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Water Management Problems in Coastal Karstic Regions - Interrelationship of Natural Processes and Human Impacts Exemplified by Raša River Estuary (Istria-Croatia)

Raša River estuary is an area characterised by dynamic natural processes inherent in the contact of surface watercourses and coastal springs with the sea. Human impact on natural processes and water quality in the area is significant while rapid sedimentation of the river deposits causes a rise of the river bottom. Extensive irrigation works in a narrow zone of the Raša River mouth, carried out in the beginning of the 20th century, dried up a shallow Krapanj Lake and a part of the coastal sea in the estuary, forming Donja Raša amelioration system. As a consequence, Raša River runoff regime as well as the water regime of several coastal springs in the vicinity (such as Fonte Gaia, Kokoti, Rakonek and Mutvica), which are successively included in the water supply system were significantly changed. Total minimum discarge of the mentioned springs is app. 0, 5 m3s-1 and some of them are having an aquifer strongly influenced by the sea waters, which causes an increase in salinity hence the periodical or even long-term brackish spring water (Blaz, Česuni). Since 18th century until 1990, Raša River watershed was considerably influenced by the coal mine in the area that is now abandoned and the shafts sunked. Population density of the study area is quite high (app. 15.000 inhabitants living in the town of Labin and the surrounding settlements), hence the water quality depends greatly on the surface runoff draining practices and the wastewater management. Karstic characteristics considerably increase sensibility of the area in terms of water quality. However, sewage system and the wastewater treatment plant with the capacity of 7.000 PE are built only for the town of Labin. This paper analyzes operation of the wastewater treatment plant in different hydrological conditions as well as the interrelationship between hydrological conditions and water quality at the locations of the springs Fonte Gaia and Kokoti that are very much influenced by urban drainage waters. Salinization of the spring waters that is partially caused by unsuitable drainage practices in Donja Raša area has also been analyzed and an overview of possible solutions are given, with an emphasis on integral approach to wastewater management. Raša River watershed, in that respect, represents a good example of high level of complexity in managing coastal areas where human impact plays an important role in runoff dynamics.

estuary; sedimentation; karst; coastal spring water quality; water supply; wastewater treatment

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