PARTICLE, COLLOID AND MULTIPHASE TRACERS: ATTENUATION AND EXTENDED RELEASE ATTRIBUTED TO DIFFERING MECHANISMS
The particle tracer, i.e. glass beads used as a sediment proxy, was highly dispersed with respect to water flow. This is due to a combination of suspended and bedload transport resulting in an irregular breakthrough observed at the corresponding spring. Short-distance in-cave sampling permitted more detailed breakthrough curves to be assessed and interim deposition to be modeled using a first-order attenuation approach. Colloid and multiphase tracers, whose reaction kinetics was previously tested in laboratory experiments, were also attenuated, though this was attributed to different processes. Given the strong and irreversible attachment of the colloid tracer to cave materials, transport is assumed to be predominantly particle-bound, which is also known for some similar-sized biotic colloids. This is consistent with the irregular breakthrough measured at the spring and may also explain the scattered occurrence of microsphere tracers observed in several karst tracing studies.
Relative recoveries over the duration of the solute breakthrough did not exceed 20% for the particles and colloids, and 40% for the multiphase tracer. Prolonged sampling, however, resulted in nearly complete recovery of the sediment tracer, including remobilization of deposits during a subsequent flood event. The behavior of particle-attached colloids is assumed to be related to similar mechanisms, though colloids may also be attached to immobile surfaces and thus be permanently stored. The sorbing multiphase tracer was subject to significant retardation and could still be extracted from cave sediments two years after injection, with long-term release comparable to those of DNAPL.