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 het erogenous porous media on the basis of the dual porosity model. In soil col umn experiments both solute and particle tracers were measured on-line at t he outflow of the column. For the on-line measurement of particles we devel oped a flow cytometer with a sensitivity of one single microsphere in 1 ml of water. The microspheres used are latex spheres of 1 mu m diameter, label ed with a fluorescent dye. The technique is based on the excitation of micr ospheres with a small diode laser. If a microsphere passes the laser focus the incorporated dye molecules emit fluorescent light. These photons are sa mpled by a Single-Photon-Counting Avalanche-Diode. The maximum flow rate th rough 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 ha s 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 contai n information on the structure of the medium. Laboratory columns were fille d with a mixture of quartzsand (250-500 mu m) and cellpore filter cylinders with an internal pore size of similar to 35 mu m. The measured breakthroug h curves show, that because of the highly reduced matrix diffusion of collo ids, their first breakthrough can be up to 2.25 times faster than the first breakthrough of uranine. (C) 1998 Elsevier Science B.V. All rights reserve d.