Lead Bioaccumulation in Root and Aerial Part of Natural and Cultivated Pastures in Highly Contaminated Soils in Central Andes of Peru

Article history: Received: 21 November, 2019 Accepted: 25 February, 2020 Online: 09 March, 2020 Lead concentration on surface soil (0-20 cm), root and aerial part of natural and cultivated pastures were evaluated, in the rainy season (March 2018), collected in 20 sites of a rural community located 20 km from the La Oroya metallurgical complex , which has been emitting to the environment particulate material with heavy metals since 1922. Lead concentration was determined by flame atomic absorption spectrometry. The data was statistically processed in SPSS 23. Lead levels in the soil, root and aerial part of the cultivated pastures were 224.75 ± 39.41, 169.13 + 58.79 and 20.73 ± 2.52 mg / kg (p <0.01). In natural pastures values were 210.87a ± 40.37, 184.36b + 52.66 and 19.47c ± 3.12 mg / kg (p <0.01). There are no differences between cultivated and natural pastures. Lead transfer factor from soil to root of cultivated and natural pastures was 0.75 and 0.87. Lead transfer factor value from soil to aerial part of cultivated and natural pastures was 0.092 in both. High lead content in soil and aerial part of the pastures used as food for high Andean cattle is a public health problem; livestock products produced in these soils would not be fit for human consumption.


Introduction
The important mining activity for Peru [1] involves the emission of particulate matter (PM) loaded with heavy metals, which are transported by air [2] and deposited in water and soil, then transferred to pastures, causing adverse effects on animals and human health [3][4][5][6].
Lead soil contamination affects its quality, destroys the power of self-purification through biological regeneration, decreases the normal growth of soil microorganisms, alters biodiversity and decreases crop yields [7].
Due to the long half-life and high potential for bioaccumulation, lead is available to grass roots causing damage to the same plant [8] generating a highly contaminated biomass with serious consequences for human health [9][10][11][12][13]. Lead (Pb) toxicity affects the central nervous system, cardiovascular, digestive and urogenital system, and causes different types of cancer [14][15][16].
The Pb absorption is affected by the concentration of other metals, pH and organic matter content [17] and bioaccumulation of chemical elements in plants depends on their concentrations in soil [10]. Bioaccumulation of toxic heavy metals pose a threat to human health, induce renal tumors, reduce cognitive, development, and increase blood pressure and cardiovascular diseases risk for adults [18,19].
The transfer factor (TF), is the relationship between the concentration of metal in the aerial part of the plant and that of the soil [20,21]. The TF quantifies the metal bioavailability and is an indicator of the extent of metal mobility [20,22]. The TF is the key parameter of the heavy metal accumulation in plants, because it is the main pathway of human exposure to soil contamination by heavy metal accumulation [20].
In the central region of Peru, for more than 90 years, soils have been contaminated by metallurgical emissions of fine particulate ASTESJ ISSN: 2415-6698 material, rich in lead and other heavy metals [24], which have been deposited in the soil and they are available for plants [25,26,17]; in this case, Pb bioaccumulates in the pastures consumed by cattle, sheep and South American camelids.
Analysis of the soil surface layer (between 0 to 20 cm) will provide valuable information to study the dynamics of contamination of Pb from soil to plant [27].
A main activity of families living in Andean communities located near the metallurgical complex is the raising of sheep, alpacas and cows in large areas of natural pastures, and there is no information on the lead content of pastures consumed by these high Andean cattle.
This research contributes information on the soil-root-plant relationship, by determining the concentration of Pb in soil, root and natural and cultivated pastures and their transfer factors (Figure 1), contrasting the content of Pb in soil and grass of the study with levels of environmental quality safety [28,29].
Results represent the most up-to-date cut in the content of Pb in the surface soil and pastures of a highly contaminated area of the central highlands of Peru. This information can be used to establish a better regulation of environmental and human health protection by Pb contamination. Figure 1 shown the sequence of the pasture contamination by heavy metals. Self-created model

Study area
Peasant community "Purísima Concepción de Paccha" (CCP-LO), of Yauli province, Junín-Peru, is situated between South Latitude 11 ° 31´03" and West Longitude 75 ° 53´58" (altitude 3,700) and located 10.2 km from metallurgical complex La Oroya (Figure 2), industry dedicated to poly-metallic transformation (copper, zinc, silver, lead, indium, bismuth, gold, selenium, tellurium and antimony), that emits fine particulate material (PM), loaded with heavy metals that pollute the ecosystem for more than nine decades [23,24]. PM rich in Pb and other heavy metals is deposited in the soil and water resources, and being easy transferred to the plants [25,26,17]. Figure 2 shown the geographical location o the CCP-LO.

Sample collection
The soil and grass samples collected for the study approximately correspond to 1 ha of natural pastures (NP) and 1 ha cultivated pastures (CP) (Figure 3).
In March 2018 (rainy season), a total of 60 samples were taken (20 parallel samples of soil, root and aerial part of natural and cultivated pastures, 10 samples for each pasture type) at twentyfive-meter zigzag intervals. Samples were taken from upper soil layer (0-20 cm) [30,31,32] of 1 ha of NP and 1 ha of CP near the CCP-LO stable ( Figure 3). Five random subsamples taken of approximate area of 1m 2 were mixed until 0.5 kg soil sample was completing in a polythene bag and then taken to the laboratory. Soil samples were air dried for 48 hours, and then sieved with 2 mm mesh to remove gravel, stone and other materials prior to analysis. Sample collection details are provided in [27,33]. The forage samples were collected from the same place where soil samples were collected and on which the cows were grazing. The forage samples were divided into root and aerial part [33] and placed in paper bags to take to the laboratory.

Lead concentration quantification
For sample digestion according to USEPA method 3050B (SW-846) to extract the metals, 1 gram of the dried sieved soil was digested with repeated additions of concentrated nitric acid (HNO3) and hydrogen peroxide (H2O2). Hydrochloric acid (HCl) was added to the initial digestate and the sample is refluxed. The digestate was diluted to a final volume of 100 mL [34]. Lead concentration was determined by flame atomic absorption spectrometry (FLAA), following the AOAC-Official Method 975.03 protocol [35]. Analysis were in duplicate and the lead concentration units are expressed in mg / kg. To determine the concentration of Pb in the roots and aerial part of the pastures the same analysis procedure was followed.
8. Add 2 mL of water and 3 mL HCl (minimum effervescence) and concentrate up to 5 mL 9. Filter and refine at a 100 mL vial. Apply the quality controls of the sample. 10. Calibration curve building and analysis the sample by flame atomic absorption spectroscopy (FLAA). 11. Report results in units of mg / kg. Quality Control: Method blank (BK), Duplicate Sample (DM) and Control Pattern (PC) were performed in high and low range for every 15 samples. For the calibration curve, Pb Sigma-Aldrich 986 + -4 mg / kg was used as standard. In addition, the Pb detection limit was 0.2 mg/kg.

Statistical analysis
Information was processed in SPSS 23. Descriptive statistics were used and variance analyzes were conducted for lead contents in the soil, root and aerial part of the plant, with Tukey significance tests at a confidence level P <0.05. To determine the difference in means between Pb in soil and grass with the maximum permissible limits, "t" tests were performed for a single sample. Maximum limits used for soil and grass were 70 and 30 mg / kg.

Lead concentration in the soil, root and aerial part of pastures
The Pb concentrations in descending order were soil> root> grass (p <0.01). The data had normal distribution patterns. There were no statistical differences between natural pastures (NP) and cultivated pastures (CP) ( Table 1, Figures 1, 2).

Transfer of Pb from soil to the root and aerial part of pastures
The transfer factors of soil Pb to the root of CP and NP were 0.75 and 0.87. The transfer factor of soil Pb to the aerial part of CP and NP was 0.092 for both.
In this study, the percentage of Pb transfer from the soil to the aerial part of the grasslands was 9.2% (Table 2, Figure 3), a percentage nine times higher than that reported in conditions of less pollution [9].

Discussion
La Oroya soils have received pollution since the metallurgical complex began operations in 1922, and has poured 0.39 Mt of Pb [36]. In 2008, in La Oroya and Paccha the average concentrations of PM2.5 were 32.4 and 20.3 ug / m 3 [37], which exceed the ECA PM2.5 = 15 ug / m3 [38], and the average concentrations of PM10 were 52.3 and 42.4 ug / m 3 , values similar to ECA PM10 = 50 ug / m 3 [38]. At the end of the last century, in La Oroya-Yauli, average values of Pb in the air were reported exceeding the upper limit by 800% (1.5 ug / m 3 ). Between 2009 and 2014, reported Pb levels in air were up to three times more than upper limit [39,40].
Heavy metals contaminate water and soil, transfer and bioaccumulate in plants, and affect human and environmental health [25,26].
Accumulated heavy metal contamination for decades in the study area would determine high concentrations of Pb in the soil, root and aerial part of NP and CP. The soils in the study area have more than 3 times of Pb than the maximum established in Peru (70 mg / kg) [28] and more than 3.5 times that established in Finland (60 mg / kg) [29]. It is reported that concentrations of Pb in the soil, between 10 and 30 mg / kg have no detrimental effects on plant growth [41].
Lead concentration in Paccha community soil is of high risk to human health, because it bioaccumulates in the crops produced. Excess Pb in soil affects its quality, microbial development and biodiversity [7] and in agri-food products it can cause a series of health problems [42][43][44][45].
Average concentration of Pb in NP and CP determined in this study (19.46 and 20.73 mg / kg), reflects accumulation of Pb over time, since the smelting began activities in 1922, as reported in other latitudes too [46]. High Pb content inhibits the absorption of all mineral elements in the plant [47], affects its performance [48], decreases food safety and quality, causing multiple adverse effects on human health [11,6,15,16].
In Caldas-Colombia, the Pb of soils adjacent to petrochemical activity is transferred and bioaccumulated in brachiaria crops, and the highest levels of Pb bioaccumulation have the following order: root> stem> leaves [25,26].
Average concentration of Pb in NP and CP in this study was 6.7 times higher than the critical value proposed by other researchers for vegetables 0.05-3.0 mg / kg [49]. Our results were similar than those observed in pastures areas near to La Oroya reported in a Missouri study [50]. The study soils have a high concentration of heavy metals compared to other parts of Peru and the world, which can lead to unwanted economic and social results.
Pb concentrations in NP and CP were similar and consistently higher than those observed in studies conducted in other latitudes [51,45]. This study shows that the bioaccumulation of Pb in the grass roots gives it control over its concentration in the aerial part of the grass. Of Pb concentration present in the soil, only 9.5% is transferred to the aerial pasture. Roots capture a large amount of Pb from the soil (> 83%), transferring 12% of Pb to the aerial part of the plant; However, concentration of Pb in the edible part of the pasture is a threat of toxicity to high Andean cattle, according to another research [52].
In areas near the refineries, Pb contents in 31 brachiaria species, is values between 9.8 and 16.0 mg / kg, being higher closer to the emission focus. In places around the exploration wells, Pb contents between 9.7 and 13.2 mg / kg are reported [25,26].
Average Pb in the pastures of the study area was lower than those recorded around sites containing lead slag in Nigeria, forage grasses had between 209-899 mg / kg, with an average of 425 ± 79.0 mg / kg [53].
Although the average Pb content in the aerial part of the pastures produced in the study area did not exceed the allowed limit of the European Union (30 mg / kg), it exceeds the normal range of 5-10 mg / kg recommended by other authors [54]. Considering as a limit value of Pb in pastures 10 mg / kg [54], the average of Pb in pastures of this study was 2 times higher, not being suitable for feeding cattle [55].
Regarding cultivated pastures, in Trifolium alexandrium, Brassica campestris and Avena sativa associations, average concentrations of Pb was between 36.5 and 60.21 mg / kg [52], values higher than the European Union toxic level (30 mg / kg) [56], and those found in this study.
In New Zealand pastures, an average value of Pb is reported in grass (10.6 mg / kg) with a range of 4.4 to 26.8 mg / kg [27]; observing that the toxicity of Pb causes a decrease in the percentage of germination, growth, dry biomass of roots and shoots, alteration of mineral nutrition, reduction in cell division and inhibition of photosynthesis [43,57].
NP and CP increase the accumulated amount of Pb in the root to a higher Pb content in soil, as observed in another study [58]. Accumulation of Pb in the aerial part of the grass is also increased to a greater amount of Pb in soil and root. Comparing the results obtained in both types of pastures, the accumulated amounts of Pb in the roots and the aerial part were similar in NP and PC. In both types of pasture, the concentration of Pb in the roots was higher than that of the shoots, which suggests that the transport of lead from the roots to the shoots is restricted and the root maintains a high Pb content in its structure, and the cumulative amounts of lead in the aerial part of the plants were greater than those reported in other studies [58].

Conclusion
Pb concentration was significantly higher in the soil than in the root and aerial part of natural and cultivated pastures used in Andean livestock food -in areas close to metallurgical emissions.
Lead level in soil and grass exceeded the toxic levels suggested by national and international standards; So, the soils and pastures are intoxicated.
The information for the ecosystem studied is useful to take measures to reduce the adverse effects on human health by the consumption of food produced in these soils. This evidence should be used to establish action plans for removal and remediation.