A mobile laser-induced breakdown detection system and its application for the in situ-monitoring of colloid migration

A mobile Laser-Induced Breakdown Detection (LIBD) system is developed for the field study of the aquatic colloid migration. The principle of LIBD is based on the plasma generation at breakdown of colloids in the laser focus region, which is then observed by either photo-acoustic detection or optical monitoring. The method provides information on the number density and average size of colloids. The present instrumentation built up particularly for the field study is applied for the aquatic colloid migration in a natural granite fracture. The study is performed within the Colloid and Radionuclide Retention project (CRR) in order to investigate the role of colloids on the radionuclide migration. For this purpose, bentonite colloids dispersed in granite groundwater are injected into a rock fracture zone with a dipole distance of 5 m. After passing through the fracture, the groundwater is guided into a flow-through detection cell of LIBD for the quantification of colloids. Another experiment is performed with the same colloids traced by tetra- and tri-valent metal ions, Th(IV), Hf(IV) and Tb(III). The recovery of colloids as found by LIDB appears to be about 55±5% of the injected colloid mass concentration, which is corroborated by ICP-MS analysis of Al present in bentonite colloids. An average size of recovered colloids is slightly decreased from the value of before injection and hence the relative number density increased in the course of the rock fracture migration. Recovered fractions of colloid-borne trace elements in the extracted groundwater are 78±8% for both tetra-valent elements and 33±3% for trivalent element of the originally traced concentrations. Plausible explanations are given for the different recoveries of heavy metal ions.