Dissolved oxygen as a tracer of flow characteristics in a karst hydrogeological system

Monitoring of various natural tracers in a spring water provide insight in groundwater flow characteristics and processes happening within the karst hydrogeological system. Electrical conductivity, temperature, turbidity, stable isotopes, organic carbon, nitrates, etc. are more and more commonly monitored on karst springs in order to provide information on infiltration mechanisms, epikarst function, mean groundwater transit times, interrelation and dynamics of fast and slow flow, etc. Supersaturation of gases or “air excess” in spring water is a phenomenon which occurs periodically or permanently in some karstic springs. Fish farming literature is well aware of this phenomenon, as gases oversaturation can cause fatal “gas bubble disease” in fish. In contrary, this phenomenon was very rarely investigated within the karst hydrogeological research. Some of the karst springs in Dinaric karst of Croatia are known among local community as occasionally fatal for fish due to gas oversaturation, but this was never monitored or researched in more detail. Krbavica Spring in Lika Region of Croatian Dinaric karst is captured for the local water supply. In a spring capture reservoir bubble formation is visible with variable intensity during different periods of the year. Within the monitoring program of multiple natural tracers in spring water, high temporal resolution monitoring of dissolved oxygen was established. Dissolved oxygen was monitored as an indicator of excess air in spring water. Preliminary analysis and interpretation of the observed data points to the gas oversaturation as a good indicator of flow characteristics within the system, e.g. indicator of “closed flow” through fractures of the vadose zone, as well as flow through dominantly phreatic conduits in the deeper parts of the system.