HYDROGEL TRACER BEADS: THE DEVELOPMENT, MODIFICATION, AND TESTING OF AN INNOVATIVE TRACER FOR BETTER UNDERSTANDING LNAPL TRANSPORT IN KARST AQUIFERS

Karst aquifers are susceptible to light non-aqueous phase liquid (LNAPL) pollution, but the factors affecting LNAPL transport are not well-understood. Although soluble tracers can be used to help understand the fate of water-soluble pollutants, LNAPLs may behave quite differently. In this study, hydrogel tracer beads were developed and tested to better address this issue. The beads are made using alginic acid, a derivative of marine algae, which forms a polymeric hydrogel in the presence of most divalent cations. The beads contain 96-98% water, are environmentally benign, and can be altered with regard to density, size, color, and fluorescence.

Buoyant beads containing hollow glass microspheres were tested alongside soluble fluorescein dye in a cave stream near Lewisburg, WV. The stream section was located in a 64-m canyon, confined by bedrock on the sides and bottom, had a water depth of 0.5 - 1.0 m, and discharge of 0.14 m³/s. After the simultaneous release of dye and beads, grab samples of stream water were collected every 20 s and analyzed for dissolved tracer by fluorometry; accumulated beads were collected every 20 s from the stream surface using swimming pool skimmers and were later counted.

The beads and dye were transported differently. Beads traveled faster over the test distance arriving after a minimum of 140 s (0.46 m/s), compared to 160 s (0.40 m/s) for the dye. Peak bead recovery occurred at 180 s (0.36 m/sec) and peak fluorescein concentration was detected at 210 s (0.31 m/s). Fluorescein results were analyzed using QTRACER2; the recovery averaged 91%. Bead recovery for duplicate tests was 58% and 72%, reflecting bead retention and an imperfect bead collection system.

The results suggest that solute tracers dispersed laterally and horizontally through the stream water but the buoyant beads were transported mainly in the zone of high surface velocity. This implies that LNAPLs would be transported rapidly through the karst section, yet a significant percentage would be effectively retained, even within a relatively “trap-free” reach of stream. Future tests will focus on longer cave sections having more traps to impede the downstream transport of the beads. The preliminary results indicate that hydrogel bead tracers can be a useful and cost effective means for modeling LNAPL transport in natural systems.