PARTICLE-SIZE DISTRIBUTIONS IN WATERS FROM A KARSTIC AQUIFER - FROM PARTICLES TO COLLOIDS

Waters from the surface hydrologic network and the spring of a karstic aquifer in Switzerland were sampled to analyse their colloidal conten t. The measurements were done weekly with a single particle counter an d were verified by other techniques. The particle size distribution (P SD) was modelled in two portions, below and above 5 mu m, using two ty pes of equation: a power law (Pareto distribution) and an exponential law. The model results matched well with the entire PSD data set by va rying the parameter values. The parameters obtained from fitting the m easured PSD curves were then interpreted in relation to environmental factors. It appears that the two parts of the curves vary independentl y. The first part of the PSD curve, relating to the smallest particles , is dependent on the pH value of the spring or the temperature of the surface brook. In contrast, the second part of the curve depends most ly on the spring discharge volume. During high flow events, the major effect of the discharge on particle size occurs during the rising limb of the hydrograph, interpreted as clays deposited in the aquifer and resuspended due to high water velocity. The contrasted behaviour of th e two parts of the PSD curves suggested that the break point in the cu rves represents the limit between colloidal and particulate behaviour. Knowing these dependencies, and the characteristics of the particulat e matter, allowed the estimation of the role of the colloids in contam inant transport. Large fluxes of suspended matter, specific to karstic aquifers, demonstrate the critical role of colloids in contaminant tr ansport, which is markedly different from what typically occurs in por ous media. (C) 1997 Elsevier Science B.V.