Heavy Metals in Agricultural Soils of San Francisco de Macorís and La Vega, Dominican Republic ()
1. Introduction
The production of quality food in sufficient quantity is of utmost importance to guarantee the nutrition of people around the world. To achieve this in agriculture, it is necessary for crops to obtain the necessary micronutrients from the soil to develop and bear healthy fruits that contribute to a healthy diet for those who eat them. There are plants that are capable of storing large amounts of metals in their roots, stems, leaves or fruits; that could be toxic when ingested by people in significant amounts (Raju et al., 2016). Heavy metals such as Zn and Cu are necessary for the good development of various plants, but they have a limit for safety. The excessive bioaccumulation of metals in fruits changes their texture and quality (Sepúlveda et al., 2020; Shahbaz et al., 2018; Martínez-Alva et al., 2020). In the same way, it happens with the deficiency of nutrients including some metals that are necessary for plants to carry out photosynthesis (Ali et al., 2011; Thi-Xuan et al., 2017). The Canadian Council of Ministers of the Environment (CCME) has produced a table or guide (CEQGs, Canadian Environmental Quality Guidelines) compiled in studies to rate the quality of agricultural soil by its heavy metal content. The United States Environmental Agency (USEPA) also developed the SQuiRTs (Screening Quick Reference Tables) table used by NOAA for marine sediments. This is of vital importance since a soil is considered contaminated when the concentrations of metals are higher than in soils that have not been intervened or influenced by human activities. If the metal concentrations at the time a mining area is evaluated correspond to those that existed before the mining activity began (Arranz-González & Alberruche, 2007), this area is not considered contaminated.
In the region of La Vega and San Francisco de Macorís, our study area, the soils are basically made up of clayey rocks composed of minerals moderately rich in iron, copper, chromium and nickel (Aiglsperger et al., 2015) many of them forming sulfides that have been sedimented by being deposited during floods (Fuentes-Hernández, 2019) and overflows of the Camú River, from the Upper Miocene to the Pliocene, resting on plutonic-metamorphic rocks composed of micritic limestones and andesites. In addition to the clays that were washed away by the Yuna River, these are rich in iron, nickel and iridium minerals (Garutiite); rhodium, nickel and arsenic (Zaccariniite) (McDonald et al., 2010; Vyzmalová et al., 2012), composed of igneous and Tuscan rocks as well as volcanic-sedimentary rocks, volcanic-plutonic rocks, metamorphic amphibolites and mafic intrusive volcanic rocks. This occurred from the Upper Jurassic, Upper Cretaceous to the recent Quaternary, where coarse-grained sand layers were previously deposited (Villanova-de-Benavent et al., 2016).
According to the hydrodynamics of the region, the soils that were formed by the sedimentary depositions of the Yuna River are characterized by the presence of the Ni mineral (peridotite) and other minerals composed of heavy metals such as iron and cobalt (Marchesi et al. 2016). While the soils of La Vega and San Francisco de Macorís due to the dragging of the Camú and Jima-Masipedro rivers could present high concentrations of metals, especially Ni for the same reason, a bioaccumulative element in the rice plant (AqeelKamran et al., 2016). Along the highway that goes from La Vega to SFM there are several towns and communities that influence the levels of metals through the waste of agricultural implements and domestic activities (Romic & Davor, 2003). For the study of trace metals in soils and sediments, techniques such as atomic absorption spectroscopy by flame or graphite furnace (AAS) (Pérez-Olvera et al., 2008), X-ray fluorescence (XRF) (Fernández-Ruiz, 2009), mass spectrometry (ICP-MS) (Sibello-Hernández et al., 2014) and atomic fluorescence spectrometry (AFS). XFR is a fast, inexpensive and non-destructive technique that is often used for soil and sediment analysis due to the advantages it offers over the other techniques mentioned. It is not so sensitive, being very useful when it is not required to determine very low values (Roca & Bayon, 1981). The methodology for preparing the sample for the analysis is very simple and as it is a non-destructive technique, the samples can be used for other analyzes by other techniques if there is a need to verify the results. This advantage over atomic absorption spectroscopy makes it very convenient because it does not use acids, fuels and does not generate polluting residues from combustion (Marguí et al., 2011). When determining heavy metals in agricultural soils of the study region, their presence is sought, in order to evaluate their levels. This information would help us to determine if the soils are contaminated when comparing it with the levels of the continental crust (Rudnick & Gao, 2003), by their concentrations with those that are normally present in uncontaminated soils that serve as a reference (Freedman, 2018). In addition to its impact on the possible bioaccumulation in agricultural products of wide consumption (Prieto-Méndez et al., 2009), imply damage to animal and human health that ingests it. In the Dominican Republic, very little is known about the content of heavy metals in agricultural soils, since the various studies that have been carried out on the composition of the rocks have been conducted for mining purposes, not environmental ones. Using the levels determined with this study as a baseline, it would be possible in the future to determine if there has been any degree of contamination caused by industrial, agricultural or domestic activities. Currently, although the levels of trace metals are very high, we cannot establish whether mining, industrial, agricultural or domestic activities have contributed because no information was found on the composition of the agricultural soils studied.
2. Study Zone
The study area is located in the Cibao region, between the provinces of La Vega and San Francisco de Macorís, with contributions of water from the Monseñor Noüel province, where there is a deposit of ferronickel (peridotite) that could change the quality of the water (Gjoka et al., 2010). The mineral is found in the basins of the Camú, Jima and Masipedro rivers. The Jima and Masipedro rivers are dammed in the Rincón reservoir; these waters are used to irrigate the rice plots of the western and southern part of the sampling area of this study (Lewis, et al., 2006). The use of the land is essentially dedicated to the cultivation of rice and livestock, with crops of small fields of sweet potato, cassava and banana. The smallest area corresponding to the province of La Vega has 10.6 km2, while the largest area located within the limits of the provinces of La Vega and San Francisco de Macorís (SFC) has an area of 71.0 km2 (Figure 1).
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Figure 1. The blue areas are the soil sampling locations; the one on the east side located in La Vega (LV) and the one on the west of San Francisco de Macorís (SFM) between La Vega. Fuente: Google Earth.
3. Materials and Methods
3.1. Sampling
The place where each soil sample was taken was referencing with GPS, preferably at an approximate distance of 250 m within a plot. A plastic shovel was used to collect approximately 1.0 kg of soil. The samples were selected from a 15 cm column and packed in plastic bags to be transported at room temperature with the humidity conditions found in the soil. When the samples are to be carried out other tests, they must be kept refrigerated to prevent volatile elements from being lost.
3.2. Drying
The samples were dried, first at room temperature, and then to remove all the humidity in a convection oven at a temperature of 100˚C.
3.3. Sample Preparation
After drying, the samples were crushed in mortars and then sieved to 75 microns. 3.0 ± 0.005 g were taken, measured in a precision balance of 0.0001 g, with this mass of soil, using a Specac press, the sample was subjected to a pressure of 15 tons/cm2, making a tablet with a diameter of 2.0 cm.
3.4. Analysis
The pellet was taken and placed on a Skyray EDX3600B brand x-ray fluorescence spectrometer (Delanoy et al., 2022). The excitation voltage of the x-ray emitting source was 40 kV and 600 μA. The spectrometer was previously calibrated using standard soil and sediment samples certified to ISO/IEC 17025 and ISO Guide 34 by Sigma-Aldrich (TraceCert; NIST, IAEA) and BAM-CRM (SRM1944, SRM2704, SRM1646a and IAEA356, SRM2710a). The quality of heavy metal determinations was verified using certified materials BCR277 and SRM1944. The NOAA-SQuiRTs and USEPA guideline values for marine sediments coincide with the Canadian Agricultural Soil and Sediment Guideline. For this reason we compare the Threshold Effect Levels (PEL) of a metal with the limits of the guide for a healthy soil in relation to this metal.
4. Results and Discussion
Two of the heavy metals that are important when evaluating the quality of a soil are Arsenic and Cadmium (Bravo-Covarrubias et al., 2020), are very harmful to human health (Londoño-Franco et al., 2016). These two metals in the soils analyzed in the two study regions were in all cases well below the TEL according to the agricultural soils guide, both from Canada (CEQGs) and from other countries (SQuiRTs). In the case of Mercury, only some samples show significant values, in the rest the values were below the detection limit, 0.2 mg/kg. The Zn in one sample exceeded the PEL and two the TEL, in the other samples the values were below the TEL, which in a general sense places these soils with low levels
of Zn in the La Vega and SFM area, so they do not pose any risk to crops. These soils can be considered not contaminated with Zn (Figure 2(d)) according to the
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Figure 2. Distribution of Cu, Zn, As andPbin the soils sampled in La Vega and San Francisco of Macorisversus their respective Probable Effect Level (NOAA-USEPA).The Cu, Zn, As and Pb values in both cases do not exceed the Probable Effect Level.
FAO and NOAA-USEPA no need intervention its (Figure 3(d)). Cu, the values in the soils did not exceed the PEL, only some values exceed the TEL value in La Vega (Figure 2(a) & Figure 3(a)), so they are considered suitable as agricultural soils in terms of this metal according to the FAO. In SFM near the town a value higher than the TEL was found; all Copper values in the SFM soils are below the PEL. This value could be linked to anthropic activities in soil management (Cordero, 1979). In the La Vega area, the majority of Cr levels exceeded the PEL (Figure 4(a)) according to the values of the FAO, NOAA-USEPA and CCME standards; with greater attention to those located north of the road that connects La Vega (LV) with San Francisco de Macorís (SFM). The same happens with those in the LV-SFM area that exceed the guide Canadian Agricultural Soils (CEQGs) in all places and the table (NOAA-USEPA-SQuiRTs), which suggests that these soils have high concentrations of Cr (Figure 4(a) & Figure 5(a)) that could affect those crops that are sensitive to the metal; as well as being a bioavailable source for bio-accumulating crops (Mesa-Pérez et al., 2015). In the LV area, several Pb values exceeded the PEL, many of them higher than the TEL, north of the highway between the towns of La Vega and SMF (Table 1 & Figure 2(d)). According to the SQuiRTs table, these soils do not need intervention in relation to Lead (Figure 3(d)). As for the soils of the SFM province, they were higher than the TEL, except for some values that were below; one value was higher than PEL. Although in the SFM area the values slightly exceed the TEL, this area, like La Vega, needs some attention, to avoid reaching the PEL according to CEQGs-SQuiRTs. In the case of Ni, almost all the values exceeded
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Figure 3. Distribution of Cu, Zn, As and Pb in the soils sampled in La Vega and San Francisco of Macorisversus their respective Intervention Level (NOAA-USEPA). Regarding Cu, Zn, As and Pb, the region does not need intervention measures.
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Figure 4. Distribution of Cr and Ni in the soils sampled in La Vega and San Francisco of Macorisversus their respective Probable Effect Level (NOAA-USEPA). The Cr and Ni values in both cases exceed the PEL.
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Figure 5. Distribution of Cr and Ni in the soils sampled in La Vega and San Francisco of Macorisversus their respective Intervention Level (NOAA-USEPA). The Cr and Ni values in both cases exceed the Intervention Level.
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Table 1. Normal concentrations in soils, threshold effect levels, probable effects and intervention level according to the NOAA-USEPA SQuiRTs table and Canadian Sediment quality guidelines CSQGs (CCME, 2014). Right, averages concentrations in study zone. The average values of Hg and As correspond to some samples where levels higher than 2 mg/Kg are found.
1Normal Value in Soil; 2Threshold Effect Level; 3Probable Effect Level; 4Intervention Level.
the PEL (Figure 4(b)) in the LV area; in this case we cannot consider it as contamination since it is to be expected because it is a region influenced by Nickel mining; only in some samples the levels were below the TEL. In the western part of the LV-SFM area the Ni exceeded the PEL; so is the PEL value in most samples. Further towards SFM the values do not reach the PEL values except for one sample. In this area the influence of Ni mining is minimal, as it is far away. Chromium and Nickel need intervention according SQuiRTs table (Figure 5(a) and Figure 5(b)); while Copper, Zinc, Arsenic, and Lead do not need it (Figures 3(a)-(d)). According to the distribution of heavy metals in LV, the Cr and Ni values were higher than in SFM (Figure 6(a) and Figure 6(b)); while Cu, Zn, and Pb (Figure 6(c), Figure 6(d) & Figure 7(b)) were found to have higher values in SFM than in LV. The distribution of As was similar in both areas (Figure 7(a)).
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Figure 6. Comparative distribution of Cr (a), Ni (b), Cu (c) and Zn (d) in the La Vega (LV) and San Francisco de Macoris (SFM) regions. To the left of each graph corresponds to LV and to the right to SFM. The colors correspond to the normal ground level (green), threshold effect level (yellow) and probable effect level (red) of each element.
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Figure 7. Comparative distribution of As (a) and Pb (b) in La Vega (LV) and San Francisco de Macoris (SFM) regions. To the left of each graph corresponds to LV and to the right to SFM. The colors correspond to the normal ground level (green), threshold effect level (yellow) and probable effect level (red) of each element.
5. Conclusion
The Cr and Ni levels in almost all the agricultural soil samples in the La Vega and San Francisco de Macorís area exceed the PEL values of the Canadian Agricultural Soil Quality Guide (CEQGs) and the Sediment and Soil Quality Guide of the United States of America USEPA-SQuiRTs-NOAA, so they are considered agricultural soils not suitable for growing crops that are bioaccumulators of these metals in their food product. The Pb in the majority of soil samples exceeded the TEL value and in the vicinity of the villages the PEL in agricultural soils, so it is necessary to consider these soils in cultivation that are bio-accumulators of lead in their food product. Elements As and Hg were found in a general sense below the respective PEL values. Zn and Cu in both areas only in one sample exceed the PEL value of the Zn, possibly due to specific anthropic activities. We consider these soils of good quality in relation to Zn and Cu. The high levels of Ni and Cr are associated with the fact that in the La Vega and Bonao zone there is a deposit of peridotite, a Ni as well as Cr forming spinel mineral. The heavy metals found in the study area were dragged from the mining area by the Camú and Jima Rivers and later deposited in the agricultural soils studied. Therefore, it is not recommended to carry out intervention measures.
Acknowledgements
To the Ministry of Higher Education, Science and Technology for their financing. To the staff of the Dean of the Faculty of Sciences of the Autonomous University of Santo Domingo, especially to Dean Radhames Silverio for his support in administrative procedures. To the assistants Kendrick Perez, Nisia Nathaly Rosa Delgado, Merali Princesa Rosario Morillo and Thara Caba for their collaborations in the preparations of the samples. To Juan Pablo Gonzalez for his support in taking the samples. Also to the reviewers and editors of this article.