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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.45 n.1 Concepción mar. 2000

http://dx.doi.org/10.4067/S0366-16442000000100017 

COPPER AND SELENIUM IN RAINWATER, SOILS AND ALFALFA 
FROM AGRICULTURAL ECOSYSTEMS OF VALPARAISO 
REGION, CHILE

I. DE GREGORI1*, G. LOBOS1, S. LOBOS1, H. PINOCHET1, M. POTIN-GAUTIER2 AND M. ASTRUC2

1Instituto de Química, Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile.
2Laboratoire de Chimie Analytique, Bioinorganique et Environnement, EP CNRS 132,
Université de Pau et des Pays L'Adour, Pau, France.
(Received: October 4, 1999 - Accepted: December 16, 1999)

ABSTRACT

Total copper and selenium concentrations were determined in rainwater, soils and alfalfa samples collected on April-May 1997 at various sites from three different zones of Valparaíso region, in Chile: Puchuncaví and Catemu valleys, both impacted by mining activities and Casablanca valley (reference site).

In rainwater Cu and Se concentrations clearly decreased exponentially with the distance from the sources and were significantly correlated.

The results obtained demonstrate that the mining activities have contributed to increase the contamination by copper in the agricultural ecosystems, particularly those located near the smelters. In all matrices from Puchuncaví and Catemu valleys copper concentrations were higher than those found for samples from the reference site. In Puchuncaví valley Cu concentrations in soils and alfalfa show close correlation and were function on the distance from the smelter.

The selenium concentration in soils and alfalfa show a very different behaviour. They were relatively constant in these matrices and do not present a clear dependency on the distance from the sources. In alfalfa they ranged between 0.10 and 0.40 mg Kg-1. These concentrations are not very high and do not seem to state problems to livestock grazing in these zones.

KEY WORDS: Copper, selenium, rainwater, soils, alfalfa, agricultural ecosystems from Chile.

RESUMEN

Cobre y Selenio son elementos esenciales para la vida humana y animal. Sin embargo, dependiendo de las concentraciones, estos pueden causar problemas de toxicidad, ya sea por deficiencia o exceso.

En el presente trabajo se determinaron las concentraciones de cobre y selenio en muestras de distintos componentes de tres ecosistemas agrícolas de la región de Valparaíso: agua de lluvia, suelos y alfalfa provenientes de los valles de Puchuncaví y Catemu, lugares donde se realizan actividades mineras metalúrgicas de importancia y del valle de Casablanca, lugar considerado como referencia. Las muestras fueron recolectadas en el período Abril-Mayo de 1997.

Los resultados mostraron que en los tres tipos de muestra las concentraciones de cobre decrecen en función de la distancia de la fuente emisora. Este mismo comportamiento fue observado para selenio en agua de lluvia. Sin embargo, las concentraciones de este elemento en suelos y alfalfa son bajas y similares (entre 0,1 y 0,4 mg Kg-1).

Se obtuvieron buenas correlaciones entre las concentraciones de cobre en los distintos componentes de los ecosistemas. Por el contrario sólo se obtuvo una buena correlación entre las cantidades de cobre y selenio depositadas por agua de lluvia proveniente de los distintos sitios de muestreo del valle de Puchuncaví.

PALABRAS CLAVES: Cobre, selenio, agua de lluvia, suelos, alfalfa, ecosistemas agrícolas de Chile.

INTRODUCTION

Although toxic elements are naturally present in the environment, industrial processes often result in increased concentrations in air, water and soil. Subsequently, these elements are taken up by plants and animals and make their way into the food chain. The contamination of the environment by trace elements concerns principally the zones where most metallurgical industries and mining activities are located; i.e. it is estimated that more tha 90% of the total industrial emissions of Cd, Cu, Pb and Zn in Poland come from the south-western and south-central region of the country, where most metallurgical activities are carried out1,2).

The consequences of such contamination are that toxic element concentrations in locally grown vegetables and crops may greatly exceed accepted standards, as may the element uptake by the consumers. This effect is more pronounced in areas close to the sources of contamination, primarily metal smelters. More remote areas may also be contaminated, because a substantial portion of toxic elements released from these sources into the environment are emitted into the atmosphere bound to small dust particles, as fumes or aerosols. Therefore, a significant pathway for the pollution of terrestrial and aquatic ecosystems by toxic elements goes through the atmosphere; according to the meteorological conditions they can be transported over varying distances before deposition in the different ecosystems. In principle, deposition of trace elements from the atmosphere can occur by dry deposition of dust particles or wet deposition with rain and snow. Wet deposition is the prevailing deposition mode, except in the immediate vicinity of a point source3).

In Chile many ecosystems have been and are still susceptible to be contaminated by toxic elements produced especially by the mining and industrial processing of ores and metals (i.e. Cu, Mo), as these activities play an important role in our country. In 1997 alone, it is estimated that copper production reached to 3.400.000 ton4), but these activities are well known as an important source of contamination by toxic elements, due to the enormous quantities of waste products emitted.

The purposes of this study were to determine the distribution and to compare the behaviour of copper, a metallic pollutant, and selenium, a non metallic pollutant, in rainwater, soils and alfalfa from agricultural ecosystems, impacted and non impacted by mining activities, in the Valparaíso region of Chile, in order to assess the input of these elements by these activities.

Copper was chosen because it is one of the most important essential elements for plants and animals and Chile is one of the most important producers of this metal in the world. Selenium was retained due to its ecotoxicological significance in animal nutrition. The amount of Se present in the diet may result in either deficient or toxic responses and it is present in such significant amounts in copper ores that are obtained as a by product of the copper electrolytic refining processes.

The average Cu concentration in the earth's crust ranges from 24-55 mg Kg-1 and the average Cu range for soils is 20-30 mg Kg-1. Atmospheric inputs of Cu to soils from both rain and dry deposition varies considerably according to the proximity of industrial emissions containing Cu and the type and quantities of wind-blown dust5). Cu in soils can be associated with soil organic matter, oxides of iron and manganese oxides, soil silicate clays and other minerals. Copper is specifically adsorbed or fixed in soils, making it one of the trace metals which moves the least5). Extractable or available Cu in soils correlates statistically with concentrations absorbed and assimilated by plants. Typically, Cu concentrations in plants range from 1-30 mg Kg-1. Cu accumulation differs among plant species and cultivates differences.

The abundance of Se in the earth's crust is reported to about 0.05-0.09 mg Kg-1 and usually Se is associated with sulphide ores, where it is incorporated into the sulphide crystal lattice by a process of isomorphous substitution of S. In the natural environment, elevated concentrations of Se in soils are associated with volcanic materials and sulphide ore bodies. Se is apparently readily oxidised during weathering, becoming more mobile with increasing oxidation state. In soil, most transformations of Se appear to be microbially mediated through process such as oxidation and reduction, immobilisation and mineralisation and methylation. Many fungi and bacteria in soils are capable of reducing inorganic Se, either to elemental or to volatile and non volatile organic compounds. Immobilisation of Se reduce its availability to plants6-11).

Cu and Se are essential nutrients for animals, but are toxic at high concentration and can enter in the food chain through plants. The primary Se source for animals is through feed. Se from soil is incorporated to plant tissues, depending on various environmental factors6-11). The total Se concentration of soil-pasture systems is important in relation to the health of grazing livestock. Deficiencies of the element result in muscular disorders and impaired reproductive performance in both sheep and cattle. Concentrations of less than 0.05 to 0.1 mg Se Kg-1 in animal feed can produce Se deficiency; toxic effects on animals can occur when Se concentration reaches 2 to 5 mg Kg-1 12-14). Because of these narrow range between requirements and toxicity, treatment and prevention must be conducted with care.

Vegetables are one of the most important components of the animals diet, specially for cattle and horses. The total elements concentrations of soil-pasture systems is important in relation to the health of grazing livestock. In this study alfalfa was selected as vegetal matrix, because it is classified as quality hay and can supply the nutritional requirements of essential elements for livestock's and horses, both types of animals grazed in this region. A quality hay, such as alfalfa, can supply the selenium nutritional requirements of a mature horse. Alfalfa has been reported as a passive Se accumulator. The average concentration is 0.03-0.88 mg Kg-1, but levels as high as 57 mg Kg-1 have been reported15,16).

EXPERIMENTAL

Study area

For this study three sampling zones were selected in the Valparaíso region, two of them affected by the emissions of mining complexes: Puchuncaví valley, an agricultural zone, at the north of Valparaíso city, receives the impact of the industrial complex "Las Ventanas", where are located both a smelter and electrorefinery plant for copper ore as well as a coal-fired thermoelectric power plant; Catemu valley located eastern of Valparaíso, in the Aconcagua river valley, that also receives the influence of a Cu ore smelter (Chagres). Both zones can be therefore regarded as locations strongly affected by emissions of mining complexes. The third selected zone was Casablanca valley, a rural and agricultural area, located at south of Valparaíso. This location is not particularly submitted to the impact of mining activities and due to its similar characteristics with Puchuncaví valley has been often selected as a reference area in all environmental problems associated with the industrial complex "Las Ventanas"17,18).

The choice of sites in both impacted zones was mede taken into account the topography and the prevailing direction of winds19). The average annual rain deposition in this zone of Chile is 200-300 mm and takes place only in the period between May-October (autumn and winter seasons). In winter the wind blows from north to west and in the other periods of the year from south to west.

A map with the sampling zones and sampling sites for soils and alfalfa is shown in Figure 1. Four sites were selected in Puchuncaví valley (La Greda, Los Maitenes, Puchuncaví and Nogales), four in Catemu valley (Catemu, San José, Santa Margarita and Panquehue) and one in Casablanca valley. For rainwater sampling, samplers in duplicate were placed at different distances from the sources: six in Puchuncaví valley, three in Catemu valley and two in Casablanca valley.

Sampling and sample treatment

Soil and vegetal samples of alfalfa (Medicago sativa L) were collected simultaneously (April-May 1997) in all samples sites (previously described). The soil samples were taken from 0 to 20 cm; in the laboratory they were air dried at ambient temperature and then sieved through a 2 mm screen. The general characteristics of the soils were determined. pH values of soils were measured using a 1: 2 suspension (soil weight: water volume); the pe values were calculated from the potentials measured in the same solutions, using Pt and calomel electrodes; organic matter (OM) was determined by dichromate oxidation followed by the titration of excess dichromate with Fe(II)20). Fe was measured by FAAS in the acid digested samples (the methodology will be described subsequently).

Alfalfa samples were randomly collected from the same sites of soil sampling, the whole plants were taken but only the aboveground tissues were analysed in this study. Samples ( ª 1 kg at each site) were cleaned with deionized water in order to remove the superficial dust particles and were stored at -20°C until their treatment. Samples were crushed and homogenised using a plastic food processor (Moulinex) specially adapted with high purity titanium blades. Homogenised samples were frozen at -20°C and then lyophilised (Lyovac GT2), for 72 h at 15°C and at 0.1 mbar pressure. Lyophilised samples were homogenised again and preserved in desiccators at room temperature until the analysis was carried out.

Duplicate rainwater samples were collected in each site during the first rainfall event in 1997 in this region (on 17 May) in home made samplers, a polyethylene funnel 21.5 cm of upper diameter joined to a polyethylene container of 4 L capacity, both acid cleaned and rinsed with water with great care. The assembly was supported in a wood framework. These samplers were stored in closed polyethylene bags that were manually opened and closed at the beginning and at the end of the rainfall, thus sampling was restricted to wet deposition. The sampler was positioned on a rack, 2 m above the ground to avoid interference of soil particles during rainfalls.

On the same day, the samplers were carefully transported to the laboratory and the total volume of rain collected in each site, pH and conductivity were immediately determined in an aliquot. The rest of the samples were acidified to pH 2 with conc. HNO3 divided in two portions, and immediately evaporated in hot plates.

Rainwater samples were digested with 2 mL conc. HClO4 and slowly evaporated until almost complete evaporation. The final volume was made to 10 mL deionized water. This last treatment was made to eliminate the possible organic matter present in the rainwater samples; it is well known that organic matter may interfere in the Se determination by cathodic stripping voltammetry21-23).

Analysis

For soil analysis, duplicate 0.8-1.0 g sub-samples were added to the PTFE vessels of Uniseal teflon high pressure decomposition systems. Samples were submitted to two different digestion methods; in the first digestion a HNO3 + HClO4 + H2O2 mixture was used (method A). In the second one only 10 mL conc. H2O2 was added (method B). The procedure followed in method A consists first in the addition of 5 mL conc. HNO3 + 3 mL conc. HClO4. In both methods after the liquid reagents were added, the teflon vessels were covered with teflon plates and allowed to stand over night at room temperature. Then, the addition of 3 mL of 30% H2O2 sp. was carried out; after 4-6 h the closed Uniseal systems were heated on a heating block, at 170°C for 3 h. The cooled samples were quantitatively filtered and evaporated on a hot plate. In method A the solution was made to 10 mL with deionized water. In method B 2 mL of conc. HClO4 were added to the filtered solutions and then heated until almost complete evaporation; finally the solutions were made to 5 mL (elimination of organic matter).

FIG. 1. Map showing the sampling zones: Puchuncaví valley (Sites: 1) La Greda 2 Km; 2) Los Maitenes 2.6 Km; 3) Puchuncaví 8 Km; 4) Nogales 26 Km); Catemu valley (Sites: 5) Catemu 4 Km; 6) San José 5 Km; 7) Santa Margarita 6 Km; 8) Panquehue 13.5 Km) and Casablanca valley.

For alfalfa analysis, 10 mL of conc. HNO3 and 5 mL of H2O2 (30%) were added to 0.5 g of lyophilised material in a teflon vessel and allowed to stand overnight at room temperature. Then 3 mL of conc. HClO4 were added and the mixture was again maintained at room temperature, for 6 h at least. Further digestion was conducted at 170°C for 3 h. Deionized water was added into the digested solution to bring the final volume to 10 mL.

Cu concentration (digestion method A for soils) was measured in all samples by flameless atomic absorption spectroscopy (FAAS) on a GBC 905 AA atomic absorption spectrophotometer equipped with a deuterium background corrector.

Se in rainwater and alfalfa samples was determined by differential pulse cathodic stripping voltammetry at a hanging mercury drop electrode on a Princeton Applied Research Assembly (Model 264A polarographic analyser, Model 303A SMDE).

In the digested solutions selenium(VI) reduction to Se(IV) was made in a 1:1 mixture of sample with conc. HCl, in a closed tube, heating at 90°C for 40 min. It is necessary to reduce Se(VI) to Se(IV), because only this last species is electroactive at the mercury electrode21-26). 0.2 to 1.0 mL of this reduced solution was placed into the electrochemical cell containing 5.0 mL of 0.1 M HCl, as supporting electrolyte. Se was quantified by the standard addition method. The diluted standard solutions of Se were prepared daily from a 1000 mg L-1 Se titrisol (Merck). To increase the sensitivity some measurements were made in the presence of 2 mg L-1 Cu(II) 24-26).

Se in soil digested by method B was quantified by graphite furnace atomic absorption spectroscopy (GFAAS) using a GBC 950 AA assembly with a deuterium background corrector with an auto sampler (PAL 3000). Measurements were made in the presence of 5 µl of 2000 mg L-1 Pd(NO3)2 used as chemical matrix modifier. The Se lamp is a Photron super lamp (l = 196.0 nm, i = 18 mA). The furnace program was: three dryng steps at 110°C (10 s); 140°C (10 s) and 250°C (10 s), an ashing step at 700°C (5 s); atomisation at 2200°C (3 s) with a cleaning step at 2400°C (1 s).

All material used in this study was acid cleaned, the reagents employed were of suprapure quality and the water used was obtained from a Nanopure purifying system (Barnstead).

All results are reported as mean ± confidence limits (p < 0.05 dry weight basis d.w.)

RESULTS AND DISCUSSION

Quality control

Determination of trace elements in environmental samples requires strict quality control of the analysis. The application of quality assurance requires the analysis of certified reference materials (CRM) that match as closely as possible the matrix type and the element concentration level of the real samples. In this case the accuracy of the analytical methodologies applied for soils and vegetals were assessed analysing different CRM. Results of the CRM analysis are summarised in Table I. For quality control in the analysis of rainwater, blank samples and also rainwater samples spiked with Cu and Se were treated simultaneously with the real samples. Copper and selenium recoveries ranged from 95 to 103% (average 101%) and from 90 to 108% (average 97%), respectively.

As can be seen, the results for both elements analysed are in agreement with the certified values. The measured values, including their uncertainties, lie within the certified values and their confidence intervals (95%), and therefore there is no reason to believe that the differences are significant27).

TABLE I. Results obtained for the CRM analysed (A: recomended values, B: acid digestion, C: H2O2 digestion).


CRM Element Certified value Experimental value

  Copper (µg g-1) 34.6 ± 0.7 33.8 ± 0.5
San Joaquin Soil Selenium (µg g-1) 1.57 ± 0.08 1.4 ± 0.2 (C)
SRM 2709 (NIST)     1.7 ± 0.2 (B)
  Iron (%) 3.50 ± 0.11 3.4 ± 0.2
Irish soil (BCR) Selenium 5.9 ± 0.6 (A) 5.5 ± 0.1 (C)
      6.0 ± 0.3 (B)
White clover Copper (µg g-1) 12 ± 1 (A)   11.9 ± 0.4
CRM 402 (BCR) Selenum (µg g-1) 6.70 ± 0.25 7.1 ± 0.6 (B)
      6.7 ± 0.4 (C)

Soils

Table II shows some characteristic of sampled soils. As can be seen the pH values of soils indicate that, according to the Blakemore classification28), all soils are neutral or slightly alkaline, with the exception of one of them, Puchuncaví as site with a moderately acidic soil. Brady29) states that the normal pH is 5-7 in soils of humid regions and pH 7-9 in the soils of arid region, taken into account that the samples zone of Chile is an arid region the pH values found can be considered as normal, and are similar to those described for this region30).

The pe values show that soils from Casablanca location differ from the others, the highest values being found for this site. However the values obtained permit to classify all of them as oxic soils. Oxic conditions usually give values in the range pe 5.1-13.5, but mostly from 6.8 to 10.1. In general, heavy

metal cations are almost mobile under oxic and acidic conditions and increasing the pH reduces their bioavailability. Redox reactions in soils are frequently slow but are catalysed by soil micro-organisms, which are able to live over the full range of pH and pe conditions normally found in soils.

All soils contain organic matter, although the amount and type may vary considerably. For the sampled soils the lowest organic matter and iron contents were found for soils from Casablanca (the reference site), statistically significant differences were obtained for this site with regard to the others.

TABLE II. Some chemical characteristics of the sampled soils.


Zone Site pH pe OM(%) Fe(%)

La Greda 7.5±0.1 6.56 2.97±0.02 2.4±0.2
Puchuncaví Los Maitenes 7.3±0.2 6.3 3.00±0.03 2.4±0.2
valley Puchuncaví 5.9±0.2 6.91 2.0±0.1 2.5±0.1
Nogales 7.3±0.1 6.19 2.20±0.03 3.5±0.4
         
Catemu 7.5±0.2 6.86 3.06±0.02 2.6±0.4
Catemu San José 8.0±0.1 6.69 1.72±0.07 2.7±0.1
valley S. Margarita 7.8±0.2 6.7 1.70±0.05 2.6±0.2
Panquehue 7.9±0.2 6.47 1.89±0.02 2.7±0.2
         
Casablanca Casablanca 7.1±0.1 7.56 1.05±0.03 1.9±0.1
valley          

pe: E(V)(NHE)/59.16; OM: Organic matter

In Table III are listed, simultaneously, the sites where the rainwater were sampled with the respective total volumes of rainfall. As can be seen the total volumes of water fall in the first 1997 rainfall, were very different at the diverse sampling zones. The lowest volumes correspond to the sites from Puchuncaví valley, zone located near the Pacific coast; Casablanca presented intermediate values and the highest volumes were collected at Catemu valley, a zone located at the proximity of the Andes mountains.

On the other hand from the pH values measured all samples must be considered as acid rains except for rain from Nogales sites. Rainwater from Puchuncaví valley shows a clear decrease in acidity with distance from the smelter. In this context it has to be taken into account that these deposited rain water which have pH range from 4.3 to 4.7 will leach and dissolve heavy metals from the dust particles deposited on the surface soils during the dry periods. Acid rains were also found in these regions in rain samples collected in 1983 at different sites from Puchuncaví and Catemu valleys31).

The high pH and conductivity values obtained for rainwater from Nogales could be due to the fact that in this zone agricultural activities are well developed and urea is usually added to agriculture soils, primarily as a source of Nitrogen.

TABLE III. Total volume and some chemical characteristics of rainwater samples (V: valley; C: city; N: north and S: south).


Zone Site Total volume (mL) pH Conductivity (µS, 25°C)

  La Greda 367 4.39 17
  Los Maitenes 353 4.47 9.5
Puchuncaví Puchuncaví-V 433 4.58 7.0
valley Puchuncaví-C 479 4.64 7.0
  Nogales-N 550 6.3 12.5
  Nogales-S 505 6.77 23.5
  Catemu 1440 4.81 6.5
         
Catemu San José 1344 4.63 7.0
valley S. Margarita 1328 4.38 11.5
         
Casablanca Casablanca-V 1060 4.61 6.5
valley Casablanca-C 1040 4.68 7.0

COPPER AND SELENIUM IN RAINWATER, SOILS AND ALFALFA

Rainwater

The amounts of heavy metals deposited by rainwater depend on the heavy metal concentration in the precipitation and the volume of rainfall. These differences in precipitation volumes can be responsible for the differences in the element concentrations between the various sampling locations, the amounts of copper and selenium found in the total volumes and also for the amounts of these elements per area unit.

TABLE IV. Volumes and amounts of selenium and copper of rainwater at different sampling sites (V: valley; C: city; S: south; N: North).


Site Distance from Deposited amount
  the Selenium Copper
  Source (Km) µg µg m2 µg µg m2

La Greda 2 (E) 0.48±0.15 13±3 64±5 1749±141
Los Maitenes 2.6 (E) 0.14±0.04 3.9±0.9 31±3 854±68
Puchuncaví-V 8 (NE) 0.12±0.01 3.1±0.3 24±4 661±104
Puchuncaví-C 9 (NE) 0.09±0.01 2.4±0.3 9±2 248±39
Nogales-S 26 (NE) 0.04±0.01 1.1±0.3 4.5±0.9 124±25
Nogales-N 28 (NE) 0.008±0.001 0.22±0.02 3.0±0.7 83±19
Catemu 4 (N) 0.03±0.01 0.8±0.3 16±2 441±40
San José 5.5 (NE) 0.010±0.002 0.28±0.04 12±1 333±37
Sta. Margarita 7 (NE) 0.013±0.003 0.36±0.07 5.3±0.8 146±22
Casablanca-V - 0.047±0.003 1.3±0.1 3.0±0.7 83±20
Casablanca-C - 0.14±0.06 3.8±0.9 4.5±0.9 124±25

Comparing the results obtained for the different zones, it can be remarked that significantly larger but also varying more widely as function of location are the selenium and copper amounts deposited in Puchuncaví valley than those obtained for Catemu valley. However, when the values from both impacted zones are compared to those observed at Casablanca, while the copper amounts deposited in all sites are highest with the only exception of Nogales N, the Se amounts for Casablanca were similar or higher than those from Catemu valley and also for the sites located at long distance from the source in Puchuncaví valley. The Se levels at Casablanca appear higher than some of those where mining activities are developed, and this suggests the presence of another unidentified type of anthropogenic source.

The different behaviour observed for the rainwater from both impacted zones can be explained taking into account that at Chagres smelter (Catemu valley) several actions were taken in order to decrease the total emissions from the smelter, specially the airborne particulate. Today emissions of fumes and dusts from the plume of the Chagres smelter chimney is practically not observable. At Las Ventanas complex some actions were also carried out to decrease emissions from the smelter, but only in 1998 the emissions to the atmosphere were drastically decreased32). It is also necessary to take into account that at Las Ventanas are located a smelter and electrorefinery plant of copper ore and also a coal fired electric power plant, recognised also as one of the major anthropogenic source of toxic elements contamination.

Analysis of rainfall near the smelter sites, as might be expected, specially at Puchuncaví valley, has demonstrated considerable enrichment of both elements. As can be seen in Figure 2, the Cu and Se amounts present in the rainwater falling in the sampling sites were very different and the amounts of both elements clearly decrease exponentially with the distance from the source:

Ln Se [µg m-2] = (2.1±0.4) - (0.11±0.02) distance (Km), r = -0.9025

Ln Cu [µg m-2] = (7.1±0.3) - (0.09±0.02) distance (Km), r = -0.9322

Both elements having the same behaviour (the slopes of both relations are similar), the amounts of Se and Cu in rainwater from Puchuncaví valley were significantly correlated (see Figure 3). Some authors have described that Se has the property to form highly insoluble compounds with metals such as metal selenides. During the smelting processes one would expect Se to be released as SeO2 and a variety of reactions can occur inside the chimney plume34).

FIG. 2. Copper and selenium amounts (µgm-2) deposited by rainfall at the diverses sites from Puchuncaví valley. (LG) La Greda, (LM) Los Maitenes, (P-V Puchuncaví-V, (P-C Puchuncaví-city, (N-S Nogales South and (N-N) Nogales North.

Soils

Cu and Se concentrations in soils and alfalfa samples are presented in Table V. It can be seen that these elements have not the same behaviour in soils from the different sampling zones. While Se concentrations in each sampling zone are low and relatively constant (0.3 to 0.4 mg Kg-1 in Puchuncaví valley (see Figure 4A) and 0.15 to 0.23 mg Kg-1 in Catemu valley) and these last values are not higher than the values obtained for soils from Casablanca valley (0.17 ± 0.01 mg Kg-1); Cu concentrations of soil samples show a wide range of values, from 28 mg Kg-1 for Casablanca soils (the reference site) to 440 mg Kg-1 at La Greda, level considered to reflect gross contamination33). All sites in Puchuncaví and Catemu valleys have Cu concentrations higher than the level found at the reference site. The most highly contaminated site, La Greda, has a Cu content up to 16 time the level found for soils from Casablanca. In Puchuncaví valley, a clear decrease is observed in Cu concentrations with distance from the smelter (Figure 4B). This situation is not so clear in Catemu valley.

Major sources of pollution such as smelters, usually yield the highest concentrations in soils within 1-3 Km of the stack with concentrations decreasing with distance.

FIG. 3. Relationships between the amounts of copper and selenium deposited by the rainfall at Puchuncaví valley.

TABLE V. Selenium and copper concentrations in soils and alfalfa from different zones of Valparaíso region.


  Soils Alfalfa  
Zone Site Se Cu Se Cu
(distance Km) (mg Kg-1 d.w.) (mg Kg-1 d.w.)

La Greda (2.0) 0.36±0.05 443±24 0.28±0.05 60±2
Puchuncaví Los Maitenes 2.6) 0.31±0.06 382±10 0.30±0.02 56±3
valley Puchuncaví (8) 0.40±0.04 143±5 0.37±0.07 41±1
Nogales (26) 0.37±0.03 89±2 0.22±0.06 8.6±0.2
         
Catemu (4) 0.22±0.03 113±3 0.15±0.02 24.1±0.7
Catemu San José (5.5) 0.20±0.02 127±5 0.10±0.03 34±3
valley Sta. Margarita (7) 0.23±0.07 183±3 0.12±0.03 82±4
Panquehue (13.5) 0.15±0.02 62±2 0.23±0.03 12.1±0.2
         
Casablanca Casablanca 0.17±0.01 27.9±0.4 0.16±0.03 9.3±0.4
valley          

At Catemu valley, Santa Margarita soils have a Cu content up to 1.5 time the level found at San José, a little village near Santa Margarita. An explanation to this fact could be that the road crossing Santa Margarita have been filling with solid wastes from Chagres smelter.

Selenium concentration in studied soils never exceeded the reported critical toxicity values (5-10 mg Se Kg-1)31), but ranges in the normal levels (0.1 - 5 mg Kg-1) and were even close to the lower limits (0.1-0.4 mg Kg-1). On the contrary, all Cu concentration values for soils from Puchuncaví and Catemu valleys are higher than those considered as critical (60-125 mg Cu Kg-1) 33), only Casablanca soils have a lower content: (28 mg Kg-1), value included in those considered as background levels in agricultural soils5). It must be also considered that the ingestion of these soils may constitute an additional source of Cu for livestock living in these zones.

On the other hand, as might be seen from Tables II and V, there is no clear correlation between the levels of Cu or Se and the soil properties such as pH, pe and organic matter content in these soils.

FIG. 4. A) Copper and selenium concentrations in soils from Puchuncaví valley. LG La Greda, LM Los Maitenes, P Puchuncaví and N Nogales.
B) Relationship between Cu concentration and the distance from the source.

Alfalfa

Copper and selenium toxicities and deficiencies have been known to cause endemic diseases in animals in many parts of the world35,36). The occurrence depends, among other factors, on the availability of the elements to plants and animals.

Although it is well established that Se is an essential nutrient for the normal functioning of animals and humans, it is less clear whether Se is essential for plants35). On the contrary, Cu has been established as a component of a number of different plant enzymes. Also deleterious effects can occur when humans and other animals consume diets that contain sub-optimal levels, as well as toxic levels of copper37). The total element concentration of soil-pasture systems is important in relation to the health of grazing livestock.

As can be seen in Table V, Se concentration in alfalfa from different sites is relatively constant for a same zone (an average of 0.3 µg Se g-1 and 0.15 µg Se g-1 for alfalfa from Puchuncaví valley (see Figure 5A) and from Catemu valley, respectively). Se concentration does not follow a clear pattern on the distance from the sources. These values are included in those reported as the average Se content of alfalfa: 0.03-0.88 mg Kg-1 and were even close to the lower limits16,38). Se toxicity in livestock can only occur at concentrations as high as 3-4 mg Kg-1 material eaten14). This pattern permits to suggest that if the alfalfa cultivated in this region is the main forage for the livestock diet, the nutritional Se requirement should be supplied.

Relatively to Cu concentrations in alfalfa, all sites have values higher than the site selected as reference (9.3 ± 0.4 mg Kg-1) with the exception of Nogales (8.6 ± 0.2 mg Kg-1). As can be seen in Figure 5B, in Puchuncaví valley the Cu concentrations in alfalfa, as the Cu levels in rainwater (Fig. 2) decrease exponentially with the distance from the source.

FIG. 5. A) Copper and selenium concentrations in alfalfa from Puchuncaví valley. LG La Greda, LM Los Maitenes, P Puchuncaví and N Nogales.
B) Relationship between Cu concentration and the distance from the source.

Typically, Cu concentrations in plants range from 5 to 20 mg Kg-1. Accumulation differs among plant species and cultivate differences. However critical Cu concentrations for plants, defined as the level above which toxicity effect likely have been proposed, ranging between 20-100 mg Kg-1 32). Only alfalfa from three sites studied have Cu concentrations lower than these levels: Nogales and Panquehue, the sampling sites studied farther away from the respective smelter, and Casablanca, the reference zone.

On the other hand, a minimum dietary requirement for cattle of 10 mg Cu Kg-1 in dry matter has been recommended36). The uptake of Cu by pasture depends not only on the amount of Cu on the plant species but also on the amount and form of Cu in soils. It must be taken into account that on Cu-contaminated soils, as in this case, Cu in the herbage rarely exceeds 20 mg Kg-1 d.w. and animals may ingest up to 10 time more Cu in the form of soil than in herbage (alfalfa). For these reasons it is possible to conclude that copper in these ecosystems must be regarded with attention, and more studies should be carried out, specially those related to the speciation and bioavailability of copper.

Relatively to Cu in alfalfa and soils from Puchuncaví valley, close correlation was found between Cu concentrations in plants and Cu in soils (see Figure 6). This would suggest that alfalfa plants are capable of accumulating Cu from soils and tend to take up larger amounts when Cu levels in soils increase, until some threshold value. For accumulator species the relationship between uptake and substrate concentration is curvilinear: uptake becomes progressively less at high concentrations.

On the other hand, taking into account that Cu amounts in rainwater from Puchuncaví valley correlates with Cu content in soils and alfalfa (Figure 7A and B) the atmospheric inputs of Cu to soils from rain deposition appear, between other factors, to contribute to input of Cu to soils.

Relatively to Se concentrations in alfalfa, as in soils, a very different behaviour from Cu was observed; Se concentrations in both matrices are similar in all sampling sites from a same zone. Accumulation factor of each sampling site was calculated as the concentration of Se in alfalfa divided by concentration of Se in soil, these factors ranged between 0.5 to 1.1; no correlations were obtained for these parameters. Areas with low to moderate Se content show no general correlation between soil Se and plant content because of the larger numbers of factors influencing the availability of soil Se to plants39).

FIG. 6. Relationships between Cu concentration in alfalfa and soils in all sampled site.

Concerning the behaviour observed for Se in alfalfa and soils from the impacted zones studied, a different trend was expected, this taking into account that Se is obtained as by-product in the copper industry located at Las Ventanas and also due to the fact that Se levels in rainwater decrease with the distance from the smelter. The similar and low levels of Se found in plants and soils could be attributed to percolation of soluble Se species to the lower soils profile or to volatilisation of Se compounds through methylation, a mechanism used by micro-organisms such as bacteria and fungi to avoid Se toxicity in the environment40). The rate and efficiency of soil Se removal by plants or micro-organisms may vary greatly depending on the source of soil Se, the plant species, the physical and chemical conditions and the culture management strategies. For these reasons more research is needed to understand the behaviour of Se in both soils and plants.

FIG. 7. Relationships between Cu concentration in: A) rainwater and alfalfa, and B) rainwater and soils in all sampled sites, from Puchuncaví valley.

CONCLUSIONS

Although the results reported here should be considered as preliminary, they contribute to the scientific knowledge and information available in Chile about two essential elements, copper and selenium, present in the different components of agriculture ecosystems, information that is not reasonably well established in our country.

The results obtained in this work have demonstrated that the mining activities developed in the Valparaíso region have contributed to increase the copper concentration in the agricultural ecosystems, specially those located in the vicinity of the smelters. It has been shown that Cu is considerably enriched in soils and alfalfa, particularly in the sites located near the smelter. This information may be taken as reference in future studies, to evaluate if the actions presently developed by the copper industries, in order to reduce the contamination of the environment have been successful.

Clearly atmospheric transport is one of the principal sources that provide Se and Cu to soils in these regions. The fact that the amounts of both elements selenium and copper in rainwater from Puchuncaví present the same distribution pattern as function of distance would be indicating that these elements arise from the same anthropogenic sources: the copper smelter ore and/or the electric power plant.

In spite of the ratio Se concentration in alfalfa to soil being neighbouring to 1, the Se concentrations found in alfalfa are not very high and do not seem to state a problem to the livestock grazing in these zones. On the contrary Cu seems to be an element that must be carefully regarded, due to the high values found in some alfalfa and soils from these regions.

ACKNOWLEDGEMENTS

This study was supported by a grant from the Fondo Nacional de Ciencia y Tecnología de Chile, FONDECYT 1.97/1289. The authors also thank the DGIP of Universidad Católica de Valparaíso and ECOS-CONICYT (Scientific Cooperation between France and Chile) Action C96E04.
______________________________
*To whom correspondence should be addressed, Fax: 56-32-273422; e-mail: idegrego@ucv.cl

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