Mine Water Discharge Chemistry and Potential Risk in a Former Mining Area

Abstract

The Maramures region, located in North-Western Romania, was a renowned center of mining and smelting in the last century. Nowadays, all the mines have been decommissioned or are under conservation and greening works, but the acidic waters from some closed or abandoned mine galleries negatively affect the nearby streams and, in some cases, the entire river system. In this study, 46 elements and 6 anion concentrations were used to assess the pollution in 12 mine water discharge samples collected in two mining areas in Maramures. The results showed high concentrations of sulfate (average 1264 mg/L) and toxic elements, namely Mn (average 25.1 mg/L), Fe (average 23.0 mg/L), and Zn (average 12.5 mg/L). The sum of the REEs concentration ranged from 1.24 µg/L to 2917 µg/L, with an average of 363 µg/L, with La, Ce, and Nd being the most abundant. High correlations were found between REEs and Li, Be, Al, Sc, V, Mn, Fe, Co, Ni, Y, SO₄²⁻, and NO₂⁻. According to the pollution index, the discharge of mine water poses different degrees of ecological risk. The health hazard index calculated for 37 elements revealed an extremely high non-cancer risk and, in addition, an increased carcinogenic risk for Cd, As, and Cr.

1. Introduction

Mine water discharges during underground operations and after mine closures or abandonment remain a global concern, with adverse environmental and public health impacts [1]. In the case of abandoned mines without a legally responsible owner, wastewater often flows downstream, with minimal or no treatment, unless local governments or other organizations assume responsibility for the mine management. These waters, also called acid mine drainage, are often acidic and are formed when metallic sulfide ores such as sphalerite (ZnS), pyrite (Fe₂S), chalcopyrite (CuFeS₂) come into contact with oxygen, water, and microorganisms, leading to sulfuric acid that can dissolve various minerals [2].

Acid mine drainage is characterized by a low pH and high metal concentrations, which can contaminate receiving waters and destroy the surrounding ecosystems [3,4]. Contamination with toxic elements like Cd, Cr, Pb, Hg, and As is considered to be one of the most prominent issues affecting water quality. Due to their non-biodegradability and tendency to accumulate, contamination by such toxic elements is a significant environmental issue worldwide, particularly when natural and/or anthropogenic processes lead to their release into ground and surface waters [5,6]. In addition, toxic elements can pass in the human body via drinking water or the food chain, affecting the human health [7]. Dietary exposure to toxic elements has been associated with cardiovascular, respiratory, renal, immunological, and oncological pathologies [8,9,10].

Rare Earth elements (REEs) include fifteen lanthanide elements, from La to Lu, together with Sc and Y due to their similar properties [11]. REEs occur collectively in various minerals in the Earth’s crust, exhibit comparable chemical properties and geochemical behavior, and are split into light REEs (LREEs) and heavy REEs (HREEs) [12,13]. The LREEs include the elements from La to Sm (La, Ce, Pr, Nd, Sm), and the HREEs comprise the elements from Eu to Lu (Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) [14,15]. REEs have multiple applications in high-tech industries (computers, smartphones, LEDs, TVs, magnets, hard drives) and are considered critical elements of strategic importance [12,16]. Although the REEs have low toxicity, their ability to bioaccumulate and persist can lead to negative effects on human health, including DNA damage, cell death, neurological disorders, oxidative stress, pneumoconiosis, changes in the bone structure, cytotoxicity, genotoxicity, and the development of fibrotic tissue [17].

Mining in the Maramures region of NW Romania dates back to ancient times, as the region is rich in complex nonferrous ores containing Cu, Pb, Zn, Au, and Ag. Nonferrous mining and ore processing flourished in the 20th century and contributed to the economic development of the region. The oldest mining basins in the area are those of Baia Mare and Baia Borsa. In these areas, base metals (Cu, Pb, Zn) and polymetallic ores were mined in extensive underground mining operations. The decline in gold and base metal prices, together with the more stringent environmental legislation in recent decades, has led to the gradual closure of ore-processing facilities and mines in the area. Although mining activity in these areas has been discontinued, the mining and ore-processing wastes and acid mine water discharges still represent a major source of environmental pollution in the area [18]. During the operational phase of underground mining, precipitation and infiltrating groundwater are continuously pumped from underground and treated, if necessary, before being discharged to the surrounding river system. After the mine closure, the underground galleries are slowly filled with water until the galleries are flooded. In contact with the gallery walls, the flood water can dissolve various elements and then reach the surface and the nearby streams. In the case of most mines, the mine water is untreated or only partially treated, mainly by neutralization and element precipitation with hydrated lime (Ca(OH)₂) or limestone (CaCO₃). Seepage from mine galleries carries metals that pollute streams and then entire rivers, with negative consequences for the ecosystems and populations through water ingestion [19,20,21]. Therefore, mine closures and environmental rehabilitation are critical to the sustainable development of the mining industry.

The current study aims to determine the concentrations of major, trace, and REEs in mine water discharge samples from several closed mines to assess the water pollution by calculating the pollution index and the human health risk to the local population by water ingestion. Thus, this investigation provides information on the risk of untreated or unsuitably treated mine water discharges and their impact on the ecological and functional quality of river systems and pollution in mining regions.

2. Materials and Methods

2.1. Study Area

Acid mine drainage is one of the most common mining wastes that can affect the surrounding environment for many years if not properly managed. In the Maramures region, NW Romania, twelve mine water samples were collected in September 2023, eight in the Baia Mare area (M1–M8) and four in Baia Borsa (M9–M12) mining areas, located approximately 150 km apart. The Baia Mare area is situated in the Gutai Mountains, and Baia Borsa is situated in the Toroiaga-Tibles Mountains, with both areas a part of the NW Carpathian Mountains [22]. The sampling sites and their description are presented in

Table 1. Sampling sites in Maramures region, NW Romania.

Sample	Mining Area	Sampling Site Description
M1	Baia Mare	Surface water from Baita River, after the confluence with Tyuzosa mine drainage
M2		Mine drainage from Tarnita mine gallery, collected near the Baita village
M3		Mine drainage from Reiner mine gallery, collected near the Cavnic town
M4		Surface water from Red Valley, after the input of Campurele mine drainage treatment plant, collected upstream Baita Valley confluence
M5		Mine drainage from Cavnic mine gallery, collected near Cavnic town
M6		Mine drainage from Herja mine gallery, collected near Baia Sprie town
M7		Mine drainage from Baiut mine gallery, collected near Targu Lapus town
M8		Mine drainage from Sasar mine galleries, collected near Baia Mare city
M9	Baia Borsa	Mine drainage from Emerik mine gallery, collected near Baia Borsa town
M10		Mine drainage from Colbu mine gallery, collected near Baia Borsa town
M11		Mine drainage from Burloaia mine gallery, collected near Baia Borsa town
M12		Mine drainage from Gura Baii mine gallery, collected near Baia Borsa town

and Figure 1.

The investigated mining areas contain volcanic rocks of the Mesozoic age, with massive sulfide deposits characterized by polymetallic mineralization. The constituent elements of sulfides of nonferrous mineralization are Pb, Zn ± Cu, Au, and Ag. Their most distinctive geochemical feature is the high content of Cd, Co, Ni, Ge, Tl, Se, Te, Bi, Sb, As, Sn, Mo, and W [19,22].

2.2. Sampling and Analytical Methods

At each location, the water samples were collected by directly immersing a pre-cleaned polyethylene bucket under the surface of the water. The pH and total dissolved solids (TDS) were measured on-site via a portable multiparameter (WTW, Weilheim, Germany). The samples were filtered on-site through 0.45 µm cellulose acetate syringe filters. For metal determination, the filtered samples were acidified to a pH < 2 with 65% HNO₃ (Merck, Darmstadt, Germany). For the other parameters, the filtered samples were stored at 4 °C until analysis.

In the laboratory, the water samples were analyzed for anions (F⁻, Cl⁻, NO₂⁻, NO₃⁻, SO₄²⁻, PO₄³⁻) using a 761 IC ion chromatograph (Metrohm, Herisau, Switzerland), after filtration over 0.45 μm cellulose acetate membrane filters. The total acidity, total alkalinity, and bicarbonate (HCO₃⁻) were determined by titration [23,24]. The metals were analyzed using a 7500 CX inductively coupled plasma mass spectrometer (Agilent Technologies, Santa Clara, CA, USA) after digestion using an Xpert microwave system (Berghof, Eningen, Germany) with 65% HNO₃. To prepare the standard solutions and dilute the samples, the ultrapure water was produced using a Direct Q3 Milli-Q system (Millipore, Molsheim, France).

The ion chromatograph was calibrated using an IC Multi-element standard I (Certipur Merck, Darmstadt, Germany) containing PO₄³⁻ (1000 mg/L), NO₃⁻, SO₄²⁻ (500 mg/L), Cl⁻ (250 mg/L), and an NO₂⁻ standard (1000 mg/L) diluted appropriately with ultrapure water. The ICP-MS was calibrated using calibration solutions (0.01–100 μg/L) prepared from multi-element standard solutions II and III (Perkin Elmer, Waltham, MA, USA). A mixed solution of Rh-Re served as an internal standard to correct for overall instrumental drift and matrix effects [14].

The accuracy of the measurements was checked using 1643f NIST-Trace in water (National Institute of Standards and Technology, Gaithersburg, MD, USA) for metal analysis and SPS-NUTR WW1 (Spectrapure standards, Manglerud, Oslo, Norway) for anions analysis. The reproducibility of the metals and anions expressed as the relative standard deviation was less than 10% and 12%, respectively, while the recoveries were 96–105% and 91–104%, respectively.

The limits of detection (LODs) were determined by multiplying three times the ratio of the standard deviation of the mean signal of 10 blank samples by the slope of the calibration curve. The LODs for metals were in the range of 0.003 µg/L (Lu)–10 µg/L (Ca, Mg, Na). The LOD for anions using ion chromatography was 0.010 mg/L for NO₂⁻, 0.100 mg/L for NO₃⁻, 0.004 mg/L for SO₄²⁻, and 0.025 mg/L for PO₄³⁻.

2.3. Pollution Indices

Metal pollution for each mine water was assessed by calculating the metal evaluation index (MEI), which indicates the overall status of water quality related to metal content [25] according to Equation (1).

MEI = ∑C_e

(1)

where C_e represents the contamination coefficient of the studied elements, calculated according to Equation (2)

C_e = c_e/MAC_e

(2)

where C_e represents the contamination coefficient of the element, c_e is the measured value of the element, and MAC_e is the maximum allowable concentration of the element. The MEI values were calculated for 3 sets of elements, for which guideline values are established in the Romanian legislation: (i) the MEI for surface waters was calculated considering the concentrations of Be, V, Cr, Co, Ni, Cu, As, Mo, Cd, Sn, Ba, Tl, and Pb and their corresponding quality standards were established by Regulation 161/2006 on the hazardous and priority hazardous substances in surface waters [26]; (ii) the MEI for wastewater to be discharged into natural receptors was calculated on the basis of the concentrations of Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Cd, and Pb and their corresponding limits established by Government Decision 188/2002 approving the pollutant load limits prior to the discharge of industrial and municipal wastewater into natural receptors [27]; (iii) the MEI for REEs was calculated based on the REEs concentration and MAC values provided by Sneller et al. [28] (

Table 2. Maximum allowable concentration (MAC) used in MEI calculation.

	MAC (µg/L)
Surface water 1	Be	V	Cr	Co	Ni	Cu	As	Mo	Cd	Sn	Ba	Tl	Pb
	0.05	1.2	2.5	0.7	2.1	1.3	7.2	3.6	1.0	2.2	200	2.0	1.7
Wastewater discharged in natural receptors 2	Al	Cr	Mn	Fe	Co	Ni	Cu	Zn	As	Mo	Cd	Pb	-
	5000	1000	1000	5000	1000	500	100	500	100	100	200	200	-
REEs 3	Y	La	Ce	Pr	Nd	Sm	Gd	Dy	-	-	-	-	-
	6.4	10.1	22.1	9.1	1.8	8.2	7.1	9.3	-	-	-	-	-

¹ quality standards for hazardous and priority hazardous substances in surface water set by Romanian legislation, Order 161/2006 [26]. ² limit values for pollutant load limits for industrial and urban wastewater for discharge into natural receptors set by Romanian Government Decision 188/2002 [27]. ³ Maximum Permissible Concentrations for Rare Earth Elements given by Sneller et al. [28].

).

The MEI values indicate low (MEI < 10), medium (10 < MEI < 20), and high (MEI > 20) pollution levels [25,29].

The potential ecological risk index (RI), initially described by Hakanson [30] for sediments, then adapted by other authors to evaluate the risk of different types of waters, was calculated according to Equation (3) [31].

RI = ∑Tr_e × C_e

(3)

where Tr_e represents the toxic response factor of the element and C_e is the contamination coefficient of the element, calculated according to Equation (2). The values of Tr were set as the toxicological factors in the risk index approach, as follows: 30 (Cd), 10 (As), 5 (Pb), 5 (Ni), 5 (Cu), 2 (Cr), and 1 (Zn) [30].

According to the RI values, the potential ecological risk is divided into five classes, as follows: RI < 150, low; 150 < RI < 300, moderate; 300 < RI < 600, strong; >600 < RI < 5000, very strong, and RI > 5000, extremely strong risk [31].

2.4. Human Health Risk Assessment to Metals via Water Consumption

To evaluate the non-carcinogenic human health risk by water consumption, the chronic daily intake (CDI) of each metal was calculated for the adult population using Equation (4) [32,33].

CDI = (C × IR × EF × ED × 10⁻³)/(BW × AT)

(4)

where CDI is the chronic daily intake (mg/kg/day), C is the concentration of the metal in the water sample, in µg/L, IR is the intake rate, i.e., the water amount consumed daily (2.5 L/day) [34], EF is the exposure frequency (365 days/year), ED is the exposure duration (70 years), BW is the average body weight (70 kg for an adult), and AT is the average exposure time, the period of time in which the concentration is regarded as constant (AT = 70 × 365 = 25,550 days).

The non-carcinogenic risk is assessed by comparing the calculated CDI with the chronic oral reference dose values (RfD) established by the United States Environmental Protection Agency [34]. The RfD is the maximum acceptable oral dose of a substance or element, below which no adverse non-cancer health effects result from a lifetime of exposure. The ratio between CDI and RfD is the hazard quotient (HQ). The potential risk to human health from exposure to multiple metals was measured by the chronic hazard index (HI) by summing all the HQs calculated for each individual metal. The HQ or HI values below 1 indicate no apparent health risks, values exceeding 1 denote the possibility of adverse health effects, and values above 10 indicate severe chronic health effects [33].

The carcinogenic risk (CR) for the water ingestion was calculated only for metals with defined carcinogenic slope factors (CSF), namely Pb, As, Cd, Cr, and REEs, using Equation (5) [35,36,37].

CR = CDI × CSF

(5)

The CR value between 10⁻⁴ and 10⁻⁶ designates that the carcinogenic risk is considered acceptable, the CR < 10^{− 6} specifies a negligible carcinogenic risk, and a CR > 10⁻⁴ indicates an unacceptable carcinogenic risk. The CSF quantitatively defines the relationship between the dose and response and is used to estimate the risk of cancer associated with lifetime exposure to a carcinogenic or potentially carcinogenic substance by ingestion and is 1.5 (As), 6.3 (Cd), 0.0085 (Pb), and 0.5 (Cr) mg/kg/day, respectively [35,37], and the CSF for REEs has been set at 3.2 × 10⁻¹² mg/kg/day for all REEs [36].

2.5. Data Analysis

The water type was determined based on the Piper diagram drawn via GW Chart software version 1.29.0.0. The diagram consists of three components: a central diamond plot—reflecting all ions along with two trilinear plots—one dedicated to cations and the other for anions [38]. Ficklin diagram was drawn to discriminate and categorize the different mine water samples [39].

Pearson correlation and principal component analysis (PCA) were used to explore the relationships between the concentrations of metals and anions. A significance level of p < 0.05 was considered statistically significant. The PCA was used to reduce data and extract a small number of independent components named PCs. The varimax rotation was applied on the PCs with eigenvalues higher than 2. To categorize the samples based on their chemical composition, Agglomerative Hierarchical Clustering (AHC) was conducted using a squared Euclidean distance and Ward linkage method. Statistical analysis was carried out using the XLStat (Addinsoft, Paris, France) software (BASIC+, 2019.3.2).

3. Results and Discussions

3.1. Hydrochemical Properties

The physicochemical analysis of the mine water samples is presented in

Table 3. Concentrations (expressed as average ± 2 × standard deviation, m ± 2 s) of elements, anions, total dissolved solids (TDS), acidity and total alkalinity in water samples (n = 12).

Parameter	pH	TDS (mg/L)	Na (µg/L)	Mg (µg/L)	Al (µg/L)	K (µg/L)
m ± 2 s (min–max)	5.28 ± 3.96 (2.34–7.49)	2216 ± 2112 (594–4150)	10,772 ± 12,104 (4523–23,941)	38,033 ± 58,636 (6468–92,489)	8083 ± 184,978 (72.8–51,934)	3434 ± 4704 (1316–8895)
Parameter	Ca (µg/L)	Mn (µg/L)	Fe (µg/L)	Co (µg/L)	Ni (µg/L)	Cu (µg/L)
m ± 2 s (min–max)	135,368 ± 130,858 (48,126–24,2061)	25,085 ± 119,198 (48.2–212,247)	22,975 ± 62,386 (372–94,569)	69.6 ± 252 (0.674–459)	56.2 ± 76.4 (11.1–135)	253 ± 702 (16,386–1236)
Parameter	Zn (µg/L)	As (µg/L)	Cr (µg/L)	Mo (µg/L)	Cd (µg/L)	Pb (µg/L)
m ± 2 s (min–max)	12492 ± 24346 (317–40469)	13.2 ± 22.8 (1.05–31.3)	9.19 ± 13.9 (2.55–22.3)	1.30 ± 3.02 (0.199–5.57)	39.7 ± 98.4 (0.339–164)	7.76 ± 15.34 (<0.010–20.1)
Parameter	Li (µg/L)	Be (µg/L)	Ba (µg/L)	La (µg/L)	Ce (µg/L)	Pr (µg/L)
m ± 2 s (min–max)	86.4 ± 177 (21.1–345)	2.86 ± 12.5 (0.024–22.1)	33.4 ± 71.8 (11.7–144)	51.6 ± 226 (0.424–404)	144 ± 674 (0.335–1198)	19.6 ± 96.8 (0.049–172)
Parameter	Nd (µg/L)	Sm (µg/L)	Eu (µg/L)	Gd (µg/L)	Tb (µg/L)	Dy (µg/L)
m ± 2 s (min–max)	57.7 ± 236 (0.193–417)	14.5 ± 61.8 (0.048–109)	4.65 ± 21.0 (0.025–36.9)	23.0 ± 102.6 (0.054–181)	3.49 ± 16.04 (0.010–28.3)	20.3 ± 95.6 (0.043–169)
Parameter	Ho (µg/L)	Er (µg/L)	Tm (µg/L)	Yb (µg/L)	Lu (µg/L)	Rb (µg/L)
m ± 2 s (min–max)	3.93 ± 18.74 (0.011–33.1)	10.1 ± 48.6 (0.021–85.8)	1.29 ± 6.24 (<0.004–11.0)	7.46 ± 36.4 (0.023–64.1)	1.10 ± 5.38 (<0.003–9.50)	25.2 ± 53.8 (3.38–80.5)
Parameter	Sr (µg/L)	Y (µg/L)	Zr (µg/L)	Sn (µg/L)	Cs (µg/L)	U (µg/L)
m ± 2 s (min–max)	502 ± 562 (110–975)	59.2 ± 268 (0.259–475	0.886 ± 1.130 (0.372–2.23)	3.41 ± 7.66 (1.10–15.1)	7.03 ± 16.54 (0.208–24.9)	2.26 ± 5.46 (<0.005–8.57)
Parameter	Th (µg/L)	Tl (µg/L)	Sc (µg/L)	Ti (µg/L)	Sb (µg/L)	V (µg/L)
m ± 2 s (min–max)	0.742 ± 1.816 (0.021–2.48)	1.69 ± 6.26 (<0.007–9.99)	11.2 ± 25.8 (1.59–49.8)	212 ± 199 (109–412)	1.28 ± 3.32 (0.107–4.86)	2.65 ± 9.62 (0.241–15.1)
Parameter	ΣLREE (µg/L)	ΣHREE (µg/L)	ΣREE (µg/L)	ΣREY (µg/L)	Acidity (mg/L CaCO3)	Alcalinity (mg/L)
m ± 2 s (min–max)	288 ± 1292 (1.05–2299)	75.3 ± 350 (0.193–618)	363 ± 1642 (1.24–2917)	433 ± 1934 (8.70–3442)	43.8 ± 85.4 (0–100)	0.875 ± 1.708 (0–2.00)
Parameter	SO42− (mg/L)	HCO3− (mg/L)	NO2− (mg/L)	NO3− (mg/L)	F− (mg/L)	Cl− (mg/L)
m ± 2 s (min–max)	1264 ± 4620 (74.4–8500)	53.4 ± 104.2 (0–122)	1.09 ± 2.10 (0.025–3.70)	1.94 ± 3.76 (0.10–5.31)	1.60 ± 3.48 (0.025–4.70)	6.63 ± 8.66 (0.690–12.5)

. The sum of La, Ce, Pr, Nd, and Sm concentrations represents the concentration of LREEs, while the sum of Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb concentrations denotes the concentration of HREEs. The sum of the 14 naturally occurring REEs was expressed as the total REEs (ΣREEs). The sum of the 14 REEs and Y and Sc was expressed as ΣREY.

The TDS varied largely, from 594 mg/L (sample M1) to 4150 mg/L (sample M8), with an average concentration of 2216 mg/L. The SO₄²⁻ concentration ranged from 74.4 mg/L (sample M10) to 8500 mg/L (sample M8), whereas the total acidity ranged from 0 to 100 mg CaCO₃/L. The pH values ranged from 2.34 (sample M8) to 7.49 (sample M10), with an average value of 5.28. In nine samples, the pH values were in the acidic range. PO₄³⁻ concentrations were below the limit of detection (0.025 mg/L) in all samples. In the mine waters, the average elements’ concentration was dominated by Ca, Mg, Mn, Fe, Zn, and Na, with average concentrations higher than 10,000 µg/L (Figure 2a). Average concentrations lower than 1 µg/L were found for Zr, Tl, and Th and, among REEs, for Tm and Lu.

The most toxic elements for living organisms in aqueous systems are Ni, Cr, Cd, Pb, As, Cu, and Zn. Some of these elements are essential (Cu, Zn, Cr) in low concentrations but, at high concentrations, can be carcinogenic, mutagenic, or teratogenic [29]. The concentrations of Mn, Cu, Zn, As, and Cd were comparable with those in the AMD from the Smolnik mine in Slovakia [32] and were significantly lower than in the acid mine water sample from the Poderosa mine in the Iberian Peninsula, Spain [40]. Also, the concentrations of Mn, Co, Ni, Zn, and As in the Crocodile River, South Africa, located in the western basin of the Witwatersrand mining area, were comparable with the concentrations obtained in the present study [41].

The concentration of ∑REE in the investigated water samples varied over three orders of magnitude and ranged between 1.24 μg/L in sample M10 and 2917 μg/L in sample M8 (average 363 μg/L and standard deviation 821 μg/L), with Ce, Nd, La, and Gd being the most abundant, accounting for the (79.2% ± 6.93%) of the total REEs (Figure 2b). The average concentration of Y was comparable to the average concentrations of Nd and La. Additionally, the concentration of LREEs ranged from 1.05 μg/L to 2299 μg/L, whereas the concentration of HREEs ranged from 0.19 μg/L to 618 μg/L, with an average of 288 μg/L and 75.3 μg/L, respectively. Like other studies [15,42], the ∑REE concentrations in mine drainage samples increased as the pH levels decreased.

The average concentration of ∑REEs in the mine waters collected from the Baia Mare area (137 µg/L) is comparable to the ∑REEs average concentration in the samples collected from the Baia Borsa area (120 µg/L) if the M8, containing the highest REE concentrations, is not accounted for. In each sample, the concentration of LREE was higher than the concentration of HREE, with the ratio ∑LREEs/∑HREEs ranging from 1.50 to 10.5 (5.41 ± 2.50). The five most critical REEs (the sum of Dy, Eu, Nd, Tb, and Y) [1] accounted for 41.7% (average) of the total weight of all REEs.

The concentration of the ∑REEs was comparable to the concentration of ∑REEs in the waste waters of Jaintia coalfield (India), Hanes mine (Romania), Esperanza mine (Spain), coal mine drainage of the Northern Appalachian coal basin (USA) from the abandoned mine sites of Pennsylvania (USA), and in the Ronneburg mine (Germany) [15,16,43]. The REE concentrations were higher than in the acid mine drainage originating from the Xingren coalfield, China, where ∑REE ranged from 118 to 926 µg/L and was higher than in the mine drainage from the inactive Zn-Pb mine of Santa Lucia (Cuba), where ∑REE ranged from 370 to 860 µg/L [15].

The variations in element concentrations in the waters studied can be attributed to composition of the host rock, natural weathering processes, the time of closure/abandonment of the mine, the effectiveness of neutralization processes with minerals such as calcite (CaCO₃) or dolomite (CaMg(CO₃)₂), or the efficiency of other types of remediation activities.

The representation of the acid mine drainage samples on the Ficklin diagram [39] showed the grouping of samples based on their pH and total element content (Figure 3).

Most of the samples are classified as “near-neutral, extreme-element”, M4 and M8 as “high-acid, extreme-element”, M7 and M9 as “acid-extreme-element”, and only samples M1 and M2 as “near-neutral-high element” and “acid-high element”, respectively. Although in other studies [31], most of the mine drainage samples are considered to be highly acidic with extreme element concentrations, in our case, the presence of several samples in class “near neutral” could be explained by the occasional use of limestone for neutralization and precipitation of elements.

3.2. Potential Ecological Risk

The MEI values were calculated based on the concentrations of Be, V, Cr, Co, Ni, Cu, As, Mo, Cd, Sn, Ba, Tl, and Pb, and the quality standards of these elements in surface water [26] varied in the range of 49.8 (M10)–1290 (M8), with an average value of 432. All the MEI values were up to 60 times higher than the threshold value of 20, indicating highly polluted waters (Figure 4a).

The MEI values were calculated using the concentrations of Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Cd, and Pb, and the limit values of these elements for wastewater for discharge into natural receptors [27], varied in the range of 1.57 (M10)–253 (M8), with an average value of 58.2. Four values were below 10 (samples M1, M2, M3, and M10), and the other eight values were above 20, suggesting low and high polluted waters (Figure 4b). The two calculation modes provide two dissimilar results; the differences are attributed to the fact that the MACs set by the legislation are very different, and only eight metals are common.

The values of MEI calculated for the eight REEs that have MACs [28] ranged from 0.228 (M10) to 476 (M8), with an average value of 62.3. In five samples (M4, M5, M8, M9, M12), the MEI exceeded a value of 20, indicating highly polluted waters (Figure 4c).

Similar to the MEI, the potential ecological risk index (RI) was calculated in two ways: (i) based on Tr and the threshold values of Cd, As, Ni, Cu, Pb, and Cr in surface water [26,30], and (ii) based on Tr and limit values for Cd, As, Ni, Cu, Pb, Cr, and Zn for industrial and urban wastewater discharges to natural receptors [27,30]. The RI values based on the surface water thresholds (Figure 5a) varied between 199 (M10) and 7546 (M12), dividing the samples as follows: M2 and M10 recorded a moderate risk; M1 and M6 fell into a strong risk; M3, M4, M7, M8, M9, and M11 exhibited a very strong risk, and sample M5 belonged to the extremely strong risk class. No sample recorded a low risk. The major contribution to the RI value was attributed to Cd (50.8%), followed by Cu (41.5%), Ni (5.71%), Pb (0.973%), As (0.784%), and Cr (0.314%), considering the average concentration of the six elements.

The calculated RI values considered the limit values for industrial and urban wastewater when discharged to natural receptors (Figure 5b) and ranged from 3.12 (M10) to 123 (M5), placing the samples in the low-risk class. The methods/mathematical models used to calculate the MEI and RI have an important drawback as only a few elements have MACs set in legislation, and even fewer elements have Tr values assigned in the literature. This fact could lead to an underestimation of the potential ecological risk by ignoring other elements present in the samples.

3.3. Health Risk Assessment via Water Consumption

Several villages and small villages are located near the mine water discharges. Residents occasionally use the water in their households for various needs, mostly for watering the gardens. Therefore, the risk has been assessed under the worst-case scenario, in which the residents near the study areas may use the water for drinking purposes. Residents may be exposed to metals through water ingestion, which may lead to health risks, including respiratory, neurological, cardiovascular, and reproductive system damage, as well as carcinogenic and mutagenic effects [17].

3.3.1. Non-Carcinogenic Risk to Metals Through Water Consumption

The values obtained for the calculated chronic daily intake (CDI), hazard quotient (HQ), and hazard index (HI) from water consumption, together with the RfD values, are shown in

Table 4. Reference doses (RfD, mg/kg/day), chronic daily intake (CDI, mg/kg/day), hazard quotient (HQ) and hazard index (HI) through water consumption (expressed as average ± 2 × standard deviation, m ± 2 s).

Element	RfD 1	CDI		HQ		HI
		m ± 2 s	Min–Max	m ± 2 s	Min–Max	m ± 2 s (Min–Max)
Al	2.86 × 10−1	0.29 ± 1.1	0.003–1.9	1.0 ± 3.7	0.01–6.5	91.8 ± 262 (4.54–466)
V	5.04 × 10−3	9.0 × 10−5 ± 3.4 × 10−4	1.0 × 10−5–5.4 × 10−4	0.02 ± 0.07	0.002–0.11
Cr	3.00 × 10−3	3.0 × 10−4 ± 4.0 × 10−4	1.0 × 10−4–8.0 × 10−4	0.11 ± 0.16	0.03–0.27
Mn	2.40 × 10−2	0.92 ± 4.3	0.002–7.6	37 ± 177	0.07–316
Fe	7.00 × 10−1	0.82 ± 2.2	0.013–3.4	1.2 ± 3.2	0.02–4.8
Co	3.00 × 10−3	2.0 × 10−3 ± 1.0 × 10−2	2.0 × 10−5–1.6 × 10−2	8.3 ± 30	0.08–55
Ni	2.00 × 10−2	2.0 × 10−3 ± 2.0 × 10−3	4.0 × 10−4–5.0 × 10−3	0.10 ± 0.14	0.02–0.24
Cu	4.00 × 10−2	9.0 × 10−3 ± 2.6 × 10−2	1.0 × 10−3–4.4 × 10−2	0.23 ± 0.63	0.02–1.1
Zn	3.00 × 10−1	0.45 ± 0.87	0.011–1.45	1.5 ± 2.9	0.04–4.8
Sr	6.00 × 10−1	0.02 ± 0.02	0.004–0.035	0.03 ± 0.03	0.01–0.06
Mo	5.00 × 10−3	5.0 × 10−5 ± 1.0 × 10−4	1.0 × 10−5–2.0 × 10−4	9.3 × 10−3 ± 2.2 × 10−2	1.4 × 10−3–4.0 × 10−2
Cd	1.00 × 10−4	1.4 × 10−3 ± 3.6 × 10−3	1.0 × 10−5–6.0 × 10−3	14 ± 35	0.12–58.4
Pb	3.00 × 10−4	2.8 × 10−4 ± 5.4 × 10−4	4.0 × 10−7–7.0 × 10−4	0.92 ± 1.8	1.2 × 10−3–2.4
U	2.00 × 10−4	8.0 × 10−4 ± 1.8 × 10−4	2.0 × 10−7 –3.1 × 10−4	0.40 ± 0.97	0.001–1.5
Li	2.00 × 10−3	3.0 × 10−3 ± 6.0 × 10−3	0.001–0.012	1.5 ± 3.2	0.38–6.2
Th	1.00	2.7 × 10−5 ± 6.4 × 10−5	8.0 × 10−7–8.9 × 10−5	2.7 ± 6.5	0.08–8.9
Tl	3.00 × 10−6	6.0 × 10−5 ± 2.2 × 10−4	2.3 × 10−7 –35.7 × 10−5	20 ± 74	0.08–119
Sb	4.00 × 10−4	4.6 × 10−5 ± 1.2 × 10−4	3.8 × 10−6 –1.74 × 10−5	0.11 ± 0.30	9.6 × 10−3–4.34 × 10−1
Zr	8.00 × 10−5	3.2 × 10−5 ± 4.0 × 10−5	1.3 × 10−5 –7.9 × 10−5	0.40 ± 0.50	0.17–0.99
Ba	2.00 × 10−1	1.2 × 10−3 ± 2.6 × 10−3	4.0 × 10−4–5.2 × 10−3	6.0 × 10−3 ± 1.3 × 10−2	2.1 × 10−3–2.6 × 10−2
As	3.00 × 10−4	4.7 × 10−4 ± 8.2 × 10−4	3.8 × 10−5–1.1 × 10−3	0.79 ± 2.1	0.06–1.9
∑REE	2.00 × 10−2	0.01 ± 0.06	4.4 × 10−5–1.0 × 10−1	0.65 ± 2.9	2.2 × 10−3–5.2
Sc	2.00 × 10−2	4.0 × 10−5 ± 1.0 × 10−5	5.7 × 10−5–1.8 × 10−3	0.02 ± 0.05	2.8 × 10−3–8.9 × 10−2
Y	2.00 × 10−2	2.1 × 10−3 ± 9.6 × 10−3	9.3 × 10−6–1.7 × 10−2	0.11 ± 0.48	5.0 × 10−4–8.5 × 10−1

¹ Based on references [44,45,46,47,48].

. Considering the maximum CDI values, the exposure to Sc, Sb, Sr, Ba, V, Cr, Ni, Mo, Zr, and Y through water consumption is lower than the RfD, confirming that there is a low risk of adverse health effects caused by their ingestion with water. On the contrary, the exposure to Cu, As, Zn, Al, Fe, Pb, U, Li, and Th slightly exceeds the RfD values, indicating the presence of risk by water consumption. The large exceedance of the RfD in the case of elements Cd, Co, Tl, and Mn indicates a high risk to the health of consumers of contaminated water. A high average value for HQ (Table 4) was calculated for Mn (37.3), accounting for 40.7% of the HI due to the high Mn concentrations measured in the mine water samples. Apart from Mn, the average values of HQ above the unit were recorded for Tl (20.1), Cd (14.2), Co (8.29), and Th (2.65), accounting for 21.9%, 15.4%, 9.03%, and 2.89% of the HI, respectively. The average values of a HQ close to the unit were calculated for Li, Zn, Fe, and Al. The highest risk was observed for the sample M8 (HI = 466), followed by M4 (HI = 207) and M5 (HI = 108), with an HI greater than 100.

The estimated daily intake of the sum of the 14 REEs was in the range of 0.044–104 µg/kg/day (average value of 13.0 µg/kg/day) and in the range of 0.311–123 µg/kg/day (average value of 15.5 µg/kg/day) for REY. For La, Ce, and Y, tolerable daily intake (TDI) thresholds of 51.3, 161.5, and 145.5 µg/kg/day were derived based on the no observed adverse effect level by the European Food Safety Authority [14,44]. For the ∑REE, the TDI set for La (51.3 µg/kg/day) was adopted [44]. The intake of REEs through water consumption was lower (except for sample M8) than the RfD for ∑REEs, as well as the TDI of 51.3 µg/kg/day [14,44]. The estimated daily intake for other metals (Al, Mn, Fe, Zn, and Sr) was considerably higher than those for REEs. The non-carcinogenic risk of REEs was the highest in sample M8 (HQ = 5.21) and the lowest in sample M10 (HQ = 0.0022), with all the values below the safe value of the unit, except for sample M8.

Recently, the European Food Safety Authority updated the risk assessment of inorganic As by ingestion in 2024 and established a reference value of 0.06 µg/kg bw per day derived from a case-control study of skin cancer. This value represents a conventional estimate of the minimum dose that may be associated with increased induction of skin cancer following exposure to inorganic As [49]. Using this reference value, the HQ varied from 0.625 to 18.6, with an average value of 7.87, indicating a high risk of skin cancer from water ingestion. A TDI of 6 µg/kg bw has been established for Sb [44]. Considering the assessed exposure to Sb, the average daily intake (0.046 µg/kg bw) from water consumption would represent only 0.76% of a TDI, indicating a negligible risk to the health of potential consumers. A TDI of 0.2 mg/kg bw per day has been established for Ba [44]. At the highest exposure (0.0052 mg/kg/day) from water consumption, only 2.6% of the TDI would be met, indicating a negligible risk of adverse health effects from Ba intake.

3.3.2. Carcinogenic Risk to Metals Through Water Consumption

The carcinogenic risk calculated for Pb, Cd, As, Cr, and REEs is shown in

Table 5. Carcinogenic risk (CR) through water consumption (expressed as average ± 2 × standard deviation, m ± 2 s).

Element	CR
	m ± 2 s	Min–Max
Pb	2.4 × 10−6 ± 4.7 × 10−6	3.0 × 10−9–6.1 × 10−6
Cd	8.9 × 10−3 ± 2.2 × 10−2	7.6 × 10−5–3.681 × 10−2
As	0.79 ± 0.0012	0.062–1.9
Cr	1.6 × 10−4 ± 2.5 × 10−4	4.6 × 10−5–4.0 × 10−4
REEs	4.1 × 10−14 ± 1.9 × 10−13	1.4 × 10−16–3.3 × 10−13

.

Regarding the carcinogenic risk, the exposure to REEs through water was negligible, with all CR values lower than 10⁻⁶. The CR values for Cd were higher than 10⁻⁴, indicating an unacceptable risk, except for sample M2, with a CR in the acceptable range. The same observation was valid for As and Cr, with CR values higher than 10⁻⁴, indicating an unacceptable risk, except for sample M11 for As and for samples M2, M3, M6, and M11 for Cr, with CRs in the acceptable range. For Pb, the CR values showed an acceptable risk (10⁻⁴ < CR < 10⁻⁶), except for samples M1, M2, M9, M10, and M12, with a negligible carcinogenic risk.

3.4. Statistical Analysis

Pearson correlations revealed significant correlations among most elements. The Pearson correlation coefficient (r) ranged between −0.026 and 1.000. The correlation between the two variables was considered strong when the 0.7 < |r| < 1.0, moderate when the 0.5 < |r| < 0.7, and weak when the |r| < 0.5 [36]. All REEs revealed strong correlations with each other and also with Li, Be, Al, Sc, V, Mn, Fe, Co, Ni, Y, SO₄²⁻, and NO₂⁻, moderate correlations with Mg, Ca, Ti, Cr, Rb, Th, pH, and TDS, and weak correlations with Na, K, Cu, As, Sr, Zr, Mo, Cd, Sn, Sb, Cs, Ba, Tl, Pb, and U. The As, Zn, Sr, and U showed positive and negative correlations with different anions. The Water pH correlated negatively with all the anions and metals (except positive correlations with Ba, Sb, Sn, Mo, Zr, Sr, Cu, Na), whereas the total alkalinity, CaCO₃, and HCO₃⁻ correlated positively. The total alkalinity, CaCO₃, and HCO₃⁻ negatively correlated with all the anions, TDS, and metals, except with Ba, Sb, Sn, Mo, Zr, Sr, Na, and U, which were positively correlated. The significant correlations among REEs and the other elements indicated their common origin.

The hierarchical relationship between the 12 samples is shown in Figure 6. The analyzed samples were grouped into two distinct branches; one main branch was represented by M8, and the second one by the other investigated samples; they were grouped into three subdivisions, namely, M9, M7, M12, and M5 in the first one, M4 and M2 in the second, and the third subdivision consisted of M11, M3, M6, M10, and M1. The natural geological background is the same in both studied areas, so it is expected that all samples will have similar compositions. In addition, all samples, except sample M8, represent natural mine drainage and sample M8 represents an artificial mine water collection channel that gathers water from multiple galleries, leaking from the Sasar mine, with various other influences, including urban and traffic impacts, and is located in the vicinity of the Baia Mare city.

The different elements and anions classified together indicate that their contents are intercorrelated. The PCA extracted six components, with eigenvalues higher than two, comprising 90% of the total variance. The biplot for the first and the second components, which account for 57.85%, is shown in Figure 7. The first component had strong loadings of all REEs, Li, Be, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Y, Th, SO₄²⁻, NO₂⁻, and TDS, accounting for 47.79% of the total variance. The second component explained 10.06% of the total variance, with strong loadings of K, Rb, Cu, Cs, Tl, pH, F⁻, and Cl⁻. The first two components suggested a clear separation between the elements and were consistent with the Pearson correlations. The third component explained 9.16% of the variance and showed positive loadings of Zr, Mo, Sn, and Sb. The grouping of all REEs into one component confirms their similar behavior in mine water discharges in the investigated areas.

3.5. Water Typology

According to the Piper diagram, the majority of the samples are classified into the CaMgSO₄ water-type category. Samples M1 and M10 are characterized as CaMgHCO₃⁻ water types, and M3 is situated between the two water-type categories (Figure 8).

M1 and M10 are included in the CaMgHCO₃^- water-type category due to the lowest amounts of Mg, TDS, and SO₄²⁻. Based on the cations trilinear plot, the samples are characterized into two water types categories: magnesium (M11) and calcium types (all samples except for M11). The anion trilinear plot indicated that the M1 and M10 are included in the bicarbonate type, while M2, M4–M9, M11, and M12 are included in the sulfate water type. Sample M3 is situated between the two water-type categories.

4. Conclusions

In order to assess the adverse effects of water discharges from derelict mines located in NW Romania, the concentrations of metals, including REEs and anions, were considered important monitoring parameters whose values are regulated in many countries. The obtained results indicated that the REEs were present in each sample, and their concentrations varied over a wide range (1.24–2917 µg/L), with the lowest value for Lu and the highest for Ce. The LREEs were observed to be more prevalent than the HREEs, accounting for (81.9 ± 8.17) % of the total REEs. The Pearson correlation matrix showed that all REEs were highly correlated (α = 0.05) with each other and with SO₄²⁻, NO₂⁻, Li, Be, Al, Sc, V, Mn, Fe, Co, Ni, and Y. The potential ecological risk index (RI) varied between 199 and 7546 and classified the samples into moderate, strong, very strong, and extremely strong risk categories, with no sample recording a low risk. The major contribution to the RI value was attributed to Cd (50.8%), followed by Cu (41.5%), Ni (5.71%), Pb (0.973%), As (0.784%), and Cr (0.314%), considering the average concentration of these elements. The estimated average daily intake of REEs (13.0 µg/kg/day) from water consumption was below the tolerable daily intake (except for sample M8) and did not pose any non-carcinogenic and carcinogenic health risks. The hazard index calculated for 37 elements, including 14 REEs, from water consumption was extremely high (91.8 ± 131), far exceeding the safe value of the unit, with the highest HQ values for Mn (37.3), Tl (20.1), Cd (14.2), Co (8.29), and Th (2.65). The highest risk was observed for sample M8, followed by M4 and M5, with HIs greater than 100. Regarding the carcinogenic risk, the exposure to Cd, As, and Cr through water resulted in an unacceptable risk for most of the samples. Further studies should explore the geological and hydrogeological conditions of abandoned mines and investigate the efficient treatment of mine drainage water and the remediation of mine-affected areas.