Distribution and Partition of Trace Metals in the Amazon Basin
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HYDROLOGICAL PROCESSES Hydrol. Process. 17, 1345–1361 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1288 Distribution and partition of trace metals in the Amazon basin Patrick T. Seyler1* and Geraldo R. Boaventura2 1 Institut de Recherche pour le D´eveloppement (IRD), Laboratoire des M´ecanismes de Transfert en G´eologie (LMTG), Universit´e Paul Sabatier, 38 rue des trente-six ponts, 31400 Toulouse, France 2 Instituto de Geociˆencias, Universidade de Bras´ılia, CEP 70910-900 Bras´ılia, DF, Brazil Abstract: The distribution of trace metals (V, Cr, Mn, Co, Cu, Zn, As, Rb, Sr, Mo, Cd, Sb, Cs, Ba, U) was investigated in surface waters and associated particulates in the Amazon mainstream (Solimoes˜ and Amazon rivers). Dissolved V, Cu, As, Sr, Ba, U correlate with major ions and appear to be predominantly derived from soluble rocks occurring in the Amazon upper basin. These elements appear conservative in waters and are progressively diluted by less-concentrated waters coming from the lowland and shield areas. A monthly time series obtained at the Obidos´ gauging station shows that temporal variability of trace element concentrations reflects the source, remobilization and/or biological processes occurring in the channel or in the surrounding floodplain lakes. The trace element concentrations in the particulate matter show a clear relationship with the location of the samples. V, Co, Cr, Mn, Sr, Cs and Ba concentrations are higher in the Solimoes˜ and the Rio Negro is enriched in Fe, Al and Zn. In the Rio Solimoes,˜ V, Cr, Mn, Co, Ni, Zn, Cs and Pb are almost entirely carried by the river particulate matter; Cu, Rb, Sr, Ba and U are transported mainly by the suspended particles, but a dissolved phase contributes to the transport. In the Rio Negro, the proportion of elements transported by the dissolved phase is higher for the whole set of elements. The implications of these results allow us to compute the fluxes from the Amazon River to the Atlantic Ocean. Copyright 2003 John Wiley & Sons, Ltd. KEY WORDS Amazon basin; trace metal geochemistry; trace metal fluxes INTRODUCTION Trace metallic pollutants, such as heavy metals (Cd, Pb, Hg, Cr, Zn, Cu, etc.) are a major issue for the quality of continental waters and of great concern in terms of pressure on both ecosystems and human health. In contrast to organic pollutants, metals do not degrade or eliminate; their physical and chemical forms change and these elements are readily remobilized into the environment by natural transformation mechanisms. An understanding of the biogeochemical processes involved in the transfer, accumulation and mobility of trace metals in pristine environments is required in evaluating the capacity of receiving waters to accommodate wastes without detrimental effects. The Amazon River system, which covers 6 ð 106 km2 and supplies up to 20% of all the river water discharged to the ocean (Molinier et al., 1997) is relatively free of industrial and agricultural interference. Due to its magnitude, the Amazon River is a prominent link in trace element cycles and quantifying its exportation fluxes to the ocean is of considerable importance. A limited number of papers deal with the distribution and partition of trace metals in the Amazon River. Martin and Meybeck (1979) and Martin and Gordeev (1986) gave a global tabulation of trace metal concentrations in particulate matter of major rivers including the Amazon, and Palmer and Edmond (1992) * Correspondence to: Patrick T. Seyler, Institut de Recherch pour le Developpement,´ Laboratoire des Mecanismes´ de Transfert en Geologie´ (LMTG), Universite´ Paul Sabatier, 38 rue des trente-six ponts, 31400 Toulouse, France. E-mail: [email protected] Received 5 January 2002 Copyright 2003 John Wiley & Sons, Ltd. Accepted 22 July 2002 1346 P. T. SEYLER AND G. R. BOAVENTURA measured dissolved Fe, Al and Sr concentrations in the Amazon mainstream and a number of its tributaries. Konhauser et al. (1994) reported the trace and rare earth elemental composition of sediments, soils and waters, mainly in the region of Manaus. Gaillardet et al. (1997) published an extensive data set for major and trace concentrations in the Amazon region extending between Manaus and Santarem. More recently, Seyler et al. (1999), Elbaz-Poulichet et al. (1999) and Seyler and Boaventura (2001) presented a comprehensive survey of trace metal forms in the Madeira and Amazon river basins. Our aim in the present work is to gain new insights into weathering and transport processes controlling the fate and flux of trace metals (V, Cr, Mn, Co, Cu, Zn, As, Rb, Sr, Mo, Cd, Sb, Cs, Ba, U) of the major streams in the Amazonian basin. MAJOR ENVIRONMENTAL FEATURES OF THE AMAZON BASIN The drainage basin of the Amazon River is characterized by a high diversity of geological formations. Three major geological provinces can be distinguished: (i) the Guyana and Brazilian shields (44% of the basin area) with metamorphic and crystalline rocks; (ii) the Andean cordillera (11% of the basin area) which consists of the Precambrian basement, formed by sediments, igneous and metamorphic rocks, overlain by Paleozoic and Mesozoic red clays and dark shales, and by carbonates and evaporites; (iii) the Amazon trough (45% of the basin area) filled with a massive layer of fluviolacustrine sediments ranging in age from Paleozoic to Pleistocene (Figure 1). The average air temperature in the Amazon is rather uniform, ranging from 24 to 26 °Cinthemajorpartof the basin, and the mean annual precipitation for the basin is 2400 mm. The Amazon climatology was described extensively by Marengo and Nobre (2001). The whole Amazon basin is covered by tropical rainforest (71%) and savannas (29%; Sioli, 1984). The soils of the Amazon basin belong mainly to the red ferralitic soil family. Their mineralogy is dominated by quartz, Al and Fe oxides and kaolinite, with a few accessory minerals such as anatase and zircon (Bernoux et al., 2001 and references cited therein). Compared to crustal abundances, these soils are more siliceous and Figure 1. Major geomorphological features of the Amazon River basin Copyright 2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 1345–1361 (2003) TRACE METALS IN THE AMAZON BASIN 1347 aluminous with considerably lower levels of major cations. Numerous podzol zones exist on the central plain (Konhauser et al., 1994). The major tributaries in the upper Amazon basin have their sources in the Andes (Solimoes,˜ I¸ca, Japura and Madeira rivers), in the sub-Andean trough (Jurua´ and Purus rivers) or in the Guyana shield (Negro and Trombetas rivers). The main tributaries of the lower course, the Tapajos´ and Xingu´ rivers, drain the Brazilian shield (Figure 1). During the period 1965–1990, the mean annual water discharge of Solimoes˜ at Manacapuru (confluence with the Rio Negro) was estimated at 103 000 m3 s1, the discharge of the Negro River at 28 000 m3 s1 and the discharge of the Madeira at 31 200 m3 s1 (Molinier et al., 1997). At Obidos,´ which is the ultimate gauging station on the Amazon river upstream of the marine influence, the mean discharge was estimated as 168 000 m3 s1. The proportion of water originated from the Solimoes,˜ Negro and Madeira rivers accounts for 95% of the total discharge at Obidos´ and varies with total discharge during the annual cycle (Guyot et al., 1999). Regarding the whole basin, up to 209 000 m3 s1 is discharging to the Atlantic Ocean (Molinier et al., 1997). The low inter-annual variability of rainfall, the size of the basin, the lag between tributary inflows and the storage capacity of the extensive floodplain (varzeas)´ are responsible for the low inter-annual variability of the Amazon hydrograph. Concerning sediment transport, the more recent results obtained by Filizola et al. (1999) give a mean annual discharge of suspended sediment close to 770 ð 106 tatObidos,´ where 97% is due to the contribution of Andean tributaries (62% from the Solimoes˜ and 35% from the Madeira). The contributions of the Negro, Trombetas, Tapajos´ and Xingu´ account for less than 3%. An examination of the suspended sediment concentrations (SSM) and discharge versus time at Obidos´ indicates that plots of the relations between sediment discharge and water discharge will form loops rather than a straight line. During the hydrological period, SSM concentrations show high frequency variations (10 days) and the sediment peak discharge precedes by three months the maximum water discharge (Guyot et al., 1999). SAMPLING AND METHODS The Solimoes,˜ as the Amazon mainstream is called between the Colombia and Brazil boundary and its confluence with the Rio Negro and the Amazon, downstream, were sampled on board R/V C. Dario, in October/November 1995 and in April/May 1997, corresponding to high water stage (HW) and low water stage (LW), respectively. Moreover, a monthly time series covering a whole hydrological cycle was obtained at the Obidos´ gauging station during the 1997 year. The sampling locations (Figure 2) were mostly the DNAEE/ANEEL stations, where water discharge was measured systematically with an acoustic Doppler current profiler (ADCP-RDI, 300 kHz) with a precision of over š5%. Suspended sediment concentrations were determined at the same time by filtering 5 l of river water, collected on three or five vertical profiles (depending on the section width) across the river section. In order to avoid contamination of the main vessel, water samples for major and trace element determination were taken from a small boat upwind the vessel, at approximately 0Ð5 m depth in the middle of the river section. Samples were collected in acid-washed polyethylene containers. The samples were filtered immediately through 0Ð22-µm Millipore filters, under a laminar air flow bench in the laboratory of the research vessel according to the procedure described elsewhere (Seyler and Elbaz-Poulichet, 1996).