USE OF HYDROGEL TRACER BEADS TO BETTER UNDERSTAND CONTAMINANT FATE AND TRANSPORT IN KARST AQUIFERS
Hydrogel tracer beads contain 96-98% water and are made using alginate, an anionic polysaccharide extracted from marine algae, which gels in the presence of divalent cations. The beads are environmentally benign, easily made, and can be experimentally altered to mimic different contaminant behaviors. Preliminary laboratory experiments examined methods that could be used to modify physical properties of the beads, such as size, buoyancy, and density. Bead size could be modified by controlling droplet size and flow rate during the gelling process. Buoyancy could be altered by incorporating various additives into the alginate matrix. The short- and long-term stability of the beads under field conditions was determined by evaluating gel integrity after incubating them in natural waters for various lengths of time.
Preliminary field tests were conducted to assess potential problems associated with bead use in natural settings. The beads remained stable in a freshwater stream and could be captured using an appropriately-sized mesh screen. Prototype beads rapidly settled to the sediment but were easily resuspended with increased stream velocity, similar to what would be expected for dense non-aqueous phase liquids. Additional tests using beads of varying density and buoyancy are underway. Further testing of the beads will be conducted in conjunction with soluble tracers in the stream in Buckeye Creek Cave, near Lewisburg, WV. This stream has a long reach with no inputs or outputs, and it flows through variable cave morphologies. Research at the cave is underway and future experiments will examine how bead movement varies relative to flow velocity and alternative tracers.