A multiparameter analysis of environmental gradients related to hydrological conditions in a binary karst system (underground course of the Pivka River, Slovenia) Chemical and bacterial gradients under different hydrological conditions were studied in a well-developed underground karst system. Water samples were collected from the main underground drainage conduit of the Pivka River from October 2013 until June 2016. The system responds quickly to external pulses (precipitation events), and is also impacted by human interventions, as is demonstrated mainly by fluctuations of sulphates, chlorides, and occasionally elevated concentrations of organic and faecal pollutants. Chemical and bacterial parameters showed a monotonous trend of decreasing concentrations from the ponor towards the interior of the karst massif during stable hydrological conditions, and a significant change during high water conditions. High flow events tend to equilibrate chemical and bacterial parameters in the underground river. Concentrations of chlorides, TOC (total organic carbon) and nitrates were the most indicative parameters describing the formation of the gradient. Stable isotopes of hydrogen and oxygen in water indicated that the main karst conduit collects isotopically different waters from the aquifer. The river water collected

The behaviour of aquifers with karst porosity is dependent on hydrological conditions. This is due to the peculiar characteristics of the groundwater flow and dynamics of hydrological processes in karst. As a result, karst aquifers are especially vulnerable to the effects of environmental change. We assessed the long-term climatological and hydrological trends and the short-term effects of increasingly frequent extreme hydrological events (droughts) for the Mediterranean karst spring Rižana in SW Slovenia. The findings predict higher mean annual air temperatures of 0.34°C decade−1, lower annual precipitation of c. 60 mm decade−1 and higher annual actual evapotranspiration (especially during spring and summer) of 32–49 mm decade−1. As a consequence, we can expect a decrease in the mean annual discharge of the spring of c. 480 l s−1 decade−1 with prolonged dry summer periods. Detailed monitoring of the physical, chemical and microbiological parameters showed that the flood pulses caused by precipitation events after a long dry period cause a significant deterioration in water quality. In such situations, contaminants stored in the unsaturated zone are flushed out and counts of coliform bacteria can reach >1400 cfu (100 ml)−1, with total Al and Fe concentrations up to 206 and 474 μg l−1, respectively. These results suggest that there should be urgent adherence to water quality standards to protect karst water sources in view of the anticipated climatological stresses. Management strategies should promote monitoring and the rational use of karst water supplies.