Hydrochemical apportioning of irrigation groundwater sources in an alluvial aquifer

River floodplains sustain irrigated agriculture worldwide. Despite generalised groundwater level falls, limited hard data are available to apportion groundwater sources in many irrigated regions. In this paper, we propose a workflow based on: hydrochemical analysis, water stable isotopes, radiocarbon contents and multivariate statistical analysis to facilitate the quantification of groundwater source attribution at regional scales. Irrigation water supply wells and groundwater monitoring wells sampled in the alluvial aquifer of the Condamine River (Queensland, Australia) are used to test this approach that can easily be implemented in catchments worldwide. The methodology identified four groundwater sources: 1) river/flood water; 2) modified river/flood water; 3) groundwater recharged through regional volcanic materials and 4) groundwater recharged predominantly through sands and/or sandstone materials. The first two sources are characterised by fresh water, dominant sodium bicarbonate chemistry, short residence time and depleted water stable isotope signatures. Groundwater sources 3 and 4 are characterised by saline groundwater, sodium chloride chemistries, enriched water stable isotopes and very low radiocarbon contents, inferred to correspond to long residence times. The majority of wells assessed are dominated by flood water recharge, linked to decadal >300 mm rainfall events and associated flooding in the region. The approach presented here provides a groundwater source fingerprint, reinforcing the importance of floodwater recharge in the regional water budgets. This apportioning of groundwater sources will allow irrigators, modelers and managers to assess the long-term sustainability of groundwater use in alluvial catchments.This research was funded by the Cotton Research and Development Corporation, Australia, Grant Number UNSW1401. The authors would like to thank Lucienne Martel for her assistance with sample collection. We would also like to thank all cotton growers who provided access to their irrigation water supply wells and the staff at the Queensland Department of Natural Resources and Mines (Toowoomba, Office) who facilitated at short notice access to the government groundwater monitoring wells. Dr. Matthias Raiber (CSIRO) is also acknowledged for conversations and assistance in the field during sampling. Chris Dimovski, Barbora Gallagher, Robert Chisari, Henri Wong, Brett Rowling and Vlad Levchenko from ANSTO are also acknowledged for their constant logistic and analytical support. Laura Scheiber and Enric Vázquez-Suñé would like to thanks Spanish Ministry of Science and Innovation (Project CEX2018-000794-S). The authors also thank Lisa Williams for editing and proofreading the manuscript.Peer reviewe