Comparison of sulfur hexafluoride, fluorescein and rhodamine dyes and the bacteriophage PRD-1 in tracing subsurface flow

We compared velocities of the subsurface flow from a mounded onsite septic system towards a depressional wetland with three types of tracer; an inert gas, sulfur hexafluoride (SF6), two fluorescent dyes, fluorescein and rhodamine WT, and a viral tracer, the bacteriophage PRD-1. The movement of both fluorescent dyes was significantly retarded in the soils compared to both SF6 and PRD-1. In experiments using injection solutions containing both a dye and SF6, fluorescein was found to move at least 3–4 times slower than SF6, and rhodamine was not observed away from the drainfield. In contrast, the velocities calculated from SF6 data are very similar to the velocities calculated from the PRD-1 data obtained during the same experiment. At a second site, the movement of fluorescein was half as fast and not as extensive as the movement of SF6. The results of these experiments indicate that fluorescent dyes may underestimate velocities of effluent from septic systems adjacent to seasonal wetlands. In contrast, SF6 was found to perform similarly to the viral tracer PRD-1.