HEAVY METAL CONCENTRATIONS IN SEDIMENTS AND SUSPENDED PARTICULATE MATTER FROM MAR CHIQUITA COASTAL LAGOON, ARGENTINA.

L. Ferrer, S. Andrade, J. Marcovecchio (Instituto Argentino de Oceanografía, Florida 4000, C.C.804, 8000 Bahía Blanca, ARGENTINA), S. DeMarco & A. Gavio (Universidad Nacional de Mar del Plata, Funes 3350, 7600 Mar del Plata, ARGENTINA)

INTRODUCTION

Coastal environments are the main target for pollutants because of the different routes of transport which converge there (Salomons & Förstner, 1984). The study of coastal lagoons systems is very important because their productivity originates in ocean-continent interaccion, which makes them a part of the most productive ecosystems in the biosphere (UNESCO, 1982).

The study of the levels and distributions of trace metals in marine of estuarine sedments showed to provide a practical way of assessing metal pollution in coastal ecosystems. By the way, suspended particulated matter (SPM) plays an important role in maintaining chemical and biological gradients in the aquatic system. Its composition and concentration reflect dynamic water-column processes such as primary production in surface waters, dissolution and degradation at bottom, and vertical and lateral particle transport (Leventer & Dunbar, 1985).

The Mar Chiquita coastal lagoon is unique on its type within the marine littoral of Argentina. It is situated 32 Km northeast of Mar del Plata City (38°00’S; 57°30’W), Buenos Aires Province, Argentina. This has an area of approximately 50 Km² and a tributary basin of 10000 Km², including several areas affected not only by farming but also from industrial activities. Moreover, numerous areas close to Mar Chiquita are intensively utilized with urbanistic, touristic or sport / recreative goals, with the corresponding effects on the aquatic system (occupation of wetlands, effluents drainage and waste disposal, human activities, etc.).

The present study includes basic information on the occurrence and distribution of cadmium, copper and zinc in surface sediments, <63µm grain size sediment fraction, and suspended particulated matter from Mar Chiquita Coastal Lagoon.

MATERIALS AND METHODS

Samples of surface water and surface sediments from Mar Chiquita coastal Lagoon were collected in six (6) stations, in order to study the spatial distribution of cadmium, copper and zinc in suspended particulate matter (SPM), surface sediments and <63µm grain size sediment fraction. The area under study and location of sampling stations are shown in Figure 1.

Surface water samples (0,2-0,8 m) were collected with a Van Dorn bottle. Samples of water were filtered through Millipore HA filters (0,45 µm pore size) for the determination of suspended matter and particulate cadmium, copper, and zinc. Before use, the filters were soaked overnight in dilute HCl and dried at 75°C until constant weight was attained. Absolute suspended particulate concentrations are determined through direct filtering. The samples of suspended particulate matter were kept in a freezer, at -20°C until analysis at the laboratory.

Samples of coastal surface sediments were collected, and were kept in plastic bags, and stored in a freezer (at -20°C) until their treatment in the laboratory. All visible organisms and shell fragments were hand removed. Samples were dried at 45 ± 5°C up to constant weight. After that, sub-samples were sieved, and the fraction smaller than 63 µm was separed.

Surface sediments and suspended particulate matter were digested in a mixture of concentrated nitric and perchloric acids (3:1), according to the method described by Dalziel & Baker (1983) modified by Marcovecchio et al. (1988).

A Perkin-Elmer Model 2380 atomic absorption spectrophotometer with air/acetylene flame was utilized to determine cadmium, copper and zinc concentrations.

Analytical grade reagents were used to built up the corresponding blanks and calibration curves, and the analytical quality (AQ) was tested against reference materials, provided by The National Institute for Environmental Studies (NIES) from Tsukuba (Japan).

Figure 1. Location of sampling stations in the study area.

RESULTS AND DISCUSSION

Suspended particulate concentrations range from approximately 5 to 65 mg/L. A large diversity of mechanisms regulates suspended particulate matter distribution in the study area (i.e., tide sea influence, freshwater contribution, wind, climate, etc.).

Analyzed metals have been recorded in most of the Mar Chiquita Coastal Lagoon samples, even though several of them were present in extremely low concentrations within the total fraction of sediments.

By the way, it must be underlined that sediment grain size distribution is not homogeneous within the coastal lagoon. So, sampling stations 1 and 2 –close to coastal lagoon outlet- have shown higher pecentages of coarse grain sediments: station 1 has presented 49% of medium sand (500 – 250 µm), while station 2 has 77% of fine sand (250-125 µm). Simultaneously both sampling stations have presented less than 5% of mud ( < 63 µm ). Unlike this, the other analyzed sampling stations have shown lower percentages of coarse sediments (i.e., sands) and higher one of mud (at least 10%).

Cadmium concentrations in total fraction sediments have varied between non detectable values and 0.16 µg/g, dry wt., while in the finest fraction did between 1.01 and 3.55 µg/g, dry wt. Moreover, cadmium in suspended particulate matter were ranged between 6.47 and 27.59 µg/g, dry wt. (Figure 2). These results are in agreement with those reported by other authors on comparable aquatic environments (i.e., Barcellos & Lacerda, 1996).

Nevertheless, in the mentioned Figure 2, it is possible to observe that a clear trend was obtained in the assessment of spatial distribution of the cadmium in SPM and total fraction sediment.

Figure 2. Cadmium concentrations in total fraction sediment, <63µ grain size sediment fraction and suspended particulate matter.

In the case of copper, concentrations in total fraction sediments ranged between 3.52 and 19.49 µg/g, dry wt., while in the finest fraction did between 21.22 and 500.97 µg/g, dry wt. By the way, those in suspended particulate matter were ranged between 121.05 and 355.14 µg/g, dry wt. (Figure 3). The maximum values of copper were registered at station 1 (close to the sea) in both <63µ grain size sediment fraction and suspended particulate matter, while the highest value of copper in total sediment did at station 4.

Figure 3. Copper concentrations in total fraction sediment, <63µ grain size sediment fraction and suspended particulate matter.

Zinc concentrations in total fraction sediments have varied between 11.78 and 39.15 µg/g, dry wt., while in the finest fraction did between 49.92 and 236.21 µg/g, dry wt. On the other hand, zinc in suspended particulate matter were ranged between 22.2 and 77.72 µg/g, dry wt. (Figure 4).

For the study area can be commented that particulate trace metal concentrations were seemed to be comparable with those measured in pristine rivers and non contaminated seawater (Wimdom et al., 1991; Lewis & Landing,1992; Nriagu et al., 1993).

Both copper and zinc have shown similar distribution patterns in Mar Chiquita Coastal lagoon, agreeing not only in SPM but also in <63 µ fraction distributions, and also presenting their peak (maximum value) at station 1, close to the coastal lagoon outlet. On the contrary, their concentrations in total sediment fraction have also shown to be minimum at station 1.

Unlike this, cadmium has presented similar distribution trends in SPM and total sediment but with the highest value at station 6 (the most inlet one). Meanwhile the <63 µ sediment fraction has shown a different behavior, with its maximum value at station 5.

The three metals analyzed have shown the highest values in the less than 63µ grain size sediment fraction. These results fully agree with the “grain size effect” (Salomons & Förstner, 1984), where the metal content is always maximum in the finest grain sediment. This fact suggests that the low <63µ grain percentage in Mar Chiquita Coastal Lagoon sediments produced a dilution effect on the total fraction content.

Figure 4. Zinc concentrations in total fraction sediment, <63µ grain size sediment fraction and suspended particulate matter.

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