Janja Kogovšek

Increase of vulnerability of karst aquifers due to leakage from landfills

Karst aquifers are very vulnerable to various pollution sources. Among these, landfills can contribute contaminants over long periods. Groundwater quality monitoring is required to assess the impact of a landfill leachate on underlying aquifer water or spring discharge. Tracer tests are useful for selecting the location and frequency of sampling. Three landfill sites in karst areas were studied with tracer tests. Additional insight was accomplished by the comparison of the tracer transfer and the breakthrough curves obtained from these tests with the results of two other tracer tests carried out in the same period in karst areas without a landfill. To explain the differences observed, the hypothesis of increased permeability below the landfill due to the presence of inorganic acids in leachates was further tested. The results of detailed, long-term monitoring of contaminated drip water in the Postojna Cave were used to verify the hypothesis. The analysis of the simultaneously increased content of calcium, magnesium, and contaminants in the drip water indicates a direct correlation between limestone dissolution and contaminants. Increased dissolution increases secondary porosity and thus permeability of the vadose zone resulting in higher vulnerability of underlying aquifers and springs in the vicinity landfills.

Impact of Motorways on Karst Waters

Impervious surfaces of road networks accumulate contaminants and pollutants, which are washed off during rain and snowstorm events into nearby waters and lands. Especially in areas with a developed traffic network, runoff from motorways is a significant source of pollution. Various protection and remediation measures were developed and implemented for preventing or at least diminishing its negative influences. Three major sources of pollution are associated with the road network: vehicles (emissions of motor vehicles, spilled and released oil, tires’ particles, de-icing agents), road characteristics and paint markers, and atmospheric depositions influenced by the adjacent land use. Besides pollutants directly associated with traffic, the impermeable surfaces can collect and drain a negligible quantity of organic waste, nitrogen and phosphorous, herbicides, pesticides and faecal pathogens. Soil infiltration treatment, building of collection basins and constructed wetlands are used as protection and remediation measures. In karst areas, motorway runoff has an amplified impact on ground water, compared to other types of landscapes. The soil layer is commonly thin or non-existent and thus soil infiltration treatment is practically inexistent. Stormwater runoff may immediately flow into the aquifer through subsurface conduit networks, fractures, sinkholes and sinking streams, which makes karst groundwater especially vulnerable to pollution. Accidents that cause greater quantities of hazardous substances to flow into the karst, for whatever reason, endanger our environment, karst waters, and even in smaller quantities also the quality of the karst springs that have been captured for drinking water supply. Especially dangerous are spills of oil derivatives, since we know too little about the flow of such substances that do not dissolve in water and are lighter than it. Based on observations of Globocec spring following the accident near Ortnek and of the Rižana Spring following the spillage near Obrov and other similar cases, we anticipate a longer retention time and washing away of oil derivatives due to the possibility of adsorption on sediments and retention in siphons.

Caves and Hydrology of the Contact Karst of Beka and Ocizla

The Beka-Ocizla cave system extends at the contact between Palaeocene limestone and Eocene flysch at an altitude of 350 m in a shallow depression called Loke. Six known caves are connected to this cave system: Ocizeljska jama (Ocizla cave), Blažev spodmol (Blaž’s rock shelter), Maletova jama s slapom (Maletova jama cave with waterfall), Jama z naravnim mostom (Cave with the natural bridge), Jurjeva jama v Lokah (Jurjeva jama cave in Loke) and S-4/Socerb. Three larger streams and a smaller one flow into the depression from flysch and sink into marly limestone. Together, they drain the flysch surface of approximately 3.5 km2. Some of the cave entrances function as periodic sinks of the surface waters that flow in from beneath the villages of Beka, Ocizla and Petrinje. The water from the entire cave system flows towards the Boljunec spring. Hydrological observations conducted in the cave include mapping of traces of the past flood events and continuous monitoring of the water level and temperature at five different locations in the cave system. A few flood events happened in past years: an extreme flood event between 17 and 19 September 2010, and flood events between October 2012 and March 2013. Special hydrological conditions during these events are also described.

Unroofed Caves Provide Important Clues to the Karst Development

Unroofed caves are old caves that were revealed on account of the lowering of the karst surface. They are preserved by their infill—mostly alluvium and flowstone. It became clear during the motorway construction undertaking in Slovenia that unroofed caves constitute a relatively common karst landform. In fact, more common than karstologists had imagined before the karst surface was uncovered through earthworks. The various types of notches occurring on the surface have long been interpreted as types of dolines or as the result of the lithological properties of rock and its fracturing. 75 km long and, on average, 25 m wide stretch of the motorway across the classical Karst gave up 350 caves, of which 90 are unroofed caves. Some of them make up the same cave system. New findings prompted karstologists to become more aware of these unique surface karst forms. In the process, numerous unroofed caves filled with all types of alluvium were discovered. There were several attempts at typification of the characteristic shapes of unroofed caves and to design partial models to explain their typical formation processes. The surface and subcutaneous dissolution of carbonate rock and its disintegration from back in the Ice Age, brought about the lowering of the karst surface. Old caves, which were formed by erstwhile water flows and are partly intersected by shafts which drain water from the permeable karst surface, pop up as either empty or filled with alluvium. The caves were formed as a part of a system of cavities in a period when impermeable rocks had enclosed the aquifer higher up, causing the underground water in the aquifer to be at a higher level. The hypothesis was that the karst topography and its remarkable systems of valleys can be traced back to former surface water throughflow. However, revealing the surface did not provide the evidence to support this hypothesis, instead obvious signs of former water throughflow in carbonate rock were identified—manifested as open and cut through old caves.

Unroofed Caves Near Kozina and Their Identification

Unroofed caves are an important karst form that makes up a part of the karst surface and epikarst, and provides us with valuable evidence of the karst development. They are old caves that became exposed by the lowering of the karst surface. In fact they are preserved by their fill—mostly fine-grained alluvium. It is also often that they feature preserved flowstone and an intact rock rim. During earthworks preceding the motorway construction, this important karst feature, also characterizing the surface, attracted special attention. Earthworks revealed that the karst surface is scattered with several distinct types of unroofed caves, which in itself are not an uncommon phenomena. The typical shapes of unroofed caves found on karst terrain could be singled out, i.e. individual doline-like forms that occur in strings, and oblong notches. Because the surface of the Karst region has lowered so dramatically, there are old caves and shafts opening up all the time in the course of the construction of the motorways. Old caves are either void or filled with alluvium. The caves were formed as a part of a system of cavities in a period when impermeable rocks had enclosed the aquifer higher up, causing the ground water in the aquifer to be at a higher level. But karstification gave rise to a drop in the water table in the aquifer—today it is 200 m and more below ground, and the karst surface is still lowering. Unroofed caves are therefore regarded as distinct surface karst forms which were in part reshaped by surface processes that make up an important part of epikarst. In the course of the earthworks for the construction of the Kozina motorway, the typical shapes of unroofed caves, passages and large cave systems carved in horizontal or inclined karst surfaces were discovered.

Palaeomagnetic Research of an Unroofed Cave Near Kozina

The unroofed cave from which samples for the palaeomagnetic analysis were taken, is located to the northeast of Kozina, near the existing Ljubljana–Koper main road, in the cutting which was formed during the construction of the Divaca–Klanec motorway. The sampled profile consists of more than 5 m of alluvium in two main sequences. The lower sequence comprised ochre-coloured sandy to clayey sediments that were app. 3 m thick. These sediments were sampled to undergo the palaeomagnetic method. The lower sequence was covered with collapsed breccia and limestone blocks, and no samples were taken from it. The profile’s lithology clearly manifests a two-phase depositing in the past. The lower sequence underwent erosion after having been deposited. Subsequently, during the collapse, the empty space in the cave filled with rubble ranging in size from rocks to blocks mixed with brown karst soil. The ochre-coloured intercalations in the upper part of the upper sequence may indicate the presence of eroded sediment comparable with the lower sequence. The thinning of cave ceilings through erosion and karst denudation triggered collapsing. The sediment originates most probably from the weathered Eocene flysch. The Kozina profile is older than the Brunhes/Matuyama boundary (0.78 million years). The distribution of individual magnetozones supports the claim that the sediment is older than the end of the Olduvai epoch (1.77 million years) as the magnetostratigraphic profile concludes with the inverse polarity magnetozone and contains two normal polarity zones.

The Karst in the Breccia of Rebrnice in the Vipava Valley

The road runs in the NW–SE direction across two landscape units: across the bottom of the Vipava Valley and the southwestern slopes of Mount Nanos, Breg and Rebrnice. The geological, geomorphological, speleological, and hydrological diversity of the Slovene karst has been demonstrated also by the study of karstification of breccia that have been forming beneath the western slope of Mount Nanos. Water, in most cases percolating diffusely through the permeable surface of rubble or breccia and into the more or less impermeable flysch bedrock, is forming young karst phenomena. The percolating water accumulates where breccia is most consolidated. Earthworks have revealed the early stages in the formation of unique dolines. Since the motorway runs diagonally to the slopes, at the same time it cuts many and various relief forms in the upper and lower sections of the slopes. Road construction has also indirectly affected many, particularly fluvial relief forms. Due to the changes in the outflow from the slopes, changes in the old and the formation of the new relief forms are likely. Characteristic types of cavities developed in the young and very porous breccia which is consolidated only in places that lie on the more or less slanting flysch, i.e. on an impermeable bedrock. The true karst cavities are small and their development was influenced by the alluvium that has been depositing on their bottom and mainly fills them. They formed in a locally and periodically flooded zone or they were paragenetically enlarged. The largest cavities formed above the contact with the impermeable flysch bedrock where the big streams joined together. Their shape reflects the varying degrees of consolidation of breccia. In areas where breccia is less solid and along fissures they rise into domes. Along fissures which emerged due to the sliding of breccia and rubble down the slanting bedrock of frequently soggy flysch, fissure caves formed diagonally to the dip direction of the slope; some of them are very long and wide enough in places to make them accessible. Their walls are mainly covered with flowstone.

The Section Between Divača and Kozina Revealed Many Characteristics of Karst Development

The motorway section from Divaca to Kozina, stretching 7.5 km, revealed 50 old caves, most of them filled with alluvium, many of which unroofed, and six of them qualifying as shafts. The old caves were once, in a time when the underground water table was still high enough, characterized by through-flowing water. At that time, the aquifer was encased in flysch high up and partly covered with it. The runoff from the flysch introduced loam and sand into the caves. After they had been dry for a while and featured flowstone deposits, flood waters filled the caves with fine-grained sediments in one of the last cave development phases. The alluvium sealed the cracks and withstood even the long periods of the aquifer development during which its surface lowered for several tens of metres. It thus appears that old unroofed caves are an important component of the karst surface, providing clues to the development of the aquifer. Seeing as today the underground water level is 200 m and more below the surface, the traces of ancient water flow through the aquifer are now preserved only in caves and their respective alluvium. There was no indication of surface water flows that could have reshaped the aquifer’s surface. Old caves therefore contain the oldest geological traces dating back to the very early periods of the karst aquifer development. The more significant caves were able to be preserved. Today, these are either hidden below the road or accessible via man-made entrances.

Great Cavernosity Between Dane and Fernetiči Points to Diverse Karst Formation

The motorway route between Dane and Fernetici penetrates the karst ridge near Sežana, running over the karst plain to the border with Italy. The karst developed in Cretaceous limestone, which is intersected by smaller faults; this was best visible during the tunnel excavation under the Tabor Hill. It is quite remarkable the extent to which the surface of this part of the karst aquifer is dotted by old caves which are filled up by fine-grained alluvium and rubble and that had, at one point or another, lost their roofs. The higher parts of the aquifer have been subject to reshaping by long-term percolating water. Old caves reveal glimpses into a time when subsurface water was close to the actual surface of today. The continuous lowering of the karst regions led to the situation where today’s surface is lower than the original karst surface was. It appears that great cavernosity came about as the result of the high permeability of this part of the aquifer. The size of the passages suggests that they were passed by larger water flows which had originally completely filled the passages. It is assumed that a remnant of a flysch nappe was somewhere in the vicinity, feeding the aquifer with surface runoff. This is also suggested by the ponor properties of some caves. The cave bottoms display meandering patterns carved out by small-scale water flows. Even after the subsurface water table had dropped, flysch layers still lingered over the limestone. Despite the water drop, high waters occasionally still reached the caves, eventually filling them up with fine-grained sediments. In the cooler periods of the Pleistocene, some of the old caves were filled up by rubble.

The Large Unroofed Cave Near Povir

The largest unroofed cave in the entire motorway route across Classical Karst was the 230 m long unroofed cave named Brezstropa jama which was unearthed near the village of Povir. It contained flowstone, stalactites, stalagmites and various sediments. Inside the cave, flowstone has been preserved in the form of crusts, massive flowstone heaps, and free-standing stalagmites, but some broken off stalactites were also found. It is possible to reconstruct a part of the development of the Brezstropa jama unroofed cave based on the shape of the walls and the sediments. The cave is a remnant of a larger cave system which drained sinking stream waters from flysch. The preserved section of the passage was located deep beneath the surface. The cave had a through-flowing sinking stream, carrying large pebbles. The large share of flysch sandstone pebbles and their size allow the conclusion that ponors were not far off. There were no relevant barriers for the water flow on its course from the ponors to the cave. The formation of stalactites/stalagmites and flowstone heaps does not reflect external influences, although the growth of the stalactites and stalagmites was interrupted several times by stages of either erosion or sedimentation. One of these cave erosion stages also left its mark on the flowstone which is over 350 thousand years old. Next, the cave was filled with fluvial sediments. The infill prevented the further filling with flowstone, the collapse of the roof and the transformation of the walls by corrosion. Thus, the cave transformation came to an end, but not that of its surroundings. For the time being, we are still unable to date the precise age of the Brezstropa jama unroofed cave. The only indirect clue is the rate with which the surface is lowering. The age of the Brezstropa jama unroofed cave is defined by the time in which the water level of free-flowing rivers, such as the one flowing through the cave, dropped from 400 m to 180 m a.s.l. It took 750 thousand to 1.5 million years for the Brezstropa jama unroofed cave’s 50–100 metre-thick roof to be corroded down, while the cave itself or its infillings are probably even older.