Articles | Volume 21, issue 11
Research article
28 Nov 2017
Research article |  | 28 Nov 2017

A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer

Charlotte P. Iverach, Dioni I. Cendón, Karina T. Meredith, Klaus M. Wilcken, Stuart I. Hankin, Martin S. Andersen, and Bryce F. J. Kelly

Abstract. Understanding pathways of recharge to alluvial aquifers is important for maintaining sustainable access to groundwater resources. Water balance modelling is often used to proportion recharge components and guide sustainable groundwater allocations. However, it is not common practice to use hydrochemical evidence to inform and constrain these models. Here we compare geochemical versus water balance model estimates of artesian discharge into an alluvial aquifer, and demonstrate why multi-tracer geochemical analyses should be used as a critical component of water budget assessments. We selected a site in Australia where the Great Artesian Basin (GAB), the largest artesian basin in the world, discharges into the Lower Namoi Alluvium (LNA), an extensively modelled aquifer, to convey the utility of our approach. Water stable isotopes (δ18O and δ2H) and the concentrations of Na+ and HCO3 suggest a continuum of mixing in the alluvial aquifer between the GAB (artesian component) and surface recharge, whilst isotopic tracers (3H, 14C, and 36Cl) indicate that the alluvial groundwater is a mixture of groundwaters with residence times of < 70 years and groundwater that is potentially hundreds of thousands of years old, which is consistent with that of the GAB. In addition, Cl concentrations provide a means to calculate a percentage estimate of the artesian contribution to the alluvial groundwater. In some locations, an artesian contribution of up to 70 % is evident from the geochemical analyses, a finding that contrasts with previous regional-scale water balance modelling estimates that attributed 22 % of all inflow for the corresponding zone within the LNA to GAB discharge. Our results show that hydrochemical investigations need to be undertaken as part of developing the conceptual framework of a catchment water balance model, as they can improve our understanding of recharge pathways and better constrain artesian discharge to an alluvial aquifer.

Short summary
This study uses a multi-tracer geochemical approach to determine the extent of artesian groundwater discharge into an economically important alluvial aquifer. We compare estimates for artesian discharge into the alluvial aquifer derived from water balance modelling and geochemical data to show that there is considerable divergence in the results. The implications of this work involve highlighting that geochemical data should be used as a critical component of water budget assessments.