Abstract
M.Sc. (Aquatic Health)
Wetlands are amongst the most productive ecosystems in the natural world. Important biogeochemical and hydrological functions, such as nutrient cycling and organic carbon sequestration, are the result of a wetlands ability to moderate groundwater flow. Colbyn Valley is an urban wetland in the heart of Pretoria, South Africa that has been under threat due to urban development and pressure from Tshwane City Council. The wetland contains a 4.68 ha component of peatland that is up to 2.4 m deep in places, which is a valuable ecological resource that requires protection. Weirs have been built to restore the wetland function and prevent degradation by erosion, yet no monitoring had been conducted to determine the success of the restoration measures.
The aim of this study was to determine if rewetting of drained channelled valley bottom wetlands by weirs creates a back flooding effect, thereby raising the groundwater flow in the wetland. Wetland function was determined by means of hydrogeomorphological classification, soil profile analysis, and hydrodynamics. Hydrodynamics is a hypothetical field based method that requires the use of flow nets or models to determine flow dynamics in the system. Isotope analysis confirms the hydrological movement as illustrated by flow models, whereas flow duration curves represent the relationship between stream flow and adjacent sites.
Peat accumulation commenced 2360 years BP during the mid-late Holocene period. LiDAR images indicated the lack of significant channelling taking place at the lowest point, therefore, clearly the Colbyn Valley was originally an unchannelled system. Although monitoring well water levels follow rainfall patterns, it was noted that in the first part of the study period that there was a delayed response to rainfall, most likely due to the high clay content of the soils, reducing permeability. The rain isotope signatures were different from the wells and piezometers, indicating that the main source for the system originated from the groundwater aquifer. During the wet season, isotope signatures indicated that the wetland and stream water were not related. The point at the peatland was the most depleted section, with groundwater being exposed to evaporation when reaching the surface. Enrichment of water during the wet season was indicative of base flow maintenance, an essential function of wetlands. The flow dynamics within the peat section could not be thoroughly examined but geological maps indicating a diabase dyke could account for the inconsistencies in flow dynamics. The repairs at weir 3 had a positive effect on water levels in adjacent sites, by raising the levels with more than 60cm. When equating the water...