Biogeochemical evolution of trace elements in a pristine watershed in the Brazilian southeastern coastal region

Establishing 'reference sites' is a difficult task and a critical factor in determining the baseline functioning of ecosystems. The information thus o btained on nutrient and contaminant background concentrations ill turn subs idizes the remediation of impacted landscapes. This paper reports a study o n metal (Cr, Cu, Pb, Zn, Mn, Fe and Al) and nutrient (C) distribution in se diments From Capivari River watershed (Praia do Sul Biological Reserve, Ilh a Grande, Rio de Janeiro State, Brazil), an area where typical SE Brazilian coastal ecosystems are located. Contrary to what one would expect from the high rate at which these ecosystems have been deteriorating in recent deca des, the study site is surprisingly well preserved. The present study was d eveloped to assess variations in heavy metal concentrations in river basin sediments, to identify the geochemical carriers of these elements, and dete rmine the influence of water quality and organic matter on their distributi on. Results showed that heavy metal distribution has been influenced by the natural biogeochemical properties of those ecosystems found in an upland-t o-lowland sequence in the watershed. Minimum and maximum total concentratio n were: 5 and 23 mg/kg for Cr; 4 and 29 mg/kg for Cu; 13 and 53 mg/kg for P b; 24 and 142 mg/kg for Zn; 54 acid 342 mg/kg for Mn, 0.8 and 7.2% for Fe; 0.5 and 4.9% for Al; 6.3 and 25% for C. The pH and E-H are the key-paramete rs in explaining total metal concentration decrease in the swamp area, wher e dissolution processes and losses through metal transport seem important. The most important geochemical carriers are Al in the basin's "continental" stretch and Fe in the estuarine portion. The data also provide evidence sh owing that organic matter is the key-parameter in Cu concentration control in the sediments through burial and accumulation processes especially in th e swamp area. Heavy metal concentrations in sediments from the study area a re generally lower than those found in similar regional ecosystems. Surface enrichment in heavy metal concentrations in collected sediment cores was n ot observed. The authors therefore conclude that this site is suitable as a 'reference site' for studies on the biogeochemistry acid ecotoxicology of SE Brazilian coastal ecosystems. (C) 2001 Elsevier Science Ltd. All rights reserved.