Solubility of TINOPAL CBS-X fluorescent dye at different EDTA concentrations and pH values: Implications regarding its applicability in field tracer tests

Abstract

TINOPAL CBS-X has been described in published literature as a fluorescent dye that is suitable for use as a tracer in groundwater investigations. However, several of these field tests have yielded unexpected results. In this study the influence that the complexation agent EDTA (ethylenediaminetetraacetate) and pH have on the solubility of TINOPAL CBS-X have been systematically investigated in the laboratory and the results obtained validated with thermodynamic calculations using the PhreeqC software. Modeling indicated that the amount of EDTA required for complete TINOPAL CBS-X dissolution is, in many aquifers, so great that its use is no longer feasible. However, softening the water by adding NaOH appears to be a promising way to increase TINOPAL CBS-X solubility. Water with a low availability of bivalent and/or trivalent ions yielded solubilities up to two orders of magnitude higher than waters from limestone aquifers with a high Ca2+ ion content. Results revealed that the spectrum of TINOPAL CBS-X applicability as a conservative tracer in aquatic environments is limited to a limited range of specific water chemistries. The long tailing in the breakthrough curve and the retarded breakthrough of TINOPAL CBS-X in a previously reported carbonate karst aquifer field tracer test was accurately reproduced by a transport model with a modified input function based on solubility modeling. Travel times and attenuation by incompletely dissolved tracers are tend to be overestimated, which can be problematic with regard to predictions of contaminant transport, or the delineation of water protection zones. The use of TINOPAL CBS-X as a conservative tracer and the concentrations required needs therefore to be carefully evaluated with respect to water-hardness, pH, alkalinity, and dilution rates. An input script for PhreeqC is provided to help estimate appropriate concentrations of TINOPAL CBS-X, thus enhancing its potential and reliability for future groundwater research.