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Hydrogeological overview of the Adriatic karst islands based on diverse parameters

In the Croatian part of the Adriatic Sea, there are 718 islands and many rock crags and reefs. The coast is very indented. Most of the terrain is built from karstified rock mass. It is a part of the "dinaric" karst, known for its tectonical disturbance, reverse faulting, and the deep and irregular karstification. 48 islands are considered as permanently inhabited. Before any kind of regional research, a classification of islands needs to be carried out. The most important factors for this classification are the island's area, indentation, and the amount of precipitation. These factors have been compiled into a new parameter: the island's hydrological size index. According to the hydrological size index, the Adriatic islands are classified into four main groups: big, medium, small, and very small islands. The hydrological size index synthesizes some simple geographical and hydrological properties of an island, and can be used only as a rough initial estimation. Many diverse parameters of the Croatian islands have been obtained by different methods. Using statistical regression, various correlations have been recognized, and some basic conclusions have been arrived at with more complex, multivariate methods. The mathematical system of hydrochemical data is very explicit, (Eigen value of the first factor 58%). Three factors have been calculated, and they are interpreted as three main natural processes: (1) mixing of the aquifer with sea water ; (2) carbonate dissolution ; and (3) pollution. Multivariate analysis has been performed for the different parameters, such as hydrological, lithological, hydraulic, geophysical, and hydrochemical. This mathematical system identifies five factors. The first factor groups a few parameters obtained by the step-drawdown test, and it is interpreted as the laminar flow factor. The second is the factor of the aquifer parameters. This is defined by the transmissivity, hydraulic conductivity, and the parameter of turbulent losses, which has a negative value. The mathematical system indicates that calculation of the aquifer parameters is less justified when there is a significant percentage of turbulent flow. The climatic-geographical factor is the third. There are group values that describe climatic and spatial properties, such as the hydrological size index, catchment surface area, distance of the site from the coast, and the concentration of chlorides (with a negative value). Thus, if there is a large catchment area and a high hydrological size index, there will be a lower concentration of chlorides in the aquifer. The fourth is the lithological factor. This is defined by the lithological composition and the interpreted resistivity (electrical tomography). The fifth factor is the factor of tidal efficiency. All considerations taken together, the mathematical system indicates that for the study of a karst terrain, an application of different methods is needed to enhance the accuracy level of interpretation.

karst hydrogeology; salt/fresh water interface; statistical analyses; multivariate analyses; Adriatic Sea; Croatia

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