Effects of mine water discharge on river sediments: metal fate and behaviour, Upper Silesian Coal Basin

The studied streams are situated in the Czech part of the Upper Silesian Coal Basin (USCB), which, with a total area of 7,500 km² (from which only 1,550 km² is in the Czech Republic), is one of the biggest hard coal basins in Europe and has a long mining history. The basin is divided into the Ostrava and Karviná sub-basins (Dopita and Kumpera 1993). The present USCB is only a denudation remnant of what was initially a much larger basin structure filled with Devonian and Lower to Upper Carboniferous sediments (Jirásek et al. 2018; Geršlová et. al. 2016).

The in-situ measurement of ADER was done using a gamma spectrometer GT-40 (Georadis Ltd.) equipped with a NaI (Tl) detector with a volume of 345 cm³, diameter 76 mm (cylinder). GT-40 is a device capable of accurate measurement of ADER values from 20.0 nSv/h to 0.5 mSv/h within an energy range of 25 keV to 3 MeV. The in-situ measurement was done by walking directly in the streams and the device was carried in hand partially submerged under the water so the detector would be still at the same distance from the sediment. Conductivity, pH and temperature were measured by Multimeter WTW Multi 350i (with accuracy ± 0.01 for pH, ± 0.5% for conductivity and ± 0.1 °C for temperature) equipped with probe WTWSenTix 41 for pH and temperature and probe WTW TetraCon 325 for conductivity measurements. The conductivity, temperature and pH of stream water were measured in a single day during active mine water discharging straight into the streams.

The samples of sediments from the streams were taken at places with elevated ADER values. The samples were dark brown to black clayey silty sand with a rare occurrence of bigger grains. Sample S1-2 contains corroded fragments of piping (up to 2 cm in size). The fine-grained river sediments were taken with a scoop and placed in plastic bags. Dry samples were sieved with a mesh size of 2 cm to remove plant debris and larger detrital pieces. The obtained fraction was used for the laboratory gamma spectrometry and particle size distribution. The Marinelli beakers with a volume of 600 ml were filled and hermetically sealed for 30 days to reach the secular equilibrium for ²²⁶Ra and ²²²Rn. For the XRF analyses, the homogenized samples were crushed to a fraction less than 0.2 mm.

Samples of water were collected into plastic sampling bottles directly from the streams and were transferred to the laboratory within 24 h and immediately analysed.

The results were processed using Microsoft Excel and QGIS 3.2.2. The spectra were evaluated using G2k and GENIE 3.2.3 (Mirion–Canberra). The results were corrected using MEFFTRAN (freeware).

Data normalisation was applied to the assessment of anomalous metal contribution using Al as a reference element. Data from the publication by Rudnick and Gao (2003) were used as the background values. The following formula was used for enrichment factor calculation (Zoller et al. 1974 and Sinex and Helz 1981):

A = the content of the element in the river sample. An = the content of the reference element in the river sample. B = the content of the element in the background samples. Bn = the content of the reference element in the background samples.

Hydrochemical modelling was conducted to study the solid–liquid interactions using PHREECQ software. This included the calculation of the mineral saturation index (SI) for both streams to compare the mineral stabilities in the water. The water composition (concentration of dissolved ions and anions), pH, and T values were used as input parameters for the SI calculation. SI is defined as an ion activity product in relation to solubility product on a logarithmic scale where SI value > 0 means saturation of mineral phases in the water, while SI values < 0 represent subsaturation.

The discharged mine waters have a stable temperature (23–25 °C) according to long-term measurements done by Diamo a.s. The pH of discharged waters was comparable (7.3–7.4), as was EC with the values of 20,700 μS/cm and 16,600 μS/cm for Karvinský potok and Stružka, respectively (Table 1). Both discharged waters (S1-W2-DIS and S2-W2-DIS) are classified as chloride types. In the cationic part, the proportion of alkali metals Na⁺ and K⁺ is more than 80 eq.%. It is generally assumed that the mine waters of USCB contain a substantial concentration of sulphates (Grmela and Jelínek 1999). These opinions are also supported by findings of sulphate minerals inside the mines (Matýsek et al. 2014) and the general assumption that coal contains an increased amount of sulphur (Deurbrouck 1972). However, the concentration of sulphates (SO₄)²⁻ in the studied mine water discharges was low (not exceeding 77 mg/l). This deficit of SO₄²⁻ in mine water can be explained as either insufficient oxidation of sulphides in the mined bedrock or reached saturation limits of gypsum and/or jarosite and their precipitation (Banks 2006). A low content of mainly pyritic sulphur in coal-bearing strata of the Ostrava and Karvina Formations was found in boreholes in the Karvina region (Vöröš et al. 2022). Thus, low sulphates content will be conditioned by a lower pyrite content contained in the rock massif. A lower amount of Ba reaching 6.4 and 15.5 mg/l was found in the discharge water of Karvinský potok and Stružka, respectively (Table 1). However, the values are not as high as from the discharge of an active mine (up to 99 mg/l, Vöröš et al. 2021). Even higher content of Ba was measured in Polish parts of USCB, where in water samples the concentration was up to 2,000 mg/l (Lebecka et al. 1994). The measured activity of ²²⁶Ra in discharged waters was 0.70 Bq/l and 0.68 Bq/l for Karvinský potok and Stružka, respectively. Both mine waters has an extreme excess of ²²⁶Ra compared to ²³⁸U (Tab. 1).

According to the ion composition the surface waters are classified as carbonate-sulphate, however, after the mine water discharge, it changes to chloride type. The pH values do not show a visible trend along both streams and are in the range of 6.8–8.1. The discharge of mine waters significantly increased the electrical conductivity values in both streams (Table 1). The subsequent development of conductivity is different for individual streams. In Karvinský potok, the EC decreases from 24,800 to 2030 μS/cm within a 5.7 km long section. In Stružka, the EC it does not fall immediately after discharge, but on the contrary rises and then falls. The activity ratio between ²²⁶Ra/²³⁸U is significantly lower (1.0–20.9) compared to its value in the discharged mine waters (93.7 in Karvinský potok and 67.3 in Struzka; Table 1).

The field ADER measurement proved an increased content of radioactivity in both streams (Fig. 2). Highest measured radioactivity (up to 4000 nSv/h), with an average of 1,831 nSv/h was found directly after mine water discharge in the Karvinský potok. No place with ADER above 200 nSv/h has been found upstream of the mine water discharge. The highest ADER values exhibited in the Stružka exceeding 2000 nSv/h were measured 200 m after mine water discharge with an average of 898 nSv/h (Fig. 2). Laboratory gamma spectrometry of bottom sediments proved that the most active radionuclides in river sediments are ²²⁶Ra and ²³²Th in both studied streams (Table 2).

Stružka river sediments were composed of silt (10.5–75.4% w/w) and sand fraction (16.1–87.5% w/w). On the contrary, in the Karvinský potok sediments the granulometric distributions of the particles were less variable and the most abundant fraction is silt fraction (Median 80.5% w/w). Sample S1-3 is an exception with the prevailing sand fraction (85.1% w/w) and 1.2% w/w content of clay fraction (Fig. 3).

The SiO₂/Al₂O₃ ratios of siliciclastic rocks can be used as a parameter sensitive to sediment recycling and the weathering process. With increasing sediment maturity, quartz is enriched compared to feldspars, mafic minerals and lithics (Roser et al. 1996). The average SiO₂/Al₂O₃ ratios in unaltered igneous rocks ranged from 3.0 (basic rocks) to 5.0 (acidic rocks). The SiO₂/Al₂O₃ ratio in the Karvinský potok ranges from 3.2 to 5.0, which confirms basic to intermediate maturity. Values of SiO₂/Al₂O₃ ratio > 5.0 in Stružka sediments pointed to increased geochemical sediment maturity. There has been found no correlation between Al and Si (r = 0.14). It follows from the above that sediment material of both rivers is made up mainly of detrital components of weathered rocks with a minor portion of soil flushes from the stream surroundings.

A strong positive correlation was found for Al and K (r = 0.92) in both streams, and this is interpreted as the presence of clay minerals or feldspars. The sample S1-2 completely deviates from the evaluated set, as it contained corroded fragments of piping with a diameter of up to 2 cm (Fe content = 204 508 mg/kg). Samples S2-1, S2-2 and S2-11 are enriched in Fe content (over 48 720 mg/kg). The different element distribution was found in samples S2-3 and S2-4 where a low content of Fe (31 955 mg/kg and 23 321 mg/kg, respectively) and an increased content of Ca (average of 35 000 mg/kg) was measured (Appendix 1). The ratio of ²²⁶Ra and ²³⁸U in the river sediments of Karvinský potok is in the range 6.3–32.7 and in Stružka in the interval 3.3–43.1 (Table 2). All evaluated sediments contain ²²⁶Ra, which is not in secular equilibrium with its parent radionuclide ²³⁸U (Fig. 4). The ratio of ²²⁶Ra and ²³⁸U in sediments decreases downstream, but a systematic gradual decrease could not be traced. This is probably due to the natural movement of sediments within the stream. The most abundant radionuclide ²²⁶Ra correlates with ²²⁸Ra and ²³²Th in all evaluated samples, which reflects the different mobility of uranium, radium, and thorium in the environment (Fig. 5). Uranium is highly soluble under oxidising conditions, whereas radium is mobile regardless of oxidation–reduction environmental conditions. Thus, radium movement in the environment is controlled by precipitation or sorption.

To assess the mutual relationship between river sediment composition and radionuclides, it was necessary to eliminate the effect of grain size. Thus, the enrichment factor (EF) using Al as the normalising element was calculated. The EF for both streams reached extreme values for Sr, Cr, Mo, Th, Pb, Zn and Cu (Fig. 6).

A mutual correlation was found between the enrichment factor of Sr and the content of ²²⁶Ra (Fig. 5, Appendix 2). This is related to their oxidation states (+ 2) which do not change under typical groundwater conditions, thus their mobility in aquifers is indirectly affected by changes in geochemical conditions leading to precipitation or sorption onto negative mineral surfaces.

Two geochemical models applied for the results from discharge mine water (MW) and mixing discharge water with surface water (MIX) were evaluated in both Stružka and Karvinský potok (Table 3). The mineral saturation indices (SI) were calculated from the quarter-month monitoring data and revealed positive values for calcite and dolomite, averaged of 0.41 and 0.97 for Stružka; averaged of 0.49 and 0.93 for Karvinský potok (MW), respectively. This observation indicates supersaturation in respect to their solubilities (Table 3). Other hydrous carbonate phases such as aragonite showed lower SI values compared to aforementioned minerals, averaged of 0.27 for Stružka, and 0.35 for Karvinský potok.

The calculated internal partial pressure of CO2 showed negative log (pCO₂) values in both streams, not exceeding log (pCO₂) for the Earth’s atmosphere -3.4. It reflects on the potential degassing of CO₂ from the solutions to the atmosphere (Boch et al. 2015). According to the results, both streams are supersaturated with respect to the amorphous and hydrous iron minerals, such as hematite, goethite and Fe-oxo hydroxides (FeOH)₃. The SI values highly fluctuated and their values differed from each other. Amorphous (FeOH)₃ exhibited the lowest SI values in Stružka and Karvinský potok, averaged of 1.71 and 2.25, respectively. Goethite and hematite showed much higher SI values in both streams with a slight difference between the two systems. This is in agreement with the observation of the first section of Stružka where Fe-hydroxide coatings were observed. The SI values for goethite and hematite were averaged of 7.56 and 17.12 in Stružka; averaged of 8.15 and 18.31 in Karvinský potok, respectively.

The variability of SI values for the mineral Jarosite-K was found in both streams. Only negative averaged SI values were observed in Stružka and Karvinský potok (− 3.99 and − 2.15, respectively). When the system does not reach the equilibrium with carbonate phases (calcite and dolomite), thus Jarosite-K remains undersaturated. Precipitation/dissolution of Jarosite-K is considered when the mine water reaches the equilibrium with carbonate phases or is mixed with surface water (positive and negative SI values, Table 3), which is controlled by various factors, such as temperature, alkalinity, content of sulphates and iron in the solution regardless of the equilibrium with these phases (Elwood Madden et al. 2012). Barium (Ba) solubility in the Stružka stream seems to be controlled with respect to barite that is found to be supersaturated (average SI > 1). According to the mineralisation, pH and the content of dominant anions, Ra can co-precipitate when incorporated in barite, gypsum or even calcite, but the co-precipitation with gypsum and calcite is negligible compared to barite (Bosbach et al. 2010; Yoshida et al. 2009; Vinson et al. 2012). A low content of sulphates is not favourable for Ra²⁺ precipitation as RaSO₄. In water with dominant Cl⁻ anion, Ra prefers to remain in solution (Beneš et al. 1982). Radium is readily and strongly adsorbed especially by the fine fraction of soils, but not in the Fe-oxide fraction. In addition, Mn solubility in the Karvinský potok is governed by the mineral phases of pyrolusite, manganite, and hausmannite that have been found supersaturated as well (average SI > 3 for manganite, and average SI > 6 for hausmannite and pyrolusite). The results of geochemical modelling lead to a conclusion that all presented supersaturated mineral phases are important for Fe, Ba, Ra and Mn removal from the discharged water in both streams.