Seasonal Variations in Ochreous Precipitates and Drainage Waters in the Grantcharitsa Tungsten Deposit, Western Rhodopes, Bulgaria

Abstract

Seasonal variations of drainage waters and ochreous products of their discharge from the closed abandoned old gallery at the Grantcharitsa scheelite deposit (Bulgaria) were studied by field and laboratory methods for the period 2019–2023. The drainage is generated under anoxic conditions and is inherently diluted (EC = 100–202 µS/cm) with S (6–12 mg/L), Si (6–22 mg/L), Na (6–10 mg/L), Fe (0.2–3.3 mg/L), and W (0.19–3.5 µg/L), at a pH 4.4–6.5 and temperature 7–11.5 °C, with dissolved oxygen DO (2.1–7.7 mg/L). The concentrations of Fe and W and the pH of the water are variable and reach their maximum values during the dry (autumn) season. It was found that such parameters as pH, Eh, DO, Fe and W content change dramatically at a distance of up to 3 m from the water outlet; the values of pH, DO and Eh are sharply increased with a simultaneous nearly 5–6-times reduction in iron and tungsten content. The decrease in the contents of these elements is associated with the precipitation of ochreous material consisting of nanoscale ferrihydrite with an intermediate structural ordering between 2-line and 6-line ferrihydrite (major phase), hematite, goethite, quartz, montmorillonite and magnetite. The formation of ferrihydrite occurs as a result of abiotic and biotic processes with the participation of iron-oxidizing bacteria. Besides Fe₂O₃ (55.5–64.0 wt.%), the ochreous sediment contains SiO₂ (12.0–16.4 wt.%), SO₃ (1.3–2.4 wt.%), Al₂O₃ (3.1–6.8 wt.%) and WO₃ (0.07–0.11 wt.%). It has been shown that drainage waters and ochreous sediments do not inherently have a negative impact on the environment. The environmental problem arises with intense snowmelt and heavy rainfall, as a result of which the accumulated sediments are washed away and carried in the form of suspensions into the water systems. It is suggested that by providing atmospheric oxygen access to the closed gallery (via local boreholes), it is possible to stop the generation of iron-enriched drainage.

1. Introduction

Drainage waters in ore deposits before, during and after their exploitation are of particular importance for environmental protection, which is why a significant number of research articles and reviews have been published on this problem, and especially on acid mine drainage AMD [1,2,3]. Recently, in addition to environmental protection, the idea of using waste products from mining operations and drainage waters as non-traditional raw materials for the extraction of critical metals has become increasingly popular [4]. Nanometric ferric oxyhydrate-sulfate minerals (as ferrihydrite and schwertmannite) are active participants in AMD as ochreous precipitates, and well recognized for their ability to control the mobility of inorganic and organic pollutants in the environment through sorption processes [5,6].

Local climatic and hydrogeological conditions can control the formation of ferric oxyhydrate-sulfate minerals because the chemical composition of the draining water can vary due to changes in temperature, precipitation and snowmelt. As is shown by Murad et al. [7], a reduced supply of water during dry periods leads to more acidic effluents of AMD favoring the formation of schwertmannite (Sokolov Coal Basin (Czech Republic)). An opposite example of the pH behavior can be found in article [8] regarding AMD in the Taebaek coal mine area, Korea. Here, the AMD sources are more acidic during the period of maximal atmospheric precipitation—August and September. In one of the AMD sources the pH falls from ~8 (April, formation of ferrihydrite) to ~3.5 (August–September, formation of schwertmannite). There are a lot of examples showing that in the AMD environment of temperate and polar climate, the pH of waters changes during the year (to one pH unit), and the solid sediments are composed of a metastable phase gradually transformed into a stable one. For example, it is noted that metastable schwertmannite formed during spring floods turns into goethite under quieter conditions—in summer (Northern Finland) [9] or in winter (Northern Pennsylvania) [10].

A wide range of pH from 2.4 (spring) to 6.5 (autumn) was measured in the drainage water from an abandoned exploration gallery at the western part of the Grantcharitsa scheelite–pyrite deposit (Western Rhodopes, Bulgaria) [11]. The discharge of this runoff is accompanied by intensive precipitation of ochreous material. Then, the water enters the local fluvial system and the water conduit, which is an environmental and health concern as the conduit is used for technical and domestic needs. The problem is compounded by the fact that in the western part of the deposit, there are other abandoned exploration mine workings (adits) and their waste dumps, which also generate acid drainage with potential negative impacts on the environment. In this paper, variations in the phase and chemical composition of ochreous sediments and hydrochemical characteristics of the drainage waters during different seasons in the period 2019–2023 in the above-mentioned abandoned gallery are studied in order to clarify the conditions for the formation of drainage and sediments, and assess possible negative environmental consequences and their solution.

2. Site Description

2.1. Local Geology

The Grantcharitsa scheelite–pyrite deposit (Western Rhodope, Bulgaria) is located in the northern part of the Grantcharitsa granitoid body (granite–granodiorite and porphyritic granodiorite, 69–67 Ma) of the Rila–West Rhodope batholith, in close proximity to the Babyak–Grashevo shear zone, which separates these granitoids from the younger medium- to coarse-grained biotite granite (40–35 Ma) of the West Rhodopes body (Figure 1) [12,13]. The ore mineralizations of the deposit are associated with pegmatoid quartz–feldspar veins characterized by an almost sub-latitudinal strike—a dip to the northwest of ~350° with a slope of ~30°, and with the host rocks in the form of vein-disseminated ores (“mineralized granitoids”). The mineral composition of the vein ores is strongly dominated by quartz, SiO₂, and microcline, KAlSi₃O₈ among the gangue minerals, and by pyrite, FeS₂, and scheelite, CaWO₄, among the ore minerals. The oxidation zone of the deposit is unevenly developed. Goethite, α-FeOOH, and jarosite, KFe₃(SO₄)₂(OH)₆, are the two most abundant supergene minerals formed by the alteration of pyrite and silicate minerals. Scheelite is changed unevenly. It is either represented by relict forms of dissolution without visible traces of secondary tungsten minerals, or is partially or completely replaced by secondary tungsten minerals from the Fe₂O₃–WO₃–H₂O system [14]. In addition to its own minerals, tungsten has been found in other minerals such as tungsten-containing goethite and hematite [15].

2.2. Relief, Climate and Historical Data on the Deposit

The relief of the deposit is high-mountainous, strongly rugged, with an altitude from 1100 to 1350 m. The climate in the region is mountainous, temperate-continental to transitional-Mediterranean. According to [16], the coldest month is January and the warmest month is July. The average annual temperature is about 10 °C. The average annual rainfall is 605 mm. Precipitation is unevenly distributed by months of the year; rainfall is most abundant in May and June and scarce in February and March. From December to April there is a snow cover of 20–45 cm.

The detailed exploration of the deposit was carried out in the period 1960–1990. During this period, more than tens of thousands meters of mine workings and wells were created in the region and reserves of tungsten ore were proven; however, the industrial mining of tungsten was not carried out. In the northern part of the Central section of the deposit, on the Grantcharitsa River, there are facilities for water intake into the Bistritsa derivation conduit, built in the 1950s as one of several to feed the Batak dam lake and the Batak hydroelectric power station. The Veziov Dol creek (Figure 1), one of the tributaries of the Grantcharitsa River, is affected by drainage from several abandoned exploration galleries and heaps of rocks.

3. Materials and Methods

3.1. Sampling and Sample Preparation

The sampling site is on the western slope of the Veziov Dol valley, at the entrance of the abandoned exploration gallery (Figure 2). The gallery was constructed in the 1960s and today its entrance is closed, covered with stones and soil and hidden by grass and large trees. The actual location of the closed gallery is indicated by the existing permanent discharge of drainage water and the presence of a puddle with ochreous sediments (Figure 2). Samples of water and ochreous material were taken in the period 2019–2023 during wet (high-water) and dry (low-water) seasons. The water sampling locations were always chosen in such a way that there was a natural ledge and a narrowing forming a channel, which ensured the receipt of a fluent of water convenient for sampling. In Figure 2, such locations are marked as point 1, point 2, point 3 and point 4. Sediment sampling was carried out near and above the water sampling point (for points 1 and 2) directly in the puddle. Waters are always sampled before the sediments.

Water samples for dissolved cations and trace metals were filtered under field conditions using a syringe (50 mL) and a 0.45 µm-membrane filter into a polyethylene bottle (100 mL) and then acidified by Suprapur 65% HNO₃ (Sigma-Aldrich, Berlin, Germany) to pH ~1. Unfiltered water samples were collected in 1 L polyethylene bottles for anion analyses. These bottles were filled with water in such a way as to exclude the presence of air. Before analyzing, the water samples were kept at 4 °C.

Samples of ochreous material were taken in the field with a plastic spoon from the surface layer of the sediment to a depth of about 3 cm and then placed in plastic bags. Samples were stored in a cool place at 4 °C before preparation and analysis. During preparation, large pieces, mainly the remains of grass stems and tree leaves, were first removed from the wet sediment using plastic tweezers and a coarse sieve (1 mm). Then the samples were dried in a laboratory oven at 50 °C for approximately 12 h (overnight). To remove possible grains of rock-forming minerals, the ochreous material was additionally sieved through a sieve of 0.1 mm. It was found that samples taken in 2019 feature an essential presence of detrital material.

3.2. Methods

Field measurements of the pH, temperature (°C), electric conductivity (EC, µS/cm), dissolved oxygen O₂ (DO, mg/L) and redox potential (ORP, mV) of the waters were taken using portable Thermo Scientific Orion Star A329-STARA3295. The field ORP readings were then converted to Eh (readings relative to a standard hydrogen electrode) using correction values specified in the instruction manual for the electrode used. These corrections ranged from +215 mV for 7 °C to +211 mV for 11–12 °C. The waters’ composition was analyzed using ICP-OES for main components such as Na, K, Ca, Mg, Si, S and Fe (SPECTROBLUE ICP-OES spectrometer, Eurotest-Control EAD, Sofia) and ICP-MS for 69 elements (main elements and microelements) (Faculty of Chemistry and Pharmacy, Sofia University) [17]. ICP-MS analyses of the water samples were performed using Perkin-Elmer SCIEX Elan DRC-e with a cross-flow nebulizer. Working standard solutions were prepared from ICP-MS multi-element calibration standard solution-2 (Ultra Scientific) and ICP-MS Multielement Standard B (High-Purity Standards). The saturation indices (SI) and percentage distributions of dissolved species in the drainage waters were determined using Visual MINTEQ software (ver.4.02) [18].

The chemical compositions of ochreous sediments were determined using LA-ICP-MS (microelements) analysis and SEM-EDX (main elements) area analysis. Ochreous materials were finely ground in an agate mortar and pressed into a pellet for analysis by LA-ICP-MS. For SEM-EDX analysis the pellets were coated with carbon. The LA-ICP-MS analysis of 50 isotopes was performed on a PerkinElmer ELAN DRC-e ICP-MS with a New Wave UP193FX LA system (Geological Institute (GI), BAS) at 5 Hz pulse rate, laser energy of 7.4 J/cm² and spot size of 100 μm. For better reliability, each sample was laser-ablated in four selected areas. NIST 610 was applied as the primary external standard for calibration of the analyzing system. SILLS software (Version 1.1.0) [19] and the Si content (as internal standard from SEM-EDX) were used for data reduction and calculation of the chemical composition.

SEM and SEM-EDX studies were carried out on a ZEISS SEM EVO 25LS with an EDAX Trident system (Institute of Mineralogy and Crystallography (IMC), BAS) at an acceleration voltage of 18 kV. SEM-EDX was performed at selected points (spot analyses) and selected areas (area analyses) using an EDAX SDD Apollo 10 EDS detector and Genesis V. 6.2. software with the ZAF correction method, and hematite (for Fe), diopside (Mg), albite (Na, Al, Si), sanidine (K), apatite (P), anhydrite (S) and tugtupite (Cl) as reference standards. SEM and SEM-EDX have also been used to characterize the microstructure and inhomogeneity of ochreous materials. For this, the materials were either directly fixed on SEM holders or pressed into pellets without any other mechanical operations, and then coated with conducting carbon.

The phase identification of ochreous materials was carried out by powder X-ray diffraction (XRD) analysis using a PANanalytical EMPYREAN Diffractometer system (IMC-BAS), Cu anode, 45 V, 40 mA, range 3–100 degrees 2Theta, step size 2Theta—0.0001. The XRD patterns were processed using HighScore 5.1 software (Malvern Panalytical). The transmission electron microscopy study of sediments was carried out on a JEOL JEM-2100 at 200 kV accelerating voltage. Bright Field TEM (BFTEM), High-Resolution TEM (HRTEM) and Selected Area Electron Diffraction (SAED) regimes were used. Natural suspensions of ochreous material were directly dropped onto Cu grids supported by holey carbon.

4. Results

4.1. Hydrochemical Characteristics of Water

Two approaches were used. The first approach included studying the change in hydrochemical characteristics of waters over time (months, years, and seasons). For this, the same sampling point 2 was chosen (Figure 2) approximately 1.5 m from the source of the drainage. Just above point 2, a small puddle located nearby was used to collect samples of ochreous sediments.

The second approach included studying the change in the hydrochemical characteristics of water along the profile from the source itself at a certain distance. Taking samples and measuring the physical characteristics of water takes a short time, and therefore it is assumed that during this time the parameters of the water do not change significantly at any of the selected points. Such points are points 1, 2, 3 and 4 (in Figure 2), located at distances from the point of water outlet, respectively, of 0, 1.5 m, 3 m and 9 m. Point 3 was chosen exactly on the border of abundant sedimentation of ocherous material.

4.1.1. Seasonal Variations of Hydrochemical Characteristics of Water

It was found that the flow rate of the flowing water varies from 1 (dry season) to 3 (wet season) L/min. Variations in the most important hydrochemical characteristics of waters are shown in

Table 1. Most important hydrochemical characteristics of the drainage waters.

t °C	pH	Eh, mV	EC µS/cm	DO, mg/L	Fe, mg/L	W, µg/L	S, mg/L	Si, mg/L	Na, mg/L	K, mg/L	Ca, mg/L	Mg, mg/L	Al, mg/L
7–11.5	4.4–6.5	203–389	100–202	2.1–7.7	0.2–3.3	0.19–3.5	6–12	6–22	6–10	0.8–1.9	5.9–11.3	1.5–2	0.03–0.26

.

The data in Table 1 indicate that regardless of the season, the water is dilute and varies in narrow the temperature range of 7–11.5 °C (Table 1). The waters have sulfate–silicate anionic and calcium–sodium cationic compositions. The measured electrical conductivity (EC) of water ranges from 100 to 202 μS/cm, which, after conversion [20] to 25 °C, gives an EC range of 147–314 μS/cm, corresponding to drinking water with a low mineral content (according to [21], the conductivity of drinking water varies in the range 50–1000 μS/at 25 °C cm; according to the European Directive [22] the upper limit for drinking water is 2500 μS/cm at 20 °C). It should be noted that the lowest value of pH 2.4, measured in the waters in April 2019, was never repeated. For this reason, it is not listed in Table 1.

Only the iron concentration exceeds the indicative value of 0.2 mg/L for drinking water according to the European Directive [22]. The established W content 0.19–3.5 µg/L is significantly lower than the concentrations (0.25–337 µg/L) of the element in drinking water in the town of Fallon, Nevada, where a cluster of childhood leukemia was localized at the beginning of the 21st century [23]. Among other trace elements in waters, molybdenum is of interest, which has a very close geochemical behavior with tungsten. Its measured contents were 0.02–3.45 μg/L.

The Visual MINTEQ software application (ver.4.02) [18] shows that the dissolved iron in the water samples is represented by Fe²⁺ (~95–97%) and FeSO₄(aq) (~3–5%). For two selected contrasting water samples, 2021-01 and 2021-07, there are different sets of mineral phases with positive saturation indices. For the water sample 2021-01 (pH = 4.43, Eh = 386 mV), chalcedony, cristobalite, quartz, goethite, hematite, lepidocrocite and magnetite have positive saturation indices. For the water sample 2021-07 (pH = 5.77, Eh = 221 mV), alunite, chalcedony, diaspore, gibbsite, goethite, halloysite, hematite, kaolinite, magnetite and quartz have positive saturation indices. Potentially all of these minerals can make up the ochreous material.

Figure 3 shows the seasonal variations in the pH value and the W and Fe contents of the water. It is interesting to note that there is a tendency for the pH of the waters to decrease during the wet season (at high waters). Close behavior with some deviations is also characteristic of the iron and tungsten contents.

A strong dependence on the season (or pH) is demonstrated by Na (Pearson’s coefficient R = +0.65 with pH), Mo (+0.82), Al (−0.71) and U (−0.80). The behaviors of other major elements in waters (Ca, Si, S) do not correlate with pH or seasons.

4.1.2. Change in Hydrochemical Characteristics of Drainage Water After It Flows Out of the Gallery

In 2023, April, the drainage waters were sampled at 4 points (Figure 2)—immediately at the outlet of the effluent and then downstream. It was found that such parameters as pH, Eh, dissolved oxygen concentration DO, iron and tungsten content (Figure 4) change dramatically at a distance of up to 3 m from the water outlet; the values of pH, DO and Eh sharply increased with a simultaneous nearly 70% reduction in iron and tungsten content. The decrease in the contents of these elements is associated with the precipitation of ochreous material.

It should be noted that all heavy metals, although with low initial contents (in µg/L) (Zn 28.5, Cu 2.43, Bi 1.64, Pb 0.58, U 0.34, Mo 1.0), drastically reduced their amounts in water together with iron (Table S1).

The obtained data for DO, Eh and Fe distributions in Figure 4 are very similar to the physicochemical profile data in [24] for the anoxic iron-rich groundwater discharge zone flowing into Ogilvie Creek, Ontario, Canada. The concentration of Fe²⁺ falls by about 70% from 2.65 mg/L to 0.08 mg/L at a distance of 3.5 m due to the oxidation of Fe²⁺ and the formation of an iron oxide mat. The process occurs with the participation of iron oxidizing bacteria Leptothrix ochracea (filamentous sheaths) and Gallionella ferruginea (helical stalks). Dissolved oxygen saturation in the drainage was shown to jump from <10% to nearly 100% over a distance of 5 m. In the studied drainage waters in the Grantcharitsa deposit, the calculated saturation of dissolved oxygen increases from 23.6% to 80.4% over a distance of 3 m (the data on maximal content of dissolved oxygen in fresh waters are from [25]).

4.2. Characteristics of Ochreous Precipitates

4.2.1. SEM and SEM-EDX Study of Microstructure and Composition of Ochreous Precipitates

All samples, regardless of the sampling season, have a specific chemical heterogeneity that is clearly visible in backscattered electron (BSE) images as light (hereafter denoted as light material) and dominated dark areas (dark material) (Figure 5a,b). The light material consists of helical elongated particles, an indication of their biological origin, as well as globular particles and their aggregates, most likely also of biological origin. SEM images of the undisturbed precipitate surface show a large number of twisted stalks (Figure 5c,b), consistent with those produced by the iron-oxidizing bacterium Gallionella ferruginea [26]. The stalks are about 2 µm wide and up to 100 mm long.

In the compositions of two materials, in addition to Fe₂O₃, we found the following (

Table 2. Representative EDX (spot analyses) of light and dark areas (in BSE signal) of ochreous materials in pressed pellets (in wt.%).

Samples	2019-04		2019-09		2020-05		2020-10
Area	Light	Dark	Light	Dark	Light	Dark	Light	Dark
Na2O	n/d	0.27	n/d	0.16	n/d	n/d	n/d	n/d
MgO	0.12	0.41	0.37	0.48	0.1	0.14	0.24	0.29
Al2O3	1.62	6.3	4.61	6.48	0.85	2.46	2.35	5.63
SiO2	9.42	20.59	16.83	20.42	14.4	11.71	11.77	15.82
P2O5	0.15	0.54	0.5	0.54	0.37	0.39	0.52	0.57
SO3	3.75	1.43	1.34	0.8	4.48	2.61	3.27	1.26
Cl	0.12	0.1	0.1	0.14	0.07	n/d	0.05	n/d
K2O	n/d	0.43	0.52	0.84	n/d	0.07	n/d	0.17
CaO	0.22	0.33	0.34	0.52	0.1	0.14	0.11	0.26
TiO2	n/d	n/d	0.12	n/d	n/d	n/d	n/d	n/d
MnO	n/d	n/d	0.07	n/d	n/d	n/d	n/d	n/d
Fe2O3	82.97	60.79	72.34	57.48	79.17	61.81	81.43	65.14
Total	98.37	91.18	97.13	87.87	99.54	79.33	99.73	89.13

): SiO₂ (the second most important component), Al₂O₃, SO₃, small amounts of MgO, CaO, Na₂O, and P₂O₅, and random MnO and TiO₂. Compared to the dark material, the light material is more enriched with Fe₂O₃ (72–83 wt.%), and contains less SiO₂ (9–17 wt.%) and relatively more SO₃. The EDX analysis results of the light material are close to 100% (97–100%), which may indicate that the material is dense and less hydrous. EDX analysis of the dark material yielded the Fe₂O₃ 57–65 wt.%, SiO₂—11–21 wt.% and sums of 79–91 wt.%, which may indicate that the material is more hydrous. Both types of material well reflect the chemistry of drainage waters. The role of silicon relative to sulfur in the composition of sediments increases. According to SEM images, the ratio between the two materials is about 30%—biotic material and 70%—abiotic material.

Silicon is a very common impurity in natural ferrihydrite [27,28]. The maximal reported content of SiO₂ in ferrihydrite is 31.5 wt.% [27]. With contents of SiO₂ between 9.42 and 20.59 wt.%, the studied ochreous precipitates correspond well with the composition of natural ferrihydrite. According to [28,29,30] and other authors, silicon co-precipitates with iron, forming siliceous ferrihydrite nanoparticles; Si most likely forms surface complexes on the particle, thereby inhibiting the further growth of ferrihydrite nanoparticles and the transformation of ferrihydrite into crystalline oxide (hematite) and oxyhydroxide (goethite) minerals. Another important component in the composition of ochreous material is aluminum, which can substitute iron in iron oxide phases [31]. The data in Table 2 show that aluminum accumulates to a greater extent in the non-biogenic (dark) part of the material.

4.2.2. Main Components and Trace Elements in Ochreous Precipitates

Trace elements were analyzed by LA-ICP-MS in pellets prepared from homogenized samples by grinding. After LA-ICP-MS measurements, the pellets were examined by SEM and EDX. The average composition of ochreous precipitates was obtained by averaging several (up to four) EDX analyses performed in scan mode. The Si content of the average compositions was used as an internal standard for recalculating LA-ICP-MS data. The obtained average data from EDX and LA-ICP-MS are presented in

Table 3. Average chemical composition of ochreous precipitates (in wt.% for oxides and in ppm for trace elements).

	2020-05	2020-10	2021-01	2021-07	2022-04	2022-07	2023-04
Na2O	0.05	0.05	0.05	0.17	0.05	0.05	0.16
MgO	0.17	0.23	0.18	0.39	0.35	0.34	0.45
Al2O3	2.55	4.41	2.83	6.17	6.67	6.38	6.63
SiO2	11.98	14.8	13.5	13.58	15.16	16.02	18.81
P2O5	0.38	0.54	0.47	0.54	0.43	0.41	0.34
SO3	3.09	1.39	1.42	1.32	1.4	0.79	0.93
Cl	0.07	0.11	0.09	0.11	0.08	0.06	0.08
K2O	0.09	0.13	0.09	0.21	0.3	0.27	0.4
CaO	0.07	0.21	0.18	0.33	0.47	0.47	0.51
TiO2	0	0	0	0.11	0.18	0.12	0.15
Fe2O3	64.1	62.72	57.71	51.07	51.3	50.6	44.21
Total	82.55	84.59	76.52	74	76.39	75.51	72.67
W	769	634	830	986	882	1106	660
Mo	187	153	203	200	146	192	114
U	9.7	9.1	10.2	15.8	14.6	17.1	17
As	13.1	15.1	19.8	10.4	9.4	10.7	8.4
Pb	64	100	91.4	110	111	97	74
Bi	9.4	12	13.8	14.2	11.5	12.1	8.4
Cu	9.9	12.7	11.5	21.3	12.4	16.6	13.5
Zn	32	52	65	78	70	65	63

.

The gross chemical compositions of the ochreous precipitates collected in different years and seasons demonstrate significant variations in the contents of the main components (in wt.%) of Fe₂O₃—44.21–64.1, SiO₂—11.98–18.81, Al₂O₃ 2.55–6.63, and SO₃—0.79–3.09. In general, the composition of the sediment corresponds to the composition of the drainage water, but the obtained variations in the contents of the main components do not correlate with the variations in the chemical composition of the water, although the highest SiO₂ content of 18.81 wt.% was obtained for the sediment in contact with water with the highest Si content—22 mg/L.

Tungsten and molybdenum are the most important minor elements in the composition of the precipitates under study (W—up to 1106 ppm, Mo—up to 203 ppm). The contents of other trace elements are of the order of a few ppm (U) to tens of ppm (Pb > Zn > As > Cu > Bi >). As can be seen from the table, the content of microelements does not fluctuate greatly.

In contrast to the drainage waters, the sediments did not show clear trends in changes in W content depending on the seasons. There is no clear relationship between the W content of the sediments and the drainage waters, but the highest W content in the sediments was found in the material produced from water with the highest W content (3.5 mg/L).

4.2.3. Phase Composition of Ochreous Precipitates According to XRD Analysis

To study the phase composition of the ochreous material using X-ray diffraction and TEM methods, samples were specially selected taking into account two main requirements: firstly, that they correspond to the different pH conditions of formation (correspond to the dry and wet seasons), and secondly, that they do not contain terrigenous impurities. The selected samples were taken in January 2021—sample 2021-1 (pH = 4.43) and July 2021—sample 2021-7 (pH = 5.57). Their chemical compositions are shown in Table 3.

The X-ray diffraction patterns of the selected samples correspond to ferrihydrite in an intermediate structural ordering between 2-line and 6-line ferrihydrite (Figure 6). The patterns contain additional peaks 0.36, 0.42 and 0.445 nm, which can be related to other iron oxide phases, and one narrow peak 0.335 nm corresponding to quartz. Chalcedony, cristobalite, quartz, goethite, hematite, lepidocrocite and magnetite have positive saturation indices and can form under the physicochemical conditions of drainage waters. The d-spacing 0.42 nm corresponds to the 110 reflection of goethite. The d-spacing 0.36 nm in the pattern of sample 2021-07 corresponds to the 012 reflection of hematite. The most significant difference between the two XRD patterns is the presence of a relatively strong and sharp peak 0.445 nm in the pattern of sample 2021-07. This peak may be associated with smectite minerals as it is shown in [32] for low-temperature hydrothermal Si-rich Fe-oxide precipitates—products of microbial biomineralization. The strong peaks 0.445, 2.56 and 1.495 of montmorillonite-22A (ICDD PDF 00-029-1499) correspond well to the pattern of sample 2021-07. This identification corresponds to the chemical composition features of sample 2021-07—it has an increased Al₂O₃ content (6.17 wt.% compared to 2.83 wt.% in sample 2021-01). In addition, the set of possible phases (with positive saturation indices) in sample 2021-07 is significantly larger than in 2021-01.

4.2.4. Microstructure and Phase Composition of Ochreous Precipitates According to TEM Study

SEM data show strong heterogeneity of ochreous sediments, which consist of at least two main components of biological and inorganic origin. In Figure 7a, a fragment of twisted stalk consistent with those produced by iron oxidizing bacterium Gallionella ferruginea (sample 2021-07) is shown. The selected area electron diffraction (SAED) (inset in Figure 7a) corresponds to a nanomaterial with a high degree of structural disorder and better matches the pattern of 2-line ferrihydrite. A high-resolution (HR) image of a fragment of this particle (Figure 7b) shows that the twisted stalk consists of nanometric globular nanoparticles of 3–5 nm in size, which exhibit the onset of structural ordering—the appearance of one-dimensional lattice fringes (indicated by white arrows).

The biologically derived elongated particles (in sample 2021-01) demonstrate a higher degree of structural order of 2-line ferrihydrite (diffraction rings in the pattern become more visible) (Figure 7c). A fragment of these particles (Figure 7d) shows the presence of well-pronounced lattice fringes corresponding to hematite (d-spacings 0.37 and 0.25 nm). Figure 7e (sample 2021-07) shows an ordered crystal of hematite with 0.35 and 0.26 nm lattice fringes (long-range order, leading to the appearance of a diffraction peak of hematite with d = 0.36 nm in XRD (Figure 6)). The polycrystalline diffraction pattern of a group of nanocrystals is shown in Figure 7f SAED with a d-spacing of 0.48 nm, which could be related to magnetite—one of the possible iron oxide phases with a positive saturation index.

5. Discussion and Conclusions

5.1. Water

A low concentration of sulfur (6–12 mg/L), the moderate concentration of other components (Si, Na, Ca, Mg), a relatively wide range of variations of pH (4.4–6.5), and the presence of Fe (0.2–3.3 mg/L) ensuring the formation of ochreous precipitates are the most important characteristics of the studied dilute drainage. Of these characteristics, the most uncertain is the wide pH range. The development of an AMD is related to the combination and availability of sulfide mineral, water, oxygen and microorganism. The high variations in the AMD systems are typical for territories with seasonally wet–dry climates such as northern Australia, where ephemeral acid mine drainage exists during the rainy season [35]. In other cases, the seasonal atmospheric precipitates modify the acidity in the permanent AMD system [8].

The studied drainage is inherently diluted and has a very low concentration of sulfur due to the restricted availability of sulfide minerals, because the other factors are present. During the autumn, the drier season (low waters), the drainage water becomes less acid to circumneutral. The drainage pH decreases significantly during snowmelt, spring flood and rainfalls (May 2020), or as a result of intensive and prolonged atmospheric precipitations during other seasons (January 2021). This creates conditions for water seepage from the ceiling and walls of the gallery, raising the level of groundwater, dissolving efflorescent salts and contacting the water with the available sulfide minerals. The water drainage becomes more intensive and can bring detrital material into ochreous precipitates, as happened in April 2019. A similar mechanism of decreasing pH was described by [35].

There is a tendency for the iron concentration to decrease in more acidic water samples—becoming less than or close to 1 mg/L (Figure 3). This trend is directly related to the seasonal changes in water and sediment characteristics discussed here. The decrease in iron content at lower pH can be explained by the simple dilution of the initial water from the incoming iron-free or poor water. In the dry season, the normal drainage is ferrous water draining the limonite rocks of the oxidation zone of the deposit, containing W-containing goethite and hematite [15]. The tungsten content in the waters sampled in the autumn season, 1–3 μg/L, is close to the metal concentrations in drainage waters found in other tungsten deposits [36].

Another very important issue is that the iron contained in the water is divalent. The flowing water is characterized by very low dissolved oxygen content, which indicates the presence of anoxic conditions in the voids of the destroyed gallery. The anoxic conditions in the study gallery and the elevated ferrous content of the drainage waters are key circumstances for understanding the processes occurring during the generation of dilute iron-rich mine drainage, as well as for predicting its development and management. The first stage in the generation of iron-rich drainage is the dissolution of iron oxides and oxyhydroxide minerals. Under anoxic conditions, this can be done with the participation of iron-reducing bacteria [37,38].

5.2. Bacteria Participation in the Ochreous Material Formation

SEM and TEM data unequivocally prove the involvement of iron-oxidizing bacteria in the formation of ocher precipitates. The large number of twisted stalks observed in SEM corresponds to those produced by the iron-oxidizing bacterium Gallionella ferruginea [26]. The accurate identification of the bacteria requires genomic analysis [39], which is not the goal of this paper. But the physicochemical conditions of the studied drainage waters (Table 1) correspond well to the favorable conditions for the life of Gallionella ferruginea [39], namely, pH 5.5–7.2, concentration of Fe²⁺ 1.6–16 mg/L, and dissolved O₂ 1–10 mg/L [39]. Recently, Gallionella-related iron oxidizers living at pH 4.4 were described [40]. Gallionella ferruginea is a microaerophile that requires Fe²⁺ for growth. During the bacterial oxidation of Fe²⁺, iron oxihydroxide precipitates, forming a stalk (Figure 5c,d), which is a function of bacteria growth. According to [39], the bacteriogenic iron oxihydroxide participates further in the autocatalytic oxidation of Fe²⁺ and accelerates the rate of abiotic iron oxidation nearly 10-fold. This mechanism well explains the existence of two types of ochreous materials: the dense material is a product of biotic (formation of stalk) and abiotic (thickening of stalks) oxidation, while the second material most probably is formed via only abiotic processes.

5.3. Phase and Chemical Composition of Ochreous Precipitates

At a qualitative level, the compositions of ochreous precipitates and water correspond to each other, but variations in sediment and water compositions do not correlate with each other. The gross chemical compositions of the ochreous precipitates are characterized by significant variations in the contents of the main components Fe₂O₃, SiO₂, Al₂O₃ and SO₃ without clear dependences from seasons. The obtained data show that ochreous precipitates have a very high capacity to purify water, which is demonstrated by the coupled behavior of iron and tungsten in Figure 4 and related to the phenomenon of co-precipitation. The presence of such environmentally critical trace elements as W, Mo, Cu, Zn, U, Pb, Bi, and As in nanoscale oxyhydrate-sulfate iron minerals is associated with the co-precipitation and adsorption processes studied by many authors [5,6,41,42].

It is assumed that during the dry season (autumn), there is a constant (normal) drainage regime, which is characterized by stable contents of Fe²⁺, Si, S, W, Mo and other elements and a pH of 5.5–6.5. This regime creates favorable conditions for the vital activity of iron-oxidizing bacteria, such as Gallionella ferruginea. The sustainable regime is disturbed by snowmelt, spring flooding and intense prolonged rain, which leads to an increase in the acidity of the water.

The hydrochemical characteristics of the studied waters, combined with the participation of iron-oxidizing bacteria, create conditions for the formation of a complex chemical and phase composition of sediments. Precipitates are characterized by an increased content of SiO_2, which is known to promote the formation of ferrihydrite [27,28,29,30]. It is also known that the presence of silicon causes the formation of a low-ordered 2-line ferrihydrite (with two very broad peaks in the XRD pattern) and retards its transformation to stable goethite [43]. The activity of iron oxidizing bacteria such as Gallionella ferruginea also causes the formation of X-ray amorphous or poorly crystalline 2-line ferrihydrite [39,44,45]. The obtained data show that the ochreous sediments are polyphase aggregates. The main phase in the sediments is ferrihydrite, with a low degree of structural order—between 2-line and 6-line ferrihydrites, which have biogenic and abiogenic origin. In addition, the sediment samples contain goethite, hematite, and most likely magnetite, the presences of which were also predicted by the saturation index calculations performed using the Visual MINTEQ software. Besides iron oxide minerals, the ochreous sediments also contain quartz and minerals of the smectite group. The polyphase composition of ochreous sediments correlates well with their complex chemical composition, containing, as main components, Fe₂O₃, SiO₂, Al₂O₃ and SO₃.

5.4. Local Environmental Hazards and Local Mine Drainage Management

First, it is necessary to clearly formulate what are the environmental hazards posed by the studied mine drainage system. One of the dangers is the risk of tungsten getting into water and the food chain. This issue is explored in detail in [23,46]. The drainage at the outlet of the gallery studied is characterized by increased contents of iron and tungsten. The study of the hydrochemical characteristics of the drainage along the profile (Figure 2 and Figure 4) shows that almost 70% of tungsten and iron was co-precipitated in ochreous sediment already in the first 3 m. It therefore appears that the drainage itself poses no danger to the local environment due to the self-purifying properties of the ochreous sediments, composed mainly of ferrihydrite.

The danger can be seen in the very fact of the formation ochreous sediments. Regardless of the fact that they purify the water from tungsten and other heavy metals, during the intense melting of snow and heavy rainfall, the accumulated sediments are washed away and transferred in the form of suspensions in the water system, reaching water intake installations of the Bistritsa derivation. The strongest carryover of such suspensions occurred in April 2019 at high waters, when the pH of the drainage water dropped to 2.4.

Various methods and approaches used to control acid mine drainage [3] are not applicable here. It is necessary to pay attention to the formation of dilute ferrous drainage. This drainage is formed in anoxic conditions by the reductive dissolution of ferric minerals from the oxidation zone. To stop the reductive dissolution of Fe³⁺ minerals, it is necessary to provide the access of atmospheric oxygen to the closed gallery. This can be achieved by local drilling or by partially opening the entrance to the gallery (very difficult).