Quantifying the role of groundwater in sustaining Groenvlei, a shallow lake in the southern Cape region of South Africa

[摘要] Eight of the 21 Ramsar-designated wetlands in South Africa are located in similargeohydrological settings as Groenvlei, a 359 ha lacustrine wetland found east of Sedgefield inthe southern Cape. Groenvlei is unique as it is isolated from the sea and neither fed nordrained by rivers. Consequently, the lake is fed only by rainfall and groundwater inflow.Losses comprise evaporation and groundwater outflow. These characteristics result in arelatively uncomplicated hydrological system that allows for the geohydrological componentto be quantified and understood. Using climatic and lake data monitored by the Departmentof Water Affairs and geohydrological data collected over a period of a decade, research wasconducted to quantify the groundwater contribution to the system and develop an improvedunderstanding of the hydrology of Groenvlei.A daily water balance based on rainfall, adjusted S pan evaporation data and lake levels wasused to compute that the nett groundwater contribution to Groenvlei amounted to about0.3 mm/d. It was shown that S pan evaporation data adjusted by coefficients prescribed byMidgley et al. (1994) should be used to quantify lake evaporation, and that the reed collartranspired 10% to 30% more during summer than evaporated from open water. No water istranspired by the reed collar in winter as the vegetation is dormant.Integrating the water balance results with steady-state Darcian flow calculations and achemical mass balance indicated direct rainfall (71.6%) and groundwater inflow along thewestern and northern boundaries of the lake (28.4%) constituted inflow into the system. Thisis balanced by evaporation from open water (61.7%), transpiration from the reed collar insummer (21.4%) and groundwater outflow along the southern boundary (16.9%). This lattercomponent 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 wascalculated only 5.7% of rainfall in the lake catchment discharges into the lake. The balance ofrain entering the subsurface is lost through terrestrial evaporation or discharges into the seavia the deeper part of the aquifer. It was interpreted that the deceptively thick vadose zoneplays a buffering role in the hydrology of the area and that evapotranspiration losses areappreciable. The importance of the reed collar was further exemplified by the retention of salts in thevegetative fringe. Salts are assimilated by the vegetation and retained in the hyporheic zoneuntil re-entrained into the main water body through wind and wave action. This results inonly part of the salt load leaving the lake along the southern boundary and affectinggroundwater quality between the lake and the sea. Further research is required to confirmthis.The results of the research allowed for tools to be developed to assess the impact ofgroundwater abstraction from the lake's catchment on lake levels and water quality. Thesetools could also be used to demonstrate Groenvlei has long since lost its connection to themarine or estuarine environments, with a new equilibrium being reached within 120 years ofdisconnect. The young lake is dynamic in character and rapidly responds to hydrologicalchange. In its short history, Groenvlei has adapted and responded to changes in both sea leveland climate, collectively resulting in the present-day system.In addition to highlighting the importance of sound conceptualisation, data quality and aconvergence 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 issustained by underlying Table Mountain Group aquifers. It was also found that thepermeability 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 aneed exists to link hydrologists and ecologists to better understand wetlands, with eachcontributing specific skills and knowledge.An important contribution of the research documented in this thesis is that the approach usedcan be applied to similar wetlands where the role of groundwater might be less obviousbecause of river flows and tidal exchange. The importance of sound conceptualization anddirect rainfall onto wetlands, quantification of evaporative losses using S pan data andcoefficients prescribed by Midgley et al. (1994), and the relationship between open waterlosses and transpiration losses are three aspects that could improve the understanding andquantification of lake �?groundwater interaction elsewhere.A limitation to understanding the geohydrology of Groenvlei is the lack of informationpertaining 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 thicknessof 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.

[发布机构] University of the Free State