Assessment of Tailings Contamination Potential in One of the Most Important Gold Mining Districts of Ecuador

Abstract

Mining waste presents significant environmental and public health risks due to the potential release of toxic substances when improperly managed. In this study, four tailings samples were taken to evaluate the environmental risks in the Ponce Enríquez mining area in Ecuador. Chemical characterization and X-ray Fluorescence Spectrometry (XRF) were used to analyze the content of potentially toxic elements (PTEs) of interest (As, Cd, Cr, Cu, Ni, Pb, and Zn), and X-ray Diffraction (XRD) for mineralogical characterization. The contamination index (IC) was calculated to assess the potential hazard associated with the content of PTEs in the mining wastes. To assess environmental risks, leaching tests were carried out to evaluate the potential release of PTEs, and Acid-Base Accounting (ABA) tests were conducted to determine the likelihood of acid mine drainage formation. The results revealed that the PETs concentration exceeded the maximum permissible limits in all samples, according to Ecuadorian regulations: As, Pb, and Cd were identified as critical contaminants. Mineralogically, quartz was the dominant phase, followed by carbonates (calcite, dolomite and magnesite), phyllosilicates (chlorite and illite), and minor amounts of pyrite and talc. The IC indicated high to very high contamination risk levels, with As being the predominant contributor. Although leaching tests met the established limits for non-hazardous mining waste, the ABA test showed that all samples had a high potential for long-term acid generation. These results underscore the need for implementing management strategies to mitigate the environmental impacts and the development of plans to protect local ecosystems and communities from the adverse effects of mining activities.

1. Introduction

Mining is an essential activity for the supply of raw materials for the economic and technological development of modern society [1]. However, when not properly managed, it can pose significant environmental challenges due to its potential to generate adverse impacts on surrounding ecosystems and communities [2,3,4].

A major concern is the possible release of potentially toxic elements (PTEs), which are present in tailings [5]. These PTEs, such as As, Cd, and Pb, are well known for their toxicity, persistence, and bioaccumulation capacity. Without proper management, they can compromise soil and water quality and pose a risk to human health [6,7,8]. PTEs persist in the environment, even after the cessation of mining activities, and their impact can extend beyond the original operation site. Therefore, it is crucial to implement effective mitigation and environmental management strategies to ensure the sustainability of mining operations and to protect natural resources in the long term [9].

Pollution from tailings is a widespread concern in mining regions around the world, where inadequate management of these wastes has led to environmental degradation and public health problems [10,11].

Recent studies conducted in Papua New Guinea [12,13] highlight the importance of environmental compliance in mining activities, particularly regarding the desulfurization of mine tailings at the Ok Tedi Mine. These studies emphasize the need to adhere to international environmental regulations when discharging desulfurized tailings into river systems. They caution that continued disposal of these materials may result in serious ecological impacts, such as alterations in river morphology, increased turbidity, and the accumulation of sediments enriched with heavy metals and dissolved contaminants.

In Ecuador, environmental legislation establishes standards for water, soil, and air quality with the aim of preventing and mitigating negative impacts resulting from productive and extractive activities [14]. However, various studies have revealed a significant gap in the implementation of effective instruments for monitoring, oversight, and environmental quality control, leading to limited and, in many cases, ineffective enforcement of regulations aimed at ecosystem preservation [15,16,17,18].

The Ponce Enríquez Mining area in southern Ecuador is an example of how mining activity can boost local socioeconomic development, but also presents environmental challenges due to ineffective mining waste management, the impacts of poorly managed, sometimes illegal or insufficiently regulated mining. Gold mining in the area has generated large volumes of sulfide-rich tailings, materials capable of producing acid mine drainage (AMD) [19]. Inefficient management of mining waste, mainly tailings, has resulted in the contamination of rivers and soils, affecting ecosystem health and local populations [20,21]. Over the past decade, the region has experienced several contamination incidents due to tailings spills into rivers. The most recent case occurred in July 2020, when the collapse of a tailings dam released approximately 50 tons of contaminants into nearby streams and rivers [22].

Previous research has reported high concentrations of PTEs in rivers, sediments and soils in the Ponce Enríquez Mining area [20,23,24]. In 2017 Sierra et al. [25] reported high concentrations of PTEs in surface detrital sediments in the Fermín, Villa, Guanache and Siete rivers, with levels exceeding the probable effect levels (PEL) for Hg, As, Cr, and Cu. In 2021 Jiménez-Oyola et al. [26] documented the presence of As and Cd in soils, as well as As, Cu and Pb in surface waters; and in 2023 Romero-Crespo et al. [27] reported a high content of PTEs in soils and in food grown in local gardens. The values previously reported exceed the environmental quality standards established in Ecuadorian legislation. Other authors like Carling et al. [20] attributed these concentrations to the release and mobilization of PTEs due to inadequate tailings management.

Previous research in Ponce Enríquez has focused on the environmental quality assessment of water resources, soils, and sediments. However, information regarding the characteristics of mining tailings and their pollution potential is limited. There is also a lack of comprehensive data providing a thorough understanding of the contaminant potential of tailings from the Ponce Enríquez mining area and the risks they may pose to the ecosystem and the local communities.

Conducting preventive studies is essential to assess the potential pollution risk of tailings, as it allows the anticipation and mitigation of environmental risks before they occur. These studies help identify the likelihood of contaminant release and the risk of AMD, both of which can severely affect aquatic ecosystems and human health if not adequately managed [28]. Techniques like Acid-Base Accounting (ABA) tests, contamination indices, and leaching tests are commonly applied across various mining settings, yielding favorable outcomes in predicting contamination and guiding corrective actions [29,30,31,32,33,34]. These methodologies provide valuable insights into the potential for long-term acid generation, which is particularly critical in sulfide-rich mining areas [35], such as the Ponce Enríquez case study. By using these approaches, it is possible to anticipate and reduce acid generation and the mobility of toxic elements, minimizing environmental impact and supporting the recovery of impacted areas.

In this context, the present study aims to evaluate the contaminant potential of tailings from the Ponce Enríquez mining field through the physicochemical and mineralogical characterization of tailings samples, quantification of the Contamination Index (IC), leaching tests, and the application of the Acid-Base Accounting (ABA) test. This integrated approach aims to bridge existing knowledge gaps regarding the environmental impacts of mine tailings and provide a solid foundation for decision-makers to design and implement effective environmental management plans. The goal is to minimize the increasing impact of mining waste, ensuring the protection of both ecosystems and the health of affected communities.

2. Study Area

The Ponce Enríquez region is one of the most important gold mining areas in Ecuador, located in the southern part of the country, in Azuay Province (Figure 1). The region covers an area of approximately 644 km² and features an altitudinal range from 13 to 3800 m above sea level, resulting in a variety of climatic zones, including warm, temperate, cold and paramo.

The region has a humid tropical climate, with annual temperatures ranging between 25 °C and 28 °C. Precipitation is seasonal, with a rainy season from December to May (during which most of the annual rainfall occurs, between 1500 and 2000 mm) and a dry season from June to November [36]. Mining activity began in the 1980s [24,37] and has increased rapidly due to the rise in gold prices [38]. In this area, mining operations are primarily carried out through artisanal and small-scale mining (ASM), currently, according to the Agency for Mining Regulation and Control (Originally in Spanish: Agencia de Regulación y Control Minero-ARCOM) there are more than one hundred mining concessions [39], which extract gold from high-temperature hydrothermal arsenopyrite-copper-pyrite veins through underground galleries [24,40]. More than half of the mining concessions have tailings dams, many of which were built near rivers without proper safety standards [41]. These tailings dams were designed using the upstream method, a technique that is not recommended due to its lack of safety, especially considering the region’s geomorphological, climatic, and seismic conditions [42]. Nonetheless, large volumes of tailings are stored on the steep slopes of the area, containing high concentrations of PTEs [19,22]. These tailings dumps are prone to erosion, particularly from heavy rainfall, which washes the material into nearby drainage systems, releasing PTEs that degrade water quality and affect other productive activities [43].

In addition to mining, other productive activities such as agriculture and livestock farming also take place in the Ponce Enríquez area. Agriculture plays a key role in the local economy, as the land is suitable for growing cacao, bananas, yuca, citrus fruits, papayas, sugarcane, corn, potatoes, and rice. The inhabitants also engage in animal husbandry, raising cattle, pigs, and poultry (including chickens, ducks, turkeys, geese, guinea fowl, among others). To the west, toward the sea, shrimp farming is practiced [44]. According to the most recent population estimates [45], the canton has a total of 22,810 inhabitants, approximately 30% of whom are engaged in agricultural activities.

3. Geological Background

The study area is located within the Azuay Mining District, in the Western Cordillera of the southern Ecuadorian Andes, and is bounded by two main drainage basins: the Río Siete basin to the south and the Río Guanache basin to the north [40,46,47]. Regional geology is dominated by alluvial deposits and by the main geological units: the Pallatanga and Yunguilla Formations, the Saraguro Group, and intrusive rocks [40,48].

Alluvial deposits are located on the western flank of the study area, which includes an alluvial terrace at the foothills of the river basins, characterized by relatively flat topography (Figure 1). On the eastern side, where the mountains begin to rise, igneous formations are observed. The Pallatanga Fm. (Cretaceous) consists of oceanic basalt, pillow lavas and blue-green hyaloclastites. It is also characterized by a propylitic mineral association that includes epidote, chlorite, and quartz, with some traces of titanite, illite, and prehnite. The Yunguilla Fm. (Upper Cretaceous-Maastrichtian) is a turbidite sequence composed of sandstones, shales, siltstones, quartz-feldspathic sandstones, and quartz arenites. The Saraguro Group (Late Eocene) mainly encompasses welded ash-flow tuffs of dacitic to rhyolitic composition, andesitic lavas, reworked volcanic material, and sedimentary rocks. There are also minor intercalations of turbiditic sandstones and shales. The Saraguro Group is divided into five formations, but only the Las Trancas Formation is exposed in the study area. Las Trancas Fm. (Oligocene) overlies the Yunguilla Formation and consists of andesitic to dacitic ignimbrites, conglomerates, argillites, muscovite, and quartz-rich sandstones.

The mining district is intersected by a network of fault systems with N–S, NW–NE, and WNW–ESE orientations (Figure 1), of Cenozoic age, which play a major role in controlling mineralization within the district [40,48]. The occurrence of ferromagnesian and carbonate deposits in this region is closely associated with magmatic activity and the presence of these fault structures in the Pallatanga Fm.

The ore deposits in this district are primarily related to gold (Au), copper (Cu), and silver (Ag), and occur within epithermal to mesothermal vein systems. At depth, mesothermal mineralization (Au-Cu-Ag ± As ± Zn ± Pb ± Bi-Te) is dominant, whereas in the upper portions of the system, epithermal open-space filling textures are observed (e.g., crustiform, colloform, banded), reflecting a gradual transition between both mineralization styles [48]. The gangue mineralogy reflects a complex hydrothermal evolution and includes a variety of silicates and carbonates such as quartz, calcite, dolomite, epidote, chlorite, sericite-muscovite, biotite, tourmaline, chalcedony, ankerite, smectite, and barite.

4. Materials and Methods

4.1. Sampling Collection

The tailings dams sampled in this study are situated along a ~7.5 km segment adjacent to the Chico River, which flows westward and discharges into the Pacific Ocean. Tailings samples were collected in October 2023 from four facilities (T-1, T-2, T-3, and T-4), operated by three small-scale gold mining companies (Figure 1). A total of 30 sub-samples, each weighing approximately 0.5 kg, were obtained using a stratified random sampling method designed to enhance spatial representativeness and minimize sampling bias. Within each tailings dam, several sub-samples were systematically collected from predefined zones (e.g., upstream, mid-slope, and downstream sectors), based on observable differences in grain size and moisture content, to account for internal heterogeneity.

Sampling was conducted manually using a clean, plastic shovel at depths between 30 and 40 cm, following the standard protocol by Smith et al. [49]. To prevent cross-contamination from surficial material potentially influenced by atmospheric deposition or anthropogenic disturbance, the uppermost layer was carefully removed prior to sampling. Samples were immediately sealed in pre-labelled polyethylene bags to avoid moisture loss and contamination, and subsequently stored at room temperature under dark, dry conditions until laboratory processing.

4.2. Characterization of Tailings Samples

4.2.1. Physical Characterization

Particle size analysis was conducted at the Soil and Construction Laboratory at Escuela Superior Politécnica del Litoral (ESPOL) with the objective of quantifying the fine fraction in the samples, following the standardized protocol of the American Society for Testing and Materials (ASTM) guidelines [50]. Subsequently, wet sieving was performed using a No. 200 sieve to isolate the fraction smaller than 0.075 mm, in accordance with ASTM D-1140 [51]. This procedure was carried out solely for granulometric purposes; the washed fractions were not used for geochemical analyses to avoid dissolution of soluble salts or release of trace elements associated with fine particles.

The material retained on the 0.075 mm sieve was then oven-dried and subjected to dry sieving, following ASTM D6913 standard [52]. This analysis included the use of sieves with mesh sizes No. 10 (2 mm), No. 16 (1.18 mm), No. 30 (0.6 mm), No. 50 (0.3 mm), No. 100 (0.15 mm), and No. 200 (0.075 mm). Granulometric characterization allowed the determination of the fine fraction proportion in the tailings, a key parameter to understand their physical behavior and potential geochemical reactivity in tropical humid environments.

The pH and electrical conductivity (EC) were measured in the Water and Sanitation Laboratory at ESPOL using a HACH (Loveland, CO, USA) HQ40D multi-parameter device on the supernatant of a mixture prepared with deionized water and tailings sample, using a liquid-to-solid ratio (L/S) of 1:1, following the methodology described by Peech [53].

4.2.2. Chemical and Mineralogical Characterization

The PTEs (As, Cd, Cr, Cu, Ni, Pb, and Zn) were determined by Monochromatic Energy-Dispersive X-Ray Fluorescence (MEDXRF) using an E-max Portable Heavy Metal Analyzer (Z-Spec, East Greenbush, NY, USA). For each tailings sample collected directly in the field, a quartered section was ground to a particle size of 850 µm to ensure sample homogeneity and accurate instrument readings. The limit of quantification was: As = 0.2 ppm, Cd = 0.05 ppm, Cr = 5 ppm, Cu = 0.5 ppm, Ni = 1 ppm, Pb = 0.8 ppm, and Zn = 0.5 ppm.

Additionally, four other samples were ground to a particle size of <10 μm. The mineralogical analysis of these samples was performed with an XRD Rigaku (Tokyo, Japan) MiniFlex 300/600 diffractometer. An X-ray tube with Cu anode material was used as a radiation source, with a Cu-Kα target tube. The primary divergent slit was 0.625° and the axial soller slit 1.25°. Scans were conducted over a 2Ɵ range of 10–110° with a total exposure time of 14 min. For the regular samples, a step size of 0.05° and a counting time of 56.9 s/step were applied. This equipment enabled the identification of mineral phases based on the crystal structure. Mineral identification was carried out using SmartLab Studio II software V4 (Tokyo, Japan) from Rigaku, employing the Reference Intensity Ratio (RIR) method for phase quantification due to its simplicity, low computational requirements, and suitability for rapid analysis using the Crystallography Open Database (COD) [54]. The resulting mineral abundances are semi-quantitative and provide approximate estimations.

4.3. Assessment of the Contamination Potential of Mining Tailings

4.3.1. Index of Contamination (IC)

This index is proposed to quantify the probability that PTEs found in tailings may contaminate soils and sediments due to erosion processes. It is calculated as the ratio of total PTE concentrations in tailings to their respective reference levels [31,55,56]. The calculation was performed using Equation (1):

$$IC={\displaystyle \frac{1}{n}}\sum _{i=1}^n{\displaystyle \frac{{\left[X\right]}_i}{N{F}_x}}(\mathrm{e}\mathrm{c}.1)$$

(1)

where Xi represents the concentration of each analyzed element in the sample, NFX is the background level of the element X, and n is the number of elements for which the concentration in the tailings exceeds the NFX. The reference levels (in mg/kg) used for the study were As (4.8), Cd (1.1), Cr (42), Cu (14), Ni (18), Pb (25), and Zn (62) [57,58].

The IC value was interpreted according to the following assessment scale: very low contamination potential (1 ≤ IC < 3.5), low contamination potential (3.5 ≤ IC < 6.5), moderate contamination potential (6.5 ≤ IC < 10), high contamination potential (10 ≤ IC < 15.6), and very high contamination potential (IC ≥ 15.6) [58].

4.3.2. Leaching Test

To determine the leaching potential of the PTEs from the tailings, the static leaching protocol proposed by the United States Geological Survey was applied [59]. Deionized water was used as the extraction fluid to simulate conditions where the waste comes into contact with solutions that have low buffering capacity, such as rainwater. The particle size was below 2 mm, and the liquid-to-solid (L/S) ratio for the test was 20:1. Following the procedure, manual agitation was performed for 5 min. After agitation, the eluates were extracted and filtered using a vacuum filtration device (0.45 µm).

The pH, electrical conductivity, and concentrations of As, Cd, Cr, Cu, Ni, Pb, and Zn were measured in the leachates by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) with a Thermo Scientific (Waltham, MA, USA) iCAP 7400 equipment, applying the EPA method 200.7, revision 4.4 [60]. Certified reference materials were used: NIST SRM 3103a, NIST SRM 3112a, NIST SRM 3114, NIST SRM 3136, and NIST SRM 3128. The recovery rates of the measurements ranged from 85% to 110%, indicating good accuracy and reliability of the leaching test results. The results obtained were compared with the reference levels for water quality according to the “Guideline for Risk Assessment of abandoned mining wastes” by Alberruche del Campo et al. [61]. Although the leaching protocol used in this study follows the static method developed by the USGS [62], the comparison with water quality guidelines is considered valid. The European guideline explicitly endorses the use of leaching tests with deionized water from sources such as the USGS and USEPA, particularly for assessing toxicity in abandoned mining environments. Therefore, despite the difference in solid-to-liquid ratios between methods (1:20 vs. 1:10), the analytical framework remains consistent and appropriate for environmental screening and prioritization.

4.3.3. Acid-Base Accounting (ABA) Test

The ABA test is a static procedure that assesses the acid-base balance of a material [5,28]. This method was applied to estimate the potential of tailings to produce or neutralize acidity [63,64]. The Net Neutralization Potential (NNP) was calculated as the difference between the Neutralization Potential (NP) and the Acid Potential (AP), serving as an indicator of whether the sample tends to generate or consume acidity. Both AP and NP are expressed in kilograms of calcite equivalent per ton of material (kg CaCO₃/t) and are determined from the total sulfur and carbon contents in the mining waste [65].

To classify materials, the following criteria are used: an NNP value less than −20 kg CaCO₃/t indicates that the material is acid-generating, while an NNP above 20 kg CaCO₃/t indicates it is acid-neutralizing. There is an uncertainty zone for this technique between 20 > NNP > −20 kg CaCO₃/t [65,66,67].

5. Results and Discussion

5.1. Physicochemical Characterization

Regarding the physical characterization, it was noted that less than 10% of the material was finer than 0.075 mm in samples T-2, T-3, and T-4. In contrast, sample T-1 contained 70% fine particles (≤0.075 mm). Samples T-3 and T-4 showed D80 values of 0.17 mm and 0.19 mm, respectively, whereas the D80 of sample T-2 was 0.37 mm; these particle sizes correspond to the fine sand range [50] (Figure S1).

Particle size is known to influence sulfide oxidation processes because finer particles present a greater surface area exposed to water and oxygen. This larger reactive surface accelerates the generation of acid mine drainage (AMD), increasing the likelihood of its occurrence [5].

Table 1. Physicochemical characterization and total content of PTEs in tailings samples from Ponce Enriquez mining area.

Parameter	MPL a	T-1	T-2	T-3	T-4
pH	8-Jun	7.17	8.04	8.09	7.3
EC (µS/cm)	200	1495	249	181.5	380
As (mg/kg)	12	2100	429.82	1200	4466
Cd (mg/kg)	0.5	1.14	0.2	1.78	5.18
Cr (mg/kg)	54	143.84	268.36	196.9	263.61
Cu (mg/kg)	25	163.14	57.82	68.11	105.43
Ni (mg/kg)	19	39.86	52.45	41.29	537.96
Pb (mg/kg)	19	71.2	2.73	49.57	410.55
Zn (mg/kg)	60	127.68	40.94	131.26	313.09
S (%)	60	1.66	0.16	0.16	1

^a Reference levels according to Unified Text on Secondary Environmental Legislation [14].

presents the pH values and the concentration of PTEs in the tailing samples. The pH ranged from 7.17 to 8.09, indicating neutral to slightly alkaline conditions. This may be due to the common practice of adding calcium carbonate to the tailings to neutralize the acidity of the waste.

The higher electrical conductivity observed in T-1 (1495 µS/cm) can be attributed to the significant content of fine particles (70% ≤ 0.075 mm), which enhances mineral-water interactions. This results from a combination of increased specific surface area, reduced permeability, and intensified geochemical processes that promote the release of dissolved ions into the solution [68,69].

All samples exceeded the MPL of PTEs, with As being the most critical according to the Unified Text on Secondary Environmental Legislation from Ecuador [14]. Sample T-4 exhibited the highest levels of PTEs, followed by samples T-1, T-3, and T-2 (Figure S2). As was the primary contaminant, exceeding the MPL by 372, 175, 100, and 35 times in samples T-4, T-1, T-3, and T-2, respectively. Ni and Pb exceeded the MPL by up to 28 times in the sample T-4. Although samples T-3 and T-4 belong to the same company, their results vary considerably, with T-4 being more critical than T-3. The discrepancy was attributed to the natural dynamic of mining operations, where these facilities often process materials from different operators, leading to significant heterogeneity in the processed material.

Furthermore, the average PTE content of the tailings samples was compared with the local background (Bi) values reported by SES [70]: 7.7 for As, 0.17 for Cd, 60 for Cu, 5.4 for Pb, and 64 for Zn, in mg/kg. Also, an enrichment of PTEs was observed in the following decreasing order: As > Pb > Cd > Zn >Cr, with enrichment values of 213.2, 20.5, 10.3, 2.1, and 1.3 times the Bi, respectively.

A comparison with the bulk rock geochemistry carried out in 10 rock samples in the Bella Rica mining area (located 10 km southwest of the sampling area) [40], which also belong to the Pallatanga Formation. The concentration ranges in the rock samples (in ppm) were: Cd (0.34–4.62), Cr (22–207), Cu (53–232), Ni (15–103), Pb (3.11), and Zn (40–1358). The average PTE concentrations in the tailings samples were found to be enriched in the following decreasing order: Pb > Ni > Cr > Cd > Cu > Zn, with respective values 171.7, 7.9, 5.2, 4.5, 3.2 and 2.9 times higher than the average values reported in the rock samples.

These results align with those reported by Salgado-Almeida et al. [17], where the As, Cu, Pb, and Zn concentrations in eight tailings samples from the Zaruma-Portovelo mining area (another significant small-scale gold mining district in Ecuador) exceeded the MPL. In Ponce Enríquez, the As concentrations exceeded up to 481 times the MPL, followed by Pb and Zn, which exceeded 300 times, and Cu up to 70 times the MPL.

5.2. Mineralogical Characterization

The mineral composition was dominated by quartz (55.56%–58.82%) across all samples. Carbonate minerals such as dolomite (3.11%–24.8%), calcite (3.58%–27%), and magnesite (5.39%) were also present. Sample T-2 exhibited the highest calcite content (27%), while T-3 showed the highest dolomite concentration (24.8%). Chlorite and illite were the main phyllosilicates identified, with T-4 containing the highest chlorite (20.98%) and illite (7.79%) levels. Pyrite was only identified in sample T-1 (1.38%), indicating an acid generation potential. Talc was only identified in sample T-4 (8.11%) (

Table 2. Semi-quantitative mineralogical composition of tailing samples.

Minerals	T-1	T-2	T-3	T-4
Quartz	55.03%	58.20%	58.82%	50.56%
Calcite	13.92%	27.00%	9.51%	3.58%
Dolomite	9.82%	5.00%	24.80%	3.11%
Chlorite	16.50%	5.60%	5.20%	20.98%
Illite	3.34%	4.20%	1.67%	7.79%
Magnesite	-	-	-	5.39%
Pyrite	1.38%	-	-	-
Talc	-	-	-	7.37%

). Most mineralization (quartz, chlorite, illite and calcite) was associated with mafic volcanic rocks, evidenced in Pallatanga Fm, which is relevant for mining operators due to its significant gold mineralization potential.

According to these results, the minerals identified were silicates, carbonates, and a sulfide. Among the carbonates, calcite, magnesite and dolomite are known for the neutralization potential, where calcite is the most significant neutralizing mineral commonly found in mining environments [71]. This is due to its broad reactivity and its ability to neutralize acids across a wide pH range [72]. In addition, their dissolution mobilizes alkaline earth elements and cations that participate in the formation of secondary solids (metal hydroxides or sulfated hydroxides) [73,74]. Dolomite and magnesite were also found and have high neutralization capacity, although lower solubility compared to calcite [75]. However, the results suggest that in the tailings studied, calcite and dolomite alone are insufficient to neutralize all acidity generated, as most samples show a high potential for acid mine drainage (AMD) formation.

Regarding silicates, quartz was identified in all samples, but it is considered an inert material in the AMD evaluation [72,75,76]; however, microfractures or irregular surfaces in quartz can serve as adsorption sites for heavy metals, although to a lesser extent than other minerals. In tailings contexts, quartz acts as a gangue mineral that dilutes the total metal concentration in the sample [77]. Micas, which are phyllosilicate minerals, are primarily composed of aluminum-magnesium or aluminum-potassium structures. They can absorb heavy metals such as Cr, Cd, and Zn on their surfaces, especially in weathered or altered environments, due to their high cation exchange capacity. Chlorite, often associated with hydrothermal alteration zones, can act as a “trap” for metals through adsorption or ion-exchange processes [78].

Talc is a geochemically neutral mineral, although it can retain small amounts of Cr due to its origin in ultramafic or metamorphic environments, where chromium is common. Studies have shown that soils from weathering ultramafic bodies exhibit high concentrations of metals such as Cr and Ni, suggesting that minerals derived from these rocks, such as talc, can contain and retain these metals [68,79]. It can absorb heavy metals from solutions rich in metallic cations, acting as a retention surface. As shown in Table 1, sample T-4 (the only one in which talc was detected) contains the highest amount of Ni and, along with T-2, exhibits the highest Cr concentrations.

Finally, regarding sulfides, only pyrite was identified, which is the most common and abundant sulfide that exists and is the main cause of AMD [75,80,81,82]. It is closely associated with the presence of arsenic and heavy metals, and it can also be associated with Pb, Cd, and Zn through co-precipitation with other sulfides. In hydrothermal environments and sulfide deposits, pyrite may contain trace amounts of arsenic, either incorporated into its crystal structure as a substituent or adsorbed onto its structure [83,84,85]. In Table 1, Sample T-4 shows the highest concentrations in almost all metals (except for Cu), followed by T-1 in Cu and As.

It is common to find only small amounts of sulfide minerals in mine tailings, as these minerals are often removed during the flotation processes typically used in gold, silver, and copper extraction. These metallurgical techniques are designed to concentrate sulfide minerals, leaving behind tailings that are relatively depleted in sulfides. However, this does not imply the absence of potentially toxic elements (PTEs) associated with sulfides, as these may not be present in crystalline form but rather remain in solution or as amorphous phases. These dissolved or poorly crystalline species can still contribute to the mobilization and environmental risk of PTEs, particularly under acidic or oxidative conditions.

5.3. Assessment of the Potential Contamination of Mining Tailings

5.3.1. Contamination Index

The contamination index (IC) results reveal very high levels of contamination, with IC values ranging from 34.96 to 204.02. In descending order, the samples with the highest contamination potential are T-4 > T-1 > T-3 > T-2 (Table S1), indicating a significant hazard to the environment. Arsenic (As) is the most polluting element in the samples, followed by chromium (Cr), zinc (Zn), and copper (Cu), while cadmium (Cd) is the least polluting.

The T-4 sample showed the highest IC value and was located near the upper reaches of the Chico River. Therefore, the potential release of contaminants from this pond could lead to increased pollution and significant impacts on the water quality in the study area. Figure 2 illustrates the percentage contribution of each PTE to the total IC. As is the primary contributor to these results.

Due to the proximity of the tailings facilities to surface water sources, these surface residues, resulting from erosion and weathering processes, may be mobilized by wind and runoff, potentially affecting surface water quality [18,26]. Therefore, effective management of mining waste deposits is crucial to mitigate environmental impacts and safeguard water resources.

The results of this study are consistent with Salgado-Almeida et al. [17], who reported very high IC levels (ranging from 25.20 to 178.50) in six tailing dams located in the upper part of the Puyango river basin, southern Ecuador. Similarly, Guzmán-Martínez et al. [31] reported high to very high IC levels (11 < IC < 220) in ten abandoned tailing dams in Spain, and Arranz-González et al. [30] documented IC > 13 in an abandoned tailing area in the Riotinto Mining District in Spain. In concordance with Thiombane et al. [86], who applied the Robust Compositional Soil Contamination Index (RCCI) to assess soil contamination in the Kedougou mining region (Senegal, West Africa), found that the highest RCCI values were observed in areas with gold mining activities, potentially leading to a relatively significant release of PTEs into the environment.

5.3.2. Leaching Test

Leaching is the process by which the PTEs in mining waste are released to the environment when they come into contact with water. Therefore, it is crucial to understand the extent of PTEs release to assess the environmental, ecological, and human health risks [32]. The data obtained in the leaching tests were compared with the reference values for preservation of freshwater quality for wildlife established in Unified Text of Secondary Environmental Legislation for Ecuador [14], and with the reference values for water quality according to the “Simplified guide closed/abandoned mining waste facilities risk assessment” [61] (

Table 3. Release of PTEs obtained (in mg/L) from the leaching test.

Parameter	MPL a	MPL b	T-1	T-2	T-3	T-4
pH	6.5–9	6.5–8.5	7.01	7.89	8.02	7.18
As	0.05	0.01	0.02	0.02	0.18	0.17
Cd	0.001	0.00025	<0.05	<0.05	<0.05	<0.05
Cr	0.032	0.05	<0.01	<0.01	<0.01	<0.01
Cu	0.005	0.01	<0.05	<0.05	<0.05	<0.05
Ni	0.025	0.02	<0.05	<0.05	<0.05	<0.05
Pb	0.001	0.01	<0.01	<0.01	<0.01	<0.01
Zn	0.03	0.12	<0.05	<0.05	<0.05	<0.05

^a Surface water quality guidelines set by the Ecuadorian legislation [14]. ^b Simplified guide closed/abandoned mining waste facilities risk assessment [61].

).

The pH of the leachates was within the range recommended in Ecuadorian water quality regulations (6.5 < pH < 9). Regarding the PTEs content in leachates, it was the element of greatest concern since it exceeds the MPL presented by Alberruche del Campo et al. [61]. Tailings T-3 and T-4 present similar concentrations of As, exceeding the MPL by at least 17 times, while tailings T-1 and T-2 doubled the permitted content of As.

As previously noted, mining wastes in the study area are derived from the processing of gold ores, mainly from arsenopyrite-copper-pyrite veins [24,40]. Since As is contained in sulfide minerals, As, together with other PTEs, can potentially be mobilized due to the weathering of tailings and sediments, since, as is known, sulfides in the presence of water, oxygen, and microorganisms can oxidize and solubilize PTEs [87]. Martínez-López et al. [88] state that the oxidation of sulfides such as arsenopyrite is the major source of As in mine tailings. That mineral can be oxidized by oxygen and Fe³⁺, with the oxidation rate by Fe³⁺ higher than that of pure pyrite. Therefore, as a further line of research, it is recommended to carry out chemical speciation studies of As in the study area, since it is known that its toxicity decreases as the degree of methylation increases (As (III) > As (V) > MA > DMA). On the other hand, according to the pH results of the leachates, it is relevant to comment that As can become mobilized even at neutral pH [89,90]. Hageman et al. [35] comment that anions such as As are usually more soluble at high pH.

The presence of As above regulatory limits underscores the need for further investigation into the sources of release/contamination and potential response strategies, as these PTEs may be entering the water system through wind-borne mobilization of fine particles or runoff. Evidence shows high levels of As in surface waters from the Ponce Enríquez mining area, with concentrations over 100 µg/L, where the MPL is 50 µg/L according to the TULSMA [91]. Additionally, As has been reported in soils, with concentrations up to 200 mg/kg [26].

The Cd, Cr, Cu, Ni, Pb and Zn content in the tailing sample leachates was below detection limits for the ICP-OES analysis; the absence of detection indicates that their actual concentration could not be quantified, which could hide levels that could exceed the MPLs and be harmful to the environment. Conversely, Cr showed concentrations below 0.01 mg/L; that is, the Cr content is below the reference MPL in all samples.

5.3.3. Acid-Base Accounting (ABA) Test

Table 4. ABA test results.

Sample	AP (kg CaCO3/t)	NP (kg CaCO3/t)	NNP
T-1	51.87	–170.5	–222.37
T-2	5.00	–250.00	–255.00
T-3	5.00	–160.00	–165.00
T-4	31.25	–160.00	–191.25

summarizes the ABA test results. All samples exhibited highly negative NNP values (<−20), indicating a strong potential for acid generation with insufficient neutralization capacity. This pattern suggests that these materials could significantly contribute to acid generation in the environment, potentially leading to environmental harm if leached or exposed to water sources.

These findings raise concerns about the potential for environmental contamination in areas surrounding the mine waste deposits. Given the high acid-generating potential in the tailings studied, it is important to consider appropriate management or treatment strategies, such as the addition of neutralizing agents or encapsulation, to limit exposure to water and air. This approach could help mitigate the environmental risks associated with their acid-generating behavior. It is well known that AMD can contain PTEs, which, due to their persistence in the environment and high toxicity, pose threats to aquatic ecosystems, biodiversity, and human health through bioaccumulation in the food chain.

6. Management Recommendations

The potential for acid generation revealed in the study highlighted the importance of mine tailings management and the lack of government monitoring policies to avoid environmental damage and public health effects. Furthermore, the large volumes of solid and liquid waste generated, combined with a shortage of qualified technicians, limited institutional capacity, and the lack of comprehensive environmental management guidelines within the mining control agency, exacerbate the problem [17,92].

The promotion of more sustainable extraction and waste management practices with strict regulations must be prioritized and implemented. The integration of the desulfurization method has the potential to reduce the acid-generating capacity of sulfide tailings. The separation of non-reactive sulfide materials from reactive sulfide concentrates is usually established between beneficiation and waste tailing management facilities through a flotation process using specific reagents [12].

These regulations should involve the implementation of stringent rules for managing mining waste, including mandatory reporting on contaminant mobilization control practices. In addition, ongoing environmental monitoring should be conducted to assess water, soil, and air quality in regions surrounding tailings sites, enabling the detection of contamination shifts and the prompt application of corrective actions. Public education is also essential, raising awareness in local communities about the dangers of exposure to PTEs and encouraging sustainable use of natural resources. Finally, the requirement and support of programs to rehabilitate polluted areas by designing and applying effective remediation and restoration plans to mitigate long-term environmental damage and help recover the health of affected ecosystems.

Public education and community engagement programs are essential, aimed at raising awareness among local populations about the risks of exposure to PTEs and fostering responsible, sustainable use of natural resources.

Moreover, the development and funding of rehabilitation programs are critical to restoring polluted areas. These programs should focus on designing and applying scientifically sound remediation and ecological restoration plans to mitigate long-term environmental impacts and support the recovery of affected ecosystems

An alliance of government with the public universities who develop interdisciplinary research could be promoted. This involves sharing study results, successful practices and cutting-edge technologies for mining waste treatment and reducing environmental harm. Additionally, sustained efforts from governments, the mining sector, and civil society are crucial to effectively tackling these issues and supporting long-term sustainable growth.

7. Conclusions

This study presents the first integrated characterization of four mining tailings ponds in the Ponce Enríquez gold mining area of Ecuador, conducted to assess the environmental risk of contaminant mobilization to the surrounding environment and the potential generation of acid mine drainage. The physical characterization revealed significant variability in particle size distribution among the samples. Samples T-2, T-3, and T-4 had less than 10% of material with a thickness below 0.075 mm, whereas T-1 had 70% fine particles. These results were consistent with the higher electrical conductivity of T-1. Samples T-3 and T-4 had finer particles, while T-2 contained coarser material, classifying it in the fine sand range. These findings highlight the distinct physical properties of the tailings, which may influence their behavior in terms of PTEs mobilization and environmental risk.

The mineralogical analysis showed that the tailings are dominated by quartz, followed by carbonates (calcite, dolomite and magnesite), phyllosilicates (chlorite and other micas), minor pyrite, and talc. This composition suggests a mixed geochemical behavior, where carbonates may provide acid neutralization potential, while pyrite is a key contributor to acid generation and heavy metal presence. The association of pyrite with As, Pb, Cd, and Zn is particularly relevant, as is the potential of chlorite and other micas to adsorb metals such as Cr, Cd, and Zn. The detection of talc, chlorite, illite and calcite is typically linked to ultramafic environments and gold mineralization related to Pallatanga Fm., and could also explain localized Cr retention. The mineral assemblage is consistent with the heavy metals detected in the samples, reinforcing the role of mineralogy in influencing the mobility and fate of PTEs in the tailings.

Chemical analysis of the tailings allowed us to identify elements with environmental relevance due to their toxicity, particularly As, whose concentration exceeds between 35 and 370 times the MLP established in environmental regulations. The ABA test indicates that the tailings are potentially acid-forming under their current conditions. In addition, the results of the leaching tests indicated potential for As leaching in all four tailings dams evaluated. This fact contributes to it being considered a hazardous waste, with a high potential to impact the environmental quality of the area if not managed properly. In the case of tailings mobilization or inappropriate storage, the generation of AMD and contamination by PTEs, mainly as a result, is likely to occur. Therefore, this study highlights the critical importance of proper management and monitoring of mining tailings to prevent significant environmental contamination, particularly from As, ensuring the protection of water quality and ecosystems in the area.

Although this study is based on the analysis of composite samples from four mine tailings facilities, the authors emphasize the significance of the findings, particularly the identification of arsenic as a critical contaminant and the high potential for acid generation in the tailings, factors that pose a significant risk to the environment and public health. In this context, future efforts should focus on expanding the sampling network and applying more robust analytical methodologies to generate more representative and reliable data. The results of this study provide valuable information for decision-makers in the Ponce Enríquez mining district, as they can support the design of future projects, environmental monitoring and control activities, and targeted intervention strategies in the area.