Bench-Scale Investigation of Fluorescein as an Applied Tracer for NAPL Impacted Groundwater

Roger Saint-Fort¹, and Darcy Bye²

1. Department of Environmental Science, Mount Royal University, Calgary, Canada

2. Trans Canada Corporation, Calgary, Canada

Abstract: In this study, the detection limit of fluorescein, a non-reactive tracer was statistically established at 0.02 mg/L. Correspondingly, the initial tracer signal-to-noise ratio was set at 5,000 times the detection limit. Empirical evidence indicates that an injected-based reagents mixture of tracer and ClO₂ as a one-step operation does not produce an effective approach towards achieving field system operation and optimization. Significant and rapid degradation of the tracer signal-to-noise ratio were induced by the presence of ClO₂ in the mixture. Greater than 90% of fluorescein tracer in the solution mixture with ClO₂ was degraded within an 8 sec time interval by the oxidant. Loss of ClO₂ ranged from 35 to 65% of the initial concentration in the systems. The ratio of fluorescein to ClO₂ mass disappearance per unity time (tracer: ClO₂: time) appears to be temperature independent, generally proportional to either the concentration of fluorescein or ClO₂ in a respective system. Such ratio ranged from 59 to 373. Breakthrough curve for the non-reactive fluorescein tracer was obtained from a bench-scale physical model. The tracer breakthrough curve was bell-shaped with increased tailing and significant spreading as the tracer progressed in time and distance. Quantifying the degree of mixing was done by means of the dimensionless Pėclet number (Pe). Pe value was 3, implying a relatively pronounced effect of the NPAL lens on the mean transit time of the tracer. In such cases, the centroid lags behind the peak concentration of the tracer mass. Locale-scale dispersion dominates fluorescein breakthrough behavior. An effect ascribed to the NAPL lens with respect to travel time distributions of the tracer cloud. Knowledge gained from this study can be applied to provide guidance in early stage field operations and valuable for the development of conceptual site model.

 Key words: tracer, NAPL, breakthrough curve; chlorine dioxide; pėclet number