Cu, Pb AND Zn DISTRIBUTION IN NEARSHORE WATERS IN SAN JORGE 
BAY, NORTHERN CHILE

DISTRIBUCION DE Cu, Pb Y Zn EN AGUAS COSTERAS DE BAHIA SAN
JORGE EN EL NORTE DE CHILE

Marco A. Salamanca¹, Andrés Camaño², Bibiana Jara¹ & Tatiana Rodríguez³

  ABSTRACT

An extensive spatial survey metal content to study the spatial distribution of Cu, Pb and Zn concentration in nearshore waters of San Jorge Bay, along the coast of Antofagasta City, Chile was carried out located near the Atacama desert, which represent the largest populated urban settlement in northern Chile (~ 260,000 inhabitants). The coastal zone of this area is under an intensive use as discharge location of pollutants of different sources. Dissolved Cu varies between 0.97 to 1.65 µg l^-1, in the southern part of the study area and in front of an industrial discharge, respectively. The dissolved Pb shows a narrow concentration range (0.019 to 0.029 µg l^-1) and Zn content varies between 1.7 µg l^-1 in the northern part of the study area to 2.0 µg l^-1, in front of the industrial discharge zone. The particulate Cu fraction pattern is similar to that of dissolved Cu, with concentrations varying between 126.8 µg g^-1 and 467.2 µg g^-1. Particulate Pb ranges between 9.8 to 33.8 µg g^-1, and Zn 149.7 to 403.5 µg g^-1. The spatial distribution of both, dissolved and particulate Cu, Pb and Zn results from local industrial sources located within Antofagasta City and San Jorge Bay current system that dilutes them as the water moves to the north. The particulate fraction is the dominant form of the metals, probably due to the dust coming from the Atacama desert, representing an important natural source of these metals to the water of the bay, particularly in those locations far from local inputs.

Keywords: Trace metal distribution, coastal waters, desert areas.

RESUMEN

En este estudio se realizó una investigación espacialmente extensiva para conocer la distribución de Cu, Pb, y Zn en el agua de la zona intermareal del sector costero de Bahía San Jorge, cerca de la ciudad de Antofagasta, que representa la mayor densidad poblacional (~ 260.000 habitantes), en el norte de Chile. La zona costera de esta bahía es intensamente utilizada como área de descarga de diferentes fuentes de contaminación. Los resultados indican que el Cu disuelto varía entre 0.97 y 1.65 mg l^-1, al sur del área de estudio y frente a la zona industrial de la ciudad, respectivamente. El Pb disuelto varía entre 0.019 y 0.029 mg l^-1 y el contenido de Zn entre 1.7 mg l^-1 en la parte norte del área de estudio y 2.0 mg l^-1, frente a la descarga de la zona industrial. La distribución de la fracción particulada de Cu es similar a la disuelta, con concentraciones que varían entre 126.8 mg g^-1 y 467.2 mg g^-1. El Pb particulado presenta un rango entre 9.8 y 33.8 mg g^-1, y el Zn entre 149.7 y 403.5 mg g^-1. La distribución espacial encontrada es el resultado de los diferentes aportes locales y del sistema de corrientes litorales que diluyen los aportes así como el agua se mueve hacia el norte. La fracción particulada es la forma más abundante de los metales, probablemente debido al aporte del polvo atmosférico proveniente del desierto de Atacama, representando una fuente natural de metales a las aguas de la bahía.

Palabras claves: Distribución de metales traza, aguas costeras, areas desérticas.
 

INTRODUCTION

Coastal areas near urban and industrial areas are affected by the waste products of both activities which, in many cases, change the natural trends of substances that have natural and anthropogenic sources. Usually, these sources are multiple and variable in nature, supplying different kinds of wastes, and thus changing the composition, distribution and abundance of pollutants in these areas (Luoma 1990; Giordano et al. 1992; French 1993).

In this context, trace metals have been one of the main group of elements whose abundance in the environment has been extensively studied, because of their toxicity and because trace metals are also produced in significant amounts, both in urban and industrial processes. These metals have natural source that reaches the coastal zone by the same biogeochemical pathways used by anthropogenic sources, mixing both signals, which complicates the understanding of their spatial distribution patterns (Chester & Murphy 1990). On the other hand, many trace elements are considered as essential for organisms and thus their relative abundance is influenced significantly by biological processes. In fact, it is well known that several trace metals such as Cd, Ni, Cu and Zn have a similar distribution pattern as algae micronutrients (Bruland 1980; Danielsson 1980; Boyle et al. 1981; Jickells & Burton 1988; Hunter & Ho 1991). This is to be interpreted as these trace elements are involved in biological cycling, and thus its distribution and abundance can reflect biological activity (Kudo et al. 1996).

In Chile the studies of trace metals in the marine system are scarce and basically shows the existence of areas of high metal concentration such as Chañaral Bay in northern Chile (Castilla 1983; Olivares & Ruiz 1991) and San Vicente Bay in central south Chile (Salamanca et al. 1986). On the other hand, the history of heavy metals in Caleta Coloso (northern Chile), Concepción Bay (central south Chile) and in Chacabuco Bay (fjords area) has been established (Salamanca & Camaño 1994; Salamanca 1996). In all these locations, anthropogenic trace metals signals has been clearly detected, indicating an alteration of metal natural supply processes. Finally, sediments cores from the fjord and Strait of Magellan area has been analyzed for heavy metals, showing natural Cd, Cu, Pb and Zn vertical distributions in sediment column (Ahumada 1996).

The present study was carried out in San Jorge Bay, along the coast of Antofagasta City, located in the Atacama desert, which represents the largest populated area in northern Chile (~ 260,000 inhabitants). The coastal ocean waters receive several discharges from municipal and industrial activities, including eight domestic effluents, three oil loading terminals, two non treated industrial discharges (one of them is the Fiscal Port where mineral concentrate is loaded) and one treated (Minera Escondida effluent). These discharges are mainly concentrated in the northern side of the city, being the Minera Escondida port facilities the only industrial activities to the south of Antofagasta City (Fig. 1).

The oceanographic conditions of San Jorge Bay are not well characterized, but the bay is dominated mostly by cold waters of the Humboldt current and by an intermittent upwelling process (Marin et al. 1993; Escribano et al. 1997; Rodriguez et al. 1996). The temperature range vary between 13.13 and 21.07ºC.

The coastal zone of the study area is under intensive and extensive use as discharge location of different sources of pollutants, so in this research an extensive spatial survey of metal content was carried out to verify the current status of Cu, Pb and Zn content in the nearshore waters of this area, both in the particulate and dissolved fractions.

Figure 1. Map of the study area, showing the sampling stations and main pollutant discharges to San Jorge Bay coastal waters. Figura 1. Mapa del área de estudio, mostrando las estaciones de muestreo y las principales descargas de contaminantes a las aguas costeras de Bahía San Jorge.

METHODOLOGY

SAMPLING

One liter seawater samples from seven locations behind the surf zone, along the coastal area of San Jorge Bay in Antofagasta, northern Chile (Fig. 1), were collected manually in narrow mouth low density polyethylene bottles (Nalgene) in June, October and December of 1996. The bottles were exhaustively cleaned involving soaking in detergent solution (2% v/v) Extran 300 for 24 h, rinsing with abundant deionized water and then a acid treatment with 3 M Merck Suprapur HCl for another 24 h, rinsing with deionized water and soaked with HNO₃ for another 24 h followed by rinsing with copious quantities of Milli-Q water. Each bottle was stored separately in re-sealable polyethylene bags. The samples were collected in triplicate and are reported as average values.

ANALYSIS

The dissolved metals fraction was obtained by pressure filtration using 0.45 µm acetate cellulose membranes pretreated with deionized water and a 3M HCl solution and rinsed with abundant Milli-Q water. A total of 1.5 l of sample were filtered, discarding the first 500 ml. A separate subsample (250 ml) was taken for Zn analysis and another subsample (500 ml) was considered for Cu and Pb determination. These samples were acidified with 0.03 M HCl and concentrated HNO₃, respectively using Merck Suprapur acids.

The solid material retained by the membrane was analyzed for particulate metals, rinsing the filters with Milli-Q water and stored frozen until analysis. These samples were digested in concentrated HNO₃ using clean teflon bombs heated for 1 h at 160ºC. After cooling the residue was made up to 25 ml with Milli-Q water to be analyzed for Cu, Pb and Zn.

Trace metals were analyzed by Anodic Stripping Voltametry (ASV) using a ISS-820 Radiometer Polarograph in the Chemistry Depart- ment of the Antofagasta University, Chile. The accuracy and precision of the analytical method were assessed using CASS-3 (National Research Council of Canada) certified standard coastal seawater. Quality appraisal data are given in Table I.

Table I. Summary of quality control data.

Tabla I. Resumen del control de calidad de los datos.

 

Element	aFound	Certified CASS-3	bAccuracy	cPrecision
	(µg 1-1)	(µg 1-1)	(%)	(%)
Cu	0.530	0.517	2.5	3.9
Pb	0.013	0.012	6.3	5.7
Zn	1.247	1.240	0.5	6.7
aMean value of 12 measurements bPercentage deviation away from certified value of CASS-3 cRelative standard deviation of CASS-3 analysis (n=12

RESULTS

The accuracy analytical results obtained for the reference material CASS-3 vary between less than 1% (Zn) to 6.3% (Pb), with a precision about 5% (Table I), indicating an acceptable accuracy and precision of the analytical techniques.

The average trace metals and total suspended solids (TSS) data obtained in the study area are presented in Table II. The average TSS does not shows large variability between sampled locations. On the contrary, trace metal present a clear spatial distribution showing higher concentrations within Antofagasta City limits, being Station 5, near by the industrial sector, the zone that present the highest metal concentrations.

Table II. Total suspended solids and average* seawater trace metal concentrations along the coastal zone of San Jorge Bay, Antofagasta area.

Tabla II. Concentración de sólidos totales suspendidos y contenido promedio* de metales en traza en agua de mar, a lo largo de la zona costera

 

	TSS (mg l-1)		Dissolved (µg l-1)						Particulate (µg g-1)
Location	000000000	c.v	Cu	cv	Pb	cv	Zn	cv	Cu	cv	Pb	cv	Zn	cv
El Lenguado (1)	15.7	25.7	0.97	12.7	0.019	29.5	1.78	22.01	126.8	41.0	4.21	102.3	149.7	31.28
Murallones (2)	15.9	20.3	1.12	06.8	0.020	57.7	1.74	31.03	173.0	42.0	9.80	111.5	177.9	73.32
Universidad de Antofagasta (3)	16.3	30.2	1.16	19.8	0.029	73.0	1.92	35.17	153.3	53.0	31.2	094.0	346.2	95.18
Puerto Antofagasta (4)	15.8	27.0	1.21	15.1	0.017	21.1	1.88	19.87	152.8	60.0	16.0	036.0	343.4	88.56
Descarga Industrial (5)	17.2	18.1	1.65	42.4	0.020	29.0	2.00	10.49	467.2	40.3	33.8	048.8	403.5	50.88
Las Rocas (6)	16.8	20.6	1.12	16.7	0.019	21.1	1.96	14.73	161.4	46.3	18.6	32.0	208.3	45.11
Los Metales (7)	16.4	31.7	1.14	29.0	0.018	22.2	1.70	46.49	152.5	31.7	07.0	86.6	179.4	53.70
*Calculated as the average value of june, october and december 1996 sampling.

Numbers in parenthesis indicate the location of sampling stations, shown in reference map.

Figure 2 presented the annual average concentration of the dissolved (upper) and particulate (lower) fractions of Cu, Pb and Zn for each sampled location. These results shows a clear spatial trend in the particulate fraction where Sta. 5 (Antofagasta Industrial Park) present the highest metal content. The dissolved fraction this location also slowed the highest content of Cu and Zn, but in the case of Pb the highest concentration was presented in Sta. 3 to the south of Antofagasta city.

Figure 2. Dissolved and particulate spatial distribution of Cu, Pb and Zn in San Jorge Bay waters. Sampling station number and its corresponding locations: (1) El Lenguado; (2) Murallones; (3) Universidad de Antofagasta; (4) Puerto Antofagasta; (5) Descarga Industrial; (6) Las Rocas and (7) Los Metales. Figura 2. Distribución espacial de Cu, Pb y Zn disuelto y particulado en las aguas de Bahía San Jorge. Estaciones de muestreo y sus correspondientes localidades: (1) El Lenguado; (2) Murallones; (3) Universidad de Antofagasta; (4) Puerto Antofagasta; (5) Descarga Industrial; (6) Las Rocas y (7) Los Metales.

Particulate copper distribution is characterized by uniform concentration along the coast with values between 100 ~ 150 µg g^-1 except in Sta. 5 where there is a clear increase up to 450 µg g^-1. The dissolved Cu, on the other hand, shows a pattern characterized by lower concentration in the southern side of the study area, increasing towards the north, reaching a maximum in Sta. 5, to decrease again in Sta. 6 and 7. However this decrease does not reach the level of southern locations.

Lead present a different spatial distribution pattern when compared with that of Cu. Particulate lead showed two locations with high concentrations, both located between Antofagasta City limits. Further south or north, the concentration decrease by about 50% (Fig 2). However, the dissolve fraction spatial distribution pattern exhibits a uniform concentration along the coast, except in Sta. 3 where there is a clear increase of this metal in sea water.

Finally, particulate Zinc showed a spatial distribution pattern comparable to that of lead, i.e., high concentration within city limits (Sta. 3, 4 and 5) and lower values to southern and northern limits of the study area. The highest concentrations corresponded to Sta. 5. On the other hand, the dissolved fraction of Zn showed large variability in its spatial distribution with increasing concentration station in Sta. 3 relative to southern stations, to decrease again in Sta. 4 again, to reach a maximum in Sta. 5, decrease again in Sta. 6, to finally reach the lowest level in Sta. 7, far to the north of study area.

DISCUSSION

SPATIAL DISTRIBUTION

The spatial distribution of Cu, Pb and Zn along the waters the coastal zone of San Jorge Bay is clearly affected by the inputs from local sources and the hydrodynamic of area. In fact, a predominant current moving towards the North running along the San Jorge Bay, in response to wind pattern, has been described (SHOA 1988). This can explain the metal distribution pattern, characterized by lower concentration in the southern part of the studied area, where there are no important trace metals sources, except Minera Escondida facilities. As the water moves toward north, receive the discharge of industrial and municipal effluents, reaching a maximum just in front to a industrial Port area, to decrease again further north.

To verify this pattern, the data on metal concentration were analyzed using exploratory analysis based on multivariate statistical techniques. Fig 3 presented the results of Principal Components Analysis (a) and Correspondence Analysis (b). Both tests shows comparable results, separating the locations in three groups. Group I includes Universidad de Antofagasta, Puerto Antofagasta and Las Rocas, which correspond to sampling location within Antofagasta City limits (Fig. 1), Group II includes El Lenguado, Murallones and Los Metales sites, located far from city limits, in fact, these points are close to external limits of the studied area. Finally, Group III considers the sampling site located in front of the discharge of the industrial Port, well separated of the other two groups. This distribution supports the reasoning that the local sources determine the abundance of the three studied metals in coastal waters of San Jorge Bay.

Figure 3. Locations groups resulting from multivariate analyses. a) Principal Component Analysis and b) Correspondence Analysis. Figura 3. Grupos de localidades resultantes del análisis multivariado. a) Componentes Principales y b) Análisis de Correspondencia.

The spatial relation for each metal was evaluated using Cluster Analysis. In Fig. 4 it is presented the resulting clusters, which are not equivalents but shows some similarities, particularly in separating the site in front of industrial Port discharge, and the relation between El Lenguado and Murallones stations, which represent the southern limit of the study area. It is interesting to note that in the case of Cu and Zn, sites located at the extremes of the bay are grouped together. On the contrary, the largest mixing of locations is shown in the Pb distribution, reflecting more scatter in the sources, which might be due to the atmospheric mobilization of this metal from a local source such as Antofagasta City.

Figure 4. Cluster diagram for Cu, Pb and Zn distribution in the coastal waters of San Jorge Bay. Sampling station number and its corresponding locations: (1) El Lenguado; (2) Murallones; (3) Universidad de Antofagasta; (4) Puerto Antofagasta; (5) Descarga Industrial; (6) Las Rocas and (7) Los Metales. Figura 4. Dendrograma de la distribución de Cu, Pb y Zn en las aguas costeras de Bahía San Jorge. estaciones de muestreo y sus correspondientes localidades: (1) El Lenguado; (2) Murallones; (3) Universidad de Antofagasta; (4) Puerto Antofagasta; (5) Descarga Industrial; (6) Las Rocas y (7) Los Metales.

METAL PARTITIONING

The partition of metal between dissolved and particulate fractions may be defined by a distribution coefficient K_D (McManus & Prandle 1996):

K_D = C_P / C_D x 0.000001

with C_P = C_P/SS, where C_P is the concentration in the particulate fraction (µg l^-1) and SS the suspended sediment concentration (mg l^-1), which represent the metal concentration in the particulate fraction (µg mg^-1 of suspended particulate material) and C_D being the dissolved concentration fraction (µg l^-1). Thus the resulting units of K_D are in ml g^-1, and the 0.000001 factor represent the conversion factor for SS in mg l^-1 to SS in g ml^-1. The resulting K_D computed as described above, are presented in Table III. The lower K_D values are shown by Cu (~ 10⁵) and the largest by Pb and Zn (~ 10⁸). In general, the distribution of trace metals in surface waters is controlled by the input function, the circulation effects and internal processes, such as output, trapping or recycling (Chester 1996). In a coastal zone, such as the present study, the metals are supply mainly via atmospheric, through terrestrial dust in suspension from the desert nearby (river run off is considered negligible, since no water discharge at all exists in the area) and local anthropogenic sources. A third sources that can supply metals to the particulate fraction is the biogenic particles produced in situ, but this source can be important only during the high production season, which occurs in spring and summer time with Chlorophyll a values between 8.1 to 155.4 mg m^-2 (Rodriguez et al. 1996). The first two source are probably the most important in supplying metals, mainly in the particulate form, which is indicated by high K_D values (~ 10⁵ - 10⁸) calculated for Cu, Pb and Zn. Considering that the TSS values vary between 15.7 and 17.2 mg l^-1 the mass of metals in the particulate fraction in fraction is an important source. In fact, McManus & Prandle (1996) indicate for the North sea K_D values less than 10⁵ show dominance of the dissolved fraction. On the other hand, the three order of magnitude difference between K_D values for the studied metals can also be a reflect of the different biogeochemical behavior of the metals (Kremling et al. 1997), particularly in the case of Pb, where the atmospheric supply can enhanced by Pb from the combustion of cars circulating in Antofagasta City. This burden of trace elements reaching the nearshore waters then is redistributed by the current system described above, which tend to transport the water towards north. This motion can also explain the decrease of Cu, Pb and Zn further north the Puerto Industrial discharge, diluting the trace metal content as the waters moves to the northern part of San Jorge Bay, producing a diminish of the concentrations to levels that are comparable to the southern situation, where the lower metal concentrations are found.

Table III. Partition coefficients of samples in the study area.

Tabla III. Coeficientes de partición en las muestras del área de estudio.

 

Location	Kd Cu (x105)	Kd Pb (x108)	Kd Zn (x108)
El Lenguado (1)	1.31	7.62	0.84
Murallones (2)	2.21	4.89	1.02
Universidad de Antofagasta (3)	1.32	10.760	1.80
Puerto Antofagasta (4)	1.26	9.62	1.83
Descarga Industrial (5)	2.83	17.160	2.02
Las Rocas (6)	1.44	9.80	1.06
Los Metales (7)	1.34	3.89	1.05

CONCLUSIONS

 

The main conclusion that can be drawn from this study is that the Cu, Pb and Zn in the coastal waters of San Jorge Bay, is in response to a local input from a industrial discharge coming from an industrial park located within Antofagasta City limits, and the current system that run from south to the north, carrying away the metal and diluting them as the water moves to the north. This explains the sharp decrease of Cu, Pb and Zn in the waters of the northern part of the bay.

The particulate fraction is the dominant form of the metals in the study area. This is expected, since the main atmospheric source of metals is dust coming from the Atacama desert, representing a natural input to the water of the bay. The other source is a local discharge from an industrial park, which also have the largest TSS content, representing an anthropogenic supply.

The distribution coefficient K_D vary between 10⁵ - 10⁸, indicating the dominance of the particulate fraction if this metal in the coastal waters of San Jorge Bay.

Fecha de recepción: 23.03.2000
Fecha de aceptación: 30.08.2000

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