Study area

The state of Durango (Figure 1) is a northern state of the Mexican Republic, between parallels 22°20’ and 26°50’ North latitude and between meridians 102°28’ and 107°12’ West longitude. This state’s area is 124,258.50 km², which is about 6.3 % of the Mexican territory. It is a traditionally mining state, with an evidence of mining activity since the Pre-Columbian period, reaching its highest peak when Mexico was a Spanish colony.

Figure 1: Study area location 

The mine started operating in 1552, when the iron deposit of “Cerro de Mercado” was discovered by the Spanish Captain Ginés Vázquez de Mercado, and the city of Durango was founded. The exploitation of the deposit began in 1828 (^{SGM, 2018}), (^{Corona et al., 2009}).

The sampling carried out at the tailings in the water dam “Boleo Estrella”, were conducted in accordance with the standard NMX-AA-132-SCFI-2016, (^{Official Journal of the Federation, 2017}). The samples were then analyzed using the US-EPA 6200 method, an “X-Ray fluorescence semi-quantitative analysis” and a pistol equipment Nitton XL3t Ultra. The use of the “alkaline digestion for hexavalent chromium” method, established in NOM-147-SEMARNAT/SSA1-2004 determine the oxidation status in tailings with the presence of chromium. In the leaching test a composite mixture was prepared and 5 replicates were performed. An atomic absorption spectrophotometer GBC Avanta and an inductively coupled plasma mass spectroscopy equipment (ICPS-MS) 7500 ce Aligent, were used for chromium according to A NOM-141-SEMARNAT-2003 standard.

The risk assessment was carried out through a version 5.0 computer program “Spatial Analysis and Decision Assistance” (SADA) as well as the maximum allowable limit values established in the NOM-157-SEMARNAT-2009 standard as the ones established by the EPA.

Theory/software calculations

SADA is a free distribution software developed by the Environmental Modeling Institute at the University of Tennessee. Among others, it includes GIS, a statistical analysis and ecological and health risk assessment tool. The risk models being used by this software, are based on the EPA guidelines risk assessment. SADA calculates PRG (Preliminary Remediation Goals), higher concentrations of a substance in a specific environment ensuring health and ecosystem protection. The PRG corresponds to minimum and admissible effects on individuals that will cause minimal effects on the rest of the population or community.

The metal concentrations (antimony, arsenic, barium, chromium (III), strontium, manganese, nickel, vanadium and zinc using the X-Ray fluorescence technique), were introduced in the software. Due to some pollutant limitations of the software, metals such as Fe, Ca, Ti and Rb were not loaded. The exposure parameters for each case used by the software are shown in Figure 2. The body weight parameters, for adults and children were modified as well as the lifetime by the values reported by ^{INEGI (2016)} for the population of Durango.

Figure 2: Parameters exposition used by SADA version 4.1 

Based on the information provided by the workers of “Cerro de Mercado” mine, the houses around the mine are built on tailings dam, people living in these houses are in direct contact with tailings, therefore for the above, this was selected as the soil matrix for further calculations. When calculations were made, the following routes were selected: Inhalation and dermal. The reason for this is that during the exploration of the population near the study site, it was found that since wind that disperses the tailings, individuals are in contact with them without even noticing it. For the selected pathways, the equations to perform the risk calculations executed by the software are as follows:

Inhalation:

Dermal:

The equations terms are shown in Table 1.

Health Index (THI) and Target Risk values used are shown in Figure 3.

Figure 3: HI and TI values used in the calculations 

A THI less than or equal to one, indicates that no adverse health effects are likely to be experienced by exposure of a chemical substance toward all routes (inhalation, ingestion and dermal) in each medium (water, soil, air, etc.), so the risk is worthless. In case value 1E-06 is higher in Target Risk, it is considered that there is no health risk (^{EPA, 2017}). In order to determine whether there is a risk for the population surrounding “Boleo Estrella” tailings dam, the THI columns and total reference concentrations (CRT) values, were compared. THI data calculated by SADA, while CRT values were obtained for arsenic, barium, nickel and vanadium cases from NOM-147-SEMARNAT/SSA1-2004, the remaining from Regional Screen Level Soils (^{EPA, 2016}).

Sampling

The tailings intake was carried out at the Boleo Estrella dam, which stopped operating 15 years ago. It has an area of 16200 m², a depth of 80 m, and 3,700,000 tons of tailings. Samples were taken in a rectangular pattern, at a depth of 30 cm, obtaining 27 samples (Figure 4a). During sampling, extensive vegetation was observed as well as various insects (Figures 4b, 4c). The quantity of tailings taken at each point was approximately 4 kg, which were placed in airtight polyethylene bags for further analysis.

Figure 4: a) Sampling points on tailings dam, b) vegetation present in dam, c) insects found during sampling 

Tailings characterization

All 27 samples had a higher pH value of 7 (Figure 5) indicating that metals present in tailings have low solubility and are unlikely to reach aquifers (^{Yang et al., 2015}).

Figure 5: Tailings sample pH values 

In the characterization of the 27 tailings samples (Figure 6) the presence of TI, Mn, Ba, V, Zr, Sr, Ni, Zn, As, Rb, Cu, Fe and Ca, was found. The concentration values of the mentioned metals do not exceed the maximum permissible limits (LMP) established in the standard NOM-157-SEMARNAT-2009. In the case of chromium, it was detected in all samples (27), where the values were higher than those established in the above-mentioned standard, while antimony (only detected in 10 samples) also exceeded the values allowed in the Mexican standard.

Figure 6: a) Leaching process, b) atomic absorption equipment, c) HPL equipment used 

In the Figure 4a, the distribution of the samples that contain antimony (samples: P5, P8, P12, P15, P19, P20, P21, P24, P25) presents a descendent pattern. In the first sampling line (from P20 to P12) it is observe that 4 samples (P20, P19, P15 and P12) had antimony, in the second one (from P27 to P11) in three samples (P25, P24 and P21), the metalloid aforementioned was detected and finally in the third sampling line antimony was detected only in P5 and P8.

Although a distribution patter it seems to be present in the dam, it is no able to claim that the antimony will probably be concentrated between the points where it was detected because other samples between those points did not have antimony, for instance, between P5 and P8 there are the points P6 and P7 and it is expected that these samples will have antimony but this is not true. Another important thing to say is that there were not significant differences in the dam’s ground where the samples P5, P8, P12, P15, P19, P21, P24, and P25 were taken.

Table 2 shows the average concentration of the metals and metalloids obtained in the samples as well as the LMP established by the referred standard.

Leaching tests

Since the presence of two metals in the tailings exceeded the LMP in dry-base, leaching tests were performed (Figure 6) as required by the Mexican regulations, obtaining that in the composite sample analyzed an antimony concentration of 2.1 mg/ml was found. This value exceeds the 0.5 mg/ml established in Mexican regulations.

The concentration value for chrome was less than 2 mg/ml, which is below the LMP. However, the standard NOM-157-SEMARNAT-2009 establishes that if the presence of any metal or metalloid in tailings exceeds the LMP, mining waste must be considered hazardous. The antimony present in “Cerro de Mercado” mine tailings, exceeds the value of the LMP and therefore this constituted a hazardous waste with a great potential to cause damage to health and the environment.

Chromium oxidation status

The performed determination test for the hexavalent chromium, showed that Cr⁶⁺ tailings samples is below the LMP established in standard NOM-147-SEMARNAT/SSA1-2004, which is 290 mg/kg. The value of hexavalent chromium was substracted from the total chromium values obtained by X-ray fluorescence, to determine the concentration of trivalent chromium. This refers to the oxidation status predominant in the samples, due to the fact that the concentration was higher than that of hexavalent chromium in all samples (Table 3).

In soils, chromium is usually found in a Cr³⁺ form (^{Zayed & Terry, 2001}), Cr⁶⁺ is present in aerated soils and, with the presence of organic matter, it is generally the most important reducing agent (^{Adriano, 2001}), and it can be reduced to Cr³⁺. “Boleo Estrella” tailings dam is not properly a soil, however, it has dried up and consolidated over the years providing sustainment to different vegetation, as well as to insects and animals, which were observed during sampling. The results obtained in chromium determination in an oxidation status are consistent to those reported by various authors. It is relevant to mention that Cr³⁺ is not known to be a potentially toxic substance, while Cr⁶⁺ is highly toxic (^{Severiche & González, 2013}).