Abstract
Abstract. Fresh groundwater on barrier islands is affected by changing sea levels and precipitation variability due to climate change and is also vulnerable to anthropogenic processes, such as contamination and groundwater over-abstraction. Constraining groundwater mean residence times (MRTs) and flow paths is essential for understanding and managing these resources. This study uses tritium (3H) and carbon-14 (14C) to determine the MRTs of groundwater along a transect across subtropical North Stradbroke Island, south-east Queensland, Australia. Hydraulic properties, major ion geochemistry and stable isotopes are used to validate residence times and to identify the processes responsible for their variability. 3H activities range from less than 0.01 to 1 TU (tritium units), which are values lower than those of local average rainfall (1.6–2.0 TU). 14C concentrations range from 62.5 to 111 pMC (percent modern carbon). Estimated MRTs determined using lumped parameter models and 3H activities range from 37 to more than 50 years. Recharge occurs over the entire island, and groundwater MRTs generally increase vertically and laterally towards the coastal discharge areas, although no systematic pattern is observed. MRTs estimated from 14C concentrations display similar spatial relationships but have a much greater range (from modern to approximately 5000 years). Water diversion and retention by lower-permeability units in the unsaturated parts of the dune systems are the most likely course for relatively long MRTs to date. The results indicate that the internal structures within the dune systems increase MRTs in the groundwater system and potentially divert flow paths. The structures produce perched aquifer systems that are 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, especially for older sand masses that have developed structures from pedogenesis and dune movement. With respect to south-east Queensland, it allows the existing regional groundwater flow model to be refined by incorporating independent MRTs to test models' validity. The location of this large fresh groundwater reservoir, in dry and populous south-east Queensland, means that its potential to be used as a water source is always high. Background information on aquifer distribution and groundwater MRTs is crucial to better validate impact assessment for water abstraction.
Highlights
Barrier islands are common landforms in coastal environments
Studies on barrier islands from the North Sea (Europe) have indicated that the groundwater systems there are not stratified (Houben et al, 2014; Seibert et al, 2018; Holt et al, 2019). In comparison to those islands, many coastal sand masses, including the barrier islands in subtropical regions of Australia, have two aquifer systems: (1) regional groundwater that is present on the entire island and (2) small, localised perched aquifer systems and lakes that develop above low-permeability layers in the unsaturated zone
Groundwater bore 14400088 is probably screened in a perched aquifer system that surrounds Brown Lake (Fig. 1), whereas all other bores are screened within the regional groundwater system
Summary
Barrier islands are common landforms in coastal environments. They boast some of the world’s highest biodiversity, provide fresh groundwater resources, and have economic value for tourism and mineral industries. Freshwater lenses develop beneath these islands until they reach hydrodynamic equilibrium with the underlying saltwater (Röper et al, 2012) The dynamics of these lenses are similar to other coastal aquifers and are affected by changes in groundwater levels due to pumping, land use change, climate variations and sea level fluctuations (Austin et al, 2013; Masterson et al, 2014; Moore et al, 2010; White and Falkland, 2009). Variability in the water table of the perched aquifer systems on the other hand results from spatially variable local rainfall and the extent or size of the perched aquifer systems These perched systems are generally small, they are currently of paramount importance to regional ecology and groundwater-dependent ecosystems. They have the potential to inhibit recharge to the regional groundwater system and to divert the flow pattern of local groundwater
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