TRACER-BASED INVESTIGATION OF GROUNDWATER FLOW AND CONNECTIVITY IN FRACTURED AQUIFERS

Abstract

The fluorescence tracer uranine is frequently applied to groundwater studies, but its behavior in fractured crystalline aquifers is still not well constrained since apparent tracer losses may be due to actual sorption on rock surfaces or changes in fluorescence intensity with temperature. The purpose of this study was to measure the temperature dependency of uranine in solution within water-groundwater mixtures as well as in batch contact with crushed gneiss from the fractured aquifer system of Freiberg (Germany), to separate the true sorption behavior of uranine onto the rock from apparent uranine concentration changes induced by temperature-dependent fluorescence quenching. Batch reactor experiments were performed with aqueous solutions of groundwater only, groundwater and uranine, and groundwater, uranine, and gneiss at initial uranine concentrations of 0.5, 5, 20, and 50 µg/L and at temperatures of 10, 25, and 55 °C. Triplicate reactors were quantified using temperature-specific calibration curves prepared in the same groundwater matrix as the samples. For each temperature, calibration standards consisted of Freiberg groundwater containing known uranine concentrations but no gneiss, so that both matrix effects and temperature-dependent fluorescence quenching were incorporated into the concentration calculation. Correcting for this fluorescence–temperature dependence, uranine exhibits no detectable sorption at concentrations of 0.5 and 5 µg/L at any temperature within the limits of analytical detection. Weak sorption is detectable at concentrations of 20 µg/L, and higher still at 50 µg/L, with up to several tens of percent sorption observed in some cases. Temperature showed only a modest and non-systematic effect on sorption, with some datasets even showing higher sorption at 10 °C relative to 25 °C or 55 °C. Overall, these data suggest that uranine is likely to behave as a conservative tracer under the chemical conditions presented here at concentrations commonly applied in the field, but that non-conservative behavior may be important at higher concentrations and should be considered in tracer tests in crystalline aquifers.