International Journal of Geosciences
Vol.05 No.03(2014), Article ID:44229,16 pages

Contamination Potential of an Urban Mine Tailings Deposit in Central Mexico —A Preliminary Estimation

Liliana Lizárraga-Mendiola1*, Diana Elizabeth Ángeles-Chávez2, Alberto Blanco-Piñón2, Màrius Ramírez-Cardona2, Francisco Javier Olguín-Coca1, María del Refugio González-Sandoval1

1Academic Area of Engineering, Institute of Basic Sciences and Engineering, Autonomous University of the State of Hidalgo, Pachuca, Mexico

2Academic Area of Earth and Materials Sciences, Institute of Basic Sciences and Engineering, Autonomous University of the State of Hidalgo, Pachuca, Mexico

Email: *

Copyright © 2014 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

Received 21 January 2014; revised 15 February 2014; accepted 11 March 2014


This study consisted of a geochemical analysis of the Dos Carlos tailings’ deposit located in the Mining District of Pachuca-Real del Monte in the state of Hidalgo, Central Mexico. The goal of the study was to determine the potential effects of this deposit on the environment and health of the population of the metropolitan area of Pachuca. Sampling was conducted from the top to the base of two raised sections at opposite ends of the deposit, and macroscopic features (profiles A and B) of these deposits were evaluated. Subsequently, mineralogical analyses of the collected samples were performed using X-ray diffraction and physico-chemical analysis of the leachates. The results were compared with the maximum permissible limits established by different national and inter- national standards for drinking water and hazardous waste. In addition, geochemical modeling was conducted using PHREEQC to calculate the distribution of aqueous species, ionic activities and saturation indices. Of the 35 samples of leachate analyzed for profile A, two samples had two to nearly three times the national permissible maximum limits of Cr6+ in drinking water, four samples had levels that exceeded these limits and 19 samples had levels that exceeded these lim- its. Of the 10 samples analyzed from profile B, six samples had levels that were up to fourteen times higher than the limit for human consumption, and six samples had levels of that were up to 28 times higher than the permissible limit. The predominant minerals found in the Dos Carlos tailings’ deposit were quartz, feldspar, clay from the kaolinite group, and calcite and orthopyroxene (enstatite-ferrosilite series). The samples from profile A indicated that minerals con- taining Agmetallic, Cd2+, Cu+, Ni2+, Pb2+ and Zn2+ were dissolved, whereas minerals containing Fetotal, Mn2+, Pb(OH)3 and Zn2+ were in a state of precipitation. In profile B, the dissolution of minerals containing Agmetallic, As5+, Cd2+, Cu+, Mn2+, Ni2+, Pb2+ and Zn2+ was observed only in a superficial sample, whereas Agmetallic, Fetotal, Mn2+, Pb3(PO4)2 and PbHPO4 were found in a precipitated state. The above-mentioned species that occurred in a state of dissolution pose an environmental threat to the groundwater in the study area; indeed, the tailings’ deposit provides the conditions neces- sary for these minerals to leach and infiltrate into the subsoil or be transported by wind, depo- sited in the soil of the surrounding area and inhaled by the nearby population, potentially com- promising respiratory health.


Contamination; Dos Carlos Tailings; Mining District Pachuca-Real del Monte

1. Introduction

In Mexico, one of the major byproducts of mining activity is the generation of millions of tons of tailings or slurries. Some mining sites in the country have had produced tailings with environmental impacts, such as those located in the Mexican localities of Taxco, Guerrero [1] - [3] ; Guanajuato, Guanajuato [4] - [7] ; Santa Bárbara, Chihuahua [8] and Tizapa, Estado de México [9] - [11] . However, there are many sites where the source of con- tamination from tailing deposits is not obvious. One example is the tailings from the mining unit of Charcas, San Luis Potosí, which does not represent potential generators of acidity and does not contain soluble toxic elements; therefore, it is not considered hazardous [12] .

The state of Hidalgo, an area with significant environmental problems, contains mining waste from the Zi- mapan district. The waste from this district is classified as dangerous because of its toxicity; in particular, even in stable forms, the arsenic content of the waste can contaminate the environment [13] . In the city of Pachuca, the accumulation of tailings in the urban area began in 1912 by order of the Department of Mining of the Secretariat of Industry and Trade [14] ; this order was aimed at storing mining waste until techniques to recover the silver contained within the waste could be improved [15] . The Dos Carlos tailings’ deposit began in response to this order. In 1959, this deposit contained approximately 55 million tons of waste [16] . Forty-seven million tons of these tailings were produced by the cyanidation of minerals from all parts of the mining district; eight million tons came from the only flotation plant of the Mining District of Pachuca-Real del Monte [17] . To date, the tailing deposits of the district have been investigated for a variety of purposes [18] [19] ; however, few investigations have evaluated the individual deposits for environmental purposes. In this context, this paper presents a preliminary investigation of the contamination potential that the Dos Carlos tailings’ deposit of the Mining Dis- trict of Pachuca-Real del Monte in the state of Hidalgo presents to the environment and the health of the sur- rounding population.

2. Description of the Study Area

2.1. Location

The tailings deposit known as Dos Carlos is located at the coordinates 20˚06'20" latitude north and 98˚42'46.8" longitude west to the north of the municipality of Pachuca in the state of Hidalgo (Figure 1). The deposit has an elevation of 2437 meters above sea level (masl) [20] .

2.2. Climate

According to the climatic classification of [21] and the modified classification of [22] , the city of Pachuca has a dry climate of subtype BS1k’w(w)(I’). The rainy season usually occurs from April to September, and maximum average precipitation levels (203.9 mm) occur in the month of May; however, in some years, the rainy season can begin in March and extend through late October or early November. In general, Pachuca has a temperate

Figure 1. Location of the study area (No scale).

climate. Its annual average temperature oscillates between 11.3˚C and 16.6˚C. The highest temperatures occur in April (maximum temperatures of 28.8˚C) and the lowest temperatures occur in January (minimum temperatures of −3˚C). Most of the year is dominated by winds that blow in a north-south and northeast-southwesterly direc- tion and reach an average speed of 22 to 24 m/s [23] .

2.3. Description of the District and Tailings Deposit

Pachuca-Real del Monte is one of the oldest silver-producing mining districts of Mexico. It is located in Sierra de Pachuca in the metallogenic province of the Mexican Neovolcanic Axis. The site is characterized by its epi- thermal polymetallic deposits of silver, lead, zinc, copper and gold [24] . Hundreds of productive veins are scat- tered along a surface with an area of 70 km2; this area has been exploited for more than 500 years and has yielded 40 million kg of silver from a total of 100 million metric tons of ore with 1% of total sulfides. These veins have a series of calc-alkaline volcanic rocks from the Oligocene-Miocene age (Figure 2) with a thickness of 2700 m [25] .

The tailings deposit, which is surrounded by housing complexes, is located in the northern part of the city, approximately 10 meters from the Sosa riverbed. Because it is directly exposed to the weather conditions, the mining waste deposit site was selected according to the area and volume of the site and its state of neglect and inactivity. We found no indication that the site is monitored, and no studies have been performed to analyze the behavior of the chemical speciation of the minerals of the site; consequently, the environmental contamination potential of these tailings and their effects on the health of the population are unknown.

3. Methods

3.1. Field Work

Two raised sections at opposite ends of the Dos Carlos tailings deposit were described based on the specifica- tions established by the NOM-141-SEMARNAT-2003 [26] , the NMX-AA-132-SCFI-2005 [27] and the NOM-052-SEMARNAT-2006 [28] . The selection criterion was that the deposit has had several periods of de- position, therefore, the profiles selected represent the oldest and most recent stage of mineral processing. The northeast end (profile A), which is approximately 5.32 m high with respect to the natural terrain, contains the most recently deposited tailings (deposited during the 1980s); the southwestern end (profile B), which is ap- proximately 7 m high, contains the older tailings deposited in the 1950s (Figure 1). Macroscopic characteristics such as the thickness of the layer or stratum, particle size, color and presence of oxidation were described in both sections (Tables 1(a) and (b)).

Figure 2. Stratigraphic column of the Mining District Pachuca-Real del Monte. No scale (Edited from [17] ).

A total of 45 samples were collected from the top to the base of the studied sections based on changes in the different aforementioned macroscopic characteristics (35 from profile A and 10 from profile B). The collected samples were sieved (mesh #60) to remove plant debris and non-tailing rock fragments. Subsequently, the sam- ples were weighed to determine their total weight in grams, placed in pre-labeled polyethylene bags and trans- ported to the Environmental Geology laboratory of the Academic Area of Earth and Material Sciences of the Autonomous University of the state of Hidalgo for analysis.

3.2. Laboratory Work

The following parameters were evaluated in the leachates: a) percent humidity, b) physical parameters such as

Table 1. (a) Macroscopic characteristics of leachates in profile A. (b) Macroscopic characteristics of profile B.



temperature, pH, electrical conductivity (EC), redox potential (Eh) and dissolved solids (measured with elec- trodes, models HI98120 and HI98129-Hanna® instruments, USA-), c) levels of ions such as Cr6+, Cu+, , , , P+, SiO2 and Zn2+, d) mineralogical contents using X-ray diffraction, e) geochemical modeling, and f) average daily dose.

3.2.1. Percent Humidity

To determine the percent humidity, the samples were sieved and weighed in the field (Ph) and desiccated in an ORL-V (O.R.L., USA) oven for 24 hours at 150˚C to eliminate the existing moisture. The samples were then weighed again (Ps). The percentage of total humidity (%th) was calculated:


3.2.2. Physical Parameters and Levels of Ions

Parameters such as temperature, dissolved solids, pH and electrical conductivity were evaluated with a Combo pH and EC electrode, model HI98129 (-Hanna® instruments, USA), that was previously calibrated with refer- ence buffers (pH 4.01 ± 0.01 at 25˚C and pH 7.00 ± 0.01 at 25˚C). The oxidation-reduction potential (ORP or Eh) was measured under the same conditions used for the pH and EC with a Hanna Instruments ORP potentiometer, model HI 98120 calibrated by the manufacturer (-Hanna® instruments, USA).

Subsequently, the samples were prepared to evaluate the above-mentioned physical and chemical parameters: 100 g of tailings were added to 500 ml of distilled water and left for 48 hours. Three vials of distilled water without tailings were used as controls in all analyses. The analytical quality was calibrated using samples of cer- tified High Purity Standards. Detection limits were as follows: Cr6+ (0.005 mg/L), Cu+ (0.001 mg/L), (0.5 mg/L), (0.02 mg/L), (0.04 mg/L), P+ (0.3 mg/L), SiO2 (0.03 mg/L) and Zn2+ (0.003 mg/L); these limits were determined using a multiparametric photometer, model HI83206 (-Hanna® instruments, USA). The error in the ionic balance was verified to be less than 10%.

The values obtained in the leachate analysis were compared with the maximum permissible limits (MPL) a) for drinking water established by the Mexican standards NOM-127-SSA1-1994 [29] , by the United States [30] and by the World Health Organization [31] ; b) for water discharged into rivers and artificial reservoirs estab- lished by the NOM-001-SEMARNAT-1996 [32] and c) for the classification of waste considered hazardous to the environment because of its toxicity according to the NOM-052-SEMARNAT-2005 [28] .

3.2.3. Mineralogical Contents

The mineralogical analysis was conducted using X-ray diffraction (XRD). Fourteen samples with oxidation or high concentrations of Cr6+, Cu+, SiO2 and/or Zn2+ were selected from each of the samples; ten of these samples corresponded to profile A and four to profile B. The analyses were conducted at the Department of Geochemi- stry of the Geological Institute of the National Autonomous University of Mexico. The diffractograms were ob- tained with a Shimadzu XRD-6000 diffractometer (Shimadzu Corp., USA) equipped with a monochromator and copper tube. Two grams were required for each sample. The samples were milled with an agate mortar and ana- lyzed in an aluminum sample holder (non-oriented fraction) at an angle of 2θ with intervals ranging from 4˚ to 70˚ and velocities of 2˚/minute.

3.2.4. Geochemical Modeling

The PHREEQC software for Windows (version 2.18.5570) was used for environmental geochemical modeling [33] . The speciation code WATEQ4F Version 2.63 [34] was selected to calculate the distribution of aqueous species, ionic activities and saturation indices based on the values of temperature, pH, Eh, and ionic concentra- tions determined in the laboratory. Using this simulation software, it was possible to analyze trends in dissolu- tion, precipitation or equilibrium of a mineral in contact with water. The activity coefficients were obtained with the Debye-Huckel or Davis equations depending on the ionic strength of the solution [35] . The product of ionic activity (PIA) is used to calculate the saturation index (SI):

, (2)

where Ksp is the solubility constant. With these data, it was possible to determine the tendency of water to dis- solve a mineral (SI < 0), precipitate (SI > 0) or maintain equilibrium (SI = 0).

3.2.5. Average Daily Dose

The risk to the surrounding population associated with the oral ingestion of Cr6+ was measured and was calcu- lated as follows:


The average daily dose (ADD) specifies the amount of contaminant that is ingested by a Mexican adult with an average weight [36] . C is the concentration of the contaminant (Cr6+, in this study mg/L), IRw is the inges- tion rate per unit time (L/day), ED is the exposure duration (years), EF is the exposure frequency (days/year), BW is the body weight of the receptor (kg), and AT is the averaging time (years), equal to the life expectancy, and 365 is the conversion factor from year to days.

4. Results

4.1. Physical Characteristics of Tailings

Profiles A and B both have materials with a heterogeneous granulometry. Profile A consists of a 5.32-m-thick material composed of particles with sizes varying from fine sand to silt fractions, with the fine sand fraction be- ing predominant (Tables 1(a) and (b)). The deposits are arranged in horizons with Munsell scale colors of light brown (6/3 7.5YR) and light gray (6/2 7.5YR) and thicknesses that vary from 1 to 37 cm; one of the horizons at the top has a peak thickness of 110 cm. In addition, the tailings have intercalated slightly darker (5/3 7.5YR) ho- rizons with thicknesses ranging from 1 to 5 cm. The coloration is not associated with granulometry. Occasional- ly, profile A has 4-cm (12A) and 33.5-cm-thick (25A) gray (7/1 7.5YR) horizons with yellowish brown (7/8 10YR) and reddish brown (5/8 7.5YR) bands that were considered to be iron oxide. Profile B has the same gra- nulometric characteristics and colorations as profile A; however, the reddish iron oxide layers (5/8 7.5YR) are more frequently encountered in profile B than in profile A. Toward the top of the deposit and distributed throughout, a 3-m-thick layer of intermediate gray color (4/1 5Y) that corresponds to a sandy loam soil type is found. On this soil horizon, vegetation from the families Cactaceae, Asteraceae, Poaceae, Leguminosae, Che- nopodiaceae, Solanaceae and Cruciferae and the species Brickelia veronicifolia, Zaluzania triloba, Solanum elaeagnifolium, Stipa tenuissima, Bouteloua curtipendula, Haplopappus venetus and Cinodon dactylon can be found [37] .

In both profiles A and B, the humidity content varies from 1% to 15%. The strata with the highest percent humidity in both profiles consist mainly of sand-sized particles that easily lose moisture and are generally not observed in an oxidation state; oxidation was detected in both of the strata consisting of fine-textured particles of silt and/or clay (Tables 1(a) and (b)).

Variations in potential of hydrogen (pH) were observed within each profile. In profile A, the pH varies from 7 to 7.64; in profile B, the pH varies from 7.27 to 7.85. In both cases, the slightly acidic medium becomes slightly alkaline. Additionally, horizons with pH values of 6 occasionally exhibit reddish to brown sheets with thicknesses of only a few millimeters, which indicate the oxidation of Fe sulfides; in contrast, these sheets are absent in the horizons with pH values greater than 6 (Tables 2(a) and (b)).

4.2. Chemistry of the Tailings

The results of the chemical analysis of the leachates (Cr6+, Cu+, , , , P+, SiO2 and Zn2+) for profiles A and B are summarized in Tables 2(a) and (b), respectively. The levels in the leachates were compared to the national [29] and international [30] [31] environmental standards for drinking water and for water discharged into rivers and artificial reservoirs [32] . These levels were also examined in the context of the classification of hazardous waste considered toxic to the environment [28] to evaluate whether the infiltration of these leachates into the ground water poses a risk to inhabitants consuming the drinking water. Of the 35 leachates samples analyzed for profile A, only samples 2A and 33A, had levels of Cr6+ (maximum = 0.014 mg/L; average = 0 mg/L) that were, respectively, twice- and almost three-fold higher than the maximum permissible limit (MPL) established for drinking water by [29] - [31] . Samples 26A, 29A, 31A and 32A had levels of (maximum = 12.85 mg/L; average = 4.25 mg/L) that slightly exceeded the MPLs established by the standards [29] [30] . Samples 1A, 3A, 5A, 10A, 11A, 12A, 16A, 17A, 18A, 19A, 20A, 21A, 23A, 25A, 26A, 27A, 29A, 31A and 32A had levels (maximum = 1.57 mg/L; average = 0.30 mg/L) that exceeded the MPL [29] [30] .

Of the 10 samples analyzed in profile B, samples 1B, 2B, 3B, 4B, 7B and 8B had levels (maximum = 146.19 mg/L; average = 38.96 mg/L) that were 14 times higher than the MPL for drinking water established by the [29] [30] and three times higher than the MPL established by the [31] . Samples 1B, 2B, 3B, 6B, 9B and 10B had levels of (maximum = 1.57 mg/L; average = 0.48 mg/L) that were up to 28 times higher than the MPLs established by the [29] and slightly higher than the MPL established by the [30] .

Table 2. (a) Concentrations from the chemical analysis of Profile A. (b) Concentrations from the chemical analyses of Profile B.



4.3. Mineralogical Analyses Using XRD

The mineralogical variability of the samples of tailings was minimal (Table 3). The predominant minerals and mineralogical groups were quartz, feldspar, clays of the kaolinite group (to 7.3 Å), as well as calcite and orthopyroxene (enstatite-ferrosilite series). Phase identification was performed using the PDF-2 database of the International Center for Diffraction Data (ICDD) with the program Jade v.5. The Powder Diffraction File (PDF) patterns associated with peak identification were 00-019-0002, 00-046-1045, 00-014-0164, 00-047-1743 and 00-019-0607, which correspond to orthoclase, quartz, kaolinite-1 Å, calcite and ferrosilite, respectively. The orthoclase pattern was associated with potassium feldspars, and ferrosilite was associated with a mineral of the enstatite-ferrosilite series. The adjustment process of the diffractometer allowed for refinement of the unit cell parameters. Phase quantification was carried out using the Reference Intensity Ratio (RIR) method [38] .

In profile A, high concentrations of quartz were found in all samples (77.3% - 91.79%), followed by orthoc- lase (4.28% - 20.5%) and kaolinite (2.2% - 10.08%); calcite (0.84% - 5.10%) was found at a depth of 250.5 cm, and ferrosilite (5.5% - 8.9%) was found only in the first 115 cm of depth. In profile B, quartz, orthoclase and kaolinite were present in the four selected samples (5.34% - 19.53%, 9.5% - 27.2%, and 58% - 76.5%, respec- tively); calcite and ferrosilite were only found in one surface level sample (1.54% and 3.4%, respectively).

4.4. Mineral Speciation and Geochemical Modeling

The results of the selected samples from profiles A and B are shown in Figure 3. In profile A, geochemical modeling showed that minerals containing Agmetallic, Cd2+, Cu+, Ni2+, Pb2+ and Zn2+ were dissolved, whereas minerals containing Fe3+, Mn2+, Pb(OH)3 and Zn2+ were in a state of precipitation. In the case of profile B, min- erals that contain Agmetallic, As5+, Cd2+, Cu+, Mn2+, Ni2+ and Pb2+ were dissolved, and dissolved Zn2+ was only found in a surface sample. Minerals containing Agmetallic, Fe3+, Mn2+, Pb3(PO4)2 and PbHPO4 were found in a precipitated state. The ionic strength in solution for samples of profiles A and B varied from 2.198−3 to 3.755−3 and from 2.662−3 to 7.777−3, respectively.

Table 3. Mineralogical composition of samples of tailings (XDR).

Blank cells indicate that the mineral species was not identified or was below the detection limit of the analytical equipment.

4.5. Risk for Human Exposure

This risk determination defines the risk of contaminants entering the body via a variety of routes (i.e., via inha- lation or oral ingestion of contaminated water). The magnitude, frequency and duration of the contact between human beings and the contaminant were estimated [39] . For this study, exposure to Cr6+ was assumed to princi- pally occur via oral ingestion of contaminated water, as levels of this toxic contaminant in the leachates ex- ceeded the MPL for drinking water. Table 4 summarizes the data used in calculations (Equation (3)) and the av- erage daily dose for Cr6+ that an inhabitant of the city of Pachuca might consume: 1.45 × 10−4 mg/L/day.

5. Discussion

The integration of the results obtained by different analyses and data from previous studies conducted in the area provides a general overview of the effect that the tailings from the Dos Carlos deposit can have on the environ- ment and, consequently, on the population of Pachuca and its urban area.

5.1. Physical Characteristics of the Tailings Deposit

The tailings deposit is predominantly composed of fractions of fine sand, with some fractions of silt and clay. [40] suggests that there is a direct relationship between fine particle size and the ease with which particles release their minerals when they come in contact with atmospheric oxygen or water. Previous work [41] reports that minerals are released more easily in fractions with sizes of 70 µm or less than in fractions with sizes greater than 70 µm. In contrast, studies carried out in a mining site in the State of Mexico [11] reported that mineral release occurred to the greatest extent in particulate fractions with sizes of 70 µm, which correspond to fine sands and represent the finest fraction observed in the tailings.

It is known that the greater the dilution imposed on the particle, the greater the separation efficiency of its chemical components [42] . Similar results were obtained in laboratory [43] , observing that tailing particulates less than 1000 µm in size had high levels of heavy metals. [44] used wet centrifugation and observed that the weight of the smallest particles in the tailings allowed the particles to more easily separate from sulfuric miner- als; however, this separation occurred only in the fraction of fine sand.

The preceding discussion suggests that the fine sands of the Dos Carlos tailings are less likely to become sa- turated in water than the silt particles; however, mineral release is favored in both fractions (although to a lesser

Figure 3. Geochemical speciation of selected minerals.

extent in the fraction of fine sands). This means that contaminants such as Cr6+, and could be re- leased every time oxygen diffuses through the pores of the tailings. [45] showed that the reaction speed and ef- fective diffusion of oxygen through the tailings are affected by factors such as the degree of saturation and the porosity of the material; more specifically, greater diffusion of oxygen throughout the pores occurs at lower de- grees of saturation. The percent humidity encountered in the different horizons of both profiles was low (1% - 15%); however, these values favor the leaching of certain minerals from mining activities. [46] indicated that when water infiltrates through this type of deposit, it acquires a similar chemical composition as the water con-

Table 4. Exposure factors for an adult Mexican living near the tailings deposit.

tained in the host rock. This type of leaching into the Dos Carlos deposit was reflected in the slightly acidic ho- rizons (pH values of 5.93 to 6.83) where evidence of the oxidation of iron sulfides was observed.

5.2. Chemistry of the Tailings Deposit

Chemical analyses of the tailings deposit identified the presence of Cr6+, , [28] - [32] ; levels of these contaminants were two, three and up to 14 times higher, respectively, than the maximum permissible limits (Tables 2(a) and (b)).

An analysis of leachates from eight samples collected on the surface (first 50 cm) of the Dos Carlos tailings [18] reported levels of Cu+ (maximum = 1.045 mg/L; average = 0.43 mg/L), Cd2+ (maximum = 0.625 mg/L; av- erage = 0.244 mg/L), Fe3+ (maximum = 4.16 mg/L; average = 1.07 mg/L), Mn2+ (maximum = 53.55 mg/L; av- erage = 29.83 mg/L), Pb2+ (maximum = 1.685 mg/L; average = 0.8 mg/L), Ag (maximum = 0.41 mg/L; average = 0.12 mg/L) and Ni2+ (maximum = 0.6 mg/L; average = 0.12 mg/L). This analysis concluded that the tailings of the Dos Carlos deposit are potential sources of acid mine drainage [18] . The values obtained in this study were compared with the values obtained by [18] and with the standards established [28] - [32] . Two, eight, three, eight, five, one and three samples had levels of Cu+, Cd2+, Fe3+, Mn2+, Pb2+, Ag and Ni2+, respectively, that exceeded the MPLs [30] [31] .

When the leachates mix with ground water, the concentrations of pollutants are diluted in the new water body, resulting in pollutant levels that are only slightly higher than the MPLs; consequently, the risk of compromising the water quality is negligible or nonexistent. However, for ions with levels that were several-fold higher than the MPL (as in the case of Cd2+), the risk is greater. Cd2+ in particular should be given special attention, as this ion can be a gastric irritant and can cause vomiting and diarrhea in humans. In some short-term studies, the oral administration of soluble Ni to animals resulted in the accumulation of this compound in the kidneys, lungs, liv- er and heart (in that order), and absorption via the lungs resulted in toxicity and carcinogenicity [47] .

The particulates in the tailings were observed to influence the chemical composition of the soil in the metro- politan area of Pachuca. [48] reported Cu+ concentrations of 34.68 - 119.88 mg/kg and Cd2+ concentrations of 0.13 - 20.07 mg/kg in the soil around Pachuca; the authors concluded that the presence of these elements was associated with activities of the Mining District Pachuca-Real del Monte and that these compounds may have come from the Dos Carlos tailings. In contrast, these authors concluded that the concentrations of Mn2+ (188 - 780 mg/kg) and Pb2+ (9.64 - 114.6 mg/kg) were associated with atmospheric emissions produced by vehicles. However, the possibility that these elements could also come from the Dos Carlos tailings should not be ex- cluded. In this context, performing atmospheric measurements to estimate the wind-borne spread of some of the ions from the tailings will be critical, as, for example, manganese can affect the nervous system in humans and cause deleterious neurological effects [49] .

As shown in the correlation matrix of the physico-chemical parameters measured in this study (Table 5), strong positive correlations (r2 > 0.50) were observed for the following pairs in the indicated order: pH and E.C. > Eh, Cu+ and Zn2+ > E.C. > pH, and, and vice versa. These correlations indicate that variations in the pH of the leachates could contribute to the dissolution or precipitation of ions, thereby producing a more acidic or neutral-alkaline environment, respectively. The correlation between Cu+ and Zn2+ is related to the type of deposit exploited; therefore, as the pH decreases, these ions will continue to be released. The presence of can be related with agriculture soil eroded that was deposited by aeolian transportation; other source for nitrates can be associated with atmospheric emissions produced by vehicles [48] .

Table 5. Cross correlation matrix of physico-chemical parameters of the tailings deposit.

5.3. Mineralogy and Speciation

The mineralogical analysis of the Dos Carlos tailings and the comparison of mineral speciation results in pro- files A and B suggest that minerals in a state of dissolution are releasing Agmetallic, Cd2+, Cu+, Ni2+, Pb2+ and Zn2+ into the environment. Minerals that contain Fe3+, Agmetallic, Pb(OH)3, ZnSiO3 and ZnO are found in a precipitated state. This indicates that certain minerals such as quartz, calcite and kaolinite could promote an atmosphere in which low levels of acidity in the deposit can be neutralized [50] . [51] stated that the interaction of calcite with fluids that contain phosphate in slightly acidic to slightly alkaline environments results in the dissolution of cal- cite and promotes the precipitation of Ca-phosphate phases. In contrast, [52] determined the acidity constants of several clays and found that the kaolinite group favors the retention of heavy metals. The mobility of certain ions in the soil could also influence the mobilization of the dissolved ions; for example, Pb2+ has negligible mo- bility in soils (or tailings) with a slightly alkaline pH, whereas Cu+ and Zn2+ have greater mobility in this type of soil [53] .

Minerals in the tailings such as kaolinite and calcite that are found at high concentrations could neutralize the dissolution of minerals containing Cu+, Cr6+, and Zn2+. [54] reported that when the pH is almost neutral (6.5 - 7), the neutralization of calcite occurs as follows: CaCO3 + 2H+ = Ca2+ + CO2 + H2O. However, it is necessary to conduct other studies to determine the neutralization capacity of these minerals in this deposit.

5.4. Oral Exposure of Cr6+

The mean daily dose of Cr6+ in groundwater (the main source of water for the population) that an inhabitant of the study area could safely consume is 1.45 × 10−4 mg/L/d. The carcinogenic potential of chrome ingested orally has not yet been determined. It has been determined that inhabitants of a village in China who consumed drink- ing water with 20 ppm of this contaminant suffered adverse health effects [55] . However, in a toxicological re- view of hexavalent chromium, the [56] reported that 5% of orally ingested Cr6+ is absorbed by the gastrointes- tinal track. Once transported into the bloodstream, a portion of this concentration can remain in the body for a long time without causing apparent damage.

6. Conclusions

This preliminary study showed that the granulometric fraction of tailings with fine sand and a lower percentage of silts and clays favors the diffusion of oxygen (atmospheric or from water infiltration). This finding was con- firmed by the fact that both profiles had moisture contents that could promote the oxidation of minerals, espe- cially in the horizons with a slightly acidic pH. Cr6+, and were found at different depths in conta- minant levels. A correlation matrix showed that the behaviors of the physical and chemical parameters of the tailings were strongly related. This indicates that the environmental conditions (diffusion of atmospheric oxygen or rainfall) affect the physico-chemical composition of the tailings. In addition, the origin of the deposit, the mining activities and the mineral separation that occur during the mining process also influence the chemical composition of the tailings. Therefore, the concentrations of the previously mentioned elements vary with pH and with the diffusion of oxygen through the pores of the tailings.

The analysis of the geochemical modeling showed that minerals containing ions characteristic of acid mine drainage were dissolved in both profiles, as it was also reported in a previous study. In contrast, minerals such as calcite and kaolinite were observed to be capable of being neutralized.

The above-mentioned findings and the concentrations of Cr6+ measured in the leachates from the tailings suggest that the oral ingestion of this Cr6+ mixed with groundwater should not cause adverse health effects. However, it is necessary to investigate the mean daily dose that the population currently ingests through inhala- tion. Therefore, the infiltration of leachates of the tailings into the groundwater and the wind-borne transport of contaminants from the tailings’ deposit present a potential risk of contamination to the environment and human population. However, a detailed chemical and atmospheric analysis is necessary to definitively confirm whether this deposit is the source of short-term contamination of the environment and the health of the population.


The authors are grateful to the Faculty Improvement Program (PROMEP) for financing through the project F-PROMEP-38/Rev-03, SEP-23-005. We also thank to the Biotechnology and Environmental Microbiology Research Group for their helpful comments.


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