Particle-size distribution as indicator for fecal bacteria contamination of drinking water from karst springs

Continuous monitoring of particle-size distribution (PSD), total organic carbon (TOC), turbidity, discharge and physicochemical parameters, together with analyses of fecal indicator bacteria, particularly Escherichia coli, made it possible to better understand the processes governing pathogen transport in karst groundwater and to establish PSD as indicator for possible microbial contamination of drinking water from karst springs. In the study area near Yverdon-les-Bains, Switzerland, tracer tests proved connection between a sinking stream draining agricultural land and several springs, 4.8-6.3 km away. Tracing and monitoring results demonstrate that (i) suspended particles (turbidity) in the spring water either originate from remobilization of sediments inside the aquifer (autochthonous) or from the sinking stream and land surface (allochthonous); (ii) allochthonous turbidity coincides with increased E. coli and TOC levels; (iii) PSD makes it possible to distinguish the two types of turbidity; (iv) a relative increase of finer particles (0.9-10 microm) indicates allochthonous turbidity and thus possible fecal contamination. The method permits to optimize water treatment and identify periods when the spring water must be rejected. Findings from other test sites confirm the feasibility of this approach.