Hydrodynamic model of hydrogeologic fracture system in Gruda ultramafic rocks, western Serbia

Abstract

A part of the Balkan ultramafic massif is situated in western Serbia and belongs to a mountain range with several prominent peaks, including Mt. Tara, Mt. Zlatibor, and Mt. Zlatar. The porosity of ultramafic rocks is generally very low, such that they are classified as nearly or completely waterless rocks. However, due to tectonic activity and exogenic processes over geologic time, some ultramafic rocks exhibit a certain level of secondary fracture porosity. Hydraulically interconnected fractures can form aquifers of the fractured type, such as the one found in Gruda on Mt. Zlatibor. On this location, the quality parameters of the groundwater in the fractured aquifer, drained via a spring called Bijela Česma, prompted detailed multidisciplinary investigations aimed at defining the elements of the aquifer for commercial water bottling purposes. Given that the investigations were comprehensive, the results allowed high-quality interpretation of the hydrogeologic conditions and provided the background for developing a hydrodynamic model of the fractured hydrogeologic system in the ultramafic rocks at Gruda. Modflow software was used for modeling. This software is primarily intended for intergranular aquifers, but no programs are available for fractured aquifers in hard rocks with pronounced discontinuities. Modeling of the fractured aquifer in the ultramafic rocks at Gruda provided the following information: size of active infiltration surface, hydraulic conductivity of the porous medium, rate of recharge, residence time of a drop of groundwater from entry to exit, graphical representation of streamlines, and the like. Despite these difficulties, the results are satisfactory. The values of the analyzed parameters are believed to be objective and indicate a certain possibility of using Modflow in hydrodynamic modeling and solving hydrogeologic problems that involve hard rocks and fractured porosity. In practice, this is especially important for sanitary protection zoning of groundwater sources that rely on fractured aquifers.