Abstract
Shallow coastal aquifers are dynamic groundwater systems. They can display considerable variability in groundwater levels over time. Complications arise when urban areas exist above such aquifers as groundwater can interact with underground infrastructure where they come into contact. Additionally, if the water table becomes elevated to the extent that it reaches the surface, groundwater flooding may occur. Low- lying coastal areas are also at the frontline of climate change. Yet the potential effects of climate change on groundwater in these environments are poorly understood. This thesis explores groundwater dynamics in a shallow coastal aquifer in the context of potential interactions with urban land use and future climate change. The South Dunedin aquifer serves as a case study for this purpose. South Dunedin consists of a low-lying urban area, bounded by ocean on two sides, with a shallow, heterogeneous aquifer of quaternary age below. The area has been identified as vulnerable to the effects of climate change and previous research has suggested that mean groundwater levels may become elevated in response to sea level rise in the future. The focus of this thesis is on understanding natural variability in the position of a water table, in particular, in response to rainfall events, and how this may interact with urban land use and the effects of climate change.
To explore this a groundwater monitoring network was established across South Dunedin and high frequency groundwater monitoring occurred between July 2012 and January 2013. Water table movements were found to display spatial variability, which can be linked to differences in sediment and therefore hydraulic conductivity between sites. Significant temporal variability in the position of the water table was also observed. A linear regression relationship has been established between rainfall and water table responses. This has allowed potential water table responses to heavy rainfall events with return periods of 2, 5, 10 and 50 years to be extrapolated. The results of this indicate that groundwater flooding could occur in response to rainfall events with return periods of 5 years or more at some sites in South Dunedin.
How the water table in South Dunedin may respond to future sea level rise has previously been modelled by the Otago Regional Council (ORC) indicating that sea level rise may force groundwater levels to rise. In this thesis the variability of groundwater levels in response to rainfall events is considered in conjunction with an elevation of mean groundwater levels due to sea level rise. These factors are found to combine to increase the risk of groundwater flooding in South Dunedin. While sea level rise alone may not result in mean groundwater levels reaching the surface for quite some time, South Dunedin could experience sporadic flooding occurring more and more often as sea level rise brings the water table closer to the surface. The closer to the surface the water table is located, the greater the opportunity for the natural temporal variability in groundwater levels due to rainfall to bring the water table to the surface, and into contact with underground infrastructure. While groundwater flooding is estimated to occur in response to one in five year rainfall events at present, with sea level rise of 0.1m or more, a one in two year rainfall event could lead to the same extent of flooding. Thus, we could consider one of the effects of sea level rise for South Dunedin as an increase in the probability of flooding due to rainfall events. There is also evidence from wastewater pipe flow monitoring suggesting that groundwater may be infiltrating into the South Dunedin stormwater and wastewater pipe network through weaknesses, and doing so at an increased rate when the water table is elevated in response to rainfall. Accordingly, this study points towards interactions between groundwater, climate change and urban infrastructure as important issues for the future management of South Dunedin.
These findings have implications for coastal cities with shallow groundwater conditions, as the problems South Dunedin faces will very likely be the problems encountered in similar environments in the future. Variability of groundwater levels is rarely represented in climate change impact studies. Yet this research demonstrates a long-term trend of sea level rise and short-term fluctuations due rainfall events interacting to affect groundwater levels. This highlights the importance of both investigating, and acknowledging, the role of natural variability when considering how climate change may impact hydrological systems.