Groundwater seepage controls salinity in a hydrologically terminal basin of semi-arid northwest Australia

[Display omitted] •Terminal basins in arid environments may have low rates of groundwater seepage.•Groundwater outflow of ~2GL/yr is sufficient to maintain moderate salinity.•Extremely low groundwater seepage can significantly impact hydrochemistry.•A dimensionless time model is more accurate than classical water budget approach. Very small groundwater outflows have the potential to significantly impact the hydrochemistry and salt accumulation processes of notionally terminal basins in arid environments. However, this limited groundwater outflow can be very difficult to quantify using classical water budget calculations due to large uncertainties in estimates of evaporation and evapotranspiration rates from the surface of dry lake beds. In this study, we used a dimensionless time evaporation model to estimate the range of groundwater outflow required to maintain salinity levels observed at the Fortescue Marsh (FM), one of the largest wetlands of semi-arid northwest Australia (∼1100km2). The groundwater outflow from aquifers underlying the FM to the Lower Fortescue catchment is constrained by an extremely low hydraulic gradient of