Research Article |
Corresponding author: Dimitar Zhelev ( zhelev@gea.uni-sofia.bg ) Academic editor: Galina Radeva
© 2022 Dimitar Zhelev, Rumen Penin.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Zhelev D, Penin R (2022) Pre-monitoring geochemical research of the river sediments in the area of Ada Tepe gold mining site (Eastern Rhodopes). In: Chankova S, Peneva V, Metcheva R, Beltcheva M, Vassilev K, Radeva G, Danova K (Eds) Current trends of ecology. BioRisk 17: 379-387. https://doi.org/10.3897/biorisk.17.77466
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The article depicts the geochemical properties of the landscapes in the Ada Tepe gold mine area before its launching. The research is conducted by examining the heavy metals (Cu, Pb, Zn, Co, Cr, Mn and Ni) content in samples of river sediments in the local landscapes. The research aims to analyse the concentration of heavy metals before the launch of gold mining. The study implements the coefficient of Clarke concentration. The deviation from the background concentrations is a ratio between the element concentration in the collected environmental samples and the Clarke value of the element. The coefficient has a scale from 0 to a particular positive value, corresponding to the level of enrichment of the sample in comparison to the background Clarke value. The values corresponding to the Clarke concentration are equal to 1, the lower values are between 0 and 1 (dispersion) and any value higher than 1 is a case of concentration (enrichment). The obtained results display the researched territory as a natural background area. The content of heavy metals in the river sediments of the researched area (mg/kg, median value) by chemical elements is Cu (15), Zn (72), Pb (17), Mn (461), Ni (35), Co (8) and Cr (60). That is the reason it could be defined as not impacted by human activities and it is not influenced by natural geochemical anomalies. Heavy metals do not pollute the researched landscapes before mining. This outcome is obtained by the geochemical content of the investigated heavy metals in the river sediments.
Ecogeochemistry, environmental impact, gold mining, landscape assessment, pollution
In 2016, the current research was conducted as an environmental pre-monitoring activity in the area of the Ada Tepe gold mining site (Eastern Rhodopes, Bulgaria). The mine itself started exploitation in 2018 after years of delay due to obstructions by local authorities and non-governmental organisations. The non-supporters of the project outlined possible environmental damage, such as contamination of rivers with heavy metals, loss of habitats, air pollution etc., that might be caused by the mine. As a result, the company Dundee Precious Metals Inc. reshaped its initial business plans to satisfy the demands of the local community for better environmental security.
In ancient times, the area of Ada Tepe was a well-known place for its metal deposits. People used to mine gold thousands of years ago. This activity caused local environmental changes such as deforestation, soil erosion and topographical transformations (
The study is focused on the concentration of heavy metals (Cu, Pb, Co, Zn, Mn, Ni and Cr) in the river sediments of particular catchments as a geochemical indicator for the environmental status of the territory. The obtained results are a baseline for future monitoring and assessment of contamination with heavy metals and human impact.
The methodological base of this investigation was the system approach (
The chemical elements'content in the various rocks and soils does not match the Clarke value of the element (the average chemical element’s content in the lithosphere). That is why the coefficient of Clarke concentration outlines the abundance or the lack of particular elements in rocks, soils and river sediments. It is a quantitative proportion between the chemical element’s content in a natural component such as a rock, soil, water, plants and the Clarke value of the element.
In the current study, the deviation from the background concentrations is a ratio between the element concentration in the collected environmental samples of river sediments and the Clarke value of the element. The coefficient has a scale from 0 to a particular positive value, corresponding to the level of enrichment of the sample in comparison to the background Clarke value. The values corresponding to the Clarke concentration are equal to 1, the lower values are between 0 and 1 (a case of dispersion) and any value higher than 1 is a case of concentration (a case of enrichment). The coefficient allows the implementation of a comparative analysis between particular areas. The current study compares the area of Ada Tepe to technogenic and natural territories in Europe and Bulgaria.
This study analysed 12 samples of river sediments for particular chemical elements (Cu, Pb, Zn, Ni, Co, Cr and Mn). The samples were collected by a standardised methodology. Every sample (500 g, grain size < 0.5 mm) was collected from the upper layer (0–5 cm) of the river-dried thalwegs during the summer when all intermittent rivers in the area dry out. We selected the locations by an analysis of spatial perspective and topographic accessibility. The locations allow interpreting the geochemical influence of the Ada Tepe itself and the influence of the tributaries that confluence the main river in that area.
The collected samples were analysed in the Geochemistry Laboratory of Sofia University. The sediments were dried, quartered, levigated in a porcelain cup, sifted through a 63 µm sieve, burned at 500 °C and dissolved by a mixture of acids (HClO4, HF and HCl). Heavy metals content in the chemical solution was obtained by the method of atomic-absorption spectrometry (Perkin-Elmer 3030).
River sediments are a geochemical indicator for the environmental status within a catchment. In recent years, many scientists have applied the basin approach in evaluating the natural processes and human impact on the environment (
The bedrock, the topography, the vegetation and the climate determine the properties of the river sediments in the area of Ada Tepe. The impact of ancient people is still visible in the features of the landscapes with some rock niches and ancient mining topography changes. The precipitation rates (760 mm per year) directly affect the river sediments'accumulation, transportation and deposition. The precipitations are at a maximum in autumn and winter. They affect the slope of the streams and the formation of sediments in the riverbeds of Krumovitsa and its tributaries. Deforested landscapes in the area enable active bedrock weathering and enforce lateral erosion, a triggering factor for increasing the solid outflow in the rivers. The numerous gullies and ravines in the local topography enable the easy accumulation of sediments alongside the river banks. The local lithology is the primary natural factor that determines the geochemical content of the river sediments. Different types of rocks specifically predetermine the variation in the mechanical and chemical structure of the river sediments.
The results from the investigated locations (Fig.
Content of heavy metals in river sediments in the Ada Tepe Mining Site area (mg/kg).
No. | Location | Coordinates | Cu | Zn | Pb | Mn | Ni | Co | Cr |
---|---|---|---|---|---|---|---|---|---|
1 | Krumovitsa River before the confluence of Kesibir River | 41°25'18"N, 25°39'21"E | 5 | 61 | 29 | 459 | 4 | 7 | 5 |
2 | Krumovitsa River (a large meander south east of Ada Tepe) | 41°25'30"N, 25°39'55"E | 5 | 130 | 19 | 206 | 6 | 5 | 11 |
3 | Krumovitsa River east of Ada Tepe (before the confluence of Kalach) | 41°26'12"N, 25°39'56"E | 24 | 46 | 10 | 553 | 55 | 16 | 95 |
4 | Krumovitsa River before the confluence of Byuyuk Dere | 41°26'34"N, 25°40'24"E | 50 | 76 | 13 | 766 | 124 | 25 | 195 |
5 | Krumovitsa River near the bridge between Ada Tepe and Krumovgrad | 41°28'06"N, 25°39'03"E | 42 | 80 | 10 | 609 | 94 | 21 | 149 |
6 | Krumovitsa River in Krumovgrad | 41°28'21"N, 25°38'54"E | 45 | 68 | 15 | 92 | 53 | 7 | 88 |
7 | Krumovitsa River north of Krumovgrad | 41°28'47"N, 25°38'30"E | 13 | 43 | 13 | 438 | 40 | 13 | 102 |
8 | Kesibir River before the confluence in Krumovitsa River | 41°25'14"N, 25°38'50"E | 8 | 147 | 39 | 2000 | 12 | 4 | 16 |
9 | A nameless left tributary of Kesibir River | 41°25'27"N, 25°38'55"E | 28 | 62 | 16 | 89 | 63 | 10 | 89 |
10 | An intermittent stream – a small left tributary to Krumovitsa River | 41°25'39"N, 25°39'09"E | 6 | 141 | 21 | 250 | 4 | 3 | 7 |
11 | Byuyuk Dere (Golemi Dol) – a right tributary of Krumovitsa River | 41°26'39"N, 25°40'44"E | 10 | 54 | 30 | 464 | 30 | 5 | 11 |
12 | Kese Dere – a left tributary of Krumovitsa in Ada Tepe | 41°26'29"N, 25°38'54"E | 18 | 155 | 28 | 490 | 13 | 8 | 31 |
Average | 21 | 89 | 20 | 535 | 41 | 10 | 67 | ||
Median | 15 | 72 | 17 | 461 | 35 | 8 | 60 | ||
Minimum value | 5 | 43 | 10 | 89 | 4 | 3 | 5 | ||
Maximum value | 50 | 155 | 39 | 2000 | 124 | 25 | 195 |
At first, the content of heavy metals of all sediments in the researched area (median value) is compared to other territories (Europe, Bulgaria), to lithological data (lithosphere, acidic metamorphic rocks in Bulgaria) and to adopted threshold environmental concentrations and predicted environmental concentrations (Table
Comparative data for content of heavy metals (mg/kg) in the river sediments.
Area | Cu | Zn | Pb | Mn | Ni | Co | Cr |
---|---|---|---|---|---|---|---|
Lithosphere ( |
47 | 83 | 16 | 1000 | 58 | 18 | 83 |
River sediments in Europe ( |
22 | 120 | 39 | 1120 | 11 | 35 | 93 |
River sediments in Bulgaria – natural background territories ( |
45 | 94 | 25 | 777 | 28 | 17 | 64 |
River sediments in Bulgaria – industrial (technogenic) territories ( |
217 | 155 | 102 | 972 | 35 | 37 | 74 |
River sediments in Ada Tepe Area (Median) | 15 | 72 | 17 | 461 | 35 | 8 | 60 |
Soils in Ada Tepe Area (Median) | 18 | 77 | 19 | 597 | 41 | 10 | 34 |
Acidic metamorphic rocks in Bulgaria – predominant in the Ada Tepe Area ( |
20 | 50 | 20 | 287 | 10 | 11 | 8 |
Threshold environmental concentrations (TEC) ( |
32 | 121 | 36 | 460 | 23 | - | 43 |
Predicted environmental concentrations (PEC) ( |
149 | 459 | 128 | 1100 | 49 | - | 111 |
The data outline the similarities and differences between the Ada Tepe area and the compared references. A geochemical spectrum (Fig.
Heavy metals in river sediments in Europe, Bulgaria and Ada Tepe area, based on the Clarke concentration.
Another analysis compares the geochemical properties of the local rock formations of acidic metamorphic rocks in the area (
We compared the river sediments of the Krumovitsa River (the main river in the catchment) in different sections of its course to clarify the spatial differentiation of chemical elements in the area (Fig.
Another more detailed analysis of the distribution of heavy metals in the river sediments alongside the Krumovitsa River focuses on seven locations (Fig.
There are no legally adopted norms for recommended environmental concentrations in the river sediments of metals and metalloids in the European Union, although there are such norms in the United States of America. The Environmental Protection Agency (US EPA) institutionalises them. The norms adopted by the US EPA consider two levels of quality for the river sediments (
The performed study highlights the environmental status of the Ada Tepe area before the start of the mining activity. There was no contamination with heavy metals of the investigated river sediments in the Krumovitsa River catchment up to 2016. The geochemical properties of the seven examined chemical elements (Cu, Zn, Pb, Mn, Cr, Co and Ni) within the local landscapes resemble a natural background territory with no traces for human impact. Local lithological specifics, but not anthropogenic activity, determine the geochemical properties of the river sediments in the catchment.
The study is a baseline for future research on the mining impact on landscapes and ecosystems. An extension of the list of investigated chemical elements is recommended to encompass the geochemical picture of the area. The effect of the ongoing mining activity in Ada Tepe must be a subject of regular independent environmental monitoring.