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Hydrogeological characteristics of the Rakovac spring recharge area, Mt. Žumberačka gora

The recharge area of the Rakovac spring is located north - northeast from the spring on the southwestern slopes of Štipkovo Hill (Mt. Žumberačka gora). Within the recharge area of the Rakovac spring, based on the hydrogeological characteristics and hydrogeological function, rocks are distinguished into two groups. In the first group are the high permeable intensively tectonized Upper Triassic dolostone deposits that form the main aquifer. To the second group belong medium permeable Jurassic (dolomitic limestone, limestone, calcarenite) and Upper Cretaceous deposits („Scaglia“ limestone) with the role of barrier to the groundwater flow. The cover deposits on the entire surface of the recharge area are thin and permeable forest soils. The recharge of the aquifer is provided by more intensive infiltration of the precipitation into higher areas of the terrain (above 700 m a.s.l.) characterized with well- developed typical karstic forms (sinkholes), thus reduce the possibility of surface drainage and increase effective infiltration. The discharge of the aquifer takes place at one permanent and descending type of spring and at two more intermittent springs whose occurrence is related exclusively to wet seasons when groundwater levels are high (Figure 1). All springs are located on the hypsometrically lower contacts of the carbonate aquifer with the less permeable deposits and are the places of spilling the "surplus" of the groundwater that accumulates by the infiltration of precipitation. The Rakovac spring occurs in the tectonized dolostones in which the preferential flow paths of groundwater flow are developed, i.e. aquifer probably discharges through a well-developed conduits. The Rakovac spring never dries which suggests a sufficiently large aquifer with a certain storage capacity due to a finely tectonized dolostone (fracture porosity). The facts that discharge at the spring increases rapidly and the water becomes muddy after a heavy rain refer to presence of cavernous porosity besides fracture porosity. Fast response of spring on rainfall event, i.e. rapid changes of discharge, also indicate the good hydraulic connection between the surface and aquifer. The ratio of minimum and maximum discharge is 1:520 which is similar to the typical karst springs. According to basic chemical composition of the spring water, the Rakovac spring belongs to CaMg-HCO3 type, due to the mainly dolomite dissolution process. Content of magnesium ion is lower than expected, indicating that some parts of aquifer are in contact with limestones or composed of limestones. The hydrogeochemical facies of spring is constant throughout the year what implies drainage from fracture system regardless of storms and wet and dry periods. Water is generally of a good quality, but the influence of anthropogenic pollution is noticed. Water is the most burdened with microbiological contamination. The main reason is the lack of sewerage system in the Poklek and Podigrišće settlements located less than 1000 m north of the Rakovac spring (in the zone with developed karstic forms and the largest effective infiltration), so waste water from the houses and stables can reach the aquifer fast and easy.

hydrogeological characteristics, spring Rakovac, Mt. Žumberačka gora, recharge area

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