Quantifying the role of groundwater in sustaining Groenvlei, a shallow lake in the southern Cape region of South Africa
Parsons, Roger Paul
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Eight of the 21 Ramsar-designated wetlands in South Africa are located in similar geohydrological settings as Groenvlei, a 359 ha lacustrine wetland found east of Sedgefield in the southern Cape. Groenvlei is unique as it is isolated from the sea and neither fed nor drained by rivers. Consequently, the lake is fed only by rainfall and groundwater inflow. Losses comprise evaporation and groundwater outflow. These characteristics result in a relatively uncomplicated hydrological system that allows for the geohydrological component to be quantified and understood. Using climatic and lake data monitored by the Department of Water Affairs and geohydrological data collected over a period of a decade, research was conducted to quantify the groundwater contribution to the system and develop an improved understanding of the hydrology of Groenvlei. A daily water balance based on rainfall, adjusted S pan evaporation data and lake levels was used to compute that the nett groundwater contribution to Groenvlei amounted to about 0.3 mm/d. It was shown that S pan evaporation data adjusted by coefficients prescribed by Midgley et al. (1994) should be used to quantify lake evaporation, and that the reed collar transpired 10% to 30% more during summer than evaporated from open water. No water is transpired by the reed collar in winter as the vegetation is dormant. Integrating the water balance results with steady-state Darcian flow calculations and a chemical mass balance indicated direct rainfall (71.6%) and groundwater inflow along the western and northern boundaries of the lake (28.4%) constituted inflow into the system. This is balanced by evaporation from open water (61.7%), transpiration from the reed collar in summer (21.4%) and groundwater outflow along the southern boundary (16.9%). This latter component invalidates claims that Groenvlei is endorheic in character. Recharge to the Eden Primary aquifer was estimated to be in the order of 20% MAP. It was calculated only 5.7% of rainfall in the lake catchment discharges into the lake. The balance of rain entering the subsurface is lost through terrestrial evaporation or discharges into the sea via the deeper part of the aquifer. It was interpreted that the deceptively thick vadose zone plays a buffering role in the hydrology of the area and that evapotranspiration losses are appreciable. The importance of the reed collar was further exemplified by the retention of salts in the vegetative fringe. Salts are assimilated by the vegetation and retained in the hyporheic zone until re-entrained into the main water body through wind and wave action. This results in only part of the salt load leaving the lake along the southern boundary and affecting groundwater quality between the lake and the sea. Further research is required to confirm this. The results of the research allowed for tools to be developed to assess the impact of groundwater abstraction from the lake’s catchment on lake levels and water quality. These tools could also be used to demonstrate Groenvlei has long since lost its connection to the marine or estuarine environments, with a new equilibrium being reached within 120 years of disconnect. The young lake is dynamic in character and rapidly responds to hydrological change. In its short history, Groenvlei has adapted and responded to changes in both sea level and climate, collectively resulting in the present-day system. In addition to highlighting the importance of sound conceptualisation, data quality and a convergence of evidence, the outcomes of this study challenged the findings of Roets’ (2008) PhD research and found no scientific evidence to support his contention that Groenvlei is sustained by underlying Table Mountain Group aquifers. It was also found that the permeability south of Groenvlei is not low and the extent of the lake catchment is 25 km2. Past research of Groenvlei has resulted in a number of misconceptions and it was argued a need exists to link hydrologists and ecologists to better understand wetlands, with each contributing specific skills and knowledge. An important contribution of the research documented in this thesis is that the approach used can be applied to similar wetlands where the role of groundwater might be less obvious because of river flows and tidal exchange. The importance of sound conceptualization and direct rainfall onto wetlands, quantification of evaporative losses using S pan data and coefficients prescribed by Midgley et al. (1994), and the relationship between open water losses and transpiration losses are three aspects that could improve the understanding and quantification of lake – groundwater interaction elsewhere. A limitation to understanding the geohydrology of Groenvlei is the lack of information pertaining to aquifer thickness. It is therefore recommended four boreholes be drilled to either bedrock or at least 100 m in depth (whichever is reached first) to quantify the thickness of the aquifer. Other limitations that require attention include: • Uneven spatial distribution of the geohydrological data; • Lack of information of losses from open water and the reed collar; and • Absence of monitored groundwater data needed to address temporal relationships between groundwater and the lake.