While drought conditions may create severe problems in terms of karst groundwater scarcity, related low flow provides prolonged residences times and enhanced natural attenuation capacity. This is particularly relevant with respect to microbial water composition and related fecal pollution. Stable base flow following long-standing drought, such as encountered in Europe during summer and fall 2018, represents the unique possibility to study in detail the related processes at work and to characterize the impact of residence time on microbial signature and water quality.
Investigations performed at the Milandre test site in the Swiss Jura Mountains document the gradual change of hydrodynamics in the course of the drought event, including the discrimination of discrete conduit flow paths becoming more apparent with decreasing discharge. Furthermore, the extreme low flow resulted in residence times that were doubled with respect to normal base flow. During this period, total cell count and fecal indicator bacteria reached lowest background values, only interrupted by sporadic pollution input linked to local rain events and reaching the active conduit through preferential flow paths despite a significant water deficit in the vadose zone.
These variations in flow and pollution dynamics, in turn, allowed for identifying the different water components and bacterial signatures. Spatial and temporal monitoring was conducted for natural conditions and complemented by manure tracing experiments during base flow. Results evidenced microbial signature being residence time dependent, with total cell count (including live/dead and LNA/HNA ratios) correlating with discharge and flow velocity. These parameters can then be used as indicators for the different flow components observed in the karst system. In the same manner, pollutions events could be characterized, and reproduced by the tracing approach.
Finally, extended residence times during low flow conditions also provide the basis for better assessing natural attenuation of both degradable solutes and fecal bacteria. In this context, repeated injection of selected bacterial and dye tracers in contrasting hydrological conditions allowed determining attenuation rates and effective degradation with time. Bacterial attenuation could also be deduced from the bacterial signature, for instance the ratio of more and less persistent strains. Nonetheless, the characterization and quantification of those processes demands further research, particularly for lasting low flow conditions as observed in 2018 and to be expected more frequently in the future.