EVALUATION OF HEAVY METALS ACCUMULATION DURING THE LAST CENTURY IN A FRESHWATER SYSTEM AT THE NORTH REGION OF THE RIO DE JANEIRO STATE, BRAZIL.

Mônica Aparecida Leite da Silva & Carlos Eduardo Rezende

Universidade Estadual do Norte Fluminense, Laboratório de Ciências Ambientais - CBB, Av. Alberto Lamego, 2000, Campos dos Goytacazes, 28015-620, Rio de Janeiro, Brasil.

E-mail: crezende@cbb.uenf.br/crezende@rol.com.br

Tel: +55 24 7263709; Fax: +55 24 7263720

ABSTRACT

The Cima Lake (CL) is a freshwater system located at the North Region of the Rio de Janeiro State. It has a historic related to agricultural and cattle raising activities. The objectives of this study were understand the vertical distribution of organic matter and heavy metals associated to the lacustrine sediments and evaluate the influence of land use on the relative contribution of organic matter sources and heavy metals to the lake sediments. Sediment cores were collected in three stations: inlet, center and outlet of the CL. The (C:N)a ratios showed that between the fifties and sixties there was an alteration in the sources of organic matter to the CL sediments. The heavy metals concentrations were similar to those described for natural systems, in spite of the system being influenced by anthropogenic sources. The Me⁺/Ti ratios indicates the existence of human intervention processes in the referred lake, showing that the origin of metals is not only from natural processes.

INTRODUCTION

The sediment capacity in accumulates compounds makes this compartment one of the most important in the contamination level evaluation of continental aquatic ecosystems. The sediment, after the fine particles deposition and adsorption processes, is the main deposit of heavy metals accumulation in aquatic environment. Its chemical, mineralogical and granulometric composition is influenced by the soils and by the watershed geology (Salomons e Föstner, 1984).

The geochronology of sediments has been very applied to study processes in lakes and their watersheds (Durham e Joshi, 1980). The analysis of the development of the heavy metals and organic matter concentrations in vertical profiles of dated sediments can be used to realize historical studies that seek relationships with the evolution of adjacent terrestrial ecosystems and the emission sources. This evolution is often affected by significant changes associated to industrial development, deforestation, mining and the increase in the energetic and population consumption (Palanques et al. , 1998).

The Cima Lake (CL) is a tropical freshwater system located at the North Region of the Rio de Janeiro State. This lake is formed by two rivers (Imbé and Urubu) and it is drained to another lake by the Ururaí channel (Figure 1). It has a historic related to agricultural and cattle raising activities since the Atlantic forest that covered the adjacent areas of the lake are being deforested and replaced by pasture and sugar cane crops.

The objectives of this study were understand the vertical distribution of organic matter and heavy metals associated to the lacustrine sediments and evaluate the influence of land use on the relative contribution of organic matter sources and heavy metals to the lake sediments.

Figure 1: Study area and sampling stations.

METHODS

Sediment cores were collected in January 1998, in three stations: inlet (1), center (2) and outlet (3) of the CL (Figure 1). They were obtained with acrylic tubes of 7 cm diameter and were sectioned into 3 cm intervals. The slices were oven dried at 60 °C.

The age of sediment layers was determined using the procedure described by Godoy et al. (1998). The ²¹⁰Pb Constant Rate of Supply (CRS) method was applied since this method is applicable to systems with variable sedimentation rates along the years.

For the determination of organic carbon and total nitrogen were used about 5-10 mg of the total sediment fraction, grinded and sieved through 63 µm in order to homogenize the bulk material. The analyzes were performed using a Perkin Elmer CHNS/O Analyzer.

The samples for determination of total heavy metals (Fe, Mn, Cu, Cr and Ti) were digested following a modified version of the procedure used by Watts & Smith (1994). About 1 g of sediment was placed in Teflon bombs with concentrated acids (HF + HNO₃ , 2:1) and kept at 100 °C for 24 hours. After the total dissolution of the sample, the acid was evaporated to almost dryness, and the residue was redissolved with HCl 0.5 M (30 ml). The extracts were analyzed by ICP/AES (Varian Model Liberty Series II).

RESULTS AND DISCUSSION

In the CL inlet was possible to date sediments buried between 1998 and 1848. In the core of CL center the age corresponded to a period between 1998 and 1850, while in the CL outlet were dated sediments correspondents to 101 years of deposition, from 1998 to 1899 (Figure 2).

The (C:N)a ratios of the CL dated sediments are shown in the Figure 3, where can be observed that in the CL outlet sediment core wasn’t possible to calculate the (C:N)a ratios in the most recent period (from 1998 to 1952), because the nitrogen concentrations were below the detection limit of the utilized equipment.

The (C:N)a ratios profiles observed in the three stations cores revealed the existence of an alteration in the LC sediments composition. It was possible note that between the fifties and sixties there was an alteration in the sources of organic matter for the CL sediments, evidencing a period marked by the ascension of the sugar-cane industries in the region and by the intense deforestation in the lake watershed.

Figure 2: Age of the CL sediment profiles. Figure 3: (C:N)a ratios profiles of the CL

dated sediments.

The heavy metals (Cu, Cr, Fe, Mn and Ti) concentration profiles of the Cima Lake sediments are shown in the Figure 4.

The heavy metals concentrations profiles of the CL inlet presented similar distribution trends, describing curves that overlapped are very resembling, despite the different greatness orders. The frequent oscillations observed are related to differences on the physical characteristics of the sediments in different depths. It was observed the greatest concentration of heavy metals in intervals where the sediments presented predominance of finer materials, corroborating what was described by Salomons & Förstner (1984).

In the CL center, Cr, Cu and Fe concentration profiles presented an inflexion in their curves in 24 cm depth, indicating a change in the depositional trend of these elements in this depth, dated to 1987. The decreasing trends observed to Fe and Mn concentration profiles are related to their redox sensitive nature that lead to a large variation in their concentrations in the upper layers. This trend was also observed to manganese concentrations in the CL outlet profile.

The occurrence of Cr, Cu and Fe highest concentrations in the deepest layers of the CL outlet sediment profile can be related to differences on the sediment type, that presented constituted by a very fine material in the deepest layers. In these layers also were observed elevated values of organic carbon (about 25 %), being the heavy metals possibly bounded with the organic matter.

The heavy metals concentrations observed to the Cima Lake sediments were similar to those described in literature for natural systems, in spite of the system being influenced by anthropogenic sources.

The Cima Lake Me⁺/Ti ratios are shown in Figure 5. The Me⁺/Ti ratios of the CL inlet presented peaks with maximum values, indicating the existence of human intervention processes in the referred lake, such as deforestation and atmospheric contamination through the sugar and alcohol industries, showing that the origin of metals is also coming from these sources and not only from natural processes. In the CL center and outlet this same trend wasn’t observed, because only manganese and iron presented maximum peaks, but these redox sensitive elements aren’t considered appropriated to historical evolution studies.

Figure 4: CL heavy metals concentration Figure 5: Me⁺/Ti ratios profiles in CL

profiles. dated sediments.

ACKNOWLEDGMENTS: CER received a grant from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and UENF – FENORTE. The authors would like to thank the support in the field and laboratory activities of technician of Laboratório de Ciências Ambientais Arizolli Antônio Rosa Gobo.

REFERENCES

Durham, R.W., Joshi, S.R. (1980), Chemical Geology, 31: 53-66.

Godoy, J. M., Moreira, I., Wanderley, C., Simões Filho, F.F., Mozeto, A.A. (1998), Radiation Protection Dosimetry, 75: 111-115.

Palanques, A., Sanchez-Cabeza, J. A., Masqué, P., Léon, L. (1998), Marine Chemistry 61: 209-217.

Salomons, W. , Föstner, U. (1984), Metals in the Hydrocycle. Berlin, Springer-Verlag, 349 p.

Watts, S.E.J, Smith, B.J. (1994), The Science of the Total Environment 146: 507-514.