engleski

Seawater Intrusion at Abandoned Coal Mines in the Labin Region, Croatia

Abstract Underground coal mines in the Labin Paleogene basin on the Istrian peninsula in Croatia ceased operation in 1988 after more than two centuries of mining. Throughout the next three years, most of the mines were flooded. The total volume of exploitation fields and stable underground corridors that had been flooded by groundwater was estimated at 12 million cubic metres. Given that the mine works had drained the surrounding karst aquifer, it was assumed that stored waters and underground recharge could be used for public water supply or for industrial purposes. To confirm this assumption, comprehensive hydrogeological research was conducted, including quantifying the geometry and volume of flooded underground works, identifying hydrogeological characteristics of the rock units, and exploring the water hydrochemistry and dynamics. Ultimately, despite favourable hydrogeological conditions and large drainage capacity, it was determined that the hydrochemical characteristics of the mine waters rendered the water unsuitable for most uses. One of the highlighted problems was the excessive chloride content. Since the mines are located in permeable carbonate rocks and their deepest parts are only 1 km from the coastline, seawater intrusion was a common occurrence at the time of exploitation. Data from this period show that the drainage water in the deepest parts of the mine consisted of 25 to 85% seawater. Although several available shafts have been investigated, the results obtained in the Labin shaft are representative of the entire mining system due to its drainage function and intensive groundwater dynamics. During low water conditions at the top of the water column (up to a depth of 60–70 m), fresh water with a chloride content of <250 mg/L prevails. The chloride content significantly increases with depth. At a depth of –200 m, it ranges from 1, 350 to 2, 010 mg/L, while at the bottom of the shaft at a depth of –350 m, the content ranges between 1, 810 and 2, 380 mg/L. During high water conditions, overflow from the shaft appears. The content of chlorides in the overflow water ranges from 194 up to 1, 200 mg/L.

Seawater intrusion, coal mines, karst

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