Abstract
South Dunedin and Harbourside are vulnerable to surface flooding during intense rainfall events. The causes of this flooding are exacerbated by the underlying shallow groundwater system, which has a high water table and low infiltration capacity. The objective of this study was to assess the origins of groundwater to discern whether saline water intrusion was occurring, or whether the system is predominantly influenced by rainwater infiltration. By understanding the origin of the groundwater in South Dunedin and Harbourside, the potential vulnerabilities to flooding now and into the future could be better understood. Analysis of in situ measurements and water samples extracted from 23 bores across the study area allowed hydrochemical analysis to be undertaken. The hydrochemical indicators used within this analysis were specific electrical conductance, major and minor ion concentrations, as well as stable δ 18O and δ 2H isotope values.
Results from this study indicate that there were three likely origins of groundwater in South Dunedin and Harbourside, marine, freshwater, and mixed origin. Rainfall was identified as the main contributing source of recharge for this groundwater system, reflected by the depleted δ 18O and δ 2H groundwater isotope values, that plotted along the local meteoric water line (with the exception of Kennedy Street). Marine origin groundwater was found to be a much less significant component of groundwater than originally thought, with only one site that showed predominantly marine water origin after analysis of groundwater hydrochemistry and end member mixing models, that site being Kennedy Street. The predominance of fresh and mixed origin groundwater in South Dunedin and Harbourside indicated that the risk of surface flooding in the area was driven by rainfall events that cannot be effectively attenuated, due to the low storativity of the underlying groundwater causing surface inundation. Mitigation measures such as creating areas where water can be stored during rainfall events, increasing permeable surfaces, and ensuring efficient stormwater infrastructure is in place should be implemented to reduce the detrimental affects of surface flooding in South Dunedin and Harbourside.