Groundwater mean residence time of a sub-tropical barrier sand island

Abstract

Abstract. Fresh groundwater on barrier islands is affected by changing sea levels, groundwater use and precipitation variability due to climate change. These systems are also vulnerable to contamination and groundwater over-abstraction. Constraining groundwater mean residence times (MRT) and flow paths are essential for understanding and managing these resources. This study uses tritium (3H) and carbon-14 (14C) to determine the MRT of groundwater along a bore transect across North Stradbroke Island, South-East Queensland, Australia. Hydraulic properties, major ion geochemistry and stable isotopes are used to validate residence times, and to identify processes responsible for their variability. 3H activities range from \u2009150 years. Recharge occurs over the entire island and groundwater MRT increase vertically and laterally towards the coastal discharge areas. MRT estimated from 14C display similar spatial relationships but have a much greater range (modern to up approximately 5000 years). Water diversion and retention by perched aquifers with underlying lower permeability units in the unsaturated part of the dune systems are so far the most likely course for relatively long MRT. The results indicate that these perched aquifer systems are probably wide spread and have a significant influence on regional recharge. The geochemical composition of groundwater remains relatively consistent throughout the island, with the only irregularities attributed to old groundwater stored within coastal peat. The outcomes of this study enhance the understanding of groundwater flow, recharge diversion and inhibition for large coastal sand masses in general. For south-east Queensland, it allows the existing regional groundwater flow model to be refined by incorporating independent MRT to test model validities. The location of this large fresh groundwater reservoir, in dry and populous South East Queensland, means its potential to be used as a water source is always high. Background information on aquifer distribution and groundwater MRT are crucial to better validate impact assessment for water abstraction.