In the Mantaro river basin, located in Central Peru, an agricultural activity developed, even supplying the capital of Peru. An intense mining activity took place in the headwaters of this basin, and the La Oroya Metallurgical Complex was built a century ago. Mining activity has left mining environmental liabilities, which directly impact the quality of the water and the soil. In this sense, it is very important to investigate the presence of heavy metals and identify the geochemical associations present in surface waters to assess the real impact on the environment. For this purpose, 30 water samples were analyzed, collected from the Mantaro River and the channels that derive water for irrigation and animal consumption. The samples were analyzed by ICP-MS techniques, inductively coupled plasma-mass spectrometry, and ICP-AES, inductively coupled plasma-optical emission spectrometry. For the evaluation of the main physicochemical parameters, the ECA has been used, as the environmental quality standard of Peru, according to the Ministry of the Environment (2017); while for the chemical quality of surface water, the quality standards of the WHO, World Health Organization, were taken as a reference, according to the WHO guide (2017), being the elements considered: Al, As, Mn, Pb, and Zn. Investigations results show that the waters of some sectors have concentrations of As and Pb, which exceed the standards established by WHO, and there are also some specific cases (Muqui canal) in which Al and Mn exceed the WHO standard. In the case of Zn, its concentrations are much lower than the WHO standard.

Keywords: heavy metals, surface water, environmental impact, geochemical associations, mining environmental liabilities

Surface waters of the Mantaro River, which runs through the central Andes of Peru, show an appreciable content of heavy metals as a consequence of impact generated by the mining environmental liabilities located in the headwaters of the basin, in the town of La Oroya,¹ where an old metallurgical plant is located. Similar investigations have been developed in Ecuador by Oviedo et al.² who also determined the impact on the soil; Bhuyan et al.³ carried out a similar investigation in Bangladesh, identifying values ​​that exceed the WHO standards in Fe, Ni, and Al. On the other hand, Giri et al.⁴ evaluated the surface water quality, determining that dissolved metals show greats seasonality, with the lowest concentrations in the rainy season. Bouguerne et al.⁵ applied multivariate statistical techniques to evaluate temporal variations in surface water quality over ten years, identifying geochemical associations important.

Likewise, Nnorom et al.⁶ investigated groundwater and surface water sources, determining Al and Fe as dominant contaminants in water bodies, coming from the shale of southeastern Nigeria. On the Chinese Loess Plateau, Xiao et al.⁷ determined that the poor quality of river water is mainly related to high alkalinity and the danger of salinity, while the main contaminants in drinking well water were As, Cr and B. Chao-yang et al.⁸ studied the distribution of Zn, Cd, Cu, Cr and As in Lake Honghu, indicating that these are below quality standards, denoting that Cr and As are derived mainly from industrial effluents. Using surface water and sediment samples from the upper Tigris River, Varol⁹ determined that all concentrations of total nitrogen, total phosphorus, As, Cd, Co, Cr, Fe, Mn, Ni, Pb, and Zn in water samples were lower than the maximum concentration allowed for the protection of aquatic life, with Cu being the metal that exceeded the allowed threshold.

Mantaro area, Oncevay (2013) reports that the surface waters contain high contents of iron and total suspended solids, as a result of the presence of mine effluents. Custodian et al. (2019, 2020) evaluated the presence of Cu, Pb, Fe, Zn, and As in surface waters of the aforementioned basin, using multivariate methods, revealing that "the Mantaro River continues to be a sink for discharges of mining waste and runoff of liabilities miners in the headwaters of the Mantaro basin”. On the other hand, Orellana et al.¹⁰ investigated soils and pastures, indicating that although the water contains leads at high levels, this does not imply a risk for the agriculture, since the bioconcentration factors were not significant. Aylas et al.¹¹ showed that the water of the Chanchas river (Mantaro basin) presents a higher concentration of pollutants during the afternoons, in alkaline pH conditions (8.69), being within the environmental quality standards for human consumption, but not for irrigation.

The water that flows downstream of the town of La Oroya¹² also does so through irrigation canals of the soils for agricultural use, which produce different vegetables that supply both the town of Huancayo and the city of Lima, the capital of Peru. In this way, it is necessary to evaluate the distribution of heavy metals and how they are associated with surface waters, identifying the most environmentally impacted areas. Results investigation mean a contribution principal to the knowledge of the environmental impact due to the heavy metals in surface waters in consequence of liability mining presence environmental.

In the collection of samples were used bottles of polyethylene with double lids. The samples were preserved with nitric acid (1:1) to ensure a pH lower than 2. The determination of metals was performed using the ICP-MS and ICP-OES techniques. Likewise, in situ readings of pH, electrical conductivity, and dissolved oxygen were taken at each sampling point, using HANNA 9828 multi-parameter equipment. ICP was developed for optical emission spectrometry (OES) by Wendt and Fassel at Iowa State University in the United States, and by Greenfield et al. at Albright & Wilson, Ltd. in the UK in the mid-1960s.¹³ Inductively coupled plasma (ICP) mass spectrometry (MS) is commonly used in various research fields, such as earth sciences, food, chemical materials, and nuclear industry, among others. The high density of ions and high temperature in a plasma provide an ideal medium for atomization and ionization for all types of samples and matrices introduced by a variety of specialized devices.¹⁴

Instruments and reagents

The content of heavy metals in surface water samples was determined using the ICP-MS equipment, Perkin Elmer model Nexion 300D; and ICP-AES, Agilent Technologies model 735-ES. In the laboratory of the Geological, Mining and Metallurgical Institute of Peru, the samples were digested with high purity nitric acid (60-62%) and high purity hydrochloric acid (30-32%), through a digestion system of the hot block (SCP Science DigiPREP) under controlled conditions of pressure and temperature, according to EPA methods, U. S. Environmental Protection Agency, 200.7, determination of metals and trace elements in water and wastewater by inductively coupled plasma atomic emission spectrometry,¹⁵ and EPA 200.8, Determination of Metals and Trace Elements in Water and Wastewater by Inductively Coupled Plasma - Mass Spectrometry.¹⁶

Physicochemical parameters in surface waters

Based on a strategic sampling, water samples were collected in 30 places located in the Mantaro River, streams, and irrigation canals; pH, electrical conductivity, and dissolved oxygen readings were taken in each one (Table 1). On the other hand, in the laboratory of the Geological, Mining, and Metallurgical Institute, the concentrations of total metals were determined (Table 1). The pH ranges between 7.3 and 8.8, which to a certain extent favors the formation of metal hydroxides due to their insolubility in alkaline media.¹⁷ According to the ECA (6.5 – 8.5), it identified that in four streams and three irrigation canals, said the standard was exceeded (Table 1). The electrical conductivity (EC) ranged from 111 to 853 µS/cm and did not exceed the ECA (2500 µS/cm), see Table 1. In the case of dissolved oxygen (DO), the ECA (≥4 mg/L) was exceeded in all the places sampled, except for the irrigation channel of the San Lorenzo district, in which this parameter is 7.58 mg/L (Table 1). From the applied multi-element techniques (ICP-MS and ICP-AES), a series of metals were quantified. However, the results of those that deserve special attention will be presented, given their environmental implications and their concentrations for the quality environmental standards considered.

Chemical quality of surface water

For the evaluation of the chemical quality of the surface waters of the Mantaro river basin, the environmental quality standards established by the WHO (Table 2) for five elements have been taken into account: Al, As, Mn, Pb, and Zn (Figure 1–10). The highest content of aluminum (2.36 mg/L) is recognized in the channel of the Muqui locality (24m-032), which widely exceeds the WHO standard (Figure 1). With the exception of the channel of the town of Muqui (24m-032), the other samples show aluminum values ​​lower than the WHO standard, including the water from the Muquiyauyo channel (24m-034), where the concentration of this metal is 0.86 mg /L (Figure 1). Regarding arsenic, it was evidenced that it is the metal that most frequently exceeded the WHO standard, in this case, 51% of the samples are greater than 0.01 mg/L (Figure 2). This undoubtedly denotes a characteristic hydrochemical pattern in the study area, expressed in the maximum statistical variability of the metals studied. The highest concentrations of arsenic occur in the canals of the towns of San Lorenzo (0.017 mg/L), Muquiyauyo (0.020 mg/L), and Muqui (0.035 mg/L), exceeding the WHO standard by up to 3.5 times (Figure 2). Manganese shows a behavior similar to aluminum, being for both cases the sample 24m-032, the one that exceeds the respective WHO quality standards (Figure 3).

Figure 1 Aluminum in surface waters in the Mantaro basin.

Figure 2 Arsenic in surface waters in the Mantaro basin.

Figure 3 Manganese in surface waters in the Mantaro basin.

The manganese contents closest to the WHO quality standard correspond to samples 24m-034 (0.34 mg/L), 24m-014 (0.34 mg/L), and 24l-004 (0.30 mg/L), as shown in Figure 3. It should be noted that the manganese variability pattern is the second-highest, after arsenic, both patterns being similar (Figures 2 and 3). As for lead, the values ​​that exceed the WHO quality standard range from 0.013 to 0.111 mg/L (Figure 4), located in the channels of the towns of Muqui, Muquiyauyo, Huamaly, and San Lorenzo, respectively. As shown in Figure 4, except for the lead samples that exceed the WHO quality standard, it is evident that the concentrations of this metal describe a relatively stationary pattern, with a plateau of 0.006 mg/L, which is reflected in the minimum statistical variability of the investigated metals. Zinc is the only metal that did not pass the WHO quality standard (Figure 5). The maximum concentration is evidenced in sample 24m-032 (0.357 mg/L), located in the channel of the town of Muqui. The zinc average in the studied waters is of the order of 0.04 mg/L, with a standard deviation of 0.07 mg/L, which describes a lower variability than the patterns of As, Mn, and Al (Figure 5).

Figure 4 Lead in surface water in the Mantaro basin.

Figure 5 Zinc in surface waters in the Mantaro basin.

Statistical correlations

Table 3 shows the Pearson correlation indices, which show that the highest positive correlations occur between Pb-Zn (0.97), Pb-Al (0.97), Al-Zn (0.95), and As-Zn (0.87), As-Al (0.79) and As-Pb (0.87). These indices allow us to postulate a geochemical association composed of Al, As, Pb, and Zn, under alkaline pH conditions.

The arsenic contents that exceeded the WHO quality standard (0.01 mg/L), are distributed between the localities of Parco and Sicaya, controlled by the surrounding alkaline environment, related to the Triassic-Jurassic carbonate formations and environmental liabilities mining, the product of the exploitation of polymetallic deposits in the headwaters of the basin. Between the towns of Muquiyauyo and Muqui, there are the highest concentrations of this metal. In the Ranra River (24m-004) and the ravine of the town of Concepción (24m-018), tributaries of the Mantaro River, the arsenic content that exceeds the quality standard can be attributed to anthropic factors and the metamorphic units of the Neoproterozoic-Devonian.

Lead is the second metal that exceeded the WHO quality standard (0.01 mg/L) in agricultural irrigation canal water, specifically between the towns of Muquiyauyo and San Lorenzo. It is not ruled out that this metal would come from the effluents discharged into the waters of the upper Mantaro river basin, and that in the study area they are used for irrigation through canals. In the channel of the town of Muqui, this metal exceeds the WHO quality standard by more than 11 times.

Unlike arsenic and lead, aluminum content did not exceed the WHO quality standard (0.9 mg/L), except in the irrigation canal of the town of Muqui, where it exceeded the quality standard by more than two times. It is related to the low relative mobility of this metal in alkaline conditions and its possible leaching as a result of irrigation, especially in the agricultural land of the aforementioned locality.

Manganese shows a behavior similar to aluminum, so the irrigation canal in the town of Muqui was the only place that exceeded the quality standard determined by the WHO (0.4 mg/L). It is probably influenced by its low relative mobility in basic media and its possible leaching from the irrigation of agricultural land in the town of Muqui.

Unlike the metals described above, zinc shows the lowest relative mobility in alkaline media, in this case very low. Zinc concentrations did not exceed the WHO quality standard (3 mg/L), with the maximum concentration being around 0.357 mg/L, located in the irrigation channel of the town of Muqui.

Based on the findings found, heavy metal content was found in the waters of the Mantaro River, some tributaries, and irrigation canals, with arsenic being the metal with the highest occurrence of concentrations that exceed the WHO standard (0.01 mg/L) these concentrations denote a range from 0.011 to 0.035 mg/L (Figure 6). Lead is next in importance, whose contents exceeded the WHO standard (0.01 mg/L), specifically in the canals of the towns of Muqui (0.111 mg/L), Muquiyauyo (0.032 mg/L), Huamaly (0.012 mg/L). L) and San Lorenzo (0.017 mg/L), as shown in Figure 7. Aluminum (2.36 mg/L) and manganese (0.50 mg/L) only exceeded the WHO standards in the channel of the Muqui locality (Figures 8 and 9). As for zinc, its concentrations do not exceed the WHO standard (3 mg/L), as shown in Figure 10. The impact of heavy metals in the waters of the Mantaro basin describes a predominantly fluvial pattern consistent with some geochemical factors such as mobility, pH, and adsorption, in addition to the main anthropic variable, in this case, the mining environmental liabilities located upstream of the investigated area. The fluvial course impacted by arsenic has an approximate length of 35 km between the towns of Parco and Leonor Ordóñez; while lead impact approximately 10 km of the river course between the towns of Ataura and Leonor Ordóñez. It is important to mention that 75% of the sampled irrigation canals are impacted by at least one of the metals under study.

It is evident that the hypotheses of authors such as Arce (2017) and Custodio et al. (2019, 2020), who state that the mining liabilities located in the headwaters of the Mantaro basin are the cause of the metals present in the waters of the Mantaro River and the channels studied. Likewise, it is shown that the waters studied are predominantly alkaline, with pH values ​​from 7.3 to 8.8, denoting a physicochemical similarity with the alkaline and polluted scenario of the Chachas River, described by Aylas et al. (2021). Bivariate statistics determined the Al-As-Pb-Zn association, which would be related to the main polymetallic ores in central Peru. Undoubtedly, it is necessary to expand the investigations regarding heavy metals in the Mantaro river basin to correlate the geochemical dispersion in other matrices such as soils and sediments.^18–22

The waters of the Mantaro River and the main agricultural irrigation canals between the towns of Parco and Leonor Ordoñez show metallic contents that exceed the WHO standards in at least one of the chemical elements studied. Arsenic is the metal that predominantly impacts the waters of the study area, this is explained by its greater relative mobility in alkaline media, related to the Triassic-Jurassic carbonate formations. Given the arsenic dispersion pattern, it is postulated that it comes from mining environmental liabilities located in the headwaters of the Mantaro basin. The same-origin is attributed to lead, which exceeded the quality standard in the irrigation canals located between the towns of Muquiyauyo and San Lorenzo. The only sample (24m-032) where aluminum and manganese exceeded the WHO quality standard is located in the irrigation channel of the town of Muqui. For both metals, this scenario is consistent with their low relative mobility in basic environments and their possible leaching as a result of agricultural irrigation. The highest concentration of zinc was identified in sample 24m-032 (0.357 mg/L), although it did not exceed the WHO quality standard. The metallic contents studied to define a marked hydrochemical association under alkaline conditions, made up of Al, As, Pb, and Zn, which suggests that these metals come from mining residues, a product of the exploitation of polymetallic deposits housed mainly in the carbonated units of the center from Peru.

To the authorities of the Geological, Mining and Metallurgical Institute - INGEMMET, for their support in the development of this research.

There are no conflicts of interest.

1. Arce S. Suelos contaminados con plomo en la ciudad de La Oroya - Junín y su impacto en la calidad del agua del rio Mantaro. UNMSM. 2017. Tesis de Maestría. 2016.
2. Oviedo R, Moina E, Naranjo J, et al. Contamination by heavy metals in the south of Ecuador associated with the mining activity. Bionatura. 2017;2(4):438–441.
3. Bhuyan M, Bakar M, Akhtar A, et al. Heavy metal contamination in surface water and sediment of the Meghna River, Bangladesh. Environ. Nanotechnol. Monit. 2017; 8:273–279.
4. Giri S, Singh A. Assessment of surface water Quality Using Heavy Metal Pollution Index in Subarnarekha River, India. Water Qual Expo Health. 2014;5(4):173–182.
5. Bouguerne A, Boudoukha A, Benkhaled A, et al. Assessment of surface water quality of Ain Zada dam (Algeria) using Multivariate Statistical Techniques. Int J River Basin Manag. 2016; 15:133–143.
6. Nnorom I, Ewuzie U, Eze S. Multivariate statistical approach and water quality assessment of natural springs and other drinking water sources in Southeastern Nigeria. Heliyon. 2019;5(1):e01123.
7. Xiao J, Wang L, Deng L, et al. Characteristics, sources, water quality and health risk assessment of trace elements in river water and well water in the Chinese Loess Plateau. Sci Total Environ. 2019;650(Pt 2):2004–2012.
8. Chao-yang L, Jing-dong Z, Fei L, et al. Trace elements spatial distribution characteristics, risk assessment and potential source identification in surface water from Honghu Lake, China. J Cent South Univ. 2018;25(7):1598–1611.
9. Varol M, Şen B. Assessment of nutrient and heavy metal contamination in surface water and sediments of the upper Tigris River, Turkey. Catena. 2012; 92:1–10.
10. Orellana E, Bastos C, Sotelo B, et al. Potential risk of Pb to children's health from consumption of cow’s milk in areas irrigated with river water contaminated by mining activity. Scientia Agropecuaria. 2019;10(3):377–382.
11. Aylas A, Campos A, Perez M, et al. Evaluation of the Quality of Drinking Water and Rivers in the Mantaro Valley, Central Peru. IOP Conf Ser Earth Environ Sci. 2021;943:012002.
12. Quispe-Zuniga MR, Santos F, Callo-Concha D, et al. Impact of Heavy Metals on Community Farming Activities in the Central Peruvian Andes. Minerals. 2019;9(10):647.
13. Xiandeng Hou, Renata S Amais, Bradley T Jones, et al. Inductively Coupled Plasma Optical Emission Spectrometry. Encyclopedia of Analytical Chemistry. 2016.
14. H Bin, S Li, GQ Xiang, et al. ‘Recent Progress in Electrothermal Vaporization—Inductively Coupled Plasma Atomic Emission Spectrometry and Inductively Coupled Plasma Mass Spectrometry’. Appl Spectrosc Rev. 2007;42(2):203–234.
15. U.S. EPA. “Method 200.7: Determination of Metals and Trace Elements in Water and Wastes by Inductively Coupled Plasma-Atomic Emission Spectrometry,” Revision 4.4. Cincinnati, OH. 1994a.
16. U.S. EPA. “Method 200.8: Determination of Trace Elements in Waters and Wastes by Inductively Coupled Plasma-Mass Spectrometry,” Revision 5.4. Cincinnati, OH. 1994b.
17. Pabón S, Benítez R, Sarria R, et al. Water contamination by heavy metals, analysis methods, and removal technologies. A review. Entre Ciencia E Ingeniería. 2020;14(27): 9–18.
18. Ministerio del Ambiente. Decreto Supremo N°004-2027-MINAM Aprueban Estándares de Calidad Ambiental (ECA) para Agua y establecen Disposiciones Complementarias. Lima: Ministerio del Ambiente; 2017.
19. World Health Organization. Guidelines for drinking-water quality: fourth edition incorporating the first addendum. Geneva: World Health Organization; 2017. License: CC BY-NC-SA 3.0 IGO.
20. Oncevay J. Cumplimiento de la normatividad ambiental por el sector minero metalúrgico y su impacto ambiental en el río Mantaro – Región Junín. UNCP. 2013. Tesis de Maestría.
21. Custodio M, Zapata F, Cruz D, et al. Potentially toxic metals in lotic systems with an aptitude for aquaculture at the watershed Mantaro River, Peru. Ambiente & Água - An Interdisciplinary Journal of Applied Science. 2019;14(1):1–14.
22. Custodio M, Cuadrado W, Peñaloza R, et al. Human Risk from Exposure to Heavy Metals and Arsenic in Water from Rivers with Mining Influence in the Central Andes of Peru. Water. 2020;12(7):1946.