears that Vankervelsvlei is sustained by direct rainfall and there is no hydraulic link between Vankervelsvlei and Groenvlei.

Highlights

Groenvlei is a brackish coastal lake known for its diverse bird life and is one of the best venues for large-mouth black bass angling
By comparing 5 chemical parameters measured at the 5 sites at Vankervelsvlei and Groenvlei to ranges considered by Brown et al (2003) as being typical of groundwater from Table Mountain Group (TMG) aquifers, Roets (2008, p. 138) concludes that ‘the hydrochemical data of the groundwater from this study suggests that Vankervelsvlei and Groenvlei are dependent on groundwater from the TMG Aquifer.’
It is concluded that Roets (2008; 2009) and Roets et al (2008a; b) failed to provide any credible scientific evidence that either Vankervelsvlei or Groenvlei are fed by discharges from the TMG Aquifer or that the 2 wetlands are hydraulically linked

Summary

Introduction

Groenvlei is a brackish coastal lake known for its diverse bird life and is one of the best venues for large-mouth black bass angling. In addition to TMG groundwater discharging from depth upwards into Vankervelsvlei, the conceptual model requires water to be discharged from the 0.5 km wetland downward into the sub surface to create and maintain the hydraulic link with Groenvlei. It is not possible for the opposing groundwater flow directions to co-exist in such a small area. The aquitard is required to support the thesis that tion is in line with the description of Vankervelsvlei presented Groenvlei and Vankervelsvlei are sustained by discharge from the underlying TMG Aquifer as the confining layer provides the mechanism needed for groundwater from depth to flow upwards and into the wetlands. The EC range presented by Brown et al (2003)) for the Peninsula Formation is much narrower (3 mS/m to 26 mS/m) than for the Nardouw Subgroup (9 mS/m to 155 mS/m), and only 25% of Roets’s data fall within the Peninsula Formation range

Findings

Discussion

Conclusions