Artificial colloid tracer tests: development of a compact on-line microsphere counter and application to soil column experiments

Multi-tracer tests with both solutes and particles allow to parametrise heterogenous porous media on the basis of the dual porosity model. In soil column experiments both solute and particle tracers were measured on-line at the outflow of the column. For the on-line measurement of particles we developed a flow cytometer with a sensitivity of one single microsphere in 1 ml of water. The microspheres used are latex spheres of 1 μm diameter, labeled with a fluorescent dye. The technique is based on the excitation of microspheres with a small diode laser. If a microsphere passes the laser focus the incorporated dye molecules emit fluorescent light. These photons are sampled by a Single-Photon-Counting Avalanche-Diode. The maximum flow rate through the instrument is 1 ml/min with a detection efficiency of up to 90%. The instrument is working stably over a time period of several weeks and has been tested under field conditions. The solute tracer we use is uranine, which is detected with a sensitive fluorimeter. Extreme sensitivity of the detectors is necessary to resolve tails of breakthrough curves which contain information on the structure of the medium. Laboratory columns were filled with a mixture of quartzsand (250–500 μm) and cellpore filter cylinders with an internal pore size of 35 μm. The measured breakthrough curves show, that because of the highly reduced matrix diffusion of colloids, their first breakthrough can be up to 2.25 times faster than the first breakthrough of uranine.