Slovenia has a rich underground world with caves large and small dotting the country. They have attracted cavers, visitors and researchers from near and afar for centuries, but also people who used them as a handy way to dump their waste. As a result, 20-30% of Slovenian caves are now thought to be polluted and cleaning them is a major challenge.

Caver and researcher Jure Tičar has often come across waste at the entrances to pit caves, which led him to consider the causes, process and extent of cave pollution in Slovenia. When as the head of service for cave protection at the Slovenian Speleological Association he started searching for data on pollution, he realised there was no systematic record keeping.

There was some information available about the degree of cave pollution in individual areas, but no data for the entire Slovenia, which is why he started collecting this data as part of his PhD project. Cave pollution poses a potential threat to underground sources of drinking water.

Tičar works with caves professionally, as a researcher of geomorphology, karstology and speleology at the ZRC SAZU's Anton Melik Geographic Institute, as well as in his free time as a caver and a member of multiple associations and expeditions.

He has organised and participated in many research trips in Slovenia and abroad. Currently, he is exploring the Skalar cave on Mt Kanin, where they are looking for connections with adjacent cave systems. If they manage to confirm the links, the Kanin cave system, estimated to be 1,900 metres deep, will officially become the deepest cave in Europe.

Over 500 new caves discovered each year

According to the Slovenian Speleological Association, 14,695 caves were registered in Slovenia in July and every year at least 300 new ones are discovered. In recent years, the latter number has risen to more than 500 per year.

They are all listed in the cave registry, a national database of all registered caves which cavers have visited and recorded in the cadastral registry. The registry, managed parallelly by the Slovenian Speleological Association and the ZRC SAZU's Karst Research Institute since 1947, is one of the oldest such projects and a unique project in the world.

The registry also includes information on the pollution of caves, but this is often obsolete because it is outdated. "The quantity and quality of data is also problematic, often describing merely a village dumpsite or at other times specifically stating the structure and quantity of the waste," Tičar said.

In his PhD thesis, he systematically arranged this data, edited it and complemented it with spatial data. He wanted to answer the questions of how widespread cave pollution in Slovenia is and what the connection is between cave pollution and spatial data.

Using a new methodology, he studied data from the archives about the situation in 6,965 caves in 17 Slovenian regions. He processed the data with the help of geographical information systems and descriptive statistics, finding that more than 2,500 of the caves from his sample, or 20% of all caves in the country, are polluted.

Tičar warns that the actual share is almost certainly much higher, as the data he collected was strongly affected by the selection of the areas studied. In the Alpine karst area, the Julian Alps, there are many caves, and many of them are clean, which affects the entire share.

However, in low-altitude karst areas, there is much more pollution. He estimates the realistic share of polluted caves could be up to 30%.

These differences occur mostly because of spatial factors - primarily the number of inhabitants in the proximity of the cave, distance to the nearest building, altitude of the entrance to the cave, distance to the nearest road, and the level of damage done to the cave.

"The main factor affecting the level of pollution is how accessible the cave. The more accessible the cave, the closer it is to roads or forest paths, the easier it is to dump large quantities of waste in it." Another very important factor is the density of the population and the proximity of settlements to the cave.

Most of the waste decades old

One of the interesting things that Tičar has discovered in his research of the history of cave pollution in Slovenia is that the first records of pollution date back to the 1689 Glory of the Duchy of Carniola, written by the celebrated polymath Janez Vajkard Valvasor.

Back in the 17th century, people used caves as dumping ground during special rituals or to get rid of waste. But in the past this was not a big problem, as they mostly dumped organic waste, wood or rocks.

However, as we move into the twentieth century, this problem starts to grow. After the Second World War the country saw intensive economic development. People suddenly had a lot of goods but no functioning system of municipal waste collection was in place.

As waste began to accumulate, the population turned to solutions that seemed most convenient. "In karst areas, unfortunately, these were caves, most often pits, where waste was easily dumped." Tičar explained.

The biggest increase in waste disposal took place in the period from the end of the Second World War until the 1990s. With modern municipal regulation, waste collection systems and the civil society addressing ecological issues, this problem is gradually and radically disappearing, Tičar believes.

Organised initiatives such as the high-profile Clean Up Slovenia campaign, have also helped raise awareness that the kind of pollution that occurred in the past is no longer acceptable. But the vestiges of the past are still there and cave pollution remains a pressing issue.

Anything from household waste to UXO found in caves

The composition of the waste in caves varies a lot. Overall, household and construction waste can be found everywhere, and even a random vehicle. The more dangerous waste are pesticides, electronic devices, industrial oils and other fluids.

Moreover, caves in karst areas were also used to dump unexploded ordnance (UXO) after the First and Second World Wars. The ordnance is still present in caves today and poses a threat because of the substances it contains and due to the risk of accidental ignition during a clean-up.

In the course of his research, Tičar has also come across extremely polluted caves. One such is a cave in Trnovo Forest, where more than 180 cubic metres of used tyres were dumped. Another example is a cave near Socerb, where several containers of spoiled salami from the port of Koper were dumped.

The absolute record-holder for pollution is the Ravnica Cave in Pivka, where four thousand cubic metres of waste has been dumped. According to Tičar, this cave will be cleaned in the near future, which will be an important milestone in tackling cave pollution in Slovenia.

Although caves are no longer being polluted to such an extent as in the past, individual cases of pollution still occur. One frequent example is the release of contaminated water from waste water treatment plants, like the one on the Bloke Plateau in south-central Slovenia.

"That is when we have problems in Križna Cave, an underground gem, which we very much protect and which is a very well preserved cave. In this case, foam shows in the water in the cave, which indicates pollution," Tičar says, adding there are many more such cases so this issue should be tackled systematically.

Apart from pollution, damage to caves is also done by vandals or profit-seekers. In the past, for example, the removal of stalactites with the intention of selling them was one such issue. Today, in some of the more easily accessible caves, large graffiti can be found, which completely defaces the natural space and destroys its value.

Professional and systemic approach crucial

The process of cleaning caves is very demanding and must be conducted only by well trained cavers who are skilled at using ropes and are used to working in caves. But they too face security risks that need to be properly addressed during preparations for clean-up.

The use of construction machinery, which allows loads to be lifted smoothly from vertical pits, greatly facilitates and speeds up the cleaning process. However, this is only possible in caves where the entrance allows it. The work in the cave itself is still mainly manual and all waste must first be placed in transport bags or special containers, properly lifted and later recycled.

The lion's share of underground cleaning is done by cavers on a voluntary basis, as part of caving clubs that organise annual clean-up campaigns. Around 20 caves are cleaned this way in Slovenia every year. Tičar thinks one good solution would be to include polluted caves in project activities, as was done in the past in the framework of the EU-sponsored LIFE Kočevsko project.

Caves with larger quantities of waste that have been polluted for decades often do not get cleaned as part of these campaigns as more people, infrastructure and financial support are needed for such projects.

"This is why cavers are striving to find systemic solutions at the state level, which would allow for part of the funds to be allocated to returning these habitats into their original state and tackling even the most critical cases of cave pollution."

The legal foundation already exists. A law on protection of underground caves that entered into force in 2004 defines the operation and management of underground caves in a specific way. The law is unique in the world.

It defines caves as natural assets of national importance, which Tičar says is a very high level of protection, and at the same time, the caves become natural assets owned by the state. A specific part of the law defines the monitoring of the condition of caves, how polluters are punished, and the need for an operational clean-up programme.

Given that 47% of Slovenia's surface is karst, legal protection of these natural environments is important, Tičar says. Historically, karst and karst phenomena began to be explored on Slovenian soil, with the origins of research dating back to the Habsburg monarchy and the construction of the railway from Vienna to Trieste.

Even more than history, legal protection of cave environments has been prompted by the realisation that the karst world gives access to a quality and sustainable supply of drinking water, and provides a very specific endemic underground habitat for various cave species.

"These two factors are very strongly accepted in society and it is through them that we address the issue of cave pollution and its impact on the quality of this groundwater," says Tičar.

Clean-up requires quality data and a plan

Although a direct connection between cave pollution and underground water pollution has not been experimentally confirmed yet, experts agree this connection is very probable. At a time when the supply of clean, drinking water is becoming an increasingly pressing global issue, the systematic clean-up of polluted caves is becoming a must.

As part of his PhD theses, Tičar has created a model for a priority clean-up of caves. The model singles out the most problematic caves in Slovenia, which are located in water protection areas or protected areas, and contain large amounts of waste or highly hazardous waste. As a result, they can be cleaned up as a priority.

Apart from obtaining pollution data from the cave registry, future field monitoring will be conducted to address the problem of obsolete data. Tičar also wants to change the way data is collected. The project is financially supported by the Public Research Agency and the Ministry of the Environment and Spatial Planning.

One of the project's objectives is to develop guidelines for cleaning operations that would tell organisations, caving associations or private operators how to meet all legal and safety rules to make sure the work is done safely and efficiently. Tičar says this area is currently unregulated.