Determination of trace metals in water from Mangueira Lagoon – RS, Brazil

The coastal region of the Rio Grande do Sul state is characterised by a large number of rivers and lagoons, with the Laguna dos Patos, Mirim and Mangueira Lagoons standing out in volume.

Mangueira Lagoon (Figure 1), an integral part of the Mirim Lagoon Basin, is located in the eastern portion of the southernmost part of Brazil. Because it is an area with unique geographical features, a diverse biological population, a particular water regime and very characteristic human population dynamics, it was recognized by UNESCO as a biosphere reserve (Jica 2000). This lagoon consists of a large shallow lake with surface area of approximately 820 km², and an average depth of 2.49 m and maximum depth of 6.5 m. The shape of the lagoon is elongated in the north-south direction, with a length of 90 km and width of approximately 10 km. The basin of contribution to Mangueira Lagoon has an area of 417 km² (Tejadas et al. 2016).

The fishing activity of the region is developed artisanally and is the main source of income of numerous families of fishermen (Caldasso et al. 2006). Most of the region is also used for the production of rice and livestock and the groundwater of the region is intensely used for the domestic supply of the rural population (Silva et al. 2009).

According to Silva et al. (2009) the territory around Mangueira Lagoon is primarily characterised by rice crop irrigation since the 1930's, with a significant water resources consumption and evidence of environmental changes.

The potential for contamination of water bodies is high in areas where rice is grown under flood conditions. Rice is one of the world's crops with the highest consumption of pesticides (Rodrigues 2014).

The use of agrochemicals in agriculture is necessary for the protection of cultivated plants, as it can expand their productive potential. However, the inadequate management of agrochemicals in crops can lead to contamination of water sources (Gunningham & Sinclair 2005).

When introduced into the environment, about 55% of the total pesticides applied do not reach the target, dispersing to other environmental compartments such as water, soil and atmosphere (Gavrilescu 2005).

According to Carneiro et al. (2015) data from the National Health Surveillance Agency (ANVISA) and the Agrochemical Industry Observatory of the Federal University of Paraná (UFPR), released during the II Seminar on Agrochemicals and Regulation Market, held in Brasilia, Federal District in April 2012, indicates that over the last 10 years the world market for agrochemicals grew 93%, while the Brazilian market grew 190%. In addition to the active toxic component, many of these products present potentially polluting elements or compounds, such as heavy metals, surfactants, emulsifiers, etc (Costa et al. 2004).

Heavy metals are the most toxic agents known and can cause harmful effects on organisms because they have cumulative, mutagenic and carcinogenic properties (Klaassen 2001).

To point out the limit from which the concentration of a particular heavy metal in the water is a threat, first of all, it is necessary to define the uses to which the watercourse is destined (Rocha & Azevedo 2015).

Based on the classification of water bodies in quality classes, the CONAMA Resolution No. 357 of March 17, 2005 (BRASIL, 2005) establishes water quality conditions and standards to be respected to ensure their prevailing uses or to meet the needs of the community.

When metal pollution reaches the water, the whole ecosystem of that site will be contaminated, because through water, metals can be absorbed by the plants and algae that are the basis of almost all trophic chains (Rangel & Sanches Filho 2014).

There are few studies that have ‘the knowledge of the characteristics of the study region’ as an objective, which makes the research of great importance for monitoring the environment and knowledge of water properties.

In this context, the objective of the present work was to evaluate the physico-chemical characteristics of Mangueira Lagoon water, such as pH, alkalinity, hardness, organic matter (OM), chlorides and conductivity as well as heavy metals – chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), zinc (Zn); and macronutrients like potassium (K), sodium (Na) and iron micronutrient (Fe) to determine the quality and environmental condition of this water resource.

The samples were obtained at five points, distributed along the Mangueira Lagoon: point one (P1) was collected where a channel drains the irrigation water leached from the rice fields (33°1′49″S and 52°42′23″W); point two (P2) (32°58′53″S and 52°40′9″W) 10 km from P1; point three (P3) (32°55′34″S and 52°38′42″W) 20 km from P1 towards the north of the lagoon; point four (P4) (33°4′18″S and 52°45′33″W) and point five (P5) at the coordinates (33°8′3″S and 52°46′8″O) to the south, 10 and 20 km, respectively, as shown in Figure 2. The sampling was performed in May 2016 (autumn), during the dry season. These sampling sites were chosen because they are possible points of entry of the contaminants and accessible by boat for taking samples.

Figure 3 shows the flowchart containing the steps from the collection to the characterize of the water samples collected at the five points of Mangueira Lagoon.

The collection of water samples was made with aid of a plastic jar at 0.5 m depth. Samples for metal determination were immediately acidified in the field with 1 mL of 65% v/v HNO₃ nitric acid Suprapur^® from Merck (Darmstadt, Germany). They were stored in decontaminated plastic bottles under refrigeration at ±4°C, then transported to the laboratory. All materials used in the treatment and storage of the samples were decontaminated in a solution of 10% (v/v) HNO₃ for 24 hours and oven dried at 105°C.

The analyzes of pH and conductivity were analyzed in the field with the aid of: pH meter (Q400A, Quimis, Diadema, Brazil) and conductivity (CD 830, Instrutherm, Freguesia do Ó, Brazil, all in triplicate). Hardness, chlorine, alkalinity and OM analyzes were measured in triplicate in the GPCA laboratory, where the hardness was determined by titration of complexation with EDTA, the chlorides determined using the method of Mohr, the alkalinity was determined by titration with hydrochloric acid in the presence of methyl orange and the OM by the method of permanganometry (APHA 2005).

For the determination of metals, all the samples were prepared and analyzed at the Laboratory of environmental contaminants (IFSul), where they were digested according to the analytical method described in APHA (2012). 200 mL of each sample was transferred to a teflon (semi-closed) beaker by adding 5 mL of HNO₃ p.a. from Merck and 3 mL of HCl p.a. from Merck. The beaker was placed on heating plate at 150°C until the volume reached 50 mL. Each sample was neutralized initially with NaOH a.m. from Merck and passed through the Chelex 100 resin column (3–4 g resin in sodium form, pH 8; 200 mesh) with a maximum flow rate of 2 mL.min⁻¹. The metals of the sample were stored in the resin and the water was discarded. Using a 2 M solution of Merck's HNO3 Suprapur^®, the metals were eluted to give a final volume of 10 mL, according to Barbosa et al. (2012) method. Na and K were determined directly after the sample was diluted to 50%.

The calibration curves were prepared from the dilution of Titrisol^® Merck with 1,000 mg L⁻¹ standard solutions of each metal analyzed. The concentration range of the standards used was 0.2 to 5.0 mg L⁻¹. The standards have undergone the same treatment of the samples to maintain the proportionality between the analytical signal and the concentration.

The HCl, HClO₄ and HNO₃ acids used in the analysis were all of analytical grade.

The samples were analyzed by flame atomic absorption spectrophotometry in a PerkinElmer AAnalyst 200 spectrophotometer for Cu, Cr, Pb, Zn, Ni and Fe. The metals K and Na were analyzed in the same equipment in the emission mode.

The operating conditions in the spectrophotometer for the metals analyzed are presented in Table 1.

Throughout the five white readings, the limits of detection (LD) and quantification (LQ) were calculated, using the sum of the mean of the blank signal plus three times its standard deviation for the LD, while the LQ was obtained by the sum of the mean of the white signal plus ten times its standard deviation (IUPAC 1997).

Table 2 shows the results obtained in the analysis of pH, chlorides, alkalinity, hardness and OM in water.

In Table 3, the values of the correlation coefficient (r²); the angular coefficient (a); the linear coefficient (b); the LD and quantification (LQ), both in mg L⁻¹ are presented. Using the merit parameters, it was possible to verify that the correlation coefficient remained above 0.988, indicating a high level of reliability of the calibration curves and the preconcentration method with the Chelex resin allows the determination of metals at the defined legislation levels.

Table 4 shows the results obtained in the analysis of the copper (Cu), zinc (Zn), lead (Pb), chromium (Cr), nickel (Ni) and iron (Fe) metals in Mangueira Lagoon water samples obtained by spectrophotometry of atomic absorption by flame and for potassium (K) and sodium (Na) by atomic emission spectrophotometry.

According to the data analyzed at five points it was possible to observe that the pH in water remained slightly alkaline, having an average of 7.46. These values are in accordance with the Class II standard of CONAMA Resolution No. 357/05 which estimates the range of 6.0 to 9.0 and with Britto (2012), who states that Mangueira Lagoon has a higher pH due to soil composition, being the home to a microalgae, Spirulina sp., beneficial to human health and capable of absorbing large amounts of pollutants from the atmosphere.

The pH in the water has an extreme importance in the control of the precipitation, mobility and bioavailability of metallic ions. When the pH value is high, the trace metal concentration in the water reduces.

The results in Table 2 shows that chlorine levels do not exceed the value established by CONAMA Resolution 357/05 for the Class II freshwater of 250 mg L⁻¹ at all the points studied. The first point presents the value of 21.3 mg L⁻¹ of Cl, which is three times lower than the others. In the natural environment, chloride comes from the dissolution of minerals or even from the intrusion of marine water, as well as from domestic and industrial effluent discharges and waters used in crop irrigation (Von Sperling 2005).

The alkalinity varied from 62.1 to 101.1 mg L⁻¹ CaCO₃ with a mean of 89.3 mg L⁻¹ CaCO₃, a value close to that found by Kist (2012), who analyzed the central area of the Mangueira Lagoon and obtained an average value of 66.85 mg L⁻¹ CaCO₃. The limestone used for soil enhancement provides carbonates and bicarbonates, which are directly related to the alkalinity of the waters, as well as the shells present in the lagoon.

In most fresh waters conductivity is between 10 and 105 mS/cm. However, this value can exceed 105 mS/cm especially in polluted waters, particularly in those that receive a large quantity of surface runoff (Arruda et al. 2015). In this study, the values are well below the considered range of pollution, being the highest value 377.9 μS cm⁻¹, similar to Andrade et al. (2012) for the same lagoon.

The parameters are correlated, indicating that at the point it has an entrance of water with a lower load of salts that gets concentrated as it enters the lagoon. The opposite behavior was observed for OM levels.

The hardness of the Mangueira Lagoon ranged from 58.7 to 116.0 mg L⁻¹, classifying the pond as: Water of moderate hardness.

Point 1 shows the highest result, 10.5 mg L⁻¹ O₂, indicating this as the main point of entry of OM. This is justified by being located near a farm, where there are agricultural activities that contribute to this organic load.

The studied metals can damage all biological activities. So, there are theoretically as many kinds of biological responses to these metals as there are types of biological activities.

The copper trace metal was found in all the analyzed points, with 7ppb being the highest concentration. This value is close to the limit stipulated in resolution CONAMA357/2005, which is 0.009 mg L⁻¹. Copper concentrations above 0.5 ppm are lethal to trout, carp, catfish, and above 1.0 ppm are lethal to microorganisms (CETESB 2012). Andrade et al. (2012) state that the surface waters of Mangueira Lagoon are more enriched in copper when compared to groundwater, probably due to anthropogenic inputs, such as the agricultural inputs used in the rice fields surrounding the lagoon. In addition to copper, other metals may be introduced into the aquatic environment through the water used in the rice plantations regions. This soil leaching can introduce heavy metals, such as Cd, Cr, Pb and Hg, into the aquatic environment, as well as micronutrients such as Fe and Mn.

Zinc showed a quantitative concentration at all points. Point 1, with the highest concentration, 27 ppb, is the one that receives the direct load, being the entrance of the flow from the farm in Mangueira Lagoon. According to Andrade et al. (2012), the volume of water drained from a cultivated area with flooded rice is approximately 1,000 m³ ha⁻¹, with water lost being the potential contaminant of nutrient sources, minerals and pesticides, including zinc.

Points one and two showed the highest concentrations of zinc as stipulated by CONAMA 357/2005, 18 ppb. Points three, four and five remained below the limit for Class II waters. These refer to reported levels of zinc to other references that indicate the natural values for surface water concentrations as up to 0.18 ppm (Odobasic 2012).

Among the analyzed points, three traces of Pb were found, ranging from 5 to 13 ppb, all below the limit established by CONAMA 357/2005 and below that found by Barbosa et al. (2012), of 2.45 μgL⁻¹, at Laguna do Patos in the same season.

Chromium is a trace element essential to human nutrition, but rarely found in natural waters. The toxicity of chromium, in relation to aquatic life, varies widely with species, temperature, pH, valence, dissolved oxygen (DO), synergistic and antagonistic effects (Sampaio 2003). In this research Cr was only found in point 1, with a concentration of 0.16 ppm, being above that established in the current legislation, which would be 0.05 ppm. By finding chromium at this point we can associate its presence with the use of fertilizers, because according to Bavaresco (2012), fertilizers may contain trace amounts of various metals and the presence of chromium can be observed in both mineral fertilizers and organic fertilizers.

The natural levels of nickel metal found in freshwater vary from 2 to 10 ppb (Azevedo & Chasin 2003). Only at points one and three was it possible to quantify this metal, the results being 7 and 6 ppb respectively. These values are below the limit established by CONAMA 357/2005, which is 25 ppb, and within the limit being considered as natural levels.

Iron was only found at point 2, with the value of 0.13 ppm, which is below 0.30 ppm, the maximum value established in CONAMA Resolution 357/2005 for Class II waters.

It is necessary to cross correlate pH data with this iron, since the oxidizing and alkaline environments promote the precipitation of Fe, while reducing conditions induce the formation of compounds. Fe (III) is relatively immobile under most environmental conditions, mainly due to its low solubility, while Fe (II) is more soluble under reducing conditions (Raiswell & Canfield 2012).

Potassium values ranged from 6.13 to 8.31 ppm in the five points analyzed. According to CETESB (2012), potassium is found in low concentrations in natural waters, however, potassium salts are widely used in industry and in agricultural fertilizers, and enter into freshwater through industrial discharges and agricultural areas. Concentrations in natural waters are usually less than 10 ppm. The value of the concentration in point 1 is the largest one found, due to the use of fertilizers in the rice fields around Mangueira Lagoon.

Natural water contains sodium in different proportions (it is a necessary element for living organisms). In surface waters the levels are usually below 50 ppm (Dovidauskas et al. 2017). The results of this research show Na values between 12.08 and 20.29 ppm, which can be considered natural, eliminating the suspicion of anthropological contamination.

Through the results of physical-chemical parameters of the water, The Mangueira Lagoon presented a low degree of contamination, but could in the future compromise the biota of the lagoon. With regard to the focus of this work, out of Cu, Cr, Fe, Ni, Pb, and Zn (heavy metals), only lead and iron had values above those established in CONAMA 357/2005. These values, however, are not at the level of impairment of the site biota, but serve as an alert because of their degree of toxicity, bioaccumulation and bioconcentration capacity. This research is of great importance due to the scarcity of data from this lagoon in the literature and, considering that this area is used for both agriculture and livestock, requires continuous monitoring of the water body in order to not compromise the local ecosystem.