Hydrogeology of the Gradole Spring Drainage Area in Central Istria

Abstract

The delimitation of a drainage area together with the accurate estimation of it’s surface area represents an important problem in dealing with karst springs. Water gushing from the fractured karst identifies the Gradole spring as a typical karst well of the rising type. The spring never runs dry, the ratio of minimal and maximal discharge being 1:15 to 1:20 or more. The surface of the drainage area and its spatial distribution have been defined on the basis of geological data; tectonics, morphology, photo-geological interpretation, hydrogeological characteristics of the rocks, the karst morphology (sinkholes, pits) and underground water tracing data. The drainage area is predominantly composed of carbonate rocks, mainly limestone, but there is also a lesser flysch component - marls and sandstones. The karst area is without any surface waterflows, in contrast to the flysch area where a hydrographic network of intermittent surface streams is developed. There are no real barriers to underground flow within the carbonate rocks. Nevertheless, the direction of flow is influenced by structural elements, the position of layers, the appearance of rocks - thin layers, shows of some less permeable rocks - dolomites, fissured and marly limestones. The confirmation of an underground connection of Cize swallow hole with the Gradole spring shows the possibility of supplementing low waters through the swallow holes from Butoniga accumulation lake.