PARTICLES IN A KARSTIC SPRING: PARTICLE SIZE DISTRIBUTION AND MOBILIZATION DURING A SNOWMELT EVENT
The investigations were carried out during a snow melt event in December 2012. Online measurements of particle concentrations and particle size distributions (PSD) were performed using a CIS 1 (GALAI). The single particle counter works in a range of 0.5 to 150 µm with a resolution of at least 0.5 µm. Turbidity, electrical conductivity, discharge and temperature were measured online as well. Additionally, anion and cation concentrations were measured. Scanning electron microscopy and x-ray diffraction were performed using filtration techniques.
During average discharge, the turbidity and the particle concentration lie in the range of thedetection limit. Both increase with discharge while the average particle size does not increase until a considerably higher-than-average discharge is reached. Particles with a diameter greater than 1 µm are detected during the whole event and contribute to about 40 % of all detected particles. Particles greater than 50 µm have a detection frequency of only 3 %. PSD shows a discontinuity at about 4 µm. To fit the PSD data two models were applied – a single power law distribution (PL) and a two-parted discontinuous power law distribution (DPL). The mean slope for the PL was -2.55 (n=90). For the DPL it was -2.20 for the smaller particles and -4.50 for the bigger particles (n=61). The breaking point of the DPL was located at 4.3 µm on average.
Smaller particles did usually reach the spring prior to bigger ones. Furthermore, a distinct correlation with discharge and largest particle diameters was observed. We could show a positive correlation of flow velocities, which is linearly related with the discharge rate in the aquifer, with the transported particles in a size range of about 10 to 50 µm.