A multi-scale investigation of an island groundwater resource under a drying climate using isotopic techniques

Coastal aquifers provide a source of water for more than two billion people, with island freshwater lenses being some of the most vulnerable coastal groundwater systems due to their reliance on rainfall recharge and their susceptibility to saltwater intrusion. This thesis makes an assessment of a freshwater lens in a drying climate through the use of hydrochemical and isotopic data at both single time point and temporal time scales in a bid to better understand the sustainability of such a resource. Groundwater and surface water samples were collected from Rottnest Island, Western Australia, on a seasonal basis between 2013 and 2016 for hydrochemical and isotopic (14CDIC, 14CDOC, 18O, 2H, 3H), analysis and compared with weekly composite rainfall samples. Carbon isotope values in groundwaters combined with the composition of DOC identified three water types, with the deepest saline groundwaters having a distinctly different composition to the other groundwaters on the island. This older groundwater may result from the slow circulation of the seawater wedge, or from remnant seawater caused by past sea-level highstands. A comparison of current and historic data, showed a reduction in the extent of fresh groundwater by approximately 1 km2 since 1977, which was primarily caused by a reduction in rainfall recharge rather than abstraction. A combination of modelling and geochemical time-series analysis shows recharge occurs both annually and from high rainfall events, with modelling showing the mean residence times of fresh groundwaters ranged from 11.5-36.8 years. The importance of temporal groundwater monitoring to constrain the hydrology and water balance of shallow, unconfined aquifers, due to seasonal variation, was also demonstrated. In conclusion, this thesis has advanced our understanding of the evolution, temporal variation, recharge and residence times of an island freshwater lens using isotopic tools, which can assist in the management of similar resources globally