Abstract: This paper discusses the roles of sediments and suspended particles/ colloids for contaminant attenuation and transport in karst groundwater and presents experimental results concerning the relations between particles and microbial contamination in a karst system in Switzerland. The test site consists of a swallow hole draining agricultural land, connected to two springs, one of which is used for water supply. A portable particle counter was used to obtain detailed time-series of Particle-Size Distribution (PSD) in spring water, along with continuous monitoring of turbidity, total organic carbon (TOC) and other parameters, and analyses of faecal bacteria (E. coli). Two types of turbidity were observed at the springs following intense rainfall: A primary signal occurred during increasing discharge and is explained by remobilisation of intrakarstic sediments due to a hydraulic pulse (autochthonous or pulse-through turbidity). A secondary turbidity is typically observed after several days and indicates the arrival of water from the swallow hole (allochthonous or flow-through turbidity) to spring, often associated with high levels of TOC, nitrate and E. coli. PSD analyses allowed further differentiation: The primary turbidity consists of a mixture of different particles sizes, while the secondary signal predominantly consists of very fine particles, because larger particles are removed by sedimentation between the swallow hole and the springs. There is good correlation between E. coli and 0.9～1.5 μm flow-through particles. A relative increase of very fine particles can thus be used as an “early-warning system” for microbial contamination. PSD monitoring could also help to study other problems in karst environments, not only water contamination, but also soil erosion.