On variable density surface water–groundwater interaction: A theoretical analysis of mixed convection in a stably-stratified fresh surface water – saline groundwater discharge zone Gudrun Massmann a, * , Craig Simmons b , Andrew Love a , James Ward b , Julianne James-Smith a a South Australian Government, Department of Water, Land and Biodiversity Conservation, Knowledge and Information Division, Research and Innovation Group, GPO Box 2834, Adelaide, SA, Australia b Flinders University, School of Chemistry, Physics and Earth Sciences, GPO Box 2100, Adelaide, SA, Australia Received 9 June 2005; received in revised form 15 February 2006; accepted 21 February 2006 Summary Understanding the discharge behaviour of saline groundwater into fresh surface water can be critical for the effective management of water resources. While variable density flow has been studied intensely in a number of settings, the role it plays on the discharge behav- iour of saline groundwater into freshwater streams is often neglected when calculating salt loads into a stream. The aim of this study was to determine what role variable-density flow behaviour plays in surface water/groundwater interaction in a stably-stratified fresh surface water/saline groundwater interface. The mixed convection ratio M, a measure of the ratio of density driven flow to advective driven flow, was defined for a matrix of one-dimensional numerical simulations that employed both varying hydraulic and density gradients. Vertical salt breakthrough into the surface water only occurred in the advection dominated cases (M < 1) and the salt flux into the surface water increased with increasing groundwater concentration until M reached a value of 1. Beyond this, when the flow was driven by the density difference between the two fluids (M > 1) vertical discharge of salt into surface water did not occur and the saltwater/freshwater interface migrated downwards with increasing density differences between the two fluids. This study therefore shows that there is a critical concentration differ- ence that maximises salt loads to a surface water body and that a density-invariant approach to estimate the salt flux into the surface water (as the product of flow velocity determined KEYWORDS Variable density flow; Mixed convection ratio; Surface water/ groundwater interaction; River salinisation; Equivalent freshwater head 0022-1694/$ - see front matter  c 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2006.02.024 * Corresponding author. E-mail addresses: massmann@zedat.fu-berlin.de (G. Massmann), craig.simmons@flinders.edu.au (C. Simmons), Love.Andrew@saugov. sa.gov.au (A. Love), james.ward@flinders.edu.au (J. Ward), james-smith.julianne@saugov.sa.gov.au (J. James-Smith). Journal of Hydrology (2006) 329, 390– 402 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jhydrol