| Karst aquifers can be highly susceptible to contamination due to a close connection to the land surface, lack of filtration through a soil zone, and rapid transmission along solutionally-enhanced flowpaths. These factors also allow for aquifer contamination by direct injection of immiscible organic compounds in the form of non-aqueous phase liquids (NAPLs). The fate and transport of NAPLs in karst aquifers is poorly understood. Tracer tests allow for qualitative characterization of hydraulic flow in the surface and subsurface, but their results may not be applicable to the movement of NAPL for which density plays a critical role. Particulate tracers have been used to predict sediment transport but they fail to mimic the range of possible NAPL densities. Therefore, this research focused on the development and field testing of hydrogel tracer beads (HTBs), a non-toxic tracer made of calcium alginate (a derivative of marine algae), to better represent the behavior of NAPLs. The density of the HTBs can be readily modified to match different types of NAPLs. Sinking- and buoyant-HTBs were released during a preliminary field test at Hazel Run, in Bruceton Mills, WV. The buoyant-HTBs traveled the fastest and were recovered at the collection site, while the sinking-HTBs beads settled to the channel bed. Comparative tracer tests using fluorescein and buoyant-HTBs were completed at Buckeye Creek Cave near Lewisburg, WV and Rhine Creek in Terra Alta, WV. Despite the two systems being very different, the field tests in both systems demonstrated that the buoyant-HTBs had a greater velocity and had a lower mean transit time than did the fluorescein. These results are similar to what other comparable research has found – that particulate tracers travel faster than dissolved solutes. Based on the comparative tracer tests, light NAPLs (LNAPLs) may travel faster than dissolved tracers and travel times determined from solute tracers may not accurately reflect the appropriate time to collect water samples. |
