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Preliminary results of hydrochemical dynamic of TOC in a karst aquifer in the northern part of the Dinaric karst in Croatia

The term natural organic matter (NOM) refers to a group of organic substances that can be found in surface water and groundwater as a result of natural processes. In a major part NOM is a product of plant degradation and various other biological activities such as the metabolic activity of algae, protozoa, and microorganisms as well as excretion of fishes and other aquatic organisms (Bolan et al., 2011). In this paper, NOM has been observed as total organic carbon (TOC). Considering that the TOC is mainly contained in the soil zone, from the hydrogeological point of view it might be used as a tracer for infiltration. The behavior of tracer varies considerably between the high and low flow periods (Batiot, 2003a, Emblanch et al., 1998). The TOC concentration, chemistry, and composition in natural water are highly variable and depend on the sources of organic matter, the chemistry of the environment, ionic strength, pH, temperature, major cation composition of the water and on the presence of microbiological and photolytic degradation processes (Bolan et al., 2011). The determination of TOC concentration in an aqueous solution, such as a sample of wastewater or potable water, is important in different fields including pollution and industrial processing situations. Measurements were conducted for 17 months, from March 2018 to July 2019 at 9 springs in the northern part of the Dinaric karst in Croatia. The study case encompasses two hypsometric levels of spring and sinking zones of two karst rivers Dobra and Mrežnica. Former studies have shown the complex and heterogeneous karst system of the study area (Biondić et al. ; 1986 ; Bojanić, 1973 ; Bahun, 1968 ; Poljak & Herak 1947 ; Prelogović et al., 2005). The whole area is significantly disturbed by the tectonic activity. Main structures and faults generally have the typical Dinaric strike (NW-SE), in some cases N-S due to neo-tectonic activity (Prelogović, 2005). The natural hydrogeological dynamic of the aquatic ecosystem was additionally disturbed by the construction of hydro-technical infrastructure like tunnels, dams, and accumulations for hydropower plants. This complex karst system is defined as the boundary zone between deep and shallow karst zone (Bojanić, 1973 ; Kovačević, 2005). In deep karst zone, groundwater is located on a significant depth with complex and unpredictable flow-paths as a result of intensive karstification. In contrast, carbonate deposits in shallow karst zone are thinner, and impermeable deposits are more common (Tertiary flysch) causing this zone to be rich in surface hydrology with shallow groundwater levels (Bahun, 1968 ; Kovačević, 2005). In order to determine hydrodynamic processes, the following parameters were measured in situ: electrical conductivity (EC), pH, oxygen and temperature of spring water on the monthly base. Springwaters have been sampled and analyzed for cations (Ca2+, Mg2+, Na+, K+), anions (Cl-, SO42-, NO3-) and TOC in the hydrochemical laboratory of the Croatian Geological Survey. The scope of this paper is to show the preliminary results and initial relationship between total organic carbon (TOC) and other observed hydrogeological parameters.

Karst, TOC, Hydrogeology, Natural tracers, Infiltration

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