Groundwater–Surface Water Interaction: Process Understanding, Conceptualization and Modelling (Proceedings of Symposium HS1002 at IUGG2007, Perugia, July 2007). IAHS Publ. 321, 2008, 67-75.

Quantify groundwater’s role in sustaining Groenvlei, a shallow lake in the Southern Cape region of South Africa

Roger Parsons

Parsons & Associates Specialist Groundwater Consultants, PO Box 2606, Somerset West, 7129, South Africa

roger@pasgc.co.za

Abstract Groenvlei is one of a series of five brackish coastal lakes along the Southern Cape coast of South Africa, but is the only one disconnected from the sea. Abstraction of groundwater for urban water supply purposes could impact the volume of groundwater discharged into the lake. Using information gleaned elsewhere regarding the role of groundwater in sustaining a shallow coastal lake and data measured on site, the role of groundwater in sustaining Groenvlei was quantified. The lake is both influent and effluent in character, and analysis of climatic data, monitored water level data of Groenvlei and groundwater data suggests the lengths of the inflow and outflow boundaries are 6155 m and 5245 m, respectively. Groundwater contributes 3.86 Mm³/year to the system, which equates to 61% of the freshwater input. Proposed groundwater use adjacent to the lake could lower the level of Groenvlei by 17 mm and is thus unlikely to significantly impact the hydrological or ecological functioning of the system.

Key words surface–groundwater interactions; Darcy’s law; water balance