engleski

Pazinska jama and its impacton the quality of springs in the valley of the river Raša

Pazin Cave is one of the most important speleological structures in Istria. It formed at the contact of impermeable deposits of Eocene flysch and permeable Upper Cretaceous limestones. The cave functions as a ponor and the Pazinčica, the largest sinking stream in Istria, flows into it. Tracer tests have shown that water from Pazin Cave emerges in karst springs along the right bank of the river Raša. For many years the Pazinčica and Pazin Cave were the recipients of all wastewater from the town of Pazin and the surrounding area. Since there was no treatment system, water quality in the Pazinčica was very poor, especially when water levels were low. A continuous inflow of pollution affected water quality in the karst aquifer from which the above springs in the valley of the Raša are supplied, including the Rakonek spring, which contributes to the water supply of Pula. With the construction of the Pazin wastewater treatment facility, the quality of the water in the Pazinčica has improved significantly, and this has been reflected in improvements in the springs. It thus has been shown that wastewater treatment is a fundamental requirement for the preservation of groundwater quality in karst areas. In 1997 Pazin Cave was the scene of one of the biggest environmental incidents ever to affect the karst area in Croatia. Despite the catastrophic dimensions of the incident in the Pazinčica stream, contamination levels monitored in the major springs downstream of the pollution site were significantly lower than expected. The main reason for this was the favourable hydrological conditions which allowed the removal of most of the contamination prior to its entry into the karst underground. Additionally, when water levels are very low, direct flows from Pazin Cave only account for 1% of the total discharge at the springs along the right bank of the. In these circumstances the pollution that penetrated the aquifer despite preventive measures being taken, was diluted by clean groundwater from other parts of the catchment area on its way to the drainage area in the valley of the Raša River. The highest concentrations of contaminants in the springs were recorded during increased water pulses (the maximum was recorded two months after the incident). This is a consequence of the hydraulic principles of karst flow, according to which in high water conditions the largest quantity of water drains through the most developed systems of underground conduits, or through zones of privileged flow. In other words, in such circumstances most outflows at springs are fed through a system of conduits originally connected with Pazin Cave. Under the given circumstances it is apparent that a major environmental incident in karst, despite occurring in the immediate vicinity of an active speleological structure, does not have to result in catastrophic consequences for groundwater quality, making it permanently unusable. With favourable hydrological conditions and timely measures, the penetration of pollution into underground conduits was prevented, but its subsequent washout during high water pulses, several months after the incident, led to the normalisation of the situation in springs located in the discharge zone.

karst, groundwater, pollution, hydrocarbon

nije evidentirano