Abstract

The Mapocho river, which crosses downtown Santiago, is one of the most important rivers in contact with a population of about six million inhabitants. Anthropogenic activities, industrialization, farming activities, transport, urbanization, animal and human excretions, domestic wastes and copper mining have affected the river, contaminating it and its sediments with heavy metals. Concentration and distribution of Cu, Zn, Pb and Cd were studied with the purpose of determining their bioavailability and their relation with the characteristics of the sediments. Freshly deposited seasonal sediments were collected from 0-8 cm depths from 6 locations (S 1 to S 6) along the 30-km long channel length, in the four seasons of year on the following dates: May 2001 (D 1, autumn); August 2001 (D 2, winter); October 2001 (D 3, spring) and January 2002 (D 4, summer). The dried samples were sifted to obtain the < 63-μm sediment fraction, since it has been shown that large amounts of heavy metals are bound in the fine-grained fraction of the sediment. Cu and Zn were analyzed by atomic absorption spectrophotometry and Pb and Cd by square wave anodic stripping voltammetry. The highest concentrations of Cu (2850 μg g - 1) were found in the northern part of the river (S 1, average D 1-D 4), near the mountains and a copper mine, and then decreased downstream to 209 μg g - 1 (S 6). Total Zn showed an irregular variation, with higher values at S 1 (1290 μg g - 1) and high values in some winter sampling (1384 μg g - 1 S 4, S 5-D 2). Pb showed different trends, increasing from S 1 to S 6 (17 to 61 μg g - 1), with the highest values in the summer samples (83 μg g - 1, S 4-S 6, D 4), and total Cd increased slightly from mean values of 0.2 and 0.5 μg g - 1. Partition into five fractions was made using Tessier's analytical sequential extraction technique; the residue was treated with aqua regia for recovery studies, although this step is not part of the Tessier procedure. The results show that Cu, Zn and Pb in the sediments were dependent on the sampling places along the river, and variation in two years was low (D 1-D 4). The highest values of total organic matter, carbonate and conductivity were found in S 6, which has the smallest size particles, while at S 1 the sediments were predominantly sand and contain larger amounts of silica. Cu associated with carbonate decreased gradually from 58% (1771 μg g - 1, S 1) to 16% (32 μg g - 1, S 6); Cu bonded to reducible fraction was almost constant (33% to 37%), and Cu associated with oxidizable fraction increased from 7% (S 1) to 34% (S 6), but copper content was lower (214 to 68 μg g - 1). Zn had a similar fractionation profile. However, Pb bound to oxidizable fraction did not show significant percent variation along the river (20% to 19%), but the amount bounded was 4 to 12 μg g - 1. The residual fraction increased from 24% to 41% (5 to 25 μg g - 1, S 1 to S 6). The distribution of Cd in the sediment was almost independent of the sampling stations and was bound to carbonate, reducible and residual fraction in similar proportion. Cu and Zn at S 1 were mainly bound to carbonates and reducible phases with 91% and 73% (2779 and 965 μg g - 1, respectively), and with a change in the pH and/or the redox potential of the sediment-water system, these contaminants could easily enter the food chain. In S 6 the amount of Cu and Zn in these phases was 50% and 53% (100 to 313 μg g - 1, respectively). © 2006 Elsevier B.V. All rights reserved.