Some organic micropollutants are so stable that they occur in high concentrations in treated wastewater that is discharged into streams. They also persist in surface waters for relatively long periods. Investigating the timescales over which these contaminants circulate through the streambed is a first step required for a better understanding of their persistence in our environment. Previous research has shown the importance of considering diurnal variability of surface discharge on the fate and transport of these compounds; however, seasonal changes can also be important, especially in arid climates where dilution occurs during the wet season, while summer months are characterised by high solar radiation and air temperatures.
In this study, we conducted approximately one-hour continuous salt tracer injection experiments during the wet and dry seasons (September 2016 and March 2017) in a small perennial, wastewater-impacted stream in South Australia. The data were first analysed using the solute transport model OTIS. Analysis of solute breakthrough curves over five stream reaches within approximately two kilometres showed strong variations in residence time distributions and hyporheic zone properties along this short stretch of the stream. Challenges of using this simple model included the strong correlation between transient storage and dispersion; therefore, sensitivity analysis was needed to better determine parameters. Ongoing work to better differentiate streambed parameters is discussed.