The influence of extreme flood event on redistribution of potentially toxic elements: a preliminary results from a former mining area

The climate change and related phenomena is one of the biggest challenges of today’s civilisation. The data shows an upward trend of occurrence of meteorological, hydrological, and climatic phenomena worldwide, with the largest increase observed in extreme hydrological phenomena, such as floods and mass movement. Intense rainfall and resulting floods can lead to erosion and transport of large amount of natural and anthropogenic material and thus influencing the spatial distribution of chemical elements on Earth surface. The redistribution of potentially toxic elements (PTEs) is of special concern as they are largely persistent, non-biodegradable, and many are known to accumulate along the food chain. The environmental consequences of remobilization and redistribution of pollutants during flood events are not yet widely recognized and understood.

 

An extreme rainfall and floods severely affected Slovenia (EU) at the beginning of August 2023, resulting in more than 10,000 landslides and spatial redistribution of large quantity of sediments, including heavily polluted ones. One of the most affected areas was the Carinthia region at the north of Slovenia. This area is strongly impacted by a 300-years of lead and zinc mining in the Mežica area. Although the mining activities have ceased 30 years ago, the environment (e.g., soil, floodplains) is still heavily contaminated with Pb, Zn, Cd, Mo, and other PTEs, and there are several mine waste deposits prone to erosion. The Geological Survey of Slovenia has been studying the geochemical characteristics of the wider Mežica area for several decades. Levels of PTEs in stream sediments were regularly monitored (every 3 years) since 2013.  

 

To determine the potential influence of extreme rainfall and floods on redistribution of PTEs in the environment that have been previously contaminated with PTEs, the following samples in the Mežica area in 2023 were collected: (1) stream sediment samples before the extreme weather event in August as a part of regular monitoring, (2) repeated samples of stream sediment after heavy storm at the end of July and after the extreme event in August at selected monitoring locations, (3) flood sediment samples along the Meža Valley after the extreme weather event in August. All samples were prepared (dried at 35 °C and sieved <0.125 mm) and analysed (determination of PTEs levels by ICP-MS after aqua regia digestion) by the same methods.

 

The comparison of PTEs levels in stream sediments from a decade long monitoring with flood sediment and stream sediment sampled after an extreme flood event illuminate the complexity of redistribution processes during such events, which may result in increase or decrease of PTEs levels. For example, the median levels of As, Cd, Mo, Pb, and Zn in flood sediments were higher than their median levels in stream sediment during usual hydrological conditions indicating erosion of contaminated areas and mine waste deposits dominated over erosion of non-contaminated materials. On the other hand, levels of PTEs at some specific sampling locations were much lower after the extreme flood event than before, indicating higher erosion of non-contaminated materials that may lead to the dilution effect.