TEMPORAL DYNAMIC OF PARTICULATE AND DISSOLVED HEAVY METALS
IN THE PARAÍBA DO SUL RIVER, RIO DE JANEIRO, BRAZIL.
Salomão*, M.S.M.B.;
Molisani, M.M. & Ovalle, A.R.C.
Laboratório de
Ciências Ambientais, Universidade Estadual do Norte Fluminense, Av. Alberto
Lamego, 2000, Horto, Campos dos Goytacazes, RJ, 28015-620, Brazil, Tel: +55 24
726-3709, Fax: +55 24 726-3720, e-mail: salomao@cbb.uenf.br
Abstract
The
temporal dynamics of particulate and dissolved heavy metals (Al, Fe, Mn, Zn,
Cr, Cu, Pb, Ni and Cd) in the basin outlet of Paraíba do Sul River was
evaluated during one year. Samples were collected fortnightly and analysis
performed by ICP-AES. Concentration ranges of heavy metals in the suspended
particulate matter (SPM) were: Al: 3.7-10.2 %; Fe:3.3-5.8%; Mn:610-1700mg/g; Zn:90-200mg/g; Cr:60-150mg/g; Cu:10-200mg/g; Pb:20-460mg/g; Ni:25-50mg/g;
Cd:0.7-7.2mg/g. For
dissolved fraction the ranges were: Al: <5-4000mg/L;
Fe:<5-2400mg/L;
Mn:<2-13mg/L (the
others elements were always below detection limit). The relation between
discharge and metal concentrations was inverted for all metals in the
particulate phase, while dissolved metals concentrations increase during high
discharge periods. For particulate phase there were three major factors
affecting metals concentrations: surface runoff input and bottom ressuspension
during high discharge periods, and phytoplankton activity during low discharge
periods. This shift in the dominant source of SPM causes a decrease in metals
concentrations with increase discharge. The variation in the dissolved
concentrations could be related with soil leaching during high discharged
periods.
Introduction
Receiving waters accepts metal introduced from a variety of natural
(e.g. weathering of soils) and antropogenic sources (e.g. by industrial
activities). The transport of metals in surface water are governing by processes of mixing and dispersion.
During such processes, metals tend to redistribute among the dissolved,
suspended particulate matter (SPM) and sediment phases (Shi et al, 1998). Sorption and desorption
process at the particulate/water interface are some of the determining factors
governing the fate and flux of chemical species from the continent to the ocean
(Guarnier et al, 1996).
The Paraíba do Sul River is a medium-sized river 1145-km long and with a
drainage basin of 55400km2 that crosses three of the most developed
Brazilian states (Minas Gerais, São Paulo and Rio de Janeiro). Carvalho et al (1999) studying the dynamic of
particulate heavy metals in the lower basin of the Paraíba do Sul River,
concluded that this river can be considered with low or without contamination,
but reinforce that during low flow periods these concentrations have to be
better monitored.
The aim of this work was to study the temporal dynamics of
dissolved and particulate heavy metals (Cd, Cr, Cu, Pb, Zn, Ni, Fe, Mn and Al)
in the Lower Paraíba do Sul River, determining the main sources of these metals
during the dry and wet seasons.
Material and Methods
Samples were collected fortnightly between October 1998 and October
1999, on basin outlet, Campos dos Goytacazes, Rio de Janeiro State
(21°45’05’’S, 41°19’32’’W). Forty liters of water were collected in the middle
of river channel. In the laboratory the SPM were sieved in stainless sieves of
63µm of size pore, to share the SPM in two class sizes: Coarse (>63µm) and
fine (<63µm). The contribution of each fraction for the total SPM was
quantified by gravimetry. Samples were freeze-dried, digested
with a concentrated mixture of HNO3 and HF (2:1) during 12 hours at
100 oC, evaporated to dryness and the resultant residue dissolved
with 0,5N HNO3. For the determination of metal filtrated fraction
the samples were collected with teflon bottles, filtered through glass fiber
membrane (GF/F, Whatman), and acidified to pH 2 using HNO3 (suprapur
Merck). Both particulate and filtrated extracts were analyzed for metal
determinations by a inductively coupled plasma spectrophotometry (ICP-AES
Perkin-Elmer Liberty Series II).
DOC determination were performed in the filtrated sub-sample (GF/F,
Whatman), preserved wit H3PO4 5%, using a TOC 5000
Shimadzu Analyser.
Results & Discussions
SPM
varied directly with water flow (Fig. 1a), with the percentage of SPM>63µ varying between 4%
to 49% (Fig. 1b). The highest values
were observed during the high discharge period (January-98), as well as in a
low flow period (August-98). The former could be related with bottom
resuspension and surface runoff, and the latter with a autochthonous primary
production, as suggested by the inverse temporal variation of particulate
carbon with discharge (Fig. 2a) and chlorophyll-a data (Figueiredo, 1999).
The metals concentrations in the
SPM fraction varied seasonally, with all metals concentrations decreasing with
the increase of the river discharge (Tab. 1). This relationship between river
discharge and metal concentration was statistically significant for Cr, Ni, Pb,
Zn, Fe and Mn (p<0.05).
Table
1.
Heavy metals concentrations in the SPM (average ± standard deviation), during low (LD)
and high discharge (HD) periods in the Lower Portion of Paraíba do Sul River
(Cd, Cr, Cu, Ni, Pb, Zn and Mn in µg/g; Al and Fe in %)(n=17, LD; n=9, HD) .
|
|
Cd |
Cr |
Cu |
Ni |
Pb |
Zn |
Al |
Fe |
Mn |
|
LD |
1.7±2.9 |
107±18 |
40±7 |
37±7 |
119±22 |
205±11 |
7.5±2.0 |
5.0±0.5 |
1264±98 |
|
HD |
1.0±1.0 |
79±12 |
34±7 |
32±5 |
31±8 |
106±17 |
6.9±1.7 |
4.2±0.7 |
838±182 |
This pattern was presumably
associated with a shift in the major source of SPM. The SPM during high
discharge period was originated mainly by surface runoff and ressuspension of
bottom sediments, that increase >63µ fraction. During low
flow period there were an higher percentage of POC produced by phytoplankton
and by input from sewage, that promoted an enrichment in metal concentrations
during low discharge periods. This hypothesis was corroborated by correlations
between heavy metals and POC concentrations (p<0.05 for Cd, Cr, Ni, Pb, Zn,
Fe and Mn; Salomão, 1999).
Al, Fe and Mn in the dissolved fraction presented a direct
correlation with discharge (e.g. Iron Fig. 3b), as well as DOC (Fig. 2b). The
other metals were below detection limit. This fact suggests that dissolved
metals and dissolved carbon had the same source. The strong correlation between
DOC, Al and Fe indicate that these metals were forming complexes with the
dissolved organic matter during high discharge periods. This association allows
metals remain in the dissolved fraction in elevated concentrations, and at the
oxidizing conditions of the fluvial water. Hence the main factor governing the
concentration of dissolved metals in the study area was probably the soil
leaching during the wet season, through surface and sub-surface runoff
(Salomão, 1999). Benoit et al (1994)
studying Texas estuaries found similar trends in the dissolved fraction, and
they speculates that was occurring a release of metals from bottom sediments
and associated pore water. This could also be an important sources of soluble
metals.
Conclusions
For particulate metals there are two factors affecting
metals concentrations: runoff input
during high discharge periods, and phytoplankton activity and sewage during low
discharge periods. This shift in the source of SPM causes an increase in the
metals concentrations during dry season.
The variation in the dissolved fraction was related with
soil leaching during high discharge periods, as well as the re-suspension of
fine bottom sediments and associated pore water.
The strong correlation between DOC, Al and Fe suggests that
these metals were forming complexes with the dissolved organic matter during
high discharge periods. This association allows metals remain in the dissolved
fraction in elevated concentrations.
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(a) (b)
Fig. 1. Relationship between discharge and SPM
concentration in the Lower Portion of Paraíba do Sul River (a). Contribution of
coarse fraction of SPM (<63µm) for the total SPM concentration along an
year.
(a) (b)
Fig. 2. Temporal variation on the
concentrations of carbon in the particulate fraction (a) and in the dissolved
fraction (b) in the Lower Portion of Paraíba do Sul River.
(a) (b)
Fig. 3.
Temporal distribution of heavy metals in the Lower Portion of Paraíba do Sul
River. Cr in the SPM (a). Iron in the
dissolved fraction (b).