Dying to find the source – the use of rhodamine WT as a proxy for soluble point source pollutants in closed pipe surface drainage networks

Abstract. Rhodamine WT (RWT), a xanthene dye, may serve as a proxy for soluble pollutants within quantitative tracing studies investigating point source contaminant transport. This study quantified the effects of altering the concentration, pH, temperature and salinity of a RWT solution on the detected fluorescence of RWT within the laboratory prior to a field release of RWT within a closed pipe urban drainage network. All RWT solutions exhibited stability and <10% variation from the expected concentration over a thirteen hour laboratory study period; pH related quenching of RWT fluorescence of up to 14.9% was observed for solutions with pH<3.9; and increasing salinity of RWT solution was found to have a negligible quenching effect. In direct contrast to previous studies RWT fluorescence was found to directly correlate with temperature of solution, and a temperature correction factor was determined and tested. The field release study succeeded in detecting RWT at concentrations two orders of magnitude greater than background fluorescence. Based on longitudinal dispersion theory, observed RWT peak concentrations were within 10% of predicted peaks.