The Soča is one of the most pristine rivers in Europe. It is crystal clean and water quality is good, but environmentalists and fishermen are concerned about possible industrial incidents and chemical pollutants, some of which may seep into the river from unlikely sources.

The chemical state of the Soča and its tributary Idrijca is regularly monitored at several points along their courses and occasional testing is performed upstream and downstream of Salonit Ahnovo, an industrial installation that has long concerned locals.

According to the Environment Agency, water quality in both rivers is good, but the biotic situation has deteriorated, especially in terms of fish numbers.

This is largely the result of excess levels of mercury and brominated diphenyl ethers (BDE), which have been used as fire retardants in a wide array of products and are a problem in surface waters in Slovenia and in Europe.

Local fishermen have observed a drop in the fish population in the Soča and Idrijca as well.

While this could partly be attributed to hidden pollution and declining water levels, the main reason for there being fewer fish lies in the numerous predators that live along both rivers, the Posočje Fishing Club has told the Slovenian Press Agency.

They agree with the Environment Agency's findings that the Soča is less polluted than it has been, to a large extent because of new wastewater treatment plants.

However, Erik Sivec from the Kobarid unit of the Fisheries Research Institute of Slovenia warns about possible hidden polluters that are not detected with water analyses, such as soaps, washing powders, and other similar substances.

"There are certainly several reasons for the drop in fish population. One main reason is the decrease in the number of habitats, since the Soča is no longer as wild, while the removal of material from the riverbed has also led to the water level being significantly lower than it used to be.

"The water still seems clear, but thorough chemical testing would definitely show worse results," Sivec says.

The water monitoring above and below Salonit Anhovo, carried out by the Environment Agency between 2021 and 2022, indicated that the cement factory is not detrimental to the water quality, since both analyses showed a good ecological state of affairs in terms of special polluters, as well as good chemical parameters. The same goes for the area surrounding the chemical factory TKK in Srpenica.

The situation on the Idrijca, which flows into the Soča in Most na Soči, is similar. According to the president of the Idrija Fishing Club, Zoran Babič, extensive pollution has not been recorded in recent years.

Nevertheless, occasionally spillages of dirty water from the wastewater treatment plant in Idrija occur, which has prompted the club to occasionally raise concern with the local utility company and the Idrija municipality.

Babič says that lower water levels of the Idrijca, as well as higher water temperatures due to increasingly hot summers, are a problem for the fish populations. And so are predators, including cormorants and ducks.

Yet the biggest cause for concern among environmentalists is occasional spillage from industrial facilities, Mateja Sattler of the civil initiative Eko Anhovo and the Soča Valley told the STA. Every year, the initiative reports three to four such incidents to the authorities.

"The biggest issue is industrial sewage in the central portion of the Soča, near the Doblar hydroelectric power plant and further downstream. A spillage incident has occurred in Srpenica, there was an oil spillage case in the Plastik Kanal factory four years ago, the Anhovo industrial zone is problematic as well.

"There is, unfortunately, not enough supervision of the conditions in industry, and the responsible institutions often react inadequately," Sattler said.