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EXCESS AIR IN SPRING WATER AS INDICATOR OF “OPEN“ OR “CLOSED“ FLOW IN KARST SYSTEM

Environmental tracers in ground water are naturally present substances and physical properties of the water that are depended on flow properties and processes happening within the groundwater flow system. Monitoring of such tracers in karst spring water enables insight in characteristics of typically extremely heterogeneous and complex karst groundwater systems. Commonly used environmental tracers include electrical conductivity, temperature, turbidity, ionic composition, stable isotopes of water, organic carbon, nitrates, etc. These tracers provide valuable information regarding infiltration mechanisms, epikarst function, mean groundwater transit times, dynamics of fast and slow flow, etc (TRČEK & ZOJER, 2010). Air saturation in spring water is not commonly monitored parameter in hydrogeological studies, although supersaturation with air, or “excess air”, is a phenomenon which periodically or permanently occurs on some karstic springs (SURBECK, 2005 ; HEATON & VOGEL, 1981). On the other hand it is a very important parameter for fish farming, as oversaturation with gases can cause fatal “gas bubble disease” in fish (MACHOVA et al., 2017 ; WEITKAMP & KATZ, 1980). Therefore, fish farming community is well aware of this phenomenon. Krbavica spring, situated near Krbavsko polje in Croatian Lika region, is known among local population as inadequate for fish farming although it is a permanent spring with a relatively stable discharge even in draught periods. Bubble formation is visible in a spring pool during the most of the year. The spring is captured for public water supply, and monitoring program on it was established to investigate properties of the karst system which feeds it. Among other natural tracers, dissolved oxygen, as an indicator of “excess air” in spring water, was also monitored in high temporal resolution. Monitoring data shows that oxygen saturation is in clear relation to discharge dynamics. During peaks in hydrograph values of oxygen saturation temporarily dropped, followed by maximum values with delay of several days. Increased oxygen saturation was interpreted as an increased proportion of flow through narrow fractures in a vadose zone in a closed flow conditions. Air bubbles captured in water infiltrating to narrow fractures gradually dissolve due to pressure rising and isolation from the atmosphere. In contrary, water flow through larger, karstified fractures and conduits in vadose zone is in open flow conditions, i.e. water pressure is equilibrated with the atmospheric pressure. Therefore, drops in oxygen oversaturation during hydrograph peaks are interpreted as a consequence of increased flow through larger openings within the vadose zone. Flow through narrower fractures is most intense after the rainfall events, when pressure rising in fractures mobilises water. In order for groundwater to remain oversaturated, conduits within the deeper parts of the system that drain water to the spring should be dominantly phreatic, i.e. situated completely below the groundwater level. It can be concluded that monitoring of air saturation in spring water can provide valuable information on the processes within the system, especially if it is combined with monitoring of discharge dynamics and additional tracers.

Karst hydrogeology, environmental tracer, “excess air”, Krbavica spring

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