Flow-through column experiments to determine the geochemical behavior of common hydrological tracers

Tracer testing is one of the most effective methods used to study groundwater flow, reservoir characteristics and subsurface properties in geohydrology. Hydrological tracer tests were conducted with the basic assumption that the tracer is chemically inert and non-reactive. However, not all tracers behave non-reactive at different pH conditions, the particular tracer may interact with mineral surfaces in the reservoir. In order to study the geochemical behavior of some common hydrological tracers flow-through column experiments were conducted at 25°C. Six common hydrological tracers were investigated, amino G acid, fluorescein, napthionic acid, pyranine, rhodamine B and rhodamine G in porous rocks consisting of basaltic glass, quartz or rhyolite at pH 3, 6.5 and 9. Homogenous porous material of fixed grain size 45-125μm were dry packed in the column to conduct flow through column experiments. Tracers were pumped at fixed flow rates for 20 minutes and switched back to experimental blank solution and the tracer concentration monitored at the outlet. The measured break-through tracer curves were compared to theoretical 1-D reactive transport simulations calculated using the PHREEQC program (Parkhurst and Appelo, 1999). The data obtained from the breakthrough curves suggest that the tracers may be reactive, non-reactive and partially reactive depending on the rock type and solution pH. The tracers that were observed to be reactive showed the influence of adsorption and desorption. The results suggest that some tracers commonly used in ground water hydrology are not suitable under all conditions as they may react with the rocks of the groundwater system.