Impacts of mining and smelting activities on environment and landscape degradation—Slovenian case studies

This paper provides an overview of the physical and chemical landscape changes that have occurred at four reference sites due to historical mining and smelting activities within Slovenia, and their comparison with similar sites around the World. Literature review has been made with the intention to identify major pollutant sources, its dispersion control factors, and effects. The four reference sites are Idrija, with more than 500‐year Hg mining and ore smelting history, the Meža Valley, also with a 500‐year PbZn mining and smelting history, the Celje area where Zn was smelted for 100 years and the Drava River alluvial plain, which is contaminated because of historical PbZn mining upstream. Based on the comparison between the four abovementioned reference sites and similar sites around the world that are situated in different landscapes and climates, we identified major sources of contamination, which are the erosion of mine and ore processing wastes, and atmospheric emissions of metal‐containing particles from smelters. In the first case, major control factors are rainfall pattern and river gradient, controlling erosion and sediment deposition patterns. In the second case, the prevailing control factors are topography and the dominant wind directions.


| INTRODUCTION
The earliest global traces of the impacts of mining on the environment, reconstructed from environmental archives, such as ice cores, peat bogs, lake and marine sediments, can be dated back to around 6500 BC. The first recorded global impact of mining was assigned to the Roman period due to Pb-smelting (Marx, Rashid, & Stromsoe, 2016). Mining and ore processing are amongst the most important impetus of human development and are regarded as the second worst global polluters today (Blacksmith Institute, 2018).
Mining and ore processing cause different changes to the landscape (Larondelle & Haase, 2012). The most important physical landscape changes are deforestation and vegetation removal, changes in relief, deposition of mining wastes (spoil and tailings), construction of supporting infrastructure, increased erosion rates, suspended materials in surface water systems, and increased rate of soil and rock instability. Chemical changes are caused by the dispersion of extracted materials or chemical agents used in mining or ore processing (flotation, extraction, etc.), which lead to changes in the chemical composition of the natural environment. Although minerals are naturally present in the environment due to outcrop weathering and erosion, mining and related activities can produce increased levels of certain elements in the environment that exceed natural levels up to 1,000-Besides anthropogenic and geological factors, landscape and climate characteristics can also influence the rate and dispersion of pollutants. Dominant wind patterns control dispersion of particulate matter, precipitation rates might control material stability, and the river gradient controls erosion and deposition patterns.
The objectives of this study are to identify dominant factors, which affect the dispersion of the pollution, caused by mining and ore processing activities. This is done by comparing four historic mining sites in Slovenia with similar contaminated sites, situated in different climate and topographic conditions (Figure 1; Table 1). Contaminant dispersion pathways and receptors are identified, and effects are linked to the landscape characteristics (i.e., topography and climate). The four reference sites are situated in the tectonically active junction between the Alps and Dinarides. The climate is warm-summer humid continental (Dfb; all climate symbols are according to the classification described in Peel et al., 2007), with the annual precipitation rates between 2,500 mm (Idrija) and 1,200 mm (Celje). Temperature inversion is the common weather phenomenon in the cooler part of the year.  In the town of Idrija (Figure 1), mercury ore was mined and processed for five centuries until 1995. These activities caused an unprecedented legacy of mercury contamination in the wider Idrija area (Teršič & Gosar, 2012;Gosar et al., 2016; (Covelli, Petranich, Langone, Emili, & Acquavita, 2017;Kotnik et al., 2015).
The influence of mercury originating from the Idrija ore deposit was also determined in the middle of Adriatic Sea (Foucher, Ogrinc, & Hintelmann, 2009), almost 500 km away.
A special characteristic of the town of Idrija is that it is built directly over the ore deposit and mine. The 4,000 m 2 large ore outcrop contains on average 3,800 mg kg −1 (Mlakar & Čar, 2009). Natural migration of mercury into the wider surroundings of the deposit is insignificant because large anthropogenic impacts dominate the Idrija town (Bavec & Gosar, 2016).
Mercury was distributed throughout the surrounding areas by atmospheric emissions from ore smelting facilities. Gaseous and particulate matter emissions were the major cause of creating the geochemical halo around Idrija, where the areas of highest Hg levels in soil are limited to the valley floor, and not on the surrounding hills . Models of the spatial distribution of mercury in Slovenian soils show that the influence of atmospheric Hg emissions from Idrija can be detected on a regional scale ; Figure 2).
Atmospheric mercury levels in Idrija were reported to be extraordinarily high at the times of ore smelting. In 1971In -1972 concentrations up to 30,000 ng m −3 were reported, and daily atmospheric mercury emissions were estimated at 20 kg (Kavčič, 1974).
Measurements in 1994  showed much lower but still rather high mercury concentrations. Concentrations above 300 ng Hg m −3 were observed near the smelting plant and mine ventilation shaft. The results of various studies showed rapid variation of mercury concentrations in air depending mostly upon changing weather conditions Kotnik, Horvat, & Dizdarevič, 2005). After the production of mercury stopped in 1995, the conditions improved (Kotnik et al., 2005).
Ore smelting waste dumps were found to be the major source of river sediment contamination. Since 1652, when the first smelter building was built in Idrija, the mercury rich waste products were deposited along the Idrijca River. The main reason for the complex spatial distribution of smelting residue dumps in Idrija and its surroundings are changes in smelting techniques over the centuries, continuously increasing quantities of processed ore accompanied by decreasing mercury content. Čar (1998) defined the locations of historic waste dumps containing mercury. Hg-binding forms in these dumps depend on the efficiency of the smelting technique and the predominant Hg species in the processed ore. In older dumps (deposited in 18th and 19th centuries), the predominant Hg species is cinnabar (crystallised Mercury (II) sulphide, HgS), whereas in dumps of the 20th century the amount of cinnabar in the material decreased due to the higher efficiency of the smelting process and the use of ore containing mostly native Hg. Leaching tests showed that although lower total Hg concentrations are found in the younger dumps, their long-term risk potential is higher than that of the older ones, which contain mostly immobile cinnabar (Biester et al., 1999).  (Biester et al., 2000;Gosar, 2008;Gosar, Pirc, Šajn, et al., 1997;Gosar & Žibret, 2011).
During high water levels, Hg-rich material was deposited on the floodplains in the lower part of the Idrijca and Soča Valley ( Figure 3). These sediments represent a large accumulation of Hg-enriched sediments (Gosar & Žibret, 2011). It was estimated that about 2,000 tons of mercury are stored in the Idrijca River alluvial sediments (Žibret & Gosar, 2006).
During the first 150 years of mercury production, numerous smaller ore smelting sites existed in the woods around Idrija. Detailed geochemical investigation of these sites proved their significance for environmental contamination (Teršič et al., 2014). The soils at these sites are highly contaminated with Hg, showing Hg loads into the percentage range. The investigations proved that Hg-loaded materials are still present at the smelting sites and are being intensively eroded and transported downstream during periods of high water levels.
Investigation of the Hg load in the Idrijca River sediments shows that the historic smelting sites still remain an important source of Hg-contaminated material today and one of the primary concerns for persistent Hg release into the aquatic ecosystem (Gosar & Teršič, 2015;Teršič et al., 2014).

| Mežica Pb-Zn mine
The    (Kladnik, 2009; Table 3).    and in abandoned mine shafts covering a total area of approximately 0.5 km 2 (Budkovič et al., 2006;Figure 5d). Some mine spoil heaps are considered unstable due to their surface topography and the underlying ground and are also subjected to significant runoff erosion and remobilisation of waste material.
Within spoil heaps mine waste material is chemically relatively stable. However, its washing out into streams may cause leaching and mobilisation of PTEs into the fluvial environment. Investigation of stream sediments in the Meža River Valley (Gosar & Miler, 2011;Miler & Gosar, 2012) showed that stream sediments are contaminated for a length of approximately 30 km downstream, mainly due to the steep stream gradient, which causes erosion of the polluted sediments. The Meža River sediments contain high median values of Pb (1,100 mg kg −1 ), Zn (1,240 mg kg −1 ), Cd (7 mg kg −1 ), As (13 mg kg −1 ), and Mo (23 mg kg −1 ).
Between 1914 and 1979, 150,000 tons of flotation tailings were discharged every year directly into Meža River (Kladnik, 2009). As a consequence, the water in the Meža River contained over 200 mg l −1 Pb, which completely destroyed all stream biota. After 1979, flotation tailings were deposited in mine shafts, and the water quality began to improve.

| Celje Zn smelter
Celje is a central Slovenian town (Figure 1) of approximately 50,000 inhabitants that is a well-known historic industrial centre. Smelting 1989 was partly the result of stricter environmental controls (Leskošek, Mašat, Eržen, & Uršič, 1998), enforced by the restoration plan, and partly due to the economic crisis at the end of 1980s.
The study of Lobnik et al. (1989) revealed a geochemical Pb, Zn, Cd, and As anomaly covering an area of 32 km 2 within the densely populated town of Celje. The urban sediment study done within the territory of Slovenia by Šajn (1999) revealed that the levels of Zn, Cd, and Pb in urban dusts (house, attic, and street dust) of Celje town exceeded average Slovenian urban levels by more than 15 times. The studies of Žibret (2002) and Šajn (2005) (Žibret, 2012, 2013; Table 4). Further study also revealed that this waste pit represents significant future risk, as extremely high levels of pollutants were measured at this site (Voglar & Leštan, 2010).
The recent floodplains reflect anthropogenic geochemical signatures, whereas the river terrace represents the historical geochemical signature of deposited material, and so can be used to differentiate between anthropogenic and natural processes as well as for the evaluation of trace element levels in soils and sediments before the industrial revolution (Halamić, Galović, & Šparica, 2003).

| DISCUSSION
Despite many potential sources of contamination, caused by mining and ore processing, the two most profound ones that are identified and investigated are erosion of mine and ore processing waste, and deposition of atmospheric particles from smelters.
The most important recognised source of contaminants, accounting the contaminant's quantities and size of contaminated area, are erosion and the transportation of mine waste and ore processing waste by water (Figure 8a). Contaminant receptors are alluvial and marine sediments, as is in the case of the Idrija and Mežica. Due to relatively large gradients of Idrijca and Meža Rivers, contaminated materials are eroded and transported downstream. In Idrija, flash flooding during thunderstorms is a common weather phenomenon, allowing the miners to continuously dump mining and ore processing waste directly into the river channel. In contrast, Almadén Hg mining district in Spain, where Hg was produced for at least 2,000 years, has cold semiarid climate (BSk) with less than 500 mm annual precipitation.
Despite similar stream gradients in Idrija, Mežica, and Almadén, the physical remobilisation by aqueous transport in Almadén is less important due to a much lower amount of rainfall (Higueras et al., 2006).  (Conesa & Schulin, 2010). Despite a dry climate, streams in the area are extremely polluted along their entire length because of the erosion of mine waste deposits. Due to short but intense rainfalls and steep gradients, Pb contents in stream sediments in La Union district are by 10-times higher than in Meža River sediments (Brotons, Díaz, Sarría, & Serrato, 2010). It is clear that not only the quantity of rainfall and river gradient but also the precipitation distribution pattern plays a crucial role in contaminated material dispersion by water.
Metal containing particles can be transported over large distances by water, as is in the case of Idrija area, where effects of mining are recorded in the marine sediments as far as 500 km away from the source (Foucher et al., 2009), or several hundred kilometres away from the mines as is in the case of Drava River. In both cases, geomorphological factors (i.e., past or present floodplain and terraces), rather than the distance from the source, determine the level of contamination (Gosar & Žibret, 2011;Šajn et al., 2011). Another example is the Kawerong-Jaba River system in Papua New Guinea, where the waste rocks and tailings have been continuously dumped in the river. Although the majority of tailings were transported into the sea, deposition patterns reveal that tailings were deposited mainly in the overbank sediments and in the area where river enters lowlands and the river gradient is reduced. The areas of lowest deposition are in confined, steeper sections, and in the swampy lowlands (Brown, 1974). Examples show that river erosion and deposi-  a consequence of atmospheric particle deposition, is recorded up to 20 km away (Žibret, Van Tonder, & Žibret, 2013). No orographic barriers are present in this case. Another historic Zn smelting district is located in a narrow valley in Palmerton, PA, USA. In this case study, the effects on soils are recorded between 12 and 39 km away, depending on the upwind or downwind direction from the smelter, with maximum Zn, Pb, and Cd levels in topsoil reaching 135,000, 2,000, and 1,750 mg kg −1 , respectively (Buchauer, 1973). The study of lead isotopic composition in the most contaminated O2 soil horizon shows that the primary source of lead is the 80-year long Zn smelting tradition in the area (Ketterer, Lowry, Simon, Humphries, & Novotnak, 2001 (Buchauer, 1973). On the contrary, in the case of water transport, complex contamination patterns in the soil and alluvial sediment profile are recorded, and deeper layers can also be affected (Gosar & Žibret, 2011;Pavlowsky et al., 2017), thus making potential clean-up operations more challenging.
Contaminants released by mining activities can have negative effects on the ecosystem and humans. Most commonly reported impacts are effects on wildlife and vegetation (Brown, 1974;Covelli et al., 2017;Kladnik, 2009;Špes, 1978), metal uptake by plants and crops, especially by vegetables with edible roots, such as carrots (Bešter et al., 2013;Roy & McDonald, 2015) and increased levels of metals in blood and urine (Jež & Leštan, 2015;Zhang et al., 2012).
Metal levels in crops in such areas can exceed reference metal levels by more than 50 times (Zhang et al., 2012).
Presented cases show that mining and ore processing industry has a high potential to produce large-scale impacts on the environment.
Major potentially toxic substances dispersion processes are runoff erosion of tailings and ore smelting wastes, and particulate matter emission from ore processing facilities. Poor environmental practices in the past resulted in large contaminated areas, where contaminated flood plains and soils present a reservoir for future dispersion of pollution. It must also be noted that it was very difficult to quantitatively compare contaminated sites, because almost every author used their TABLE 5 Identified sources, pathways, and receptors in the case of the Drava River alluvial plain contamination due to past mining activities, and their effects on the landscapes and possible effects on humans own sampling, analytical, and data interpretation methodology. Thus, the comparison in this paper is based more on qualitative assessment.
It is important to know past environmental burdens, caused by metal pollution, which can help to better utilize contaminated areas and also utilize prevent further spreading of contamination today. On a global scale, such considerations can help to prevent dispersion of harmful substances from current and future mining operations around the globe.

| CONCLUSIONS
This study focuses on the impacts of mining and ore processing activities on the environment. Four reference study areas are presented and compared with similar sites around the world in order to identify most important sources of contamination, main transport mechanisms of pollutants, receptors, major landscape control factors, and effects.
The most important identified contamination sources, accounting the contaminant quantities and size of the contaminated areas, are mining and ore processing wastes, and their runoff erosion. Receptors are floodplains and marine sediments, whereas contamination distribution pattern is dominantly controlled by precipitation and river erosional and depositional patterns. Impacts of such contamination dispersion pathway can be detected several hundred kilometres from the source. cessing on the environment and landscape but also to allow better understanding of how the environment responds to these changes, allowing us to identify their long-term impacts and how historical changes influence the present-day biosphere and humans.