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Tracing tests are one of the most powerful tools in investigation of karst system hydrogeological properties (GOLDSCHEIDER & DREW, 2007). Tracing typically consists of injection of artificial tracer in a system of karst conduits and afterward monitoring of tracer concentration in discharging water of selected springs. Therefore, it directly reveals underground flow directions, velocities and dispersion within the system. However, considerably different results obtained from repeated tracings of the same sinkhole are common in karst areas (e.g. KUHTA et al., 2010 ; STROJ et al., 2010). Significant differences are commonly present in apparent groundwater velocities and dispersion, and sometimes even in groundwater directions. Inconsistencies in results of the repetitive tests are usually interpreted as a consequence of different hydrologic conditions within the system during performance of the tests. However, methodology of tracer monitoring in Croatia significantly evolved during 1980s: from detection of tracer using quartz lamp to detection using a laboratory spectrofluorometer. If properly used, spectrofluorometer enables much lower detection limit, i.e. much more reliable tracer detection at low concentrations. In recent years usage of field fluorometers also enabled high temporal resolution of sampling, and consequently determination of very detailed tracer concentration curves. Typically, tracer concentration curves show characteristic and relatively regular shapes, regardless if they are single or multi peaked. Therefore, highly irregular shape of tracer curves obtained by quartz lamp at concentrations close to detection limit gives rise to doubt in positive tracer detection. Within the karstic catchment of the Gacka river springs, several tracing tests were carried out over the time period from 1950s till present. Present knowledge about this regional and complex karst system is largely based on the results of these tests. Recently, a multi-tracer test in medium-low water conditions was performed. Usage of field fluorimeters together with hydrological monitoring enabled acquiring detailed tracer concentration and recovery curves. Despite relatively high tracer recovery, apparent ground flow velocities were much lower in comparison to the previous results. Comparison of results obtained in medium-low flow and in high flow conditions is particularly valuable for comprehensive estimation of karst system properties. Determined flow directions were quite different in comparison to some of the older results (reviewed in PAVIČIĆ, 1997), while they were consistent with more recent ones (KUHTA et al., 2010 ; KUHTA & FRANGEN, 2013). It can generally be concluded that caution is needed during interpretation of relatively old tracing test results considering determined tracer concentrations to detection limit ratio and concentration curve shape. |
