Introduction

Mexico has a wealth of metallic and non-metallic minerals throughout its territory (^{Dirección General de Minas, 2015}). This has caused problems arising from mineral extraction and the processes of mining activity, reflected mainly in the northern area where the soils are more polluted (^{Mendez & Maier, 2008}). In the state of Zacatecas, located in the northern center of the country, mining activity began in the sixteenth century, which has generated large amounts of waste deposits known as environmental liabilities (ELs). The problem of this activity is the historically accepted practice of abandoning mines and ELs when the minerals of interest are exhausted (^{Himley, 2014}; ^{Unger, Lechner, Kenway, Glenn, & Walton, 2015}); in 2013, there were 3 241 abandoned ELs in Mexico (^{Secretaría de Medio Ambiente y Recursos Naturales [SEMARNAT], 2014}). These ELs are a source of heavy metals, which are dispersed by the wind and water (^{Adriano, 2001}; ^{Navarro et al., 2008}), since they are associated with clays migrating as composite particles in the sediments (^{Balderas-Plata, Gutiérrez-Castorena, Carrillo-González, Ortiz-Solorio, & Lugo-de la Fuente, 2006}), thus increasing the risk of pollution. The presence of metals in the soil and waterbodies increases the chronic health risks of living organisms, including humans (^{García-Rico et al., 2016}; ^{McSwane, French, & Klein, 2015}).

The Real de Ángeles mine, located in Noria de Ángeles, Zacatecas, is constituted by alternating sandstones, siltstones and argillites (^{Servicio Geológico Mexicano [SGM], 2016}). The risk of pollution depends on the associated rock and the host rock of the deposit. The main minerals in the mine are: argentite (Ag₂S), sphalerite (ZnS), chalcopyrite (CuFeS₂), arsenopyrite (FeAsS), pyrrhotite (Fe₁-XS), argentiferous galena (PbS·Ag), freibergite (3Cu₂Ag·S·Sb₂S₃), tetrahedrite (Cu₃SbS_4-3), anglesite (PbSO₄) and cerussite (PbCO₃) (^{Bravo-Nieto, 1988}). The above shows the possible presence, perhaps at pollutant levels, of lead, zinc, arsenic, silver, iron, strontium and antimony in ELs; the presence of these heavy metals in soil, dust and water could put public and environmental health at risk (^{Balderas-Plata et al., 2006}; ^{García-Rico et al., 2016}; ^{McSwane et al., 2015}). In this context, the objective of this research was to determine the concentration of potentially toxic minerals (Hg, Pb, Cd and As) in two environmental liabilities and in waterbodies that are adjacent to or on these liabilities in Noria de Ángeles, Zacatecas.

Materials and methods

The study was conducted in Noria de Ángeles, located in the southeast of the state of Zacatecas, where there are two ELs (Figure 1). The first (historical) dates from the colonial era (^{Arellano, 1970}), covers an area of approximately 2 ha, has no protection and is within the urban area; adjacent to this EL is a spring that forms a stream, with the water used by domestic livestock. The second EL (recent) began its activities in 1982 and corresponds to the Real de Ángeles mine. This EL covers a 360-ha area and has an approximately 30-m-thick protective layer; however, it has areas without protection and with outcrops due to water erosion. In this EL there are waterbodies adjacent to and above it that are used as drinking troughs for domestic livestock. The climate is semi-dry temperate (BS₁kw) with summer rains; according to weather station data for Villa González Ortega, Zacatecas, the average annual temperature is 13.7 °C, average annual rainfall is 342.6 mm and potential evapotranspiration is 2 023.4 mm (^{Comisión Nacional del Agua [CONAGUA], 2017}).

Figure 1 Location of environmental liabilities and waterbodies sampled in Zacatecas. UTM and Datum WGS84 projection. Source: Instituto Nacional de Estadística y Geografía (^{INEGI, 2015}). 

The ELs were sampled in August 2015, following the specifications of Mexican Standard NMX-AA-132-SCFI-2006 (^{Secretaría de Economía [SE], 2006}). A reference site (22.56497° NL - 101.653022° WL) located in Salinas, San Luis Potosí, was also sampled. In the historic EL and reference site, three, 50-m transects were located; every 10 m a sample was taken at a depth of 0 to 20 cm. The direction of each transect was chosen based on the last three digits of 200 MXN banknotes, which were considered as degrees. The recent EL is "protected" with a 40-cm layer of uncontaminated material; however, it has outcrops due to water erosion or areas that were not properly protected. Therefore, sampling was carried out in 15 of these outcrops. In each one a 1-m² square was established where gravel, vegetation and mulch were removed; the corners and the center were sampled, with which a composite sample was obtained (^{SE, 2006}).

In June 2016, three water samples were taken from a spring adjacent to the historic EL (1), and from three waterbodies located above (2) or adjacent (3 and 4) to the recent EL (Figure 1), according to NOM-230-SSA1-2002 (^{Secretaría de Salubridad y Asistencia [SSA], 2003}) and NOM-147-SEMARNAT/SSA1-2004 (^{SEMARNAT, 2007}). Samples were placed in sterile disposable bottles, 1 mL of concentrated nitric acid was added per 100 mL of sample and they were kept at 4 °C until analysis.

The EL and reference site soil samples were analyzed for lead, mercury, cadmium and arsenic, in accordance with NOM-021-SEMARNAT-2000 (^{SEMARNAT, 2002}). The presence of these metals and metalloid in the water was determined in accordance with NOM-127-SSA1-1994 (^{SSA, 2000}), using atomic absorption equipment (Aurora Instruments Model AI-1200).

Metal and metalloid concentrations in the historical and recent ELs and in the reference site were subjected to an analysis of variance in a completely randomized design using the PROC GLM of SAS (^{Statistical Analysis Software Inc. [SAS], 2015}) and Tukey’s range test (^{Steel & Torrie, 1980}). The minimum, maximum and average values ​​of metals and metalloid in the samples were obtained for each waterbody.

Results and discussion

Table 1 contains the average values of the metals in the ELs of Noria de Ángeles, Zacatecas, as well as the permissible limits established by NOM 147 SEMARNAT/SSA1-2004 (SEMARNAT, 2004) and the USEPA (^{United States Environmental Protection Agency, 2017}). This table shows that the Hg concentration was higher (P < 0.05) in the ELs than in the reference site, but it did not exceed the maximum permissible limits established by the standards.

With respect to Cd, the maximum concentrations in the three sites exceeded the limits of the aforementioned standards; however, the average value was only higher in the recent EL, being statistically higher (P < 0.05) than that of the other sites.

On the other hand, the maximum and average As values in the three sites were higher than the maximum permissible limits established in the two standards. It should be noted that although the As concentration in the reference site was lower than in the EL, the value was almost five times higher than the maximum allowable, which indicates a source of contamination of surface waterbodies, soil, and forage that domestic livestock consume and, therefore, represent a risk to the health of animals and the human population (^{Kyunghee et al., 2013}). The historical EL had the highest As concentration (P < 0.05).

The maximum and average Pb values in the historical EL exceeded the maximum permissible limits of both standards; in the recent EL, the values exceeded only the limits established by the USEPA (2017). The high Pb concentrations are related to the argentiferous galena (PbS·Ag), anglesite (PbSO₄) and cerussite (PbCO₃) minerals present in a deposit (^{Bravo-Nieto, 1988}). With respect to the reference site, the Pb concentration was found to be well below the permissible limits. The Pb and As values were lower than those found by ^{Navarro et al. (2008)} in abandoned mine deposits.

According to the results, the Pb, Hg and Cd pollutants of the ELs are mainly of anthropogenic origin since the concentrations were higher than in the reference site. On the contrary, it is deduced that the As has anthropogenic and geological origins, since its content in the reference site was higher than in the recent EL, but less than in the historical one. Both ELs constitute a threat to public health and wildlife, through the ingestion of dust with a high content of heavy metals, mainly As, Cd and Pb (^{García-Rico et al., 2016}; ^{Jeong-Hun & Kyoung-Kyoon, 2013}) since these are near (recent EL with poor protection) or within (historical EL without protection) urban areas.

Table 2 shows the Pb, Cd, Hg and As concentrations in the waterbodies. The concentrations of these elements exceeded, in all cases, the maximum permissible values established by the Ministry of Health (^{SSA, 2003}) and ^{USEPA (2009)}. The Cd, Pb and As values found in this study are higher than those found by ^{Steller, Domínguez-Mariani, Garrido, and Avilés (2015)} in water samples taken from abandoned minefields in Huautla, Morelos, where the concentrations do not exceed 1.0 mg·L^-1. By contrast, in water sources of other abandoned mines, ^{Elyaziji, Khalil, Hakkou, Benzaazoua, and Alansari (2016)} and ^{Oyarzún, Maturana, Paulo, and Pasieczna (2003)} found values similar or much higher than those found in the present work. Water contaminated by metals is common in abandoned mining areas, due to the weathering of minerals (^{Elyaziji et al., 2016}; ^{Steller et al., 2015}).

The waterbodies in the Noria de Angeles’ ELs pose a major risk to domestic livestock and wildlife that consume water directly from these places. In addition, the toxic elements could be bioaccumulating in products such as meat and milk and thus pose a threat to the health of local people who consume these products (^{Nava-Ruíz & Méndez-Armenta, 2011}; ^{Nouri & Haddioui, 2015}).

Conclusions

The concentrations of Pb, Cd and As of the environmental liabilities (ELs) and the concentrations of Pb, Cd, Hg and As in the waterbodies exceed the permissible limits established in national and international standards. Therefore, the ELs and waterbodies pose a risk to the health of the local population, wildlife and domestic animals, mainly due to dust inhalation and contaminated water ingestion.